相控陣探頭

1.超聲聲束的特性
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.
常規縱波超聲探頭工作的方式如同可發出高頻機械振動的活塞，探頭產生的這種振動即為聲波。在壓電換能器晶片（通常稱作晶片）被施與電壓時，垂直于晶片表面的方向會受壓變形。電壓消失后，一般在一微秒之內，晶片反彈，產生機械能脈沖（見圖2-1）。
樣道理，如果晶片受到射入超聲波的壓力，也會在其表面產生電壓。這樣，同一個壓電晶片既可以作為鏟哦生脈沖發生器，又可以充當超聲脈沖接收器。

如果晶片是矩形，則擴散規律如下：

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：如果把單晶探頭看作一個向被測介質2推動的活塞聲源、單片磁盤、或平板，則其產生的聲波從數學角度上可以被定型為從大量點源發出的聲波的總和。這種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.
聲束擴散：原理上講，由探頭產生的聲波會沿直線傳播，直到遇到材料介質的邊緣。但是如果聲波的聲程長度比其近場距離長，則聲束的直徑還會增加，會像聚光燈的光束一樣發生擴散。

All transducer of the kind most commonly used for ultrasonic NDT have the following fundamental functional properties:
所有最常用于超聲無損檢測的探頭在功能方面都有以下基本屬性：
1. Size：the diameter or length and width of the active transducer element,which is normally housed in a somewhat larger case.
尺寸：尺寸是指激活探頭晶片的直徑，或者晶片的長度和寬度。晶片通常被置于比它稍大一點的外殼中。

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.
頻率：頻率是指一秒鐘內聲波完成振動周期的次數，通常用千赫茲（KHz）或兆赫（MHz）表示。頻率越低，穿透力越強；頻率越高，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.
帶寬：帶寬是指處于特定波幅范圍內的頻率響應的部分。在這種情況下，應該注意的是典型的NDT探頭不會生成單一頻率的聲波，而只能在以額定的頻率值為中心的頻率范圍內生成聲波。工業標準將這個帶寬確定在-6dB(或半波幅)的位置處。
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.
波形持續：波形持續是指每次探頭被脈沖觸發后生成的波動周期的數量。窄帶寬探頭要比寬帶寬探頭生成的波動周期數量多。
5. Sensitivity: The relationship between the amplitude of the excitation pulse and that of the echo received from a designated target.
靈敏度：靈敏度是指激勵脈沖波幅與從指定目標反射的回波波幅之間的關系。

二 ，超聲聲場近場和遠場
The sound field of a transducer is divided into two zones: the near field and far field (see Figure 2-4).
探頭的聲場被分為兩個區域：近場和遠場（見圖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.
近場是指接近探頭的區域。在這個區域中聲壓反復幾次達到最大值和最小值。這個區域的終端為軸上最后一次出現最大聲壓值得位置。這個位置到探頭表面的距離表示為N，即近場距離。近場距離代表探頭的自然焦距。
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:
遠場是近場距離（N值）以外的區域。在這個區域，隨著聲束直徑的擴展及能量的消散，聲壓逐漸降低為零。近場距離是探頭頻率、晶片大小以及被測材料的聲束互相作用的一個函數。通過以下公式，可以為相控陣檢測中常用的正方形或長方形晶片，計算出這個函數值：

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