TABLE OF CONTENTS We will start our analysis of the PWAS phased array with the simplest configuration: we will discuss the 1-D, a.k.a. linear, phased array (Fig. 11.3). Arrays are made up of a number of elements, usually identical in size and arranged along a line, at uniform pitch. The wave pattern generated by the phased array is the result of the superposition of the waves generated by each individual element. By sequentially firing the individual elements of an array transducer at slightly different times, the ultrasonic wavefront can be focused or steered in a specific direction. Thus, inspection of a wide zone can be achieved by electronically sweeping and/or refocusing without physical manipulating the array.
Once the beam steering and focusing have been established, the detection of internal flaws is done with the pulse-echo method (Fig. 11.4). A pulse, consisting of a smooth-windowed tone-burst of duration t p, is transmitted toward the target. The target reflects the signal and creates an echo, which is detected by the radar. By analyzing the phased-array signal in the interval ( t p , t p + t 0), one identifies the delay, ?, representing the time-of-flight (TOF) taken by the wave to travel to the target and back. Knowing TOF and wave speed allows one to precisely determine the target position.
