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. 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. Inter-layer cracking generally nucleated from the maximum inter-layer shear stress locations. 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 as a result of inter-layer crack kinking into the adjacent layer. The resulting intra-layer mode-I cracks accelerated and may branch if they attained high speeds. For homogenous-layered systems, intra-layer cracks appeared in the form of cracks radiating from the impact site. When these cracks approached an interface, inter-layer cracks were 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 before mode-I cracks reached the interface. Also, significant dependence of the failure characteristics on impact speeds and interfacial strengths was found. For heterogeneous three-layer systems subjected to a high impact speed, two shear shock waves associated with the intersonic inter-layer cracks were observed. 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. The deflection/penetration behavior of an in-coming 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.