TEMPERATURE DEPENDENCY OF BEREMIN'S PARAMETERS FOR 20M_nM_oN_i5-5 MATERIAL

摘要

The mechanism of cleavage fracture in reactor structural material is significantly different from ductile fracture. The cleavage fracture in a material usually originates from micro-cracks, which are formed by different mechanisms. The micro-cracks are formed due to non-homogeneous distribution of plastic deformation within the grains, called slip-initiated cleavage. The cracked grain boundary carbides also are the sources to originate the micro-cracks. This occurs when the stress normal to the planes of carbide particles is sufficiently high. The fracture takes place by the formation of micro-cracks and their extension with little global plastic deformation. The cleavage process is stress controlled and consumes little deformation energy and hence, the crack grows unstably fast. In this case the local fracture criterion is generally based on a critical cleavage stress, which is statistical in nature. Based on the weakest link assumption and Weibull statistics, Beremin developed a model for analyzing brittle fracture process by local approach. The two parameters of the model are material properties and can be determined from notched tensile tests at low temperatures and their finite element analysis. Once these parameters are determined, these can be used to predict probability of cleavage fracture initiation in a component. In the present work, temperature dependency of Beremin's parameters 'm' and 'σ_u' has been investigated for 20MnMoNi55 structural material. Tensile tests have been conducted on specimens having central groove at two different -50℃ and -100℃ temperatures. Thirty specimens have been tested in each set. Image processing technique has been used for measuring diametrical contraction from on-line images. The analytical study has been carried out to determine Beremin's parameters using the statistical variation in load v/s diametrical contraction. For this purpose, finite element analysis has been carried out to determine Weibull stress during fracture. The experimental probability of failure of each specimen has been calculated by ranking them based on the diametrical contraction at the time of fracture. Such a table along with the corresponding Weibull stresses have been used to find out Beremin's parameters using "maximum likelihood method". The temperature dependencyrnof these parameters is then investigated for the present material.