However, the correlation effect over time in SUR model is not taken into account. In a standardlinearmodel, we often assume the error terms are identically independent distributed.
It is better to use the standardlinearmodel (SLM) simulating the earth medium crust. In a short period of time it shows elasticity, and it shows rheological properties in a long time.
Thus, a nonlinear viscoelastic constitutive relation for cement mortar based on standardlinear solid model considering the evolution of the damage internal variable is established accordingly.