铸钢轧辊超声波探伤非相关显示的分析及排除

介绍铸钢轧辊超声波探伤中非相关显示的出现机理、出现位置的理论计算和试验分析及其排除方法。     

Continuous surface wave and impact methods of measuring the stiffness and density of railway ballast

Continuous surface wave and transient impact excitation tests were performed on strips of ballast to monitor their compaction. The ballast's shear wave velocity, hence the ratio of its shear modulus and density, increased with depth. Two types of transient excitation tests were carried out: firstly an impact load was applied to the ballast and the resulting transient acceleration measured at various positions. Secondly, two accelerometers at different distances from the impact excitation were used to study the effects of compaction on the transient output signal amplitude and its attenuation. Measured shear wave velocities showed good correlation between amount of compactive effort and ballast stiffness.     

Automatic defects classification — a contribution

The objective of this work is to provide a contribution to defect classification. More precisely, we try to prove that it is possible to identify and classify defects of different types using the pulse-echo technique. The classification process makes use of the time and frequency domain responses of the ultrasonic echo signals acquired from different specimens simulating defects with three different shapes (cylindrical, spherical and planar with rectangular cross-section) and sizes. Although the final goal is the characterisation of practical defects (for instance, voids, cracks, delaminations, and so on) appearing in composite materials during manufacturing and in service, we first use the already mentioned reflectors for simplicity reasons. In these experiments 66 reflectors are used with water as matrix material. The inclusion (reflector) materials are brass, copper, steel and polystyrene. From the time domain signals we extract three features, namely, pulse duration, pulse decay rate and peak-to-peak relative amplitude of the third cycle. From the spectra of the echoes we extract the frequency for maximum amplitude and the standard error estimate from the deconvolved spectrum responses.All experimental signals were obtained using only one normal incident ultrasonic transducer aligned to maximise the direct reflected signal. In spite of the fact that this kind of configuration does not provide complete information about the characteristics of the geometries being studied, all the extracted features proved to be important discriminating factors of the geometrical classes considered, as will be demonstrated by making use of a pattern recognition technique for classification.     

材料薄层超声无损测厚方法评述

介绍了四种典型的材料薄层超声测厚方法——脉冲回波法、时域反射场分析法、兰姆波法及表面波法，阐述了四种测厚方法的基本原理，并结合具体的研究实例，对不同测厚方法的优缺点和适用条件进行了综合评述。     

小直径铜棒的超声波自动检测系统应用

针对φ2．5～15mm的铜质棒材,采用脉冲反射法的表面波和穿透法的纵波相结合的方法,可以较好地解决薄壁小直径铜质棒材的检测方法。研制了小直径铜棒超声自动检测系统。系统主要由上位机（即PC机）、下位机（即单片机控制系统）、发射／接收电路、探头和机械系统组成。上位机通过RS232串口与下位机通信,从而完成系统控制和数据采集。试验表明,系统可以检出φ2．5～15mm铜质棒材上直径为φ0．5mm,深度为1／4D的人工平底孔,可以实现定性定量检测。     

Ultrasonic Guided Waves Scattering Effects From Defects in Adhesively Bonded Lap Joints Using Pitch and Catch and Pulse-Echo Techniques

In this article a method to evaluate defect dimensions in adhesively bonded lap joints based on the measurement of scattering effects of ultrasonic guided waves is presented. A simplified theoretical model is proposed which was initially tested in plates with through holes. The experimental results obtained using both pitch-and-catch and pulse-echo techniques for 500 kHz and 1 MHz frequencies confirm the validity of this model. To evaluate the lap joint defects, a set of samples with artificial defects were manufactured and the form and dimensions were confirmed using C-scan ultrasonic images. With the same methodology used in through-hole analysis, scattering effects of defects were measured. The results obtained with the pitch-and-catch technique with 1 MHz transducers allow us to say that an estimate of defect dimensions could be done by using the proposed model with reasonable accuracy and according with the predictions.     

数字实时脉冲回波法声速测量系统的研制

使用主要硬件为波形发生器和经过二次开发的数据采集卡,设计了一种新型的实时数字脉冲回波法声速测量系统。系统检测第一回波和第二回波的第2或3个波峰的零点,设计根据两个零点的时间差求得超声波声速,为声速的测量以及其它超声检测技术提供了一种新的思路。     

脉冲回波法与水听器法测量超声探头声场参数比较研究

针对美国ASTM E1065中提及的脉冲回波法与水听器法,通过在<10MHz频率范围内的实验,比较了这两种方法在评价平面超声探头与聚焦超声探头声场参数的一致性。实验表明,针对超声探头辐射声场参数的测量,在合理选择小球参数的基础上采用脉冲回波法与水听器法具有较好的测量一致性,而水听器法则提供了一种具有量值可溯源的超声探头评价与校准方法。     

Ultrasonic measurements of undamaged concrete layer thickness in a deteriorated concrete structure

Ultrasonic wave propagation in deteriorated concrete structures was studied numerically and experimentally. Ultrasonic single-side access immersion pulse-echo and diffuse field measurements were performed in deteriorated concrete structures at 0.5 MHz center frequency. Numerically and experimentally it is shown that the undamaged layer thickness in a deteriorated concrete structure is measurable using pulse-echo measurements when the deterioration depth is larger than the wavelength. The signal overlapping, which occurs in the thin deteriorated layers, can be overcome using diffuse field measurements or a pattern matching technique. The ultrasonic experimental data were shown to be in good agreement with the widely used phenolphthalein test for concrete degradation.     

Ultrasonic detection and sizing of compressed cracks in glass- and carbon-fibre reinforced plastic composites

Nondestructive evaluation of compressed cracks is a major challenge. A quantitative study of the effect of crack-tip closure on the pulse-echo ultrasonic sizing of delaminations in fibre-reinforced polymer-matrix composites (FRP) is presented. In particular, this study focuses on the interaction of ultrasound with a closed crack or kissing disbond, and their effect on the ultrasonic inspectability of FRP laminates consisting of carbon and glass plies. The compression of laminar cracks in these two different laminate types is clearly detectable via both pulse-echo and through-transmission ultrasonic measurements, but the reflected ultrasonic pulses in the two material types exhibit markedly different behaviour. The glass-fibre laminates show a drop in the reflected signal for crack openings up to approximately half the crack growth load, whereas the corresponding carbon-fibre laminates show the expected increase in the reflected signal as the crack opens. The origins of the observed effect of crack closure on the reflection and transmission of ultrasound are analysed in detail to ascertain possible mechanisms responsible for these effects.