Effect of stress-strain-law transients on formability

The effect of work-hardening changes, of the type that follow a rapid change of strain state or strain rate, on formability has been investigated using finite element modeling (FEM). A standard sheet tensile test was chosen as an arbitrary forming operation and a rigid-viscoplastic FEM program was employed for analysis. In each case, a transitional region of true strains between 0.1 and 0.2 was modeled. Transients involving lowered work-hardening rates (negative value of An) promoted increased strain localization and premature failure while transients with increased work-hardening rates enhanced formability. Total engineering elongations (50 mm gage length) varied between 20 pct and 46 pct, illustrating the importance of the transient far beyond the transient region. The controlling factor is the effective work-hardening rate in the transient region rather than the change in strength coefficient or flow stressper se. A direct stress offset of the flow curve { } also affects strain localization through a change in effective work-hardening rate.     

Härteprofile als Hinweis auf verschiedene Festigkeitssysteme im Holz

1. Sandblasting of hardwood and softwood cross-sections reveals unexpected hardness profiles, which may provide an extension to the conventional division of wood into annual rings, earlywood and latewood or zones of growth.
2. Hard wooden parts which occur in cross-sections and which presumably contribute towards its strength, can also be traced in longitudinal cuts in the form of ridges.
3. It is assumed that tree species build up different mechanical systems which can be varied during wood formation as the need arises.