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Hygrothermal ageing has detrimental effect of the fatigue delamination growth (FDG) in carbon fibre reinforced polymer laminates, and may increase the crack growth rate by a factor of ∼5. The paper examines, how this degradation for Mode I fatigue delamination is affected by the severity of the ageing conditions. Fatigue delamination tests for R = 0.1 and R = 0.5 are conducted after ageing (1) at 70 °C 85 % relative humidity (RH) and (2) immersion in 70 °C water bath (WB). Paris-type FDG characterisation is derived, in the form, which accounts for the effect of fibre bridging. It is demonstrated that parameters of FDG degradation do not differ for these two types of hygrothermal ageing. The physical reasons for this are examined using dynamic mechanical thermal analysis (DMTA) and fractographic analysis, which revealed similar irreversible degradation of the material near the fibre/matrix interface and in the matrix itself, and the similar damage mechanisms in fatigue delamination. Furthermore, this study can highlight the importance of obeying similitude principles in FDG characterisation, and provide extra information for the ISO standard development for mode I fatigue delamination in unidirectional carbon fibre reinforced polymer composites. [ABSTRACT FROM AUTHOR]

Abstract In this paper, compressive material tests on unconfined/confined resin and steel reinforced resin were experimentally evaluated in order to validate the numerical results. The uniaxial model which combines damage mechanics and Ramberg-Osgood relationship is proposed in this paper to describe the uniaxial compressive behaviour of resin and steel reinforced resin. Numerical homogenization is conducted to predict the tensile and shear behaviour of steel reinforced resin after validated by compressive material test results. The friction angle β , the ratio of the yield stress in triaxial tension to the yield stress in triaxial compression K, and the dilation angle ψ of the linear drucker-prager plastic model are obtained based on experiments and numerical homogenization simulation. The confinement effects on resin and steel reinforced resin could be effectively simulated by combing above parameters and uniaxial compressive model. Finite element simulations on unconfined/ confined resin and steel reinforced resin material tests were conducted to validate the material parameters proposed in this paper. A good agreement is observed, indicating the model and parameters proposed in this paper could be effectively used in the finite element simulation of injected bolts. [ABSTRACT FROM AUTHOR]

A comprehensive understanding of the complex damage mechanisms in composite laminates is of great importance for establishing material failure models and designing structural damage tolerance. This paper proposed a combined numerical computation and in-situ experimental study to comprehensively investigate the influence of ply orientation and loading conditions on the microscopic damage behaviors of composite laminates. In terms of numerical computation, to overcome the limitations of current monolayer Representative Volume Element (RVE) models, 3-D two-layer RVE models were developed to characterize the intra-ply and inter-ply damage behavior of composite laminates. In-situ tests were conducted on orthotropic laminates under transverse tensile and out-of-plane shear loads, and the physical processes of crack initiation and propagation were analyzed in conjunction with numerical simulation results. The research findings revealed the contributions of the matrix, fibers, and their interfaces to the damage behavior dominated by matrix cracking in composite laminates. The extension processes and final morphologies of matrix cracks were analyzed and a summary of the propagation patterns of matrix cracks was provided. The proposed 3-D two-layer RVE model effectively captured transient failure processes that cannot be observed in in-situ tests, and the numerical computation results were consistent with ex-situ observations after specimen failure. [ABSTRACT FROM AUTHOR]

Many biological materials such as tendons and muscles contain helical fibers. In this paper, the fracture behavior of such chiral composites is investigated through a combination of theoretical analysis and finite element simulations. A mesoscopic fracture mechanics model of helical fiber-reinforced biological composites is presented, with the effects of interfacial damage and fiber breakage. A cohesive law is adopted to characterize the interfacial damage induced by the relative slipping between the fibers and the matrix. The theoretical model agrees well with the numerical results. The optimized fiber radius that can maximize the fracture toughness of the composites is determined. The effects of interfacial (e.g., bonding strength and energy dissipation) and material properties (e.g., strength and elastic modulus) on the resistance to crack propagation are revealed. Our results show that the composites reinforced by helical fibers exhibit comprehensively excellent mechanical properties, e.g., simultaneous high strength, stiffness, and fracture toughness. This work not only helps understand the structure–property interrelations of biological chiral composites, but also provides inspirations for designing high-performance engineering materials. [ABSTRACT FROM AUTHOR]

This paper investigates tensile behavior of through thickness reinforced carbon/epoxy lap joint composite laminates, reinforced with steel z-pins and staples, arranged in two rows parallel to the overlapping edges, via experimental testing. Acoustic emission (AE) monitoring is employed during the displacement-controlled tensile tests to monitor damage propagation during loading using the Vallen AMSY-5 measurement system, with two piezoelectric sensors being mounted at the laminate surface. Furthermore, machine learning algorithms are integrated to process AE data, enabling the recognition and prediction of failure mechanisms. Fractographic analyses were performed to observe the nature of damage post-failure. The experimental research was enriched with capturing high-resolution pictures of total crack propagation length growth using a high-resolution photocamera. The performed empirical tests demonstrated that the unstable propagation of a crack along the bonding interface has led to an eventual breakdown of both unreinforced and reinforced joints. An increase in the full displacement and load at failure was clearly detected for both z-pins and staples with a noticeable decrease in crack growth length, while a higher performance was illustrated for staples in comparison to z-pinned and unpinned. [ABSTRACT FROM AUTHOR]

Thermal expansion properties of textile reinforced composites with certain structure symmetries can be efficiently calculated by a size-limited unit cell. In this paper, a general approach is developed for the establishment of such a unit cell model. For the derivation of unit cell boundary conditions, three rules are summarized according to the displacement fields in translational, reflectional and 180° rotational symmetric structures under a uniform temperature change loading. The application scope of present unit cell method is clarified from the thermal and mechanical point of views. Three typical composites, i.e., unidirectional fiber reinforced composite, plain woven composite and multi-harness (4HS, 5HS, 6HS, 7HS and 8HS) satin woven composites are then studied, and four, three and two size-reducing unit cells are formulated, respectively. The thermal expansion behaviors of each composite are analyzed, and the effective thermal expansion coefficients are predicted. The influence of structure symmetries on the deformation pattern of unit cell models is clarified. The numerical models are validated by the identical results obtained from unit cells of different sizes and also by the results available in literatures. The approach developed in this paper can be applied to thermal expansion studies of any other composites with relevant structure symmetries. [ABSTRACT FROM AUTHOR]

Bending-active elastica beam is a structural configuration that is based on the elastic deformation of an initially straight beam. This deformation occurs when horizontal displacements are applied to a sliding support, causing the beam to bend into an arched shape. Previous research has focused on the stability of small-scale bending-active structures, thus not considering material strength, which is crucial in real-scale applications and structural design. This paper investigates the short-term structural behavior of bending-active elastica beams using pultruded glass fiber-polymer composite profiles, as used in real-scale structures. A series of short-term bending and service load application experiments were performed considering different bending degrees and loading scenarios. These results were used to validate finite element modeling. They demonstrated that steeper bent beams experience material failure, while shallower ones exhibit snap-through buckling. Material failure initiates on the tensile side of the beams, with cracks initiating at locations of maximum curvature. Higher bending degrees and symmetric loading result in higher maximum loads than lower bending degrees and asymmetric loading. A strain-based failure criterion, which can serve for structural design, is derived. [ABSTRACT FROM AUTHOR]

In this paper, crack growth in orthotropic materials subjected to dynamic loading is numerically studied using an adaptive phase-field method. The study starts with a coarse structured mesh and the adaptive refinement strategy based on a user-defined threshold on the phase-field variable is proposed for computational efficiency, and variable-node elements are employed to treat the hanging nodes as a result of local adaptive refinement. The Hughes-Hilbert-Taylor (HHT) time integration scheme is adopted for temporal discretization. The directionality of orthotropic materials is represented by a penalized second-order structural matrix, which is incorporated in the crack face energy density. Through numerical examples, the influence of the material orientation on the dynamic crack growth in orthotropic materials is studied and the reliability of the proposed framework is validated. • An adaptive PFM is proposed to simulate dynamic crack growth in orthotropic materials. • Time integration method HHT is combined with a staggered solving scheme. • A robust local adaptive refinement strategy is used based on a phase-field threshold. • Variable-node elements are used to overcome incompatibility in the refined mesh. • Performance of the proposed framework is demonstrated by numerical examples. [ABSTRACT FROM AUTHOR]

This work presents a novel numerical methodology to model the degradation and failure of composite materials like GFRP submitted to monotonic and high cycle fatigue loads. This is done by using the Serial–Parallel Rule of Mixtures homogenisation technique together with a proper mechanical characterisation of the constituent materials of the composite. This paper also proposes an efficient way of estimating the fatigue properties of each of the material constituents (fibre or matrix) to comply with the experimental results obtained at composite level; this enables to estimate the fatigue strength of any stacking/orientation of fibres with only one mechanical characterisation of the material properties. A comparison of the results obtained analytically and experimentally for GFRP is presented. The results show the applicability and accuracy of the proposed methodology in this field. [ABSTRACT FROM AUTHOR]

The property measurement and structure optimization of composite materials are difficult topics due to the diversity of combinations of composite constituents and complexity of their application environments. The distribution of composite constituents and heat sources can cause heat aggregation phenomenon which may lead to failure of materials. In this paper, we first propose a deep learning-based surrogate model (DLBSM) which can quickly and accurately achieve the end-to-end prediction between the layout of composite materials under multiple heat sources and its temperature field. The prediction result depicts that the coefficient of determination for the maximum and average temperature of all cases exceeds 0.996. Then, the layout optimization is transformed into a combinatorial optimization problem, and the DLBSM is combined with optimization algorithm to optimize the maximum temperature, temperature gradient, and uniformity of the temperature field. The optimized maximum temperature and temperature gradient are significantly reduced, while the temperature uniformity is improved. These enhancements effectively reduce the probability of failure in composites. This approach can significantly improve the efficiency of thermal behavior prediction of composite and its layout optimization compared with finite element method (FEM). [ABSTRACT FROM AUTHOR]

• Mode II fatigue crack growth is characterized at the steel-to-composite interface. • Displacement and load controlled ENF and 4ENF test methods are compared. • Novel method is proposed to determine crack growth by shear strains from DIC. • Friction and geometrical non-linearity is considered by finite element models. In this paper, mode II fatigue crack growth properties of the composite-to-steel interface are characterised through different test configurations, namely ENF and 4ENF tests. Different loading types including force control and displacement control methods are compared. An innovative shear strain based method is proposed for monitoring the mode II crack growth at the bi-material interface through Digital Image Correlation (DIC). A 3D finite element model with Virtual Crack Closure Technique (VCCT) is built and used for obtaining the strain energy release rate (SERR) to investigate the effect of geometrical nonlinearity, friction at the interface and steel yielding, as well as to verify the mode mixity. The results show that the standard 3-point bending ENF specimen can be unstable under force control and sweeps narrow SERR range by a single test under displacement control. The 4-point bending 4ENF test shows stable crack propagation and clear SERR developing trend. More pronounced geometrical nonlinearity and friction effect exist for 4ENF test which can be considered when interpreting the Paris curves by a nonlinear finite element model. [ABSTRACT FROM AUTHOR]

The USAF report on the risk analysis of aging aircraft fleets notes that the operational life of individual airframes is seldom equal to the design life of the fleet and that the life of an aircraft fleet tends to be determined more by its inherent operational capability and maintenance costs rather than by the number of flight hours specified at the design stage. As such this paper focuses on whether the USAF approach to risk assessment can be used for airframes repaired with a composite patch/doubler. To this end the present paper describes a test program designed to study the effect of adhesively-bonded composite repairs to fatigue cracks that, prior to repair, have grown from small naturally-occurring materials discontinuities. This study reveals that crack growth in composite repairs conforms to the exponential growth equation used in the USAF approach to assessing the risk of failure. Furthermore, the exponent, ω, in the exponential growth law can be determined from the crack growth history associated with the unrepaired specimens and the simple reduction in the stress due to the application of the composite patch/doubler, using the ‘cubic rule’ that was previously used to assess crack growth in the RAAF F/A-18 (Hornet) fleet. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation on epoxy resin-carbon fibers composites interleaved with Nylon 6,6 nanofibers. In particular, the paper focuses on the effect of the thickness of the nanoreinforce into two types of laminae: unidirectional (UD) and plain wave (PW). The effectiveness of the nanoreinforce has been addressed by comparing critical and propagation energy release rates, calculated by testing samples under Mode I and Mode II fracture mechanic loads. Experiments show a general improvement in delamination resistance when the nanofibers are interleaved. Nevertheless slightly different behaviour has been found between the two types of lamina: micrographs of crack paths have been used to explain the reinforce mechanisms and such differences, suggesting a strong interaction between the nature of the fabric and the thickness of the nanointerlayer. [ABSTRACT FROM AUTHOR]

In the conventional methods, the failure load and failure mode of composite pre-tightened tooth connections are determined by experiment. To determine the failure load and failure mode of the composite pre-tightened tooth connection without performing experiments, a new method based on the characteristic lengths is presented in this paper. This method involves three steps: first, based on the concept of the method to determine the shear characteristic length, the compressive characteristic length is defined and the characteristic length of composite tooth can be determined; second, the stress distribution of the composite joint is analyzed by the use of a finite element method; finally, combined with failure criterion and the stress distribution of the composite joint, the failure load and failure mode of this joint can be determined by the compression factor f c and shear factor f s . The prediction methods based on the characteristic lengths are validated by the tests for composite joints with different parameters, and the results indicate that the methodology proposed in this paper can accurately and effectively predict failure loads as well as failure modes in composite pre-tightened tooth. [ABSTRACT FROM AUTHOR]

The strengthening of damaged reinforced concrete (RC) beams through near surface mounted (NSM) fiber reinforced polymers (FRPs) has been shown to be a suitable method in practice. Availability of this technique is based on maintaining the adhesion between FRP rods and the concrete surface of grooves, both undamaged and damaged by cracking of beams. Controlling the status of RC beams strengthened with NSM FRP rods may be carried out through non destructive experimental tests measuring dynamic parameters such as frequency values. This paper analyses experimental vibration of four RC beam models without strengthening and strengthened with NSM carbon-FRP rectangular rods after damage due to bending cracking of concrete by static tests. The investigation of vibration was carried out considering RC beams at different static loading damages in different constrain conditions: two beam models were studied assuming free-free end condition and the other two with hinge-hinge end condition. Envelope of Frequency Response Functions (FRFs) obtained through dynamic testing are shown in the paper and changes of frequency values are compared and discussed. Finally, a comparison between experimental and theoretical dynamic values obtained by finite element analysis is developed to verify the control of RC beams strengthened with NSM CFRP rods. [ABSTRACT FROM AUTHOR]

In laminated composite materials fibre–matrix debonding, as an initial damage mechanism, can initiate a damage sequence that can result in catastrophic failure of its structure by promoting intermediate damage mechanisms. This paper presents an in-depth experimental investigation into each of these damage mechanisms and how they transition from one state to the next, beginning at the micro-scale with fibre–matrix debonding and crack coalescence, to transverse ply fracture at the meso-scale through to formation of macroscopic delamination. In-situ SEM micro-mechanical testing is used to determine the onset of the aforementioned damage mechanisms and to follow their progression in laminates of both [0/90] s and [90/0] s stacking sequences. The damage progression of [0/90] s specimens is presented first, followed by the more progressive failure of [90/0] s specimens, yielding an in-depth analysis of both rapid and more progressive damage growth, respectively. The intralaminar cracking and delamination of [0/90] s laminates was found to be instantaneous and provided limited opportunity to characterise damage progression. For [90/0] s laminates, damage progression was much more progressive and various factors such as fibre positioning were shown to influence debonding initiation and crack path development before catastrophic failure and so these laminates form the bulk of the analysis presented for this paper. [ABSTRACT FROM AUTHOR]

This paper presents an experimental and numerical investigation on the mechanical response of the Nomex honeycomb core subjected to transverse loading. Here, a series of tensile, stabilized compressive and step-by-step compressive tests were carried out, also a meso-scale finite element modelling method was developed to simulate the resin-paper-resin layered honeycomb cell walls by employing explicit shell elements. Through the analysis of the test results, the brittle fracture behaviour of the phenolic resin coating is recognised as a main reason of the honeycomb collapse. Both the strength and modulus of the honeycomb core in tension are higher than those in compression, due to the local buckling of the thin cell walls at a quite low level of compressive loading. From the numerical analysis, it was found that the volume of the resin coating has a positive effect on the collapse strength of the honeycomb core, however has no influence on the collapse strain. Moreover, the modulus of the resin coating has a positive effect on the collapse strength but a negative effect on the collapse strain. In addition, the strength of the resin coating has positive effects on both the collapse strength and strain of the honeycomb core. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental program on precast reinforced concrete wall panels (PRCWP). These panels were damaged under cyclic lateral loads and thereafter retrofitted or rehabilitated and retested. The experimental program was conceived to analyse the possibilities of using FRP materials for strengthening the PRCWP affected by seismic action. The fibre reinforced polymer (FRP) composites are frequently used in strengthening structural elements because of their superior characteristics and simple technology. The existing literature lacks information concerning reinforced concrete walls (RC) retrofitted by FRP composites compared to other structural members. This paper presents various effective strengthening solutions that can be applied to damaged elements. The retrofitting solutions consist in use of EBR-CFRP strips, combined EBR-CFRP strips with NSM-CFRP plates, textile reinforced mortar (TRM) using glass fibre grid, and TRM using carbon fibre grid. The solutions were proposed with the aim of restoring the wall shear resistance and to provide the confinement effect at the ends. The experimental results indicate that the performance of the elements, repaired and strengthened, were almost equal to or higher than the reference elements in terms of load bearing capacity, stiffness and energy dissipation capacity. A more ductile behaviour compared to the reference elements was recorded for the rehabilitated and retrofitted elements. [ABSTRACT FROM AUTHOR]

The aim of this paper is the analysis of vibration of undamaged and damaged Carbon Fibre Reinforced Polymer (CFRP) beam elements. Experimental free vibration of CFRP cantilever beam elements were investigated by dynamic tests; damage of CFRP specimens were locally due to double notches close to the fixed end with different reduction of section and bending stiffness. Frequency values were extracted by transformed signals in frequency domain using the Fast Fourier Transform (FFT) technique. A comparison was made of the experimentally extracted frequencies at each damage level and in relation to the single positions of the accelerometer. The present paper illustrates the envelope of Frequency Response Functions (FRFs) obtained by the experimental dynamic tests and the changes of natural frequency values correlated to the damage degree of CFRP beam element. Theoretically, the dynamic behavior of damaged CFRP cantilever beam is studied with an appraisal model adopting an elastic hinge as equivalent concentrated damage. Theoretical data is defined and discussed in comparison to the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the results of fracture mechanical tests on carbon reinforced composites. The evaluation of the experimental data leads directly to material properties for the further application in numerical crash simulations of automotive structures. If fibre bridging was detected within the composite’s failure surface, it could be quantitatively considered in the material input data for numerical simulations. The fracture mechanical tests have been conducted at two different velocities differing about several orders of magnitude to account for rate effects. Finally, the paper further presents a first validation of the extracted material models by the application of the identified material cards in numerical simulations. [Copyright &y& Elsevier]

Abstract: Conventional laminated glass has a relatively low post-fracture strength and stiffness, which imposes several constraints on the structural use of glass in buildings. This paper proposes a new generation of laminated hybrid-glass units, built-up from plies of chemically strengthened glass, conventional polymer interlayers and heat treated/annealed glass, that aim to outperform conventional laminated glass units. The paper describes the experimental investigations on the novel laminated hybrid-glass units subjected to quasi-static out-of-plane loads and presents the corresponding analytical models developed to characterise the load–deflection response of the units, both in the unfractured and post-fractured states. The experimental data shows that laminated hybrid-glass units can achieve significant post-fracture stiffness and their post-fracture strength can equal or exceed the strength at first fracture. The analytical models are successfully validated and the equivalent shear modulus approach developed in this paper facilitates future numerical analysis and optimisation of these units. [Copyright &y& Elsevier]

Abstract: This paper deals with modeling of the first damage mode (i.e. matrix micro-cracking) in helicopter rotor or wind turbine blades and how this effects the overall cross-sectional stiffness. The helicopter rotor or wind turbine rotor system operates in a highly dynamic and unsteady environment leading to severe vibratory loads present in the system. Repeated exposure to this loading condition can induce damage in the composite rotor blades. These helicopter rotor or wind turbine blades are generally made of fiber-reinforced laminated composites and exhibit various competing modes of damage such as matrix micro-cracking, delamination, and fiber breakage. There is a need to study the behavior of the composite rotor system under various key damage modes in composite materials for developing Structural Health Monitoring (SHM) system. Each blade is modeled as a beam based on geometrically non-linear 3-D elasticity theory. Each blade thus splits into 2-D analyzes of cross-sections and non-linear 1-D analyzes along the beam reference lines. Two different tools are used here for complete 3-D analysis: VABS for 2-D cross-sectional analysis and GEBT for 1-D beam analysis. The physically-based failure models for matrix in compression and tension loading are used in the present work. We detect the matrix cracking using two failure criterion: Matrix Failure in Compression and Matrix Failure in Tension which are based on the recovered field. We set the strain variable which drives the damage variable for matrix cracking and this damage variable is used to estimate the reduced cross-sectional stiffness. The procedure presented in this paper is implemented in VABS as matrix micro-cracking modeling module. To investigate the matrix failure model, three examples are presented which illustrate the effect of matrix cracking on cross-sectional stiffness by varying the applied cyclic load. Finally, the stiffness degradation of composite cross-section of rotor blade due to matrix micro-cracking is correlated to the life of composite rotor blade using damage accumulation model. At the end, an empirical equation is given for the Stress Intensity Factor (SIF) to simulate the matrix micro-crack growth in future. [Copyright &y& Elsevier]

Abstract: This paper is focused on laminated glass plate designed with the “sacrificial ply” concept: The glass layer that collects the live loads is considered as broken (i.e., it is not considered), independently of whether or not it is fractured. Accordingly, the load-bearing system is composed of: Heath-strengthened (or annealed) glass layer, plus interlayer, plus toughened (tempered, or chemically-strengthened) glass layer. Hence, the laminated glass plate is hybrid. The first part of the paper derives the constitutive law of the interlayer materials that belong to the latest generation. Then, the behavior of the laminated glass plate is predicted by using this new constitutive law in lieu of the constitutive law of traditional interlayer materials, and the results are analyzed. The second part discusses the results of a theoretical analysis conducted on products available on the architectural marketplace that encompass glass structural applications. These results prove that the ultimate load is dictated by the toughened glass only if the stiffness of the interlayer surpasses a critical value; otherwise, it is dictated by the less resistant glass. However, the critical value is very high. Moreover, the results prove that assessment cannot use practical or simplified expressions (e.g., effective thickness), since these expressions provide the maximum stress in the plate but not in each layer. [Copyright &y& Elsevier]

Abstract: A method based on the Refined Zigzag Theory (RZT) to model delamination in composite laminated beam structures is presented. The novelty of this method is the use of one-dimensional finite elements to discretize the geometry of the beam. The key property of this beam element, named LRZ [1], is the possibility to capture the relative displacement between consecutive layers which occurs during delamination. The fracture mode that the LRZ element is capable to predict is mode II. In order to capture the relative displacement using the LRZ element it is necessary to adapt the RZT theory as presented in this paper. The mechanical properties of the layers are modeled using a continuum isotropic damage model [2]. The modified Newton–Raphson method is used for solving the non-linear problem. The RZT theory, the LRZ finite element and the isotropic damage model are described in the paper. Also, the implicit integrations algorithm is presented. The performance of the LRZ element is analyzed by studying the delamination in a beam for two different laminates, using the plane stress solution as a reference. [Copyright &y& Elsevier]

Abstract: This paper investigated the failure mechanism of fibre composite T-piece specimens under mixed mode loading conditions by simulation of multi-delamination using a modified cohesive model with a coupled damage scale and added thermal effects. A mixed mode load includes pulling and bending. Multi-delamination was simulated in the way modelling the dominated multi-delamination in the deltoid region of T-piece. Failure predictions in the mixed mode loading case, individual bending and pulling case together with part of tested solutions were presented in this paper. It was exposed that the interaction between pulling and bending in the mixed mode loading case plays an important role in the failure mechanism of T-piece. The effect of thermal initial crack in the deltoid of T-piece on prediction of the capacity to resist mixed mode loading was also investigated. A significant effect of thermal initial crack on delamination was found in the mixed mode loading case. This investigation supplied important information for the design of T-piece related composite components under mixed mode loading conditions. [Copyright &y& Elsevier]

Abstract: Hollow bridge piers, particularly those built before the seventies, often have insufficient shear capacity due to inadequate transverse reinforcement details. Therefore, special attention must be given to this very important aspect when reinforced concrete (RC) piers with hollow sections are analysed and retrofitted. This paper covers the experimental analysis of retrofit solutions using CFRP sheets along the piers’ entire height to prevent shear failure. Experimental cyclic tests were carried out to evaluate the shear retrofit strategy efficiency on a set of RC piers with square hollow sections. This work also covers the study of design procedures for CFRP shear retrofitting and the evaluation of the associated ductility capacity improvement. The various transverse reinforcement detailing scenarios were assessed to determine their shear-failure prevention efficiency. The corresponding cyclic response behavior was also evaluated. The most relevant experimental information is presented in the paper, such as the evolution of the outer damage pattern. Finally, shear retrofit solutions, with a 40% increase over the maximum flexural force, show that this strategy is adequate to allow satisfactory ductility behavior. [Copyright &y& Elsevier]

Abstract: In this paper, the layerwise model for laminates with imperfect interfaces and enhanced in Part 1 of this work is confronted with experimental results. The model calculations are validated by comparing its sliding predictions in double lap adhesive joints to the sliding measurements performed in a previous paper. The model predictions agree with the experimental results. Finally, the model is applied to the failure analysis in double lap joints and T-peel joints exhibiting adherend, adhesive and cohesive failures. The model calculations and pertinent failure criteria provide accurate predictions and may become a helpful tool suitable to the design of adhesive joints. [Copyright &y& Elsevier]

Abstract: This paper uses a bio-inspired design strategy based on tree branch joints to improve the damage tolerance of co-cured composite T-joints. The design of tree branch joints at different length scales from the microstructural to the macro-length scale was investigated. X-ray computed tomography of a pine tree revealed three main features of tree branch joints which provide high structural efficiency and damage tolerance: integrated design with the branch embedded into the centre of the trunk; three-dimensional fibril lay-up in the principal stress directions; and variable fibril density to achieve iso-strain conditions through the joint connection. Research presented in this paper adapts the embedded structural feature of tree joints into a carbon/epoxy T-joint. The flange plies were embedded to 25%, 50% and 75% of the depth of the skin of the composite T-joint to mimic the design of tree branch joints. Experimental testing revealed that the bio-inspired T-joint design with integrated adherends had increased normalised inelastic strain energy (defined as ductility), increased normalised absorbed strain energy to failure, and higher load-carrying capacity following damage initiation (damage tolerance) compared to a conventionally bonded T-joint. However, these improvements were achieved at the expense of earlier onset of damage initiation in the T-joints. [Copyright &y& Elsevier]

Fiber reinforced cementitious matrix (FRCM) composites represent an effective alternative to fiber reinforced polymers (FRP) to strengthen existing concrete and masonry structures. When a single layer of fiber textile is employed, FRCMs generally failed due to debonding at the matrix-fiber interface and the presence of friction/interlocking residual stresses was observed for some composites. Therefore, the study of the stress-transfer mechanism is a fundamental topic to understand the behavior of FRCM composites and provide reliable design procedures. In this paper, the bond behavior of FRCM composites is studied by an analytical approach based on the use of an elasto-brittle bond-slip law that accounts for the possible presence of friction stresses. A stress and an energy criterion are put forward to describe the stress-transfer mechanism distinguishing between the bond and friction contributions. Finally, a comparison between the results of the analytical approach and corresponding experimental load responses is provided, showing that the methodology proposed is a fast and easy tool to estimate the bond behavior of FRCM composites. [ABSTRACT FROM AUTHOR]

Abstract Glass structures are often used in industries utilising large structural topologies. These structures are typically manufactured by post-curing subcomponents together, using a chopped strand mat layer at the interface. To predict failure of these joints requires an accurate assessment of the material and fracture properties. In this paper two industrially manufactured top-hat stiffened panels are tested to determine the fracture behaviour at the component level. This highlights that the variability seen in fracture properties at coupon level is less evident in structural component response. Then a previously developed set of material properties is used to accurately model the structural response, crack initiation and debonding of the panels under four point bend using Finite Element Analysis which gives final failure at 6.2 kN and a 4.4% error compared to the experimental results which exhibits final failure at 5.94 kN. The specific fracture properties tested and R curve are shown to be critical in assessing crack initiation and propagation with considerable error, 14.5%, provided by data assumed from the literature. [ABSTRACT FROM AUTHOR]

Abstract Fiber reinforced polymers (FRPs) subjected to mode I fracture show important toughening due to the development of large scale bridging (LSB). Experimental studies of this phenomenon in unidirectional carbon/epoxy laminates using double cantilever beam specimens, demonstrate important differences in R-curve response for inter- and intralaminar fracture. Post fracture observation of composite's cross-section pointed out dissimilar fiber bundle size and shape, as the main origin of their differences. In the present paper, representative volume elements with the composite's constituents, based on the actual material microstructure, and homogenized 2D finite element models were developed to study the effects of microstructure on the first stage of damage leading to LSB development in carbon/epoxy composites under mode I fracture. The differences between inter- and intralaminar fracture were investigated along with the influence of fiber dispersion and the presence of interply and intraply resin-rich zones. The numerical simulations captured different microcrack morphologies for inter- and intralaminar fracture, supporting the experimental observations, while parametric studies showed the influence of the microstructure in the formation of LSB. In particular, fiber dispersion within a ply and resin rich zone between plies play significant roles in mode I fracture and can be used to control toughening mechanisms in FRPs. [ABSTRACT FROM AUTHOR]

Abstract This paper proposes a new damage index for the crack identification of beams made of functionally graded materials (FGMs) by using the wavelet analysis. The damage index is defined based on the position of the wavelet coefficient modulus maxima in the scale space. The crack is assumed to be an open edge crack and is modeled by a massless rotational spring. It is assumed that the material properties follow exponential distributions along the beam thickness direction. The Timoshenko beam theory is employed to derive the governing equations which are solved analytically to obtain the frequency and mode shape of cracked FGM beams. Then, we apply the continuous wavelet transform (CWT) to the mode shapes of the cracked FGM beams. The locations of the cracks are determined from the sudden changes in the spatial variation of the damage index. An intensity factor, which relates to the size of the crack and the coefficient of the wavelet transform, is employed to estimate the crack depth. The effects of the crack size, the crack location and the Young's modulus ratio on the crack depth detection are investigated. [ABSTRACT FROM AUTHOR]

Abstract Disbond damage propagation in honeycomb core sandwich structures is investigated numerically and experimentally. A fully parametric two-dimensional finite element model of a disbonded honeycomb core sandwich specimen is presented. Energy release rate and mode-mixity were numerically determined using the Crack Surface Displacement Extrapolation (CSDE) method. An advanced method was adopted to obtain the homogenized mechanical properties of the honeycomb core based on the geometry of a single honeycomb cell and the material properties of the cell wall paper. The numerical model was benchmarked against CFRP/Nomex® Single Cantilever Beam (SCB) specimen tests and a closed-form semi-analytical model. The results show a close agreement between analytical, numerical and experimental energy release rate, as well as analytical and numerical mode-mixity. An extensive sensitivity analysis was also carried out and the effects of the geometry and the material properties of the SCB specimen on the energy release rate and mode-mixity have been investigated. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we studied the selective activation of intrinsic cohesive elements by using controllable multi-point constraints (MPCs) for fracture analyses of laminated composites. Cohesive elements inserted between bulk elements were deactivated by tying the cohesive nodes using MPC, which were selectively activated during analysis only for the failure region by releasing the constraints. A strategy for the systematic definition and release of MPCs that considers the composite failure modes was developed. When applied to the fracture analysis of laminated composites, the selective activation strategy was found to alleviate the artificial compliance problem of intrinsic cohesive elements while producing results that matched accurately those of conventional intrinsic cohesive elements. [ABSTRACT FROM AUTHOR]

Abstract The influence of nano-organic montmorillonites (nano-OMMTs) on mode I and mode II interlaminar fracture toughness of continuous glass fibre reinforced ammonium polyphosphate polypropylene (CGF/PP/APP) composites was experimentally studied in this paper. Nano-OMMTs modified CGF/PP/APP unidirectional prepreg composites were fabricated by hot-melt impregnation process. To investigate the effect of nano-OMMTs on the interlaminar fracture toughness of composites, double cantilever beam (DCB) test and end-notched flexure (ENF) test were conducted to obtain the mode I initiation interlaminar fracture toughness (G IC init.), mode I propagation interlaminar fracture toughness (G IC prop.), and mode II initiation interlaminar fracture toughness (G II C init.) of samples, respectively. Through comparison against to the neat CGF/PP/APP composites, with the addition of 6.5 wt% nano-OMMTs into PP matrix, the G IC init. , G IC prop. , and G II C init. of composites can be enhanced from 0.502 kJ/m2, 0.746 kJ/m2 and 0.623 kJ/m2 to 0.712 kJ/m2, 1.119 kJ/m2, and 1.195 kJ/m2, respectively. Finally, it was found that nano-OMMTs in matrix can act as "pins", which could absorb energy in the forms of nano-particles pullout and enhance the interlaminar fracture toughness values in DCB and ENF tests. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with the problem of numerical prediction of bird strike induced damage on aeronautical structures. The problem of soft body impacts has been tackled by applying a hybrid Eulerian Lagrangian technique, thereby avoiding numerical difficulties associated with extensive mesh distortion. Eulerian modeling of the bird impactor resulted in a more realistic behavior of bird material during impact, which has lead to an enhanced response of the impacted structure. The work presented in this paper is focused on damage modeling in composite items of aeronautical structures. The bird impactor model and damage modeling approaches have been validated by comparison with experimental gas gun results available in the open literature, while the complete damage prediction procedure has been demonstrated on a complex airplane flap structure finite element model. [Copyright &y& Elsevier]

Abstract: In a companion paper in this issue (Pankow et al., 2010 ), experimental results for the deformation response of 3D glass fiber textile composites subjected to shock wave pulse pressure loading were presented. In this paper, finite element models are developed to simulate the deformation response and damage development observed in the experiments. Two levels of models are presented, with the first focused on a layered, homogenous orthotropic model that examined the bulk macroscopic deformation response. In the second model, each layer is represented to capture the interactions between fiber tows and matrix, taking into account the discrete non-homogeneous material distribution in each layer. Both models incorporated a user defined subroutine within the commercial software ABAQUS to capture matrix micro-cracking, which is responsible for damage development and growth. Results from the computational models correlated well with experimental results and observed locations of matrix micro-cracking. [ABSTRACT FROM AUTHOR]

Abstract Due to the weave structure of woven textile composites, unstably finite crack growths are observed in the quasi-static interlaminar fracture tests for unidirectional woven textile composites. The unstable crack growth speed and finite crack growth length are up to 60 m/s and 24 mm, respectively. A double compliances method is used to measure the fracture toughnesses for the static and dynamic crack growths. It is found that the double compliances method is very accurate and much simpler than the ASTM standard methods for determining the static fracture toughness. With the help of a high speed camera and the double compliances method, the fracture toughness of the unstable crack is given in this paper. [ABSTRACT FROM AUTHOR]

Abstract An interlaminar crack-jump event is one of the complex failure phenomena in laminated composite structures. This paper examines the mechanics of the interlaminar damage process leading to crack-jump event in CFRP composites under mode-II loading condition. A series of end-notched flexure tests of CFRP composite were conducted to create a standard interface failure, and an unstable interface fracture that led to crack-jump event. FE simulation of the composite tests was created using a new FE model-based construction and CZM theory in combination with a hybrid experimental-computational approach to assess the interface damage and crack-jump events. The FE model of the standard test predicted a short range crack-jump event instead of a gradual interface crack growth, coincided with the load drop in the structural response. A constant value of interface damage dissipation was predicted at the time of fracture for all composite cases. The unstable crack-jump event occurred due to the release of high strain energy in the composite structure while the interface underwent cracking process. [ABSTRACT FROM AUTHOR]

Abstract The paper presents the failure analysis of compressed channel section profile made of GFRP material. Three symmetrical arrangements of layers were considered: [90/0/90/0]s, [90/-45/45/0]s, [45/-45/90/0]s. In order to characterize the failure process, visual inspection, ultrasonic phased array, non-destructive testing and computer microtomography were used. Damage analysis based on studies of external and internal damage of investigated structure allowed for identification of characteristic failure modes and mechanisms. It was shown that the dominant form of destruction in the quasi-orthotropic system are matrix cracks coupled with the presence of extensive delamination while in quasi-isotropic systems more concentrated damage was noticed, in which the cracking/rapture of fibers localized in the column’s corners determined the initiation of the failure mechanism and the carrying capacity loss. In addition, it was noted that the fiber directions of the outer layers determined the directions of matrix cracks and delamination propagation. [ABSTRACT FROM AUTHOR]

Abstract High velocity transverse impact on reinforced composites is a matter of interest in the automotive, aeronautical and biomedical sectors. Most existing studies have addressed this problem by single isolated impacts; however, this work deals with the distinction between single, sequential and simultaneous impacts on composite structures. This paper proposes an experimental methodology to study the mechanical behaviour of materials under single and multi-impact loadings. The overall objective is to investigate the mechanical response of short carbon fibre reinforced PEEK when is subjected to single and multiple high velocity impacts. Experimental tests are conducted covering impact velocities from 90 m/s to 470 m/s. Energy absorption, damage extension and failure mechanisms are compared to assess additive and cumulative effects in high velocity impact scenarios. Experimental results show that the specific deformation and fracture mechanisms observed during multi-hitting events change with impact velocity. Compared to the behaviour of unreinforced thermoplastics, short fibre reinforced composites present significant limitations at velocities close to the ballistic limit, but multi-hit capability is observed at high impact velocity when the damage is mainly local. As key conclusion, the ballistic limit obtained in single impact test cannot be extrapolated to sequential and simultaneous tests. Multi-impact tests, especially close to the ballistic limit, are necessary to guarantee the structural integrity of composite structures in realistic impact scenarios. [ABSTRACT FROM AUTHOR]

Abstract According to various experimental results, stiffened panels under compressive loading are prone to debonding between the skin and the flange of the stringer. In this paper, a novel two-way global-local coupling approach is presented that is able to model progressive separation of the skin and stringer in stiffened CFRP panels under compression. The main goal of this methodology is to examine skin-stringer debonding at two levels of accuracy, taking advantage of the fast calculations at the global level and assessing in detail the damage propagation at the local level. First, critical areas are defined in a global model with a standard mesh, and local models with a considerably finer mesh are created by means of a submodeling technique. Secondly, a local model analysis is conducted, in which cohesive elements are applied to simulate debonding. Particularly important is the appropriate information exchange in both directions between the different steps of the coupling analysis. Averaged degraded properties are defined at the local model level and transferred back to the global level. The applied compressive load is increased and induces a progression in skin-stringer separation. The global-local coupling loops are repeated until panel failure occurs. The approach is applied to a case of a representative one-stringer stiffened panel and to a stiffened panel for which test results are available. A good correspondence with reference results and test results demonstrates the effectiveness of the global-local approach presented. [ABSTRACT FROM AUTHOR]

Abstract This paper proposes an acoustic emission (AE) based real-time health monitoring framework to efficiently identify the probable damage initiation/propagation locations in advanced sandwich composite structures. Towards this, numerical simulations and laboratory experiments on damage-induced AE-wave propagation in an aramid honeycomb composite structure have been carried out using a randomly selected sensory network. The simulation results are successfully validated with laboratory experiments. Eventually, the damage-source/AE-source regions are efficiently identified by applying an evolutionary algorithm – Particle-Swarm-Optimization based monitoring framework, which uses the registered AE-signals from the sensory network. A thorough assessment of different AE-source locations was carried out to evaluate the performance and the robustness of the proposed online monitoring strategy. The results clearly represent the efficiency of the framework for localizing the AE-source locations in such advanced and complex structures. Moreover, the proposed framework is reliable, independent of sensor positions, and not dependent upon the operator’s expertise. [ABSTRACT FROM AUTHOR]

Abstract This paper presents an investigation on flutter speed of cracked composite plates. This work is divided into two sections: (a) variation of crack length at a fixed location on the plate, and (b) variation of crack location on the plate with a fixed crack length, modelled as a unidirectional composite for 0 0 , 90 0 and 135 0 orientations. Mori-Tanaka homogenization model is applied to obtain the effective composite constitutive properties as the function of fiber and matrix volume fraction. Doublet Lattice Method (DLM) is used to calculate the unsteady aerodynamic forces, i.e., lift distributions. It is found that the existence of small crack ratio on the composite plate (less than 0.4) has triggered an increment of the flutter speed. To support this statement, flutter response modes for each crack ratio are plotted, where the structure appears to be more stiffened than the undamaged plate. However, the crack results in the reduction of flutter speed when the crack ratio reaches 0.5. For the crack location assessment, the flutter speed increases as the crack location moves from the root to the tip due to the reduction of flutter frequency. The results show a good agreement with the validation using Strip Theory considering unsteady aerodynamics. [ABSTRACT FROM AUTHOR]

Abstract This paper presents the effects of size, curvature and free edges of laboratory lap joints on the debond fracture behaviour of joints that more realistically represent fuselage skin structures than conventional flat, narrow specimens. Finite Element Analysis is used in conjunction with Cohesive Zone Modelling (CZM) to predict the strength of selected joint features. The modelling approach was verified by simple single lap joint geometry. Four realistic joint features were then modelled by this validated modelling approach. The results show that moderate curvature has negligible effect on the peak load. There is a significant difference in the load vs displacement response of flat lab coupon joints with free edges and realistic curved joints with constrained edges. Further detail design features were investigated in this study, including (i) the joint runout and (ii) the presence of initial damage (thumbnail delamination). The modelling results show that the joggle configuration has an effect on the distribution of interlaminar stresses that affect the damage initiation and propagation. Fracture behaviour from different initial crack geometries associated with wider specimens has been simulated. From a design standpoint, an expansion of modelling capability is suggested to reduce the number of component tests in the traditional test pyramid. [ABSTRACT FROM AUTHOR]

Abstract In this paper, the Young’s modulus and failure mechanics of nanotube-reinforced aluminum nanocomposites has been investigated using the Bridging Cell Method (BCM). The advantage of the BCM for multiscale modeling of CNT-Al nanocomposites is in its quasi-static formulations in concurrent multiscale environment with specialized local quasi-harmonic approximations for thermal contributions. The model has been initially verified by coupling the atomistic-continuum domains in a representative volume element of the nanocomposite. The multiscale method sets the nanotubes as part of the atomistic domain, while the rest of the domain is kept in the continuum domain and modeled using the finite element method. The types of nanotube reinforcements considered in this study are single-walled nanotubes (SWNTs), multi-walled nanotubes (MWNTs) and a bundle of SWNTs. It was found that the overall strength of the nanocomposite was similar between the different reinforcements, but the toughening mechanisms differed significantly between the three. The model has been tested for two different crystalline planes in the aluminum crystal to observe the degree of toughening on different atomic planes where it was found that the essential plane and the cleavage plane had similar ultimate tensile strength, however the role of toughening was less prevalent in the cleavage plane case. [ABSTRACT FROM AUTHOR]

This paper presents a numerical and experimental study on the intralaminar tensile fracture toughness of carbon fiber reinforced composite subjected to high strain rates. As there is no standardized testing procedures for intralaminar fracture toughness characterization of composites at high strain rates, there is a clear need to design specimen geometries, testing apparatus and data reduction schemes that allows the characterization of the fracture toughness of composites in the dynamic regime. Initially numerical studies were performed based on finite element simulations in order to investigate the viability of its construction for different testing configurations to characterize the intralaminar toughness of composite laminates. A comparative study is presented showing the advantages and disadvantages of each testing configuration. A new data reduction scheme based on modifications in the ASTM standard, accounting for material anisotropy and specimen finite geometry effects is suggested. Experimental tests were carried out, using the proposed specimen configuration at different strain rates in order to investigate the strain rate effects using a modified version of the Split Hopkinson Pressure Bar. Fractography analyses using Scanning Electron Microscopy(SEM) have been also performed in order to investigate the strain rate effects on the failures mechanisms of the composite material studied herein. [ABSTRACT FROM AUTHOR]

Micromechanical simulation of composite material failure requires a pressure-dependent failure model for the polymeric matrix. Available pressure-dependent damage formulations assume a certain shape of the stress-strain law under uniaxial loading. However, upon close inspection none of the available formulations is able to reproduce the assumed shape. This implies that input values for the fracture energy cannot be recovered exactly. In this paper, a new methodology for developing consistent pressure-dependent damage models for polymeric materials is presented. Using this method the predefined shape of the stress-strain relation of an element with localized deformation under uniaxial tension can be exactly reproduced which enables further to recover the exact amount of energy dissipation consistent with the input toughness. The methodology is demonstrated for two different softening laws, namely linear and exponential softening. These models are applied to the damage analysis of unidirectional continuous fiber-reinforced composites. The formulation is validated by simulation of a test for Mode-I fracture energy characterization and comparing the load-displacement response with that obtained with cohesive elements. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a theoretical analysis of an external matrix crack located in a unidirectional fibre-reinforced elastic solid modelled as a transversely isotropic material. The presence of matrix cracking with fibre continuity introduces bridging action that has an influence on the stress intensity factors at the crack tip of the external crack. This paper presents a model for the bridged crack, where the fibre ligaments induce a constant displacement-dependent traction constraint over the external crack. This gives rise to a Fredholm integral equation of the second kind, which can be solved in an approximate fashion. We examine the specific problem where the bridged external circular crack is loaded by a doublet of concentrated forces. Numerical results are presented to illustrate the influence of the fibre–matrix modular ratio and the location of the loading on the bridged-crack opening mode stress intensity factor. [Copyright &y& Elsevier]

• This paper discusses the importance of similitude in analysis and in laboratory tests. • Plotting da/dN versus ΔG yields curves that are a function of adhesive thickness. • Hence two cracks with the same value of ΔG, and G max , will NOT grow at the same rate. • The Hartman-Schijve crack driving force Δκ can overcome this shortcoming. • FM94 crack growth tests at different temperatures lie on a single da/dN v Δκ curve. Structural adhesives are widely used for joining composite components in many industries and crack growth in such materials is far more likely to occur when they are subjected to repeated cyclic loading than to monotonic loading. Whilst the Hartman-Schijve equation for fatigue crack growth (FCG) has been shown to hold for cohesive crack growth in adhesives under Mode I, Mode II and Mixed-Mode I/II loading, little attention has been paid to its ability to capture the effects of the thickness of the adhesive layer or the test temperature. The present paper examines FCG, that occurs cohesively through the adhesive layer, in two toughened epoxy adhesives typical of those used in the automotive and the aerospace industries. Firstly, it is established that when the FCG rate, da/dN , curves are expressed as a function of Δ√ G, or Δ G , where G is the energy release-rate, then the crack growth curves are a function of the thickness of the adhesive layer. It is then shown that this dependency vanishes when da/dN is expressed as a function of the crack-driving force, Δ κ , as defined by the Hartman-Schijve equation. Secondly, it is shown that similar observations are recorded when the effects of test temperature are considered. Therefore, it is suggested that the parameter Δ κ appears to be a valid similitude parameter. [ABSTRACT FROM AUTHOR]