Comparison of laboratory techniques for evaluating the fracture toughness of glassy polymers

Several test methods were employed to determine polymer fracture toughness (𝒢 Ic , the opening‐mode strain energy release rate) at room temperature. The materials used included DGEBA epoxies and those modified by the addition of CTBN elastomers. Double‐cantilever beam specimens were used to determine the fracture toughness both of bulk resins and of an adhesive layer bonded between two aluminum half‐beams. The adhesive fracture toughness of a 0.025‐cm bond was slightly less than the bulk 𝒢 Ic value, attributed to the bond thickness effect. Fracture toughness of bulk resins was also evaluated by using both rectangular and round compact tension specimens. The results, when compared with those obtained with the bulk double‐cantilever beams, are quite acceptable. The thickness of compact tension specimens, ranging from 0.64 to 1.0 cm, might not give pure plane‐strain conditions, and thus some plane‐stress contribution to 𝒢 Ic should be expected for the tougher materials. Izod impact tests were also carried out to determine sample fracture toughness at high loading rate.