应力比对含缺陷选区激光熔化TC4合金稳态疲劳裂纹扩展速率的影响

Effects of Stress Ratio on the Fatigue Crack Growth Rate Under Steady State of Selective Laser Melted TC4 Alloy with Defects

选择含有2种不同微观缺陷的选区激光熔化TC4合金,定性研究了缺陷尺寸对稳态阶段疲劳裂纹扩展速率的影响规律,并对缺陷尺寸较小的合金,在不同应力比(R=0.1、0.3和0.5)下进行稳态阶段疲劳裂纹扩展速率对比研究。在疲劳裂纹扩展速率(da/dN,其中,a为裂纹长度,N为应力循环周次)和应力强度因子范围(ΔK)关系的基础上,利用Paris公式拟合分析,结果表明,缺陷尺寸增大导致da/dN增大,即Paris公式中的系数m不变,C增大;而随着R增大,ΔK减小,da/dN增大,同时da/dN曲线在低ΔK时汇集,即Paris公式中的系数m增大,C减小,且m和lgC之间存在线性关系,该关系不受R的影响。最终结合疲劳损伤机制,对微观缺陷和R引起的不同变化规律进行了分析。

TC4 alloy components with complicated geometries can be directly fabricated using selective laser melting(SLM)at a low cost.These components are often used under complex service conditions.Thus,it is important to investigate the effects of the stress ratio(R)on the fatigue crack growth(FCG)rate(da/dN)in SLM TC4 alloys with defects at the steady state to develop guidelines for damage-tolerance design and fatigue life assessment.In this work,SLM TC4 alloys containing two different microdefects were used to qualitatively examine the effect of the defect size on the da/dN at the steady state.In addition,comparative studies using an alloy with smaller defect sizes were performed at the steady state and at R=0.1,0.3,and 0.5.The relationship between the da/dN and stress intensity factor range(ΔK)was plotted and analyzed by fitting the Paris formula.The results show that the Paris formula parameter,m,is constant and the parameter,C,increases,which means that the increase in the defect size increases the da/dN.The da/dN increases with an increase in R,and the da/dN curves converge at lowΔK,which are reflected in the increase in the parameter,m,and the decrease in the parameter C.Additionally,there is a linear relationship between m and lgC(the common logarithm of C),which is not affected by R.Finally,the change patterns in the da/dN caused by the microdefects and R were analyzed along with the fatigue damage mechanisms.