声波全息照像 翻译 2

1、第25篇 100813 3.7 声波全息照像 1.光学全息照像是现在众所周知的三维数据存储在一个二维的记录面上的一种方法，并作为超声波脉冲相干辐射的形式，可以对超声应用类似的技术。但是，必须记住，超声无损检测中使用的波长很长，和要检测的缺陷的大小差不多。超声波光圈大小也只有几个波长，所以纵向图像分辨率可能不是很理想。 2.线性声全息原理是超声波传感器扫描沿着一条直线并产生一个宽声场平面扫描线。收到的回音乘以两个参考信号，对传输的脉冲进行0度和90度的改变，所以在多次沿孔径采样会收到真正的和虚显示的回波。数据存储在计算机中，通过使用二次相函数；计算机定义一个重建的平面和一个观察的角度，并计算缺陷

2、的声场强度区域。可以从配置文件的超声强度场得到缺陷的尺寸。声全息技术的主要缺点是在超声场方向的分辨率低，但一定程度上克服这一聚焦探头，或由多频全息术，或合成孔径聚焦技术。 3.声全息的早期作品是使用浸泡系统和能产生两个交叉的超声场的超声波换能器的试样，一个传感器生产参考光束；在干涉区的液面上产生静态表面位移，也就是激光束产生全息图的样本。后来，电子参考光束投入使用，但这些系统都被已经合成孔径扫描的计算机重建技术取代。3.8合成孔径聚焦技术（SAFT） 4.从扫描传感器收集和处理数据已经提议了许多方法，以模拟一个更大的传感器有更好的分辨率和信噪比。SAFT，超声波传感器的应用，是从合成孔径雷达系

3、统理论得到的。理论上，可以考虑不规则的屈光接口中的标本及复杂角散射的缺陷。 5.SAFT使用脉冲超声、记录两波的振幅和相位，不需要引用函数。在数字化以后，从一系列探测器位置的数据可以被存储（图3.5）。 从探测器到缺陷的传输时间，探测器在不同的位置，是通过一个基于时间的算法来更正；景深效果是可以克服的聚焦算法和不平衡性接口材料的表面相校正算法；系列脉冲从每个探测器的位置发射的平均噪音被减小。实际孔径大小是理论的大型传感器的大小，当然不可以是无限大的，如超声波束探测器定位在孔径的末端仍必须达到该漏洞。6.声全息SAFT的主要优势似乎可能在纵向分辨率的改善，虽然这只能通过复杂的电脑程序来获得。3.

4、9超声成像系统 7.试块的超声波“图像”可以通过已经描述的C-扫描显示，通常在浸泡池中，通过探头扫描常规栅格的表面。然而，这种机械扫描速度太慢，并且光束直径极为有限限制了横向分辨率。如果使用的是聚焦超声探头，该字段宽度更小，和横向分辨率提高了，但机械扫描时间将更长时间。这就需要电子扫描，以及多元线性阵列传感器来加速检测进程。 8.传感器必须内置一些延时装置，以便从一个点在试样的信号在同一时刻到达的所有元素。这样的多阵列系统发现医学超声成像的广泛使用，但尚未为工业应用。这种扫描可以是机械或电子扫描阵列相控阵。 9. 在相控阵中，元素间距必须小于波长的一半，以避免虚假衍射瓣，所以制造困难；然而，在

5、帧速率，线密度，视场之间还要有一定有用的灵活度。现役系统一般工作在3和15MHz之间。 图3.705 合成孔径聚焦技术（SAFT）：为了简单起见数字化和时间-传输距离校正后，只显示三个探头位置。用的是集成信号。 10. 在金属直接超声成像方面已研制了一系列设备，从具有超声敏感的电视类相机面板管，通过使用超声波镜片，电脑重构图像。超声波镜片可制成固体材料，或用薄的，塑形的外壳填充液体制成。透镜设计类似于光学透镜设计，应根据不同材料的超声速度来选择的屈光指数。 11.几种用来使超声图像可见的方法被，如下。（1） 液表漂浮法（如同提到的简单的声全息系统。）（2） 弹光效应：在合适的透明固体，偏振光可

6、以使超声波可见，在压力下形成光的双折射。（3） 纹影方法：更改光的折射，在透明材料时，承受压力变化，可以做可见超声场。（4） 由电场的液晶可以刺激变得混浊和光的散射，超声波可以产生同样的效果。（5） 从光和超声波束在液体后者可以被视为光纤光栅光栅折射的变化相互作用的衍射的影响。如果满足布拉格衍射条件。 其中是光束角度，是光波长，是声波波长，衍射光进行光代表性的声场。 12. 直接图像可视化技术是有吸引力的，他们能实时成像，并且设备，如径向扇形扫描仪，扫描沿径向线从传感器阵列的中心扩展，已在医学超声应用非常成功。延迟线系统很复杂，只有少量元素上的传感器，所以模拟数字转换器得到更快和更便宜，数字计

7、算机技术似乎可能会用于下一代探测器，已经描述在合成孔径系统之下。 13。需要检查快速移动的物体，如人的心脏，相应的的应用在工业无损检测中还没有出现；因此可以允许较长的图像恢复时间。目前，大多数医用电子方式扫描超声系统使用比电视栅格图像较小的行数。一些成像问题，可以通过使用压电薄膜材料氟乙烯构成的传感器阵列消除。3.10超声波衰减测量 14.近代的技术发展已经简化了在较宽的频率范围内精确测量金属超声衰减的问题，似乎有可能衰减数据可能被用来预测有关的材料属性，以便强度计算。 15.宽带、高频传感器，由于快速上涨时的脉冲星是由石英缓冲杆耦合到试块。试样厚度通常很小（1-20mm）。 平行面，大大超出

8、传感器。从标本回来的反射声波被捕获并处理，来校正在接口的衍射损失。关于频率f，平均粒度、D和衰减的曲线就形成了。不同的值，就会有不同的关联，普遍的形式是 其中a和b是常量。 这种关系很复杂，因此一直是理论和实验研究的主题。 16.对于低频(>>D)，似乎衰减主要是由于几何散射，因为整个晶粒（grain）作为散射单位。在这一区域，和f4.成正比。 在高频 (<< D)，散射是一个扩散的过程，是独立的频率，并在中间区域，由于最初的平面波前的相位差异所造成的大量“相散射”。在这一区域，与 f2.成正比。 17. 这些数据主要从单晶材料获得，同时它对多晶合金材料还是有效的，由于

9、混乱，有一个额外的衰减影响，这最终有可能涉及超声衰减测量疲劳强度的属性。至少，超声衰减可用于检测材料性能，但目前似乎有超声波应力波相互作用，材料微观结构和金属机械失效方面的理论不足，。 18.已经有超声衰减和某些钢的抗冲击强度，衰减和断裂韧性，以及纵波速度与抗拉强度的铸铁相关的经验。使用单个不锈钢样本的断裂韧性会随着温度的变化而变化，这听上去超声衰减和断裂韧性的相关性是不好的，断裂韧性的变化远远大于衰减的变化。原因被认为是在其他材料性能的变化，如屈服应力，也会影响断裂韧性，而超声衰减，主要取决于晶粒的大小。3.11应力测量 19.超声速度测量可用于测量金属应力。这项技术应用并不容易，如超声波速

10、度随压力变化很小，并且不同的材料也不一样，但超声脉冲准确上升时间的能力使得该技术技术可行，通过几个实际应用，例如钢螺栓和残余应力测量中的轴向载荷的测量。超声速度也随温度变化，所以试样温度必须准确控制。然后技术的真正的限制是，在很多材料超声脉冲变成了扭曲的，所以测量的精度降低。 20，避免此问题的技术是通过改变频率相位差的常数来测量双颜色之间的相位差。例如使用小试样，用水浴，然后接收脉冲前后表面的重叠。 21. 应力的存在旋转极化平面的极化剪切波，并且旋转角度和压力的大小有相关性。因此这种旋转测量可用于测量平均量超声波束在固体材料内部的应力。 22. 测量残余应力的更先进的技术是使用剪切波在两个

11、相互垂直的方向上极化。这些波速度相近，所以发生干涉，因此随着传感器旋转，当偏振平面平行、垂直轴压力，干扰消失了。一旦这个轴是已知的，就可以从速度计算实际的压力。 23. 瑞利表面波声弹效应也被用于测量在不锈钢管道的残余应力。使用急救医疗助理传感器，以消除耦合问题和提高可重复性。使用1.5兆赫的频率和校准曲线上开发的测试样本，然后管道是使用应变计来测量。在零压力的瑞利波速度是未知的，是微观结构变化的影响，所以主要的使用则相对而非绝对测量技术。3.12临界角反射率 24.如果从液/固界面的反射率来衡量，有一个非常尖锐的最小反射率在瑞利临界角（图3.6）。从这个角度的知识，瑞利和Lamb波的速度可以

12、确定，最低的形状是受固体表面层的衰减特性，或固体的表面层的接口属性，或接口之间两种不同材料的属性的影响。超声波测向仪进行测量和大部分的金属临界角的值可以确定±1分钟，从而使表面波速度可以测量到±1ms-1。因此，可将该技术用于测量灰铸铁抗拉强度，措施对奥氏体钢冷作模具的条件，研究热处理或纹理，并衡量然后厚度薄表层，如胶结厚度。25. 最小反射的超声波频率和超声衰减，晶粒尺寸有关，但关系极为复杂，至今还没有发现哪种实用的应用来进行该测量。 图3.6临界角反射率水不锈钢界面显示一个尖锐的最低反射 原文：3.7 Acoustic holographyOptical hologra

13、phy is now well known as a method of storing three-dimensional data on a two-dimensional recording surface, and as an ultrasonic pulse is a form of coherent radiation, it is possible to apply similar techniques to ultrasound. It must, however, be remembered that ultrasound as used in NDT has a long

14、wavelength, of the same magnitude as the size of the defects to be detected. The ultrasonic aperture size is also only a few wavelengths, so longitudinal image resolution is likely to be poor.。The principle of linear acoustic holography is that the ultrasonic transducer is scanned along a line and p

15、roduces a wide sound field in the plane of the scan line. The received echoes are multiplied with two reference signals, one and shifted against the transmitted pulse, so producing a real and imaginary part of the received echo at a number of sampling points along the aperture. The data is stored in

16、 a computer and by using quadratic phase functions; the computer defines a reconstruction plane and an observation angle, and calculates the ultrasound field intensity at the region of a defect. From the profile of this ultrasound intensity field, the defect size can be derived. The main disadvantag

17、e of acoustic holography is the poor resolution in the direction of the ultrasound field, but this can be overcome to some extent with a focusing probe, or by multi-frequency holography, or by synthetic aperture focusing techniques.-Early work on acoustic holography used an immersion system with two

18、 ultrasonic transducers with overlapping ultrasound fields at the specimen, one transducer producing the reference beam; the interfering fields produced a static surface displacement on the liquid surface, which was optically illuminated with a laser beam to produce a hologram of the specimen. Later

19、, an electronic reference beam was used, but both there systems appear to have been superseded by computer reconstruction techniques with synthetic aperture scanning.3.8 Synthetic aperture focusing technique (SAFT)Various methods have been proposed for collecting and processing data from a scanned t

20、ransducer, so as to simulate a much larger transducer having much better resolution and signal-to-noise ratio. SAFT, applied to an ultrasonic transducer, is derived in principle from synthetic aperture radar systems. In theory, it is possible to take into account irregular refractive interfaces in t

21、he specimen and complex angular scattering by defects.SAFT uses pulsed ultrasound, records both wave amplitude and phase, and does not need a reference function. The data from a series of probe positions is stored, after digitization (Fig.3.5). The time of flight from the probe to the defect, for di

22、fferent probe positions, is corrected by a time-based algorithm; depth of field effects can be overcome by a focusing algorithm and the uneven nature of the material interfaces by a surfacephase-correction algorithm; noise is reducer by averaging a series of pulses from each probe position. The prac

23、tical aperture size; which is the size of the theoretical large transducer, can not of course be infinitely large, as the ultrasonic beam from the probe positioned at the extremities of the aperture must still reach the flaw.The main advantage of SAFT over acoustic holography appears to be the impro

24、vement which is possible in longitudinal resolution, although this is obtained only by complex computer programs.3.9 Ultrasonic imaging system 7. An ultrasonic image of a specimen can be obtained by the C-scan display already described, by scanning a probe over the surface, usually in an immersion t

25、ank, on a regular raster. However, such mechanical scanning is slow, and the transverse resolution is severely limited by the beam diameter. If a focused ultrasonic probe is used, the field width is smaller, and transverse resolution is improved, but mechanical scanning times will be still longer. E

26、lectronic scanning is needed, together with a multi-element linear array of transducers, to speed up the process.8. Some time-delays must be built into the transducer elements so that a signal from one point in the specimen reaches all the elements at the same moment. Such multi-array systems have f

27、ound widespread use for ultrasonic imaging in the medical field, but not yet for industrial applications. The scanning can be mechanical or electronic using a steered array (phased array).9. In a phased array, the element spacing must be less than half the wavelength to avoid spuriousFig.3.705 Synth

28、etic aperture focusing technique (SAFT): only three probe positions are shown for simplicity after digitization and time-of flight correction, the signals are integrateddiffraction lobes, so there are difficult fabrication problems; however, there is also a useful flexibility between frame rate, lin

29、e density, and field of view. Systems working between 3 and 15MHz are now in use.10. A whole range of equipment has been proposed for direct ultrasonic imaging in metals, from television-type camera tubes with a faceplate sensitive to ultrasound, through the use of ultrasonic lenses, to computer-rec

30、onstructed images. Ultrasonic lenses can be made of solid materials, or with liquids inside a thin, shaped skin. Lens design is analogous to optical lens design, with appropriate values of refractive indices, based on the ultrasonic velocities in the different materials.11. Several methods have been

31、 used to make a spatial ultrasonic image visible, as follows.(1) liquid surface levitation(as mentioned briefly for acoustic holography systems).(2) Elasto-optical effects: in suitable transparent solids, polarized light can make ultrasonic wave visible, by becoming optically double refracting under

32、 stress.(3) Schlieren methods: the change in optical refractive index, when a transparent material is subjected to a pressure change, can be used to make visible an ultrasonic field.(4) Nematic liquid crystals can be stimulated by an electric field to become turbid and scatter light, and ultrasonic

33、waves can produce the same effect.(5) Diffraction effects from the interaction of a light beam and an ultrasonic beam in a liquid the latter can be regarded as an optical grating with changes in refractive index along the grating. If the Bragg diffraction condition is fulfilled. Where is the beam an

34、gle, is the optical wavelength, and is the acoustic wavelength, the diffracted light carries an optical representation of the acoustic field. 12 。 Direct image visualization techniques are attractive in that they produce instantaneous image displays, and equipment such as the radial sector scanner,

35、with a scan along radial lines extending from the centre of the transducer array, has been very successful in medical ultrasonic applications. The delay-line system is complex and is only possible for a small number of elements on the transducer, so that as analogue/digital converters get faster and

36、 cheaper, digital computer techniques seem likely to be used for the next generation of imagers, as already described under synthetic aperture systems. 13. The need to examine rapidly moving objects, such as the human heart, is not present in industrial non-destructive testing applications; hence lo

37、nger image build-up times can be tolerated. As present, most electronically-scanned ultrasonic systems for medical applications use a much smaller number of lines to form the image than a television raster, so that even with interpolation routines the image appears crude. For imaging, some of the pr

38、oblems in constructing transducer arrays can be eliminated by using a plastic sheet piezoelectric material PVDF.3.10 Ultrasonic attenuation measurements 14. Recent technical developments have simplified the problems of accurate measurement of ultrasonic attenuation in metals over a wide range of fre

39、quencies and it seems possible that attenuation data might be used to predict material properties which are relevant to strength calculations.15. A broadband, high-frequency transducer, driven by a fast-rise-time pulsar is coupled to a specimen by a quartz buffer rod. The specimen thickness is norma

40、lly small (1-20mm). Parallel-sided,and considerably wider than the transducer. Several return echoes from the back of the specimen are captured and processed, to correct for diffraction losses and reflections at the interfaces. Curves relation frequency, f, mean grain size, D, and attenuation, , can

41、 then be potted. For different values, different relationships appear to hold, generally of the formWhere a and b are constants. The relations are therefore complex and continue to be the subject of both theoretical experimental investigations.16.With low frequencies (>>D), it appears that att

42、enuation is mostly due to geometric scattering, in the sense that the whole grain acts as the scattering unit. In this region, is proportional to f4. At very high frequencies (<< D), scattering is a diffusion process and is independent of frequency, and in the intermediate range there is phase

43、 scattering due to the phase differences on the initially plane wave front caused by the large number of paths associated with each elementary portion of the wave front. In this region, is proportional to f2.17.This data has been obtained largely from work in pure materials and while it is also vali

44、d for polycrystalline alloyed materials, there is an additional attenuation effect due to dislocations, which may eventually make it possible to relate ultrasonic attenuation measurements to fatigue strength properties. At the least, ultrasonic attenuation can be used to monitor material properties,

45、 but at present there seems to be insufficient theory on the interaction of ultrasonic stress waves, material microstructure, and mechanical failure in metals.18.There have been claims for empirical correlations between ultrasonic attenuation and the impact strength of certain steels, between attenu

46、ation and fracture toughness, and between compressional wave velocity and the tensile strength of cast iron. Using a single sample of stainless steel in which the fracture toughness could be varied with temperature, it was sound that the correlation between ultrasonic attenuation and fracture toughn

47、ess, was not good, the variation in being much larger than the variation in attenuation. The reason was thought to be variation in other material properties, such as yield stress, also affecting, whereas ultrasonic attenuation is mostly dependent on grain size.3.11 Stress measurement 19 .Ultrasonic

48、velocity measurements can be used to measure stress in metals. The technique is not easy to apply, as the change in ultrasonic velocity with stress is very small and varies with different materials, but the increased ability to time the arrival of ultrasonic pulses accurately () has made this a feas

49、ible technique, with a few practical applications, such as the measurement of axial loads in steel bolts and residual stress measurement. The velocity of ultrasound also varies with temperature, so that specimen temperatures must be accurately controlled. The real limitation of then technique is tha

50、t in many materials the ultrasonic pulse becomes distorted and so the accuracy of measurement is reduced.20. A technique which avoids this problem is to measure the phase difference between two-tone bursts, by changing the frequency to keep the phase difference constant. Small specimens are used, in

51、 a water bath, and then pulses received from the front and back surfaces overlap. 21. The presence of stress rotates the plane of polarization of polarized shear waves and there is some correlation between the angle of rotation and the magnitude of the stress. Measurement of this rotation can theref

52、ore be used to measure internal stress in a solid averaged over the volume of material traversed by the ultrasonic beam. 22. A more advanced technique for measuring residual stress is to use shear waves polarized in two mutually perpendicular directions. These waves have slightly different velocitie

53、s and so interfere, so that as the transducer is rotated, the interference vanishes when the polarizing planes are parallel and perpendicular to the stress axis. Once this axis is known, the actual stress can be computed from the velocities.23.The acousto-elastic effect with Rayleigh surface waves has also been used to measure residual stress in s