Evaluation of the influence of the level of stresses in the material on its in-service damageability

Abstract

620.178.15/179.119:539.4 and V. P. ShvetsWe study the degree of influence of the level of stresses in the material on its in-service damageability according to the parameters of spread of the characteristics of hardness. The full unloading effect on the process of accumulation of defects in the material is analyzed.Keywords: stresses, hardness, plastic strains, complete unloading, degree of damage to structures, and the coefficient of homogeneity. Introduction.To estimate the material stressed state of the structural component in different stages of its operation, it is customary to use different methods [1]. For the realization of some of these methods, one must perform the unloading of a structural component in the stressed state and cut out a test specimen for investigations [2]. According to the other methods, the analysis of the state of the material can be performed without damaging the product [3]. The major part of works aimed at the diagnostics of the operation state of the material are carried out for products in the stress-free (unloaded) state. In this case, the accumulated results inadequately reflect the actual state of the material and the response of its structure to the range of operating loads.Thus, it is of interest to study the degree of influence of the level of stresses in the material on its in-service damageability and analyze the possibility of application of the data on the degree of damage to the structure of the stress-free material to the evaluation of its operation (stressed) state. Moreover, it is necessary to clarify the degree of influence of the forced operations of complete unloading (periodically performed in the course of scheduled maintenance for the diagnostics of the product) on the accumulated estimates. This is explained by the fact that the operations of partial and complete unloading performed even for low levels of stresses equal to (0.6-0.7) σ 0 2 . , as well as the repeated operations of unloading lead to changes in the mechanical characteristics of structurally sensitive materials [4,5].Experimental Procedure. The specimens were subjected to repeated static deformation under uniaxial tension with simultaneous measuring of hardness in their working part (in the zone of uniform deformation) in loaded and unloaded states for the same levels of plastic strains. However, it is difficult to use the measured absolute values of hardness for the evaluation of the influence of the level of stresses in the material on its damageability in view of the weak correlation of hardness with the structure of the material [6]. Therefore, the state of the structure of the material is studied by the LM-hardness method [7] according to which the degree of damage to the material is estimated by using a parameter characterizing the spread of values of hardness in mass measurements. As this parameter, we take the Weibull coefficient of homogeneity m given by the formula

Determination of Stresses and Strains in Elastoplastic Deformed Body from Hardness Characteristics

“…The tests were repeated static ones. In each loading stage, stresses s, strains e, and residual strain e res were determined, and multiple HRB hardness measurements of a material, which was in the loaded and unloaded state, were made, from which hardness spreading parameters, the Weibull homogeneity coefficients, were calculated [28].…”