Probabilistic Strain-Life Analysis

If needed, please go to the Constant Amplitude section for a review of the general terms and terminology related to Strain-Life fatigue analysis.

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.

Experienced user mode is off. Turn experienced user mode on for a more concise form.

Click on the button below to learn by example:

Description of Distribution Types

Loading

Loading on structures frequently follows a Normal or LogNormal distribution. In a well controlled situation, such as may be encountered in a test track or in an electric motor, the coefficient of variation COV is typically 0.1. The COV increases to as much as 0.5 for uncontrolled customer usage. In the absence of any other information, a reasonable value of 0.2 or 0.3 may be assumed. If you enter a value of zero for either the normal or the uniform distribution types, you should enter the standard deviation for the scale parameter. This allows you to generate a distribution around zero.

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

Material

Material properties are the most common source of uncertainty in a fatigue life calculation. Fatigue is a weakest link process and fatigue cracks start from the most severe flaw in the material. There is considerable variability in fatigue lives often a factor of 2 - 10 in fatigue life. But because of the strong dependance of fatigue life on stress or strength, the COV of material properties is on the order of 0.1 - 0.2.

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 curve plot. Registered users may also save this material in their personal database by clicking the "Save Material" button.

Name
Type
        Distribution Type Scale Parameter Correlation Coefficient
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

The various modifying factors sometimes play an important role in calculating the fatigue life. A normal distribution is typically used with a coefficient of variation, COV, of 0.1 as a reasonable value.

        Distribution Type Scale Parameter
Surface Factor kSF = or

Stress Concentration Factor

The stress concentration factor, Kt, is subject to any uncertainties in the model used to obtain its value as well as any variability in the structure. A normal distribution is typically used with a coefficient of variation, between 0.05 and 0.3.

        Distribution Type Scale Parameter
Stress Concentration Factor Kt =  

Either specify Kf directly or enter Kt and the radius.

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

Calculate