A Mixed-Mode Fracture Analysis of (±25/90_n)_s Graphite/Epoxy Composite Laminates

An investigation of the fracture mechanisms and behavior of sub-laminate cracking in graphite/epoxy composite laminates is presented. A series of laminates of the form (±25/90_n)_s is chosen for analytical modeling and compared with experimental results. By varying the number of 90-deg plies, indicated by n, several important and distinct fracture modes are identified. The thickness of the 90-deg layer is the single parameter that separates these fracture events. The present work focuses in particular on the mechanisms of transverse ply cracking and free-edge delamination.

The energy release rate approach of classical fracture mechanics is applied to describe the crack initiation fracture process. The cracking process is numerically simulated using a finite-element procedure formulated within the framework of ply elasticity and on the assumption of a generalized plane-strain state. The numerical procedure explicitly calculates the Mode I, Mode II, and Mode III components of the strain-energy release rate as a function of crack length. Unit mechanical and unit thermal load conditions are solved, and the results are superposed to obtain the general load case. The analytical model predicts the sequence of occurrence of the fracture modes and the critical onset loads.

The theoretical model is then correlated with published experimental data including tension tests on (±25/90_n)_s (n = ½, 1, 2, 3, 4, 6, and 8) laminate coupons manufactured using the T300/934 graphite/epoxy system and with fracture tests using double-cantileverbeam and cracked-lap-shear specimens. Good agreement is obtained between the theoretical and experimental results. The general nature of the present analytical method is readily applicable to composite laminates of more complicated structural geometry or loading conditions or both.