International Journal of Fracture 27 (1985) R17-R21. © 1985 Martinus Nijhoff Publishers, Dordrecht. Printed in The Netherlands. RI7 CAUSTICS UNDER COMPRESSIVE LOADS FOR CRACKED PLATES Bahram Farahmand Structural Integrity, 3150 Alameda Expressway San Jose, CA 95118 USA tel: (408) 978 8200 Demetrios D. Raftopoulos The University of Toledo Toledo, Ohio 43606 USA tel: (4i4) 537 2661 An attempt has been made here to investigate the stress distribution in the vicinity of the tip of an open edge and center crack under com- pressive loading. The transmitted and reflected caustics were obtained by impinging a monochromatic convergent light beam from a He-Ne laser on the tip of the crack of a loaded specimen. The crack under study is in the compressive field. It was found that in the case of pure bending, as it was in the case of uniaxial compression, the caustic of mode I can be seen upon impinging a monochromatic convergent light beam at the tip of the compressed crack. The specimens that were used for uniaxial compression had a width b = 5.0 cm and thickness t = 0.4 cm. Cracks in the specimens were made with a cutting saw having a rim thickness of 0.03 cm. For compression, the length of the cracks in each specimen were a = 0.5, 1.5, and 2.0 cm for edge and center crack. Experiments for the pure bending case necessitate specimens with 24 cm length and 6.35 cm width and the use of a special attachment in the loading machine to assure pure bending. In both cases of single and double edge cracks subjected to pure bending, the cracks were situated in the compression field and in the middle of one of the long edges of the specimen. For a single edge crack, specimens with crack lengths of 0.6, i.i, 1.6, 2.0, 2.3, 2.7, 3.0, 3.4, 3.6, 3.75, and 3.9 cm and for double edge crack specimens with crack lengths 0.6, i.i, 1.6, 2.1, and 2.6 cm were utilized. The cracked specimen was placed between two metallic plates for com- pressive loading. A monochromatic convergent beam from a He-Ne laser was employed with a focal point falling between the specimen and the measuring screen. Photographic pictures of the reflected and transmitted caustics, at equal distances from the specimen, for different steps of loading, are shown in Fig. (la, b) for an edge cracked specimen having crack length a = 2.0 cm. It is interesting to note, as in the case of tension [i] the measured diameter of the reflected caustic was always greater than the transmitted caustic when both were measured at equal distance from the specimen. Thereafter, the ratios of the transverse diameter of the trans- mitted and reflected caustics were investigated when the screens were located at equal distance from the specimen and behind the focal point of the convergent beam. For a fixed specimen-screen distance and for various compressive applied loads, constant ratios were observed, see Fig. (i). Thus~ it was determined that in the case of compression, just as in the case of tension [i], the ratio of the transverse diameter of the two caustics (reflected and transmitted), when the two edges of the crack are not in contact, is constant. Int Journ of Fracture 27 (1985)