粗糙度引入的Ⅱ型裂纹扩展的剪切阻力

ROUGHNESS-INDUCED SHEAR RESISTANCE OF MODE Ⅱ CRACK GROWTH

通过混合(Ⅰ/Ⅱ)型疲劳裂纹扩展门槛值和裂纹闭合效应的测试.获得双相钢纯Ⅱ型疲劳门槛值是其纯Ⅰ型的1.9倍:其纯Ⅱ型分枝裂尖正应力强度因子范围为纯Ⅰ型的2.2倍说明由同一粗糙度引入裂纹面的剪切阻力,对Ⅱ型裂纹扩展的阻碍程度将大于此粗糙度诱导裂纹闭合效应,降低裂尖Ⅰ型有效驱动力的贡献,即粗糙度对Ⅱ型裂纹扩展的阻碍比对Ⅰ型的大Ⅱ型分量在近门槛值的疲劳裂纹生长过程中能够起两个重要的作用:(1)增大裂尖塑性以至加速裂纹生长;(2)引入裂纹表面接触以至减小裂纹扩展速率本文通过裂纹闭合效应的测试,定量估算了剪切阻力,Ⅱ型摩擦剪切应力强度因子范围(△K_(Ⅱ.f))要远远大于纯Ⅰ型的闭合应力强度因子范围(△K_(ICl)),说明粗糙度的引入加大了第二种作用,从Ⅱ型裂纹角度出发起到了屏蔽裂尖的功能.

The mixed-mode (Ⅰ/Ⅱ) fatigue crack growth threshold was studied. The fa-tigue crack growth threshold value of pure mode Ⅱ in dual-phase steel(DPS) was found to be1.9 times of that of pure mode Ⅰ; the tensile stess strength factor range of pure mode Ⅱbranch crack tip is 2.2 times of that of pure mode Ⅰ. The roughness-induced shear resistanceimpedes the mode Ⅱ crack growth; while roughness-induced crack closure reduces the drivingforce of mode Ⅰ crack tip; the resistance of the former is larger than that of the latter. Themode Ⅱ component in near-threshold fatigue crack growth may increase the plasticity ofcrack tip which accelerates the crack growth and introduce crack surface contact and rub toreduce the crack propagation rate. The difficulty for quantifying the shear resistance is readilysolved through measurement of crack closure in this paper. The friction shear stress strengthfactor range of pure mode Ⅱ still is far larger than the closure stress strength factor range ofpure mode Ⅰ. It is illustrated that the roughness enlarged the second role and played a role ofshielding crack tip from mode Ⅱ crack.