(a) Contrast the manner in which stress relaxation and viscoelastic creep tests are conducted. (b) For each of these tests, cite the experimental parameter of interest and how it is determined.

Answer: Stress relaxation and viscoelastic creep tests are both methods used to examine the viscoelastic behavior of materials. In a stress relaxation test, a constant strain is applied to the material, and the stress required to maintain this strain is measured as a function of time. The experimental parameter of interest is the stress relaxation modulus, determined by measuring the stress in the material over time at a constant strain and plotting it as a function of time. In a viscoelastic creep test, a constant stress is applied to the material, and the strain that occurs as a result is measured as a function of time. The experimental parameter of interest is the creep compliance, determined by measuring the strain in the material over time at a constant stress and plotting it as a function of time. The main difference between these tests is the variable that remains constant (strain for stress relaxation tests and stress for viscoelastic creep tests).