Fatigue crack closure under triaxial stress constraint—I. Experimental investigation

Abstract Experimental investigation has been done for fatigue crack closure in 7050-T7451 aluminum alloy and 30CrMnSiNi2A high strength steel. CT specimens with thicknesses of 2, 3, 4 and 8 mm are used and four techniques of closure measurement are adopted in the test, namely the COD-load compliance technique, back face strain-load compliance technique, near tip strain gauge technique and the indirect technique based on fatigue crack propagation rate (FCPR) measurement. The uncertainties associated with different techniques of closure measurement as well as the thickness effects are analysed. It is shown that the global-compliance based methods cannot reflect the shielding effect of closure upon the crack tip strain intensification sufficiently. The near tip strain gauge method is more sensitive and can give a better result if the gauge lies in an optimum position. For materials in which the striation mechanism is dominant the indirect methods based on FCPR or fatigue striation measurement can predict the closure level very well.