Development of Fatigue Crack Closure with the Extension of Long and Short Cracks in Aluminum Alloy 2124: A Comparison of Experimental and Numerical Results

Abstract

The development of crack closure during the extension of long and short fatigue cracks has been investigated in a 2124 aluminum alloy using both experimental and numerical procedures. Specifically, the growth rate and closure behavior of long (∼17 to 38 mm) cracks in compact C(T) specimens and through-thickness physically-short (∼50 to 40 μm) cracks in single-edge-notched SEN(B) bend specimens have been examined experimentally from threshold levels to instability (over the range ∼10−12 to 10−7 m/cycle), and results compared with those predicted numerically using an elastic-plastic finite-element analysis of fatigue crack advance in plane strain. It is shown that the numerical analysis consistently underpredicts the magnitude of crack closure for both long and short cracks, particularly at near-threshold levels; an observation attributed to the fact that the numerical procedures can only model contributions to closure from cyclic plasticity, whereas, in reality, significant additional closure arises from the wedging action of fracture surface asperities and corrosion debris. Such closure is shown to provide the predominant mechanism for rationalizing differences in the growth rate behavior between long and physically short cracks, although other factors, such as the nature of the singularity and the extent of local plasticity, are deemed potentially to be important.