黄铜薄板单向拉伸第Ⅱ类尺寸效应

Second class size effects on brass sheet in tensile test

以黄铜薄板为研究对象,通过对不同厚度、不同宽度试样的单向拉伸,研究厚度和宽度变化对薄板单向拉伸过程应力和塑性性能的影响。实验结果表明,厚度变化对应力的影响十分显著。当厚度小于10倍左右内禀尺寸,薄板变形不均匀性加剧,不断增大的几何必需位错加强了薄板的硬化,流动应力随厚度的减小而增大,表现出明显的第Ⅱ类尺寸效应。但是当应变增大到0.013以后,几何必需位错所占比例不断减小,其对材料的硬化作用明显减弱。此外,随着厚度的减小,薄板的断裂机制由韧窝微孔聚集断裂转变为沿晶断裂,板料的塑性性能持续下降。拉伸试样宽度减小使流动应力略微下降,但对塑性性能影响不大。

The size effect is the major subject in the research on micro forming of metal sheet.In this paper,tensile tests of brass sheet with various thicknesses and widths were performed to investigate the size effect on the flow stress and plastic properties.The results showed that the flow stress was sensitive to the brass sheet thickness.The flow stress was hardened by the geometrically necessary dislocation that continually increased with the inhomogeneous deformation when the brass sheet thickness decreased to the level of 10 times of the characteristic length.The yield strength and tensile strength increased with the decrement of thickness.But the hardening effect of the geometrically necessary dislocation density on the flow stress was weakened because of the decrement of the ratio of the geometrically necessary dislocation density to the statistically stored dislocation density.The fracture surface analysis of tensile specimen revealed the size effect mechanism of elongation.The elongation of the brass sheet with thickness less than 50μm was lower because its fracture mechanics was intergranular fracture.As the fracture mechanics was dimple fracture and the holes postponed the fracture,the elongation of the thicker brass sheet was good.