材料期刊网

用断裂力学方法研究了Ti-6Al-4V合金退火板材在蒸馏水中的疲劳裂纹扩展,测定了疲劳裂纹扩展速率dα/dN同应力强度因子幅度△K,温度和频率的关系。同时进行了空气中的对比疲劳试验。并用扫描电镜观察了疲劳断口形貌。实验表明,疲劳裂纹的扩展在水中比在空气中快。其扩展速率随温度升高而降低,随频率降低而增加。在10Hz频率下裂纹扩展速率同温度的关系可表示为(dα/dN)=Ae~(Q/RT)(△K)~(2.7),其中Q=6.2kJ/mol。疲劳断裂面上的延性条纹和裂隙条纹同载荷循环有近似一一对应关系;而脆性条纹的间距则比相应的宏观扩展速率大几倍。本文认为,水中疲劳裂纹扩展的加快,是水同钛合金裂纹表面的反应产生的氢所造成的;温度和频率对疲劳裂纹扩展速率的影响,可以用应变感生氢化物机制得到说明;进而提出,应变感生氢化物的形成是裂纹扩展的速率控制过程。

The environmentally assisted fatigue crack growth for annealed Ti-6Al-4V alloy plates in distilled water has been studied by using fracture mechanics method. Fatigue crack growth rates da/dN were determined as a function of stress intensity factor range AK from 20 to 45 MPam~(1/2), temperature from 273 to 353K and frequency from 0.1 to 75 Hz at a load ratio of 0.11. Reference data were obtained in laboratory air at room temperature and a frequency of 10 Hz. Futhermore, SEM fractographic studies have been carried out to observe the fatigue fracture morphologies. The experimental results showed that fatigue crack growth in water is enhanced as compared with that in air. The relationship between da/dN and △K follows the Paris equation. Fatigue crack growth rates were found to decrease with increasing temperature and to increase with decreasing frequency. The temperature dependence of crack growth rate at a frequency of 10 Hz can be described as:da/dN=Ae~(Q/RT)(△K)~(2.7), where Q=6.2 kJ/mol. Fatigue fracture surface is composed of relatively flat regions and irregular fracture regions. The flat regions are covered with three kinds of fatiguestriations: ductile striations, fissure striations and brittle striations. The irregular fracture regions are heavy plastic deformation areas, combined sometimes with small areas of striations. The ductile and fissure striations show an approximate one to one correspondence with the load cycles, whereas the spacing of the brittle striations is several times larger than the corresponding macroscopic growth rate. Enhancement of fatigue crack growth by water is believed to result from embrittlement by hydrogen that is produced by the reaction of water with the crack surface of Ti alloy. The observed temperature and frequency effects on fatigue crack growth rate can be explained in terms of a strain-induced hydride mechanism. Formation of strain-induced hydride is considered to be the rate-controlling process for crack growth.