45号钢氢致脆断试验与GTN模型数值模拟研究

Experimental and GTN Model-based Numerical Study on Hydrogen-induced Brittle Fracture of 45 Steel

采用电化学方法对试样充氢后再进行单轴拉伸,在此基础上研究45号钢的氢致脆断问题.试验观察到,随充氢时间增加,试样的延伸率和平均断裂应变减小,断口韧窝数目减少,准解理区域增加.但该改变逐渐减缓,充氢大于72 h若继续充氢试样延性和断裂应变几乎不再变化.基于试验观测,将HEDE机制和受氢影响的材料微孔洞演化机制引入GTN本构模型.模型考虑了随氢浓度增加微孔洞形核体积分数增加、形核和聚合提前等影响因素.利用该模型对不同充氢试样的拉伸断裂进行有限元数值模拟,结果表明,随氢浓度增加断裂时试样延伸率和断裂收缩率下降,而且试样启裂点的位置受氢浓度分布变化而变化,它们可合理解释和再现试验观测到的现象.

The 45 steel was investigated for transition of fracture mode due to hydrogen concentration change under uniaxial tension by experimental tests and numerical modeling. The tests were conducted with a series of solid round bar specimens of 45 steel after different durations of hydrogen charging by the electrochemical method. It was observed from the tests that with the increase of hydrogen charging time, the elongation and average fracture strain measured by the tensile test decreased, the number of dimples in fracture decreased, and the area of quasi-cleavage increased. However, the change rate was lowered with hydrogen charging time. When the charging time was more than 72 hours, the ductility and fracture strain of the specimens did not decrease significantly any more. The hydrogen diffusion process within the specimens was analyzed by numerical modeling, and the results showed that the hydrogen concentration in specimens increased with the hydrogen charging time and reached a stable saturated state gradually. Based on the phenomena observed in the tests and the numerical modeling results, the hydrogen-enhanced decohesion(HEDE) mechanism and the evolutionary mechanism of microvoid affected by hydrogen, were introduced into the GTN constitutive model. Taking into account the increase of microvoid nucleation volume fraction with hydrogen concentration, as well as the earlier occurrences of microvoid nucleation and coalescence, the relationship between the damage parameters and hydrogen concentration was established for the GTN model. The damage parameters of GTN model were set according to the hydrogen concentrations calculated for the specimens, and the finite element modeling for tensile fracture of different hydrogen charging specimens was carried out. The tensile fracture response curve, fracture elongation and contraction of the specimens obtained by modeling, as well as the neck contour deformation at fracture for the specimens, were very similar to or consistent with the measured results. The modeling results also showed that the position of the crack initiation point changed with hydrogen concentration distribution, which was also consistent with the observations.