A stress of 6.7 MPa is employed to one type of a polymer under constant strain . After 40 days at 20 ° C the stress is reduced to only 8.4 MPa . When the same polymer is heated to 40 ° C the relaxation time is 40 days . 1- Calculate the relaxation time constant for this material at 25 ° C ? 2- Estimate the stress after 50 days at 25 ° C ? 3- Estimate the stress relaxation activation energy for the same polymer

1) Assuming that the moduli and volume fractions of fiber and matrix materials are known, obtain the rule of mixtures in order to calculate the longitudinal Young's (elasticity) modulus of a unidirectional lamina by use of iso-strain assumption

At room temperature, most polymers have similar fracture toughness compared to metals. O True False

For a glass/epoxy lamina with a 70% fiber volume fraction using the properties in the tables provided below (a) Find the ultimate tensile strength. (b) Find the minimum and critical fiber volume fractions related to (a). (c) Find the compressive strength when the matrix undergoes extension mode and when the matrix undergoes shear mode, which one controls the compressive failure? Property Axial modulus Transverse modulus Axial Poisson's ratio Transverse Poisson's ratio Shear modulus Axial tensile strength Axial compressive strength Transverse tensile strength Transverse compressive strength Shear strength Epoxy Glass Units 3.4 85 GPa 3.4 85 GPa 0.3 0.20 0.3 0.20 1.308 35.42 GPa 72 1550 MPa 102 1550 MPa 72 1550 MPa 102 1550 MPa 34 35 MPa