513.2杆件变形能的计算 1轴向拉伸或压缩 U=W=-P1/ P2I 2EA 轴力N是x的函数时aU N(x)dx P 2EA N(x)dx 2EA 应变能密l=og 度 2E4 §13. 2 杆件变形能的计算 1 轴向拉伸或压缩 U =W= Pl 2 1 EA P l 2 2 = 轴力N是x的函数时 EA N x x U 2 ( )d d 2 =  = l EA N x x U 2 ( )d 2 应变能密 度  2 1 u = 2E 2  = P l  l