Excelling Fatigue Analysis in SolidWorks: Understanding Cyclic Loading and Fatigue Failure

Welcome back to our series on Finite Element Analysis (FEA) using SolidWorks! In today’s post, we’ll dive into the world of Fatigue Analysis and how to use it effectively in SolidWorks. By the end of this blog, you’ll understand what fatigue analysis is, why it matters, and how to perform it using SolidWorks. 

Let’s get started! 

What is Fatigue Analysis? 

Fatigue analysis evaluates whether a component will experience fatigue damage when subjected to varying loads over time. In real-world applications, structures and materials rarely experience a constant load. Instead, they are subjected to loads that vary, such as alternating forces, vibrations, or impacts. 

To understand fatigue analysis, it’s essential to differentiate between fatigue failure and fatigue damage: 

• Fatigue Damage: This occurs when microscopic cracks form on the surface of a material due to repeated loading. Over time, these cracks grow and eventually cause failure. 
• Fatigue Failure: A sudden and catastrophic fracture of a component after it has undergone a large number of cycles of varying loads, even if the applied stress is below the ultimate yield strength of the material. 

Why Does Fatigue Failure Occur? 

Fatigue failure is driven by the cyclic application of loads. As loads are repeatedly applied and removed, microscopic cracks form and grow. Eventually, these cracks cause the material to fracture. 

For example, imagine a steel shaft under repeated loading. Small cracks form over time, and eventually, the shaft breaks, even though the loads never exceeded the yield strength. This process is known as fatigue failure. 

To predict when a component might fail, engineers perform fatigue tests, applying cyclic loads and counting how many cycles the material can withstand before failure occurs. The results are plotted on an S-N curve (stress vs. number of cycles), which gives engineers a clear picture of the material’s fatigue life. 

The S-N Curve and Endurance Limit 

The S-N curve (also known as the Wöhler curve) is a critical tool in fatigue analysis. It plots the stress (S) on the Y-axis and the number of cycles to failure (N) on the X-axis. For a given material, the S-N curve shows how many cycles a component can endure before it fails under a particular stress level. 

Endurance Limit: For some materials (especially ferrous metals like steel), the S-N curve flattens out at lower stress levels. This flat portion is known as the endurance limit. It indicates the stress level below which the material can theoretically endure an infinite number of load cycles without failing. Materials like aluminum, however, do not have a true endurance limit, as the S-N curve does not flatten out. 

Applications of Fatigue Analysis 

Fatigue analysis is crucial in industries where components are subjected to repeated cyclic loads. Here are a few examples: 

1. Spinal Implants: These provide support to the spine and must endure repeated bending loads as a person moves. Fatigue analysis ensures the implants can withstand many cycles without failing. 
2. Turbine Blades: Turbine blades experience high levels of cyclic stress as they spin, making fatigue analysis critical to prevent catastrophic failure. 
3. Aircraft Wings: The wings of an airplane flex during takeoff, landing, and flight. Fatigue analysis ensures they can endure the repeated stress cycles they experience. 
4. Power Transmission Shafts: These shafts experience varying loads as they transmit mechanical power. Fatigue analysis ensures they won’t fail over time. 

Performing Fatigue Analysis in SolidWorks 

Let’s move on to the practical part: performing a fatigue analysis on an S-hook in SolidWorks. An S-hook is commonly found in chains and is subjected to cyclic loading when used. 

Step 1: Set Up the Model 

We start by loading the S-hook model into SolidWorks. The S-hook is a small, simple part, but it’s crucial in many applications, such as holding two ends of a chain together. 

1. Open the S-hook model in SolidWorks. 
2. Activate the Simulation Add-In if it’s not already enabled. 

Step 2: Perform a Static Analysis 

Before running a fatigue study, we need to perform a static analysis to determine the initial stress distribution. Here’s how to set it up: 

1. New Study: Create a Static Study and name it “Static Hook.” 
2. Material: Assign pure gold as the material (for demonstration purposes). 
3. Fixtures: Fix one end of the hook to prevent it from moving. 
4. Loads: Apply a 200 Newton force on the other end of the hook, simulating the load the S-hook will experience. 
5. Run the Static Analysis: This will give us the stress distribution, which will be used in the fatigue analysis.

Step 3: Set Up the Fatigue Analysis 

Now that we have the static stress results, let’s move on to the fatigue study. 

1. Create a New Fatigue Study: Right-click the static study and create a Fatigue Study. 
2. Select Fatigue Type: SolidWorks offers four different types of fatigue studies: 

• Constant Amplitude: The load remains constant in magnitude but alternates in direction. 
• Variable Amplitude: Both the load magnitude and direction vary over time. 
• Harmonic Fatigue: The load follows a sinusoidal pattern, gradually increasing and decreasing. 
• Random Vibration: Used for components subjected to random vibrations (e.g., aircraft parts or turbine blades). 

For this example, we’ll use the Constant Amplitude study. 

• Set Up the Load: Use the results from the static analysis and set the loading type to fully reversed, which means the load alternates between positive and negative values with a constant magnitude. 
• SN Data: Right-click the model and apply SN data. SolidWorks will automatically generate an SN curve based on the material’s elastic modulus. 

Understanding the Results 

Once the fatigue analysis is complete, you’ll get two key results: 

1. Damage: This result shows the areas of the component that have been most damaged by fatigue. In our case, the damage will likely be concentrated around the bend of the S-hook, where the stress is highest. 
2. Life: This result shows how many cycles the S-hook can endure before failure. For the S-hook in this analysis, the fatigue life is around 10 cycles under a 200 Newton load, indicating that it will fail quickly. 

Conclusion 

In this post, we explored the fundamentals of fatigue analysis, including how to perform a fatigue study using SolidWorks. We covered key concepts like fatigue failure, the S-N curve, and the endurance limit. 

In your next project, consider running a fatigue analysis to ensure your components can endure the loads they’ll face over time. If you’d like to practice this analysis, download the S-hook model provided and experiment with different loads and materials. 

Stay tuned for more advanced FEA topics in future posts! 

Happy simulating! 

This blog is part of our ongoing series on FEA Simulations using SolidWorks. If you missed the previous posts, check them out here.  

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