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A new method is developed in this paper to deal with the thermomechanical response of continuous fiber-reinforced composites. Treating the matrix as an elastic-perfectly plastic solid, the analytical formulae of the deformations and stresses of the matrix are obtained from the plasticity theory, axisymmetric equilibrium equation, and stress–strain and strain–displacement relations. The fiber is taken to be an anisotropic, elastic material, and the formulae calculating its deformations and stresses are also presented. The boundary conditions and the consistence of deformations and stresses between the fiber and matrix, and between elastic and plastic regions of the matrix are employed to determine the unknown constants in the analytical formulae. With the developed method, the thermomechanical stress distributions in composites reinforced with circumferentially orthotropic, radially orthotropic and transversely isotropic fibers are investigated, and how the elastic-perfectly plastic property and different materials of the matrix affect the thermomechanical response of the composites is discussed. For the thermomechanical loads and composite systems given in this paper, the elastic-perfectly plastic property of the matrix can reduce the compressive stresses in the fiber, and the tensile circumferential and axial stresses in the matrix. A strong matrix can raise the compressive stresses in the fiber, and the tensile circumferential and axial stresses in the matrix. [ABSTRACT FROM AUTHOR]

All the five independent elastic properties/constants of micro- and nano-structured hierarchical and self-similar random irregular honeycombs with different degrees of cell regularity are obtained by analysis and computer simulation in this paper. Cell wall bending, stretching, and transverse shearing are the main deformation mechanisms of hierarchical honeycombs. The strain gradient effects at the micro-meter scale, and the surface elasticity and initial stress effects at the nano-meter scale are incorporated into all the deformation mechanisms in the analysis and finite element simulations. The results show that the elastic properties of hierarchical random irregular honeycombs strongly depend on the thickness of the first-order cell walls if it is at the micro-meter scale, and that if the thickness of the first-order cell walls is at the nano-meter scale, the elastic properties of hierarchical random irregular honeycombs are not only size-dependent, but are also tunable and controllable over large ranges. In addition, the geometrical properties of nano-structured hierarchical random irregular honeycombs are also tunable and controllable. [ABSTRACT FROM AUTHOR]

Wood fibres constitute the structural framework of e.g. wood, paper, board and composites, where stiffness and dimensional stability are of importance. An analytical modelling approach has been used for prediction of hygroelastic response, and assessment of the stresses in thick-walled cylinder models of wood fibres. A wood fibre was idealised as a multilayered hollow cylinder made of orthotropic material with helical orientation. The hygroelastic response of the layered assembly due to axisymmetric loading and moisture content changes was obtained by solving the corresponding boundary value problem of elasticity. A simple solution scheme based on the state space approach and the transfer matrix method was employed. This was combined with an analytical ultrastructural homogenisation method, used to link hygroelastic properties of constituent wood polymers to properties of each layer. Predicted hygroelastic response captured experimentally measured behaviour. Fibres that were constrained not to twist showed a stiffer response than fibres allowed twisting under uniaxial loading. It was also shown that the ultrastructure, i.e. the microfibril angle, will control the hygroexpansion in the same way as it controls the compliance of the cell wall. Qualitative failure trends comparable with experimental observations could be established with stress analysis and a simple plane-stress failure criterion. [ABSTRACT FROM AUTHOR]

The very complex, multi-level hierarchical construction of textile composites and their structural components commonly manifests via significant property variation even at the macro-level. The concept of a “meso-volume” (introduced by this author in early 1990s) is consistently applied in this work to 3-D stress/strain and failure analyses of 3-D woven composites at several levels of structural hierarchy. The meso-volume is defined as homogeneous, anisotropic block of composite material with effective elastic properties determined through volumetrically averaged 3-D stress and strain fields computed at a lower (“finer”) level of structural hierarchy and application of generalized Hooke’s law to the averaged fields. The meso-volume can represent a relatively large, homogenized section of a composite structural component, a lamina in laminated composite structure, a homogenized assembly of several textile composite unit cells, a single homogenized unit cell, a resin-impregnated yarn, a single carbon fiber, even a carbon nanotube assembly. When composed together, distinct meso-volumes constitute a 3-D Mosaic model at the respective hierarchy level. A multi-scale methodology presented in this paper first illustrates 3-D stress/strain analysis of the Mosaic unidirectional composite, computation of its effective elastic properties and their further use in 3-D stress/strain analysis of the Mosaic model of 3-D woven composite Unit Cell. The obtained 3-D stress/strain fields are then volumetrically averaged within the Unit Cell, and its effective elastic properties are computed. The predicted effective elastic properties of 3-D woven composite are compared with experimental data and show very good agreement. Further, those effective elastic properties are used in 3-D simulations of three-point bending tests of 3-D woven composite; theoretical predictions for central deflection show excellent agreement with experimental data. Finally, a 3-D progressive failure analysis of generic 3-D Mosaic structure is developed using ultimate strain criterion and illustrated on the 3-D woven composite Unit Cell. The predicted strength values are compared to experimental results. The presented comparisons of theoretical and experimental results validate the adequacy and accuracy of the developed material models, mathematical algorithms, and computational tools. [ABSTRACT FROM AUTHOR]

Despite recent research exploring the elastic properties of avian keratins, data on failure properties are less common in the literature. In this paper we present data on the failure properties and moduli of both avian feather and claw keratin in tension and the modulus of claw keratin in compression. Increased water content acts to decrease stiffness and strength but to increase strain at failure. The modulus of claw did not differ significantly when tested under tension and compression. [ABSTRACT FROM AUTHOR]

The mechanical response of various combinations of non-linear viscous elements (dashpots) is analysed in this paper. It is assumed that the properties of i-th element can be described by the relation σi = Ki ξ , where σ i is the stress, ξi is the strain rate, Ki and mi are constants 0 ≤ mi ≤ 1). Parallel, series and combined combinations are considered. The main object is to find out the potential for various combinations to describe the sigmoidal variation of the flow stress, σ, with the strain rate, ξ, which is a typical feature of superplastic materials. It is found that elements are connected in parallel as well as elements are connected in series are characterised by non-sigmoidal dependency of the net stress σ on the net strain rate ξ. At the same time a mixed combination with two elements in parallel connected with a third one in series is shown to describe the sigmoidal curve with reasonable accuracy. [ABSTRACT FROM AUTHOR]

The influence of uniaxial and biaxial strains on electronic and transport properties of phosphorene nanoribbons (PNRs) is investigated within the tight-binding Green's function theory by including an iterative procedure. For this purpose, we use tensile and compressive strains and employ the electronic band structure, effective mass and current-voltage curve, to explore the transport mechanism of PNR devices. Our results based on the band structure show that pseudogap in zigzag PNR (zPNR) is tunable under the strains. Hence, the tunneling transmission is controllable between two edge states of zPNRs. Obviously, we observe that the compressive strains can disturb the electron distributions leading to the induced charge polarization in armchair PNRs (aPNR). Also, in some particular cases there is massive-massless Dirac fermion transition in aPNRs. We found that the p-type zPNR devices can be designed under the compressive strains and the n-type aPNR devices can be modulated under the tensile strains. Our results show that the strains can be used as a way to control and improve the negative differential resistance phenomena in zPNRs. In aPNRs the current and threshold voltage under a finite bias can be changed several times with the uniaxial and biaxial strains. Thus, PNRs can be utilized for the development of flexible electronic and field effect transistor nanodevices. [ABSTRACT FROM AUTHOR]

In this study, experimental data have been obtained which reveal taking place of complex physical processes under conditions of the structural superplasticity not only in the grain boundary, as it is commonly believed, but also in the grain body. It was established for the first time that superplastic deformation of the eutectic alloys is accompanied by significant changes in their Young’s moduli. It was demonstrated that under superplastic deformation the observed changes of the elastic properties of the eutectic Sn-38wt%Pb alloy under study are caused mainly by the decomposition of supersaturated solid solutions on the basis of the alloy components and the relaxation of internal stresses. It was also found that the viscous dislocation-diffusion non-conservative flow is actively developing in the eutectic alloys under conditions of superplasticity, in contrast to the existing ideas about grain boundary sliding as the main mechanism of the matter transport. The experimental results obtained are important for the deeper understanding of the physical nature of the structural superplasticity effect. [ABSTRACT FROM AUTHOR]

A hybrid model of homogenization of elastic anisotropic porous materials with non-spherical pores was proposed. Instead of the localization strain tensors as a ratio of average strain of a phase and composite, the model considers the effective volume tensors of averaged strain as the ratio of the strain amounts in the phase and composite volumes. The effective volumes of the averaged strains of anisotropic porous material with non-spherical pores are expressed in terms of the phenomenological models of an isotropic material with spherical pores. The results of calculation of elasticity moduli of the transversely isotropic porous materials are in close agreement with the experimental data and simulation results. [ABSTRACT FROM AUTHOR]

Identifying the parameters of a crystal plasticity model requires the use of grain scale experimental data. The objective of our work is to develop a robust procedure to identify the model using information at the scale of the crystals. In this study, an in situ experimental measurement has been developed for the parameter identification of crystal plasticity models. During the experimental stage, the total $${\left( \epsilon ^t\right) }$$ and elastic $${\left( \epsilon ^e\right) }$$ strain fields of an Al-alloy specimen with around 12 grains were measured at the same time. The total strain fields were determined by digital image correlation. For this, a speckle-painting was applied on the sample surface which was tracked to derive the total deformation of the specimen surface under loading. The elastic strains were calculated from X-ray diffraction measurements. Yet certain experimental difficulties had to be solved in order to achieve these simultaneous measurements. Besides results and analysis, the corresponding uncertainties during each measurement were quantified as well. [ABSTRACT FROM AUTHOR]

In this work, a study on the influence of morphologic texture on the residual stress determination by diffraction in metallic materials with cubic and hexagonal symmetry is proposed. To this end, elastic self-consistent model has been developed to properly take into account the morphologic texture. Extreme crystallites morphologies (sphere, disc and fibre) were studied, and coupled with the crystallographic texture to reflect the combined effect of morphologic and crystallographic texture in elasticity. In the case of morphologic texture, a stronger influence than the crystallographic texture on the estimated residual stresses (several tens of MPa difference) was observed. We propose a methodology through a scale transition model to take into account the influence of these different morphologies in the stress analysis by diffraction methods. The main purpose of this work was to make the best choice for lattice planes ( hkl) used for residual or internal stress analysis, in elasticity, depending on the morphologic (and crystallographic) texture of the polycrystal, especially when the usual X-ray Elasticity Constants (XECs) are used instead of the stress factors. [ABSTRACT FROM AUTHOR]

Thermo-hydro-mechanical densified balsam fir ( Abies balsamea (L.) Mill.) wood used in this study was made by compressing wood in the radial (R) direction (ray direction) at a temperature of 230 °C and a compression ratio of 60 % for 20 min, which significantly increased its density and some mechanical properties. However, the elastic parameters of densified wood in the R and tangential (T) directions, [moduli of elasticity ( E and E), shear modulus ( G), and Poisson's ratios ( μ and μ)], have seldom been reported because of the relatively small thickness of a densified wood specimen. This study measured the E, E, G, μ, and μ of densified balsam fir wood using a digital image correlation method, which was used to measure the strain field of a specimen under compressive loading. The corresponding five elastic parameters of undensified balsam fir wood were also measured. The experimental results were: (1) E was 284 MPa, which was about 66 % lower than that of undensified fir; (2) E was 2551 MPa, which was about ten times larger than that of undensified fir; (3) G was 21 MPa, which was about 50 % lower than that of undensified fir; and (4) μ and μ were 0.104 and 0.331, which were about 75 and 10 % lower than those of undensified fir, respectively. [ABSTRACT FROM AUTHOR]

Following previous study on non-microcracked porous ceramics (SiC and alumina), we studied the micro and macrostrain response of honeycomb porous microcracked ceramics under applied uniaxial compressive stress. Cordierites of different porosities were compared. Both macroscopic and microscopic strains were measured, by extensometry and neutron diffraction, respectively. Lattice strains were determined using a single diffraction peak (steady-state neutron source) in both the axial and the transverse sample directions. Complementarily, we measured the macroscopic Young's modulus of these materials as a function of temperature, at zero load, using high-temperature laser ultrasound spectroscopy. This allowed having a non-microcracked reference state for all the materials investigated. Confirming our previous study, we observed that macrostrain relaxation occurs at constant load, which is not observed in non-microcracked compounds, such as SiC. This relaxation effect increases as a function of porosity. Moreover, we generally observed a linear dependence of the diffraction modulus on porosity. However, for low and very high applied stress, the lattice strain behavior versus stress seems to be influenced by microcracking and shows considerable strain release, as already observed in other porous microcracked ceramics. We extended to microcracked porous ceramics (cordierite) the macro to microstrain and stress relations previously developed for non-microcracked ceramics, making use of the integrity factor (IF) model. Using the whole set of data available, the IF could also be calculated as a function of applied stress. It was confirmed that highly porous microcracked materials have great potential to become stiffer and more connected. [ABSTRACT FROM AUTHOR]

Due to the typical large variability in the measured mechanical properties of flax fibres, they are often employed only in low grade composite applications. The present study aims to investigate the reasons for the variability in tensile properties of flax fibres. It is found that an inaccuracy in the determination of the cross-sectional area of the fibres is one major reason for the variability in properties. By applying a typical circular fibre area assumption, a considerable error is introduced into the calculated mechanical properties. Experimental data, together with a simple analytical model, are presented to show that the error is increased when the aspect ratio of the fibre cross-sectional shape is increased. A variability in properties due to the flax fibres themselves is found to originate from the distribution of defects along the fibres. Two distinctive types of stress-strain behaviours (linear and nonlinear) of the fibres are found to be correlated with the amount of defects. The linear stress-strain curves tend to show a higher tensile strength, a higher Young's modulus, and a lower strain to failure than the nonlinear curves. Finally, the fibres are found to fracture by a complex microscale failure mechanism. Large fracture zones are governed by both surface and internal defects; and these cause cracks to propagate in the transverse and longitudinal directions. [ABSTRACT FROM AUTHOR]

Cellular materials from hollow bronze spheres were prepared and investigated. The main aim was to determine the effective Young’s modulus of cellular materials by means of the dynamic resonant method, and the next goal was to demonstrate the possibility of using sandwich samples for such determination. For this purpose, two kinds of samples were prepared: quasihomogeneous rods consisting entirely of a cellular material, and sandwich rods containing the cellular material only in a core. Resonant frequencies of the fundamental flexural modes of all these samples were measured, and corresponding flexural rigidities were evaluated. Obtained values of flexural rigidities were subsequently utilized for calculation of the effective Young’s modulus of cellular material. A reasonable agreement between the modulus values determined by means of quasihomogeneous and sandwich samples was obtained. This indicates that the sandwich samples could be used for measuring the material properties of particular layers. [ABSTRACT FROM AUTHOR]

The article investigates on using lysine triisocyanate (LTI) as an additive for Poly (lactic acid) (PLA)/Poly (ε-caprolacton)(PCL) polymer blend to improve impact fracture properties. Impact fracture properties of PLA/PCL/LTI blend is evaluated and compared to those of PLA/PCL blend to assess the effectiveness of LTI addition. Effect of LTI addition on fracture micromechanism is also investigated by observing their fracture surfaces using a scanning electron microscope.

Reports the derivation of elastic properties of a Poisson-Shear material for infinitesimal deformation based on a combined hexagonal honeycomb and re-entrant microstructures. Computation of the elastic coefficients by taking the double derivative of the potential energy of hinge rotational stiffness with respect to the orthogonal strains.

Environment-friendly fiber-reinforced composites were fabricated using ramie fibers and soy protein isolate (SPI) and were characterized for their interfacial and mechanical properties. Ramie fibers were characterized for their tensile properties and the parameters for the Weibull distribution were estimated. Effect of glycerol content on the tensile properties of SPI was studied. Interfacial shear strength (IFSS) was determined using the microbond technique. Based on the IFSS results and fiber strength distribution, three different fiber lengths and fiber weight contents (FWC) were chosen to fabricate short fiber-reinforced composites. The results indicate that the fracture stress increases with increase in fiber length and fiber weight content. Glycerol was found to increase the fracture strain and reduce the resin fracture stress and modulus as a result of plasticization. For 10% (w/w) of 5 mm long fibers, no significant reinforcement effect was observed. In fact the short fibers acted as flaws and led to reduction in the tensile properties. On further increasing the fiber length and FWC, a significant increase in the Young's modulus and fracture stress and decrease in fracture strain was observed as the fibers started to control the tensile properties of the composites. The experimental data were compared to the theoretical predictions made using Zweben's model. The experimental results are lower than the predicted values for a variety of reasons. However, the two values get closer with increasing fiber length and FWC. [ABSTRACT FROM AUTHOR]

The force resisting the opening of a crack in a brittle matrix composite that is bridged by ductile fibers was studied ( Acta Mater. 46(18) (1998) 6381; Acta Mater. 45(9) (1997) 3609). to gain a generic understanding of the crack-bridging process by ductile reinforcements. The matrix was alumina, initially containing a parallel array of fine cylindrical holes. Molten Al was cast into the holes to produce the fibers in situ. A crack was gently introduced to traverse the specimen. The matrix halves were pulled apart in a controlled manner to open the crack. The resisting force increased proportionally to the crack opening over a wide range until a force plateau was reached. Thereafter the force diminished very gradually until failure intervened. Analysis of this counter-intuitive behavior indicated that the excellent adhesion between the fiber and the matrix in combination with the large thermal expansion mismatch must have led to extensive but spotty debonding already from the start of the start of the crack opening. In spite of the well-known ductility of the fibers, the bridging showed quasi-elastic behavior over much of the crack opening. Necking appeared to be suppressed until the separation approached failure. Detailed modeling is offered to provide interpretation of this observed behavior. [ABSTRACT FROM AUTHOR]

A non-destructive method based on measurements of ultrasonic wave velocities and Young's modulus is proposed for quantification of creep cavitation in silicon nitride. Tensile creep tests of silicon nitride were conducted at 1400°C in air and the tests were periodically interrupted to measure the longitudinal and transverse ultrasonic wave velocities and Young's modulus. The velocities and Young's modulus decreased linearly with tensile creep strain. The volume fraction of cavities was estimated from the values of the ultrasonic wave velocities and Young's modulus, and compared with the cavity volume predicted from tensile creep strain. The dependence of Young's modulus on volume fraction of cavities is discussed. [ABSTRACT FROM AUTHOR]

A model for the large strain deformation of polyethylene under uni-axial tension is described. Based on observations made by other researchers, the model describes hardening at large strains by considering dislocation nucleation and reorientation of slip planes within the crystalline phase. The temperature and strain-rate sensitivity of this behaviour are predicted with reasonable accuracy in a pipe-grade high-density polyethylene. The model has only three adjustable parameters which are the crystal shear modulus, the Young's modulus and a critical dislocation length. [ABSTRACT FROM AUTHOR]

Applying the finite deformation theory to a solid, which possesses either cubic or isotropic symmetry at stress-free natural state and is subsequently loaded homogeneously in uniaxial direction, one obtains a stress (or strain) dependence of the Young's modulus, Poisson's ratio, and a volume (or density) change, together with a nonlinear elastic relation between stress and strain. These are all expressed in terms of the second and third order elastic constants of the solid material. These expressions are illustrated with examples of cubic silicon crystal, isotropic carbon steel, Pyrex glass, and polystyrene at the relaxed state. [ABSTRACT FROM AUTHOR]

Most of adhesively bonded joints are under complicatedly distributed triaxial stress in the adhesive layer. For the estimating of the strength of adhesively bonded joints, it is crucial to clarify behavior of yield and failure of the adhesives layer under triaxial stress conditions. Two types of the adhesively bonded joints were used in this study: One is the scarf joint which is under considerably uniform normal and shear stresses in the adhesive layer, where their combination ratio can be varied with scarf angle. The other is the butt joint with thin wall tube in which considerably uniform pure shear can be realized in the adhesive layer under torsional load conditions. These joints can cover the stress triaxiality in adhesive layers of most joints in industrial application. The effect of stress triaxiality on the yield and fracture stresses in the adhesive layer were investigated using the joints bonded by three kinds of adhesives in heterogeneous and homogeneous systems. The results showed that both the yield and failure criterion depend on the stress triaxiality and that the fracture mechanism of the homogeneous adhesive is different from that of the heterogeneous one. From these experimental results, a method of estimating the yield and failure stresses was proposed in terms of a stress triaxiality parameter. [ABSTRACT FROM AUTHOR]

Cartesian three-dimensional braiding as a method of preforming for hybrid composites has been investigated. The fundamental case of a two-sided hybrid 3-D braid was chosen. Hybrid preforms, along with a corresponding set of non-hybrid preforms for control, were fabricated using a Cartesian braiding method. The preforms were consolidated through a Resin Transfer Molding process and prepared for characterization and mechanical testing. Characterization of the braided hybrid composite specimens included yarn packing and deformation within an assumed unit cell, and measurement of constituent tow fiber volume fraction using digital image analysis. A comparison study of the elastic performance of Kevlar/epoxy and carbon/Kevlar hybrid composites was carried out. The tension test results show a near-linear stress-strain relationship for both specimen types within the range of the applied load. The tensile modulus for the carbon/epoxy and hybrid composite were found to be 41 GPa and 74 GPa, respectively. In addition, the Poisson ratio of near unity for both specimen types strongly suggests a fiber dominated material response. The difference in hybrid composite transverse strain due to the differing constituent fiberous materials is found to be appreciable. It is believed that this discrepancy in Poisson contraction, between the carbon and Kevlar sides of the specimens, causes the propagation of transverse cracks [primarily within the carbon tows] and ultimately leads to catastrophic composite failure. Composite ultimate strength and strain to failure were found to be 793 MPa and 1.9% for the Kevlar/epoxy sample and 896 MPa and 1.1% for the carbon/Kevlar hybrid. [ABSTRACT FROM AUTHOR]

The effect of misfit strain on the equilibrium aspect ratio of isotropic islands has been investigated by using numerical calculations of the elastic strain energy of spherical caps on a substrate. The effects of three dimensionless quantities were investigated: the ratio of the surface energy to strain energy, the modulus mismatch and the ratio of the interfacial energy to the surface energy of the island. It was shown that values of these parameters that tended to increase the strain energy dominance resulted in larger equilibrium aspect ratios (height-to-width ratio). The effect of modulus mismatch was also studied. It has been shown that as the modulus of the substrate increases relative to the island (keeping all other parameters constant), the aspect ratio increases. Furthermore, it has also been shown that island-to-island interactions occur over very short ranges, becoming negligible when the midpoint of the islands are separated by more than two diameters. [ABSTRACT FROM AUTHOR]

Analysis of a specific idealised model demonstrates the limitations of the equivalent elastic crack procedure when it is applied to internally pressurised crack problems. [ABSTRACT FROM AUTHOR]