Low Cycle Fatigue vs. High Cycle Fatigue

Low Cycle Fatigue vs. High Cycle Fatigue

Fatigue is a very important problem in engineering design. And also, the calculations of fatigue for most materials are somewhat intriguing. So, these calculations are studied in two classes engineering; low cycle and high-cycle. Here we explain the low-cycle fatigue vs. high-cycle fatigue terms. 

Low cycle and high cycle fatigue endurance limits.
Endurance limit is very clear in this S-N curve example(Image Source: Researchgate.com).

What is the Low Cycle Fatigue?

Firstly, low cycle fatigue is regime where stress cycles are below 1000. There are lots of systems that show low-cycle behavior such as set screws on shafts, truck wheel studs, and automotive glove compartment latches. 

At the low cycle stress, the device will survive 1000 cycles. In this case, designers tend to design the parts in an infinite life approach. In the infinite life approach, stress levels are not high as any fatigue will occur. And the stress levels at low-cycle are much higher than the high-cycle. 

You Can Use the Static Design Approach

Furthermore, in the design of low-cycle fatigue, we can use a static load design approach. Static load design calculations are made by the consideration of the yield strength of the material where the maximum stress is observed without any elastic deformation. Also, safety factors are used in the static design of the components which will probably eliminate the low-cycle fatigue problem. 

High-Cycle Limits for Steel

Moreover, we defined the fatigue limits for steels well according to their maximum allowable stress values. We defined the limit between the low-cycle and high-cycle fatigue below.

The low-cycle fatigue limit for general types of steel is approximately 0.9 times of ultimate fracture strength in bending loading conditions. 

In the axial loading condition of the steel, the limit is defined as 0.75 times of ultimate fracture strength of the steel. 

This value is 0.70 times the ultimate fracture strength of steel in torsional loading. 

What is High Cycle Fatigue?

High cycle fatigue is the fatigue stresses of materials between the 10^3 and 10^7 stress cycles. Between these stress cycles, the stress levels lower below the yield strength of the material. 

High cycle is also classified as finite and infinite life of materials. Few materials are showing infinite life endurance limit stresses such as titanium alloys and ferrous materials. Below this endurance limit, the material shows any fatigue. 

There is no infinite life for any material. The stress levels below the endurance limits lead to failure of the material at gigacycles. Gigacycles are a very high cycle of use and there is no machine work in gigacycles. So, we consider gigacycles to be infinite life of the components and materials.

In the S-N curves of materials, the slope of the low is higher than the high-cycle fatigue


Finally, we can explain the general difference between the low-cycle fatigue vs. high cycle fatigue as above. 

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