Fatigue and Fracture
Fatigue and Fracture
Introduction
Fatigue is a type of fracture, which occurs in materials subjected to varying and changing stress over time. This is a key problem in modern devices and machines that are being used today. It is mainly caused by the environment where the materials are being utilized. Materials that have repeated loading and unloading tend to experience fatigue. Cracks start forming on the surface if the load is above a particular threshold. With time, the crack will continue expanding and reach a critical size where the structure will suddenly fracture. Failure occurs at lower stress because of fatigue (Nikbin, K & Farahmand, B 2008). Fatigue is estimated to comprise of approximately ninety percent of the metallic failures.
Fatigue and fracture
For fatigue and fracture to happen, there is a portion of time varying stress that should be tensile in nature. The resistance of material fatigue involves implementing a well characterized strain or stress cycle. The resistance of a material fatigue can also be affected by the frequency of the stress that has been used.
In order to design against fatigue failure, thorough education and supervised experienced is required in material engineering. Approaches to life assurance of mechanical parts, which show increased degree in sophistication, have three principles that can be used. The first is designing to have stress below the threshold of fatigue limit (Gjonaj, M 2000). The second is instructing the user to ensure periodical inspection for cracks replacing the crack once it reaches a critical length. The third principle is designing for a fixed life where the user is instructed to replace the structure with a new one.
Cracks of fatigue that have started propagating can be stopped through drilling holes in the fatigue crack path. However, this practice is not generally recommended because the hole tends to represent a factor of stress concentration that depend on the geometry and the size of the hole. A change of the material that is used in part can also help in improving the life of the fatigue. Replacement and redesigning of the parts are techniques that can also be used in reducing the fatigue problem if not eliminating it totally. For example, composite equivalents are replacing the helicopter rotor blades and the propellers in metals. The equivalents are lighter, and they are also much resistant to fatigue.
Factors to consider
In order to prevent fatigue and fractures, it is significant to consider material behavior, service loads, and critical stresses when designing a machine. When analyzing the service loads, careful evaluation should be done by the engineer. The cycle load histogram of the machine can be used to represent the factors. This should represent the environment, conditions, and the malfunctions that are expected during the service life. It is necessary to determine the vibration cycles and the various loads for each component of the machine during the expected life of the machine. The designer should draw a histogram on the load on the data that is available and the past experience.
Once a histogram is drawn, preliminary design is made where computation of critical fatigue stress is done (Rolfe, S & Barsom, J 1999). Factors that should be taken into consideration include all the stress concentration that might exist at the point in question. The second factor that should be considered is the state of stress. That is whether the stress is pulsating or if it is a superimposed alternating stress on a steady stress. The effect of the stress inadvertently introduced in the assembly is a factor that should be considered. The last factor to be considered is the effect of the manufacturing tolerance on stress.
In order to prevent fatigue and fracture, material behavior is an essential factor to put in consideration. When determining if a material will fail under stress, it is essential to have knowledge on the material behavior under all environmental conditions. That may affect fatigue inducing stresses. One of the factors that many designers have overlooked that can reduce fatigue strength is galling or fretting at riveted, bolted, or press fit joints that are subjected to alternating loads (Paris, C & Jerina, K 2000). Making the joint to be tighter is expected to improve its strength. Galling or fretting is the only condition which helps in reducing a material par value. For accurate assessment of the strength of the fatigue, it is advisable to have a fatigue test of the entire structure.
Conclusion
From the analysis on fatigue failures, it has been determined that most of the failures are usually caused by the factors that are not related to the inbuilt fatigue strength of the material. In order to prevent fatigue and fracture, it is essential to make sure that there is constant inspection of the material. This will make it possible for one to spot parts that have been cracked and replace them fast before the crack gets critical.
Reference
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