Real-time acquisition of fatigue crack images for monitoring crack-tip stress intensity variations within fatigue cycles

Abstract

Digital images of fatigue crack behaviour have been acquired in real time at high cycle fatigue rates (77 Hz) using a high-speed camera at 1000 frames/s. Digital image correlation has then been used to determine the crack-tip position and stress intensity variations (K_I and K_II) within selected cycles. This has been achieved for a pre-cracked aluminium compact tension (CT) specimen subjected to constant load amplitude fatigue crack cycling. The crack-tip displacement field has been determined at 14 points within each cycle. In this proof of concept study, despite noise in the inferred displacement fields, by least squares fitting the displacement field rather than the strain field to the Muskhelishvili's form crack-tip stress field, the crack-tip stress intensity inferred from the measured crack-tip displacement field was found to good accuracy (around 0.2 MPa m^1/2). Furthermore, the observed sinusoidal variation was in excellent agreement with the nominal ΔK obtained from the applied fatigue amplitude confirming the accuracy of the method; a drift of around 0.42 MPa m^1/2 was observed in K_mean from cycle to cycle. No evidence of closure was observed at low K. In principle this method permits the identification of crack-tip closure and crack growth, as well as the application of constant ΔK_eff cycling, through intermittent monitoring throughout the duration of a fatigue test. The current method is well suited to more complex cases where crack closure, residual stresses, or more complex geometries mean that it is difficult to predict ΔK_effa priori and a number of potential applications of the method are identified.