PetroStreet

Some comparative points on Creep and Fatigue... keep filling in.

Fatigue
[*]Material tends to crack
Due to cyclic stresses
"Measured" by crack size

Creep
[*]Material tends to deform
Constant loads
"Measured" by elongation

The best definitions of these terms:

Fatigue
If a stress less than a materials tensile strength is applied, it will not break; if the same load is removed and reapplied several times it may eventually break. Repeated bending of a paper clip to the point of failure is an example of a type of fatigue. The strength of a material to withstand repeated load applications is called fatigue strength.

Creep
If a load below a material’s tensile strength is applied at room temperature, the material initially elongates as the load is applied; sustaining the same load, however, causes no further measurable elongation. Yet if the same load were to be applied at an elevated temperature, the material would continue to gradually elongate as long as the load was maintained. This behavior is called creep. Maintaining the load long enough would eventually cause the material to rupture.

Where on a Process Plant can we expect Creep and Fatigue failures?
Bullets would help.
After this particular question, we can then go about their inspection. So areas of concern please.

Fatigue can occur under following scenarios:
a. Cyclic pressure conditions (changes in internal pressure)
b. Variations in flow (steam hammering, water hammering, cavitation)
c. System changes and other external factors (induced vibrations from compressor etc.)
d. Continuous high temperature conditions (due to pipe movement, uneven expansion, extra anchoring etc.)
Compressor discharge piping, piping under high temperature conditions, steam piping are the main areas prone to fatigue failure. Improper flexibility stress analysis can also lead to fatigue failures under high temperature conditions where pipe movement is improperly restricted.

Creep failure is likely to occur in materials operating at elevated temperature at which time dependent properties get activated. Process piping code ASME B31.3 provides provisions for such piping as operating at elevated temperature conditions and refers to ASME Sec. II part D Table 1A for calculation of Tcr (critical temperature) so that it may be evaluated whether elevated temperature conditions apply or not.
In process plants, creep is likely to occur on those materials used in reformers, cracking furnaces, fire tube boilers, fired pressure vessels etc.

It would be good if you add real time photographs and relevant experience also. Anyway, a good thread initiated.