The viscoelastic response of a common orthopedic polymer is represented using a Maxwell model, so that the spring constant represents the material elastic modulus E (MPa) and the dashpot constant represents the material coefficient of viscosity, ? (MPa-s). A uniaxial tensile test is performed to examine the creep response of the material. A step increase in tensile stress of 6.5 MPa is applied at time 0 and maintained for 20 seconds. The sample reaches a maximum engineering strain of 15% at 20 seconds. At 20 seconds the stress is removed, and the strain immediately drops to a level of 6%.
a. Sketch the stress-time and strain-time curves, labeling the transition points with actual values for stress and strain.
b. Using the data from your graphs, calculate the values for the elastic modulus, E, and the coefficient of viscosity, ?.