Comparison between Creep Test and Tension Test of Engineering Materials

This article will give you the basic differences and conceptual comparison between creep test and tensile test. Creep Strength and Tensile Strength are two major mechanical properties of a material. Creep test is used to find out the behavior of the material under creep loading and tension test which is usually done in a Universal Testing Machine (UTM) is carried out to find out the behavior of the material under tensile loading. You are requested to have the basic knowledge of stress-strain curve and creep curve before reading this article. The basic comparison is as follows.

Creep Test Vs Tension Test

• Creep is a slow and progressive permanent deformation of a material at constant stress and at a constant temperature of about 0.4 times the melting point of the material. Whereas in the tension test, the deformation is not that slow and progressive and stress is not constant. It is gradually increasing and we are not much concerned about the temperature factor.

• Both the specimen in the creep test and tension test will obey Hooke's law when it is initially subjected to load. In the tension test, once the load is applied above the elastic limit or stress applied is above yield stress, the body will start permanent plastic deformation under a gradually increasing load. But in creep test, as time pass on, the specimen will manifest permanent deformation even at a constant load.

• Both creep test specimen and tensile test specimen looks like the same but the only difference is that the creep test requires a constant temperature furnace to conduct experiments above room temperature. However, the tension test does not require such arrangements as we are not interested in temperature variation of tensile stress.

• In creep test, we are interested in a creep curve which is plotted as a function of permanent. strain and time. Whereas in the tensile test we are interested in the stress-strain curve under the elastic limit. 

Interesting observations from Creep Curve

A creep curve is a curve drawn as a function of permanent strain and time. It can be divided into 3 regions namely primary creep region, steady state creep or secondary creep region and rapid creep region or tertiary creep region. The following are the important observations in each of these creep regions.

• Primary creep region is similar to a cold worked region and strain hardening dominates in this region due to plastic deformation. Hardness increases as a result of the arrest of slip due to plastic deformation and resistance to permanent deformation increases. So in primary creep region ductility decreases as the time goes on and the slope of the creep curve in primary creep region gradually decreases. 

• In the steady state creep region, as the specimen is exposed to a constant temperature for more time, the material starts to soften due to thermal influence and this balances the strain hardening effect in this region and thus the curve is normally flat in this region or in other words slope remains constant.

• In rapid creep region, softening effects due to thermal influence dominates over strain hardening and thus the slope of the graph increases in this region. So you can see a considerable rise in the permanent strain as the time goes on in this region.

• If we are keeping the stress constant and varying the temperature, we will observe a rapid increase in creep as the temperature is increased. Moreover, the secondary creep region will get vanished.

Methods of protecting material against creep

• Use material having high melting point.

• Use the material having coarse grain structure.

• Use dispersion strengthened materials.

• Materials having high oxidation resistance usually exhibit high creep resistance.

• Use single crystal materials.

• Dendritic structures show better creep resistance than equiaxed structures. This is because grain boundary sliding which results in creep is prevented by the irregular geometry of dendritic structures.

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