Su Su Wang

Fracture Mechanics for Delamination Problems in Composite Materials

A fracture mechanics approach to the well-known delamination problem in com posite materials is presented. Based on the theory of anisotropic laminate elasticity and interlaminar fracture mechanics concepts, the composite delamination problem is for mulated and solved. The exact order of the delamination crack-tip stress singularity is determined. Asymptotic stress and displacement fields for an interlaminar crack are obtained. Fracture mechanics parameters such as mixed-mode stress intensity factors, KI, KII, KIII, and the energy release rate, G, for composite delamination problems are defined. To illustrate the fundamental nature of the delamination crack behavior, solu tions for edge-delaminated graphite-epoxy composites under uniform axial extension are presented. Effects of fiber orientation, ply thickness, and delamination length on the interlaminar fracture are examined.

Fatigue Damage and Degradation in Random Short-Fiber SMC Composite

Damage accumulation and cyclic degradation in a random short-fiber SMC com posite subjected to tensile fatigue loading are studied. Fatigue damage in various forms of microcracking is examined. The transient nature of the nonlinear, monotonic stress- strain curve is investigated first, and subsequent property degradation and hysteresis loop changes are examined. Contrary to the behavior of certain metals and polymers, a cyclic stable state is never reached in general; cyclic softening is always observed in this class of materials. Owing to the random microstructure of the SMC material, the fatigue damage is viewed as being macroscopically homogeneous and uniform, and the damage growth is treated in a continuous sense. A parameter is then introduced to define the degree of the homogeneous damage. A power-law relationship among the rate of damage evolution, loading variables, and cyclic history is established. The homogeneous fatigue damage decreases rapidly with the loading cycle due to combined effects of rapid depletion of microcrack initiation sites and presence of various crack arrest mechanisms.

An analysis of interface cracks between dissimilar isotropic materials using conservation integrals in elasticity

Abstract A new method of analysis is presented for studying the mixed-mode interface crack between dissimilar isotropic materials. The method of approach is formulated on the basis of recently developed conservation laws in elasticity for nonhomogeneous solids and fundamental relationships in fracture mechanics of interface cracks. A solution procedure for the analysis is established and shown to be computationally efficient and operationally simple, involving only known auxiliary solutions and evaluation of conservation integrals along a path removed from the crack tip. An important feature of the present approach is that the crack-tip stress intensity factor solution for each individual fracture mode can be determined accurately and -conveniently by information extracted in the far field. Numerical examples, whose solutions are available in the literature, are presented to demonstrate the accuracy, convergence, and related characteristics of the current approach.

The Extension of Crack Tip Damage Zones in Fiber Reinforced Plastic Laminates

The size and character of the damage zone at the tip of sharp notches in fiber reinforced plastic laminates have been investigated. The variables studied were the stress intensity factor, specimen size, laminate thickness, ply thickness, ply orientation, and fiber properties. The damage zone consists of subcracks parallel to the fibers of each ply, in some cases accompanied by delamination between plies. The damage zone is found to increase in extent approximately in proportion to K2 I up to fracture for notch-sensitive laminates. For notch-insensitive laminates, a point is reached where the zone spreads rapidly across the entire specimen prior to fracture. A strong dependence of damage zone size and fracture toughness on ply thickness, fiber orientation, and fiber properties is demonstrated and discussed.

Penetration Mechanics of Textile Structures

Abstract : This report reviews those aspects of wave propagation and dynamic fracture relevant to the penetration mechanics of textile structures intended for use in personnel ballistic protection, and then describes the development and implementation of numerical analyses for use in instances for which closed- form analyses are intractable. These numerical treatments are used to assess the manner in which fiber material properties influence ballistic resistance, and this is done by performing simulations of missile impact on four fabrics of actual interest: ballistic nylon, Kevlar 29, Kevlar 49, and graphite. Following this parametric materials study, the numerical treatment is extended to include the effect of linear and non-linear viscoelastic relaxation on fabric response to impact. Finally, a special purpose computer code is described which was developed to study stress wave effects occurring at fiber crossovers.

An analysis of delamination in angle-ply fiber reinforced composites

An analytical investigation of the mechanics and failure modes of delamination initiated from a service-induced surface flaw in angle-ply fiber reinforced composites is presented. The analysis employs a hybrid-stress finite element method including a crack-tip singular element with its field variables expressed by Muskhelishvilis complex stress functions. Solutions are obtained for delaminated composites with various laminate parameters. The results elucidate unique and important characteristics of delamination crack-tip response and interlaminar stress transfer mechanisms. Of particular interest are the mixed-mode stress intensity factors associated with the delamination crack. The influence of ply orientation on KI and KII and their effects on subsequent crack extension are discussed.

Interlaminar fracture of random short-fiber SMC composite

The interlaminar fracture behavior of a random short-fiber SMC composite is studied both experimentally and analytically. In the experimental phase of the study, the random short-fiber SMC-R50 composite with different thicknesses is used. The commonly used double-cantilever-beam (DCB) fracture test is employed to evaluate the mode-I interlaminar fracture toughness. In the analytical portion of the research, owing to significant deformation in the DCB test, a geometrically nonlinear analysis is introduced to account for large deflection and nonlinear load-deflection curves in the evaluation of the interlaminar fracture toughness. For the SMC-R50 material studied, the interlaminar toughness of the composite is found to be an order of magnitude higher than that of unreinforced neat resin due to unusual damage mechanisms ahead of the crack tip and significant fiber bridging across crack surfaces. The effect of com posite thickness on the interlaminar fracture is appreciable. The influence of the SMC material microstructure on interlaminar crack resistance is discussed in detail.

Stress Intensity Factors for Anisotropic Fracture Test Specimens of Several Geometries

Stress intensity factors have been obtained for single-edge-notched, double-edge-notched, and double cantilever beam fracture toughness test specimens using a two-dimensional hybrid stress model finite element analysis. The degree of anisotropy is shown to have a significant effect on the stress intensity factor in some cases, with differing effects for different specimen shapes; however, effects of anisotropy are relatively constant for varying crack lengths of a given shape. An experimental K-calilbration for the double cantilever beam specimen is in good agreement with the analytical prediction, and the effect of geometry on the applied load to cause crack propagation is accurately predicted by the analysis.

Fatigue Crack Propagation in Random Short-Fiber SMC Composite

An investigation of the kinetics of fatigue crack growth in a random short-fiber rein forced SMC composite is presented. Experiments were conducted on notched SMC- R50 composite specimens subjected to cyclic uniaxial tensile and mixed-mode fatigue loading. Fatigue crack propagation rates were studied in terms of fracture mechanics parameters. A power-law relationship between the crack growth rate and the cyclic stress intensity factor range was obtained for the SMC-R50 composite under various fatigue loading modes. The value of the exponent in the power-law equation was found to be much higher for the composite than values reported for metals and some structural polymers. A fractographic study of fatigue fracture surfaces was also con ducted. Mechanisms of fatigue crack growth in the SMC-R50 composite were iden tified. A mechanical model is then proposed for further study of the mechanics of fatigue crack growth in random short-fiber SMC composites.

Influence of fibre properties on ballistic penetration of textile panels

Abstract A number of computer simulations have been performed in order to assess the ballistic penetration resistance of a series of textile panels. The results indicate that the rate of energy absorption of the panel increases monotonically with the fibre modulus, but that very high modulus material tends to exhibit poor impact resistance due to its low breaking strain. Aramid fibre seems to exhibit the best combination of high modulus while still maintaining reasonably high breaking strain. A ‘master-curve’ description of impact response has been developed from the computer results, which may be useful in minimising the number of full computer simulations necessary in the design of impact protective devices.