Testing and analysis of composite skin/stringer debonding under multi-axial loading

A consistent step-wise approach is presented to investigate the damage mech anism in composite bonded skin/stringer constructions under uniaxial and bi axial (in-plane/out-of-plane) loading conditions. The approach uses experim ents to detect the failure mechanism, computational stress analysis to dete rmine the location of first matrix cracking and computational fracture mech anics to investigate the potential for delamination growth. In a first step , tests were performed on specimens, which consisted of a tapered composite flange, representing a stringer or frame, bonded onto a composite skin. Te sts were performed under monotonic loading conditions in tension, three-poi nt bending, and combined tension/bending to evaluate the debonding mechanis ms between the skin and the bonded stringer. For combined tension/bending t esting, a unique servohydraulic load frame was used that was capable of app lying both in-plane tension and out-of-plane bending loads simultaneously. Specimen edges were examined on the microscope to document the damage occur rence and to identify typical damage patterns. For all three load cases, ob served failure initiated in the flange, near the flange tip, causing the fl ange to almost fully debond from skin. In a second step, a two dimensional plane-strain finite element model was d eveloped to analyze the different test cases using a geometrically nonlinea r solution. For all three loading conditions, computed principal stresses e xceeded the transverse strength of the material in those areas of the flang e where the matrix cracks had developed during the tests. In a third step, delaminations of various lengths were simulated in two locations where dela minations were observed during the tests. The analyses showed that at the l oads corresponding to matrix ply crack initiation computed strain energy re lease rates exceeded the values obtained from a mixed mode failure criterio n in one location. Hence, unstable delamination propagation is likely to oc cur as observed in the experiments.