Low Cycle Fatigue vs. High Cycle Fatigue: Key Differences

This article explores the differences between low cycle fatigue and high cycle fatigue, two key concepts in understanding material failure under dynamic loading.

What is Fatigue?

Introduction:

Fatigue is the phenomenon of failure occurring due to dynamic and fluctuating loads. This type of failure has become increasingly prevalent due to the widespread use of complex machinery and structures in various applications like bridges, automobiles, pumps, and turbines. In fact, it’s estimated that fatigue accounts for about 90% of all service failures caused by mechanical issues.

Fatigue failures typically occur after extended periods of repeated stress or strain cycles. Key factors contributing to fatigue failure include:

• High maximum tensile stress
• Large variations in applied stress
• A sufficiently high number of applied stress cycles

Fatigue behavior is generally categorized into two main domains: low cycle fatigue and high cycle fatigue.

Low Cycle Fatigue

Here are the key characteristics of low cycle fatigue:

• High Loads: Low cycle fatigue is associated with relatively high applied loads.
• Elastic and Plastic Strain: During each cycle, the material experiences both elastic and plastic strain.
• Short Fatigue Life: This type of fatigue has a relatively short lifespan.
• Cycle Range: Low cycle fatigue typically occurs at less than 10^4^ to 10^5^ cycles (N < 10^4^ to 10^5^ cycles).

High Cycle Fatigue

Here are the key characteristics of high cycle fatigue:

• Lower Stress Levels: High cycle fatigue is associated with lower stress levels compared to low cycle fatigue.
• Elastic Deformation: In high cycle fatigue, the deformations are predominantly elastic.
• Longer Fatigue Life: This type of fatigue generally has a much longer lifespan.
• Large Number of Cycles: A large number of cycles are needed to induce fatigue failure in high cycle fatigue.
• Cycle Range: High cycle fatigue is associated with fatigue lives greater than 10^4^ to 10^5^ cycles (N > 10^4^ to 10^5^ cycles).

In summary, low cycle fatigue involves high loads and plastic deformation, leading to failure in a relatively small number of cycles, while high cycle fatigue involves lower stresses and primarily elastic deformation, leading to failure after a large number of cycles. Understanding the differences between these two types of fatigue is crucial in designing robust and durable mechanical components and structures.