钉孔挤压强化三维弹塑性有限元分析

On Three-Dimensional Finite Element Analysis of Coldworked Fastener Hole Using Flow Theory

采用三维增量弹塑性有限元方法对钉孔挤压强化过程进行了模拟计算,给出了残余应力、应变及位移的分布.计算结果表明,应力、应变沿厚度的分布具有不均匀性,并可用来说明挤压强化钉孔的疲劳源的可能位置等情况.

Shao et al[1] analyzed the conditions under which fatigue crack source of coldworked fastener hole did not occur on the edge of the hole but occurred in the sheet material close to the edge. But the 2-dimensional deformation theory used by Shao to obtain residual stresses neglected the variation of residual stresses across the thickness of the sheet. Only 3-D theory can reflect such variation of residual stresses. Besides, the theoretical analysis of Zhang et al[7] showed that flow theory was better than deformation theory in the computation of residual stresses of coldworked fastener hole. Therefore the authors in this paper use 3-D flow theory to calculate residual stresses of coldworked fastener hole. The authors propose the 'Coldworking Displacement Compatible Method' for iterative calculations. From the calculations, the following conclusions can be drawn: 1. When D_m is the diameter of mandrel. D_k is the diameter of hole, and coldworking amount δ=(D_m-D_h)/ D_k is larger than 0.02 (δ for coldworked fastener hole is usually Jarger than 0.02) then flow theory needs to be used. 2. When sheet thickness h is equal or greater than 6 mm, then both circumferential residual stress σ_(θr) and radial residual stress σ_ vary conspicuously across sheet thickness. When δ=0.0342 and h=6mm,σ_(θrmax)/σ_(θrmin)≈1.5. Evidently, 3-D model instead of 2-D one should be used when h>6 mm. 3. The non-uniformity of residual stresses σ_(θr) and σ_(rr) can explain quantitively why the fatigue crack source occurs in the sheet material close to the edge.