Machine Design

Our real concern is how to design a component so that failure by fatigue could be precluded.We have noted earlier that-Materials response to fatigue loading is characterized by its S-N behavior obtained througha standard test-The most important factors that affect the fatigue performance (strength) are also noted inthe previous lecture.-Standard test conditions do not account for all these factors.-Components in real use will be subjected to different or varied conditions.In order to design for satisfactory fatigue life (prior to testing actual components), goodpractice requires that the "laboratory" Endurance Limit value be reduced by severaladjustment factors. These reductions are necessary to account for:(a) the differences between the application and the testing environments, and(b) the known statistical variations of the material.This procedure is to insure that both the known and the unpredictable factors in theapplication (including surface condition, actual load, actual temperature, tolerances,impurities, alloy variations, heat-treatment variations, stress concentrations, etc. etc. etc.)will not reduce the life of a part below the required value. Please read that paragraph again,and understand it well.An accepted contemporary practice to estimate the maximum fatigue loading which aspecific design can survive is the Marin method, in which the laboratory test-determined ELof the particular material (tested on optimized samples) is adjusted to estimate themaximum cyclic stress a particular part can survive.This adjustment of the EL is the result of six fractional factors. Each of these six factors is calculated from known data which describe the influence of a specific condition on fatigue life. 

Those factors are:

(a) Surface Condition (ka): such as: polished, ground, machined, as-forged, corroded, etc.Surface is perhaps the most important influence on fatigue life;

(b) Size (kb): This factor accounts for changes which occur when the actual size of the part or the cross-section differs from that of the test specimens;

(c) Load (Kc): This factor accounts for differences in loading (bending, axial, torsional) between the actual part and the test specimens;

(d) Temperature (kd): This factor accounts for reductions in fatigue life which occur when the operating temperature of the part differs from room temperature (the testing temperature);

(e) Reliability (ke): This factor accounts for the scatter of test data. For example, an 8% standard deviation in the test data requires a ke value of 0.868 for 95% reliability, and 0.753 for 99.9% reliability.

(f) Miscellaneous (Kf): This factor accounts for reductions from all other effects, including residual stresses, corrosion, plating, metal spraying, fretting, and others. 

These six fractional factors are applied to the laboratory value of the material endurance limit to determine the allowable cyclic stress for an actual part: Real-World Allowable

Cyclic Stress = ka * kb * Kc * kd * ke * kf * EL

Thus designers are now able to tackle this situation by applying as many modification factors as possible so that most important deviations of the real design condition from the standard test conditions are accounted. So the next part of the discussion will deal with the endurance strength modification factors.

Credit : IIT Madras