Much of the focus in the world of Finite Element Analysis (FEA) centers on designing a structure to withstand a single application of a static load. Frequently, however, structures are subjected to repeated cyclic loading, which can lead to the initiation and growth of cracks, often culminating in sudden fracture. For many common loading scenarios, the fatigue module of Ansys Mechanical can be used to accurately predict the fatigue life of a part. For more advanced cases, Ansys Ncode Designlife is required. In this blog post, we will discuss three approaches to fatigue, when the Ansys Mechanical fatigue module is adequate, and some situations that require a more advanced analysis in Ansys Ncode Designlife.
There are three main theoretical approaches to fatigue: the crack growth approach, the stress life approach, and the strain-life approach.
The Crack Growth Approach
The crack growth approach directly considers crack size and the rate of crack growth to predict the number of cycles required for the crack to grow to a critical size (usually determined by linear elastic fracture mechanics). This approach can be handled with Ansys’ “Separating Morphing and Adaptive Remeshing Technology (SMART) and is a topic for another blog post. We will focus here on the stress-life and strain-life approaches which can be utilized with the Ansys fatigue module.
The Stress Life Approach
The stress-life approach, developed by August Wöhler in the late 1800s to assess the fatigue life of railroad components, was the first method of fatigue calculation and is still commonly used today. It involves testing smooth material specimens under constant amplitude stress. By testing many specimens at different stress levels, a stress-life (SN) diagram can be developed that shows the number of cycles to failure (N) for a given stress amplitude. An example S-N curve for aluminum is shown in Figure 1. This empirical curve can also be represented in the form of an equation called Basquin’s rule. The stress-life approach is mainly applicable to high cycle fatigue.
Figure 1: An example of an SN curve for Aluminum with an Ultimate Tensile Strength of 320 MPa
The Strain Life Approach
In the 1960’s, the strain-life method was developed to better predict the fatigue life for components which experience plastic deformation. Even in components that remain nominally elastic, localized plastic deformation can often occur near notches. In the strain-life method, specimens are tested under strain control. In contrast to the stress-controlled method discussed in the previous paragraph, during strain-controlled testing, the force applied to the specimen during testing is adjusted to maintain a constant strain amplitude throughout the test. Because the total strain life approach effectively combines the stress life curve (in the form of Basquin’s rule) with the Coffin Manson expression, it can be used for both high cycle and low cycle fatigue.
So why use the stress-life approach for high cycle fatigue when the total strain life approach can handle both low cycle and high cycle fatigue? One answer is that stress-life fatigue data is widely available. Using the strain-life approach may require costly material testing if test data is not available. Another consideration is that the strain life approach requires the use of an equation fit to the test data, which has the potential to introduce some error into the fatigue life calculation.
Figure 2: Plot of Strain Amplitude vs Reversals to Failure used in Strain-Life Analysis
Ansys Mechanical Fatigue Module vs. Ansys Ncode DesignLife
So, when should you use Ansys Ncode Designlife instead of the fatigue module in Ansys Mechanical? See below for some guidance.
When Ansys Mechanical Fatigue Module can be used:
- Constant amplitude, proportional loading scenarios
- Non-constant amplitude proportional loading scenarios using rainflow cycle counting
- When it is acceptable to use Neuber’s plasticity correction to convert strains calculated in an elastic analysis into a plastic strain for use in a strain-life analysis
- When stress gradient notch corrections are not needed in a stress-life analysis
When to Ansys Ncode DesignLife is required:
- For multiaxial nonproportional loading, requiring a more advanced approach such as the critical plane method
- When it is desired to use a plasticity correction other than Neuber’s correction, such as Hoffman Seeger
- When it is desired to use the actual plastic strain calculated in an elastic-plastic simulation
- When it is desired to use the actual time history of the stress/strain
- When stress gradient notch corrections are necessary in a stress-life analysis