Multiaxial Fatigue Life Prediction In HCF Including Non-proportional Hardening

Along with the rapid development in aviation, a higher level requirement on the service life of theaircraft is proposed by countries in the world. The deformations of most force-bearing components ofthe aircraft under the fatigue loading are predominantly elastic deformation, but the loading paths areoften non-proportional. These components designed by direct application of uniaxial fatigue theorygenerally can not meet fatigue strength design requirements. Therefore, the in-depth study ofmultiaxial high-cycle fatigue behavior of metallic materials under complex loading plays an importantrole in developing advanced aircraft structural design theories.First, this paper discussed the calculation of shear stress parameter of thin-walled tubular specimens.The shear stress refers to engineering shear stress, acting in the orthogonal tangential and axialdirection of the gage section. In the most of fatigue life prediction models, the maximum shear stresson the outer surface of the tube was chosen as the torsion stress. Based on fatigue mechanisms, a newparameter model considering supporting effect and local cyclic hardening was proposed. Fatigueexperiments of LY12CZ aluminum alloy specimens were carried out and the results were used tocheck the accuracy of shear stress calculations.Second, some commonly used multiaxial fatigue damage parameters under nonproportional loadingwere systemically reviewed. The damage parameters were classified into two groups, equivalentdamage parameter and direct damage parameter. Equivalent damage parameter derives frommultiaxial stress or strain with the additional cyclic hardening into account, and directs damageparameter deduced from multiaxial fatigue mechanisms.4kinds of parameters were tested with140groups of multiaxial fatigue test data from11kinds of materials.Last, based on micro concepts of dislocation slip under nonproportional loading, a newnonproportional coefficient considering the elects of the secondary slip bands on multiaxial fatiguelife was proposed. Two multiaxial fatigue assessment methods of Itoh’s and Susmel’s were modifiedby the new nonproportional coefficient. These modified assessment methods were tested with140groups of multiaxial fatigue test data from11kinds of materials.