~ Pergamon Dynamic Decohesion of Bimaterials: Experimental Observations and Failure Criteria

s--Some results of high speed interferometric measurements on dynamically propagating interfacial cracks are presented. Polymethylmethacrylate (PMMA)/steel bimaterial specimens of the one-point bend type are used. They are impact loaded using either a drop weight tower device or a high speed gas gun. This results in two distinct groups of experiments at different loading rates. In all cases very high crack propagation speeds are seen. In gas gun experiments, terminal crack tip speeds of up to 1.5 C~ MMA, where C~ T M is the shear wave speed of P M M A , are measured. In addition, high accelerations (~107 g, where g is the acceleration of gravity) are observed and reported. Theoretically predicted near tip fields are used to extract experimental values of the dynamic complex stress intensity factor histories in each test. Using the fitted histories of the complex stress intensity factor, a dynamic crack growth criterion relating the energy release rate to phase angle and crack tip velocity is proposed and discussed. 1. I N T R O D U C T I O N The increased use of composite materials in engineering applications in recent decades has become an incentive for extensive research, both basic and applied, into the various failure modes of such materials. It was soon recognized that the macroscopic performance of composite materials was intimately related to effects occurring at the interface between the different phases of the composite. This led to a recent explosion of research in interfacial fracture effects in bimaterial systems. The first study of interfacial fracture was made by Williams (1959). He found an oscillatory stress singularity present at the tip of a crack lying on an interface between two elastic solids. Until recently, only a few researchers had provided some explicit solutions of the stress field in particular interracial fracture problems (Sih and Rice, 1964; England, 1965 ; Erdogan, 1965 ; Rice and Sih, 1965). In all these solutions, asymptotically an oscillatory stress singularity was found, confirming Williams's (1959) result. The recent resurgence of work on interface fracture has resulted in many more contributions. Most notable are those of Rice (1988) and Shih (1991). Up to this point most of the research activity on interface fracture is on issues regarding the quasi-static failure of bimaterial systems. To understand fully the catastrophic nature of composite materials' failure, which is important in many engineering applications, a good understanding of dynamic interfacial fracture mechanics is also necessary. Due to the complexity of the latter problem, however, only a few theoretical investigations have been made so far (Goldshtein, 1967 ; Brock and Achenbach, 1973 ; Willis, 1971, 1973 ; Atkinson, 1977; Yang et al., 1991 ; Deng, 1993). To our knowledge, only two preliminary experimental studies on this subject are currently available (Tippur and Rosakis, 1991 ; hiu et al., 1993). In the current work we investigate the essentially unexplored area of experimental dynamic interracial fracture mechanics. The general objective sought by this investigation is the understanding of the highly transient dynamic interfacial crack initiation, growth and arrest in bimaterial systems. In particular, in the context of the present paper, it is intended to provide a summary of recent experimental observations on the issue of dynamic t Present address: Department of Mechanical Engineering, University of Delaware. Newark, DE 19716, U.S.A.