What is Fatigue Life and Why Calculate It?

Fatigue life refers to the number of loading cycles a material or component can withstand before failure occurs. Unlike static failure, where a part breaks due to a single application of force exceeding its yield strength, fatigue failure happens over time under cyclic or fluctuating loads. This calculator utilizes Basquin's Equation to estimate the number of reversals to failure based on stress amplitude and material properties.

Understanding the Parameters

To use the Fatigue Life Calculator effectively, you need to understand the inputs:

• Stress Amplitude (σ_a): This is half the difference between the maximum and minimum stress in a cycle. It is the primary driver of fatigue damage.
• Fatigue Strength Coefficient (σ'_f): A material property roughly equal to the true fracture stress, representing the intercept of the S-N curve.
• Basquin Exponent (b): Also known as the fatigue strength exponent, typically ranging from -0.05 to -0.12 for metals.
• Mean Stress (σ_m): The average stress in a cycle. If not zero, it can significantly accelerate fatigue (accounted for here using the Morrow correction).

How to Use This Tool

1. Enter the Stress Amplitude experienced by your part in Megapascals (MPa).
2. Input the Fatigue Strength Coefficient specific to your material.
3. Provide the Basquin Exponent (usually a small negative decimal).
4. Click "Calculate" to see the estimated number of cycles before the material is expected to fail. If the result is extremely high (e.g., > 10⁷), the material may be operating within its endurance limit.

Frequently Asked Questions

What is the S-N Curve?

The S-N curve (Stress vs. Number of cycles) is a graphical representation of the fatigue life of a material. It helps engineers determine the maximum stress level a part can handle for a specific service life.

How does Mean Stress affect fatigue?

Tensile mean stress (positive) reduces fatigue life by pulling the atomic structure apart, while compressive mean stress (negative) can actually increase fatigue life by keeping cracks closed.