Phased array ultrasonic evaluation of ultra high strength steel weldments for aerospace applications

Ultra high strength steels viz. Maraging 250 grade steel, that are used for realising rocket motor casings have a tendency to undergo brittle fracture in the presence of cracks.  Thus, it becomes imperative to resort to fracture mechanics for determination of critical crack sizes that could cause a catastrophic structural failure.  The flaw sizes that are characterised as critical are then taken as the minimum detectable flaws for NDT. Ultrasonic NDT is an effective method to detect small tight cracks available in weldments. Weld defects are sized/evaluated by comparison  with standard reference notches listed in  AMS 2632 viz.  E, F and G Notches. The weld defects occur with a wide spectrum of geometric shapes and orientation. Hence there is possibility of missing defects of certain orientation and depth when carrying out Ultrasonic inspection with a single angle UT Probe. To overcome this problem during conventional ultrasonic inspection, multiple probes of various beam angles viz. 45deg, 60deg and 70 deg are used to inspect the same weldment. This results in multiple scanning of weldments which is further time consuming. Moreover certain type of defects that have a favourable orientation to the beam may not give the right amplitude echo so as to size it with the above said standard notches. An advanced Ultrasonic Inspection using Phased Array Ultrasonic Testing is implemented where sectorial scanning using various angles is performed on maraging steel weldments. The detection of the various SAE AMS standard reference notches are demonstrated on the welded plates. Phased array Sector scan result is compared with local A-Scan result to arrive at the geometric attributes of the defect.