Issue 50

Ch. F. Markides, Frattura ed Integrità Strutturale, 50 (2019) 451-470; DOI: 10.3221/IGF-ESIS.50.38 468 with caution. For example, the photos of caustics in Figs. 9(c1) and 9(c2), though they seem more representative and perhaps more adequate than those of Figs. 9(b1) and 9(b2), are actually refer to the case the material has enter plasticity, in the contact region, so that any measurements should not be so simply used in obtaining the contact length (this is readily verified by unloading and reloading the specimen whence it is seen that the new contact caustics are by no means of an elastic nature). Finally, it should be noticed that the general formulae including double and single caustics obviously refer also to opaque materials as particular cases of the transparent ones, provided their faces are properly polished. The latter could be of crucial importance, especially for specimens made of marble [50] and natural building stones [51], the mechanical response of which is usually studied using the Brazilian-disc test (due to insuperable difficulties encountered in the implementation of direct uniaxial tension tests, caused due to the unique nature of such materials [52-56]) employing either traditional or innovative experimental techniques [57-60]. A CKNOWLEDGMENTS he author would like to express his deep gratitude to his Professor Kallimassiotis-Pazis, D. N., for his unreserved help and support in conducting the experimental protocol; his long experience in the application of optical methods in the investigation of the elastic equilibrium of various commercial materials were very crucial in obtaining reliable results and improve the quality of this paper. R EFERENCES [1] Manogg, P. (1964). Anwendung der Schattenoptik zur Untersuchung des Zerreissvongangs von Platten, Dissertation, Freiburg, Germany. [2] Manogg, P. (1964). Schattenoptische messung der spezifischen bruchenergie waehrend des bruchvorgangs bei plexiglas, in: Proceedings of the International Conference on Physics of Noncrystalline Solids, edited by J.A. Prins, Delft, The Netherlands, pp. 481–490. [3] Manogg, P. (1966). Investigation of the rupture of a plexiglas plate by means of an optical method involving high speed filming of the shadows originating around holes drilled in the plate, Int. J. Fracture, 2(4), 604–613. [4] Manogg, P. (1966). Die lichtablenkung durch eine elastisch beanspruchte platte und die schattenfiguren von kreis- und risskerbe, Glastechnische Berichte, 39, pp. 323–329. [5] Theocaris, P.S. (1970). Local yielding around a crack tip in plexiglas, J. Appl. Mech., 37(2), pp. 409–415. [6] Theocaris, P.S. and Gdoutos, E.E. (1972). An optical method for determining opening-mode and edge sliding-mode stress intensity factors, J. Appl. Mech., 39(1), pp. 91–97. [7] Kalthoff, J.F., Winkler, S. and Beinert, J. (1976). Dynamic stress intensity factors for arresting cracks in DCB specimens, Int. J. Fracture, 12(2), pp. 317–319. [8] Theocaris, P.S. (1981). Elastic stress intensity factors evaluated by caustics, In: Mechanics of Fracture, edited by G. C. Sih, Sijthoff and Noordhoff, Amsterdam, The Netherlands, 7(3). [9] Papadopoulos, G.A. (2011). New formula of experimental stress intensity factor evaluation by caustics, Int. J. Fracture, 171, pp. 79–84. [10] Yazdanmehr, A. and Soltani, N. (2014). Evaluation of stress intensity factors of rounded V and U notches under mixed mode loading, using the experimental method of caustics, Theor. Appl. Fract. Mec., 74(1), pp. 79–85. [11] Theocaris, P.S. and Andrianopoulos, N.P. (1981). Dynamic three-point bending of short beams studied by caustics, Int. J. Solids Struct., 17(7), pp. 707–715. [12] Papadopoulos, G. A. (1990). Dynamics caustics and its applications, Opt. Laser Eng., 13(3–4), pp. 211–249. [13] Georgiadis, H.G. and Papadopoulos, G.A. (1992). On the method of dynamic caustics in crack propagation experiments, Int. J. Fracture, 54(2), pp. R19–R22. [14] Xiong C.Y., Yao, X.F., Fang, J. (1999). A study of dynamic caustics around running interface crack tip, Acta Mech. Sinica, 15(2), pp. 182–192. [15] Yao, X.F., Xu, W., Jin, G.C., Yeh, H.Y. (2004). Low velocity impact study of laminate composites with mode I crack using dynamic optical caustics, J. Reinf. Plast. Comp., 23(17), pp. 1833–1844. [16] Yao, X.F., Zhao, H.P., Yeh, H.Y. (2005). Dynamic caustic analysis of propagating mode II cracks in transversely isotropic material, J. Reinf. Plast. Comp., 24(6), pp. 657–667. T