1-112 Section 1Creep and Stress Relaxation Creep and stress relaxation are related time- and temperature-dependent phenomena, with creep occurring under load control and stress relaxatio
Trang 11-112 Section 1
Creep and Stress Relaxation
Creep and stress relaxation are related time- and temperature-dependent phenomena, with creep occurring under load control and stress relaxation under deformation control In both cases the material’s temper-ature is a governing factor regarding what happens Specifically, for most metals, the creep and relaxation regimes are defined as high homologous (relative, dimensionless) temperatures, normally those above half the melting point in absolute temperature for each metal Thus, solder at room temperature creeps significantly under load, while steel and aluminum do not However, some creep and relaxation may occur even at low homologous temperatures, and they are not always negligible For polymers, the creep regime is above the glass transition temperature This is typically not far from room temperature Figures 1.6.13 and 1.6.14 show trends of creep and stress relaxation in the large-scale phenomenon region
Stress vs rupture life curves for creep may be nearly linear when plotted on log-log coordinates (Figure 1.6.15)
FIGURE 1.6.12 Practical fracture mechanics with NDE: nearly instantaneous measurement of crack size and the
actual stress intensity factor via advanced thermoelastic stress analysis The member’s loading (including boundary conditions) need not be known to obtain reliable data using this method.
FIGURE 1.6.13 Creep under constant load dε/dt = A(σ)n A and n are material parameters.
FIGURE 1.6.14 Stress relaxation under constant deformation σ = σ 0e –Et/η E and η are material parameters.