Propagation of the front of fast cracks in hard polymers

Abstract

3. V. V. Moskvitin, Plasticity under Alternating Loads [in Russian], Moscow State Univ. (1965). 4. V. V. Moskvitin, Cyclic Stresses in Structural Elements [in Russian], Nauka, Moscow (1981). 5. R. M. Shneiderovich, Strength during Static and Pulsating Loading [in Russian], Mashinos- troenie, Moscow (1968). 6. N. G. Bychkov, A. N. Petukhov, and I. V. Puehkov, "Certain features of the kinetics of deformation of structural materials during cyclic elastoplastic deformation," Probl. Prochn., No. ii, 7-11 (1986). 7. I. A. Birger, "Deterministic and statistical models of endurance," in: Problems of the Keliability of Aircraft [in Russian], Mashinostroenie, Moscow (1985), pp. 105-150. 8. I. A. Birger, "Prediction of safe life under low-cycle fatigue," Probl. Prochn., No. 10, 39-44 (1985). 9. Yu. M. Temis, "Evaluating plasticity and creep in engineering calculations," in: Analy- tical and Numerical Methods of Solving Boundary-Value Problems of Plasticity and Visco- elasticity [in Russian], UNTs AN SSSR, Sverdlovsk (1986), pp. 100-106. i0. A. P. Gusenkov, Strength under Isothermal and Nonisothermal Low-Cycle Loading [in Rus- sian], Nauka, Moscow (1978). PROPAGATION OF THE FRONT OF FAST CRACKS IN HARD POLYMERS Yu. A. Kostandov and S. I. Fedorkin UDC 620.117.5:539.219.2 Experimental investigations of the dynamic failure of linear and cross-linked polymers such as polymethyl metacrylate (PMMA) and cured epoxy resins showed that in specimens with more stored elastic energy (this corresponds to higher nominal static stresses) the crack accelerates more rapidly to its limit speed [1-3]. The mean rate of crack growth increases from zero, and the time of acceleration usually amounts to some tens of microseconds. Various solutions in determining the limit crack growth rate obtained within the frame- work of the theory of elasticity practically lead to the same results which are confirmed by some experimental data. There exists nevertheless information contradicting the assertion that the limit crack growth rate is dependent solely on the elastic constants of the material. It is assumed that the limit crack growth rate is also dependent on the nature of the energy supply [i]. In connection with that the dependence of the limit growth rate of fast cracks, propagating under conditions of pulsed effects, on the parameters of the process of failure requires thorough investigation. Numerous experimental data testify to the jumpwise nature of the propagation of the crack tip in dynamic failure of materials. Some researchers [i, 5] ascribe this to the effect of stress waves reflected from the boundaries of the specimen. The same effect is apparently also the cause of the j umpwise motion of a crack in splitting cyrstals, which is accompanied by the transition of the crack into neighboring planes [i, 6]. That is why only some publications [2, 3, 7] contain data on the jumpwise nature of crack propagation not connected with the interaction of the crack with the boundaries of the speci- men. In the present work we investigated the nature and speed of propagation of the front of cracks initiated by the detonation of a conductor by a current pulse [8] in specimens of materials ED-2OMA, ED-20MTGFA (epoxy resins cured by maleic and methyl tetrahydrophthaleic anhydrides) and PMMA that are free of static stresses or are prestressed. The specimens were rectangular plates 4 mm thick; halfway along their length there was a shaped notch with radius of curvature 25 mm and radius at the tip 0.05 mm. Into the notch we placed a strip of tin plate which detonated when capacitors charged to high voltage were discharge on it. The dimensions of the plates were chosen such that the incidence of elastic waves reflected from the boundaries of the specimen into the zone of registration was pre- vented. M. V. Frunze Simferopol University, Simferopol. Translated from Problemy Prochnosti, No. 9, pp. 23-26, September, 1988. Original article submitted November 24, 1986. 1156 0039-2316/88/2009-1156512.50 9 1989 Plenum Publishing Corporation