where ðdFUL=dhÞh¼ht
is the slope of the unloading curve
at h ¼ ht (maximum displacement).
It is also possible to calculate the relative contributions
of the elastic and plastic responses of the material
once a curve fitting procedure is completed. The total
work of compression (Wt) (the area under the loading
curve) and the elastic work of compression (We) (the area
under the unloading curve) are given by the following
equations (Briscoe & Sebastian, 1996):