One experimental method of determining the maximum stresses in splitting

Abstract

y in an impact wave), also requiring, however, conversion from the value of the mass velocity to the maximum stresses o, in the plane of splitting, since measurement of the impact wave parameters with the use of a pressure sensor is done at the sample--"soft" material interface. In view of the proportionality of the measured (pressure) and determined (the stress ~,) parameters the question arises of the possibility of directly finding the maximum tangential stresses o, in the plane of splitting, based on recording of the profile of the load wave by a pressure sensor. In connection with this let us consider in the acoustic approximation the interaction of the waves and the formation of splitting with the use of t vs x and p vs u curves (Fig. I). The impact of two plates (for simplicity we will assume similar materials) leads to propagation in both directions from the surface of contact of load waves with the condition I behind the front. In reflection from the free surfaces of the striker and the sample two unloading waves I-0~ and I-2 occur. They propagate toward each other and, in their interaction in the material of the sample, an area of tensile stresses ABDC is formed. Under their action with a stress corresponding to condition 3 splitting occurs, the right boundaryofwhichlies on the last characteristic DC. The appearance of additional free surface in the development of failure changes the character of the drop in pressure in the unloading wave moving to the right. Existing means of measuring the pressure profile of impact waves, such as dielectric, manganin, and quartz sensors, are capable of recording only a compressing load and, therefore, the plan of the test must provide for the action of only compressing forces in the planes of their location. If at some distance s from the free surface of the samples there is located a base of the same material with a pressure sensor mounted within it or on its surface, then in impact of the free surface with the base two compression waves with the condition ~ behind the front occur. With arrival at this boundary of that portion of the unloading wave 1-01, which has passed through the area of splitting, the pressure starts to drop to a value corresponding to condition 4 and then, as the result of the appearance of splitting and the formation of a new free surface, it theoretically must again increase to condition I, but for various reasons it is sometimes somewhat less. The pressure profile of the impact wave in the material of the base has the form shown in Fig. 2. It is not difficult to see that the difference in pressures in conditions I and 4 will accurately correspond to the value of the maximum stresses at splitting @,, i.e., ~, = Pl -- P4. The pressure profile measured in this manner is qualitatively similar to the change in the velocity of the free surface [6] and the pressure profile at the boundary with the soft material [4]. The wave analysis made is true until the moment of arrival of lateral unloading waves at the sensor and assumes impact of the sample free surface with the base at a moment of time