Biaxial fatigue evaluation on element free faces

To evaluate biaxial fatigue life using the principal axes approach or maximum damage approach, the durability solver applies two assumptions: free face and proportional loading.

Based on these two assumptions, the durability solver uses two different approaches to perform biaxial fatigue evaluations on element faces:

• Von Mises effective amplitude approach — This approach is used for stress life, maximum principal strain life, and Smith-Watson-Topper life criteria.

• Brown-Miller approach — This approach is used for the maximum shear strain life criterion.

If you select the biaxial loading option for fatigue evaluation on element faces, the durability solver uses the stress or strain histories in the two directions on element faces to calculate the damage. It assumes that the loading is proportional, and that the ratio between the two principal stresses remains constant on element faces during the event.

Free face assumption

To be consistent with the cyclic test specimen, the durability solver assumes that the structural surface is traction-free. It uses the plane stress to decide the two effective principal axes on element faces. Stress normal to the element face is zero.

Proportional loading assumption

The durability solver assumes that the loading is proportional, so that it can count the stress or strain cycles in one direction and decide the corresponding ones in other directions. This assumption implies:

1. The principal axes found remain unchanged during the loading: directions 1 and 2 are on the element face, and 3 is normal to the element face.

2. The ratio between two principal stresses on an element face is constant.r = σ2/σ1 = constant

3. The ratio between maximum shear strain, γm, and the corresponding normal strain εn is constant.rε = εn/γm = constant

For more information see:

• Principal axes approach

• Maximum damage approach

• Stress biaxial ratio

• Strain biaxial ratio

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Von Mises effective amplitude approach

Brown-Miller approach

Stress biaxial ratio

Strain biaxial ratio

Biaxial fatigue evaluation for beam elements

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Biaxial fatigue evaluation on element free faces, Simcenter 3D 2021.1 Series

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