Abstract
Successful estimation of the service life of a structure or component which is subjected to a complex history of loading, depends on a suitable cumulative-damage summation technique. A general technique must be capable of predicting the effects on fatigue life of geometry, mean stress or strain, occasional overloads or overstrains, frequency of cycling and environment. As a contribution towards the general solution, this paper describes the utilization of a fatigue-damage summation method which incorporates two of these variables, mean stress and overloads. The method is tested for complex load histories in mild-steel specimens in the intermediate to long-life range.
A four-part constant-stress-amplitude testing program was carried out consisting of: (a) tests with a constant mean stress, (b) tests on prestrained specimens, (c) tests with a mean stress applied in one block of cycles, and (d) tests with a mean stress applied in frequent regularly spaced short blocks.
The constant-mean-stress results, which include several values of mean stress both tensile and compressive, are reduced to a single curve on a stress-life plot with either of two simple parameters (from Morrow and Smith, et al.) as ordinate. The prestrained specimen tests result in another curve lower than the first, showing the reduction in life due to the few initial cycles of high strain.
These two effects are then incorporated into a cumulative-damage summation technique which is based on the well-known Miner's rule. The two curves on the stress-life plot are the foundation for the subsequent summations. The technique assumes that the first application of an overload causing appreciable plastic straining reduces the remaining fatigue life. A significant point is that this assumption apparently holds for both tensile and compressive overloads. The accuracy of this technique is demonstrated for a wide range of stress conditions and loading histories.
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Abbreviations
- \(\sigma _a\) :
-
stress amplitude
- \(\sigma _o\) :
-
mean stress
- \(\sigma _f\) :
-
true-fracture stress
- \(\sigma _{cr}\) :
-
equivalent completely reversed stress amplitude
- \(\sigma _{max} ,\sigma _m\) :
-
maximum tensile stress
- R :
-
ratio of minimum to maximum stress
- N f :
-
cycles to failure
- 2N f :
-
reversals to failure
- b :
-
fatigue-strength exponent
- \(\varepsilon _a\) :
-
strain amplitude
- \(\varepsilon _f ^\prime\) :
-
fatigue-ductility coefficient
- \(\varepsilon _o\) :
-
mean strain
- Δσ:
-
stress range
- Δε:
-
strain range
- E :
-
modulus of elasticity
- n i :
-
number of cycles applied at leveli
- N fi :
-
number of cycles to failure at leveli
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Watson, P., Topper, T.H. Fatigue-damage evaluation for mild steel incorporating mean stress and overload effects. Experimental Mechanics 12, 11–17 (1972). https://doi.org/10.1007/BF02320784
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DOI: https://doi.org/10.1007/BF02320784