In this paper, we have proposed an evaluation method for the initiation conditions of heat crack under thermal shock using the tensile test of a notched specimen. In some materials, such as hot rolling rolls and dies for aluminum die-casting, there is a problem of heat crack initiation, and the evaluation of heat crack resistance using test pieces was difficult until now. The thermal stress and stress gradient that occurred in such material were analyzed by the finite element method (FEM). The maximum stress when tensile stress was applied to the material with a notch was compared to the stress gradient of the thermal stress. As a result, the quantitative evaluation during the initiation conditions of a heat crack for materials became possible.
The fatigue ratio of lamellar pearlite steel used for railroad rails is lower than the ratio of general steels. The reason for this has not yet been clarified. It is reported that the direction of the lamellar microstructure in the lamellar pearlite steel affects the fatigue characteristics. In order to clarify the microscopic deformation behavior of the lamellar microstructure considered to affect the fatigue characteristics, static tensile tests of the lamellar pearlite steel were conducted. Using the plastic replica technique, the microscopic deformation behavior i.e. the initiation and propagation of microcracks was observed. Based on the experimental results, transgranular cracks and intergranular cracks were observed. The obtained main results are as follows. (1) The transgranular crack originated in the discontinuous area in the lamellar pearlite microstructure. The transgranular crack initiates in a 45 degree direction to the axis of tension. The transgranular crack does not break through the grain boundary. (2) The intergranular crack does not propagate, but opens. (3) The difference in the material characteristics affected by heat treatment is the total strain of the crack initiation.
In order to establish a strength evaluation method of a high strength material with a notch which has an inclusion, tensile tests of materials, which have various notch radii, and fracture toughness tests for casting high speed steel were performed. The validity of considering that an inclusion is equivalent to a crack and considering the influence of the interference between an inclusion and a notch on the tensile strength using the fracture mechanics was examined. The tensile fracture conditions were evaluated by defining one formula which calculates the stress intensity factor that used the area parameter model as the tensile fracture conditions K IB (ρ). As a result of the testing, as for the high strength notch material containing many inclusions, the notch root maximum elastic stress at the time of the fracture varied, and it was shown clearly that the cause was the size scatter of an inclusion. Moreover, a strength evaluation with little scatter was possible by factoring K IB (ρ) into the fracture conditions. That is, it was clearly shown by considering that an inclusion is equivalent to a crack which is effective when considering the influence that the interference between an inclusion and a notch has on the tensile strength using a stress intensity factor.
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