Investigation Of The Size Effect With Regard To The Fatigue Strength Of Typical Al Alloy Mechanical Elements In Aeronautical Structures

As a consequence of the increasing size of modern aircraft designs, the size effectimpacting structural material, i.e. the decrease in material strength for larger scales, hasa growing importance in the aeronautical industry. In this paper, two mechanicalelements representative of common aeronautical structures-square panels with a centeropen-hole and lugs made of Al alloy2024T351-were put to fatigue tests, and theirfatigue strength at105cycles was obtained to investigate the size effect. The analysis ofthe results shows that one part of the samples exhibits a regular size effect, and byfitting the data with the empirical formulas we can formulate promising fatigue criteria.The other part of the fatigue strength results shows a reversed size effect, i.e. increasingfatigue strength with increasing scales. Various factors are identified for the appearanceof the reversed size effect, with special emphasis on the characteristics of the crackpropagation. Some of the samples were put to SEM observation, showing that all thefatigue cracks were originated from the point with heaviest stress concentration, themanufacturing edges and the surface defect points introduced by fretting corrosion.Using experimental data from the tests carried out on the project and thoseprovided in the literature, data fitting was implemented according to different modelstaking into account the size effect, with special attention to the stress gradient. Severalfatigue criteria were built for open-hole specimens and lug specimens. Among thesemodels, the power law model, Peterson’s and Neuber’s model yield promising results inpredicting fatigue strength on mono-dimensional curve fitting problems at a constantstress concentration factor. Meanwhile, the critical distance theory and other2-Dmodels fail to predict the results with a satisfying accuracy, due to the limited size of thedatabase and the significant scatter of the results. Moreover, only the power law modeland1-D Peterson’s model remain consistent with the reversed size effect.Bushes are strongly recommended when the lug connector is considered in the realapplication to avoid fretting corrosion. In addition, it is suggested that only the regularsize effect should be considered in the process of designing large-scale aircraftstructures, since it provides a safe correction on the fatigue strength.