3-DIMENSIONAL ANALYSIS AND RESULTING DESIGN RECOMMENDATIONS FOR UNIDIRECTIONAL AND MULTIDIRECTIONAL END-NOTCHED FLEXURE TESTS

Results are presented from a theoretical investigation of the effects of stacking sequence on the energy release rate in laminated composite end-notched flexure test specimens. Deflections and energy release ra tes of unidirectional and multidirectional ENF specimens are obtained by classical laminated plate theory, shear deformable plate theory, an d three dimensional finite element analyses. It is shown that the dist ribution of energy release rate varies across the front of an initiall y straight delamination. The percentage of mode II and mode III energy release rates for the specimen, as well as the local peak values of t he mode II, mode III and total energy release rates that occur at the specimen's free edges are shown to correlate with a nondimensional rat io comprised of the specimen's flexural rigidities. The results of the study are used as a basis for a proposed ''ENF test design procedure' ' that may be used for the determination of appropriate specimen stack ing sequences and test geometries for studying delamination growth at interfaces between plies at various orientations. The test design proc edure minimizes the contributions to the energy release rate from resi dual thermal stresses, geometric nonlinearities, local mode II concent rations at the specimen's flee edges and local mode III effects.