Fatigue Life Prediction Of Structures In HCF Region Under Complex Stress Field

Along with the rapid development in technology and economy, a higher leval requirement on the service life of the aircraft is proposed by countries in the world. In this situation the deformation of most force-bearing components of the aircraft under the fatiuge loading are predominantly elastic deformation (frequently defined as high-cylce fatigue), but the loading paths are often non-proportional. In addition, the presence of notches or other kind of discontinuity features in these components generate stress concentration, which leads to complex multiaxial stress field even when the externally applied load is uniaxial. However, the multiaxial high-cycle fatigue failure mechanism is not yet systematically established domestic and abroad up to now, and mulitaixal high-cycle failure analysis under the complex multiaxial stress field as well as fatigue life prediction have not come to perfection. Therefore, the in-depth study of multiaxial high-cycle fatigue behavior of metallic materials under complex loading plays an important part in developing advanced aircraft structural design theory.First, several common multiaxial high-cycle fatigue criteria are reviewed. In light of different macroscopic and mesoscopic scale phenomena of the fatigue failure, the different criteria are classified into three categories namely, approaches based on the stress invariants, critical plane approaches and mesoscopic integral approaches. According to the fitness analysis of five common group fatigue damage parameters under three typical cyclic loading cases, the shear stress amplimtude and the maximum normal stress acting on the critical plane prove to be damage parameters of multiaxial fatigue criteria more suitably. Several proportional and non-proportional tension-torsion fatigue tests were carried out on thin-walled tubular specimens made of LY12CZ aluminum alloy and 30CrMnSiA steel. Axial tension-compression, pure torsion, in-phase, 30°/45°/60°/90°out-of-phase loading paths were selected. The above experimental analysis also confirms the dependent relationship between these two damage parameters and fatigue life. Then a new multiaxial high-cycle fatigue criterion based on a nonlinear combination of these two damage parameters was proposed. The proposed criterion corrects the mean stress effect.and extends the applicable range for more metals. Five sets of multiaixal proportional and non-proportional experimental fatigue limit data published in the literatures are used to validate the proposed criterion. It is demonstrated that the proposed criterion gives satisfactory results for all five different kinds of steels including hard steel and mild steel. The capability of fatigue life prediction for the proposed model was checked against the three different kinds of metals including the above experimental data of LY12CZ aluminum alloy and 30CrMnSiA steel. The predicted results show good agreement with experimental results.Second, according to the early stage investigation of crack path in the presence of notches, a new equivalent critical distance method based on critical plane concept was proposed. A characteristic stress calculated at this equivalent critical distance on the critical plane was used to formalise the notch multiaxial high-cycle fatigue criterion in terms of Point Method and Line Method. And this method avoids the drawback of the independence between the notch geometry and the critical distance constant introduced by Taylor etc. The proposed criterion was applied to verify the uniaxial and multiaxial fatigue limit data obtained from three types of notched specimens with various notch geometry. It is demonstrated that the proposed criterion gives satisfactory results.Last, the traditional stress severity factor (SSF) approach is almost unable to obtain the SSF value under complex geometry and loading boundaries. An equivalent SSF approach based on the superposition of two orthogonal uniaxial stress fields in the region of the pin-hole was proposed. The equivalent SSF value is deduced by the force equilibrium equations on the finite width plate with a hole, thus the effects of complex geometry and loading boundaries are both considered. The fatigue life of a multi-fastener plate of an aircraft tank and typical bolt joints are analyzed and verification test is made as an example. The predicted fatigue life is in good agreement with the experimental results.