Experimental studies of Mode I energy release rate in adhesively bonded width tapered composite DCB specimens

The Mode I energy release rate, GI, derived from conventional uniform width double cantilever beam (DCB) test data depends on crack length. Data acquisition and analysis from such experiments can be simplified if GI is independent of crack length, particularly at high loading rates. Previous research by others has shown that width tapered double cantilever beam (WTDCB) specimens made from aerospace grade carbon/epoxy composite laminates without adhesive layers exhibit critical energy release rates which are independent of crack length. The purpose of the present investigation was to determine whether or not adhesively bonded WTDCB specimens of automotive type sheet molding compound (SMC) composites show similar behavior. It was observed that the critical energy release rate was practically independent of crack length for unidirectional carbon/epoxy WTDCB specimens without adhesive layers but not for unidirectional carbon/epoxy, unidirectional E-glass/epoxy or automotive SMC WTDCB specimens with adhesive layers, and that self-similar crack growth only occurred in unidirectional carbon/epoxy WTDCB specimens without adhesive layers.