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研究了拉伸过载对一种铁镍基合金高温下裂纹扩展的影响。过载试验是用4mm厚、中心切口的板材样品在恒定载荷下进行的。裂纹长度用电位法进行监测。用延滞循环数N_D所表示的过载效应随温度升高而降低。即使到700℃仍然比较明显。延滞循环数N_D受过载应力强度因子K_(o.p.)、过载时的裂纹长度α、过载比r以及材料屈服强度σ_y等因素的影响,其关系可用下式来表达N_D=N_oexp[m·(r/α·σ_y)~2·K_(o.p.)] 合金采用了两种热处理制度,高塑性低强度的合金A比低塑性高强度的合金W表现出较大的延滞效应。用体视显微镜和扫描电镜对过载试验后的样品断口进行了观察,给出了断口形貌示意图,提出过载损伤区的概念,并用断裂力学的观点进行了解释。
The effect of tensile overload on the crack propagation at a high temperature of an iron-nickel-base alloy was investigated. Overload tests were performed on 4 mm thick, center-cut sheet samples under constant load. Crack length is monitored by potentiometry. The overloading effect, expressed as the number of delay cycles N_D, decreases with increasing temperature. Even to 700 ℃ is still more obvious. The number of delay cycles, N_D, is affected by such factors as the overload stress intensity factor K op, the crack length α at overload, the overload ratio r and the yield strength σ y of the material. The relationship can be expressed as N_D = N_oexp [m · (r / α · σ_y) ~ 2 · K_ (op)] alloys, two kinds of heat treatment systems were used, and alloy A with high plasticity and low strength exhibited larger hysteresis effect than alloy W with low plasticity and high strength. The fracture of the sample after overload test was observed with stereomicroscope and scanning electron microscope. The sketch of the fracture surface was given and the concept of overload damage zone was put forward. The fracture damage was explained from the viewpoint of fracture mechanics.