The influence of size effect on the ductile fracture in micro-scaled plastic deformation

In macro-scaled plastic deformation processes, viz., macroforming, the so-called ductile fracture has been extensively studied in terms of the physics, mechanisms, affecting factors and the prediction criteria of ductile fracture. In micro-scaled plastic deformation processes, or microforming, all of these are relatively new and have not yet been extensively investigated. In tandem with this, the applicability of the traditional fracture criteria in micro-scaled plastic deformation and how the size effect affects the deformation and fracture in the microforming processes are critical. Using micro scale flanged upsetting as a case study process, the fracture in the process is studied via experiment and finite element (FE) simulation. The FE simulation is conducted using the established model based on the widely accepted surface layer model in microforming. Both the physical experiment and simulation show that the size factor has a significant effect on fracture formation in micro-scaled plastic deformation. Based on the proposed surface layer model, the fracture in micro scaled plastic deformation is predicted by considering the size factor in Cockcroft fracture criterion and the results are corroborated and verified by experiments. It is found that the ductile fracture affected by size effect is difficult to occur in microforming in the same deformation conditions under which the fracture happens in macroforming scenario. The research thus provides an in-depth understanding of the fracture in micro-scaled plastic deformation.