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A general theory is developed for determining the sizes, shapes, and locations of the double caustics produced in statically loaded birefringent plates containing Mode-I cracks. Particular attention is paid to the effect of the first few higher-order, non-singular stress terms on the determination of the stress-intensity factor KI associated with the singular stress field.
It is found that the transverse diameters of the inner and outer parts of the double caustic have an average value essentially equal to the transverse diameter of the single caustic produced by an optically isotropic material having the same optical constant. Furthermore, with the superposition of a constant (tensile or compressive) stress parallel to the crack, each part of the double caustic deforms independently but in such a way as to maintain this average transverse diameter.
Other higher-order effects are also investigated and it is concluded that birefringent materials offer the potential of “feature extraction” concerning crack-tip stress fields, in addition to being suitable for the accurate determination of KI-values experimentally.
caustics, shadow-spot patterns, Mode-I deformation, linear elastic fracture mechanics, optical methods, crack-tip stress fields
Associate professor, University of Illinois at Urbana-Champaign, Urbana, Ill.
Head, Structural Reliability Section, Marine Technology Division, Naval Research Laboratory, Washington, D.C.,