Dynamic fracture behavior of homogeneous and functionally graded syntactic foams

Syntactic foams are a mixture of micro-/nano-structured hollow balloons and a matrix material. They offer good specific strength and energy absorption characteristics for structural applications. The microscopic porosity of these foams provide insulating, damping and dielectric characteristics as well. Unlike conventional foams, the macroscopic isotropy of syntactic foams is also beneficial in simplifying mechanical design.; This dissertation deals with dynamic fracture behavior of syntactic epoxy foams when subjected to impact loading. Of particular interest is the role compositional gradients play in the failure response when used as a core material of a sandwich structure. In the first phase of this research, homogeneous and functionally graded syntactic epoxy foams have been developed and elastic properties, tensile strength and crack initiation toughness are characterized. Guided by the measurements, an analytical model based on strain energy balance and micromechanics is proposed for predicting tensile strength and crack initiation toughness. In the next phase, mode-I crack initiation and growth behaviors are experimentally investigated in monotonically graded foam sheets using high-speed photography and optical interferometry. Two experimental configurations, first one with a crack on the stiffer side of the sample but impacted on the compliant side and the second one with a crack on the compliant side but impacted on the stiffer side, are considered separately. The measurements reveal higher crack tip loading rates and lower crack decelerations in the latter case. The compositional gradients primarily affect these behaviors while fracture is controlled by the local fracture toughness. Additional clarity on the fracture behavior is obtained by resorting to supplementary elasto-dynamic finite element simulations. The linear elastic fracture mechanics concept of crack tip biaxiality/constraint has been extended to these nonhomogeneous materials. An earlier loss of negative crack tip constraint is identified with higher crack tip loading rate in graded foams having a crack on the compliant edge. In the final phase of the research, the feasibility of incorporating the concept of functional grading of foam core in a sandwich structure is demonstrated by developing two dynamically equivalent sandwich structures having identical average acoustic impedance. Numerical and experimental simulations reveal substantially lower normal and shear stresses at the core-skin interfacial regions and lower crack tip loading rates in sandwich structures with functionally graded core.