Learn how to perform a fatigue analysis including ocean wave behavior calculated by Ansys Aqwa.

## Understanding Fatigue Analysis

Most machine failures are a result of loads that vary over time rather than static loads. Such failures typically occur at stress levels significantly lower than the material's yield strengths. Therefore, relying solely on the static failure theories can lead to unsafe designs when dynamic loads are present. The S-N diagram (Wöhler diagram) became the standard for representing the behavior of materials subjected to fully reversed loading cycles and is still in use, even though there are now other measures of material strength under dynamic loads.

Fatigue failures always begin as a crack, which may have been present in the material since its manufacture, or perhaps developed over time due to cyclic deformation around stress concentrations. It has been demonstrated that practically all structural members have discontinuities, ranging from microscopic to macroscopic, introduced in manufacturing or the fabrication process. Fatigue cracks usually initiate as a notch or other stress concentrator. Then, it is critical that dynamically loaded parts be designed to minimize stress concentrations.

Classifications

• There are three phases or stages of fatigue failures: Crack initiation (short-lived), crack propagation (majority of the part's life), and sudden fracture due to unstable crack growth (instantaneous).

• At present, three fatigue failure models are used, each with its place and purpose. They are: the stress-life approach (S-N), the strain-life approach (ε-N), and the linear elastic fracture mechanics (LEFM) approach.

• Based on the number of stress or strain cycles expected to be applied to the part during its lifetime, Fatigue is classified as either low-cycle fatigue (LCF) or high-cycle fatigue (HCF) regime.

Loads that vary over time can cause fatigue failures. Then, understanding the repetitive loading caused by water waves on offshore structures is crucial for gaining insights into their structural behavior.  The loading depends on the wave type (regular, irregular, long- short-crested), height (amplitude), period (or frequency), direction, and water depth. Moreover, Structures' mass and inertia properties play an important role in the response. Finally, the stress values and material properties allow calculating fatigue results using a proper theory.

## Hydrodynamic analysis: Diffraction and Radiation

To enhance wave fatigue analysis, engineers have developed advanced techniques that provide more accurate and detailed insights into the behavior of ocean waves. Some of these techniques include:

• Numerical simulations: They provide valuable information about the wave loading, structural response, and fatigue damage accumulation.

• Fatigue testing: By subjecting the components to cyclic loading similar to that experienced in the ocean, engineers can assess their fatigue resistance and durability.

• Structural health monitoring: This real-time data can help identify potential fatigue damage and facilitate timely maintenance and repairs.

• Wave measurement technologies: Allows to collect accurate and detailed data on wave characteristics that can be used to improve wave fatigue analysis and enhance the design of fatigue-resistant structures.

Numerical approach: Ansys Aqwa

Ansys Aqwa performs the Diffraction/Radiation modeling which provides an integrated environment for developing the primary hydrodynamic parameters required for undertaking complex motions and response analyses. Three-dimensional linear radiation and diffraction analysis may be undertaken with multiple bodies, taking full account of hydrodynamic interaction effects that occur between bodies. Floating and fixed structures (such as breakwaters or gravity-based structures) may be included in the models. Computation of the second-order wave forces (full quadratic transfer function matrices) permits use over a wide range of water depths.

In the Hydrodynamic Diffraction analysis, an incident wave with a given amplitude, direction and frequency interacts with the structure. Due to this interaction, the waves experience Diffraction (the bending effect around obstacles or corners) and Radiation (the generation of new waves). If we combine these effects, the results are those shown below. If we combine these effects, the results are those shown below. If we combine these effects, the results are those shown below. If we combine these effects, the results are those shown below.

Incident Wave

Diffracted Waves

Final result

Demo: Structural and Fatigue results

The geometry of the Ship shown above is used to calculate the hydrodynamic pressure in Ansys Aqwa. Different wave direction and frequency values are also  The results are transferred to a 'Static Structural' module for the stress and fatigue analyses. The next figures show a summary of the workflow:

1. Geometry preparation (Ansys SpaceClaim, Ansys Discovery)
The geometry must be prepared previously for this study, although recent Ansys versions have modified this task to make it faster and easier.

2. Diffraction (Ansys Aqwa)
The figure presents the direction and frequency values included. The maximum frequency depends on the mesh size.

3. Stress contours (Ansys Mechanical)
In this example, the results are calculated for an entire period in the cycle. There are more options available.

4. Factor of Safety (Ansys Mechanical)
The setup of the Fatigue Module must be carefully performed and include realistic values in the setup.

The complete workflow is then presented as follows:

April 25, 2024