Experimental Mechanics @ Michigan State University
		Abstracts on Caustics
A STUDY OF RESIDUAL CAUSTICS GENERATED FROM FATIGUE CRACKS R. A. Tomlinson and E. A. Patterson Fatigue and Fracture in Engineering Materials and Structures(1997), 20, (10), 1467-1479 Abstract: The method of caustics was used to determine the stress intensity factor of fatigue cracks in steel compact tension specimens. Under zero load a residual caustic was observed at the tip of a fatigue crack indicating the presence of a residual stress field. Caustics were generated at increasing static loads and the stress intensity factors were compared with those predicted by theory. It was found that the difference between each measured stress intensity factor and its corresponding theoretical value was a constant for the range of loads. This difference was shown statistically to be equal to the stress intensity factor determined from the residual caustic. The proposed mechanism for the formation of this residual caustic was probably due to crack tip plasticity effects and not due to crack closure. It was concluded that residual caustics can be measured to quantify crack tip behaviour in fatigue cracks and have been shown to be a useful tool in the measurement of residual stress fields.
A GENERAL METHOD FOR AUTOMATED ANALYSIS OF CAUSTICS J. Carazo-Alvarez and E. A. Patterson Optics and Lasers in Engineering (1999) Vol.32, pp.95-110 Abstract: In most published work on the optical method of caustics, experimental results were obtained by manually measuring some characteristic dimensions of the shadow pattern and, therefore, used only a limited amount of data from the caustic. However, some previous work has been done using digital image processing and light intensity plots combined with least-squares techniques to analyse the shape of a caustic obtained from a particular problem. All of these methods have been developed to solve only one specific problem. This paper highlights the essential guidelines of an automated method, which has the novelty of generalising the whole analysis, such that only the parametric equations of a particular problem are required to solve it. In the proposed method the image is digitised, the central line of the caustic pattern is identified, and the parameters in the mapping equation of the caustic are determined. The method has been validated, and the aim of this paper is to describe the key features of the method, i.e. the optical arrangement, digital image processing, and numerical analysis.
THE EFFECTS OF SURFACE TOPOGRAPHY ON THE METHOD OF CAUSTICS R. A. Tomlinson and E. A. Patterson Experimental Mechanics (1999) Vol 39, No. 4, pp. 335-342 Abstract: An investigation was performed to establish the effects of surface topography on the determination of stress intensity factors of opaque components using the method of caustics. Theoretical predictions of caustics produced by curved surfaces were generated and validated by experiment. The errors introduced for stress intensity factors determined at various radii of curvature were assessed. It was found that relatively small surface curvatures caused significant errors in the stress intensity factor. These errors were affected by the initial radius, load and material properties
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