Crack Closure Measurement by the Optical Method of Caustics

Wallhead, Ian (1995). Crack Closure Measurement by the Optical Method of Caustics. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000fb5e

Abstract

This work presents an investigation into the applicability of the optical method of caustics as a tool to study the role of crack closure in fatigue crack propagation. The technique has been applied to centre-cracked panel specimens of 2024-T3 aluminium alloy under a range of fatigue loading conditions which are expected to induce crack closure to varying degrees.

The optical method of caustics is an experimental technique which is primarily applied to the measurement of stress intensity factors by quantifying out of plane surface deformations local to the crack tip. In this work the technique has been used to measure stress intensity factors throughout various fatigue cycles so as to quantify the range of stress intensity factors actually experienced by the crack tip, and thus determine ΔK_eff. In order to accomplish this the technique has required substantial development both to measure low stress intensity factors where closure effects are known to predominate, and to measure them with sufficient resolution to quantify ΔK_eff.

These improvements comprise the application of an accurately collimated laser (with output beam wavefront error of approximately λ/5 at 633nm), the specification of optically flat specimen surfaces (with a surface form error of approximately λ/4 at 633nm over the measurement area), together with the use of a CCD camera image processing system for recording and measuring the caustics. Most importantly, however, the experimental arrangement of the technique has been dramatically modified for this study to produce a “split beam caustics” set-up which has been found to substantially increase the accuracy of the technique.

Using these developments it has been shown that the method of caustics can detect premature material contact in the wake of a fatigue crack. This is manifest as an increased tensile stress field ahead of the crack which can exist even under a small far field compressive loading. However, the quantification of crack closure using this technique is not in agreement with other experimental evidence or theoretical models. Furthermore, in stark contradiction with contemporary theory, fatigue crack growth rate is found to be independent of the stress field range around the crack tip (conventionally described in terms of a stress intensity factor range, ΔK) under certain conditions, such following a single tensile overload.

Close observation of the crack tip stress field under conditions of closure has revealed that its spatial function is such that it cannot truly be expressed in terms of a stress intensity factor which assumes the stress field to vary as K/2πr. Consequently, it is concluded that successful application of ΔK_eff as the fatigue crack driving force is derived on phenomenological rather than physical grounds.