What is creep test : Measure and Signifiance

Hi pals 

Wonder what is creep test?What does it measure and what is the significance? 
 It is basically study of creep deformation at constant load (Stress) or constant true stress of material at significantly high homologous temperature for reasonable time.  

What is creep test?

A method of evaluating progressive deformation of material at constant stress/strain and constant temperature. Basic record of this test is plot of creep strain verses time.

What is significance of creep test?

The strength of material decreases with increasing temperature. This is caused due to reasons such as Mobility of atoms increases rapidly with temperature and High mobility of dislocations.

Hence, the uses of metals at elevated temperatures have numerous problems. Following examples of applications which uses high temperature. Boilers, Steam turbines, High temperature furnaces, Powerful engines, steam pipe lines, etc

For such applications high temperature strength characteristics is important. Most engineering materials exhibit tensile property independent of time at room temperature. However, at elevated temperature tensile property is dependent on both strain rate and time of exposure to elevated temperature. Hence, at elevated temperature creep will occur and metal will undergo time dependent increase in length which could be dangerous in service life. 

Creep resistance is increased by the finely dispersed precipitates. Ni based alloys contains small amount of Al and Ti which forms fine precipitates by forming intermetallic compound of Ni3Al and Ni3Ti. Carbides increase the creep resistance of steels.

Below are examples of alloys which have high creep resistance.

Austenitic Steel 316, Nickel based Astroloy, Inconel, etc

Creep test -

Standard for Creep Test – ASTM E139 – 70

While doing creep test, Constant Load is applied to the tensile test specimen at constant temperature. Further strain (elongation) is measured with respect to the time.

Below fig shows the typical shape of Creep Curve. Creep curve is divided in Primary Creep, Steady State / secondary creep and Tertiary Creep.

Creep Curve

Minimum Creep rate = The slop of the curve

What Is creep curve-

1.      Initially drastic elongation (change in length)

2.      Further creep rate decreases with time

3.      Then reaches steady state creep

4.      At the last stage creep rate increase with time and gets fracture.  

What does creep test measure: 

In creep test constant load testing is generally used for engineering purposes as this situation most accurately represents service loading conditions.Constant true stress testing is used to study deformation mechanisms.