Factor of Safety

Factor of Safety

From the stress-strain curve we note that ultimate stress is the maximum value of stress, the material withstands before failure.

So, should we allow the material to reach its ultimate stress value while in operation?

The answer is No..! as we would like to keep some scope for safety, since the estimated operation loads may be exceeded at time. Also, the ultimate stress value may be different from that of the tested specimen.

Hence, the actual stress is kept much below the ultimate stress limit. In fact, ideally the stresses should be kept within the elastic limit. The upper limit up to which the stresses are permitted is known as the permissible stress or allowable stress or safe stress or working stress.

The ratio of Ultimate stress to safe stress/Working Stress know as Factor of Safety. It works as a protective shield against failure of material under stress.

The factor of safety for a component or structural part has to the be chosen very carefully. If chosen too small, the possibility of failure becomes dangerously large and if chosen very large, the design can become uneconomical. Generally, the choice of factor of safety for a component is influenced by the flowing considerations:

Material Properties:

        Material used for a component may not be homogeneous, i.e., it may not have the same properties throughout. The composition, strength and its dimensions may have small variations during its manufacture. The properties are also altered when a material undergoes various heat treatments.

Type of Loading:

        A designer may keep in mind the type of loading (static, impact, reversal, etc.) a structure is going to withstand; yet there are possibilities of inaccuracy in his estimates. There can also be alterations of use of a structure with time.

Number of Loading:

       The actual number of loading during the life of a component may be different from the expected loading at the time of designing. The ultimate stress usually decreases with increase in the number of loading. This phenomenon is known as fatigue and plays a key role in designing.

Type of Expected Failure:

         Sometimes, the collapse of a structure may occur suddenly and there is no prior indication as in case of brittle materials. In such cases where there is possibility of sudden failure, a higher factor of safety has to be used then where there is possibility of pre-warning in the form of yielding of the material.

Method of Analysis:

         As all relations and analyzing methods are based on certain assumptions, it is appropriate to include some factor of safety for such type of assumptions and simplifications.

Maintenance:

        All mechanical components and structures require good maintenance during their lifetime. However, to consider poor maintenance, rusting, corrosion, decay and other natural uncertainties in future, some margin has to be included.

Few more piping resources...

1. Stress Analysis of Column Piping System - 01
2. Stress Analysis of Column Piping System: PIPING &COLUMN MODELLING IN CAESAR-II 
3. Basics of Pipe analysis: Objectives
4. Basics of Pipe analysis: Principal stresses
5. Piping Stress Basic
6. Piping Stress analysis