While doing fatigue calculation stress life gives higher damage, but the strain life approach gives good life.The stress value is less than yield point.Is it valid to use strain life approach for a stress value below yield stress?How to justify it.
What kind of material do you investigated? The nature and defomational peculiarities of material (metallic, polymeric, ceramic) are important.
The stress-life approach just described is applicable for situations involving primarily elastic deformation. Under these conditions the component is expected to have a long lifetime. For situations involving high stresses, high temperatures, or stress concentrations such as notches, where significant plasticity can be involved, the approach is not appropriate.
If the amplitude of the total strain is such that we have significant plasticity, the lifetime is likely to be short (Low Cycle Fatigue or LCF; strain life approach). If the stresses are low enough that the strains are elastic, the lifetime is likely to be long (High Cycle Fatigue or HCF; stress-life approach).
As you are well aware, the experimental data support that compressive mean normal stresses are beneficial and tensile mean normal stresses are detrimental to fatigue life. This has been observed under conditions when the mean stress levels are relatively low compared to the cyclic yield stress and the fatigue behavior falls in the long-life regime where elastic strain is dominant. In conjunction with the local strain life approach, many models have been proposed to quantify the effect of mean stresses on fatigue behavior. The commonly used models in the ground vehicle industry are those by Morrow, and by Smith, Watson, and Topper. The equations are empirically based and should be compared with test data to determine which model is the most appropriate for the material and test conditions of interest. The two models are described below:
1- Morrow has proposed a relationship when a mean stress is present.
This equation implies that mean normal stress can be taken into account by modifying the elastic part of the strain–life curve by the mean stress. The model indicates that a tensile mean stress would reduce the fatigue strength coefficient whereas a compressive mean stress would increase the fatigue strength coefficient. The equation has been extensively cited for steels and used with considerable success in the long-life regime when plastic strain amplitude is of little significance.
2- Smith, Watson, and Topper proposed a method that assumes that the amount of fatigue damage in a cycle .
Considering the above mentioned and as you expressed, yes, while doing fatigue calculation stress life gives higher damage and meanwhile the strain life approach gives good life. depending on the type of material when the stress value is less than yield point it may be valid to use strain life approach for a stress value below yield stress. This can be justified by experimental test on the material in question or by applying carefully the numerical simulation by modelling the member.
I hope I have answered your question or have given you at least some ideas regarding your question.
All the best.
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