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This article presents a new beam model that employs a recursive derivation procedure that enables the user to set the order of the governing differential equations as an input parameter, without the need for ad hoc assumptions or methodologies. This article employs a novel system of kinematic variables, section constants, and section functions that facilitate the development of higher-order beam models that retain a clear philosophical link to classical beam models such as Euler–Bernoulli beam theory and Timoshenko beam theory. The present beam model is a type of equivalent single layer beam model, wherein section constants are used to model the global stiffness characteristics of the beam, and section functions are used to recover sectional fields of displacements, strains, and stresses. The present beam model is solved for several example beams, and the results are compared to the results of finite element analyses. It is shown that the present beam model can accurately predict the deformed shapes and stress fields of each of the example beams. This article also reveals an interesting peculiarity of the elastic potential energy that pertains to any unidimensional beam model that is governed by differential equations that are of finite order.

The tensile responses of three monolithic composites (aramid, carbon, and glass) and three different interply hybrid composite laminates (aramid/carbon, glass/aramid, and glass/carbon) were studied at quasi-static and intermediate strain rates. Monolithic composites were found to be more sensitive to strain rate than hybrid counterparts. The hybrids with glass layers had the highest sensitivity among the hybrid laminates. Tensile strengths of aramid, carbon, and glass monolithic composites improved by 15.2 %, 10.1 %, and 22.2 %, respectively, as the strain rate was increased from 0.001 s−1 to 10 s−1, while increases in tensile strengths of aramid/carbon, glass/aramid and glass/carbon hybrid laminates were 8.8 %, 17.7 %, and 5.2 %, respectively. Hybridization of glass with aramid gave the highest failure strain gain. Carbon layers located at the centre of the hybrid laminates resulted in increased tensile strength.

The rheology of the polymer matrix plays a crucial role in determining the quality of thermoplastic laminates. To highlight this aspect, we prepared laminates based on polyamide 11 (PA11), a bio-based matrix with tricky rheology, and basalt fiber fabrics. The PA11 viscosity rapidly increases over time due to degradation phenomena. Samples were prepared by two procedures differing in terms of duration and intensity of the hot compaction step. Morphological analyses revealed that the fast procedure ensures a better impregnation of the fabrics, with a substantial reduction of the volume of voids (1%, against 9% of the slow procedure). This reflects in considerable increases of the flexural modulus (ca. + 20%) and strength (ca. + 60%) and a significant reduction the extent of impact damage. This study provides useful guidelines for a correct selection of the processing parameters based on the knowledge of the rheological behavior of the polymer matrix.

International audience; This paper aims to develop new natural fiber-based composite material for antenna and microwave applications. The biocomposite material has been fabricated from natural flax fiber fabrics and polymer resin using the vacuum infusion manufacturing process. Dielectric properties of the developed material were obtained by microwave characterization using the free-space measurement method over a wide frequency range (2GHz-16GHz). The NRW technique was used to extract the dielectric properties of the developed biocomposite. A dual-band microstrip antenna for 2.45/5.2-GHz WLAN applications was modeled, and fabricated by a screen printing process. Measurement and theoretical investigations show a good agreement and validate the concept of a biosourced antenna on a natural fiber-based substrate.

Experimental fatigue tests performed on riveted carbon fiber laminates showed that the specimens, after some thousands of cycles, heated up, reaching around the rivet holes high temperatures. These temperatures if not properly controlled could be detrimental for the joint health. In order to understand the relative importance of all the mechanical and geometric parameters involved in the joint manufacturing, a numerical model of the joined carbon fiber laminates was developed using a commercial finite elements code. A simplified analytical model of heat transfer was then used to simulate the rivets thermal behavior under the assigned operating conditions: this model allowed comparisons with finite elements and can be used for simple thermal simulations. The numerical and analytical results were found in a good agreement with the available experimental results. The present research conclusions can be used for drawing better design rules in the definition of riveted carbon fiber laminates.

This study presents the results of an investigation on the tensile behaviour of hybrid polymer composites under different strain rates. Glass/carbon, aramid/carbon, glass/aramid, and glass/aramid/carbon hybrid laminates were produced using vacuum assisted resin transfer molding method with epoxy resin system. Uniaxial tensile testing was performed to determine the tensile strength, modulus and failure strain of the hybrid laminates under quasi static (0.001 s‒1) and intermediate (5 and 10 s‒1) strain rates. Tensile strength and elastic modulus of hybrid composites increased with increasing the strain rate. Hybrid laminates with glass fibre were more sensitive to the strain rate. Carbon layers located at the centre of the hybrid laminates resulted in increased tensile strength, indicating the major role of stacking sequence on the behaviour of hybrid composites. Scanning electron microscope (SEM) was used to examine the fracture surfaces of the laminates. The extent of damage propagation was significantly broader at intermediate strain rates.

In this study, high-pressure laminates (HPL) impregnated with phenol-formaldehyde (PF) resins enriched with kraft lignin were developed. Pulverised kraft lignin was added to the commercial PF resin in the amounts of 1% and 5% (solid to solid). Laminates were manufactured using pressure impregnation of the resins into the papers and using hot pressing of HPL in a laboratory press. Laminates with a lignin content of 1% (L-LPF-1) showed the highest bending strength (72.42 MPa) and Brinell hardness (9.41); they also exhibited the best moisture uptake (9.61) and thickness swelling after immersion in water (3.32%). Except for impact bending, laminates with a lignin content of 5% (L-LPF-5) had worse properties. However, the differences between the variants are mostly not statistically significant and are comparable with the results of commercial PF resin. Scanning electron microscopy confirmed the homogenous structure of produced laminates and the occurrence of cohesive failures in ruptured L-LPF-1 laminates, whereas in ruptured L-LPF-5 laminates adhesive failures were also observed. Based on the conducted research it can be said that the utilisation of kraft lignin as an additive to PF resin (in the amount of 1%) has a positive effect on the produced HPL.

This article presents a new higher-order beam model. The present beam model is governed by differential equations that are similar to those present in some existing higher-order beam models; however, the present beam model makes use of a novel method of calculating the transverse shear stiffness, which facilitates the calculation of a shear-warping stiffness without the need for an assumed warping displacement field, and without introducing any additional kinematic variables. The present beam model also facilitates the recovery of the distributions of longitudinal normal stresses and transverse shear stresses. The authors postulate that the bending and shear terms in first-order shear deformation theory represent the first two terms in an infinite series that would constitute an ideal one-dimensional beam model, and it is suggested that the present beam model constitutes the first four terms in this hypothetical infinite series. The present beam model is solved for several example beams, and the results are compared with those of existing classical and higher-order beam models, as well as computational results from finite element analyses. It is shown that the present beam model is able to accurately represent deformed shapes and stress distributions pertaining to beams that exhibit non-trivial shear compliance as well as non-trivial shear-warping stiffness. In the case of laminated composite beams comprising a large number of laminae, the present beam model offers a level of analytical fidelity that is comparable to that of existing zigzag beam models; however, unlike zigzag beam models, the present beam model is equally well suited for the analyses of beams comprising any number of laminae.

The paper presents results of investigations of welding sheets of AA2519-Ti6Al4V, a difficult-to-joint components materials, produced by explosive welding with a thin technological interlayer of AA1050. The joining process leads to the formation of intermetalics in the vicinity of joint and generates significant residual stresses. In the next step the laminate was subjected to a heat treatment process in order to improve the mechanical properties by precipitation hardening. This treatment should not be carried out before welding because of negative influence on a ductility of the aluminum alloy. Material in this state was subjected to the tests of chemical composition, microstructure, and microhardness. A tensile test was carried out with accompanying strain analysis by the digital image correlation (DIC) method. Moreover, the residual stresses were determined which were measured by using two methods, the X-ray diffraction and the hole drilling. This approach made it possible to measure the residual stresses both in the plane parallel to the surface and in the cross section of the laminate.

Ultrasonic based inspection of thin-walled structures often requires prior knowledge of their mechanical properties. Their accurate estimation could be achieved in a non-destructive manner employing, e.g., elastic guided waves. Such procedures require efficient approaches for experimental data extraction and processing, which is still a challenging task. An advanced automated technique for material properties identification of an elastic waveguide is proposed in this investigation. It relies on the information on dispersion characteristics of guided waves, which are extracted by applying the matrix pencil method to the measurements obtained via laser Doppler vibrometry. Two objective functions have been successfully tested, and the advantages of both approaches are discussed (accuracy vs. computational costs). The numerical analysis employing the synthetic data generated via the mathematical model as well as experimental data shows that both approaches are stable and accurate. The influence of the presence of various modes in the extracted data is investigated. One can conclude that the influence of the corruptions related to the extraction of dispersion curves is not critical if the majority of guided waves propagating in the considered frequency range are presented. Possible extensions of the proposed technique for damaged and multi-layered structures are also discussed.

In this work, a fast explicit method, easy to implement numerically, is proposed in order to compute the effective behavior and the distribution of stresses in a general N-phase laminate made of parallel, planar and perfectly bonded interfaces. The solutions are exact for a homogeneous far-field loading and work for an arbitrary number of phases, a general linear anisotropic elasticity, as well as different uniform thermal and plastic strains in the phases. A simple direct analytical formula is also derived to compute the stress in a given phase once the effective behavior of the laminate is known. Moreover, the correctness of the proposed method is checked by comparisons with finite element simulation results on a same boundary value problem, showing excellent agreements. An application of the method is performed for a near-β titanium alloy with elongated grains, by comparing the level of internal stresses for different elastic loadings within a N-phase laminate made of 100,000 orientations and a 2-phase laminate of equal volume fraction with maximal elastic contrast. Interestingly, the maximum von Mises stress of the 2-phase laminate is always the lowest, which is explained by a volume fraction effect. Finally, comparisons with elastic self-consistent models considering oblate spheroidal grains of different aspect ratios are performed.

In-plane elastic and interlaminar properties of composite laminates are commonly obtained through separate experiments. In this paper, a simultaneous identification method for both properties using a single experiment is proposed. The mechanical properties of laminates were treated as uncertainties and Bayesian inference was employed with measured strain-load curves in compression tests of laminates with embedded delamination. The strain–load curves were separated into two stages: the pre-delamination stage and the post-delamination stage. Sensitivity analysis was carried out to determine the critical properties at different stages, in order to alleviate the ill-posed problem in inference. Results showed that the in-plane Young’s modulus and shear modulus in elastic properties are dominant in the pre-delamination stage, and the interlaminar strength and type I fracture toughness in interlaminar properties are dominant in the post-delamination stage. Five times of property identification were carried out; the maximum coefficient of variation of identified properties was less than 1.11%, and the maximum error between the mean values of the identified properties and the ones from standard experiments was less than 5.44%. The proposed method can reduce time and cost in obtaining multiple mechanical properties of laminates.

The steady-state energy release rate for tunneling cracks under mixed-mode loading is determined using finite element analyses. The balanced and symmetric laminate layup $$[0/\theta /0/-\theta ]_s$$ is investigated, where the tunneling crack is located parallel to the fiber direction of the central off-axis oriented layer. It is found that for the steady-state situation, a simple energy balance calculation of the released energy of the separated crack surfaces for a fully developed crack gives the same value as the average value of a detailed J-integral analysis of the crack front. Furthermore, the crack front mode-mixity, obtained by the same energy balance calculation, was found to give a good prediction of the average mode-mixity found from a detailed stress intensity calculation along the crack-tip. Based on this simplified energy balance approach, the energy release rate is determined for all angles $$\theta \in {]}0;90]^{\circ }$$ for orthotropic elastic properties ranging from typical low modulus glass fiber reinforced polymers to high modulus carbon fiber reinforced polymers. The predicted results can be used to investigate the influence of the layup angles on static and fatigue tunnel crack evolution in composite materials used within, e.g. automotive, aerospace, and wind energy applications.

Carbon-fibre/epoxy-matrix composites used in aerospace and vehicle applications are often susceptible to critical loading conditions and one example is impact loading. The present paper describes a detailed experimental and numerical investigation on the relatively low-velocity (i.e.

Barely visible impact damage (BVID) occurs in composite laminates\ud subjected to low-velocity impact. They can then exhibit significant effect on\ud mechanical performance of laminates. Previously, it is shown, analytically\ud and experimentally, that BVID occurs at a critical energy level and below\ud this energy level there is no induced damage. However, repeated impact\ud may cause BVID even below the critical energy level. This paper is a novel\ud investigation that deals with the cyclic behaviour of quasi-isotropic\ud glass/epoxy laminated composites under indentation, which is a quasistatic version of low-velocity impact. In particular, this study aims to\ud investigate the ply thickness effect on matrix crack-induced delamination\ud damage in the case of laminated composites under cyclic quasi static\ud indentation loadings. The effect of different parameters, such as load level\ud and ply thickness, on the damage evolution were here investigated. Tests\ud were performed according to the ASTM 7136 standard. Since the glass\ud layer was translucent, it was also possible to visually inspect the matrix\ud delamination during the tests. The laminates were subjected to load levels\ud lower than the critical load level, while there was no evidence of damages\ud when samples were indented just once. However, by increasing the number\ud of cycles, matrix crack-induced delamination appeared in the samples. In\ud brief, it was observed that the ply thickness and energy level have\ud significant effects on the intensity of the induced damage.

Barely Visible Impact Damages (BVID) caused by impact phenomena during maintenance and in-service missions can be a critical issue for composite structures. BVIDs may induce delaminations and intra-laminar damages, which can considerably reduce stiffness and strength of composite laminates leading to component replacement or repair. This work is aimed to numerically and experimentally investigate the mechanical behaviour of a repaired aeronautical composite stiffened panel subjected to compressive loading conditions. A numerical activity has been performed by introducing Finite Element models in the ABAQUS FEM environment with the aim to numerically simulate the mechanical behaviour of the investigated stiffened panel configuration. Indeed, a pristine panel configuration and a repaired panel configuration have been numerically modelled. In the repaired configuration a stepped bonded patch has been used to simulate the repair process. Then, an experimental testing activity has been performed on the repaired stiffened panel configuration and experimental data have been compared to results obtained from the numerical studies for model validation purposes. The validated numerical model has been used to perform a sensitivity analysis on the compressive behaviour of repaired stiffened panels with patches to assess the influence of stacking sequence and sizes of the repair patches on the panel's mechanical behaviour. Although other components are involved in the tightness of the repair patch, the work proved that the stacking sequence of the patch play the main role in the effectiveness of the process in recovering the mechanical performances and overall behaviour of the component.

The numerical evaluation of elastic guided wave (EGW) phenomena is an important stage in the development and configuration of ultrasonic-based non-destructive testing/structural health monitoring (NDT/SHM) systems. To reduce the computational costs, which are typical for EGW simulations in laminated composite structures, and to make the corresponding parametric analysis possible, the latter could be treated by employing an effective single-layer model with homogenized anisotropic material properties. The present study investigates the applicability of such an approach to simulate EGW excitation, propagation, scattering, and sensing in laminate composite structures, which are among the typical problems for ultrasonic-based NDT/SHM. To this end, two homogenized models have been implemented: the well-known static long-wave homogenization approach and the advanced Lamb wave homogenization method, where the effect of angular and frequency dispersion of EGWs is taken into account. To illustrate their performance, in-plane elastic guided wave excitation and sensing with surface-mounted piezoelectric transducers as well as wave scattering by a T-shaped stringer in cross-ply symmetric anisotropic laminates are examined by employing a recently developed semi-analytical hybrid approach. The limits of the applicability of both homogenized models are demonstrated and discussed via the comparison with the multi-layered model. The general conclusion from the obtained results is that only a qualitative, although computationally efficient, description of the EGW phenomena in the considered composites can be achieved using single-layer models.

This article explores wood-fiber-based fabrics containing Lyocell yarn in the warp and Spinnova–Lyocell (60%/40%) yarn in the weft, which are used to form unidirectional all-cellulose composites (ACC) through partial dilution in a NaOH–urea solution. The aim is to investigate the role of the yarn orientation in composites, which was conducted by measuring the tensile properties in both the 0° and 90° directions. As a reference, thermoplastic biocomposites were prepared from the same fabrics, with biobased polypropylene (PP) as the matrix. We also compared the mechanical and thermal properties of the ACC and PP biocomposites. The following experiments were carried out: tensile test, TGA, DSC, DMA, water absorption test and SEM. The study found no significant difference in tensile strength regarding the Spinnova–Lyocell orientation between ACC and PP biocomposites, while the composite tensile strength was clearly higher in the warp (Lyocell) direction for both composite variants. Elongation at break doubled in ACC in the Lyocell direction compared with the other samples. Thermal analysis showed that mass reduction started at a lower temperature for ACC, but the thermal stability was higher compared with the PP biocomposites. Maximum thermal degradation temperature was measured as being 352 °C for ACC and 466 °C for neat PP, and the PP biocomposites had two peaks in the same temperature range (340–474 °C) as ACC and neat PP combined. ACCs absorbed 93% of their own dry weight in water in just one hour, whereas the PP biocomposites BC2 and BC4 absorbed only 10% and 6%, respectively. The study highlights the different properties of ACC and PP reference biocomposites that could lead to further development and research of commercial applications for ACC.

Large displacement peel was studied for cases where a compliant substrate leads to a large value of the root rotation. An existing simplified beam model to calculate the peel fracture energy was modified to allow for a kinematic hardening beam model of the foil. The steady-state peel force and the root rotation were used as input data to the resulting analytical beam model. Test results from the literature were analysed. A more elaborate finite element model was also studied, using cohesive elements for the interface layer between the foil and the substrate. The cohesive zone parameters used were the fracture energy, the cohesive strength and a shape parameter. An optimization scheme for the cohesive zone parameters was developed and optimized against experimental steady-state peel force and root rotation. The optimization scheme was effective to characterize the cohesive parameters. The method yields similar values of fracture energy for the two peel angles, with the one for being slightly higher than for . The difference in fracture energies for different peel angles suggests that the fracture energy can be mode dependent. CC BY 4.0Published online: 15 Dec 2019

Microstructural effects on the energy release rate and the mode mixity for tunneling cracks are investigated. A detailed microstructure is based on scanning electron microscopy of a real laminate. Based on a newly developed two-dimensional off-axis finite element formulation, the dependency of the Mode I, Mode II, and Mode III energy release rates on a range of different layup orientations are investigated. First, the influence of the fiber-matrix microstructures on the energy release rates is assessed and compared to results from homogenized models that exclude explicit representation of the microstructure. A finite element-based homogenization is found to provide consistent and accurate predictions, whereas analytical homogenization methods yield greater discrepancies with respect to a basic stress-based validation case and are thus not recommended in the context of tunneling crack modeling. Secondly, realistic non-straight crack shapes are included in the model resulting in similar energy release rates as for straight cracks in the corresponding fully homogenized models. However, modeling the realistic non-straight cracks without accounting for the specific microstructure provides too low energy release rates when compared to the realistic cases with microstructures.

Recently, effects of rapidly developing technology have been seen in the dentistry. Advances in materials enable minimally invasive treatments in which dental tissue is preserved in order to provide esthetics.. Porcelain laminate veneer technique, one of the minimally invasive treatments, is the most popular treatment today. Porcelain laminate veneers stand out with having translusity close to tooth and high biocompatibility properties. In this case report, stages of porcelain laminate veneer technique are described in detail.

Vertical-axis wind turbines (VAWTs) are attractive tools for wind energy extraction particularly suitable for small consumers or off-grid areas. Although their geometry is simple (here, rectangular blade of constant airfoil is assumed), aerodynamic analysis may be quite complex. Computational fluid dynamics (CFD) approach is employed for the estimation of rotor aerodynamic performances. This paper provides a review of possible multi-objective optimization strategies for the design of small-scale VAWT laminate blades in terms of its main structural parameters: ply-order and ply-number. Numerous structural analyses of the composite turbine blades were performed by finite element method (FEM). Multi-criteria constrained optimizations, by an evolutionary method - particle swarm optimization (PSO), were performed with respect to blade total mass, maximum blade tip deflection under static loading, computed natural frequencies and failure index along the blade. By combining different input and output parameters (cost functions and constraints) a large variety of feasible solutions can be achieved.

The proposed material design of BaTiO3/Al2O3/ZrO2 laminate structure predetermined for energy harvesters taking advantage of residual stresses developed during processing was prepared by electrophoretic deposition. The main aim of developed residual stresses is to enhance overall mechanical reliability of piezoceramic functional layers and/or to enhance piezoelectric effects acting in the laminate. The concept of co-sintered BaTiO3 piezo ceramic functional layers with protective ZrO2 and Al2O3 layers is based on strongly bonded layers. In this contribution will be described particular behaviour of the specific material configuration BaTiO3/Al2O3 laminate where an interface interlayer among other effects was formed. The influence of sintering conditions on the microstructure development of the laminate as well as the formation of the interlayer was investigated. The relationship between observed microstructural changes and resulting mechanical properties as hardness and indentation elastic modulus was analyzed by means of nanoindentation technique. The cracks propagation through the individual layers and specific formed interfaces were observed and analyzed. The crack deflection due to the presence of developed residual stresses during the cooling stage of sintering as well as the consequence of microstructural changes on mechanical properties was confirmed.

In order to study the thread tensile performance of carbon fiber composite laminates, the connection between the test piece, connecting bolts, bushings, and the composite matrix, was leveraged for loading, and combined with an ultra-sound scanning imaging system, experiments were carried out on the dynamic response to record the failure behavior of the laminate structure of equal thickness. The effects of different pull-off loading strengths on the dynamic failure process, deformation profile, midpoint deformation, failure mode, and energy dissipation ratio of the thread were studied. The results show that (1) with the increase in pull-off strength, the response speed of mid-point deformation increases, the thread deformation mode changes from overall deformation to partial deformation, and the localized effect increases, accompanied by severe matrix and fiber fracture failure; (2) the thread energy dissipation ratio ascends with increasing pull-off strength and exhibits three distinct stages, i.e., elastic deformation, central fracture, and complete failure, which are directly related to the structural failure mode; (3) the failure load increases with the increment of the thickness of the laminate, and the maximum failure surface of the specimen will move from the upper layer of the laminate to the lower layer along the thickness direction; (4) the deformation velocity of the midpoint augments with the increase in the tensile rate, which can be included as a factor to assess the tensile properties of carbon fiber composites.

Numerical methods, including machine-learning methods, are now actively used in applications related to elastic guided wave propagation phenomena. The method proposed in this study for material-properties characterization is based on an algorithm of the clustering of multivariate data series obtained as a result of the application of the matrix pencil method to the experimental data. In the developed technique, multi-objective optimization is employed to improve the accuracy of the identification of particular parameters. At the first stage, the computationally efficient method based on the calculation of the Fourier transform of Green’s matrix is employed iteratively and the obtained solution is used for filter construction with decreasing bandwidths providing nearly noise-free classified data (with mode separation). The filter provides data separation between all guided waves in a natural way, which is needed at the second stage, where a more laborious method based on the minimization of the slowness residuals is applied to the data. The method might be further employed for material properties identification in plates with thin coatings/interlayers, multi-layered anisotropic laminates, etc.

The chapter investigates the effect of the essential manufacturing parameters on the surface quality of uncoated carbon fibre reinforced composites (CFRP) used as car body panels with visible surfaces (Class A properties). A series of CFRP laminates maunfactured by the RTM technique are investigated varying in the fibre volume content (30–60\(\%\)), reinforcement material (woven fabrics vs. unidirectional fibre reinforcements), curing temperatures (40–120 \(^{\circ }\)C), additives (SiO\(_2\)-nanoparticles as matrix fillers) and using a surface finish applied as an in-mould coating. The laminate surfaces are characterized by roughness analysis (white-light interferometry) and wave-scan measurement to quantify the influence of the different manufacturing parameters on the surface quality. Especially the used resins are intensively characterized concerning thermal properties (CTE) and total resin shrinkage. These results correlate very well with the performed analysis of surface roughness. It is found that the fibre print through effect is significantly reduced by realising low total resin shrinkage and an even distribution of resin and fibres at the surface. Thus, using of unidirectional fibre reinforcement (no weft or sewing threads; very fine filaments), low curing temperatures (slow curing processes) and an in-mould coating are most successful for reduction of fibre print through effect and getting surface similar to Class A properties. In addition, the surface quality is quite positively affected by the application of nanoparticles and also strongly controlled by roughness of tooling.

In this study, post-harvest Dioscorea alata stem fiber was rated, extracted and treated with 2 % concentration of Sodium hydroxide (NaOH) for 4 h. Three replicate samples of Dioscorea alata stem fibre/epoxy composites (DASFEC) were prepared as per ASTM D3039 and tested under tension with a crosshead displacement rate of 2 mm/min. The laminate in-plane strains (ex0,ey0,eyx0), out-plane strain due to the bending with the curvatures (kx, ky, kxy), strains (ex, ey, exy) and ply stresses in material axes (σ1, σ2, τ12) were calculated using Classical Laminate Theory. The longitudinal tensile strength, yield strength, shear strength and Elastic modulus of Dioscorea alata stem fiber/epoxy lamina (DASFEL) were respectively estimated as 94.2 MPa, 44.4 MPa, 18.1 MPa and 3552.5 MPa. Fourier Transform Infrared Spectroscopy (FTIR) Analysis captured the O-H present in cellulose II at 2750.3 cm−1, 3001.3 cm−1, 3279.849 cm−1and 3813.16 cm−1 peaks with weak intensity on the average. Energy-Dispersive X-ray Spectroscopy (EDS) analysis showed the high presence of Carbon (C) and Oxygen (O) with small deposit of Silicon (Si). The damage onset for individual ply was detected by both Maximum Stress Theory and Tsai-Hill Theory in ply 2 at 90 degrees while the overal laminate strenght (σx) was estimated as 64.7 MPa. The study established that DASFEC is desirable for car interior panel design.

RESUMEN: La idea de este trabajo surge del estado de la técnica de la tecnología ortopédica. De forma tradicional los métodos de fabricación de la ortopedia siguen basándose en gran medida en la experiencia manual y práctica. Los técnicos ortopédicos llevan a cabo un proceso iterativo de prueba y error hasta obtener el diseño correcto para cada paciente. Esto implica tiempos más largos de espera para un tratamiento eficaz y un mayor consumo de material. En contraposición, el objetivo de este trabajo consiste en poner a disposición de la ortopedia conceptos de la ingeniería de construcción ligera y, junto con el diseño asistido por ordenador, desarrollar de una forma más automatizada una órtesis para levantar el pie adaptada a la carga. Este diseño se basa en el uso de un material compuesto de fibras infinitamente largas y matriz termoplástica. Para cumplir con este objetivo el presente trabajo se compone de siete capítulos. Tras la introducción al tema, los capítulos Segundo y Tercero conforman el marco teórico para entender qué es exactamente una órtesis de levantamiento de pie y cómo ha de cumplir su función de apoyo durante la marcha. La finalidad de esta órtesis es el tratamiento del pie caído. Aquí se incluyen conceptos de anatomía y biomecánica necesarios para la determinación de las condiciones de contorno que caracterizan la fase de la marcha donde la órtesis se encuentra más solicitada. Es bajo dichas condiciones donde interesa evaluar el comportamiento de la órtesis. Tras esta aproximación, se tomarían las medidas del paciente mediante un escáner. Para este trabajo, sin embargo, un escáner 3D de la parte inferior de una pierna ha sido facilitado por el Instituto de Construcción ligera de la Universidad Técnica de Dresden (ILK). Después, trabajando sobre dicho escáner se construye el modelo de la órtesis utilizando herramientas de modelado CAD. En el Cuarto capítulo se presentan tres modelos realizados con el programa Solidworks 2019 y tras una evaluación de éstos se elige el más adecuado. En el Quinto capítulo, tras el estudio de posibles combinaciones de materiales compuestos de matriz termoplástica con fibras de refuerzo se definen tres materiales y se calculan sus propiedades elásticas. Finalmente, mediante una matriz de evaluación se comparan las distintas opciones, obteniendo como resultado la elección del material que compone la estructura de la órtesis: Una combinación de un 60 % de Polipropileno y un 40 % de fibras de vidrio. Por último, mediante el uso del análisis de elementos finitos, se simula el comportamiento de la órtesis para añadir las correcciones necesarias hasta lograr un diseño óptimo de forma virtual. Las propiedades mecánicas y de procesamiento de este tipo de órtesis están muy influenciadas por la estructura del refuerzo de las fibras. Dada la versatilidad en el diseño de estos materiales compuestos, se plantea un estudio paramétrico sobre la base de la combinación del material escogida. Todo ello con el fin de definir el material compuesto que mejor se adapte a la carga de la órtesis diseñada. Trabajando con Abaqus/CAE se configuran tres materiales compuestos diferentes. Los dos primeros sirven para comparar el efecto de diferentes direcciones de fibras. Mientras que entre el segundo y el tercer material sólo varía el espesor de las capas unidireccionales que componen el compuesto en forma de laminado. Tras la preparación y ejecución del análisis de elementos finitos para cada uno de los tres diseños se llevan a cabo las dos comparaciones. En ambas se evalúan dos resultados de la simulación: Por un lado, los desplazamientos producidos en la estructura de la órtesis y por otro, las tensiones generadas en las diferentes capas de los compuestos. De la primera comparación se deduce que frente a un compuesto con todas las fibras orientadas en la dirección de los mayores esfuerzos es preferible un reparto de las fibras orientadas en más de una sola dirección. Así se obtiene un material ligeramente más flexible con una mejor distribución de tensiones y valores más bajos. De la comparación de espesores, se puede concluir que cuanto mayor es el espesor, más rígido es el material y por lo tanto mayores son las tensiones generadas. Llevar a la órtesis al mismo estado de deformación requiere una mayor carga en el caso de la órtesis más gruesa y esto conduce también a un mayor desplazamiento total de su estructura. No obstante, esto no es deseable. Con la realización de este trabajo queda demostrado que es posible diseñar un material compuesto para una órtesis que se adapte mejor a sus cargas. Sólo después de haber obtenido este diseño virtual adaptado se llevará a cabo el proceso de fabricación. Una opción contemplada es la fabricación de un producto semielaborado, como el material compuesto en forma de laminado, que seguidamente será sometido al proceso de moldeo necesario hasta obtener la forma de la órtesis para levantar el pie cuyo rendimiento está más garantizado que el de aquellas producidas de forma artesanal en la ortopedia. Sin embargo, antes de que el diseño propuesto pueda ser fabricado en la práctica han de considerarse varios aspectos que no se contemplan en este trabajo, así como posibles correcciones. Algunos de estos se comentan en el Séptimo capítulo. ABSTRACT: The idea for this work arises from the state of the art of orthopaedic technology. Traditionally, orthopaedic manufacturing methods are still largely based on manual and practical experience. Orthopaedic technicians carry out an iterative process of trial and error until the correct design is obtained for each patient. This means longer waiting times for effective treatment and increased material consumption. In contrast, the aim of this work is to make lightweight engineering concepts available to orthopaedics and, together with computer-aided design, to develop a more automated anklefoot lifting orthosis adapted to the load. This design is based on the use of an infinitely long fibre composite, consisting of infinitely long fibres and a thermoplastic matrix. In order to meet this objective, this work is structured in seven chapters. After the introduction to the topic, second and third chapters form the theoretical framework for understanding what exactly an ankle-foot lifting orthosis is and how it is to fulfil its supporting function during walking. The purpose of this orthosis is to treat the drop foot. Here are included anatomical and biomechanical concepts necessary for determining the boundary conditions which characterize the phase of walking where the orthosis is most in demand. It is under these conditions that the performance of the orthosis should be evaluated. After this approach, the patient's measurements would be taken by means of a scanner. For this work, however, a 3D scanner of the lower leg was provided by the Institute for Light Construction at the Technical University of Dresden (ILK). Working on the scanner, the model of the orthosis is then constructed using CAD modelling tools. The fourth chapter presents three models made with the Solidworks 2019 programme and after an evaluation of these, the most appropriate one is chosen. In fifth chapter, after studying possible combinations of thermoplastic matrix composites with reinforcement fibres, three materials are defined, and their elastic properties calculated. Finally, an evaluation matrix is used to compare the different options, resulting in the choice of the material that forms the structure of the orthosis: a combination of 60% polypropylene and 40% glass fibres. Finally, through the use of finite element analysis, the behaviour of the orthosis is simulated to add the necessary corrections until an optimal design is achieved virtually. The mechanical and processing properties of this type of orthosis are essentially influenced by the structure of the fibre reinforcement. Given the versatility in the design of these composite materials, a parametric study is proposed based on the combination of the material chosen. The aim is to define the composite material that best suits the load of the designed orthosis. Working with Abaqus/CAE, three different composite materials are configured. The first two serve to compare the effect of different fibre directions. While between the second and third material only the thickness of the unidirectional layers that make up the composite in the form of a laminate varies. After the preparation and execution of the FEA for each of the three designs, the two comparisons are carried out. In both, two simulation results are evaluated: On the one hand, the displacements produced in the orthosis structure and on the other, the stresses generated in the different layers of the composites. From the first comparison shows that, compared to a composite with all the fibres oriented in the direction of the greatest stress, a distribution of the fibres oriented in more than one direction is preferable. This results in a slightly more flexible material with a better stress distribution and lower values. From the thickness comparison, it can be concluded that the greater the thickness, the more rigid the material and therefore the greater the stresses generated. Bringing the orthosis into the same state of deformation requires a higher load in the case of the thicker orthosis and this also leads to a higher total displacement of its structure. However, this is not desirable. This work shows that it is possible to design a composite material for an orthosis that is better suited to its loads. Only after obtaining this virtual adapted design will the manufacturing process be carried out. One option considered is the manufacture of a semi-finished product, such as the composite material in the form of a laminate, which is then subjected to the necessary moulding process until the shape of the orthosis is obtained, the performance of which is more guaranteed tan that of those produced by hand in orthopaedics. However, before the proposed design can be manufactured in practice, several aspects must be considered that are not contemplated for this work, as well as possible corrections. Some of these are discussed in Chapter Seven. Grado en Ingeniería Mecánica

Laminate structures composed of stiff plates and thin soft interlayers are widely used in aerospace, automotive and civil engineering encouraging the development of reliable non-destructive strategies for their condition assessment. In the paper, elastodynamic behaviour of such laminate structures is investigated with emphasis on its application in ultrasonic based NDT and SHM for the identification of interlayer mechanical and interfacial contact properties. A particular attention is given to the practically important frequency range, in which the wavelength considerably exceeds the thickness of the film. Three layer model with spring-type boundary conditions employed for imperfect contact simulation is used for numerical investigation. Novel effective boundary conditions are derived via asymptotic expansion technique and used for analysis of the peculiar properties of elastic guided waves in considered laminates. It is revealed that the thin and soft film influences the behaviour of the laminate mainly via the effective stiffnesses being a combination of the elastic moduli of the film, its thickness and interface stiffnesses. To evaluate each of these parameters separately (or to figure out that the available experimental data are insufficient), a step-wise procedure employing the effective boundary conditions is proposed and tested versus the laser Doppler vibrometry data for Lamb waves in Aluminium/Polymer film/Alumunium structure. A good agreement between theoretical and experimental data is demonstrated for a certain symmetric laminate specimen. The possibility of using film-related thickness resonance frequencies to estimate the film properties and contact quality is also demonstrated. Additionally, the rich family of edge waves is also investigated, and the splitting of fundamental edge waves into pairs is revealed.

Made available in DSpace on 2019-10-06T16:53:29Z (GMT). No. of bitstreams: 0 Previous issue date: 2018-11-01 The aeronautic structures normally operate under high levels of hygroscopic moisture from the surrounding environment at different temperature ranges while in service. Under such conditions, the behavior of laminate composite submitted to cyclic or static loadings can change drastically. In order to understand those effects in stitched fabrics, fatigue tests with open-hole specimens were carried out with a stress ratio of R = −1 and R = 0.1. The specimens were fatigue-tested as provided (environmental conditions) and after exposed to hygrothermal weathering conditions. Based on evidences from recent studies available in the open literature, e.g., effect of water diffusion on epoxy matrix, the overall results indicated a significant reduction in stiffness after the specimens are exposed to hygrothermal effects. The reduction in matrix stiffness, in this case, enhanced the fatigue strength in tension–tension load (R = 0.1) when compared to the specimens in normal conditions. The opposite occurs for the specimens loaded with stress ratio of R = −1, in which the delamination mechanisms changed during the loading reversion from tension to compression that promoted early delamination. Therefore, this process reduced the fatigue life of the specimens under hygrothermal condition. Then, by fractographic investigation, it was verified fracture patterns that regard to mode II damage in R = −1, in which mode II fracture toughness is known to decrease in the presence of water molecules. Universidade Estadual Paulista (Unesp) - Instituto de Ciência e Tecnologia IEAMar/UNESP - Instituto de Estudos Avançados do Mar Departamento de Materiais e Tecnologia Fatigue and Aeronautic Materials Research Group Faculdade de Engenharia Universidade Estadual Paulista (Unesp) Fatec-Faculdade de Tecnologia de Pindamonhangaba, Caixa Postal 1 Divisão de Engenharia Aeronáutica Instituto Tecnológico da Aeronáutica - ITA, Praça Marechal do Ar Eduardo Gomes, Vila das Acácias Universidade Estadual Paulista (Unesp) - Instituto de Ciência e Tecnologia IEAMar/UNESP - Instituto de Estudos Avançados do Mar Departamento de Materiais e Tecnologia Fatigue and Aeronautic Materials Research Group Faculdade de Engenharia Universidade Estadual Paulista (Unesp)

International audience; This work presents an original broadband method able to determine the homogenized mechanical properties of orthotropic composite plates in their main directions. This method is based on a vibratory inverse method and do not need using Eigen mode. This technique is derived from the Force Analysis Technique (FAT), originally designed to identify vibration sources on bars and plates. The main advantage of this method is that it does not require any specific preparation and can be carried out on a real and complex structure. Moreover it is non-destructive and non-invasive method and only requires the excitation by a classical electromagnetic shaker. The main characteristic mechanical parameters (Young’s moduli and damping loss factors) are identified at all measured frequencies and not only at the Eigen frequencies as compared to others measurement techniques. An experimental validation shows a good correlation with the expected values for the Young’s moduli of the test plate. Working on a complex structure shows that the method is applicable for various configurations of operating conditions and leads to a good determination of the mechanical properties of a planar component.

In this work, two numerical procedures based on Finite Elements Method (FEM) have been developed in order to simulate the Lamb wave propagation in Low Velocity Impact (LVI) damaged CFRP (Carbon Fibre Reinforced Polymer) laminate. The former (softening representation), usually adopted in literature, consists of modelling LVI damages by lowering the elastic material properties which allowed investigating the Lamb wave propagation at different stages of LVI damages evolution. The latter, proposed in this paper, conversely to the first one and the most of techniques presented in literature, consists of simulating Lamb wave propagation in a plate characterized by an initial stress-strain state and the related failures carried out by a previous impact simulation involving the same model. Such technique allows a better damage modelling and, consequently, overcoming the damage modelling approximations introduced by the former strategy; the lowering of the elastic material properties leads to a bad damage modelling which does not allow reproducing accurately what happens in the reality. Such procedure allowed investigating the Lamb wave propagation at different impact energy levels. The interaction between Lamb waves and damages has been investigated under three central frequencies of the actuation signal: 150 kHz, 200 kHz and 250 kHz which resulted in interesting observations to minimize the effect of the first lamina's fibres orientation on the wave propagation velocity. It is well known that different wave propagation velocities along fibres and matrix lead to different RMSD (Root Mean Square Deviation) damage index values, even if the sensors are mounted at the same distance from the damage location, resulting in wrong or less accurate information about the identification of both damage size and location during the post-processing phase. Moreover, the relationships between the RMSD damage index values, recorded at different instants of time of the impact history, and the impactor phases has been achieved. Finally a comparison between the results achieved by the two investigated strategies has been carried out and presented here.

Laminate composites are commonly used for the production of critical mechanical structures and components such as wind turbine blades, helicopter rotors, unmanned aerial vehicle wings and honeycomb structures for aircraft wings. During the manufacturing process of these composite structures, zones or areas with weak bond strength are always issues, which may affect the strength and performance of components. The identification and quantification of these zones are always challenging and necessary for the mass production. Non-destructive testing methods available, including ultrasonic A, B, and C-Scan, laser shearography, X-ray tomography, and thermography can be useful for the mentioned purposes. A comparison of these techniques concerning their capacity of identification and quantification of bond defects; however, still needs a comprehensive review. In this paper, a detailed comparison of several non-destructive testing techniques is provided. Emphasis is placed to institute a guideline to select the most suitable technique for the identification of zones with bond defects in laminated composites. Experimental tests on different composite based machined components are also discussed in detail. The discussion provides practical evidence about the effectiveness of different non-destructive testing techniques.

The paper describes a general analytical sensitivity analysis method for the composite laminated plates and shells, which is applied to both classical and first-order shear deformation theories and based on the finite element methods. The first derivatives of engineering constants of lamina with respect to fibre volume fractions are computed based on the micromechanics theory. The first derivatives of extensional, bending and bending-extensional coupling stiffnesses of laminate with respect to fibre volume fractions and fibre orientations are derived using classical and first order theories of laminate. Computational methods for the first derivatives of total stiffness and mass matrices are proposed based on finite element method. The analytical sensitivity analysis methods for static responses, eigenvalues and eigenvectors are described using the total stiffness and mass matrices, and their first derivatives. Sensitivity analyses of composite laminated plate and shell are demonstrated. The merits and disadvantages are also discussed.

This work presents some static analyses on reinforced thin-walled tapered structures made of composite material. These applications are performed through a refined one-dimensional model based on the Carrera Unified Formulation. This formulation uses polynomial expansions to describe the displacement field over the cross-section of the beam. In this way, a quasi three-dimensional solution can be obtained. In the present work the cross-sectional kinematic has been described using the Lagrange polynomials. The use of such models allows any component of the structure to be modelled separately and then the complex structure can be obtained thanks to the so-called component-wise approach. Different aeronautical structural components, gradually more complex, have been studied. The stress and displacement fields due to simple loads have been obtained. The results have been compared with those obtained by means of a commercial FEM tools using one-, two- and three-dimensional elements. The results obtained show how the present approach can deal with complex structures such as tapered aeronautical components. The use of refined beam models allows complex stress fields to be accurately evaluated that is composite materials can be investigated.

In this study, we discuss the effects of the diameter and position of a hole on the dynamic response of a thin-walled cantilever beam made of carbon-epoxy laminate. Eigen-frequencies and corresponding global and local eigen-modes were considered, where deformations of the beam wall were dominant, without significant deformation of the beam axis. The study was focused on the circumferentially uniform stiffness (CUS) beam configuration. The laminate layers were arranged as [90/15(3)/90/15(3)/90]T. The finite element method was employed for numerical tests, using the Abaqus software package. Moreover, a few numerical results of the structure’s behaviour, with and without a hole, were verified experimentally. The experimental eigen-frequencies and the corresponding modes were obtained using an experimental modal analysis, comprising the LMS system with modal hammer. We found that the size and location of the hole affected the eigen-frequencies and corresponding modes. Furthermore, even a small hole in a beam could significantly change the shape of its local modes. The numerical and experimental results were observed to have high qualitative compliance.

In this work, an analysis on the influence of fibre-orientation in the progression of delamination was carried out. The study was focused on a cracked beam according to an End Notch Flexure test configuration. The Serial/Parallel Mixing Theory was used as the composite constitutive law along with a continuum damage model to simulate delamination. This mathematical approach was implemented in a custom-made FEM code and applied to model the delamination progression. The model was validated with two sets of experimental results from the literature involving 0 degrees fibre-oriented laminates. After validation, a beam made of plies oriented between 0 degrees and 90 degrees was analysed. The model predicts the loss of stiffness and strength due to the increment of fibre angle. In addition, the stress concentration phenomenon at the crack tip and delamination onset was properly represented. The difference observed in the progression of delamination along the width of the beam highlights the need for a three-dimensional representation to model fibre-oriented beams. This research was financially supported by the project DPI2017-86324-R of the Ministerio de Economía y Competitividad de España.

The propagation of plane-strain harmonic waves in multilayered plates is studied using a multiscale approach which couples an equivalent first-order single-layer theory and a discrete-layer cohesive-interface model using a homogenization technique to enforce continuity conditions on tractions at the layer interfaces. The model captures the effects of the inhomogeneous material structure and the presence of interfacial imperfections on local fields and global behavior through homogenized equilibrium equations which depend on the kinematic variables of the equivalent single-layer theory only. The equations are tractable and frequency equations and dispersion curves are derived in closed-form for plates with an arbitrary number of layers, interfacial imperfections and layup anisotropy. Among the advantages of the multiscale treatment over classical structural approaches: prediction of lowest modes is accurate over a larger range of frequencies and material inhomogeneities; the correction factors in the low-frequency regime are independent of the layup and accurate explicit expressions are derived for the first two cut-off frequencies; zig-zag displacement fields and jumps at the imperfect interfaces are reproduced; changes in modes of propagation and reduction of the cut-off frequencies due to softer layers, adhesives or delaminations are captured.

The subject of study was a thin-walled C-section made of carbon fiber reinforced polymer (CFRP). Column was subjected to eccentric compression in the established direction. In the computer simulation, the boundary conditions were assumed in the form of articulated support of the sections of the column. Particular studies included an analysis of the effects of eccentricity on the critical force value. The research was conducted using two independent research methods: numerical and experimental. Numerical simulations were done using the finite element method using the advanced system Abaqus®. The high sensitivity of the critical force value corresponding to the local buckling of the channel section to the load eccentricity was demonstrated.

Trials have been conducted to investigate the shape memory capability of an interleaved composite consisting of carbon fibre reinforced epoxy laminae and polystyrene interleaf layers. It has been shown that the composite can be readily re-shaped by deforming it at an elevated temperature and then cooling the composite in the deformed state. On re-heating, the composite almost fully returns to its original shape. One potential application of the shape memory capability of the interleaved composite is in deployable structures and a simple structure has been manufactured to demonstrate this possibility.

Analytical formulation and solution of stress analysis of composite plate subjected to thermo mechanical loading for various ply orientation and thickness of lamina are studied. The main aim of the paper is to investigate how mechanical and thermo mechanical loading would affect the stress ratio and stress distribution of composite plate. The plate is composed of layers of glass-epoxy composite and the orientation of the layers is assumed to be antisymmetric about the neutral axis of the laminate. The plate is subjected to a combined mechanical loading of tensile force and moment along x direction. The thermo mechanical stress is calculated for different ply orientation and thickness ratio, subjected to a temperature change and mechanical loading. The effect of number of lamina and varying thickness of laminate on the stress ratio and stress distribution is studied. The results in this paper are obtained by use of MATLAB Programming and by Finite element software ANSYS 14. Results obtained from both the methods are compared. Such type of loading finds wide application in aircraft flying at high altitude, marine application, medical devices etc.

The local mechanical properties of hierarchically reinforced laminates of graphene-isotactic polypropylene (iPP) and carbon fibres are investigated by nanoindentation. Carbon fibre fabrics are alternated with films of iPP reinforced with different graphene contents and types (pristine and modified with iPP brushes). The distribution of the mechanical data obtained around one isolated carbon fibre at the boundary with the polymer layer is compared with a similar analysis far away from any fibre. In the case of neat iPP, a high E′ tail can be identified with a micrometre-sized interphase showing distinct properties from those of the polymer matrix. With increasing amount of graphene, the E′ distribution including the high E′ tail progressively shifts to higher values. It is suggested that graphene platelets accumulate at the front of the ply due to the filtering effect of the carbon fibres giving rise to a transition region, superimposed on the interphase, with enhanced properties. Nanoindentation studies reveal that the chemical modification of graphene by short-chain iPP lowers the reinforcement of the nanofiller, both near and far away from the fibres front. On the other hand, indentation push-in tests suggest that modified graphene increases the resistance of the laminate under shear stress conditions. However, from the point of view of overall performance, it lowers the electrical conductivity across the laminate. The work offers a discussion of the role of graphene type and content on the properties of the multiscale composite., Financial support from MINECO (Ministerio de Economía y Competitividad), Spain, under grants MAT2013-47898-C2-1-R, MAT2013- 47898-C2-2-R and MAT2017-88382-P is gratefully acknowledged. PEJ and SQ also acknowledge MINECO for a FPI fellowship. We are also indebted to the Department of Crystallography (IQFR, CSIC) for providing Xray diffraction facilities. Authors wish to thank Dr. Nuria García (ICTPCSIC) for profilometry measurements and Mr. David Gómez of the Characterization Service of ICTP-CSIC, for SEM studies.

This paper presents an experimental study on the effect of interfacial fibre orientation and interleaved thermoplastic veil on Mode II interlaminar fracture toughness of 5-harness satin woven carbon fibre reinforced polymer composite laminates. Three-point End-Notched Flexure tests were carried out to determine delamination resistance, GIIC, of specimens with five fibre orientation biases and two veil densities at the midplane. Results show that delamination resistance of 5-harness satin woven laminates depends on the layup configurations at the midplane with 90/45 fibre orientation bias exhibiting the greatest resistance. The delamination resistance enhancement from polyphenylene sulfide (PPS) veil interleaves is also fibre orientation dependent but a further increase of the veil density from 10 gm−2 to 20 gm−2 offers little extra benefit. Fracture surface morphologies were examined under SEM to understand the failure mechanism and fracture process of the woven laminate under the combined effects of the interfacial fibre orientation and the veil density. Fibre orientation relative to the delamination path, surface texture misfit, and veil density are the three main contributors identified for the variation of delamination resistance of 5HS woven laminates.

Invariant based approaches have been proposed to simplify the design and analysis of composite materials, in particular, for generating design allowables. Of these invariants, Trace, more recently referred to as Tsai's Modulus, a material property derived from the plane stress stiffness matrix, can be used to help with preliminary design. However, in literature this has mainly focused on unidirectional composites. The present work extends the application of invariants to woven and hybrid composites, i.e. the latter made with both unidirectional and woven laminae, and provides appropriate validation for the use of invariants in design. A novel approach to implement Tsai's Modulus and determine the three-dimensional stiffness matrix of a woven specimen is proposed. Omni failure envelopes have been proposed in literature and these have been extended to accommodate hybrid specimens with both unidirectional and woven laminae. This work shows that Tsai's Modulus and invariants can be used as an initial design tool in order to establish the likely properties of a given layup and material combination for use in preliminary design. Property predictions at the laminate level, Ex, Ey, νxy and Gxy, were within ±10% of experimental values for highly complex laminate stacking sequences and material combinations.

Displacement and stress analysis of an all-round simply supported piezoelectric laminate has been carried out with a new model. Formulation part has been developed using elasticity approach and the methodology is free from any a-priory assumption in thickness direction. The mathematical model is a set of mixed first ordered ordinary differential equations (ODEs). Solution is obtained using numerical integration in through-thickness direction. Accuracy of the proposed model is assessed by comparing numerical results for single layer piezoelectric plate as well as for multi-layered laminates with exact solutions available in literature and are found to be in good agreement with the same. Additional investigation has been performed to establish a few future references. (C) 2016 Elsevier Ltd. All rights reserved.

The prediction of delamination onset is a challenging task in the design of thick tapered composite laminates, where multiple ply terminations (“drop-offs”) are present. This paper addresses the development of a global-local finite element-based design approach for tapered laminates, whereby layered Timoshenko beam models are employed to predict delamination initiation from individual drop-offs. This modelling strategy provides a fast and conservative method for evaluating the strength of tapered composite laminates. Parametric test cases are presented in order to validate the methodology and understand its limitations. Finally, the application of the tool to a relatively thick tapered composite test specimen comprising multiple ply-drops is demonstrated. Keywords: Composite laminate, Ply drop-off, Delamination, Numerical analysis

Polystyrene-interleaved carbon fibre reinforced epoxy composites exhibiting controllable stiffness have been manufactured. These composites undergo reductions in flexural stiffness of up to 99% when heated above the glass transition temperature T g of the interleaf layers. Potential applications for such materials include their use in morphing and deployable structures. Flexural tests at room temperature indicated that improvements in adhesion between the polystyrene and CFRP layers are required to prevent premature failure of the composites at low shear stresses. Here we investigate how modification of the interleaf layer improves the interlaminar shear strength of the laminates without affecting the stiffness loss at elevated temperatures. Two poly(styrene-co-maleic anhydride) (SMA) films with different maleic anhydride content were prepared and used as interleaf films. Thick adherend shear tests showed that the adhesion strength more than doubled, while flexural tests showed that composites containing SMA interleafs had more than twice the apparent flexural strength of composites containing pure polystyrene layers at 25 °C and yet still undergo significant reductions in stiffness at elevated temperature.

A potential increase of the out-of plane structural efficiency of laminated composites may be obtained by using 3D reinforcements technologies. In this paper, a numerical study is presented focusing on the effects of a single reinforcement seam of stitches (selective stitching) along the edge of a stringer foot, in a stiffened composite panel. A Finite Element Model has been developed in order to simulate the mechanical behaviour of the stiffened composite panels with selective stitching. The numerical model has been preliminary validated by comparison with experimental data on three-point bending tests on a skin-stringer configuration. The numerical results, in terms of load vs applied displacements, have been found in good agreement with the experimental data proving the effectiveness of the introduced numerical model. The numerical results have confirmed the potential beneficial effects of stitching in terms of delay of the crack initiation and growth.

Double cantilever beam (DCB) specimens fabricated from 15 plies of a plain woven prepreg (G0814/913) arranged in a multidirectional (MD) layup were tested by means of constant amplitude fatigue cycles under displacement control. Four different displacement cyclic ratios were used, namely 0.1, 0.33, 0.5 and 0.75. The delamination propagation rate da/dN was calculated from the experimental data and plotted with respect to different functions of the mode I energy release rate G I . When one of those functions, denoted here as ΔK I was used, all of the data obtained for the different displacement ratios collapsed into one master curve.

Finite element calculations are performed to model failure of a carbon/epoxy composite laminate loaded in tension in the through-thickness direction, and to predict the dependence of failure loads upon specimen size. The spatial variability of the inter-laminar strength is modelled by introducing different types of discrete random fields of material tensile strength. Fracture processes are modelled using the cohesive segment method, within the extended finite element framework of ABAQUS Standard. Monte Carlo Simulation are conducted on different realisations of the random fields; the predicted responses are compared to previously published measurements and to reference FE simulations, in which the material strength is taken as uniform and equal to the measured average. The comparison shows that the modelling approach presented here provides more accurate predictions of the structural failure loads and their dependence on size, as well as capturing the failure modes observed in experiments.