DETECTION LIMITS FOR SINGLE SMALL FLAWS AND GROUPS OF FLAWS WHEN USING FOCUSED ULTRASONIC TRANSDUCERS

The performance of ultrasonic imaging systems, using line-focussed tra nsducers, was evaluated theoretically by employing a simple technique based on convolution. The analysis considered the response of single r eflectors, which are mainly voids, and also multiple reflectors locate d both on a single plane and on multiple planes, which represent a por ous layer or bond-line. The limits of detection for each of the above cases, in the presence of different levels of acoustic noise in the ob ject under test were also calculated. The results presented are expres sed as a function of a dimensionless flaw size, incorporating the ultr asonic frequency and beam F-number. The results can be used to identif y the best match between ultrasonic equipment and a specific imaging p roblem. They can also serve to guide decisions regarding the most suit able detection method for use with the chosen equipment. If the inspec tion is properly designed, surprisingly small flaws could be reliably detected even in objects where significant levels of incoherent acoust ic noise are found. The method and results presented in this paper for line-focussed transducers fit qualitatively the case of spherically f ocussed transducers. The method is highly versatile and it can be exte nded to cover a range of problems, outside the scope of this paper, in cluding, quantitatively, that of the spherically focussed transducer b y employing a formulation which uses more computer time and memory.