An Optimized Method for Thermographic Determination of Fatigue Limit in Commercially Pure Titanium
Fatigue limit is one of the most important properties of engineering materials.The long test time and the large number of specimens are required for the determination of the fatigue limit.The infrared thermography methods are developed to determine fa-tigue limit on basis of intrinsic dissipation.Two curves were utilized to interpolate experimental data by Luong,one for stresses below and the other for stresses above the fatigue limit,and the corresponding intersection was indicated as the fatigue limit.Whereas,it was not clearly specified how to recognize the experimental points below and above the fatigue limit in details,so that possibly failed in de-termining the reliable fatigue limit.Luong thermographic method was optimized to predict the fatigue limit.In order to illustrate the in-ternal mechanism of the above optimized method,the relationship was investigated between intrinsic dissipation and microstructure evolution in this paper.Commercially pure titanium TA 1 was investigated on fatigue limit,and the static tensile tests were performed to obtain stress-stain curve.In order to measure the fatigue limit by stair-case method,stress-controlled fatigue tests with a sinusoidal load were carried out on specimens.Thermography method based on stepwise loading procedure was used to rapidly obtain the fatigue limit.The temperature increment of the specimen surface was detected using an infrared camera during the fatigue test.Obviously,it was necessary to previously identify the border between the experimental data below and above the fatigue limit for Luong method.For conveniently practical applications,the elastic limit was proposed to divide the experimental data into two groups,one group for the elastic stress and the other for inelastic stress.To make clear the relationship between intrinsic dissipation and microstructure evolution from elastic strain to plastic strain,the microstructure of specimen subjected to different stresses loading was observed by the position-tracing method.Based on the above experiments,the following results were got:(1)In the use of the above optimized Luong method,the fatigue limit was determined as 323 MPa.It could be seen that compared with the fatigue limit 309 MPa obtained by the stair-case method,the corresponding result determined by above method was only 4.5％larger.(2)In observation of the microstructure evolu-tion,it was found that the microstructure of the commercially pure titanium TA1 specimens evolved from reversible motion(anelastici-ty)to irreversible motion(plasticity)under the action of the stress below 300 MPa(including 300 MPa).That corresponded to the transition of the intrinsic dissipation mechanism.
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