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Creep and stress relaxation

Posted by admin on July 24th, 2012
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Creep and stress relaxation are critical concerns when designing structural parts that are subject to Long-term loading.

Creep

Regardless of the rate at which the initial load is applied, if a constant load is continued, the structure will continue to deform. This long-term, permanent deformation is called creep, as plotted in Figure 9.

FIGURE 9. A typical creep in flexure curve. Note that the creep is dependent on load and time. In order to design parts that are subject to long-term loading, designers must use creep data in an effort to ensure that the parts do not rupture, yield, craze, or simply deform excessively over their service life. Although creep data exist for many resins at specific times, stress levels, and temperatures, each individual application must use the data that correlate with the type of stress and environmental conditions that the part is subjected to during service. Since the process of individual testing for long periods of time is not feasible and the stress and environmental conditions are difficult to predict over the long term, methods for interpolating and extrapolating shorter information are necessary. Engineers typically have to enter creep databases provided by resin suppliers to obtain time-strain data, then perform interpolation and extrapolation procedures to develop a complete nonlinear isochronous stress-strain curve, as shown in Figure 10. These curves are then used in place of short-term stress-strain curves when designing for applications involving long-term static loading.

Creep modulus

The time and temperature-dependent creep modulus, Ec, as a function of constant stress,  , and time and temperature-dependent strain,  (t,T), as defined below, can be used in design calculations for constant stress or strain-stress relaxation applications.

Other factors associated with creep are:

The rate of creep and stress relaxation will increase with increases in temperature.

If the load is continued long enough, rupture may occur. This is called stress cracking.

High internal (residual) stress should be considered along with the external stresses.

Stress relaxation

Stress relaxation is a corollary phenomena to creep. If the deformation is constant, the stress resisting that deformation will decrease with time. The physical mechanism that causes a plastic to undergo creep also applies to the phenomenon of stress relaxation. Figure 10 illustrates that at a fixed strain, the stress decreases with the elapsed time.

FIGURE 10. These isochronous (fixed-time) curves demonstrate stress relaxation at a constant strain (deflection).

 

Author: ZHILIAN MOULD
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