Time dependent effects

Composites are generally not-susceptible to time dependent effects such as creep and stress relaxation when loaded in the fibre direction at ambient temperatures. However, if the matrix is loaded directly or indirectly, especially at elevated temperature, then time dependent deformation may need to be considered. Creep is defined as the time-dependent strain exhibited by a material under the action of a constant stress. Creep is characterised as a function of time by measurement of the creep compliance which is determined by dividing the time-dependent strain by the level of constant stress. Stress relaxation is analogous to creep but is the time-dependent stress of a material when a constant strain is applied. At low levels of load or strain, these phenomena are completely recoverable processes on removal of the applied load or strain. At higher levels when the load is removed, irrecoverable deformations may occur under load. Irrecoverable strain, sometimes called permanent set, may be accompanied by time-dependent damage development such as the formation and growth of transverse matrix cracking. Another time dependent effect is environmental stress corrosion (ESC) which is where sudden failures can occur in a composite subjected to a low applied load within a harsh chemical environment. In some instances time dependent effects can be beneficial, for example where stress relaxation relieves residual thermal stresses around a bolt hole.