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.

CRACK CLOSURE IN SMALL FATIGUE CRACKS—A COMPARISON OF FINITE ELEMENT PREDICTIONS WITH IN‐SITU SCANNING ELECTRON MICROSCOPE MEASUREMENTS

“…This is in agreement with the conclusion obtained for through thickness cracks by Ritchie et al [40], who found that for a coarse-grain 2124 aluminium alloy the magnitude of fatigue crack closure measured experimentally for both long and short cracks was consistently higher than that obtained from an elastic-plastic finite element analysis under plane strain conditions. In addition, it can be seen from Eq.…”

“…If the stress level at which the COD on the specimen surface (@ = 0) is reduced to zero is defined as the closure stress, the above results indicate that the plasticity-induced fatigue crack closure stress on the specimen surface for three dimensional semi-circular cracks does not change with a change of fatigue crack length. For through thickness cracks, the independence of crack closure stress on crack length for plasticity-induced closure has been suggested by several previous studies [20,21,24,40]. One interesting indication from Figs 4(a) and 4(b) is that on reaching a fixed COD (non-zero), the smaller the crack is then the higher is the applied stress.…”

Load interaction effects during fatigue crack growth under variable amplitude loading—a literature review. Part II: qualitative interpretation

“… Schematic representation of typical behaviours of small cracks under CA loading in smooth and notched specimens 122 …”

“…Many investigators have suggested that crack closure can be a major factor in causing some of the differences between the growth of long and short cracks 94 , 109 , 121 –124 . A crack freshly initiated from a microstructural defect or in an intense slip region has no previous fatigue history and therefore no wake of plastic deformation.…”

Fatigue damage criteria classification, modelling developments and trends for welded joints in marine structures

“…Anomalous crack growth at notches (Figure 7) is often illustrated below the long crack threshold (e.g. Ritchie et al 1988, Janssen et al 2002 and suggested to be a result of missing crack growth driving force (correction) components like K t or Y n . Plasticity induced crack closure, i.e.…”