多应变速率下尺寸效应对黄铜微尺度塑性变形本构及损伤演化的影响

Influence of Size Effect to Constitutive Model and Damage Evolution in Micro Scaled Plastic Deformation under Different Strain Rate

准静态微尺度塑性变形中,特征尺寸效应与晶粒尺寸效应会直接影响韧性断裂的应变能及断裂强度,其直接原因是由于表面层自由晶粒所占比例随着样品尺寸的缩小或晶粒尺寸的增大而逐渐增加从而影响流动应力。在高应变速率下,特征尺寸效应和晶粒尺寸效应对金属流动应力和损伤的影响则较少被提及。通过分离式霍普金森压杆试验得出高应变速率下黄铜微尺度等温流动应力,以Johnson-Cook本构模型为基础构建了包含尺寸系数的微尺度高应变速率本构模型,并将其与Freudenthal非耦合断裂准则相结合,提出了多应变速率微尺度混合损伤模型,量化了不同应变速率下特征尺寸效应及晶粒尺寸效应对金属损伤的影响,为飞刀切削样品表面改善提供了理论依据。

In micro scaled plastic deformation,feature size effect and grain size effect will affect strain energy and fracture strain of ductile fracture.The main reason of this effect is the portion of the free grain of the surface layer increases when the specimen size increases or grain size decrease,and thus affects the flow stress.However,the influence of these two size effects to the flow stress of micro scaled plastic deformation under high strain rate is not revealed.In tandem of this,the isothermal stress-strain relationship of high strain rate is obtained via the split Hopkinson pressure bar(SHPB),and a new micro scaled constitutive model of high strain rate based on Johnson-Cook constitutive model with size factor is established.A hybrid fracture model,which combines this micro scaled constitutive model of high strain rate and Freudenthal fracture criterion,is thus proposed.In-depth understanding of the influence of these two size effects to fracture exist in plastic deformation under different strain rates is revealed,and methodology for surface improvement of ultra-precision raster fly cutting is provided.