Abstract: Determination and characterization of crack closure under low cycle fatigue loads were difficult in engineering practice. The characterization law and models of critical state of crack closure were studied based on magnetic property parameters. Graded loading and unloading experiments were carried out by using Q235B steel as the experimental materials. The TSC-5M-32 metal magnetic memory detector was used to collect magnetic characteristic parameters. The effects of different plastic zones at the crack tip and the crack tail on the magnetic field were investigated. The changing law of the synthetic magnetic characteristic parameter ΔHp at the critical state of crack closure was analyzed by comparing the critical load of crack closure determined by the flexibility differential method. The results show that the cracks close up when the load is less than Pop critical point， and at the same time ΔHp has a minimum inflection point； the crack is in the closure critical state when the load equals to the Pop， and at the same time ΔHp jumps sharply upward with turning points； the cracks open fully when the load is greater than Pop， and at the same time ΔHp fluctuates around at a higher level. Furthermore， the characterization model of crack closure state was established based on magnetic property parameters. The verification results show that， for the loading and unloading phases， the maximum relative errors between the model predicted values and measured values are 9.398% and 11.549% respectively， which provides a new method to judge and characterize the crack closure states for engineering practice. 

Key words: crack closure, magneto-mechanical effect, magnetic property parameter, characterization model, ferromagnetic material

摘要： 针对工程实际中低周疲劳裂纹闭合状态难以判定与表征的问题，研究了基于磁特性参数的裂纹闭合临界状态表征规律及模型。以Q235B钢为实验材料，进行分级加载、卸载实验，采用TSC-5M-32金属磁记忆检测仪采集磁特性参数，研究了裂尖裂尾不同塑性区对磁场的影响，通过对比柔度微分法，确定裂纹闭合临界载荷来分析裂纹闭合临界状态磁特性参数ΔHp变化规律。结果表明：载荷小于裂纹闭合临界点Pop时裂纹闭合，ΔHp出现极小值拐点；载荷等于Pop时处于裂纹闭合临界状态，ΔHp向上剧烈跳变出现转折点；载荷大于Pop时，裂纹完全张开，ΔHp维持在较高的水平上下波动。最后在实验的基础上，建立了基于合成磁特性参数ΔHp的裂纹闭合状态表征模型，验证结果表明，模型加载阶段和卸载阶段的模型预测值与实测值的最大相对误差分别为9.398%和11.549%，为工程实际中判断并表征裂纹闭合状态提供新的思路。

关键词: 裂纹闭合, 力磁效应, 磁特性参数, 表征模型, 铁磁性材料