On the analysis of the interlaminar strain-energy release rates in ring-fracture-test specimens

An approach to analyze delamination in a circular composite beam specimen loaded in bending is presented. First, an experimental investigation of the interlaminar fracture behavior of unidirectional glass fiber reinforced polyester under mixed mode (I/II, opening + sliding) loading conditions was performed. For this purpose, the ringfracture-test was used. The test was derived from a modified unidirectional glass fiber reinforced polyester ring specimen, which originally was used for investigation of the interlaminar tensile strength. The ring-shaped fracture-test specimens were produced by filament-winding technology. The ring specimen contains a tangential starterdelamination crack, being initiated from a strip of Teflon foil, which was added during the winding process. A radial cut was made in each ring at the opposite side of the starter crack, realizing a kind of horseshoe specimen. During the test, this specimen was subjected to opening forces. The crack propagated along the fibers (in circumferential direction) in a symmetrical manner with respect to the starter foil. A linear-elastic fracture-mechanics model is applied to interpret the crack-mechanical data recorded during the ring-test. The value of the interlaminar fracture energy is considered as a function of the length of the propagating circular crack. The total strain-energy release rate is decomposed into opening (mode I) and shearing (mode II) components. The mode partitioning shows that the ring-fracture-test gives variable mixed mode (I/II) ratios. The obtained values of the strain-energy release rate are compared with finiteelement analysis results. It is found that the ring-fracture-test represents a new and useful approach for investigating the mixed mode (I/II) fracture behavior of unidirectional glass fiber reinforced polyester.