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Non-destructive evaluation of plasma sprayed functionally graded thermal barrier coatings 总被引:2,自引:0,他引:2
Acoustic emission (AE) as a non-destructive evaluation technique has recently been used in a number of studies to investigate the performance and failure behavior of plasma sprayed thermal barrier coatings. The mechanism of coating failure is complex, especially when considering the composite nature of the coating. In the present paper, the thermal shock tests with in situ acoustic emission are used to study the cracking behavior of plasma sprayed functionally graded thermal barrier coatings. Each thermal cycle consists of 8 min heating in the furnace at 1000°C and 8 min cooling from 1000°C to the room temperature by a compressed air jet. The AE signals are recorded during the quench stage. Three, four and five layer functionally graded coatings have been tested. The results show that the five layer functionally graded coatings appear to have the best thermal shock resistance in the specimens tested, because of the gradual changes in material properties. Higher AE energy counts and cumulative counts recorded by the tests are associated with the macro-crack initiation and growth. On the other hand, micro cracking and phase transformation only give rise to lower AE signals. 相似文献
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Investigation of acoustic emission signals under a simulative environment of grinding burn 总被引:2,自引:0,他引:2
Qiang Liu Xun Chen Nabil Gindy 《International Journal of Machine Tools and Manufacture》2006,46(3-4):284-292
Grinding burn is a common phenomenon of thermal damage that has been one of the main constraints in grinding in respect of high efficiency and quality. An acoustic emission (AE) technique was tried in an attempt to identify grinding burn on-line. However, the AE features of grinding burn are relatively weak and are easily obscured by other AE sources. This paper presents an investigation of the AE features of the thermal expansion induced by laser irradiation, which was designed to simulate grinding thermal behaviour. By using wavelet packet transforms, AE features at the grinding burn temperature can successfully be extracted without other mechanical interferential factors. Such thermal AE features provide a firm foundation for analysing and monitoring the AE features of grinding burn. 相似文献
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声发射技术在氧化膜破裂监测中的应用 总被引:2,自引:0,他引:2
在高温腐蚀领域,声发射技术是一种最有效且具有极高灵敏度的监测氧化膜破裂的技术.通过分析声发射计数,可以判断氧化膜首次发生破裂的时间及最后破裂的程度.如对声发射频谱进行分析,则可以判别氧化膜内发生塑性变形、开裂及剥落的过程.利用声发射技术还可以测量氧化膜/合金界面处预存物理缺陷大小及分布.此外,将声发射技术与其它技术结合,可建立全新的分析测试系统.综述了声发射技术用于监测氧化膜破裂的原理和方法,以及多种性能测试方面的应用与新发展. 相似文献
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N. H. Faisal R. Ahmed R. L. Reuben B. Allcock 《Journal of Thermal Spray Technology》2011,20(5):1071-1084
This work presents an in situ monitoring of HVOF thermal spraying process through an acoustic emission (AE) technique in an
industrial coating chamber. Single layer thermal spraying on substrate was carried out through slits. Continuous multilayer
thermal spraying onto the sample without slit was also conducted. The AE was measured using a broadband piezoelectric AE sensor
positioned on the back of the substrate. A mathematical model has been developed to determine the total kinetic energy of
particles impacting the substrate through slits. Results of this work demonstrate that AE associated with particle impacts
can be used for in situ monitoring of coating process. Results also show that the amplitude and AE energy is related to the
spray gun transverse speed and the oxy-fuel pressure. The measured AE energy was found to vary with the number of particles
impacting the substrate, determined using the mathematical model. 相似文献
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An experimental rig based on the use of infrared quartz lamps has been developed to monitor the degradation mechanisms causing
failure of thermal barrier coatings (TBC) under thermal-cycling conditions. An acoustic emission (AE) technique monitored
these degradation mechanisms, and advanced signals processing identified the key parameters that classify the AE signals according
to the long-term behavior of the TBC. The AE technique enabled the localization of degradation sources inside the TBC with
a linear resolution of ∼5 mm by the use of two transducers fixed at both ends of the sample. Furthermore, sample zones of
high AE activity showed typical vertical cracks at the surface and delaminations at the interface between the ceramic and
the bond-coat layer. Vertical cracks were induced preferentially during the heating period of the thermal cycles when the
ceramic coating was in a tensile-stress state, while delaminations were induced during the cooling period when the TBC was
in a compressive-stress state. 相似文献
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《Journal of Materials Processing Technology》2014,214(8):1617-1626
A detection method of laser-induced thermal damage–surface burn, rehardening and residual stress, was studied in this work. Artificial thermal damage was produced to various steels, e.g., AISI 1045, ASTM A36 and AISI 304, by virtue of laser irradiation. The aim of the present work is to identify thermal damage through sensor and feature extraction techniques. Acoustic emission(AE) sensor and ensemble empirical mode decomposition(EEMD) method were employed for this purpose. A de-noising method was proposed to eliminate noises from original AE signals, based on EEMD. Quantified thermal damage features were obtained. Results evidenced a strong correlation between AE features, i.e., RMS value of the reconstructed acoustic emission signal, and surface burn, residual stress value, as well as hardness of steels. The present work could be used as a potential and indirect approach for thermal damage detection. 相似文献
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Modal analysis of acoustic emission signals from CFRP laminates 总被引:2,自引:0,他引:2
As a result of its continuous and in situ detection capabilities, the acoustic emission (AE) technique is the prime candidate for damage monitoring in loaded composite structures. None of the AE analysis techniques used in laboratory studies has, however, proven to be capable of consistently dealing with the difficulties encountered in larger structures: large amount of data, the elimination of noise sources and the influence of wave propagation effects (attenuation, dispersion). This work will use the modal acoustic emission (MAE) technique as a more intelligent and efficient way of analysing AE results. AE waveforms obtained during tensile and bending testing of CFRP laminates will be presented. It will be demonstrated how taking into account the modal nature of AE waves can in future lead to more quantitative and accurate results. 相似文献
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Kaiser effects in acoustic emission (AE) behavior of composite laminates under repetitive thermal cyclic-loads are quantitatively analyzed to identify AE source mechanisms. The repetitive thermal loads brought about a large reduction, i.e. an exponential decrease, in AE total ring-down counts and AE amplitudes. It was thought that generation of most thermo-AE events during the first thermal cycle was not caused by crack propagation, but by secondary micro-fracturing due to abrasive contact between crack surfaces. For subsequent thermal cycles, on the other hand, a small number of weak thermo-AE events were generated due to frictional sliding contact. Such behavior of thermo-AE showed different characteristics according to specimen types and the maximum temperature in the thermal load cycles. 相似文献
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The acoustic emission technique is a very promising non-destructive and online capable approach for the detection of damage events in metal forming processes. The feasibility of using this testing method for online monitoring of cold forging processes of the case hardening steel 1.7321 (20MoCr4) was experimentally proven. In this paper, exemplary upsetting tests under varying test conditions were continuously monitored to analyze the relationship between cracking and resulting AE as well as to determine ideal AE measuring parameters. Finally, the critical forming stage of an industrial cold forging process for manufacturing of drive bevel gears is examined by applying acoustic emission technique. Cracking and tool wear were detected by analyzing the distribution of the AE parameters energy, hits and amplitude over the forming process. 相似文献
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Acoustic emission (AE) signals propagated across turbine runners incur attenuation due to propagation distance and structure interface. Wavelet packet transform (WPT) was used to analyze the attenuation characteristics of AE signals. A series of experiments had been carried out on an HLA286a-LJ-800 Francis turbine runner and AE signals were obtained from standard pencil lead break sources. The experimental results indicate that the WPT technique is an effective tool in extracting the attenuation characteristics. The feature packet can reflect the attenuation characteristics of signals across the runner. As a result, the pressure on data transmission and storage can be decreased by extracting feature packet coefficients using the wavelet packet technique, which indicates the feasibility to apply the AE technique to monitor the running state of turbine runners on site. 相似文献
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An acoustic emission (AE) technique was used for the determination of the onset of cracking of thermal spray self-fluxing
NiCrFeBSi coatings under tensile loading using a 4-point bend testing apparatus. These coatings were flame sprayed on 42CrMoS4
cylinders having different diameters. Two different post-treatment fusing processes, induction, and flame fusing, were used.
Along with the investigations of the effect of cylinder diameters and fusing processes onto the cracking resistance of the
coatings, the effect of the same two parameters on the residual stresses was also investigated. Results show that, independently
of the diameter of the cylinder, the flame-fused coatings possess a higher cracking resistance than their induction-fused
counterparts, i.e., that the strain to fracture is higher for the flame-fused coatings. A correlation between the strain to
fracture and the residual strain in the coatings has been established. This study points out that the combination of an AE
technique with a bending test apparatus shows some major benefits to obtain important information on the relative ductility
of thermal spray coatings. 相似文献
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The onset of breakaway oxidation and in-situ cracking and spalling of the oxide layer formed on 2 1/4Cr-1Mo steel have been detected by the use of the acoustic emission (AE) technique. Various AE parameters, viz., acoustic emission event counts vs time, acoustic emission counts vs time, and voltage level vs time have been used to detect these phenomena. It has been found that variations in AE parameters are marginal during heating at 600, 700, and 800°C, and a sudden rise in these parameters occurs during cooling. Increase in AE activity during cooling has been related to spalling of the oxide layers. At 900 and 950°C, a considerable increase in AE parameters (except voltage level) has been detected after certain times at the respective temperatures. By comparing with corresponding thermogravimetric curves, this increase in AE parameters has been attributed to the onset of breakaway oxidation. A very large increase in AE parameters during cooling from these temperatures has been related to in-situ cracking. 相似文献