An engineer must routinely assure that designs will endure anticipated loading histories with no significant change in geometry or loss in load-carrying capability. Anticipating service-load histories can require experience and/or testing. Techniques for load estimation are as diverse as any other aspect of the design process.

The design or allowable stress is generally defined as the tension or compressive stress (yield point or ultimate) depending on the type of loading divided by the safety factor. In fatigue the appropriate safety factor is used based on the number of cycles.

Also when wear, creep, or deflections are to be limited to a prescribed value during the life of the machine element, the design stress can be based upon values different from above.

The magnitude of the design factor of safety, a number greater than unity, depends upon the application and the uncertainties associated with a particular design. In the determination of the factor of safety, the following should be considered:

1. The possibility that failure of the machine element may cause injury or loss of human life

2. The possibility that failure may result in costly repairs

3. The uncertainty of the loads encountered in service

4. The uncertainty of material properties

5. The assumptions made in the analysis and the uncertainties in the determination of the stress-concentration
factors and stresses induced by sudden impact and repeated loads

6. The knowledge of the environmental conditions to which the part will be subjected

7. The knowledge of stresses which will be introduced during fabrication (e.g., residual stresses), assembly, and shipping of the part

8. The extent to which the part can be weakened by corrosion

Many other factors obviously exist. Typical values of design safety factors range from 1.0 (against yield) in the case of aircraft, to 3 in typical machine-design applications, to approximately 10 in the case of some pressure vessels.

It is to be noted that these safety factors allow us to compute the allowable stresses given and are not in lieu of the stress-concentration factors which are used to compute stresses in service.

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