Compressive Loading Effect And Overload Effect Of Aluminum Alloy Fatigue Crack Propagation

Aluminum alloy materials and structures in practical engineering environment,especially in the civil and military aircraft flight conditions, bear complextension, tension-compression under variable amplitude loading. Study onfatigue crack tip parameter, stress field, displacement field, plastic deformationnear the crack tip based on fracture mechanics, are the internal factors associatedwith fatigue crack growth rate. Under the complex loading conditions, thecompressive loading effect and overload effect exsist widely in the fatigue crackgrowth of aluminum alloy. The compressive loading effect denotes the promotingeffect of the compression loading on the fatigue crack growth of aluminum alloy.The overload effect denotes the overload retardation effect, delayed retardationeffect and no retardation after overload under tension-compression loading. Theexisting theories, such as the widely used Paris equation,crack closure theory andthe crack tip residual stress theory, howerver, can not give a unified theoryinterpretation and quantitative prediction for the compressive loading effect andthe overload effect effects of the fatigue crack growth of aluminum alloy undercomplex loading conditions. The analytical solution of crack tip parameter cannot been obtained using fracture mechanics and elastic-plastic mechanics methodis an effective method, the numerical solution of crack tip parameter can beobtained by the elastic plastic finite element method. In this thesis, the elasticplastic finite element modeling and incremental plasticity damage theory are usedto study on the mechanism of the compressive loading effect and the overloadeffect in aluminum alloy fatigue crack growth. The prediction models with thecompressive loading effect and the overload effect are established. The predictionmodels give the unified physical interpretations and mathematics forms. Thefinished work as follows:(1) The finite element models with static crack and dynamic crack were established respectively. The comparative analysis is performed for theparameters near the crack tip under the tension-tension loading and the tension-compression loading. The mechanism of the compressive loading effect isdiscussed. The change of the parameters near the crack tip is analyzed to denotethe mechanism of overload effect in aluminum alloy fatigue crack growth.(2) The two-parameter model reflect the relation of the reverse plastic zonenear the crack tip and the loading state，that is established by finite-elementanalysis plans. The aluminum alloy fatigue crack propagation rate consideringthe effect of the compressive load using the combination of the two-parametermodel of the reverse plastic zone and the incremental plastic damage theory isobtained. The Estimation method of the parameters of the tension-compressionincremental plastic damage model was developed based on the improved leastsquare method.(3) Based on the elastic plastic finite element modeling with an extended crack,the mathematical model of the plastic zone and the reverse plastic zone afteroverload are developed using the Willenborg’s residual stress theory. It wasestablished of the predicted model of the fatigue crack propagation under thetension-tension loading using the incremental plastic damage theory, named astension-tension overload incremental plastic damage model.(4) Based on the tension-tension overload incremental plastic damage model,the predicted model of the fatigue crack propagation considering the compressiveload effect under the tension-compression loading was established, using theincremental plastic damage theory, named as tension-compression overloadincremental plastic damage model.(5) The fatigue crack propagation experiments are performed in order to verifythe new predicting model of the thesis. The parameters of the tension-compression incremental plastic damage model were fitted using the improvedleast square method after the fatigue crack propagation experiments of the LY12-M aluminum alloy under the tension-compression fatigue load. The contrast ofthe predicted results and the experimental results denote that the predictionsagree well with the experimental observations and proved the rationality of themodel. Under the tension-tension and tension-compression loading the singleoverload fatigue crack propagation experiments are performed. The results indicate that the fatigue crack propagation goes through two phases of theretardation and delayed retardation. Under the tension-compression loading whenthe compressive load is relatively small the retardation is not obvious. While thecompressive load is relatively large the fatigue crack propagation goes through atransient accelerated propagation. The following propagation is the same as theone of the constant amplitude. The contrasts of the predicted results and theexperimental results under the tension-tension and tension-compression with asingle overload denote that the predictions agree well with the experimentalresults. The new predicted model succeed in averting to give danger predictionsof Paris law and Willenborg model due to not considering the compressiveloading effect and overload effect of delayed retardation.