Why creep failure occurs

Tertiary creep: It occurs when the creep life is almost exhausted, voids have formed in the material and the effective cross-sectional area has been reduced.

The creep rate accelerates as the stress per unit area increase until the specimen finally fails. Different creep stages and their respective material conditions are shown in below figure 2.

At elevated temperatures and stresses, much less than the high-temperature yield stress, metals undergo permanent plastic deformation called creep. Figure 3 shows a schematic creep curve for a constant load; a plot of the change in length verses time. The weight or load on the specimen is held constant for the duration of the test. There are four portions of the curve that are of interest:. The initial creep temperature of carbon steel, C Fatigue is a situation in which component is subjected to cyclic loading.

Design Stress that is Endurance strength used in fatigue loading is much less than Yield strength and ultimate strength of material. For Example It is difficult to break a wire by stretching but if we apply a cyclic load and bend unbend the wire a number of times it breaks easily. Creep is a situation in which a component experiences deformation under constant load with time as it is put into use. Best example to illustrate this is that electrical cables are taught tight when they are installed but after some time they experience sagging due to self weight.

Graphitization is a change in the microstructure of certain carbon steels and 0. During Graphitization, these changes takes place in certain grades of Carbon steel Depending on the applied stress, duration and temperature, this deformation may increase to the point where failure occurs. Creep of a turbine blade may cause the blade to contact the casing, resulting in failure of the blade.

Alternatively, a boiler tube may reach a critical effective cross section where the material can no longer withstand the applied stress and therefore failure occurs, see Figure 1. The failure was thought to be a combination of issues, one of which being creep. This occurs after long-time exposure to temperature and stress. As a result, there is little reduction in cross sectional area and a thick-walled fracture occurs. Non-destructive replication metallography is an effective means of determining the presence of long-term creep damage.

Furthermore, the platelets of iron carbide in the pearlite structure of carbon steels will thermally degrade to spheroidised iron carbide as a result of long-term overheating. How to Control Corrosion by Improving Design. Why is Stainless Steel Corrosion Resistant?

An Introduction to Hydrogen Embrittlement. An Intro to Pipeline Corrosion in Seawater. Follow Connect with us. Sign up. Thank you for subscribing to our newsletter! Connect with us. Creep Failure. What Does Creep Failure Mean? Corrosionpedia Explains Creep Failure As an example of creep failure, jet engine turbine rotors and steam generators can experience creep due to increased temperature and stress exposure.

For metals, the creep curve consists of three regions: The first is the primary or transient creep region, where the material experiences an increase in creep resistance or strain hardening. It is important to note that at this point, as the material is strained, deformation becomes more difficult.

The second stage is secondary or steady-state creep.