Dynamic crack propagation and damage evolution in functionally gradient materials

An experimental and analytical study has been conducted to investigate the dynamic crack propagation and damage evolution in functionally gradient materials (FGMs). FGMs are engineered material in which the composition and properties are varied spatially to optimize the material performance. Failure of such nonhomogeneous solids has received very little attention and this dissertation addresses the crack propagation and damage evolution in FGMs under dynamic loading conditions.; As a first step, dynamic fracture in FGMs graded in layers was studied using the technique of dynamic photoelasticity combined with high-speed photography. Since FGMs suitable for photoelastic studies are not commercially available, a scheme for preparing layered FGM was developed using polyester resin and plasticizer. The elastic, fracture and optical properties of the polyester-plasticizer sheets were characterized as a function of plasticizer content. The FGM was then prepared by bonding together layers having different plasticizer content. The dynamic fracture in these FGMs was investigated to identify the effect of four different fracture toughness profiles on crack jump distance, crack velocity and stress intensity factor history. The results indicated that for the same level of strain energy, an increasing toughness profile resulted in a smaller crack jump distance. The stress intensity factor history in the layered FGMs exhibited an oscillatory behavior.; The structure of the crack-tip stress fields for dynamic fracture in continuously graded FGMs was then investigated. Asymptotic expansion of the stress field around a crack propagating at constant velocity in FGMs was developed for two different types of increasing cenosphere volume fraction. The quasi-static fracture toughness increases up to a certain volume fraction of cenospheres and then decreases.; Finally, the concept of functionally modifying two-layer FGMs to improve their performance under impact loading was investigated. Four different geometric modifications incorporated into the impact face of the FGM were considered. The effect of these modifications on the pattern and extent of damage in the FGM was investigated. The results indicated that the geometric modifications alter the nature and progression of damage in the material and helps in distributing the impact load onto a larger area of the FGMs.