What is Creep?
Creep is a very slow flow or shifting of material causing permanent shape change at stress levels below the yield point in all metal alloys at elevated temperatures.
The deformation at higher temperatures progresses through three distinct phases: primary, secondary and tertiary. The three phases are shown schematically in Figure 1, “Low-Temperature and High-Temperature Creep Strain,”1 which plots Low-temperature (solid line) and high-temperature (dashed line) creep strain of a material on loading versus time at constant engineering stress and temperature.1
At higher temperatures, primary creep—which is the shortest phase (in terms of time)—begins at a relatively fast, diminishing rate. Secondary creep, the longest phase, progresses at a constant rate. There is no loss of strength during the primary and secondary phases. The tertiary phase begins when the rate begins to increase. Its progress is comparatively rapid, and the rate continues to increase. The material loses strength during this phase until it permanently changes shape and failure occurs.
- Boyer, Howard E. Atlas of Creep and Stress-Rupture Curves, ASM International, 1988.
Creep must be considered when:
- Applied loads are continuous.
- Operating temperatures and stress levels are above the threshold continuously or for long intermittent periods.
Creep is not a consideration under the following conditions:
- Applied loads are short-term intermittent or reversing.
- The operating temperature and stress do not reach the threshold for the alloy.
- The operating temperature and stress exceed the threshold for only short durations.
Expression of Creep
Creep is expressed as the stress required to produce a prescribed strain at a prescribed temperature over a prescribed time period. A reference point for secondary creep is 1% elongation at 100,000 hours (approximately 11.4 years). Data are usually taken on specimens subjected to uniform tensile stress and uniform bending moments. The data generated are difficult to apply quantitatively in real world applications because they are based on pure loads with no variations in load or temperature. The data are useful for eliminating some alloys from consideration and for comparing the relative merits of several alloys in a given application.
See Figure 1, “Stress vs. Temperature for 0.1% Creep Strain,” for a comparison of creep profiles at varying temperatures for the most common aluminum, magnesium, and zinc die casting alloys.
Relaxation is a variant expression of creep. The term applies to conditions, such as interference fits and swaged joints, where stress (and consequently the retained load) diminishes while there is essentially no change in dimension.
Where an analysis of operating conditions indicates that creep can occur, the following design procedures will minimize the effects and often achieve an acceptable design:
- Design to reduce or eliminate bending, tension and shear and keep the material in compression. For example, through bolting with steel bolts, nuts and washers is preferable to bolts in tapped holes because the thread loads are in the steel rather than the die casting.
- Reduce stress levels in critical areas. For example, washers under nuts and bolt heads and increased depth and diameter of threads increase the area of load-carrying metal.
- Utilize additional methods of retention at joints, such as bonding or staking, where members are retained by press fits.
- Product Design and Development for Magnesium Die Castings, Dow Chemical USA, Midland, Michigan, 1984.