微桥法基体裂纹的线性模型及薄膜断裂应变测试

Accurate measurement of critical strain and stress of a thin film when it fractures is crucial during testing of fracture toughness. In microbridge testing method, tensile displacement of a film is in consistent with the crack opening distance ( COD ) of substrate pre-crack at position where it anchors. However, a mathematical model for COD is difficult to deduce in mechanics due to the complex mechanical condition cause by the non half infinite boundary and the existence of an arresting hole near crack tip. High resolution scanning electron microscope (HR-SEM) is used to examine the COD of substrate pre-crack. Both of the opening distance (δ_s1) at crack end and the opening distance (δ_s2) at the edge of arresting hole were accurately measured. Results show that δ_s1 and δ_s2 varies linearly when δ_s1 is in range of 9~35 μm. A stationary virtual crack tip was then hypothesized so that a linear relationship of COD of substrate pre-crack was subjected. Film tensile displacement can then be easily determined according to δ_s1 and the position the film anchored. A mathematical mode for fracture toughness of a film was derived via superposition of uni-axial tensile stress and the pure bending stress the film endured during tensile testing. A test for CuZr amorphous thin film was carried out, and 1. 08~1. 70 MPa•m^{1/ 2} of K_IC is obtained. Results reveal that test values of K_IC vary with the length of the pre-crack on film specimens(a / W), which may be resulted from the stain softening of amorphous metallic glass.