Experimental Study of Strain-Softening Stage in Materials

Abstract

Physical theory of reliability is based on research into degradation processes of various origins which take place in a material of a stressed construction. Experimental evaluation of parameters carried out for such processes is a practically important problem by itself. One of the approaches to solving this problem is related to the studies into the stage of material softening due to deformation. This paper analyzes the issues of experimental validation of material softening properties in terms of a phenomenological approach to the problem of structural fracture. Results of deformation analysis for the “machine – model specimen” system, using catastrophe theory are used to form requirements for carrying out experiments which investigate the softening stage of materials. The success of such experiments – which should include recording a branch descending to zero on a computer diagram – is possible when small specimen, made from structurally heterogeneous materials, are strained in a sufficiently rigid testing machine. Thus, the conditions for manifestation of the softening stage connect properties of the material with properties of the load-applying system. Therefore, the material's limiting state – preceding the fractured – also depends on the conditions of loading, and the criteria of that fracture would be nonlocal. In consideration of the results of diagrams plotted from various bases for deformation measurement, a necessity of utilizing local material characteristics for calculation purposes is discussed. As an example of using the complete diagrams for determining kinetics of material degradation from external load, the results of specimen testing, which follows a cyclic training, are cited.

On Durability of Structurally Inhomogeneous Materials

“…It means that no equilibrium states are present after the loss of strain stability, and a specimen is fracturing dynamically. If the uniform law of strength distribution were substituted with a normal or Weibull distribution, a cusp catastrophe should follow [10]. In which case, an uncontrollable transition of the system (catastrophic jump) from one equilibrium state to another would be observed.…”

“…To this date, certain rigid testing machines have been designed and more than ten appliances proposed to increase the rigidity of loading devices for plotting CSSDs [6,10,14]. An efficient and convenient ring device [11] was used to plot CSSDs for materials of various classes on specimens of 2 mm in diameter and length of their working area (Fig.…”