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基于相对法技术评价工程材料高温与超高温弹性模量
中文摘要

工程材料广泛应用于建筑工程、航空航天、交通运输、机械制造、能源化工等领域,均涉及到材料在高温与超高温(HT&UHT)环境下的服役,弹性模量是重要的力学性能参数之一,准确地测试评价工程材料的HT&UHT弹性模量对高温材料的研制、高温构件的经济合理设计与安全服役至关重要。然而,在材料HT&UHT弹性模量测试技术研究领域,仍存在一些亟待解决的问题,例如:尚未建立块体材料、厚壁管材与涂层材料HT&UHT弹性模量的测试评价技术。本文基于相对法技术和材料力学理论,针对以上三种材料制品形式,提出了HT&UHT弹性模量测试方法,并基于此研究了一些典型工程材料弹性模量—温度的演变规律及其变化机制。 首先,本文利用相对法技术实现了块体材料HT&UHT变形的准确测量,继而实现了块体材料HT&UHT模量的测试评价。i)结合三点弯曲试验与四点弯曲试验,提出了相对三点弯曲法与相对四点弯曲法(统称为挠度修正法),即将去除下支承辊的梁试样作为参比试样,利用参比试样的横梁位移增量扣除试验机的系统误差,然后通过弯曲试样与参比试样的横梁位移间接测得梁试样的真实挠度变形,代入所推导的计算公式可得梁试样的HT&UHT弹性模量。氧化铝陶瓷室温~1300℃弹性模量测试结果表明,脉冲激励技术(IET)与挠度修正法测得的弹性模量—温度变化曲线基本一致;且IET测试结果较挠度修正法测试结果略大(符合文献报道),由此证明了挠度修正法的正确性与可靠性。利用相对三点弯曲法对C/C复合材料室温~1700 ℃弹性模量的演变规律进行了研究,表明了相对三点弯曲法的易行性与有效性。ii)结合压缩试验,提出了相对压缩法,通过高温压缩试验与高温参比试验的横梁位移增量间接测得块体试样的高温压缩变形,继而可得其高温压缩模量。室温下利用电感量仪测得的多孔刚玉—莫来石质耐火材料压缩模量与相对压缩法的测试结果相近,二者仅相差2.57%,由此证明了相对压缩法的正确性与可靠性。继而利用了相对压缩法测得了多孔刚玉—莫来石质耐火材料室温~1300 ℃压缩模量。 其次,针对厚壁管材HT&UHT弹性模量的测试技术难题,本文将径向力与切向力对管材试样变形能的影响考虑在内,基于曲杆分析法,提出了适用范围更广的广义缺口环法与广义闭口环法。分别利用三点弯曲法、缺口环法与广义缺口环法、闭口环法与广义闭口环法测得了同一批次石英玻璃梁试样与管材试样的弹性模量与弯曲强度,测试结果表明:i)对于缺口环压缩试验,测试弹性模量时,在r/R>0.55(r为内半径,R为外半径)的范围内,缺口环法与广义缺口环法测得的模量值相近;而当r/R≤0.55,须利用广义缺口环法的测试环试样的弹性模量;测试弯曲强度时,当r/R≤0.75,广义缺口环法测得的弯曲强度更加准确(与三点弯曲法测试结果更相近)。ii)闭口环压缩试验测试弹性模量时,当r/R≤0.75,广义闭口环法的模量测试值更准确;测试弯曲强度时,当r/R≤0.70,广义闭口环法测试结果更准确。 利用广义缺口环法、广义闭口环法、三点弯曲法分别测得了水泥砂浆梁试样与管材试样室温弹性模量与弯曲强度,试验结果表明三者测试结果相近,由此证明了广义缺口环法与广义闭口环法测试管材试样力学性能的准确性与可靠性。进一步结合了相对法技术,提出广义缺口环相对法与广义闭口环相对法,实现了管材HT&UHT弹性模量测量。试验测得了水泥砂浆环试样-70~800℃弹性模量、石英玻璃室温~1200 ℃弹性模量与石墨环试样室温~2100 ℃弹性模量,测试结果表明了广义缺口环相对法、广义闭口环相对法测试管材HT&UHT弹性模量的正确性与良好的可操作性。 最后,提出了分别适用于梁试样表面(单面、双面)涂层与管材表面(外侧、内侧、两侧)涂层HT&UHT弹性模量测试评价的三点弯二次相对法与缺口环二次相对法。分别利用三点弯二次相对法与缺口环二次相对法测得了石墨基体梁/环试样表面化学气相沉积碳化硅(CVD-SiC)涂层室温~2100 ℃弹性模量,二种测试方法测得的CVD-SiC涂层模量值相近,二者互相证明了二次相对法测试涂层HT&UHT弹性模量的正确性与有效性。室温下利用电感量仪与二次相对法测得的涂层弹性模量相近,由此也证明了二次相对法的正确性与可靠性。本文提出的二次相对法原位(涂层粘结于基体)测得的涂层模量是涂层真实、整体的弹性模量,是根据材料的宏观力学响应参数测得的,不同于压痕法测得的局部模量和剥离后涂层的弹性模量;且测试精度取决于试验机横梁位移,不受测试环境温度的限制,可应用于HT&UHT环境条件下。试验测得了8YSZ热障涂层室温~ 800 ℃弹性模量、CVD-SiC涂层室温~2100 ℃弹性模量,并对其模量一温度演化规律及变化机制进行了研究分析。 此外,针对管材三种涂层结构形式,提出了管材涂层弯曲强度的测试方法,即缺口环法测试外侧涂层弯曲强度;闭口环法测试内侧涂层弯曲强度;缺口环法/闭口环法测试两侧涂层弯曲强度。利用三点弯曲法结合相对法技术对石墨基体梁试样表面CVD-SiC涂层的弯曲强度进行测试,同时利用本研究所提出的方法测得了石墨基体环试样表面同种CVD-SiC涂层的弯曲强度。试验测得的管材涂层弯曲强度与梁试样表面涂层弯曲强度基本相近,由此即可证明管材涂层弯曲强度测试方法的正确性与可靠性。 关键词:相对法;高温与超高温;弹性模量;工程材料;涂层

英文摘要

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 Al₂O₃ ceramics from room temperature to 1300 ℃ 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 ℃ 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 ℃ 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 ℃ to 800 ℃, the modulus of quartz glass tubes from room temperature to 1200 ℃, and the modulus of graphite ring pieces from room temperature to 2100℃ 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 ℃. 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 ℃ and the modulus of CVD-SiC coating from ambient temperature to 2100 ℃ 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. Keywords: Relative method; High temperature and ultra-high temperature; Elastic modulus; Engineering materials; Coatings

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