An Experimental Study on Surface Crack Growth under Mode-I Load

摘要

Surface crack growth in plates made of aluminum 7050 alloy is studied by the Rubber Impression Method (RIM). RIM is an experimental technique used to estimate the three-dimensional crack profiles by inserting and taking out gel-state rubber from the crack opening during the test. Far-field tensile loading was applied to the surface-cracked thin plates with various crack geometries. Three stages of fracture are found; (1) flat stable tearing close or up to through thickness (2) unstable lateral tearing with flat tunneling transitioning to slant fracture and (3) unstable fully slant fracture. Crack profiles measured by the RIM were compared with the crack extension marked by fatigue loads. Generally, reasonable agreement is obtained except the region adjacent to the free surface. The Crack Tip Opening Displacement (CTOD) at the center of the crack was obtained by examining the crack opening profiles at different load levels. The results indicate that the shallow cracked specimen has relatively high value of CTOD at the initiation of stable crack growth. Due to complex geometry of the three-dimensional crack the RIM showed its limits in precise measurement of the crack tip details. Nevertheless the RIM could be an effective way to study the overall behavior of ductile crack growth.