1989 Volume 38 Issue 428 Pages 514-519
A fractographic study has been carried out on fatigue crack growth from a semi-elliptical surface crack under shear loading. The fatigue tests of rectangular bar specimens with a semi-elliptical surface crack were made on an aluminum alloy 2017-T4 and a weldable structural steel SM41A, using the four-point-shear loading method. SM41A showed the tensile mode growth with complicated three -dimensional crack growth surfaces, and 2017-T4 showed the shear mode growth which occurred, forming flat crack growth surfaces, in the same plane as the pre-crack. The macroscopic shapes of these crack surfaces formed by the tensile mode growth and by the shear mode growth have shown good agreement with those predicted by σmax criterion and by τmax criterion, respectively. The microscopic features of these fatigue crack growth surfaces were observed by using SEM. The tensile mode crack growth surfaces of SM41A were covered with striation pattern. The factory-roof-like surface, which was formed at the crack front under mode III-predominant deformation, was found to consist mainly of striation-like pattern. The shear mode crack growth surfaces of 2017-T4 showed rather smooth and flat feature with some steps and streaks. The direction of these streaks was found to be in accordance with the direction of the maximum shear stress at the crack front.
