用有限元法计算了Ⅱ型缺口前端的应力场和应力场强度因子KⅡ。结果表明,最大正应力和最大三向应力所在位置(θ=-110°)和最大剪应力位置(θ=80°)并不重合;最大正应力的指向和缺口延长线成α=10°。 实验表明超高强度钢Ⅱ型缺口试样能产生氢致滞后开裂,在水溶液中也能发生应力腐蚀。氢致裂纹和应力腐蚀裂纹都在最大三向应力所在位置形核,但裂纹的取向并不和该处的最大正应力垂直,而是指向该处的剪应力方向。如果没有氢,则裂纹在最大剪应力所在位置处形核,并沿最大剪应力方向扩展,即开裂角α=5°。 无论是氢致滞后开裂,还是应力腐蚀,Ⅱ型试样的规一化门槛应力强度因子均比Ⅰ型试样的相应值要高,断口形貌则和Ⅰ型试样的基本相同。
The stress field around the notch tip of a mode Ⅱ notched specimen was analyzed by means of finite element method and the stress intensity factor K_(?)was calculated by J-integral. The results indicate that the maximum shear stress is located at 0=80°and its direction is α=5°, the maximum principal stress σ_1 and the maximum hydrostatic stress are both atθ=-110° and the direction of σ_1 is α=10°. Hydrogen induced cracking (HIC)and stress corrosion cracking (SCC) of a high-strength steel 34CrNi3MoA(T. S=1500MPa)under Mode Ⅱ loading were investigated using notched specimens. The results show HIC and SCC under Mode Ⅱ loading initiated at the maximum hydrostatic stress site. However, cracking is oriented along the shear stress direction at the site, not normal to the direction of maximum principal stress component. On the contrary if the specimens are loaded to fracture in air under Mode Ⅱ loading, cracking oringinates at the maximum principal stress site around the notch tip and the cracking direction coincides with the direction of the maximum shear stress. The above mentioned facts indicate that hydrogen induced delayed plastic deformation is a necessary condition for HIC; and the nature of SCC for high-strength steel in 3.5% NaCl solution is HIC. The results show that HIC and SCC under Mode Ⅱ loading can occur during dynamic charging with hydrogen and in 3.5%NaCl solution respectively. The normalized threshold stress intensity factors under Mode Ⅱ loading during dynamic charging in 1N H_2SO:+0.25g As_2O_3/l solution and in 3.5% NaCl solution are K F/K_(?) X=0.1 and K_Ⅱ SCC/K_(?) X=0.45, respectively. The corresponding values under Mode Ⅰ loading are K_Ⅰ H/K_Ⅰ X=0.02 and K_(Ⅰ)SCC/K_(Ⅸ)=0.37, where K_(ⅡⅩ)and K_(Ⅸ)are critical values loaded to failure in air under Mode Ⅱ and Mode Ⅰ loading respectively. Thus, (K_Ⅱ H/K_(ⅡⅩ))/(K_(Ⅰ) B-/K_(Ⅸ))=5 and (K_Ⅱ SCC/K_(Ⅸ))/(K_Ⅰ SCC/K_(;Ⅸ))=1.2 A typical intergranular fracture was observed during HIC and SCC under Mode Ⅱ and Ⅰ loading. But the fracture surfaces of specimens failed in air are composed of dimples for both kinds of loading.
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