Experimental study of plate bending: application to HLE steels

Bending, as a process, is the operation of cold forming a flat material, with or without lubrication, by exceeding its elastic limit. In this case, stress and strain are no longer proportional and are linked by a non-linear law. Part of the deformation is no longer reversible. Once the tools have been retracted and the stresses released, the elasticity returns and permanent deformation persists, resulting from the irreversible sliding of certain crystallographic planes relative to one another.

This operation differs from stamping in that lateral stresses are zero. The "expansion-reduction" duality, the two stress modes involved in deep drawing, is not generally encountered. The most influential parameters in a bending operation can be divided into three categories:

• machine parameters: speed, power, etc. ;

• tooling parameters: radius, stroke, angle, clearance, etc. ;

• material parameters: formability, elastic properties, anisotropy, etc.

The finished product is sensitive to these three types of parameters; nevertheless, depending on the final destination of the part and the quality criteria required, there may be a preponderance of one parameter over another, or of one family over the other two. In modern design, requirements other than mechanical and service ones come into play. The aesthetic appearance of a product obtained by shaping is increasingly sought-after. All these manufacturing constraints are reflected in the part itself. The geometrical and mechanical condition of the bent product will subsequently have a major influence on the parts' service life.