Modifying Factors


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Also may be called 'modification factors', or 'endurance limit modifying factors'. The symbols C and k are both used in literatures to represent modifying factors. Modifying factors take into account different effects that apply to the production of a part. They relate the endurance limit of a tested specimen to that of an actual production part, given the same material is used in both the test specimen and the production part.


Details

The results of an R. R. Moore test are from the special case of a mirror-polished 0.25 in diameter specimen under fully reversed bending. To denote this, the endurance limit found using the R. R. Moore test is often given a prime, S'e. The endurance limit needed for design situations, Se, must take into account differences in size, surface finish, and so on.

For many years the emphasis of most fatigue testing was to gain an empirical understanding of the effects of various factors on the baseline S-N curves for ferrous alloys in the intermediate to long life ranges. The variables investigated include:

- Size (or Size effects)
- Type of loading
- Surface finish
- Surface treatments
- Temperature
- Environment

The results of these tests have been quantified as modification factors which are applied to the baseline S-N data.

Se = S'eCsizeCloadCsurf.fin.⋅⋅⋅

This modified endurance limit tends to be conservative.

The modification factors are usually specified for the endurance limit, and the correction for the remainder of the S-N curve is not as clearly defined. The general trend is for these modification factors to have less effect at short lives. At the extreme limit of monotonic loading they all approach a value of 1. A conservative estimate is to use the modification factors on the entire S-N curve.

It is very important to remember that these modification factors are empirical models of a phenomenon and may give limited insight into the underlying physical processes. Great care must be taken when extrapolating these empirical modification factors beyond the range of data used to generate them.



The rotating-beam specimen used in the laboratory used to determine endurance limits is prepared very carefully and tested under closely controlled conditions. It is unrealistic to expect the endurance limit of a mechanical or structural member to match the values obtained in the laboratory. Some differences include:
- Material: composition, basis of failure, variability
- Manufacturing: method, heat treatment, fretting corrosion, surface condition, stress concentration
- Environment: corrosion, temperature, stress state, relaxation times
- Design: size, shape, life, stress state, stress concentration, speed, fretting, galling

Factors have been identified that quantified the effects of surface condition, size, loading, temperature, and miscellaneous items. The question of whether to adjust the endurance limit by subtractive corrections or multiplicative corrections was resolved by an extensize statistical analysis of a 4340 (electric furnace, aircraft quality) steel, in which a correlation coefficient of 0.85 was found for the multiplicative form and 0.40 for the additive form. A Marin equation is therefore written as:

Se = kakbkckdkekfS'e

where:
ka = surface condition modification factor
kb = size modification factor
kc = load modification factor
kd = temperature modification factor
ke = reliability factor
kf = miscellaneous effects modification factor
S'e = rotary-beam test specimen endurance limit
Se = endurance limit at the critical location of a machine part in the geometry and condition of use

When endurance tests of parts are not available, estimations are made by applying Marin factors to the endurance limit.


Related
▪ L - Power Equation for Stress-Life
▪ L - Endurance Limit and Ultimate Strength