Dynamic Failure Characteristics in Layered Materials and Structures

Systematic investigations were carried out to understand the general nature of dynamic failure mechanisms in layered materials and structures such as composite and sandwich structures, thin films, layered armors and layered rock. A series of impact experiments on model-layered specimens were conducted using high-speed photography and dynamic photoelasticity. For the first time, the sequence and interaction of two major dynamic failure modes in layered materials-inter-layer cracking and intra-layer cracking were revealed in real time. For heterogeneous three-layer systems, shear-dominated inter-layer cracking was always the first failure event for specimens subjected to low-speed impact. Interlayer cracking generally nucleated from interfacial locations where the inter-layer shear stress acquired a local maximum. Depending on impact speed and bond strength characteristics, inter-layer cracks were very transient and often became intersonic even under moderate impact speeds. Intra-layer cracking always initiated after the development of inter-layer cracks as a result of inter-layer crack kinking into the adjacent layer. The resulting intra-layer mode I cracks often accelerated and branched as they attained high speeds, causing core layer fragmentation. For homogenous-layered systems composed of bonded layers of Homalite, intra-layer cracks appeared in the form of cracks radiating from the impact site. As soon as these cracks approached an interface, interlayer cracks were often induced depending on the angle between the crack path and the interface. Direct experimental evidence of the dynamic equivalent of "Cook-Gordon mechanism" was recorded, i.e., two intersonic interfacial cracks nucleated and propagated along the interface before a fan of mode I incident cracks was ever able to reach the interface. Also, significant dependence of the failure characteristics on impact speeds and interfacial strengths was found. For the heterogeneous three-layer system subjected to a high impact speed, two clear shear shock waves associated with the intersonic inter-layer cracks were observed at the specimen center. Shock waves were also observed along the interface in heterogeneous three-layer systems featuring weak and ductile bonds. The impact momentum and loading duration were identified as two important parameters in damage spreading for a given impact energy. Motivated by the experimental observations of crack deflection/penetration at an interface, a novel wedge-loaded impact specimen was designed to explore the basic mechanics nature of this phenomenon. The deflection/penetration behavior of an incoming dynamic crack at an interface was found to depend on the interfacial angle and the interfacial fracture toughness. A dynamic fracture model, together with an energy criterion, were proposed and were found to agree reasonably well with the experimental observations