Question: Write short note on creep testing of material.
- As seen in the above setup, the apparatus consists of loading mechanism which is balanced over a lever.
- The specimen is kept between the loading arrangements in between furnace.
- The strain in the furnace is observed by telescopic arrangement.
- For all variety of loadings the strain is observed and plotted against the time.
- The curve is specific for a constant value of loading and constant value of temperature.
Basic observations for creep loading done on metal is as follows
- An initial steep rate that is at least partly of elastic origin, from point "0" to point "A"
- This is followed by a region in which the elongation or deformation rate decreases with time, the so-called transient or primary creep, from region "A" to "B". The portion from point "0" to point "B" occurs fairly quickly. Here the creep rate is slow as strain hardening is more.
- The next portion of the creep curve is the area of engineering interest, where the creep rate is almost constant. The portion from "B" to "C" is nearly linear and predictable. Depending on the load or stress, the time can be very long; two years in a test and several decades in service, here the strain hardening is balanced and hence we get uniform creep rate.
- The fourth portion of the creep curve, beyond the constant-creep-rate or linear region, shows a rapidly increasing creep rate which culminates in failure. Even under constant-load test conditions, the effective stress may actually increase due to the damage that forms within the microstructure.
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