Various configurations of edge delamination tension (EDT) test specimens were manufactured and tested to assess the usefulness of each configuration for measuring interlaminar fracture toughness. Tests were performed on both brittle (T300/5208) and toughened-matrix (T300/BP907) graphite reinforced composite laminates. The mixed-mode interlaminar fracture toughness G_c was measured during tension tests of (30/−30₂/30/90_n)_s, n = 1 or 2; (35/−35/0/90)_s; and (35/0/−35/90)_s layups designed to delaminate at low tensile strains. Laminates were made without inserts so that delaminations would form naturally between the central 90° plies and the adjacent angle plies. Laminates were also made with Teflon® inserts implanted between the 90° plies and the adjacent angle (θ) plies at the straight edge to obtain a planar fracture surface. In addition, Mode I interlaminar tension fracture toughness G_Ic was measured from laminates with the same layups but with inserts in the midplane, between the central 90° plies, at the straight edge. All of the EDT configurations were useful for ranking the delamination resistance of composites with different matrix resins. Furthermore, the variety of layups and configurations available yield interlaminar fracture toughness measurements, both pure Mode I and mixed mode, needed to generate delamination failure criteria.

The influence of insert thickness and location, and coupon size on G_c values were evaluated. For toughened-matrix composites, laminates with 1.5-mil (38.1-μm) thick inserts yielded interlaminar fracture toughness numbers consistent with data generated from laminates without inserts. Coupons of various sizes yielded similar G_c values. The influence of residual thermal and moisture stresses on calculated strain energy release rate for edge delamination was also reviewed. Edge delamination data may be used to quantify the relative influence of residual thermal and moisture stresses on interlaminar fracture for different composite materials.