Eish talks about: Modes of failure and material damage mechanisms

Written by Keith Cain

Eish talks about:

Modes of failure and material damage mechanisms

The following are some examples of some of the most common material damage mechanisms:

1) Embrittlement. This type of damage can be caused in some materials by irradiation, temper-embrittlement, or by caustic, hydrogen or hydrogen sulfide rich environments.

2) Fatigue. This is a damage process whereby embedded or surface cracks can initiate and grow under fluctuating stress (e.g. due to applied loads, thermal stress variations and vibrations). Members containing stress concentrations (e.g. flaws, geometric discontinuities) are particularly susceptible. For these reasons, welded components can give much lower fatigue lives than plain material under the same loading.

3) Corrosion fatigue. This is a type of damage similar to fatigue, but the onset of cracking and crack growth rates are accelerated by the corrosive medium.

4) Creep cracking. This is internal and/or external cracking due to operating temperatures greater than the creep threshold temperature, which is a function of the material. Components subjected to creep conditions have a finite life and the damage rate is a function of temperature, stress and time.

For this reason, the finite life of highly stressed components and welds within a structure can be much lower than the plain areas subject to membrane stress alone.

5) Internal corrosion. Material loss can take many forms, such as pitting corrosion, crevice corrosion, localized corrosion, general corrosion, etc., and is mainly due to the contents of the system, including possible impurities.

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