3. Assume the behavior of a creeping material to be represented by the Burger's body given below. The material parameters are: E = 20 GPa, n = 10,000 GPa days, E = 50 GPa, m = 5,000 GPa-days. (a) Find a mathematical expression for the Creep Compliance J in terms of time in days (Note: the unit should be /MPa.) (b) The material is placed under the following stress history (stress unit is MPa): Stress 20 10 E 0 50 Derive expressions for the strain for: (i) 0 < time < 50 days (ii) 50 < time < 100 days (iii) time > 100 days Specifically, what are the strain values? (i) at time = 0 100 Time (day) (ii) at 50 days, before and after loading increase (iii) at 100 days, before and after unloading Draw, schematically, the variation of strain with time up to 150 days.