互易法与表面脉冲法校准声发射传感器的一致性探讨

声发射传感器的校准是声发射定量检测技术的前提,互易法和表面脉冲法是目前通常采用的两种校准方法,为探讨其一致性,从互易常数入手,比较表面波互易常数和表面脉冲法中的等效互易常数。通过加载力的等效处理、利用激光测振仪测量质点振速,获得法向速度和所加力脉冲的时域和频域分量,计算得到等效互易常数。同时,建立互易法和表面脉冲法校准系统,对实验结果进行对比分析。互易常数与等效互易常数的计算结果和基于校准系统的实验结果表明,互易法和表面脉冲法之间具有较好的一致性,电信号激励声发射换能器所引起的径向谐振产生的影响,对于互易校准而言可以忽略。     

声发射传感器表面波灵敏度的比较法校准

声发射传感器的校准是声发射技术的重要课题,基于表面波的比较法校准是声发射传感器校准中的常用方法,其原理简单,校准效率高,适用于声发射传感器的日常校准。本文针对比较法校准,优化了校准中使用的声发射源和声场,构建了完整的声发射传感器比较法校准系统,编写了相应的校准程序,实现了传感器表面波灵敏度的校准,并具有较好的重复性和稳定性。     

平面波耦合腔中热传导和声泄漏对低频互易校准的影响

针对低频耦合腔互易校准中存在的热传导和声泄漏问题,通过理论计算和不同长度耦合腔的系列实验研究,探讨其对互易校准结果的影响规律。首先阐述了互易校准原理及电容传声器低频时的频响特征,传声器均压孔在声场外时,低频声压灵敏度随着频率降低而逐渐增加;其次,对发射传声器-耦合腔-接收传声器构成的耦合腔系统中的热传导修正因子进行计算,结果表明,随着耦合腔长度变长,热传导修正因子变小,不确定度分量变小;通过不同长度的耦合腔组成的互易校准系统研究声泄漏的影响并探讨其优化方案,实验结果表明,10 mm和15 mm耦合腔对互易校准结果影响较小,重复性高,该实验结果与传声器的低频响应特性一致。     

磁电式振动速度传感器灵敏度校准的测量不确定度评定

磁电式振动速度传感器（以下简称“传感器”）是用来测量机械振动量的一种传感器,它利用电磁感应原理将机械振动速度量转换成电压量输出,这种传感器广泛应用于机械、电力、航空等领域。传感器的主要技术指标有速度灵敏度、频率响应、幅值线性、横向灵敏度比、动态范围和温度响应等。其中,速度灵敏度的校准是确保振动速度量值准确可靠的一项主要指标。对传感器灵敏度校准结果的测量不确定度评定如下：     

高静水压下自由场水声声压标准装置的研究

为了精确测量高静水压下自由场中的水声信号,在高压消声水池系统基础上建立了高静水压下自由场水声声压标准装置.装置由水声信号发射系统、电声信号测量系统、控制和管理系统及互易测量构件设备所组成,能够在2～200 kHz频率范围内,0.1～10 MPa静水压力下用球面波互易校准原理校准标准水听器的自由场灵敏度,校准值的扩展不确定度为0.8 dB(P=95%).     

电涡流式位移传感器动态灵敏度测量不确定度评定

文章介绍了电涡流式位移传感器动态灵敏度的校准原理和方法,依据JJG 644—2003《振动位移传感器检定规程》,采用比较法振动标准装置对传感器的动态灵敏度进行校准。在数学建模和影响因素分析的基础上,对被校传感器动态灵敏度的测量不确定度进行了评定,得到灵敏度得测量不确定度为1.9%(k=2)。     

LS2F传声器的高频自由场互易校准

为满足空气超声声压量值的溯源需求,利用PULSE分析仪和互易校准仪,采用时间加窗技术对频域信号转换得到的时域信号进行处理,建立了LS2F传声器的高频互易校准装置,实现了实验室标准传声器的20~51 kHz的自由场灵敏度校准。通过对比时间加窗处理前后的实验数据,分析了驻波和墙体发射波对传声器灵敏度的影响。结果表明,高于40 kHz时,时间加窗处理技术能够减小1.0 dB的偏差。     

舰载声纳换能器的就地校准

本文介绍了一种可以对舰载声纳换能器进行就地校准的方法。本方法与普通互易校准不同,它既能对换能器的灵敏度幅值也能对其相位随时进行就地校准,校准精度只取决于电流和电压的测量精度。     

加速度冲击校准研究

文章将加速度冲击校准分为绝对法、相对法和比较法进行介绍,并给出了几种半正弦冲击脉冲激励装置的激励范围及校准装置的测量范围和测量不确定度。所提出的加速度类比法系冲击校准新方法,是相对法和比较法的结合,能对冲击加速度传感器灵敏度、幅值线性度与频率响应等进行较为准确的检定与校准。     

声表面波标签的相参阅读器设计

现有与声表面波标签匹配的阅读器通常采用非相参的超外差结构,并通过开关截断以产生激励脉冲信号。由于开关闭合时刻和本振信号初相位的随机性,该方案所测回波相位为一个不确定的值,无法与距离等待测量建立函数关系。本文设计了一种相参阅读器,在发射链路使用功分器将信号源产生的信号分为两路同相信号,并把其中一路引入接收链路的下混频器作为本振信号,由此可使回波相位为一确定值。实际制作了阅读器电路,测试了发射、接收链路,并结合声表面波标签进行了实验,通过与矢量网络分析仪测量结果对比,表明了该阅读器的有效性。     

Acoustic Emission Sensor Calibration for Absolute Source Measurements

This paper describes sensor calibration and signal analysis techniques applicable to the method of acoustic emission (AE) and ultrasonic testing. They are particularly useful for obtaining absolute measurements of AE wave amplitude and shape, which can be used to constrain the physics and mechanics of the AE source. We illustrate how to perform calibration tests on a thick plate and how to implement two different mechanical calibration sources: ball impact and glass capillary fracture. In this way, the instrument response function can be estimated from theory, without the need for a reference transducer. We demonstrate the methodology by comparing calibration results for four different piezoelectric acoustic emission sensors: Physical Acoustics (PAC) PAC R15, PAC NANO30, DigitalWave B1025, and the Glaser-type conical sensor. From the results of these tests, sensor aperture effects are quantified and the accuracy of calibration source models is verified. Finally, this paper describes how the effects of the sensor can be modeled using an autoregressive-moving average (ARMA) model, and how this technique can be used to effectively remove sensor-induced distortion so that a displacement time history can be retrieved from recorded signals.     

Localization of a ``Synthetic' Acoustic Emission Source on the Surface of a Fatigue Specimen

The finite geometry of a laboratory specimen influences a measured acoustic emission waveform because of reflections, transmission, and mode conversion at the interface and boundaries of the specimen, thus making it difficult to determine the location of an acoustic emission (AE) source. The objective of this investigation is to develop a model experiment to identifiy the exact source location on the surface using ``synthetic' AE signals. The AE event is generated by a short local thermal expansion. This expansion is produced by the absorption of a short laser pulse which provides a noncontact and broad-band generation of elastic waves. The signals are detected by a noncontact, broad-band, and high-fidelity sensor: a laser interferometer. The triangulation with several detectors is replaced by a single probe laser interferometer located at different coordinates under reproducible conditions. The recorded signals are analyzed by wavelet transform in order to determine the arrival times of waves for several frequency levels. These arrival times are used to quantify the location of the AE source in the surface as well as the velocity of the most dominant feature, the Rayleigh wave, and the time lag between the instant of the AE and the recording of the signal. The accuracy of the method is demonstrated by comparing the identified source location with the exact one.     

Damage monitoring of reinforced concrete frames under seismic loading using cement-based piezoelectric sensor

Damage process monitoring of concrete structures using acoustic emission (AE) technology has been drawn more and more attention due to its powerful capability. In this paper, a brand new cement-based piezoelectric composite sensor was introduced with improved performance, particularly concerning its AE signals detection capacity in the high frequency domain. Such sensors were embedded into the foundation of reinforced concrete (RC) frames during construction, and appointed to monitor concrete damage due to shake table excitation. A standard ground motion record of 1999 Taiwan earthquake with 840 gal and 1300 gal peak ground accelerations were employed in the tests as the excitation inputs. The signals detected by the sensors were stored and analyzed by commercial available data acquisition devices. Advanced stochastic signal analysis methods were adopted to effectively interpret the frequency domain components and identify the useful information representing the damage processes of the RC frames. The results were compared with the eye observations of structural damage and corresponding cyclic loading tests results. It is shown that the health monitoring method using cement-based piezoelectric composite sensors and advanced stochastic signal analysis are capable of detecting and evaluating the damage process of RC frames due to seismic loading. An effective damage indicator of the RC frames is possible to be evaluated from extracted AE information.     

Near-Field Acoustic Emission Sensing Performance of Piezoelectric Film Strain Sensor

This article deals with near-field acoustic emission (AE) signal sensing with a low-profile piezoelectric film strain sensor. In general, AE signals can be represented as a summation of moment tensor (dipoles or double couples) weighted Greens’ functions. Basic theories of the Green's function and moment tensor are introduced first. The formulation presented here extends the AE elastodynamic solution to stress-wave induced surface strain response in half space. As a special case with potential use for sensor calibration, stress wave-induced surface strain response to a surface pulse load is presented. To verify the derivation, experiments were carried out with glass capillary breakage on a large steel block. The experimental result matches the theoretical prediction fairly well. Based on the surface pulse case study, the characteristics of strain and displacement signals are illustrated for both P and Rayleigh wave arrivals, which could provide insights for such strain sensor design and implementation. Due to the finite sensing area of piezoelectric film strain sensor, its aperture effect cannot be neglected in practical use, especially in higher frequency AE signal sensing, which is also investigated in this article.     

Optimization of Positioning of an Acoustic Emission Sensor for Monitoring Hot Forging

Optimization of a sensor location for effective characterization of a hot forging process using acoustic emission (AE) signals is discussed in this paper. Acoustic emission signals generated during forging operations on an aluminium alloy were recorded using three sensors simultaneously by mounting them on the top bolster, bottom bolster, and bottom die of the press. The AE signals with maximum sensitivity could be detected with a sensor attached to the bottom die in preference to the other positions. Using AE parameters, the forging process could be differentiated into three regions, i.e., 1) yielding of the workpiece material, 2) intermediate deformation region, and 3) filling of the die. The results show that the optimum position of the AE sensor for monitoring hot forging is found to be the bottom die of the forging press.     

Reciprocity calibration of impulse responses of acoustic emission transducers

By means of reciprocity calibration in Rayleigh-wave and longitudinal-wave sound fields, frequency characteristics of amplitude and phase of absolute sensitivity of acoustic emission transducers were measured on the basis of the newly derived complex reciprocity parameters, and the impulse responses were obtained through inverse Fourier transform. Calibration results were confirmed with supplemental experiments in which the fracturing of a pencil lead was utilized for the source of elastic waves. Impulse responses of acoustic emission transducers to both the Rayleigh-wave and longitudinal-wave displacement velocities were determined by means of purely electrical measurements without the use of mechanical sound sources or reference transducers.     

Acoustic emission calibration instrumentation

An automatic calibration instrument was developed to improve the calibration accuracy of acoustic emission (AE) instrumentation. The instrument facilitates calibration by automatically displaying the maximum value of the stress used to produce AE calibration pulses and the AE pulse. A repeatable AE signal was generated by breaking a graphite rod on the test assembly. By measuring the breaking load of the rod and the maximum root-mean-square values of the resulting AE signals, a relative calibration of the AE measurement system was achieved. Low-noise amplifiers and filters were developed to improve the sensitivity of AE measurements by more than one order of magnitude over commercially available instruments; thus, the smaller signals obtained in parts testing can be detected     

用于非线性兰姆波检测的高灵敏度宽带光纤光栅传感器

光纤传感器因其灵敏度高,已逐渐应用于超声检测的研究中,但大多数光纤传感器的频带响应范围有限,约为几百k Hz,很难检测到更高频率的信号。所提出的光纤布拉格光栅(Fiber Bragg Grating,FBG)传感器的高频检测范围可以达到4 MHz左右,大大提高了其检测带宽范围。文中将传感器应用于304不锈钢板兰姆波的非线性检测,同时与传统超声换能器的检测结果做对比。实验结果表明,用脉冲波激励信号时,FBG传感器可以检测到钢板兰姆波的基频到五倍频信号,表明FBG在检测兰姆波非线性上是有很大潜力的。     

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