Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 5M

Year: Dec 2015

Creep Testing Equipment

Objective:

To determine the creep limit or the limiting creep stress, defined as the stress that will just not break the specimen when applied for an infinite period of time at a specific constant temperature. Continues change in the deformation of metal at elevated temperature when stressed below the yield point.

Test procedure:

Here the tension test run at constant load and constant temperature specimen like as tension test specimen. With the specimen spot welded, one platinum wire and one platinum tube. The wire slides inside the tube, reference marks on both observed through a single telescope at the middle, the elongation can be measured on a scale inside the telescope. The specimen is subjected to constant loading through a system of dead weight & levers. A tubular, electrically heated furnace is made to fit around the specimen. Elongated end of the specimen may have thermocouple in each for temperature measurement purpose.

Creep Curve:

It is plotted between the % of elongation/ strain and the time for the entire duration of test.

1. Primary creep/ Transient creep:

It is a decreasing creep rate because of the work hardening process resulting from deformation. It is similar in its mechanism to delayed elasticity. It is recoverable one.

2. Secondary creep/ steady state creep:

The deformation continuous at an approximately constant rate. It may be essentially viscous (or) plastic in character, depends upon stress level & temperature. In this stage, a balance exists between rate of work hardening and rate of softening because of recovery (or) recrystallisation.

3. Tertiary Creep:

If the stress is sufficiently high & temperature also high there is a tertiary stage in which the creep rate accrderates until fracture occurs. It is because of structural changes occurring in the metal.