Using equation 5.19, a possible expression for damage is:

where W^ is recoverable elastic energy of the damaged material. When the deformations are less than the initial threshold (£0) the material is elastic and all the energy is recoverable, which implies that Wf = Wf, and therefore d = 0. In the limit of damage, Wf ^ 0, which implies d = 1. A simple way to evaluate d is to adopt a postpeak stress function. According to Rotls (1991), the strain-softening curve of concrete must be concave. By considering an exponential function as proposed by Lubliner et al. (1989):

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