Phased Arrays for Detecting Cracking in Bolts

Phased arrays can be used for detecting cracking (and other defects) in high tensile bolts. The technique uses a “fingerprinting” approach to the bolts. In contrast to current code procedures where an operator manually scans a normal beam probe over a bolt and interprets the results, this phased array technique uses an S-scan and a mechanized scanner. Specifically, the phased array probe is mounted in contact on the head of the bolt, and performs a small angle longitudinal wave S-scan down the bolt. The array is mounted in a mechanized and encoded jig for 360 o rotation round the bolt. Signals are received from the threads, bolt geometry and from mode conversions. In essence, the Sscan gives a fingerprint of the bolt at all depths and orientations when the operator scans round the bolt collecting data. Any perturbations from defects or cracks are typically very visible. Interpretation is much simpler than with alternative bolt inspection techniques. Some examples from calibration and cracked bolts will be shown. The technique can be fine-tuned for different bolt dimensions and features. INTRODUCTION Current methods of inspecting bolts typically involve calibrating as normal, then scanning round the head of the bolt using normal beam probes (1). Interpretation of the results has proved difficult, as signals are reflected from geometry, bolt threads, and mode conversions occur, as well as reflections from cracks and corrosion. In addition, the usual ultrasonic issues arise: operator variability, no stored data, rough bolt surfaces, inconsistent coupling, no position data. In view of the difficulty of bolt inspection, perhaps it is not surprising that there are not a lot of alternative techniques available. One of the few is the Centrifugally Guided Wave Technique (CGWT) from Southwest Research Institute (2). This technique essentially floods the bolt with ultrasound by using a large transducer on the end. Signals return from threads, geometry, mode conversions and defects as expected. However, it is possible to work out which defects are anticipated for a given bolt geometry from wave physics, and hence deduce if defects are present or not. Unfortunately, the analysis may not be trivial. In contrast, phased arrays have proven to be readily applicable to bolt inspections. Phased arrays are a relatively new technology which permits electronic scanning, sweeping and focusing of the ultrasound beam. Phased arrays are well described elsewhere (3), and will not be covered here. Bolt inspection can be performed with portable, encoded phased array units, as shown in Figure 1. The initial phased array bolt investigations have been performed on cracked bolts from various sources (3). The principles remain the same, whatever the industry or application. Figure 1 Photo of Olympus NDT OmniScan performing an L-wave S-scan. For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=70 6th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components October 2007, Budapest, Hungary