Question

Explain the importance of the failure and fatigue analysis, and how these two are conducted in the industries.

What are the problems and uncertainities in doing these analysis?

Minimum Words required: 3000

Please dont just copy from net, write in your own words

Answer 1

The performance of any material in service doesnot remain the same at all the time. With time, it degrades and deteriorates. Life depends on several factors such as inherent properties of materials, load or stress system, environment and maintenance. Hence it is very important to anticipate and plan for possible failure prevention in advance. The design of a component often asks to minimize the possibility of failure. The failure of metals is a complex subject which can only be dealt with fracture or other relevant phenomenon. Therefore, it is important to understand the different types of mechanical failure i.e. fracture, fatigue, creep, corrosion, wear etc.

The objective of a fatigue test is generally speaking to determine the fatigue life and/or the danger point, i.e. the location of failure of a test-piece subjected to a prescribed sequence of stress amplitude. By simplifying and idealizing the test conditions, it would be possible to vary one or a few of the factors, which influence the fatigue life and to state their effects. Even if these conditions are fulfilled, there will always remain a number of unknown and uncontrollable factors which produce a large scatter in fatigue life even of test-piece which are considered to be identical.

There are 2 basics for a classification of the different methods of fatigue testing.

1. The sequence of stress amplitude

2. The nature of the test-piece.

3.1 Classification based on the sequence of stress amplitudes

This is the simplest sequence of amplitude obtained by applying reversals of stress of constant-amplitude to the test-piece until failure occurs. Different specimens of the test series may be subjected to different stress amplitude but for each individual item, the amplitude will never be varied.

General Classification of Fatigue Testing

Based on the three categories above, fatigue testing maybe divided base on the objective of the test, the three categories are:

1.  Material type test

Test of the material type are useful for a comparison of the behavior of different materials subjected to repeated stresses, of the effects of various manufacturing processes, of the behavior of material in various environment, of various simple geometrical factors such as different sizes and shapes, of notches and different surface finishes

2. Structural type test

This type of test may be useful for a comparison of components made from different materials, of different design and of structure fabricated by different procedures. They may also be used for revealing stress concentration and for developing better designs or better fabrication procedures.

3.Actual service type test

They are tests usually used as reliability test or quality test, mainly for fault finding or verifying a new component in the machine or structure. Fatigue tests completely different in type from the above mentioned tests are those which have as an objective a study of the initiation and propagation of fatigue cracks, this requires complex knowledge of fracture mechanics