高强度钢手工焊接后过程的声发射实时监测研究

本针对实际工艺中使用的高强度钢，重点对手工焊接的冷却过程利用声发射技术进行实时监测。本次试验是和先前在不同型号的钢板在手工焊接过程中所产生的声发射信号进行比较的基础上进行的，主要目的是寻找高强度钢在手工焊接的冷却中所产生的活性裂纹在冷却阶段的声信号特征。     

Q235钢板局部腐蚀声发射信号特性研究

为解决腐蚀声发射源特征提取和识别的难题,直接从理论上证明了腐蚀声发射监测的有效性并推导出声发射信号特征。气泡破裂声发射信号幅值近似与气泡半径平方和液位高度成正比,频率与气泡半径成反比;钢板及其腐蚀产物开裂声发射信号幅值与开裂位置局部应力强度成正比,频率与裂纹扩展速度成正比,与裂纹扩展距离成反比。用低频和高频两套声发射系统,同时长时间监测Q235钢板在10%FeCl₃·6H₂O、10%FeCl₃·6H₂O加0.01 mol/L HCl混合液、5%CuSO₄·5H₂O溶液中的腐蚀情况,辅以监测钢板及钢板腐蚀产物开裂作为验证实验。通过对声发射信号的参数及谱分析,得出不同声发射源可以通过撞击数及功率谱在频域的分布来有效区分。实验结果与理论分析相吻合,研究结果对腐蚀声发射监测技术具有重要指导意义。      

Corrosion behavior of X-70 pipe steel in near-neutral pH solution

The mechanism of near-neutral pH stress corrosion cracking (SCC) of natural gas pipelines has not been well-established since the first accident was found in the 1980s. In particular, the role of hydrogen in near-neutral pH SCC has remained unknown. In this work, cyclic voltammetry was used to comprehensively investigate the fundamentals of the electrochemical corrosion reactions occurring at the steel/solution interface in diluted, 5% CO₂/N₂-purged, near-neutral pH bicarbonate solutions. It is shown that there is no stable oxide film formed on the steel surface in near-neutral pH solution. The dissolution-based cracking mechanism does not apply for near-neutral pH SCC of pipelines. The formation of a metastable Fe(OH)₂ deposit layer shows a catalytic activity on hydrogen evolution reaction, indicating that a significant amount of hydrogen could be generated under near-neutral pH condition. The presence of corrosive anions in the soil electrolyte enhances both the anodic polarization of the steel and the cathodic hydrogen evolution reaction, resulting in an increased hydrogen evolution rate. The introduction of oxygen could form a stable oxide film on the surface of steel, resulting in the loss of the surface catalytic effect on hydrogen evolution reaction. Thus, a hydrogen-based mechanism does not apply for SCC in the presence of oxygen.     

Influence of cement flow and aggregate type on the mechanical and acoustic characteristics of porous concrete

The influence of cement flow and aggregate type on the mechanical and acoustic characteristics of porous concrete is systematically investigated in the present study. Three levels of cement flow (80%, 110%, and 140%) and five types of aggregates (normal aggregates of 8-13 mm and 13-19 mm and lightweight aggregates of 4-8 mm, 8-12 mm, and 12-19 mm) are used, and effects of the application of AE admixtures in paste were also studied. Single-layered and double-layered porous concrete specimens are fabricated to examine the effect of different layer configuration on the acoustic characteristics. For the purpose of comparison, the void ratio, compressive strength, and sound absorption coefficient of the specimens are used as evaluation parameters. Based on the findings of the study, a sound absorbing porous concrete with a maximum absorption coefficient of approximately 1.00 is developed, and the minimum absorption coefficient of the ‘double-layered porous concrete’ structure is shown to be more than 0.60 with a frequency of 400 Hz or above, considering the tolerant error.     

304不锈钢点蚀声发射监测研究

为研究304不锈钢在高于常温条件下的点蚀声发射特性,对70℃下6%氯化铁溶液中304不锈钢点蚀过程进行了声发射监测。采用参数和波形分析相结合的方法处理信号,并通过点蚀形貌观察进行验证。结果表明声发射撞击和能量随时间逐渐增加,在某一时段达到峰值,随后下降并维持平稳状态。信号波形主要由幅度、能量较大的低频段(<100kHz)弯曲波和幅度、能量较小的高频段(>100 kHz)扩展波构成。研究结果对304不锈钢高于常温条件下声发射点蚀监测具有一定意义。      

Corrosion behaviour of AISI 304 stainless steel with Cu coatings in H2SO4

The work addresses the influence of cementation and electrodeposition of copper coatings on the corrosion resistance of AISI 304 stainless steel immersed in 30 wt.% H₂SO₄ at temperatures of 25 and 50 °C. Corrosion process was evaluated by gravimetric tests, DC measurements and electrochemical impedance spectroscopy (EIS). The specimen surfaces were analysed by scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction. The corrosion performance of AISI 304 stainless steel in sulphuric acid solution was greatly improved by copper coatings. The amount of copper deposited by the cementation process was sufficient to protect the stainless steel of corrosion. A greater amount of copper obtained by electrodeposition treatments does not supply further improvement in the corrosion behaviour. The improved corrosion resistance is related to copper dissolution at the initial stages of immersion tests and the presence of Cu²⁺ in the solution, which makes the medium more oxidizing, increasing the stability of the passive layer. In addition, the presence of copper at the surface reduces the overpotential of cathodic reaction, enabling the transition from an active region to the passive one.     

混凝土声发射信号源定位精度的细观模型计算分析*

混凝土内部在断裂时会产生相应的声发射信号,通过这一信号,混凝土内部损伤断裂区域可以被定位。然而,混凝土是一种多相非均匀材料,而声发射定位算法是基于均匀介质假设进行计算的,因此采用该算法对其定位会产生一定的误差,有必要从细观的角度研究混凝土非均匀性对定位精度的影响。本文基于随机骨料模型以及时差定位算法（基于遗传算法）,建立了一种用于估计混凝土声发射定位误差的定量估算模型。本文采用该模型定量估算分析了混凝土细观组分对其声速的影响,并在该分析结构的基础上继续分析了骨料含量以及声速取值偏差对定位精度的影响。     

Noise reduction algorithm of corrosion acoustic emission signal based on short-time fractal dimension enhancement

The general corrosion and local corrosion of Q235 steel were tested by acoustic emission(AE) detecting system under 6%FeCl_3-6H_O solution to effectively detect the corrosion acoustic emission signal from complex background noise.The short-time fractal dimension and discrete fractional cosine transform methods are combined to reduce noise.The input SNR is 0~15 dB while corrosion acoustic emission signals being added with white noise,color noise and pink noise respectively.The results show that the output signal-to-noise ratio is improved by up to 8 dB compared with discrete cosine transform and discrete fractional cosine transform.The above-mentioned noise reduction method is of significance for the identification of corrosion induced acoustic emission signals and the evaluation of the metal remaining life.     

Rate effect on mechanical properties of hydraulic concrete flexural-tensile specimens under low loading rates using acoustic emission technique

Acoustic emission (AE) waveform is generated by dislocation, microcracking and other irreversible changes in a concrete material. Based on the AE technique (AET), this paper focuses on strain rate effect on physical mechanisms of hydraulic concrete specimens during the entire fracture process of three point bending (TPB) flexural tests at quasi-static levels. More emphasis is placed on the influence of strain rate on AE hit rate and AE source location around peak stress. Under low strain rates, namely 0.77 × 10⁻⁷ s⁻¹, 1 × 10⁻⁷ s⁻¹ to 1 × 10⁻⁶ s⁻¹ respectively, the results show that the tensile strength increases as the strain rate increases while the peak AE hit rate decreases. Meanwhile, the specimen under a relatively higher strain rate shows a relatively wider intrinsic process zone in a more diffuser manner, lots of distributed microcracks relatively decrease stress intensity, thus delay both microcracking localization and macrocrack propagation. These phenomena can be attributed to Stéfan effect. In addition, further tests, namely the combination of AE monitoring and strain measuring systems was designed to understand the correlation between AE event activity and microfracture (i.e., microcracking and microcracking localization). The relative variation trend of cumulative AE events accords well with that of the load-deformation curve.     

Monitoring acoustic emission (AE) energy of abrasive particle impacts in a slurry flow loop using a statistical distribution model

Slurry erosion has been recognized as a serious problem in many industrial applications. In slurry flows, the estimation of the amount of incident kinetic energy that transmits from particles suspended in the fluid to the containment structures is a key aspect in evaluating its abrasive potential. This work represents a systematic investigation of particle impact energy measurement using acoustic emission (AE), as indicated by a sensor mounted on the outer surface of a sharp bend, in an arrangement that had been pre-calibrated using controlled single and multiple impacts. Particle size, free stream velocity, and nominal particle concentration were varied, and the amount of energy dissipated in the carbon steel bend was assessed using a slurry impingement flow loop test rig. Silica sand particles of mean particle size 225–650 μm were used for impingement on the bend with particle nominal concentrations between 1 and 5% while the free stream velocity was changed between 4.2 and 14 ms⁻¹.     

Beamforming array techniques for acoustic emission monitoring of large concrete structures

This paper introduces a novel method of acoustic emission (AE) analysis which is particularly suited for field applications on large plate-like reinforced concrete structures, such as walls and bridge decks. Similar to phased-array signal processing techniques developed for other non-destructive evaluation methods, this technique adapts beamforming tools developed for passive sonar and seismological applications for use in AE source localization and signal discrimination analyses. Instead of relying on the relatively weak P-wave, this method uses the energy-rich Rayleigh wave and requires only a small array of 4–8 sensors. Tests on an in-service reinforced concrete structure demonstrate that the azimuth of an artificial AE source can be determined via this method for sources located up to 3.8 m from the sensor array, even when the P-wave is undetectable. The beamforming array geometry also allows additional signal processing tools to be implemented, such as the VESPA process (VElocity SPectral Analysis), whereby the arrivals of different wave phases are identified by their apparent velocity of propagation. Beamforming AE can reduce sampling rate and time synchronization requirements between spatially distant sensors which in turn facilitates the use of wireless sensor networks for this application.     

Role of hydrogen in stress corrosion cracking of austenitic stainless steels

Effects of hydrogen charging on the stress corrosion cracking (SCC) behavior of 304 and 310 stainless steels under sustained load were investigated in boiling 42% MgCl₂ solution. The cracking was accelerated by the incorporation of hydrogen into the steel without altering the crack growth mechanism. The fact that the active dissolution is almost unaffected by the hydrogen charging and tensile stress indicates that the phenomenon of hydrogen-promoted SCC is unlikely a result of hydrogen-facilitated active dissolution. In contrast, hydrogen significantly promotes anodic dissolution in the potential range where the active-to-passive transition occurs. The electrochemical noise detected in the SCC process implies that the crack propagation process is discontinuous and hydrogen charging can raise the frequency of film breakdown at the crack tip. These observations suggest that the hydrogen-promoted SCC may result from the hydrogen-induced passivity degradation.     

Cohesive zone modelling of anodic dissolution stress corrosion cracking induced by corrosion product films

Damage mechanics based on the cohesive zone model were applied to study the anodic dissolution stress corrosion cracking (SCC) in flat and U-shaped edge-notched specimens. The simulation results show that corrosion product films (CPFs) facilitate crack initiation in SCC due to the CPF-induced stress and CPF rupture. In the flat specimen, SCC susceptibility increases with the CPF thickness and CPF Young’s modulus, while it decreases with CPF fracture strength. For the U-shaped edge-notched specimen, the normalised threshold stress intensity factor K_ISCC/K_IC decreases with the CPF thickness and notch depth.     

声发射特性与材料断裂韧性相关性研究

以高强度铝合金的断裂过程为研究对象，利用先进的声发射试验系统，考察了拉伸试样和带有预制疲劳裂纹的DCB试样在断裂时的声发射事件特性，揭示了材料断裂韧性和声发射特性 间的关系.试验结果表明，材料稳态裂纹扩展起始于K_1C并发射出许多幅值超过 阈值35 dB的信号，材料裂纹失稳扩展时爆发出强烈的声发射信号，材料断裂的累积声发射能与宏 观断裂能呈线性关系. 关键词： 声发射 断裂韧性 铝合金     

Guided ultrasonic waves for non-destructive monitoring of the stress levels in prestressed steel strands

The safety of prestressed civil structures such as bridges, dams, nuclear power plants, etc. directly involves the security of both environment and users. Health monitoring of the tensioning components, such as strands, tendons, bars, anchorage bolts, etc. is an important research topic and a challenging task bringing together the non-destructive evaluation (NDE) and civil engineering communities. This paper deals with a guided ultrasonic wave procedure for monitoring the stress levels in seven-wire steel strands (15.7 mm in diameter). The mechanical and geometrical characteristics of the prestressed strands were taken into account for optimizing the measurement configuration and then the choice of the guided ultrasonic mode at a suitable frequency. Simplified acoustoelastic formulations were derived from the acoustoelasticity theory according to either calibration test or in situ measurement. The results from acoustoelastic measurements on the seven-wire steel strands are presented and discussed in the case of calibration tests and industrially prestressed strands. They show the potential and the suitability of the proposed guided wave method for evaluating the stress levels in the tested seven-wire steel strands.     

蒸养再生骨料混凝土断裂声发射特征*

为研究蒸汽养护对再生骨料混凝土断裂过程损伤特征的影响,开展了再生骨料混凝土三点弯断裂试验,并采用声发射技术对损伤过程进行了监测分析。结果表明,蒸养降低了再生骨料混凝土断裂峰值荷载,断裂过程中声发射信号强度低于标准养护试件。声发射振铃计数、撞击数、b值等参数能够准确反映再生骨料混凝土断裂过程损伤演变规律。加载上升阶段,声发射信号活动性很低,试件内以微裂纹的形成为主,加载至荷载峰值,声发射振铃计数累计值增长呈现平缓阶段,混凝土内微裂缝的聚合会持续一段时间。峰值后荷载迅速降低阶段声发射信号最活跃,为裂缝迅速发展的过程。峰后荷载缓慢降低的过程,声发射信号活跃性降低,裂缝扩展至试件顶部区域,主要是宏观裂缝的开展,不再产生新的裂缝。     

早龄期荷载及低温作用下的混凝土声发射特征试验研究*

井壁混凝土由于施工与环境特殊性导致其裂缝扩展情况异常,本文通过对不同早龄期荷载、低温条件下混凝土轴压破坏过程的声发射检测,分析井壁施工环境对混凝土内部裂缝产生、发展的影响。研究表明,早龄期荷载、低温单独作用时,与标养混凝土主要差异在轴压过程中的声发射初始阶段,对混凝土内部的原有收缩裂缝有所影响。当两者耦合作用时,早龄期荷载影响比低温作用显著,并且在早龄期荷载较大时会出现声发射信号"倒置"现象。     

Sonic cracking of blue-green algae

Algae are aquatic organisms classified separately from plants. They are known to cause many hazards to humans and the environment. Algae strands contain nitrogen-producing cells that help them float (heterocysts). It is hypothesized that if the membranes of these cells are disrupted by means of ultrasound, the gas may be released analogous to sonic cracking, causing the strands to sink. This is a desirable ecological effect, because of the resulting suppressed release of toxins into the water.We subjected small quantities of blue-green algae of the Anabaena sphaerica species to ultrasound of frequencies and pressures in the clinical diagnostic range, and observed the changes in brightness of these solutions over time. Blue-green algae were forced to sink at any ultrasonic frequency we studied, supporting our hypothesis that heterocysts release nitrogen under ultrasound insonification in the clinical diagnostic range.Although the acoustic fields we used to eradicate blue-green algae are perfectly safe in terms of mechanical index, the acoustic pressures surpass the NURC Rules and Procedures by over 35 dB. Therefore, caution should be taken when using these techniques in a surrounding where aquatic or semi-aquatic animals are present.     

Critical ruptures

The fracture of materials is a catastrophic phenomenon of considerable technological and scientific importance. Here, we analysed experiments designed for industrial applications in order to test the concept that, in heterogeneous materials such as fiber composites, rocks, concrete under compression and materials with large distributed residual stresses, rupture is a genuine critical point, i.e., the culmination of a self-organization of damage and cracking characterized by power law signatures. Specifically, we analyse the acoustic emissions recorded during the pressurisation of spherical tanks of kevlar or carbon fibers pre-impregnated in a resin matrix wrapped up around a thin metallic liner (steel or titanium) fabricated and instrumented by Aérospatiale-Matra Inc. These experiments are performed as part of a routine industrial procedure which tests the quality of the tanks prior to shipment. We find that the seven acoustic emission recordings of seven pressure tanks which was brought to rupture exhibit clear acceleration in agreement with a power law “divergence” expected from the critical point theory. In addition, we find strong evidence of log-periodic corrections that quantify the intermittent succession of accelerating bursts and quiescent phases of the acoustic emissions on the approach to rupture. An improved model accounting for the cross-over from the non-critical to the critical region close to the rupture point exhibits interesting predictive potential. Received 6 July 2000     

高强混凝土单轴压缩声发射频率特征试验研究

为研究高强混凝土破裂前声发射信号的频率特征,对C60、C70、C80高强混凝土试件进行单轴压缩下的高、低频双通道声发射试验,得到破裂过程的力学参数和声发射参数,探求高强混凝土不同加载阶段声发射信号频率的分布特征。研究表明,三种高强混凝土在峰值应力前,高、低频通道声发射信号均集中在特定的频段内;临近峰值应力时,高、低频通道的声发射信号频率向低频段移动,同时优势频段内的频率趋于分散,这可作为预测高强混凝土破坏的前兆信息。