Advanced and Approximate Strain-Life Data

Advanced Strain-Life Data

In this step, you define the parameters required to generate an advanced Strain-Life curve for Strain Based Fatigue Life (EN) analyses. Click the Advanced radio button in the Strain-Life Data section of the Step 2 dialog box. Or, if this radio button is already selected, click the View/Edit button. This section of the dialog box only appears if the Strain Based Fatigue Life (EN) analysis option has been selected in Step 1. The following dialog box will appear.

Note: For metals included within the Fatigue Wizard material database, the advanced strain-life data is already defined. For user-defined materials, if these advanced parameters are unknown, use the Approximate strain-life data method.

Approximate Strain-Life Data

There are various methods for approximating strain-life material data from the more readily available monotonic data, such as tensile strength, elastic modulus, and so on. The method we use in Fatigue Wizard is the same one used by Seeger/Beste. It is present in many texts about strain based fatigue calculation. Basically this technique approximates the strain life curve (strain based coefficients) using the following formulae:

Steels:

• K' = 1.65 * UTS
• n' = 0.15
• Sf = 1.5 * UTS
• b= -0.087
• c= -0.58
• Ef = 0.59 (if UTS/E<0.003)
• Ef = 0.59*(1.375-125*UTS/E)

Other Metals:

• K' = 1.61 * UTS
• n' = 0.11
• Sf = 1.67 * UTS
• b= -0.095
• c= -0.69
• Ef = 0.535

The following image illustrates these parameters.

The addition of two curves defined by the parameters entered in the Fatigue Wizard dialog box creates the Strain-Cycle curve (red).

Viewing the Strain-Life Curve

Click the View Material Graph () button to see the Strain-Life curve. This action is identical regardless of whether the Approximate or Advanced S-N Data option is selected. A typical curve is shown in the following image.