PHASED ARRAY AND CHANGES TO ULTRASONIC TESTING OF STRUCTURALWELDS IN CANADAMost inspection codes worldwide have made strides to introduce newer technologies in the past few decades. Phased array ultrasonic testing (PAUT) is one major advancement which is now common practice. This article attempts to distill the technology down to a few main talking points, highlight the introduction of PAUT into CSA W59-18, and discuss the implications on inspection results.

BACKGROUND

Prior to 2018, the technique used for ultrasonic testing in CSA W59 was drawn from the U.S. code AWS D1.1, which used an approach developed in the late 1960s. The technique is easily recognized using large search units and a fixed estimate of attenuation which requires only a single side drilled hole to set reference sensitivity. As sound travels it naturally loses power just a flashlight beam los-es intensity over distance. Being able to equally gauge indications both near and far from the probe is critical in evaluating the work-manship of a weld. In order to establish a fixed estimate of sound attenuation, the technique placed very strict limits on the frequency, size and shape of the transducer. Additionally, specific inspection angles were prescribed for weld joints of different thicknesses, largely regardless of weld geometry.The technique, paired with very stringent acceptance criteria, has the formed the basis of ultrasonic weld inspection of structural welds in North America for decades.

THE PLOT TWIST

After that nice introduction, you knew there was a “but…” and here it is. Any technique that prescriptive is basically immovable when it comes to evolving technologies. Something as simple as calibrating for actual attenuation to provide the ability to use more suitably-sized probes, long used in the ASME world, wasn’t possible. As well, angle-specific inspection procedures effectively ruled out phased array and its “lots of angles at once" approach. The demand for improved probability of detection, fewer false calls, and better repeatability are things that manufacturers,construction companies, and inspection firms all strive for. Quality and efficiency are constantly evolvingand improving. Eventually, the code had to change for the times.

INTRODUCTION OF ANNEX X

In 2018, Annex X was added to CSA W59 to allow manual (e.g. hand scanning) phased array and con-ventional UT using a modified sensitivity calibration. This is referred to as the TCG technique in CSA W59-18,as opposed to the long standing fixed attenuation technique. The TCG technique is designed to providethe same quality levels as fixed attenuation, and the code permits that either technique may be used. The fixed attenuation technique remains part of the code for now as it has been for decades.

Annex X was designed to be used essentially as a standalone set of instructions to reduce the amount of cross-referencing to Clause 8 as possible.

OK, BUT WHAT ABOUT PHASED ARRAY?

Phased array ultrasonic testing simply divides a transducer into many small elements, allowing each

one to act as an independent transmitter and receiver or as a group. By varying the timing of the pulse to each element, the resulting wave front can be steered through a range of angles.

Three beams of different angles are shown in Figure 1 below. The blue lines represent the relative time delay issued to each individual element.

In the center image, all the beams are fired at the same time, thus all the delays are identical and no beam steering is performed. This essentially gener-ates a planar wave front simulating a conventional UT transducer. After refracting at the interface between the wedge and the steel, the resulting beam angle is 55°, which is controlled only by the cut angle of the wedge.

On the left, the elements near the top of the transducer are fired first (represented by shorter blue delay lines), while the elements at the bottom are fired last (longer blue lines). The resulting composite wave front steers slightly downwards, generating a 45° refracted angle in the steel plate shown.

Similarly, by reversing the delay laws, the beam can be steered upwards resulting in a 70° refracted angle as seen in the last image.

Typically, beams are directed at 1° increments throughout a range of angles. Each set of delays used to produce a beam of a certain angle is referred to as a focal law.

By compiling a series of focal laws similar to those above over a wide angle range (e.g. 45° to 70°, in 1° increments), the result is a swept angle (sectorial, or S-scan) which provides coverage over a much largerarea than conventional UT as shown in Figure 2.

Fig. 1: Phased array focal laws generating beams at three different angles.

A WINTER ANALOGY: BECAUSE WE’RE CANADIAN

Conventional ultrasound is like cleaning off just one bare strip on your icy windshield, then driving down the highway squinting, moving your head side to side trying to tell if the oncoming lights are two little cars or one really large oncoming semi-trailer.

THE PHASED ARRAY VIEW

The view most associated with PAUT is the S-scan. The S-scan is a composite of all the A-scans defined in the focal laws. It is the most used view in PAUT weld inspection but must be used in combination with the individual A-scans at each angle to properly evaluate indications.

Fig. 2: Phased array angular sweep.

Fig. 3: View through a conventional UT windshield.

Fig. 4: View through a phased array windshield

PHASED ARRAY IS LIKE CLEANING OFF THE WHOLE WINDSHIELD.

Fig. 5: Phased array S-scan view. Fig. 6: Individual A-scans of all focal laws.

The S-scan above shows an indication from a simple side-drilled hole in a calibration block. The hole was round, but the indication is not, and instead looks like a red and yellow banana. Because sound spreads out as it propagates, a line drawn from the entry point along a beam angle that looks like it “just misses” the hole will in fact still reflect a little. This beam spread is the primary reason why indications on the S-scan appear elongated.

As well, the beam reflects off the nearside faces of discontinuities and only a portion of the reflected beam may return to the wedge and be detected. This means that the images seen in the S-scan may not exactly resemble the actual shapes of the defects, but are used rather for detection and characterization rather than forming a precise image.

The tightness of the beam and the ability to keep the smearing effect to a minimum is aided by use of a higher number of elements (e.g. 32 versus 16) which improves beam steering. However, “more is not always better”, as using a transducer with simply more elements will create a wider beam, which on thinner components can complicate matters as the entire thickness of the specimen is flood-ed with sound and differentiation between the top and bottom surfaces becomes difficult. Match-ing the transducer and focal law parameters to the weld geometry is key.

MANUAL VS. AUTOMATED TESTING

In CSA W59-18, manual phased array has been implemented in Annex X. The term manual testing implies that the technician is manipulating the probe by hand. This is the typical “weld scrub” used in conventional ultrasonic inspections of structural steel weldments. In manual testing, the probe can be freely oscillated and rastered (moved back and forth, left and right) to peak the signal in the A-scan and accurately

determine the position and depth of the defect. In many cases, a slight oscillation will result in a higher indication level. The data produced during manual scanning are typically screen images showing an S-scan with a weld overlay and a corresponding A-scan at the angle representing the highest indication rating. Phased array as well as conventional ultrasound are both commonly used for manual testing.

Automated testing requires one or more probes positioned in a fix-ture which is moved laterally along the length of the weld. The data is recorded during acquisition. The term “automated” is somewhat of a misnomer, as the scanning sys-tem may be moved manually or by the aid of motors. In either case,

the probe(s) are positioned at set distances from the weld centerline for the length of the scan. Prior to testing, a scan plan is conceived in which the technician evaluates the beam coverage for the weld and determines whether one or more stand-off positions are required for full volumetric inspection.

Since the probes in automated testing are fixed in place, they cannot be rastered or oscillated. Thus a proper scan plan showing probe placement and beam cov-erage is critical for an accurate and repeatable inspection. At this time, automated phased array testing has not been implemented into CSA W59, although it remains (as it has for years) an option in Clause 8.2.12 requiring engineer approval.

Fig. 7: Actual phased array weld scan plan and data image.

Fig. 8: Scan plan example.

With automated testing, it is possible to display the data in several different views which can provide much more detailed record of the weld integrity than static images from manual testing.

CALIBRATION

The sensitivity of the equipment must be set prior to inspection. As sound attenuates as it travels,signals are amplified as they travel to produce equal sensitivity across the entire sound path. A typical calibration block for phased array will include side-drilled holes or notch reflectors at various distances from the entry surface.

A plot of attenuation is called a DAC (Distance Amplitude Curve), which the equipment will automati-cally display as an overlay on the

A-scan. On analog-era machines,the DAC was physically drawn onthe CRT display using a wax marker.The height of indications along thesound path are compared to theheight of the DAC.

More common in the present day is the use of a TCG (Time

Corrected Gain), which is simply a mathematical correction applied to a DAC in order to bring all signalsup to the same level. In this manner,indications from reference reflectorsat various distances are all normal-ized at the same screen height mak-ing detection and evaluation easier.

Fig. 9: Various PAUT views clockwise from top left: S-scan, End view, Top view, A-scan.

Fig. 10: Phased array probe on a side-drilled hole calibration block.

ACCEPTANCE CRITERIA TYPES

There are two main approaches to acceptance criteria. The names of each are not immediately descriptive, but can be simplifi ed as follows:

Workmanship (or, “how loud is that sound?”)Evaluating the severity of a defect on the amount of sound it refl ects is referred to as workmanship criteria. This is the simplest approach and the most commonly used. There is a threshold level above which indications are evaluated. Indications below the threshold are ignored. Usually the indication level and the defect length are the main parts in evaluating for acceptance or rejection.

Workmanship criteria may be applied to both conven-tional UT and PAUT. The amount of sound that a defect refl ects is of course highly dependent upon defect type, location, orientation, size, and beam angle.

Fracture Mechanics (or, “tell me everything!”)Encoded inspections on equipment such as critical service pressure vessels may require more detailed information on the weld fl aws than simply “how loud” and “how long”. Measuring the distance from the surface, the through-wall height, the length, distance from other fl aws and proper characterization requires considerably more time and expertise than a workmanship approach. A fracture mechanics approach usually also requires a demonstration for procedure qualifi cation.

CODE COMPLIANCE AND PHASED ARRAY TESTING

Phased array has been accepted worldwide in many codes and standards. In North America, CSA W59, AWS D1.1 and ASME Section V all include sections on phased array testing.

In 2018, CSA W59 adopted the use of manual PAUT alongside the ability to use conventional probes other than the traditional “snail” and “big red brick” which been a staple in structural steel weld inspections for nearly 50 years. Either technique can be used: the original “Fixed Attenuation” technique from Clause 8 with the large probes or the “TCG Technique” in Annex X which permits both manual PAUT and conventional UT. Use of the TCG technique for W59 does not require the engineer’s approval, but shall be used only by agreement between the contractor and the welding inspection organization.

Fig. 11: DAC view of sample indication. Fig. 12: TCG view of same indication.

CSA W59-13 and earlier…Clause 8.2

CSA W59-18Clause 8.2

Annex XFig. 13: Typical search units compatible CSA W59-13 and earlier.

Fig. 14: Typical search units compatible with CSA W59-18.

The TCG technique in Annex X permits the inspector to choose the inspection angle best suited to the weld. This is the natural course of action for most inspection codes worldwide, and of course provides a natural fit for PAUT where multiple angles are used at once.

Adoption of automated PAUT, where encoded digital records can be stored, analyzed and archived, will eventually be adopted in W59. However, acceptance levels for automated UT should be distinct from those of manual UT.

Inspections of pressure vessels and piping in accor-dance with ASME Section VIII and B31 are quite commonly performed using phased array and have been used with those codes for many years.

PHASED ARRAY AND PROBABILITY OF DETECTION

Phased array is no doubt a more comprehensive tool for ultrasonic inspection of welds. You can see more at once with PAUT than you can with conventional

ultrasound. The integration of PAUT through W59 Annex X and the resulting multitude of inspection angles will provide a much clearer picture of the weld integrity. But this does not equate to failing more welds. Whatever was in the welds before will still be there, but more modern inspection techniques like the TCG technique and PAUT will help with probability of detection as it will with reducing false calls.

Anyone involved on the manufacturing, construction, or inspection side of the industry can attest that the human factor is the major variable in inspection results. Phased array requires a full understanding of ultrasonic testing, but results are less ambiguous than conventional ultrasound where only the A-scan view is used. The effects on probability of detection and failure rates vary significantly due to the human factor. Training, experience, and qualification are key.

Paul Holloway, P.Eng. MAScPresident, Holloway NDT & Engineering Inc.