Investigation into the influence of processing and manufacturing method on the elastic properties of structural stainless steel
One of the most common production routes for stainless steel structural members is cold working and cold-formed sections are the most common type of stainless steel section currently available. These materials are particularly sensitive to the effects of cold working and can experience significant changes in their mechanical properties compared with the annealed material. Considerable attention has been given to strength enhancements which are induced during the forming process of cold-worked sections, especially in the corner regions, and also the residual stresses induced during forming. However, the current project is focussing on the less-studied effect of cold working on the elastic properties such as Young’s Modulus (E) and poissons ratio (υ).
So far, it has been shown that the Young’s modulus of austenitic and ferritic stainless steels decreases with cold-work as a function of equivalent plastic strain, gradually approaching an asymptotic value. The saturation level is similar to that of mild steel, but the rate of decrease is up to five times slower for stainless steel.
Sheet metal forming
In sheet metal forming simulations it is possible to predict with high accuracy the forces involved in the forming process, how the material deforms and behaves, including its yielding, hardening and formability, predict if the forming process will lead to material instabilities such as wrinkles, non-uniform localised thinning, on how friction and lubricant affect the forming, etc.
There are many different forming operations in sheet metal forming processes making the process very challenging from a simulation point of view. These operations include for example drawing, re-drawing and reverse drawing, trimming, ironing, hydroforming and spring back just to name a few. The purpose of these forming operations is to avoid necking, wrinkling or fracture in the formed part by selecting an optimized strain path. This optimized strain path is commonly assessed through the use of strain-based or stress-based Forming Limit Diagrams (FLD) and fracture criteria, where the principal strains or stresses need to lie below the forming limit curve.
Necking followed by fracture in a cup drawing and reverse drawing sheet metal forming process
In metal cutting processes one major mechanism that is taken into account in the modelling is the formation and development of strain localisations such as shear bands. Material experiences localised necking followed by damage and fracture. The study of the development and evolution of these material imperfections falls very well within the scope of the Structural Integrity theme.