Relations entre courbes limites de formage et variations de densité

Relationship between forming limit curves and density changes

IRSID, Saint-Germain-en-Laye

Abstract

One of the possible methods to characterize the drawability of sheets is the use of forming limit curves at fracture FLCF. Given the difficulty of their experimental determination, numerous attempts have been made to forecast them by computation. At the same time, ductile fracture criteria have been established. They are written based on macroscopic mechanical parameters but the critical values of these factors are deduced from a physical condition: critical porosity or main contribution of positive stresses. The link between damage and FLCF becomes also natural. Some fracture criteria (Hancock and Mackenzie, Oyane, Latham and Cockcroft) have been used to calculate these FLCF; the curve iso-mean stress cm has been also calculated because am is the essentiel factor governing the growth of damage. These curves have been compared with those determined by Grumbach and Sanz (intrinsic curves without minimum in plane strain). Calculus forecasts decreasing FLCF e, = F (E2) similar to straight lines with a slope between -0,5 and -0,9 (ductile fracture criteria) and -2,3 (iso-aj. Experimental slopes are between — 1 and —1,5. Iso density curves have been determined for two steels with a high precision device of density change measurements. IRSID and littérature results show that these curves have an extremum for the plane strain z2 = 0, either a maximum or a minimum. These curves indicate that the damage growth is the result of not only mechanical, but also metallurgical factors. In particular, the depen- dancy of damage growth with the strain path is influenced by the microstructure and the degre of severity of different strain paths depends on the steel.