Introduction
Non-destructive testing (NDT) of welds with phased array ultrasonic testing (PAUT) and more recently the total focusing method / full matrix capture (TFM/FMC) are two excellent techniques to detect, locate and size weld defects. Compared to ultrasonic weld testing, PAUT provides more efficient volumetric testing, increases weld defect probability of detection (POD), and should improve flaw sizing given the same NDT inspector skill level. The Total Focusing Method/Full Matrix Capture (TFM/FMC) capture should provide comparable POD compared to PAUT with some potential gains in defect resolution, sizing, and even classification of weld defects. This article compares PAUT and TFM data and describes the advantages of data redundancy for increased POD and reduction of false calls.
Quick Introduction to PAUT and TFM/FMC
Phased array testing is an advancement of conventional shear wave ultrasonics. Both non-destructive testing methods use ultrasonic waves to inspect weld. While conventional shear wave inspection uses a single wave angle PAUT uses a range of shear waves, typically in the 45-to-70-degree range, for weld inspection. Many more shear waves over a spectrum of angles will impinge upon a weld defect and a result the likelihood that defects are detected is increased. Phased array testing steers beams to the desired angle pulsing the individual elements of PAUT transducers at different time.
The total focus method / full matrix capture (TFM/FMC) is appropriately named. The manner in which TFM/FMC data is acquired, however, is very different compared to conventional PAUT. During data acquisition, a data matrix of A-scans is generated. The FMC matrix is populated by pulsing with a single transducer element while receiving with all elements. The inspection area of interest is pixelized within the weld to the desired resolution size. TFM is applied to each A-scan received to calculate contribution, if any, to each pixel in the inspection area. Calculations vary depending on the TFM waveset selected. The main theoretical advantages of TFM/FMC are improved resolution and decreased sensitivity for defect orientation.
Phased Array Ultrasonic Testing (PAUT) Data
Example PAUT data is shown in the video below. The typical display for an Olympus X3 encoded scan show the A-scan on the top right, S-scan in the top right, and C-scan across the bottom. The displayed PAUT A-scan is selected using the S-scan data cursor. The PAUT S-scan displayed depends on the C-scan data cursor. As the data is replayed from left to right, three weld defects are observed. In order of appearance, a lack of fusion (LOF), second LOF, and base metal crack are observed. The first LOF is observed just on the inside of the weld bevel. The second LOF is observed just past the weld bevel. Lastly, the base metal crack is observed about a quarter inch from the top weld bevel.
Figure 1: PAUT A-scan, S-scan, and C-scan of weld.
Total Focus Method/Full Matrix Capture (TFM/FMC)
Example TFM/FMC data is shown in the second vide below. The typical display for an Olympus X3 encoded scan shows the top view to the left and the side view to the right. The weld bevel profile is displayed in both views. While the side view is the traditional cross-sectional view of the weld, the top view is generated similarly to a conventional ultrasonic thickness testing C-scan based on the gated amplitude. This is quite different that the PAUT C-scan whose vertical axis is the focal law. The data displayed in the TFM side view is dependent on the data cursor position in the top view. As the TFM/FMC data is replayed from left to right, three weld defects are observed. In order of appearance, a lack of fusion (LOF), second LOF, and base metal crack are observed. The first LOF is observed just on the inside of the weld bevel. The second LOF is observed just past the weld bevel. Lastly, the base metal crack is observed about a quarter inch from the top weld bevel.
Figure 1: PAUT A-scan, S-scan, and C-scan of weld.
Data Redundancy and Processing using PAUT and TFM/FMC
TechKnowServ Corp. (TKS) develops robust and customized non-destructive testing (NDT) procedures to improve and enhance inspection performance and reduce to some degree human dependence on interpretation. By analyzing the redundancy in PAUT and TFM/FMC data spatially within the weld it is possible to locate, size, and classify weld defect with high degree of confidence while decreasing the potential subjectivity involved through human interpretation of a single scan.
Requirements for Data Quality
The proposed data redundancy approach is contingent on quality data and must be acquired by well-trained and qualified PAUT and TFM/FMC technicians. TKS supports these efforts through our rigorous phased array and TFM/FMC training classes. Our 80-hour phased array training class, 80-hour TFM/FMC training class, and 40-hour TOFD class prepare the student to accurately assess welds with ultrasonic testing.