F F △l W≡FA=FAL AL F. -AT △l EA V==FR△L 2EA 应变能密度:单位体积内的应 F△L 变能 eVa A L 2l 1 l F l l F F O l W = FL = FN L 2 1 2 1 V FN 2 1  = EA FN L L L = EA FN L 2 2 = 应变能密度:单位体积内的应 变能 V V v   = AL FL = 2 1  2 1 =