双轴载荷比对含裂纹加劲曲板剩余强度的影响

Influence of Biaxial Loading on the Residual Strength of Cracked Stiffened Curved Panel

用弹塑性有限元方法,以裂尖张开角参数为断裂准则分析了双轴载荷比对含裂纹加劲曲板中扩展裂纹尖端应力场、塑性区形状和尺寸,加劲桁条中应力分布及加劲曲板结构剩余强度的影响。发现双轴载荷比的增大使曲板的曲率增大,明显改变了扩展裂尖处应力分布、塑性区的形状和尺寸,虽可提高起裂名义应力,但却使整个结构的剩余强度明显降低。加劲曲板结构的最终破坏是因蒙皮中裂纹快速失稳扩展所致,而这种失稳扩展是由加劲曲板内侧裂尖局部处材料的塑性垮损开始的。

Lu [5] in 1987 reported the results of her analysical study and some experimental study of the influence of uniaxial loading on the residual strength of cracked stiffened curved panel. Actually Lu's experiments were conducted under biaxial loading conditions. Another problem was Lu employed LEFM (Liner Elastic Fractue Mechanics) method in her analyt- ical study. Naturally Lu's calculated values are not in good agrcement with her experimen- tal data. Before presenting the three new conclusions of the author's research, some concise des- cription of the research done will be given in the next paragraph. In this paper, the entire process from the initiation of crack growth up to the instability of crack extension is simulated for the case of biaxial loading. Different from the LEFM used by Lu, an elastic plastic finite element method based on incremental theory of plastici- ty is adopted to account for large strain. Again, different from the criterion employed by Lu, CTOA (Crack Tip Opening Angle) is used as the fracture criterion controlling the sta- ble crack extension. This paper's three new conclusions, believed to be helpful in engineering design, are: (1) The analytical method presented and CTOA fracture cricrion adopted are suitable for the residual strength prediction of cracked stiffened structure. (2) The increase of BLR (Biaxial Loading Ratio) can remarkably change the stress field, the shape and size of plastic zone around growing crack tip, can reduce the stress in longitudinal stringers, and can increase the curvature of stiffened curved panel. These changes lead to the reduction of the maximum load that can be carried. (3) The ultimate failure of cracked stiffened curved panel is mainly caused by the in- stability of crack extension in the skin of the stiffened curved panel. This instability of crack starts first from the plastic collapse of the material that yields around growing crack tip on the inside of cracked stiffened curved panel.