Investigation On Effects Of The Mean Strain On Mechanical Behavior And Random Fatigue Life Prediction Of An Aerospace Aluminum Alloy

Aluminum alloy××××(Al-Mg-Cu) has been widely used in the field of aeronautics. With the increasing requirements for the speed and maneuverability of the aircraft in recent years, service conditions of the materials become more and more complex, generally there is large mean strain/stress in the aviation materials during their service. Due to the remarkable effect of the mean strain/stress on the low cycle fatigue life of materials, it is of important practical significance for the lifetime determination of new general aviation structures to study the low-cycle fatigue mechanical behavior of××××aluminum alloy under cyclic loadings with typical tension/compression mean strain (stress) as well as the related life prediction method.Two parts research work have been carried out in this paper under the support of the militarily industrial project under grant of GFJG-112108-E81002and NSFC project under grant of50975254.(1) Low-cycel fatigue tests of the aluminum alloy××××with four strain ratio (R=-l,-0.06,0.06,0.5) have been performed to study the effects of the mean strain on the cyclic hardening and softening characterizations, the relaxing behavior of the mean stress, cyclic stress and strain relations, strain-life curves, Masing properties, etc. It is shown that the cyclic stress response of the aluminum alloy is closely related to the applied mean strain. The alloy exhibits mainly the cyclic softening characteristics under symmetrical cycling, while the alloy presents the cyclic hardening followed by stability performance under the asymetrical cycling. It is also found that the aluminum alloy occurrs the relaxion of the mean stress under the asymetrical cyclic loading. The larger the mean stress relaxes, the larger the applied strain amplitude is. Whereas, the mean stress relaxes almost to zero when the applied strain amplitude is less than0.4%.It is further obtained that the relaxing behavior of the alloy under various strain ratio can be well described by Maxwell model. Based on Coffin-Manson model, the relationships between strain and life of the aluminum alloy have also been obtained for various strain ratios. It is shown that a double linear characteristic occurs for the plastic strain versus fatigue life and the the strain fatigue parameters such as the fatigue strength coefficient, σf’, fatigue strength exponent, b, fatigue ductile coefficient, εf’, fatigue ductile exponent, c, are related with the strain ration. In this study, the Masing property of the alloy has also been studied by using two methods. The results show that the aluminum alloy x x x x posseses Masing property at various strain ratios.(2) In terms of the local strain/stress method, an approach for predicting the fatigue life of notched components under random loading was studied, where the random loading spectrum was treated using Downing model, the local stress/strain histories were calculated using the united model, the fatigue damage was estimated using three damage models including Manson-Coffin model, SWT model and equivalent strain model, and the damage accumulation was treated using Miner’s rule. A program for this life prediction flow was further compiled. The random fatigue tests of two-type notched specimens of the aluminum alloy x x x x were performed to study the established random fatigue life prediction method. It is shown by comparing the test results with the prediction results that the equivalent strain model that takes into account the effct of both the mean strain and mean stress on the fatigue properties can obtain the best agreement prediction results with the tets life (the relative error between the prediction life and testing life is within50%). This model for fatigue damage calculation can be thus emphasized in the lifetime predition of the aeroplane structures.Microstructural changes of fatigue specimen were examined using a light optical microscopy (LOM) and a transmission electron microscopy (TEM). In the case of LOM, a slip phenomenon was observed for various fatigue conditions. While in the case of SEM, it was found that the cracks were initiated on the surface of specimen. With the increasing of strain ratio and strain amplitude it appeared multi-fatigue crack initiations and characteristic of brittle fracture.