At the

proportional limit state, SDR of the two-layer reinforced cushion was similar to that of the one-layer reinforced cushion, while at the stable state, there was a big difference between them.

This is due to the fact that the

proportional limit and the Yield point are not coincident and the Yield point is taken at 0.2% offset.

The difference lies in the attempt to use equations developed for materials that have a

proportional limit on materials that do not have a true

proportional limit and not considering the properties of copper strain hardening.

Therefore, there is a concern that bending properties, including Young's modulus,

proportional limit stress, and bending strength, cannot be evaluated appropriately by the conventional bending tests.

To simplify the calculations, it was taken, that cyclic

proportional limit [[bar.s]sub.T] is independent on [[bar.e].sub.con] [1], the number of semicycles k and on stress ratio [r.sub.[sigma]].

For high loads, stress and strain exceed

proportional limit and elastic plastic strain occurs.

*

Proportional limit. The point where the curve starts to deviate from a straight line.

Figure 1 shows the initial portion of a tensile stress-strain curve that identifies the

proportional limit. This is the last point on the stress-strain curve where the stress and strain are in proportion.

No difference in modulus of elasticity and modulus of rupture, work to

proportional limit, work to maximum load, or fiber stress at

proportional limit between samples steamed following treatment and those only treated was shown.

For many materials this

proportional limit takes place even below 0.5% strain.

The research conducted herein focused on nondestructive identification of damage in wood that has occurred via bending load in excess of the

proportional limit, yet below breaking strength.