Buckling and propagation of a delaminated composite beam in bending

Delamination is considered one of the most critical damage modes in composite materials. It has been studied extensively for in-plane compression of composite structures. Delamination in composite beams under pure bending is presented here. Experiments have been performed to determine the critical moment when the delamination buckles open (snap-buckling) and the moment at which propagation of the delamination occurs. A mathematical model has been adapted to predict the snap-buckling moment level and the bifurcation behavior. Mixed mode analysis has been applied to calculate the moment for delamination propagation. The snap-buckling and propagation moment levels were successfully predicted. Fatigue tests were performed to check the influence of the snapbuckling mode on the threshold moment for propagation. Nomenclature a = The ratio between sublaminate thickness and total thickness of the laminate A = Cross-sectional area of the beam E = Young’s modulus G = Strain energy release rate I = Second moment of area L = Length of the delamination M = Applied bending moment Mpr = Moment for propagation Q1 = Amplitude of buckled sublaminate Q2 = Amplitude angle of the sublaminate rotation at its end Qo = Amplitude of initial imperfection t = Total thickness of the laminate UB = Bending strain energy US = Stretching strain energy wi = Displacement function for subalmainte i y = Distance from surface to the neutral axis of the beam � = End shortening of the end sublaminate over the delaminated region 1 � = Total end shortening for the thin sublaminate 2 � = Total end shortening for the thick sublaminate