| The engineering materials are widely used in the constructional engineering,aerospace,aviation,traffic and transportation,machine manufacturing,energy,chemical industry,and other fields,and they are needed to be serviced in the high temparature and ultra-high temperature?HT&UHT?conditions.Elastic modulus is one of the key mechanical properties.The accuate evaluation of HT&UHT modulus plays an important role in the temperature-resistant material developing,economy reasonable design and safe service of the high temperature components.However,in the research field of the evalution of the HT&UHT modulus,there are some urgent issues demanding prompt solution,e.g.,the measuring techniques for evaluating the HT&UHT elastic modulus of bulk materials,thick walled tube materials and coating materials are not yet established.In this dissertation,the testing methods for determining the HT&UHT modulus of the above three kinds of material products were proposed based on the relative method and the theory of material mechanics.Also the temperature dependence of elastic modulus of some typical engineering materials and its variation mechanisms were studied by the proposed methods.First,the HT&UHT deformations of the bulk materials were determined accurately by the relative method,and then the HT&UHT modulus of bulk materials can be obtained.i)The relative three-point bending method and relative four-point bending method?collectively called deflection correction method?were proposed to determine HT&UHT modulus of beam samples,based on the three-point bending test and four-point bending test.The systematic errors of loading system were deducted by the cross-beam displacement of the reference sample without supporting rollers.The real deflection of the beam samples can be tested indirectly by the cross-beam displacements of the bending sample and reference piece,and then the HT&UHT modulus of beam sample can be obtained by substituting the measured deflection into the derived calculation formulas.The test results of the elastic modulus of Al2O3ceramics from room temperature to 1300°C show that the measured temperature dependence of modulus tested by the impulse excitation technology?IET?and deflection correction method are similar,and the modulus measured by IET are slightly larger than that determined by deflection correction method?which is consistent with the results of the existing literature?.Thus,the validity and reliability of the deflection correction method is verified.Also the evolution of elastic modulus of C/C composites from room temperature to 1700°C were studied.The test results demonstrated the easiness and validity of the relative three-point bending method.ii)the reative compression method was presented to test HT&UHT compressive modulus of bulk materials,based on the compression test.The HT&UHT compressive deformation can be determined indirectly by the cross-beam displacements of the HT&UHT compressive test and reference test,and then the HT&UHT compressive modulus can be obtained.The room temperature compressive modulus of the porous corundum-mullite refractory were deternined by the inductance micrometer and relative compression method,respectively,and the two test results are slightly different with 2.57%relative deviation.The room temperature results demonstrate the validity and reliablity of the relative compression method,and the compressive modulus of corundum-mullite refractory from room temperature to 1300°C were measured by this new method.Second,to solve the test technical problems for evaluating HT&UHT modulus of thick walled tubes,the general split ring method and general closed ring method with wider application range were proposed based on the analytical method for curved bar,by taking into account the effect of the radial and tangential forces on the strain energy of the tube sample.The three-point bending method,split ring method and general split ring method,closed ring method and general closed ring method were all used to measure the elastic modulus and bending strength of the beam samples and tube samples made of the same quartzs glass,and the test results show that:i)For the elastic modulus testing by split ring compression test,the modulus of quartzs glass tested by split ring method and general split ring method are close when r/R>0.55?r is the inner radius and R is outer radius?,and the general split ring method should be used to measure the modulus of the ring samples with r/R?0.55.For the bending strength testing by split ring compression test,the bending strength measured by the general split ring method are more accurate?i.e.,more close to the bending strength evaluated by three-point bending method?when r/R?0.75.ii)For the elastic modulus testing by closed ring compression test,the modulus measured by the general closed ring method are more accurate when r/R?0.75.For the bending strength testing by closed ring compression test,bending strength measured by the general closed ring method are more accurate when r/R?0.70.The general split ring method,general closed ring method and three-point bending method were all used to determine the elastic modulus and bending strength of cement mortar beams and tubes at room temperature.The test results measured by the three methods are close,and this demonstrates that the general split ring method and general closed ring method are accurate and reliable to determine the mechanical properties of tube samples.Combined with the relative method,the relative general split ring method and relative general closed ring method were proposed to measure the HT&UHT elastic modulus of tube specimens.The elastic modulus of cement mortar tubes from-70°C to 800°C,the modulus of quartz glass tubes from room temperature to 1200°C,and the modulus of graphite ring pieces from room temperature to 2100°C were obtained by the experiments,and the correctness and good operability of the relative general split ring method and relative general closed ring method were verified by the experimental results.Finally,the three-point bending double relative method was proposed to test HT&UHT elastic modulus of the single-face and double-face coatings on the beam samples,and the split ring double relative method was presented to measure HT&UHT modulus of the outer-side,inner-side and double-side coatings on the tube samples.The three-point bending double relative method and split ring double relative method were both used to determine the elastic modulus of the chemical vapor deposition silicon carbide?CVD-SiC?coatings on graphite beams/tubes from room temperature to 2100°C.The test results show that the coating modulus tested by the two double relative method are similar,and the correctness and validity of the double relative method can be verified each other by the results measured by the two methods.Also the correctness and reliablity of the double relative method is verified by that the coating modulus evaluated by the inductance micrometer and double relative method are close at ambient temperature.The modulus of the attached coatings in situ tested by the double relative method is real and overall,and they are determined according to the macroscopic mechanical response parameters.So the measured modulus are different from the local modulus tested by indentation and the modulus of the free-standing coatings.The test accuracy of the double relative method depends on the cross-beam displacement without limit to the test temperature,so the double relative method can be used in HT&UHT environments.The elastic modulus of 8YSZ thermal barrier coating from room temperature to 800°C and the modulus of CVD-SiC coating from ambient temperature to 2100°C were tested by the double relative method,and their modulus evolution and variation mechanisms were studied.Moreover,the test methods for evaluating the bending strength of tube coatings were proposed for the three patterns of tube coatings,i.e.,the bending strength of the outer-side coatings was measured by split ring compression test,the bending strength of the inner-side coatings was determined by closed ring compression test,and the bending strength of the double-side coatings can be evaluated by split/close ring compression test.The bending strength of CVD-SiC coaing on graphite beam substrates was tested by the three-point bending test combined with the relative method,and the bending strength of the same CVD-SiC coatings on graphite tube substrates was measured by the proposed method in this dissertation.The measured bending strength of the tube CVD-SiC coatings is similar to that of the beam SiC coating,thus,the validity and reliability of the test method for determining the bending strength of tube coatings is verified. |
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