Mechanical property degradation of a Nicalon fiber reinforced SiNC ceramic matrix composite under thermal shock loading

The degradation of mechanical properties of a SiC fiber reinforced SiNC ceramic matrix composite due to thermal shock by water quenching have been investigated. Post thermal shock tensile tests were performed to determine the degradation of mechanical properties of this composite. In situ acoustic emission (AE) tests were also conducted. The tensile tests data and acoustic emission data were correlated. The AE signal indicated a sudden increase in AE events at critical points in the stress–strain relationship. The effects of thermal shock temperature and the number of thermal shock cycles on the mechanical properties, and on the AE responses were also evaluated. It was observed that an increase in either factor resulted in more AE responses. Fracture damage in the tensile test specimens was examined by Scanning Electron Microscopy. It was observed that the failure mechanism changed as the thermal shock temperature increased. The fracture surfaces of the specimens tested without thermal shock indicated an extensive fiber pullout while the thermally shocked specimens showed reduced fiber pullout.     

Monitoring of thermal shock induced crack growth in a feed water nozzle corner of a reactor pressure vessel by acoustic emission

In the framework of a reactor safety research programme, thermal shocking of a clad feed water nozzle has been carried out during simulated operating conditions (temperature = 300°C, pressure = 11 MPa). The objective of these trials is to contribute to the understanding of crack initation and propagation under realistic thermal shock conditions, and to monitor crack initiation and growth by NDT on-line as well as off-line. Up to now the nozzle corner has been shocked 5200 times. The shocked area was monitored by acoustic emission (AE). During the last 800 thermal shocks new AE equipment with optimized capabilities for detection, location of AE sources and interpretation of AE signals has been used. This AE system and the results obtained during the last 800 shocks will be described. One important result is that it is possible to separate crack-growth signals from friction noise of the cracks by signal analysis when the AE signals are received with broad-band transducers.     

Evaluation of thermal shock resistance of cordierite honeycombs

A comparative study on thermal shock resistance (TSR) of extruded cordierite honeycombs is presented. TSR is an important property that predicts the life of these products in thermal environments used for automobile pollution control as catalytic converter or as diesel particulate filter. TSR was experimentally studied by quenching (descending test) the heated samples to water or by heating (ascending test) with an oxyhydrogen flame along with crack detection by acoustic emission (AE) method. TSR was also calculated by using coefficient of thermal expansion (CTE), modulus of elasticity (MOE) and modulus of rupture (MOR) of the honeycomb samples. Cordierite honeycombs of 200 and 400 cpsi were used for the above study. It was observed that the trends of TSR were same for both the experimental methods as well as by calculation. The ascending test method showed lower TSR values compared to water quench method due to early detection of cracks by AE. Finite element method (FEM) was also used to evaluate the thermal stress distribution in solid cordierite using thermal shock test data. It was observed that the maximum thermal stress calculated by FEM was lower than the strength of the material; therefore, the solid cordierite did not fail during such tests.     

Non-contact measurement of acoustic emission in materials by laser interferometry

The non-contact measuring system of acoustic emission (AE) by laser interferometry was developed to detect AE signals and analyze microfracture quantitatively during materials testing. The capability of this system was estimated by comparison between simulated AE signals due to glass capillary breaking and calculations using the finite element method. The systemcould measure AE signals during practical tensile tests of carbon fiber reinforced plastics. This technique was also applied to the thermal cycle test of ceramic/metal coatings, and AE signals during cooling were successfully detected and analyzed by a deconvolution method to evaluate quantitatively the microfracture process.     

Non-contact measurement of acoustic emission in materials by laser interferometry

The non-contact measuring system of acoustic emission (AE) by laser interferometry was developed to detect AE signals and analyze microfracture quantitatively during materials testing. The capability of this system was estimated by comparison between simulated AE signals due to glass capillary breaking and calculations using the finite element method. The system could measure AE signals during practical tensile tests of carbon fiber reinforced plastics. This technique was also applied to the thermal cycle test of ceramic/metal coatings, and AE signals during cooling were successfully detected and analyzed by a deconvolution method to evaluate quantitatively the microfracture process.     

Application of fracture surface topographic analysis technique to thermal fatigue crack extension in stainless steel vessel

The fracture surface topographic analysis (FRASTA) technique is applied to investigate the propagating process of fatigue crack in a model vessel made of type 304 stainless steel due to repetitive thermal shocks for 1000 cycles. FRASTA is a procedure for reconstructing the process of crack extension by comparing topographic features of coupled areas of opposing fracture surfaces via computer. Based on the result, fracture mechanics analysis is made to estimate the relationship between crack depth and applied number of thermal cycles.

During the thermal transient test, acoustic emission (AE) is monitored using four channel AE sensors. The estimated number of initiating cycles and propagating behavior of macroscopic crack by fracture mechanics analysis is compared with the result of AE monitoring examination, and the usefulness of the FRASTA technique is discussed.     

SUS304不锈钢拉伸过程中的声-热像特征

测量了含有Ⅰ型裂纹的SUS304不锈钢试样的单轴拉伸过程中的塑性变形和断裂。分析了裂纹尖端区域的塑性变形和断裂过程。结果表明:SUS304的各向异性在断裂阶段对声发射信号影响较大;红外热图像中的温度分布与塑性应变率有关;通过声发射参数和红外热图像可以从微观和宏观两方面分析裂纹尖端区域的塑性变形和断裂。     

静力试验中C/SiC构件集成式热结构的声发射特性及与材料测试的对比静力试验中C/SiC构件集成式热结构的声发射特性及与材料测试的对比

在对C/SiC复合材料常温拉伸试验的损伤表征和声发射演化特性进行简要分析的基础上,针对构件集成式设计带来的复杂性特点,对某C/SiC热结构在常温静力试验中的声发射特性进行了分析。给出了试验中声发射特征参数的分类特点。根据试验加载中声发射信号规律,给出了结构的损伤演化模式。通过与材料级声发射信号的对比,判断了试验加载与热结构极限破坏载荷之间的裕度。由此将声发射测试从材料级别的研究延伸到大型C/SiC热结构。      

Evaluation of the use of envelope analysis and DWT on AE signals generated from degrading shafts

Vibration analysis is widely used in machinery diagnosis. Wavelet transforms and envelope analysis, which have been implemented in many applications in the condition monitoring of machinery, are applied in the development of a condition monitoring system for early detection of faults generated in several key components of machinery. Early fault detection is a very important factor in condition monitoring and a basic component for the application of condition-based maintenance (CBM) and predictive maintenance (PM). In addition, acoustic emission (AE) sensors have specific characteristics that are highly sensitive to high-frequency and low-energy signals. Therefore, the AE technique has been applied recently in studies on the early detection of failure. In this paper, AE signals caused by crack growth on a rotating shaft were captured through an AE sensor. The AE signatures were pre-processed using the proposed signal processing method, after which power spectrums were generated from the FFT results. In the power spectrum, some peaks from fault frequencies were presented. According to the results, crack growth in rotating machinery can be considered and detected using an AE sensor and the signal processing method.     

Acoustic emission from stress corrosion cracks in aligned GRP

Acoustic emission (AE) produced by the propagation of stress corrosion cracks in an aligned glass fibre/polyester resin composite material has been recorded. Tests have been carried out over a range of crack growth rates and the variation of AE with crack velocity/applied stress intensity has been examined. The main source of AE is fibre fracture and there is a one-to-one relationship between the number of fibre fractures and the number of high-amplitude AE signals. This enables crack growth to be monitored directly from acoustic emission. The amplitude of AE signals produced by fibre failure appears to be proportional to the fracture stress of the fibres, although further analysis requires a greater understanding of the generation, transmission and detection of AE signals. This work demonstrates that stress corrosion cracking is an ideal source for the study of AE produced by fibre fracture without complications caused by interface effects, such as fibre debonding or pullout.On leave from the Technical University of Wroclaw, Wroclaw, Poland.     

Straining of a Bi-2223/Ag ribbon studied by the method of acoustic emission

The process of cooling of a Bi-2223/Ag ribbon after secondary sintering at 1000 K is studied by the method of acoustic emission (AE). The acoustic response is observed below 500 K and increases in intensity with decreasing temperature. It is established that the AE signal variation reflects the plastic flow in the silver coating caused by a difference between the thermal expansion coefficients of silver and the ceramic core of the Bi-2223/Ag ribbon.     

压缩载荷下陶瓷材料声发射特性的实验研究

对陶瓷材料在两种压缩加载下破坏过程中的声发射特性进行实验研究。三点弯曲和巴西劈裂实验中,采用多通道声发射系统监测了陶瓷试件的破坏过程。对声发射信号进行分析,结果表明,声发射能量数、事件数、振铃数等参数都反映了材料内部损伤演化扩展直至试件宏观断裂的过程。利用声发射参数可以定位陶瓷试件的破坏位置;试件断面的宏细观分析表明,陶瓷的脆性破坏机制主要是颗粒附近的微裂纹开裂。     

Failure processes within ceramic coatings at high temperatures

Plasma sprayed coatings have a complex structure which is produced by the overlaying of many molten or semi-molten particles in the diameter range of 20 to 120 μm. There is a need to characterize the failure behaviour of coatings and this has been carried out by using acoustic emission (AE) methodology. Coatings of NiCrAIY bond coat with a zirconia-12 wt% yttria overlay were applied to discshaped specimens of U-700 alloy. A waveguide of 1 mm diameter platinum was TIG welded to the specimen and allowed it to be suspended in a tubular furnace. The specimen was thermally cycled to 1150° C and the AE monitored. One method of examining the AE is from the viewpoint of the accumulative count data. It is also convenient to establish the temperatures for “initial” AE and “significant” AE (i.e., the temperature at which 100 counts is exceeded) so that coatings may be compared. Several other analyses have been carried out with the aim of establishing parameters which are related to the crack size and crack population. These studies have been used to postulate types of cracking mechanisms which may occur in plasma sprayed coatings during thermal cycling. It is shown that microcracking gave rise to a large amount of AE. However, this coating still survived more thermal cycles than a coating which exhibited macrocracking events. Data of this nature will be presented and the results discussed.     

Failure during thermal cycling of plasma-sprayed thermal barrier coatings

The thermal cycling behavior of plasma-sprayed ZrO₂?12wt.%Y₂O₃ coatings was studied. Coatings were produced with and without bond coats of Ni-Cr-Al-Zr and in some cases the substrates were heated to above the optimum temperature prior to spraying. The coatings (attached to the substrate) were thermal cycled to 1200 °C and their cracking behavior was followed by acoustic emission (AE) techniques. It was possible to examine the failure mechanisms by statistical analysis of the AE data and to evaluate the influence of preheating and bond coating. It is shown that the AE spectrum changes when a bond coat is used because of the presence of microcracks which, in turn, dissipate energy and improve the coating integrity. The preheating effect is reflected by a decrease in the peak count rate and an increase in the temperature at which AE activity is initiated.     

Monitoring thermal shock of alumina and zirconia-toughened alumina by acoustic techniques

Acoustic emission (AE) and acousto-ultrasonics (AU) have been employed to monitor the thermally induced damage which occurs when alumina and zirconia-toughened alumina ceramics are quenched from elevated temperatures into aqueous solutions. The AE and AU parameters were correlated with the microcracking and the strength degradation associated with the thermal shock treatments.     

Acoustoelastic effect in stressed heterostructures

Mechanical stresses influence the phase velocity of acoustic waves, known as the AE (acoustoelastic) effect. In order to calculate the AE effect of biaxially stressed layered systems, we extended the transfer matrix method for acoustic wave propagation by considering the change of the density, the influence of residual stress, and the modification of the elastic stiffness tensor by residual strain and by third-order constants. The generalized method is applied to the calculation of the angular dispersion of the AE effect for transverse bulk modes and surface acoustic waves on Ge(001). Our calculations reveal that the AE effect significantly depends on the propagation direction and can even change sign. The maximal velocity change occurs for transversally polarized waves propagating parallel to the [110] direction. For the layered Ge/Si(001) system, the AE effect is investigated for Love modes propagating in the [100] and [110] directions. The AE effect increases rapidly with increasing layer thickness and almost reaches its maximal value when the wave still penetrates into the unstressed substrate     

Fracture process of thermally shocked discontinuous fibre-reinforced glass matrix composites under tensile loading

Fracture mechanisms of discontinuous carbon-fibre-reinforced glass matrix composites were experimentally studied for specimens with initial damage induced by thermal shock. First, matrix cracking due to thermal shock was observed using both optical microscopy and scanning acoustic microscopy (SAM) to reveal the damage state. Secondly, tensile stress-strain behaviour and acoustic emission during tensile tests were measured for specimens with and without thermal shock. The progress of microscopic damage during tensile loading was also investigated using both replica and in-situ SAM techniques. Finally, macroscopic transient thermal stresses during thermal shock were calculated using finite-element analysis. It is proved that the fracture process of the composite specimen with thermal-shock-induced cracks is different from that of the virgin specimen. This difference in fracture processes is attributed to the difference in the evolution of matrix cracking, which is affected by pre-existing microcracks in the matrix. This revised version was published online in November 2006 with corrections to the Cover Date.     

基于小波变换的金刚石单晶生长声发射信号的降噪分析

依据小波变换的基本原理和方法,采用阈值降噪法对高温高压条件下金刚石单晶生长过程的声发射信号进行了小波降噪处理。结果表明:采用小波阈值降噪法能较好地消除金刚石单晶生长过程中声发射信号中的噪声,而且还保证了本身的声发射特性没有被破坏,从而可以更加准确地分析金刚石单晶的生长过程。     

Determination of J–R curves of thermoplastic starch composites containing crossed quasi-unidirectional flax fiber reinforcement

Thermoplastic starch (MaterBi) composites reinforced with quasi-unidirectional flax fiber in cross-ply (CP) arrangement were produced by film stacking followed by hot pressing. These composites, containing various amount of flax, failed ductilely with pronounced crack growth. Therefore, to determine their fracture mechanical behaviour the J-integral resistance curve concept (J–R) was applied. As the crack growth could not be traced, attempt was made to use the located acoustic emission (AE) events for that purpose. It was established that weighting and smoothing the located cumulative AE amplitudes the crack path can be correctly reconstructed. This was proved by collating the AE results with those derived from infrared thermographic (IT) inspection. Knowing the crack propagation at each point of the force–displacement curves the J–R curves could be determined. Both critical or initiation J-integral and tearing modulus went through a minimum with increase of flax content in the composites.