An experimental investigation of fracture at an interface between two epoxies

The growth of a crack located at the interface between two linearly elastic solids is investigated experimentally. The requirements for the test pieces are strong bonding between the two materials, a well-defined, planar interface, and a pronounced difference in the stiffness between the two materials. To attain these requirements, castable liquid epoxy resins are used. It is demonstrated that the manufacturing process, which follows previously established procedures for the bi-material solid composed of Solithane, is also applicable for epoxy. To investigate the toughness of the interface crack, the measurement of crack speed and the estimation of stress intensity factors are carried out for several different temperatures. Master curves of crack speed for either of the two materials and for the bimaterial are presented. The experimentally obtained fracture toughness data are compared with the expected values by Knauss's model (1971). By varying the mode mixity at the crack tip, it is found that the crack might advance by kinking into the soft material or by propagating along the interface itself, depending on the applied loading conditions. Although the number of data points is small, the fracture data gathered from tests performed at two different temperatures indicate that rate effects significantly influence the kinking behavior. Crack tip speeds after kinking are also recorded.