Noncontact optical measurement of CTOA and CTOD for interface crack in DCB test

The crack-tip opening angle (CTOA) and the crack-tip opening displacement (CTOD) are used as fracture parameters for characterizing the resistance of a material to fracture and predicting the instability of crack propagating fracture process. Digital image correlation method (DIC) was used to measure the displacement fields in a double cantilever beam (DCB) test with an initial interfacial crack. An apparent crack-tip was determined using the y-displacement along two parallel lines of the two crack flanks, which in turn enables the CTOA and CTOD to be evaluated. The experimental results revealed that the extension process for interface crack is divided into stable and unstable crack extension phases which changed alternately in the DCB test. The crack growth rate (CGR) varied from one stable phase to another, but was a constant value within each stable phase. The CGR, however, increased gradually over time within the unstable phases. The CTOA and CTOD did not increase synchronously in the loading process: the CTOA begins to increase at the first stable crack growth phase, whereas, the CTOD increased rapidly with the CTOA until entered into the second phase of unstable crack propagation. The above crack propagation behaviour on the bi-material corresponds to the interface crack transition, which is transformed from the tearing crack Mode I gradually into the mixed Mode I/II crack form.

► The CTOA and CTOD of the interface crack in DCB test was measured using DIC.
► The extension process of interface crack at odds into stable & unstable crack extension phases, which appear alternately.
► The crack growth rate (CGR) remains constant within each stable phases and changes from one another.
► At unstable phases the CGR increases gradually with time.
► The CTOA and CTOD are not increased synchronously in the loading process.