Fatigue Analysis
Fatigue, as it is used in the science of materials, refers to progressive structural damage of a material when exposed to extended cyclic vibrational loadings. Fatigue life refers to the maximum number of cyclic loadings a material can withstand before the structural damage occurs. In order to conduct fatigue analysis, one needs to have prerequisite knowledge of plastic analysis, advanced dynamic analysis and non-linear analysis.
One of the most important components of a design process is the analysis of the vibration modes of the design. Structures may be susceptible to vibration and cause unexpected, premature failure. Detailed fatigue analysis may determine the potential for failure where materials and structures are subjected to rapid cyclic vibrations.
Structures exposed to periodic seismic or wind loadings should be designed with cognisance of their natural frequencies and vibrational characteristics, often by utilising Finite Element Analysis (FEA).
Fatigue may also be due to other factors such as surface finish. A rough finish may cause microscopic stress concentrations that might lead to small cracks; in time, small cracks become big cracks. Temperature is another factor: high temperatures tend to decrease resistance against fatigue. Residual stresses caused by manufacturing methods such as welding and casting decrease fatigue strength as well. Changes in a part's diameter and cross section, such as recesses and notches, create stress concentrations where cracks may start. Of course, the very material that a part is made of has its own structural properties and these play a primary role.
An aircraft's service life is usually determined by the fuselage's fatigue life. The fuselage is cyclically pressurised and depressurised throughout the aircraft's life to maintain cabin pressures. The constant changes of pressure and the fuselage's reaction to it means that the aircraft may become prone to structural failure. Fatigue analysis projections at the design stage will set a maximum safe service life based on these calculations.