J.M. Vasco-Olmo

Experimental evaluation of plasticity-induced crack shielding from isochromatic data

Abstract. The plasticity-induced crack shielding effect is evaluated during fatigue crack growth using transmission photoelasticity. The proposed methodology is based on the evaluation of the stress intensity factors calculated from the analysis of the isochromatic fringe patterns observed at the vicinity of a crack tip. Four different mathematical models describing the crack tip stress fields (namely models based on Westergaard’s, Williams’s, and Muskhelishvili’s equations and a new model called Christopher–James–Patterson) have been employed. Thus, a comparative study to evaluate which of the models is more suitable for fatigue crack shielding evaluation has been performed. A set of fatigue experiments on polycarbonate middle-tension specimens at different R-ratios have been conducted. Experimental results reveal the presence of plasticity-induced crack shielding on growing fatigue cracks for specimens tested at a low R-ratio. In addition, a retardation effect on the fatigue growth rate has been observed due to the shielding effect induced by the plasticity generated both at the crack tip and along the crack flanks. All these results highlight the enormous potential of transmission photoelasticity for the evaluation of plasticity-induced crack shielding on growing fatigue cracks.

Interpretation of Plasticity Effects Using the CJP Crack Tip Field Model

This paper will outline the development of a model of crack tip fields that represents an innovation in incorporate the influences on crack tip displacement and stress fields of the zone of local plasticity that envelops a growing fatigue crack. The model uses assumed distributions of elastic stresses induced at the elastic-plastic boundary via wake contact and compatibility requirements, and defines a set of modified elastic stress intensity factors to characterise the crack stress or displacement tip field. In particular, recent work will be presented that compares the interpretation of plasticity-induced shielding obtained from trends observed in KR and KF with values of so-called ‘crack closure’ obtained via traditional strain gauge determination.

Experimental methodology for the quantification of crack tip plastic zone and shape from the analysis of displacement fields

The current work presents a novel methodology for the experimental quantification of the crack tip plastic zone during fatigue crack growth. This methodology is based on the application of yield criteria to estimate the area and the shape of the plastic zone at the crack tip. The implementation of the proposed methodology requires the use of strain maps calculated from the differentiation of the displacement fields obtained by digital image correlation (DIC). Stress maps can subsequently be inferred from both von Mises and Tresca yield criteria. Fatigue tests and associated measurements of plastic zone size and shape were conducted on a compacttension specimen made from commercially pure titanium at R ratio of 0.6. In addition, the ability to predict the shape and size of the experimentally observed crack tip plastic zone has been explored using three different analytical elastic crack tip models [Westergaard, Williams and Christopher- James-Patterson (CJP)]. This analysis indicated that the CJP model provided the most accurate prediction of the plastic zone and shape.