CREEP TESTING

Creep Testing

     The slow and progressive deformation of a material with time at a constant stress is called creep.  The creep testing determines the continuing change in the deformation of a material at elevated temperature when the stress is below the yield point.  The creep test results are useful in design of machine parts [e.g. furnace parts, turbine blades etc], which are exposed, to elevated temperature.

Test procedure

     A creep test is simply a tensile test run at constant load and temp.  A creep test is required to measure and record stress, strain, temp and time.  Then total creep or percentage elongation is plotted against time for the entire duration of the test and the result is a creep curve.  Fig (1) shows and typical creep curve for a long time and high temperature creep test, it shows four stages of elongation.

1) Instantaneous elongation following the application of load

2) Primary creep

3) Secondary creep

4) Tertiary creep

.

Fig 

Creep- Testing Equipment or machine

  Fig

Fig shows a system for testing creep. 

1. The specimen is spot welded with one platinum wire and one platinum tube.  The wire slides inside the tube, reference marks on both are observed through the single telescope at the middle. Elongation can be measured on a scale inside the telescope. 

1. The specimen is subjected to constant loading or a constant stress through a system of dead weight and levers etc. 

2. A tubular electrically heated furnace is made to be fit around the specimen may have the thermo couple of each end for temp measurement purpose