Підвищення опору руйнуванню зразка з тріщиною внаслідок обробки імпульсним магнітним полем

Abstract

Наведено методику та результати експериментальних досліджень впливу обробки імпульсним магнітним полем (ІМП) на опір руйнуванню компактних зразків з попередньо вирощеною тріщиною втоми, виготовлених зі сталі 45. Перед обробкою ІМП зразки механічно навантажували розтягом до рівня напружень у вершині тріщини, при яких значення коефіцієнта інтенсивності напружень нижче за його критичне значення (Кс). У результаті обробки ІМП у зразку відбувається релаксація напружень за рахунок пластичної течії у вершині тріщини, що призводить до її притуплення. Результати випробувань оброблених зразків показали істотне підвищення опору руйнуванню в порівнянні зі зразками в початковому стані.

Experimental technique and results of investigations of the effect of pulsed magnetic field (PMF) treatment on fracture resistance of the fatigue pre-cracked compact tension (CT) specimens are presented. Each specimen, made out of a hot-rolled sheet of medium carbon steel 45, was cut by the electric discharge machine. The initial starter notch in the specimens, perpendicular to the direction of rolling, was performed. To avoid possible residual stresses in the specimens after their manufacturing operations annealing was used. A natural sharp fatigue crack in CT specimen was created by cyclic loading. Before PMF treatment, in order to form the initial elasto-plastic stress in the crack tip and to reduce the electrical contact between its edges, treated specimen was mechanically preloaded in tension using special pool-rods. The load was carried to the stress level in the crack tip at which the value of stress intensity factor does not exceed its value on the last step of loading at growing of a natural fatigue crack in the specimen. The pulse magnetic field was provided by discharging a bank of high voltage capacitors through connected in a transmission line circuit with inductors, installed on both sides of the specimen. The parameters of current discharge were monitored using a calibrated non-contact inductive probe, called the Rogowski coil, placed around one of the current feeding wires and recorded on a personal computer (PC) with pre-installed high-frequency analog-digital card. For all specimens the same parameters of current discharge were used. According to measurements the discharge current was about 11 kA for the total duration of 10 ms. Direct measurement of the load, applied to the specimen and temperature rise in the vicinity of fatigue crack during PMF treatment were also recorded. After treatment irreversible relaxation (significant force decreasing) and negligible increase in temperature in the vicinity of the crack were observed. After testing at room temperature CT specimens in the initial state and specimens after PMF treatment, the values of maximum stress intensity factor Kmax and strength ratio of specimen Rsc were obtained. Test results showed a substantial increase of the fracture resistance of the treated specimens in a comparison to the specimens in an initial state. Stress relaxation, caused by plastic flow in a vicinity of the crack, takes place due to PMF treatment and provokes a crack blunting. This fact was confirmed by increasing the value of a crack opening displacement of the treated specimen in comparison with its initial state. Increasing of fracture resistance should be mainly associated with blunting of the crack due to plastic flow in its tip, caused by the relaxation of initial elastic-plastic stress under electroplastic and magnetoplastic effects