Aircrafts experience variable amplitude loading. The load cycle is called GAG (Ground-Air-Ground) cycle. GAG cycle follow standard load sequence (also called Standard load-time histories). Standardised load-time histories (SLH) were developed in Europe and US to help Aerospace Industries with appropriate load sequence. Two well known SLH used in the Aerospace Industry are TWIST (Transport WIng STandard) and FALSTAFF (Fighter Aircraft Loading STAndard For Fatigue evaluation). The load sequence is generation is deliberately avoided in this website as it is the sensitive information to be put in the public domain. For more details, refer Ref [1].
The load sources can be Gust, Maneuvre, Ground loads (Pre flight and Post flight load scenarios).
For the general calculations, load or stress spectra needs to be constructed by the analyst. Some best practices are detailed below.
Structures experiencing only the shear load (Shear panels for example) needs to be analysed at most care. The shear stresses irrespective of their sign cause the same damage. Therefore, it is necessary to take either the absolute value of shear for the analysis.
The components such as lug behave differently under tension and compression. The damage for the lug is caused mostly by the in-plane loads. The net section of the lug carry the tension load. However, the compression load is transferred as bearing and therefore, the direct compression loads does not cause fatigue damage in the lugs.
Word of Caution: The transverse loads on the lug irrespective of the direction cause same amount of damage, therefore, the negative loads shall not be ignored. It is necessary to consider the negative loads without sign in such scenarios.
Components such as bolts subjected to bending stresses can damage at one of the extreme fibres depending on the magnitude of the tension/compression loads. If the negative loads are dominant, then it is necessary to reverse the load spectra.
The shear loads also play an important role in the fatigue damage. The shear loads needs to be treated irrespective of the direction of the shear. Therefore, it is advisable to consider the absolute shear loads for the fatigue analysis when the component is shear critical. Analysis considering pure shear must account Kt=4.0 due to superposition of the Kt from the Mohr circle.
The joints experiencing the shear loads distribute equal loads on all the fasteners unlike the joints transferring the tensile loads. Therefore, care must be taken when analyzing such joints.
[1]. Techniques to Generate and Optimise the loads Spectra for an Aircraft, S0502_P0285, 3rd International Conference on Integrity, Reliability and Failure, Porto/Portugal, 20-24 July 2009, Krishna Lok Singh and D. V. Venkatasubramanyam
PRACTICAL FATIGUE AND DAMAGE TOLERANCE 