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Phased Array, TOFD, and TFM/FMC: Comparing Methods for Weld Lack of Fusion Detection

Phased array ultrasonic testing (PAUT), time-of-flight diffraction (TOFD) ultrasonic testing and full matrix capture (FMC) / total focusing method (TFM) are the three advanced ultrasonic non-destrictive testing techniques used routinely for full penetration weld inspections.  In this article, the application of PAUT, TOFD, and FMC/TFM to a single vee steel plate butt weld is studied with emphasis on inspection setup fundamentals, data presentation, and performance.  While these three techniques use the the same ultrasonic waves,they are applied with different nuances that are important to understand.  This article provides the reader with an excellent first introduction to PAUT, TOFD, and FMC/TFM  performance comparisons.

Description of Steel Plate Weld

The single vee weld joined two 0.375" thick carbon steel plates.  The welding process included the implementation of lack of fusion (LOF) along the bevel  in the upper half of the plate.  The LOF was approximately 0.30" long and 0.15" high.  A second lack of fusion was inserted in the root along the vertical surface.  The root LOF was 0.50" long and 0.15" high.  This weld flaw broke the bottom surface of the plate. 

Phased Array Ultrasonic Testing for Lack of Fusion

The PAUT weld inspection was setup using an Olympus 5L16 probe with an S-scan angular range of 45 to 70 degrees. THE PAUT scan plan is shown below. The first leg was used mainly for a root pass inspection. The volume of the weld was covered in the second leg which also included the heat affected zone (HAZ).

Figure 1: Phased array (PAUT) scan plan for 0.375” thick steel welded plate.

The PAUT scan length shown is approximately 3.5 inches and encompasses both the root lack of fusion and mid-thickness lack of fusion. Both weld defects were detected clearly with the phased array setup. The root lack of fusion is observable from roughly 0.80” to 1.27”. In the S-scan, it is easiest to discriminate between the root LOF and the actual root in the third leg. At the end of the first leg, it is difficult to spatially differentiate between the root and root LOF. The third leg state provides excellent data redundancy for confirmation. The height of the root LOF was measured at 0.12”. The lack of fusion in the upper half of the plate was detected with the PAUT focal laws in the 50-60 degree range over the 2.70 to 3.10 scan axis length. The height was measured to be 0.070” in the S-can.

Time-of-Flight Diffraction Testing for Lack of Fusion

The time-of-flight diffraction (TOFD) inspection was performed with 10 MHz, 0.25” diameter, transmitter and receiver. The TOFD probe center spacing was calculated to be 1.5 for the 0.375” thick welded plate. The target focal point was set to 2/3 the plate thickness.