基于声发射定位技术的矿柱破坏规律实验研究(英文)

声发射技术是研究岩石损伤破坏的良好工具。通过与矿柱相似的材料试验研究,对微裂纹的时空演化规律进行研究。断铅试验结果表明:所提出的基于最小二乘法和Geiger算法的声发射组合定位算法的定位精度良好,能满足岩石破坏过程声发射监测的需要。声发射定位结果直观地反映岩样内部裂纹初始、扩展的空间位置,对于研究岩石破坏机理具有重要意义。在受载条件下,矿柱与顶板相接位置最容易产生应力集中,出现声发射群集现象,其最终破坏结果与声发射定位时间群集区域吻合良好,说明矿柱与顶板相接位置属于宏观破坏发生的危险区;当岩石进入塑性变形阶段后,声发射率明显下降,出现"平静期",可以作为岩石失稳破坏的前兆特征。     

碳纤维含量对Al2O3+C/ZL109混杂复合材料摩擦磨损性能的影响

利用挤压铸造法制备了A1203 C／ZLl09短纤维混杂金属基复合材料，并探讨了A1203纤维体积分数为12％时，C纤维含量对该混杂复合材料摩擦磨损性能的影响。结果表明：随着C纤维体积分数的增加，复合材料的摩擦因数和磨损率逐渐降低。12％A1203和4％C短纤维的协同作用使复合材料从轻微磨损到急剧磨损的临界转变载荷比基体合金提高了1倍。当载荷低于临界载荷时，复合材料的主要磨损机制为犁沟磨损和层离，C纤维的加入有利于磨损表面裂纹尺寸的减小。但随着载荷的逐渐增加并发生严重磨损时，基体和复合材料的磨损机制均为严重的粘着磨损甚至局部熔化磨损。     

SiC／C层状陶瓷的断裂行为研究

利用流延法成膜和热压烧结工艺制备出了SiC层和石墨层交替排列、层厚均匀、界面清晰的SiC／C层状陶瓷。力学性能与单相SiC陶瓷相比，SiC／C层状陶瓷的抗弯强度略有降低，但断裂韧性却得到了大幅度的提高。层状陶瓷的载荷-位移曲线不同于单相陶瓷的1次性断裂曲线，而是呈锯齿状；同理论计算所得载荷-位移曲线相似。层状陶瓷的断裂路径呈台阶状，裂纹遇到弱界面时发生分叉产生并行扩展裂纹、晶须状微裂纹以及裂纹遇到弱界面时被吸收。认为裂纹偏转、裂纹分叉、裂纹并行扩展、裂纹遇弱界面被吸收等是SiC／C层状陶瓷的几种主要增韧机制。     

Fracture mechanism characterization of cross-ply carbon-fiber composites using acoustic emission analysis

The sequence of microscopic fracture mechanisms in locally loaded cross-ply carbon-fiber composites was studied by analyzing acoustic emission (AE) signals in combination with the modal analysis of Lamb waves, using microscopic and ultrasonic examination of the specimen after load interruption. The first 70 AE events were analyzed, which were detected during the initial loading segment when the first sudden load drop and gradual load recovery were observed. Characteristics of the detected waves were compared with the S₀- and A₀-mode Lamb waves produced by a spot- or line-focused YAG laser. The internal damage progression of the composite specimen was determined to be the fiber fracture in the front lamina, transverse cracks in the mid-lamina, delamination and splitting.     

Strength and Wear Behavior of Mg Alloy AE42 Reinforced with Carbon Short Fibers

In addition to the advantage of the lightweight of magnesium alloys, magnesium composites have moderate strength and elastic modulus. The proposed application of magnesium composites as diesel truck pistons makes it necessary to assess their wear performance. Little research data have been discussed on wear behavior of Mg alloy AE42 matrix and its composites. Thus, this paper reports wear behavior of magnesium alloy AE42(Mg–Al–Mn—RE; rare earth) and its composite AE42-C, which contains 23 vol% of randomly oriented carbon short fibers. Materials characterization, including density measurements, hardness testing, microstructures investigation, and compression testing at temperatures of 25, 150,and 300 °C, were conducted. Wear tests were performed under various loads and sliding distances. Wear mechanisms were also proposed based on the examination of worn surfaces using optical microscopy and scanning electron microscopy equipped with EDX(energy-dispersive X-ray spectrometry) analysis system. The hardness of AE42-23 vol% C composite is twice the hardness of the Mg matrix alloy AE42. Significant improvements to yield stress and compressive strength at temperatures of 25, 150, and 300 °C of the composite versus the AE42 alloy are achieved. Wear resistance of the composite is improved considerably versus that of the Mg alloy AE42 at the various sliding distances. Smearing of graphite on the worn surface produces a lubricating film that delays change from mild to severe wear of the composite, especially at high loads. EDX analysis of the worn surface shows oxidation of the matrix alloy at higher wear loads, and this mechanism decreases in the presence of carbon fibers under the same loads. Abrasive wear, oxidation, and plastic deformation are the dominant wear mechanisms for the alloy matrix AE42, whereas mainly abrasive wear is the wear mechanism of AE42-23 vol% C composite under the proposed testing conditions.     

Monitoring the tensile deformation in real unidirectional Kevlar-epoxy composites

Two acoustic emission (AE) parameters, event count rate (ER) and the skewness of the peak amplitude distribution (PAD), were found to correlate with mechanical and damage mechanisms in unidirectional Kevlar-epoxy composites loaded in tension. The ER of the AE reached a local maximum at about 0.3σ_cu (σ_cu= composite tensile strength), a minimum at about 0.6–0.7σ_cu, and increasing ER rates were noted as failure stress approached, with a peak at final failure.Along with our proposed approach to evaluate the PAD as a tool for characterizing processes, damage mechanisms and failure modes, the third statistical moment of the PAD, the skewness, was found to be able to distinguish between various mechanisms. From the beginning of loading until completion of the fibre straightening process, 0.6–0.7σ_cu, the value of the skewness steadily decreased, and then gradually increased to final failure. This behaviour is explained by a model based on the non-elastic mechanisms which contribute to the monitored AE. At final failure, the PAD's skewness value was affected by macroscopic failure modes. When this mode comprises fibre fracture with little matrix and interface splitting, the skewness decreased; when fibre failure was accompanied by matrix and interface splitting, its value changed slightly or even increased.     

Flexural destructive process of unidirectional carbon/carbon composites reinforced with in situ grown carbon nanofibers

Unidirectional carbon/carbon (C/C) composites modified with in situ grown carbon nanofibers (CNFs) were prepared by catalysis chemical vapor deposition. The effect of in situ grown CNFs on the flexural properties of the C/C composites was investigated by detailed analyses of destructive process. The results show that there is a sharp increase in the flexural load-displacement curve in the axial direction of the CNF-C/C composites, followed by a serrated yielding phenomenon similar to the plastic materials. The failure mode of the C/C composites modified with in situ grown CNFs is changed from the pull-out of single fiber to the breaking of fiber bundles. The existence of interfacial layer composed by middle-textured pyrocarbon, CNFs and high-textured pyrocarbon can block the crack propagation and change the propagation direction of the main crack, which leads to the higher flexural strength and modulus of C/C composites.     

Friction and wear properties of copper matrix composites reinforced by tungsten-coated carbon nanotubes

Carbon nanotubes (CNTs) were coated by tungsten layer using metal organic chemical vapor deposition process with tungsten hexacarbonyl as a precursor. The W-coated CNTs (W-CNTs) were dispersed into Cu powders by magnetic stirring process and then the mixed powders were consolidated by spark plasma sintering to fabricate W-CNTs/Cu composites. The CNTs/Cu composites were fabricated using the simi-lar processes. The friction coefficient and mass wear loss of W-CNTs/Cu and CNTs/Cu composites were studied. The results showed that the W-CNT content, interfacial bonding situation, and applied load could influence the friction coefficient and wear loss of W-CNTs/Cu com-posites. When the W-CNT content was 1.0 wt.%, the W-CNTs/Cu composites got the minimum friction coefficient and wear loss, which were decreased by 72.1% and 47.6%, respectively, compared with pure Cu specimen. The friction coefficient and wear loss of W-CNTs/Cu composites were lower than those of CNTs/Cu composites, which was due to that the interfacial bonding at (W-CNTs)-Cu interface was bet-ter than that at CNTs-Cu interface. The friction coefficient of composites did not vary obviously with increasing applied load, while the wear loss of composites increased significantly with the increase of applied load.     

Simulation and experimental study on thermal deep drawing of carbon fiber woven composites

Carbon fiber woven composites are composed of carbon fiber woven and resin matrix. To reduce the manufacture cost, thermal stamping, a new forming technology, was proposed and investigated to fabricate composite part. The mechanical properties of carbon fiber have great influence on the deformation of carbon fiber composites. In this study, shear angle–displacement curves and shear load–shear angle curves were obtained from picture frame test. Thermal deep drawing experiments and simulation were conducted, and the shear load–displacement curves under different forming temperatures and shear angle–displacement curves were obtained. The results show the compression and shear between fiber bundles are the main deformation mechanism of carbon fiber woven composite. The maximum shear angle for the composites in this study is 33°. In the drawing process, the forming temperature affects the drawing force, which drops rapidly with the increasing temperature. The suitable forming temperature in deep drawing of the carbon fiber woven composite is approximately 170 °C.     

B2O3 modified SiC–MoSi2 oxidation resistant coating for carbon/carbon composites by a two-step pack cementation

To protect carbon/carbon (C/C) composites against oxidation, a B₂O₃ modified SiC–MoSi₂ coating was prepared by a two-step pack cementation. The microstructure and the oxidation resistant property of the coating were studied. The results show that, the as-received coating is a dense structure, and is composed of α-SiC, β-SiC and MoSi₂. The B₂O₃ modified SiC–MoSi₂ coating has excellent oxidation resistant property, and can protect C/C composites from oxidation at 1773 K in air for more than 242 h. The failure of the coating was considered to arise from the existence of the penetration cracks in the coating during the slow cooling from 1873 to 673 K.     

Influence of SiC nanowires on the properties of SiC coating for C/C composites between room temperature and 1500 °C

To prevent carbon/carbon (C/C) composites from oxidation between room temperature and 1500 °C, a dense SiC nanowire-toughened SiC oxidation resistant coating was prepared by a two-step technique composed of chemical vapor deposition and pack cementation. SiC nanowires could effectively baffle the propagation of the microcracks and avoid the formation of the through-thickness microcracks in the original coating. The results indicated that, after introducing SiC nanowires, the coefficient of thermal expansion of the coating was decreased between 100 and 1500 °C, and the oxidation protective ability for the coated C/C composites was improved largely between room temperature and 1500 °C.     

Effects of microwave assisted heating of carbon nanofiber reinforced high density polyethylene

Carbon nanofiber and carbon nanotube reinforced polymer composites have shown promise due to their enhanced mechanical, electrical, thermal, and dielectric properties. In this study, vapor grown carbon nanofiber reinforced polyethylene composites were exposed to microwave radiation in a conventional resonance cavity microwave oven to explore the possibility of using microwave energy to assist the processing of carbon nanofiber reinforced thermoplastic composites and determine the effect of microwave radiation exposure on material properties. Average temperatures of up to 40 °C above ambient were measured after 5 min of microwave exposure. The effect of high power microwave radiation on the physical integrity of the composites via dynamic mechanical analysis, tensile tests, and dielectric analysis is presented. A drop of 50% in failure strain has been observed for composites with 15 and 20 wt% nanofiber concentrations after exposure to microwave radiation.     

Kaiser effects in acoustic emission from composites during thermal cyclic-loading

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.     

ZL109铝合金及其复合材料干滑动磨损表面及亚表面的观察与分析

利用预制体挤压浸渗法分别制备了Al2O3或C短纤维单一增强以及两者混杂增强ZL109金属基复合材料，探讨了基体合金及其复合材料的干滑动摩擦磨损行为，并对其磨损表面及亚表面形貌进行了观察和分析。结果表明：C纤维在复合材料中起到一定的自润滑作用，Al2O3和C短纤维混杂增强复合材料的磨损率低于基体合金和单一纤维增强的复合材料。与基体相比，单一Al2O3增强复合材料从轻微磨损到急剧磨损的临界转变载荷明显提高，经C纤维混杂后，其临界载荷进一步提高。经磨损表面及亚表面的观察与分析表明：纤维增强对基体合金的磨损机制没有明显影响，在轻微磨损阶段，主要磨损机制为犁沟磨损和层离，亚表面分为3个区域：表层未脱落的剥离层、亚表面裂纹形成区以及未受影响区。发生严重磨损时，磨损机制转变为严重的粘着磨损，此时表层未脱落的剥离层遭到破坏。     

Methods of surface carburizing and alloying steel by means of carbon fiber materials

Carbon fibers and textiles based on them are being used increasingly to reinforce composites. In this article we report the results of inetallographic research on the surface layers formed by passing an electric current through a hardened steel part and a fabric or felt made of a carbon-fiber material on its surface.Vinnitsa Polytechnical Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 35–39, January, 1994.     

Damage Characterization of Glass/Epoxy Composite Under Three-Point Bending Test Using Acoustic Emission Technique

Acoustic emission (AE) technique is an efficient non-destructive method for detection and identification of various damage mechanisms in composite materials. Discrimination of AE signals related to different damage modes is of great importance in the use of this technique. For this purpose, integration of k-means algorithm and genetic algorithm (GA) was used in this study to cluster AE events of glass/epoxy composite during three-point bending test. Performing clustering analysis, three clusters with separate frequency ranges were obtained, each one representing a distinct damage mechanism. Furthermore, time-frequency analysis of AE signals was performed based on wavelet packet transform (WPT). In order to find the dominant components associated with different damage mechanisms, the energy distribution criterion was used. The frequency ranges of the dominant components were then compared with k-means genetic algorithm (KGA) outputs. Finally, SEM observation was utilized to validate the results. The obtained results indicate good performance of the proposed methods in the damage characterization of composite materials.     

Behavior of oxide scales on 2.25Cr-1Mo steel during thermal cycling. II. Scales grown in water vapor

The acoustic emission (AE) technique has been applied to identify scale cracking during thermal cycling of tubes of 2.25Cr-1Mo steel. The scale morphology and failure mode were investigated by light and electron optical methods. The scale formed at 600°C in water vapor consists of an outer magnetite and an inner, chromium-containing spinel layer. Cooling leads to tensile stresses in the scale that cause macro and microcrack formation in the scale. At constant-cycle parameters, a characteristic set of crack length and crack density is established. Changes in the cycle parameters also change the crack length and crack density. The experimental results can be described by a model developed by Hasselmann assuming a large number of noninteracting microcracks in a ceramic plate.     

大坝混凝土动态弯拉声发射特性试验

为了研究大坝混凝土在不同加载速率下的弯拉声发射特性,并进一步理解不同加载速率下混凝土的损伤发展过程,在MTS试验机上以四种不同的加载速率进行了无开口湿筛混凝土梁的三弯点试验。结果表明,随着加载速率的提高,声发射撞击累计数减少,而撞击率峰值增加,低振幅（35-40 dB）的声发射信号所占比例有所增加,声发射振铃数、信号能量和持续时间的平均值都有提高的趋势;在低加载速率时,混凝土在破坏过程中产生较少数的微裂缝,其破坏机制受少数主要裂缝（跨中位置）控制,而在高加载速率时,微裂缝分布在较宽的范围之内。