Mixed Mode (K_I+K_II) Stress Intensity Factor Measurement by Electronic Speckle Pattern Interferometry and Image Correlation

Abstract

The surface displacement fields of a fatigue precracked compact tension sample under tensile load were registered by electronic speckle pattern interferometry and image correlation. The in-plane elastic strain fields calculated from the displacement data were used to obtain the first stress invariant, which for the case of plane stress is proportional to the real part of the first complex potential in Muskhelishvili's approach. Solutions for the stress fields around the crack tip, KI and KII were sought in the form of the Fourier series using Muskhelishvili's complex stress functions. The Fourier series coefficients were calculated from the displacement data using multiple point overdeterministic method (MPODM). The nominal and inferred KI values differ by around 10%; this is probably due in part to mixed mode (KII) loading introduced by some degree of misalignment during the experiment.

Towards a new model of crack tip stress fields

“…The development of this model is a natural progression from the work of two of the authors (Pacey et al 2005) and hence photoelasticity has been used, however the concepts can be extended to thermoelasticity as shown by Diaz et al (2004) and to image correlation by Shterenlikht et al (2004) so that the analysis of fatigue cracks in metals becomes viable. However, although thermoelasticity has been used to obtain K during a fatigue cycle, as yet neither technique has been used to monitor K during the cycle.…”

Crack paths under mixed mode loading

“…Shterenlikht et al [38] developed the method used in photoelasticity by Nurse and Patterson [19] to accurately determine mixed-mode stress intensity factors using full field ESPI and image correlation data. An advantage of these techniques is that minimal specimen preparation is required, only using the painted or abraded surface of the component, unlike reflection photoelasticity and moiré where a coating or grating has to be bonded to the surface.…”

Mid-thickness studies of the stress intensity factor in the bulk of bainitic steel

“…Accordingly, the tool can be used under small scale yielding (SSY) conditions. For other full-field techniques such as thermo-elasticity [3], photo-elasticity [1], digital image correlation [28] or electronic speckle patter interferometry [4], this is achieved by collecting data from outside the plastic zone. This effect is studied here by including and excluding the data from the plastic zone in the data used for estimating the SIF.…”

“…he experimental characterisation of crack tip fields is normally done using surface techniques such as photoelasticity [1], Moiré interferometry [2], thermo-elastic stress analysis [3], electronic speckle pattern interferometry [4] or digital image correlation [5,6]. For thin components, the surface behaviour is normally representative of the T complete behaviour [7].…”