Study On The Experimental Techniques Of Fatigue And Fracture For Metal Materials Based On Digital Image Correlation

Fatigue is one of the main failure modes of stressed engineering structures.Failure caused by fatigue of structural components may cause catastrophic consequences.Therefore,the fatigue life prediction of metal components is of great significance in engineering applications.Specially,low cycle fatigue,thermo-mechanical fatigue and fatigue crack propagation are experimental methods for evaluating fatigue life of metal materials.Digital image correlation method is a non-contacting optical method which is widely used to directly measure the deformation and displacement of the surface of the stressed body with high accuracy and strong adaptability.In this dissertation,the advances of digital image correlation have been discussed and the optical extensometer has been applied to fatigue test of metal materials.The main contributions of the thesis are listed in the following sections:(1)Low-cycle fatigue test of thin steel sheet is difficult when using mechanical extensometer since requirement of gage length may cause buckling while the compressive load applied.To solve this problem,a buckling prevention device was designed,and a microscopic optical extensometer based on digital image correlation has been developed to measure the strain amplitude in low-cycle fatigue tests of the material.The optical system can output strain in real-time with novel searching scheme.(2)The mechanical properties of cast aluminum alloys at elevated temperature were investigated with experimental methods including uniaxial tensile properties,low-cycle fatigue properties,and high-temperature creep tests.The optical extensometer was used to monitor the dynamic strain during the experiments.The results show that the elastic modulus,yield strength,and ultimate strength of the cast alloy decrease with increasing temperature.In the low-cycle fatigue test,the fatigue strength coefficient decreases with increasing temperature,while the fatigue strength index increases.(3)The thermo-mechanical fatigue tests of cast aluminum alloys between 50-200?,50-250?,and 50-300 ? were evaluated,respectively.The prediction models were established based on different methods.The optimal model has been selected to accurately predict the thermal mechanical fatigue life of this material.(4)A practical tool for evaluating fatigue crack growth has been studied.The long focal distanceoptical microscope and displacement extensometer are replaced by the optical extensometer.The crack length is calculated based on the flexibility method,and the crack propagation rate is calculated with the incremental polynomial method.The relationship between the crack growth rate and variation of the stress intensity is accordingly resulted.The specific software has been developed and applied to the test of TRIP steel.The effectiveness of this method has been verified by comparison with the experimental results obtained by the mechanical extensometer.