What is High Cycle Fatigue?
The repetitive and cyclic use of engineering materials and metals can often lead to a degradation of the material, otherwise referred to as “fatigue.” The failure of many mechanical components of large equipment can often be attributed to such material fatigue. Equipment managers are able to recognize fatigue by visible deformations to the equipment including cracks and fractures. There are three types of recognized material fatigue, of which include thermal mechanical fatigue (TMF), low cycle fatigue (LCF) and high cycle fatigue (HCF)1. HCF is characterized by elastic strains that exhibit low amplitude and high frequency, often which occurs following repetitive bending of the material.
When designing industrial equipment, researchers take several different approaches to ensure the overall safety of the equipment, as well as its ability to withstand fatigue and subsequent tragedies and loss of revenue that can occur when a piece of equipment fails. Such designs are based on material testing methods that measure stress life (SN), strain life (EN) and linear elastic fracture mechanics (LEFM) of the material through computer-aided engineering programs.
The Role of Cylinder Heads
Within internal combustion engines, cylinder heads, which are typically located at the top of the engine block, play an important role in the function of the overall system as it houses components such as intake and exhaust valves, springs and lifters, as well as the combustion chamber. The three types of cylinder heads include flathead, overhead valve and overhead camshaft cylinder heads, each of which can be made of cast-iron or aluminum material, depending upon the specific system in which they are being used in.
As a result of their continuous exposure to high thermo-mechanical stresses present in their working environment, cylinder heads are highly susceptible to cracks as a result of engine overheating. Cracks in cylinder heads can lead to leaked coolants, leading to failure of the engine to cool efficiently and subsequent overheating.2
Oil can also leak into the combustion chambers when cracks arise in cylinder heads. Such fatigue symptoms that are seen in overheated engines can lead to costly repairs for the engine, therefore improving the designs of cylinder heads to prevent such detrimental material defects are essential to ensure their optimal performance.
Current Efforts to Improve Fatigue in Cylinder Heads
A recent research effort supported by the Ford Motor Company analyzed current high-cycle fatigue tests in engines in an effort to improve the ability of cylinder heads to resist residual stresses. To do this, the analysis procedure stimulated a heat treatment of the engine to calculate the residual stresses that occur in the cylinder head under these conditions. A redistribution of the stress followed by increasing the machine simulation. Following these preliminary treatments, the cylinder head was then assembled directly into the engine and subjected to the physical operation loads that are found in a typical engine usage.
In addition to calculating the fatigue factors of the engine, the specific properties of the aluminum material in which the cylinder head was made of was also considered to fully understand its susceptibility to high cycle fatigue damage3.
The results presented in this study demonstrate an effective method to fully evaluate the ability of cylinder heads that are used in engine applications, such as for automobiles and industrial equipment, provide an adequate means of assessment for future design purposes. Further research in this area of equipment engineering and design is imperative in order to discover new materials that can withstand the high pressure and temperatures that these cylinder heads are exposed to during each use.
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- “Design to Prevent Fatigue” – Solid Works
- “What is a Cylinder Head?” – Buy Auto Parts
- “Cylinder Head Design Process to Improve High Cycle Fatigue Performance” Chen, X., Brewer, T., et al. SAE Technical Paper. (2017). DOI: 10.4271/2017-01-1074.