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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 fatigue crack growth of Ti-6Al-4V alloy annealed sheet in distilled water was studied by means of fracture mechanics. The relationship between the fatigue crack growth rate dα / dN and the magnitude of stress intensity factor △ K, temperature and frequency was measured. At the same time carried out in the air contrast fatigue test. The morphology of fatigue fracture was observed by SEM. Experiments show that fatigue crack growth is faster in water than in air. Its rate of expansion decreases with increasing temperature and increases with decreasing frequency. The relationship between the crack growth rate and temperature at 10 Hz can be expressed as (dα / dN) = Ae ~ (Q / RT) (ΔK) ~ (2.7), where Q = 6.2kJ / mol. The ductile fringes and crack fringes on the fatigue fracture surface have approximately one-to-one correspondence with the loading cycles, while the distance between the brittle fringes is several times larger than the corresponding macroscopic propagation velocity. This paper argues that the accelerated development of fatigue cracks in water is caused by the reaction of water with the cracks on the titanium alloy surface. The effect of temperature and frequency on the fatigue crack growth rate can be explained by the strain-induced hydride mechanism. The formation of strain-induced hydride is the rate-controlling process of crack propagation.