Study On The Coupled Creep-fatigue Characteristic Of Aluminum Alloy And The Application In Life Prediction Of Diesel Engine Piston

Aluminum alloys have been widely used in diesel engine pistons due to the highspecific strength, low density and good casting properties. In recent years, the maximalcombustion pressure and gas temperature in the combustion chamber has been increasedsignificantly with the increasing power density and rostating speed of diesel engines, whichaggravates the loading situation of aluminum alloys. Under the severe loading condition,creep damage will be induced in the aluminum alloy and interacts with the fatigue damagenonlinearly and complexly, leading to coupled creep-fatigue failure of materials.The present work studies the coupled creep-fatigue behaviors of aluminum alloy basedon experimental test, theoretical derivation and numerical simulation. The main purpose ofthe study is to analyze the characteristics of creep deformation and fatigue behavior, and theinfluence of temperature and load on the cyclic deformation and fatigue behavior ofaluminum alloy. Besides, it is intended to confirm the microcosmic damage mechanism andmicrostructure evolution process of aluminum alloy under fatigue loading, with theassistance of metallography observation and fracture surface analysis. Moreover, by usingthe damage analysis method of contimuum damage mechanics, a creep-fatigue lifeprediction model will be developed, which accumulates the creep damage and fatiguedamage nonlinearly. Finally, based on the model and experimental data, the creep-fatiguelife prediction model will be applied in diesel engine piston under the load-changingsituations.The main research contents and conclusions are as follows.(1) Research on the creep-fatigue behavior of aluminum alloyMechanical tests of the aluminum alloy under uniaxial loading are conducted and theresults show that the ultimate tensile strength at high temperature decreased significantly,indicating that the mechanical strength is greatly influenced by temperature. Besides, theinfluence of temperature and laod on creep deformation is also analyzed by conductingcreep tests, and the numerical simulation of creep deformation is carried out throughsecondary development in ABAQUS.The high temperature low cycle fatigue tests are conducted, and the cyclic softeningbehavior of the aluminum alloy is detected. Besides, the degree of softening behaviorincreases as temperature increases. Under the tested temperatures, the fatigue life of the studied aluminum alloy increases as temperature increases. Furthermore, the correlationbetween plastic strain energy and fatigue life is investigated through Halford-Marrowmodel, and it shows that the ability of the aluminum alloy to absorb and dissipate strainenergy increases with increasing temperature.(2) Analysis on the microcosmic mechanics of fracture and damageThe analysis of fracture surfaces, microstructure evolution and damage cumulation hasbeen conducted by metallography observations. It is found that there are numerious tearedges and intercrystalline micro cracks on the fracture surfaces, which demonstrate theinteraction between creep damage and fatigue damage. Besides, as temperature increases,the number of tear edges and intercrystalline micro cracks increases significantly.Furthermore, these tear edges and micro crack are of benefit to decrease the stress intensityand crack propagation, which can be used to explain the increased fatigue life at highertemperature from the microcosmic perspective.(3) Construction of the creep-fatigue life prediction modelBased on the continuum damage mechanics, the creep damage and fatigue dmage areaccumulated nonlinearly and the creep-fatigue life prediction model has been constructed.Besides, the model parameters are calibrated by test data. Then, the creep-fatigue life arepredicted with the proposed model. All the predictions are within the scater band of factor2,which demonstrates the accuracy of the proposed life prediction model.(4) Creep-fatigue life prediction of diesel engine piston under load-changing conditionConsidering the characteristic of cyclic load under alternate engine loads, acreep-fatigue life prediction procedure has been proposed. Then, taking as an example, thecreep-fatigue life of diesel engine piston under alternate loads of rated loading conditionand maximal torsion loading condition is predicted. It is found that the fatigue life of pistonis1388and the fatigue failure is proned to occur in the throat edge of the piston.The present research has investigated the coupled creep-fatigue characteristic ofaluminum alloy, by the mechanical strength, creep and fatigue properties analysis and at thesame time with the assistance of microcosmic microstructure observations and coupleddamage analysis. Then, a creep-fatigue life prediction model has been constructed andapplied in the life prediction of engine piston. In the research, an analysis methodology forcreep-fatigue life prediction, including material properties analysis and practicalengineering application, has been proposed, which is of value for assessing thecreep-fatigue reliability of components.