On the Formability/Instability of Stretch-Forming Sheet Metals

Abstract

Manufacturing processes of aircraft, automobile and electronic components involve a lot of stretch-forming of sheet metals. The processes contain a large amount of straining which, in turn, may lead to plastic instabilities such as necking, slip formation or shear banding. Understanding the instability phenomenon requires the development of constitutive description of material characterizing the plastic instability and post uniform deformation behavior. The previous work on plastic instability usually lacks the experimental verification of the theoretical models. This may be owing to the complexity involved in designing a suitable experimental methodology for accurate measurements of strain in the locally deforming zone as the deformation progresses rather rapidly once the local neck sets in. A review of the important methodologies and analyses that were and are used in investigating instability of sheet metal in uniaxial tension is presented in this paper. Some supporting results are also presented.

A novel spatio-temporal method for determining necking and fracture strains of sheet metals

“…In sheet metal forming, localised necking usually appears after onset of diffuse necking and terminates at fracture [43]. According to the Hill's criterion [44], a necking band forms along the zero extension direction.…”

“…4(a)) after the onset of diffuse necking, at F ⁄ = 86%, 90%, 96.5%, 98% and 100%. It can be seen that necking proceeds from being diffuse and gradually concentrates to a single band before fracture, and strain distribution at before onset of diffuse necking, so the major strain at the onset of diffuse necking ( 1−DN ) is independent of gauge length [43]. However, the major strain at fracture ( 1−F ) is highly sensitive to gauge length due to heterogeneous strain distribution around the necking band at the fracture time F .…”

Error Estimation of the 3D Reconstruction of a Displacement Field from DIC Measurements

“…From a macroscopic point of view, it is essential to characterise and model the mechanical behaviour of metallic parts up to large deformations, enabling an accurate simulation of manufacturing processes. However, the occurrence of plastic instabilities at large deformations, such as diffuse and localised necking [1,2], introduces additional challenges to the constitutive modelling. Necking makes the deformation non-uniform and the stress state inside the specimen triaxial, eventually leading to fracture.…”

Adaptive Stretch-Forming Process: A Computer Vision and Statistical Analysis Approach

“…This process gradually produces severe plastic deformation (SPD) due to the increased stress; therefore, strain distribution must be considered [5]. As a result, the strain increases by a specific amount, depending on the mechanical properties of each material [6][7][8][9][10][11][12]. Another aspect is the deformation, since materials behave differently when elastic or plastic deformation occurs [2,13].…”