Xuefeng Yao

Caustic analysis of stress singularities in orthotropic composite materials with mode-I crack

Abstract The stress singularities of mode I crack tip in orthotropic composites are studied by the method of caustics. The parametric equations of the caustic and its initial curve surrounding the crack tip are derived by means of elastic crack solutions of orthotropic composites and the basic principle of reflective caustics. Theoretical formulae of determining the mode I stress intensity factor are obtained. Theoretical caustics and initial curves for three kinds of orthotropic composites are simulated. Some characteristics of stress singularity at crack tip are analyzed. In comparison with numerical results, the caustic experiment is performed for crack along the material axes in glass-fiber–woven epoxy composites under three-point-bend loading.

Carbon nanotube films change Poisson’s ratios from negative to positive

In this paper, a discovery is reported that carbon nanotube (CNT) films can change in-plane Poisson’s ratios from negative to positive during a uniaxial tensile loading. First, in situ experimental investigations of the deformation fields about the CNT films fabricated using vertically aligned CNT arrays are performed by digital speckle correlation method, the novel phenomenon for changing Poisson’s ratio from negative to positive during the stretching process is discovered. Furthermore, the physical mechanisms for changing the Poisson’s ratio from negative to positive are explained based on the interactions among CNTs. Finally, a potential engineering application for designing intelligent connector is proposed based on the intriguing Poisson’s ratio of CNT films.

Inversion and decoupling of thermo-mechanical deformation in anisotropic materials using the virtual fields method

In this paper, the inverses of both the elastic constants and the thermal expansion coefficients of anisotropic materials are used to decouple thermo-mechanical deformation using the virtual fields method (VFM). First, a method for inversing and decoupling the thermo-mechanical deformation field under three-point bending loading is proposed based on the VFM. Second, thermo-mechanical coupled deformation of the three-point bending specimen is simulated by means of the finite-element method, and the inverse of the thermo-mechanical elastic constants are used, which agree well with the reference value. Finally, the influences of the noise level on the accuracy of the thermo-mechanical constants from the VFM are discussed. The results will provide a new method for characterizing thermo-mechanical elastic constants and decoupling the thermo-mechanical deformation of anisotropic materials under high-temperature environments.

Visualization of crack tip behaviour in functionally gradient materials using coherent gradient sensing (CGS)

In this paper, the coherent gradient sensing (CGS) method combined with the reflection mode is used to study the crack tip singular field problem in a functionally gradient material (FGM). Based on the physical principle of CGS and the elastic stress field of the crack tip in the FGM, the governing equations on the CGS fringe information and deformation gradient at the crack tip are developed. Numerical simulations of the optical effect for the material with different considerations like the location of the crack, crack mode (mode I, mode II, mode I+II), gradient exponent and nonsingular stress are performed using the CGS fringe pattern. The influences of these considerations on the size and geometrical configuration of the CGS fringe loop at the crack tip are analysed. These results will be useful in the understanding of the crack tip singularity in FGM, as well as fine design and safety evaluation of FGM.

Computational analysis of fatigue behavior of 3D 4-directional braided composites based on unit cell approach

The unit cell of 3D 4-directional braided composites were built up using ABAQUS.The fatigue behavior of the 3D 4-directional braided composites was analyzed.Effect of braiding angles and fiber volume fraction of on fatigue behavior was analyzed. The fatigue behavior of 3D 4-directional braided composites was investigated based on the unit cell approach. First, the unit cell models of 3D 4-directional braided composites with different braiding angles and fiber volume fraction were built up using ABAQUS. Then, the fatigue behavior of the 3D 4-directional braided composites was analyzed, and the effect of fatigue loading direction on the fatigue damage evolution and fatigue life was studied. Finally, the effect of braiding angles and fiber volume fraction of the unit cell on the fatigue behavior of 3D 4-directional braided composites was analyzed. These results will play an important role for evaluating the fatigue behavior of 3D 4-directional braided composites in engineering.

Experimental study of damage and fracture of cancellous bone using a digital speckle correlation method

Cancellous bone is a widespread structure in a creatural body, for instance, in the femoral head and spondyle. The damage evolution and crack growth of cattle cancellous bone were studied under three-point-bending load conditions. A series of speckle images with deformation information surrounding the crack tip were recorded, and the full-field displacement distributions were obtained at different loading levels by means of digital speckle correlation method (DSCM). Characterizations of the damage deformation and fracture of cancellous bone were analyzed. These results provide some useful information for studying the fracture behavior of cancellous bone.

Dynamic caustic analysis of a propagating mode-III crack in functionally graded material based on a higher-order transient crack tip expansion

In this study, the influence of the material gradient on the dynamic mode-III crack tip singularity in a functional gradient material (FGM) is analyzed by means of the optical method of caustics. The elastic constants and, and the density of the FGM are assumed to vary in an exponential manner paralleling the crack, while the Poisson’s ratio is assumed to be constant. The correlation among the stress intensity factor, the material gradient, and the crack velocity is established using the equations of the dynamic caustics and the initial curves for the moving mode-III crack tip under a higher-order asymptotic expansion equation. The dynamic stress intensity factors (SIFs) of the mode-III propagating crack tip is evaluated by the characteristic distance of the caustics and the correction factors including the velocity effects and the material gradient effect. The theoretical caustics and the respective initial curves for different material gradients are simulated under the K-dominant condition. The influence of the material gradient on the shape and size of the caustics and the initial curves is analyzed. These works are helpful to understand the mechanical behaviors of the advanced polymer reinforced composites.

Low Velocity Impact Study of Laminate Composites with Mode I Crack Using Dynamic Optical Caustics

The dynamic evolutions of stress singularities at mode I crack tip in laminate composites are studied by combining optical caustic with high-speed schardin camera. Theoretical caustics and initial curve for two different fiber laminate composites are simulated. Some characteristics of stress singularity at crack tip are analyzed. On the other hand, the generation and the subsequent evolution of dynamic failure around the crack tip are visualized by the optical shadow spots. The characteristic sizes of local stress concentration are extracted and the stress intensity factors at crack tip are determined. The local response and failure evolution mechanism of laminated composite after impact are analyzed by means of a series of optical caustic image and the deformation of composites.

Influence of material orientation on crack tip stress singularity in orthotropic composites

The crack tip (mode I, mode I + II) stress singularities in orthotropic composites are studied by the method of reflective caustic. Based on the caustic parametric equations and its initial curves surrounding the crack tip derived by the elastic crack solutions of composites and the principle of reflective caustic, theoretical caustics and initial curves at mode I and mixed mode (I + II) crack tip of fibre orthotropic composites are simulated. Influences of material orientation on crack tip singularities in composites are analysed. The static experimental caustics of mode I and mixed mode I + II crack tip in composites are obtained when the initial crack direction is parallel to the main axis of the orthotropic material, i.e. θ = 0°. The results indicate that the experimental caustics obtained from the actual fibre composites have a remarkable difference compared with the simulation caustics based on the fracture theory of continuum orthotropic composites, which provide an important physical foundation for understanding the singular stress characterizations at the crack tip of composite materials.

Gas Leakage Prediction of Contact Interface in Fabric Rubber Seal Based on a Rectangle Channel Model

ABSTRACT In this article, the gas leakage prediction through the contact interfaces between a fabric rubber seal and a metal platen is studied based on numerical simulation. A rectangular channel model based on the contact interface is established. The theoretical prediction about the gas leakage for the initial rectangular channel verifies the result from the computational fluid dynamics (CFD) method. Both the finite element method (FEM) and CFD are used to simulate deformation of the fabric rubber seal and the gas leakage, respectively. The influence of rubber thickness and the modulus of the fabric rubber seal under different loading displacements and different contact stresses is studied. In addition, the rubber thickness does not influence the mass flow rate any more under the same contact stress for the normal rubber seal (h2 ≥ 2,000 μm).