Failure mechanisms in inhomogeneous materials

One of the applications of FGM is an effective and mass efficient armor composite. Understanding of dynamic failure mechanism of FGM under rod impact is critical to the optimal design of FGMs. In our research, a multi-layered impact model with cohesive elements is used to investigate the failure characteristics of FGM structure subjected to high velocity rod impact. The metal-ceramic FGM layer acts as a protective layer backed with a metal substrate. The focus of this work is to predict the spontaneous crack propagation behavior in the FGM layer and the energy partition during the impact process. Our results show that the mode-II fracture is dominant in the FGM layer during the impact. The influence of material gradation profile and mode dependency of the material on the energy absorption and fracture behaviors is also explored. Total dissipated energy in the FGM layer is considerably increased with the fraction increase of the metal phase, and the cracks are difficult to propagate within metal-rich region. This work enables the establishment of quantitative relationship among fracture behavior, material gradation and impact loading rate, and provides a computational tool to optimize material gradation design of composite armors.;Fracture parameters of elastic-plastic FGM are difficult to be directly measured through experiments. A new inverse method is proposed to extract the variation of the separation stress sigmamax (denoted with parameter alpha) and the separation energy of the metal phase Gamma om for metal-ceramic FGM through indirect experimental records. This method is based on Kalman filter procedure, and includes four tasks: (1) Specify domain/size of unknown state parameters; (2) Choose suitable measurements for input; (3) Prepare reference data sets of certain points within the domain through finite element simulations; (4) Select optimal covariance values in Kalman Filter process. This method is tested with the double cantilever beam model by using "pseudo measurements" obtained through finite element simulations with known Gammaom and alpha. Different measurements including strain (epsilony), reaction force (P), crack opening (deltam) and crack advance (Deltaa) have been tested. With different measurements, the convergence behaviors of estimates are different. Using more than one measurements usually provides better accuracy than using single measurement. Using combined measurements (epsilony + P), which are relatively easier to measure, provides enough accuracy on determining alpha and Gamma om.;Plasma sprayed zirconia coatings are characterized with their porous microstructure. Two numerical approaches are developed to model these porous coatings. One of them is based on small angle neutron scattering (SANS) technology. In this approach, the porous structure of the coatings is artificially rebuilt according to the collective micro-structural features determined by SANS studies. This approach is suitable to model a large domain (e.g. entire coating), and only averaged pore morphology will be considered. The other is based on image analysis, in which finite element model is generated directly from micro-structural images with the program of OOF (developed by NIST). This approach is more suitable for a relatively small domain, and realistic micro-structural features can be modeled. (Abstract shortened by UMI.)...