In mechanical design, fatigue has a very important place. Because of the fatigue, unexpected cracks and failures can occur with stress levels that are lower than the yield stress. A very big amount of research and development activities are given to mechanical parts to prevent or estimate the fatigue behavior of a part. Here we explain the general aspects of fatigue and the place of fatigue in mechanical design.
What is Fatigue?
Fatigue is the development of internal cracks and defects through time with the application of cyclic stresses which are generally lower than the yield stresses of materials. In the early technologies such as around the industrial revolution, different kinds of metals and steel are produced and used in different structural applications. But in most applications, structural parts are deteriorated because of unknown events. A metal bridge suddenly deteriorates by a very low load such as a passing of a small car without giving any warning.
After these events, fatigue mechanics started to improve, and different fatigue and crack growth estimations are developed.
Important Points of Fatigue in Mechanical Systems
There are lots of theoretical and experimental applications to estimate the fatigue life of an engineering system. But before starting to understand these fatigue approaches, we need to understand some important phenomena about fatigue.
First of all, there are no universal or general theorems or calculations of fatigue to calculate the exact crack failure of the designed systems. In general, these estimations are dependent upon the experimental studies.
Fatigue in Ductile and Brittle Materials
Fatigue failures are much more important for the brittle materials than the ductile materials. Because fatigue failure takes place abruptly for brittle materials without giving any warnings. But for ductile materials, there are some warning signs that fatigue would occur. Preliminary deformations and cracks are showing the imminence of the fatigue growth in ductile materials.
Fatigue for Different Materials
For different materials, the fatigue design approach differs. There are experimental methods developed for cardol steel materials but these experimental methods do not apply to other materials.
Because different materials have different fatigue properties.
Stress cycles and the number of stress cycles are very important parameters in fatigue crack and fatigue failure. According to the stress that is applied to the part, the number of cycles that will fatigue occurs will decrease. So, engineers are trying to estimate which cycle of loadings causes fatigue cracks for different materials.
Propagation of Cracks in Fatigue Failuıre
In general mechanical fatigue incidents, crack propagation starts from the surface of the material. Possible surface defects are the starting points of the cracks and incisions which are the reason for fatigue cracks. So, surface characteristics are very important in fatigue propagation.
How to Improve Fatigue Performance of Materials and Parts?
In the design phase of fatigue, there are different approaches are applied to prolong the fatigue life of the design. These applications are;
- Improving the total length of the crack in which fatigue failure occurs,
- Improving the total time that fatigue crack occurs,
- Slowing down the fatigue crack propagation time,
- Improving the surface characteristics of parts to prevent fatigue failure.
By using different methods, the fatigue performance of the developed parts and systems can be improved.
Total Lenght of the Crack
The total length of the crack where fatigue fracture takes place is a very important parameter in the fatigue design. In general, materials that have the best fracture toughness values are the most durable materials to fatigue fracture. If the fracture toughness is high, the feature of withstanding failures when there is a crack improves. So, there is a direct relationship between the total length of the crack and the fracture toughness of materials.
Crack Occurance Time
To improve the fatigue performance of a part or material, the occurrence time of the fatigue crack is also a very important parameter. If the crack occurrence time is long, the fatigue performance will be better. To minimize the crack occurrence, compressive surface residual stresses are applied by applying different processes such as shot peening. Small metal beads are thrown to the surfaces of the mechanical parts to impart surface residual stresses to improve the total tensile stresses required to crack propagation.
Crack Propagation Time
This is not about the crack occurrence. This time is about the crack propagation time after the crack occurs. To improve the crack propagation time, surface characteristics are improved. To prevent the propagation of the cracks, the grain boundaries are produced in a parallel direction to the surface of the part. So, grain boundaries are used as barriers to the crack growth paths.
The surface characteristics of the material have a very important effect on fatigue performance. If the surface characteristics such as smoothness of the surface are improved, the possibility of crack propagation is prevented. Extra processes are applied to improve the surface characteristics such as grinding and polishing.
Cyclic Stresses in Fatigue Calculations
The situation of the cyclic stresses in mechanical fatigue is a very important factor. Different types of stresses can occur and cause fatigue on material or part. These stresses are; axial stresses which are tensile and compressive, bending stresses, and torsional stresses.
To see the cyclic stress situations of a mechanically loaded system, you can analyze the general situation of cyclic stresses by using this cyclic stress calculator.
The cyclic application of these stresses can cause fatigue situations in materials and parts. In different mechanical applications, cyclic applications of these stresses can occur on materials.
Experimental and Theoretical Methods to Calculate Fatigue Strength
There are various theories are developed to calculate the fatigue strength of different materials. But these theories generally can not be generalized to further calculations of different materials with different surface characteristics. Because of this reason, fatigue calculations are depending on much more experiment-based results. Also, the development of possible general fatigue experiments is a very hard thing. In general engineering applications, engineers are considering the worst-case scenarios and they produce their designs according to these worst-case scenario. But the worst-case scenario in the fatigue is the worst condition where crack occurs and fatigue fracture takes place. So in only one cycle of loading, fatigue fracture takes place.
There are several fatigue strength tests are developed. One of them is Moore’s machine for the calculation of fatigue strength. In this machine, a specific geometry of cylindrical test specimen is prepared and loaded with the pure bending condition. For different surface characteristics and loading conditions, lots of kinds of experiments are made to obtain very strict data that shows the material’s specific fatigue strength. By using this data, engineers can design parts or systems that have specific fatigue strength and fatigue life.
According to the manufacturing techniques that are used to produce a specific part, the average and worst surface characteristics where the crack initiators would occur are used as test surfaces. The different loading conditions are also specified according to the application.
Fatigue Growth at Microstructural Level
At the microstructural level, there are different microstructural fatigue crack growth behaviors that occur in the ongoing time to the fatigue failure. Near the origin of the fatigue crack, fatigue crack occurrence is very slow according to the surface finish on the part. When fatigue cracks are developed over time, the development of cracks are being faster with the ongoing time. Visible striations can be seen in a typical fatigue crack after the fatigue fracture takes place. When the required amount of striations occurs, an abrupt brittle fatigue fracture takes place.
The S-N Curves in Fatigue
S-N curves are very important plots of the fatigue of the materials. The S-N curves are curves that show in which stress level fatigue failure would occur. S is the stress level and N is the number of cycles that which this stress is applied.
There are different types of S-N curves. In the first type, there is a limit that shows the fatigue limit of the material. Stress levels below this fatigue limit value, there is no fatigue failure for that material in theory. Some materials are not showing fatigue limits. There is always fatigue failure that occurs for these materials at every stress level.
Also, there are other forms of fatigue failure such as corrosion fatigue and thermal fatigue. But we are dealing with mechanical applications and others are out of our scope here.
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