Constant Amplitude Strain-Life Analysis

Although most engineering structures and components are designed such that the nominal stresses remain elastic, local stress concentrations often cause plastic strains to develop in regions around them. The strain-life method assumes that the smooth specimens tested in strain control simulate fatigue damage in local region around the stress concentration.

Use of the strain-life analysis method is limited to situations where crack nucleation and the growth of small microcracks consumes the majority of the service life.

Enter as much data as you know. If it is not enough, you will be asked for more. Sections with a light blue background represent the minimum required data to begin calculations. Other data may become necessary as calculation proceeds. Pressing the button provides help in the form of an equation or default information for a parameter.

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Loads can be entered as either the maximum and minimum values or as the stress range and mean stress.

Stresses or strains entered may be elastic-plastic. You can use elastic finite element or other elastic calculations as input by selecting (elastic) units for stress or strain. Examples include input from elastic finite element models and strength of materials calculations such as bending beams. In this case, a plasticity correction will be made to the input stresses or strains before computing the fatigue life using Neuber's Rule.

Loading Units
Maximum   Smax or emax =
Minimum   Smin or emin =
OR
Range   ΔS or Δe =
Mean   Sm or em =

Material

Strain-life curves and cyclic stress-strain curves are needed for this analysis.


You may load a material from the database by selecting it and clicking on "Load Material", or browse the database by clicking the "Material Property Finder" button, or specify individual properties directly. Clicking "Material Property Estimator" will show the default properties that are computed from the input values.

For registered users, the Material Property Estimator will display a plot of the data. Registered users may also save this material in their personal database by clicking the "Save Material" button.

Name
Type
Fatigue Strength Coefficient σf′ =
Fatigue Strength Exponent b =
Fatigue Ductility Coefficient εf′ =
Fatigue Ductility Exponent c =
Elastic Modulus   E =
Fatigue Limit   SFL =
Fatigue Limit Reversals   2NFL = Reversals
Cyclic Strength Coefficient   K′ =
Cyclic Strain Hardening Exponent n′ =
Ultimate Strength Su =

Surface Finish

Fatigue usually starts at the surface so that the quality of the surface finish is very important. The surface finish becomes even more important as the strength of the material increases.

Either specify the surface factor directly or choose a finish from the drop-down box. If you don't know, a default value of 1 will be used.

Surface Factor kSF = or

Stress Concentration Factor

All mechanical components are structures contain some form of stress concentrators which can cause cracks to form. The theoretical stress concentration depends on geometry and relates the local maximum stress to the nominal or average stress through a stress concentration factor.

Stress Concentration Factor Kt =  

Small stress concentrations are less effective in fatigue than predicted by Kt. A fatigue notch factor (effective stress concentration in fatigue) is used to account for this effect. It is related to the size of the local stress gradient and material strength.

Either specify Kf directly or enter Kt and the radius.

Use Kf in analysis?
Fatigue Notch Factor   Kf =
Radius r =

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