相控阵探头

1.超声声束的特性
Conventional longitudinal wave ultrasonic transducers work as a piston source of high-frequency mechanical vibrations, or sound waves. As voltage is applied, the piezoelectric tranceducer element(often called a crystal) deforms by compressing in the direction perpendicular to its face.When the voltage is removed, typically less a microsecond later,the element springs back, generating the pulse of mechanical energy that comprises an ultrasonic wave(See Figure 2-1). Similarly, if the element is compressed by the pressure of an arriving ultrasonic wave, it generates a voltage across its face. Thus a single piezoelectric element can act as both a transmitter and receiver of ultrasonic pulses.
常规纵波超声探头工作的方式如同可发出高频机械振动的活塞，探头产生的这种振动即为声波。在压电换能器晶片（通常称作晶片）被施与电压时，垂直于晶片表面的方向会受压变形。电压消失后，一般在一微秒之内，晶片反弹，产生机械能脉冲（见图2-1）。
样道理，如果晶片受到射入超声波的压力，也会在其表面产生电压。这样，同一个压电晶片既可以作为铲哦生脉冲发生器，又可以充当超声脉冲接收器。

如果晶片是矩形，则扩散规律如下：

Wave formation:While a single element transducer can be thought of as a piston source, a single disk, or plate pushing forward on the test medium, the wave it generates can be mathematically modeled as the sum of the the waves from a very larger number of point sources. This derives from Huygens’ principle……
波前形成：如果把单晶探头看作一个向被测介质2推动的活塞声源、单片磁盘、或平板，则其产生的声波从数学角度上可以被定型为从大量点源发出的声波的总和。这种理解源自惠更斯原理……

Beam Spreading: In principle, the sound wave generated by a transducer travels in a straight line until it encounters a material boundary. But if the sound path length is longer than the near-field distance, the beam also increases in diameter,diverging like the beam of a spotlight.
声束扩散：原理上讲，由探头产生的声波会沿直线传播，直到遇到材料介质的边缘。但是如果声波的声程长度比其近场距离长，则声束的直径还会增加，会像聚光灯的光束一样发生扩散。

All transducer of the kind most commonly used for ultrasonic NDT have the following fundamental functional properties:
所有最常用于超声无损检测的探头在功能方面都有以下基本属性：
1. Size：the diameter or length and width of the active transducer element,which is normally housed in a somewhat larger case.
尺寸：尺寸是指激活探头晶片的直径，或者晶片的长度和宽度。晶片通常被置于比它稍大一点的外壳中。

2. Frequency: The number of wave cycles completed in one second, normally expressed in KHz or MHz. Most industrail ultrasonic testing is done in the 500KHz to 20MHz frequency range. Penetration increase with a lower frequency, while resolution and focal sharpness increase with a higher frequency.
频率：频率是指一秒钟内声波完成振动周期的次数，通常用千赫兹（KHz）或兆赫（MHz）表示。频率越低，穿透力越强；频率越高，分辨率和聚焦力度越高。
3. Bandwidth: The portion of the frequency response that  falls within specified amplitude limits. In this context, it should be noted that typical NDT trancducers do not generate sound waves at a single pure frequency,but rather over a range of frequencies centered at the nominal frequency designation. The industry standard is to specify this bandwidth at the -6dB(or half amplitude) point.
带宽：带宽是指处于特定波幅范围内的频率响应的部分。在这种情况下，应该注意的是典型的NDT探头不会生成单一频率的声波，而只能在以额定的频率值为中心的频率范围内生成声波。工业标准将这个带宽确定在-6dB(或半波幅)的位置处。
4.Waveform duration: The number of wave cycles generated by the transducer each time it’s pulsed. A narrow bandwidth transducer has more cycles than a broader bandwidth transducer.
波形持续：波形持续是指每次探头被脉冲触发后生成的波动周期的数量。窄带宽探头要比宽带宽探头生成的波动周期数量多。
5. Sensitivity: The relationship between the amplitude of the excitation pulse and that of the echo received from a designated target.
灵敏度：灵敏度是指激励脉冲波幅与从指定目标反射的回波波幅之间的关系。

二 ，超声声场近场和远场
The sound field of a transducer is divided into two zones: the near field and far field (see Figure 2-4).
探头的声场被分为两个区域：近场和远场（见图2-4）。
The near field is the region close to the transducer where the sound pressure goes through a series  of maximums and minimums, and it ends at the last on-axis maximum at distance N from the face. Near field distance N represents the natural focus of the transducer.
近场是指接近探头的区域。在这个区域中声压反复几次达到最大值和最小值。这个区域的终端为轴上最后一次出现最大声压值得位置。这个位置到探头表面的距离表示为N，即近场距离。近场距离代表探头的自然焦距。
The far field is the region beyond N where the sound pressure gradually drops to zero as the beam diameter expands and its energy dissipates. The near field distance is a function of the transducer’s frequency and element size,and the sound velocity in the test medium,and it can be calculated for the square or rectangular elements commonly found in phased array testing as follows:
远场是近场距离（N值）以外的区域。在这个区域，随着声束直径的扩展及能量的消散，声压逐渐降低为零。近场距离是探头频率、晶片大小以及被测材料的声束互相作用的一个函数。通过以下公式，可以为相控阵检测中常用的正方形或长方形晶片，计算出这个函数值：

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