Fatigue Crack
Fatigue is a fundamental and ubiquitous structural problem, especially in aircraft and railroad structures, as well as in many components that are exposed to repetitive loading. In aircraft fuselage and wings, this problem is further complicated by the properties of aluminum and its alloys, which are the most suitable materials for these applications.
Fatigue is the condition of a material that results from the accumulation of cyclic stresses induced from service loads. The fatigue process involves the formation and growth of microscopic cracks in the material. These cracks can grow until the point of failure due to a lack of material remaining in the region between the cracks. Fatigue crack growth is one of the main failure mechanisms of structural metals.
Fatigue cracks can form from many sources, including improper stress relief and thermal treatments, overloads, and manufacturing irregularities like casting flaws. Once initiated, fatigue cracking can spread slowly or quickly, depending on the frequency, duration, and magnitude of the applied loading, as well as on other factors such as the material, stresses, environment, and internal defects in the material itself. If left unchecked, it can lead to catastrophic failure of an otherwise safe structure or component.
To minimize the risk of fatigue failure, components and structures must be designed with an adequate fatigue margin. This involves predicting the fatigue life of a structure by accounting for the effects of cyclic loading, and incorporating these effects into the design process. Fatigue crack growth can be predicted using finite element analysis, crack propagation models, and tests which use experimental samples which have been subjected to cycles of controlled loadings.
The most reliable way to measure fatigue failure of a structure is by performing a test in which the material is subjected to a cyclic load that is representative of actual conditions until a specific level of cracking has been reached. This can be done using either a full-scale structure or an appropriately sized laboratory specimen. Testing is expensive and time-consuming, but provides the most reliable data on fatigue behavior and crack growth.
Fatigue testing has become an important part of new designs, and existing components and structures on aircraft, railroads, and other applications must also be periodically inspected to detect any fatigue cracks. Visual inspection, low energy X-ray, and eddy current methods are all well-established methods used to detect fatigue cracks in metals. These methods can quickly provide information on the presence and location of cracks in a structure, which can then be evaluated to determine if further investigations or repairs are needed.
Fatigue cracks can cause catastrophic failure of a structure or component if they are not properly identified and addressed. By designing with a proper fatigue margin, performing regular inspections, and monitoring fatigue performance with laboratory testing, the catastrophic effects of fatigue cracking can be greatly reduced.
