材料期刊网

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。

Effects of a single overload on fatigue crack growth rate have been investigated at elevated temperatures in an Fe-Ni-base superalloy which has different levels of strength and ductility through different heat treatments. All the overload tests were conducted under the condition of constant load with CCT specimen. Crack length was monitored by using d. c. potential-drop technique. It is clear that the effect of overload represented by retardation cycles, N_D, is decreased at elevated temperatures but still of significance up to 700℃. The peak level of overload stress intensity factor, K_(o.p)., the overload ratio, r, the material yield stress, σ_y, and the crack length, a, at which an overload is applied have great influence on the post overload retardation, behaviour of fatigue crack growth. The retardation cycles, measured at 650℃, can be expressed by the following equation:N_D=N_0exp[m(r/aσ_y)~K_(o.p.)]Conditions of heat treatments have great influence on the effects of overloads. Alloy A with higher ductility and lower yield strength shows greater retardation effect than alloy W under the same overload conditions. The fracture surfaces of specimens subjected to overloading at elevated temperatures have been observed by using optical steromicroscope and SEM along with the measurements of the change of potential drops during the overload tests. Some explanations have been given.