ADAPTING CSA W59 ULTRASONIC INSPECTIONS FOR USE WITH ... · ADAPTING CSA W59 ULTRASONIC INSPECTIONS...

ADAPTING CSA W59 ULTRASONIC INSPECTIONS FOR USE WITH DISTANCE-AMPLITUDE TECHNIQUES

Paul Holloway Holloway NDT & Engineering Inc.

Andrew Crawford Buffalo Inspection Services

Sean Keay SNC-Lavalin

Viwek Vaidya Techno Vogue Inc.

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NDT in Canada 2017 Conference (June 6-8, 2017)

The year was 1969…


The year of Abbey Road

and Pierre Elliot Trudeau


Since then some things have changed…


… but ultrasonic testing of structural welds in North America remains the same today as it was in 1969

STAYING THE COURSE



W59 bases ultrasonic testing on techniques developed in the late 1960’s, which CSA adopted in the early 80’s.

Since then, there has been significant progress in UT yet North American structural codes have failed to keep pace.

We are at a point where the rate of change is fast heading vertical; to stay with the status quo will ultimately render us uncompetitive.

“FIXED ATTENUATION” TECHNIQUE



The most recognizable component of the existing technique used in CSA W59 and AWS D1.1 is the use of a very large probe.

It’s common in virtually every other code in the world to select the transducer to best match the application. But here in North America, we’ve been using the same style of transducer on all welds, for nearly 50 years.


Another recognizable component is the estimate of attenuation:

2 decibels per inch

Attenuation is the rate at which sound loses power as it travels through the steel.

By restricting the size and frequency of the transducer, attenuation is assumed fixed at 2 decibels/inch.


Sensitivity is set on just one hole in an IIW-type block. Attenuation is then accounted for mathematically based on the distance traveled, at a rate of 2 dB/inch.


Another key feature of the existing technique is the procedure tables.

The code is very specific on what angles to use and which faces you can scan from, regardless of weld geometry.

DRAWBACKS



The main drawbacks of the existing technique are:

• Large, awkward probes which are not equally suited to thin and thick weld geometries

• Angle specific procedures that do not take into consideration weld geometry

• A basic practice built on 1960’s technology which is fundamentally inflexible to change


BUT…The fixed attenuation technique has an excellent track record and has served us well for a long time.

Industry has been reluctant to address changes in technology… “why fix what ain’t broke?”.

And here we are, nearly 50 years later, using an inefficient technique based on unnecessary approximations. The technique is simply awkward and outdated compared to the technology of today. Change is inevitable.

ALTERNATE TECHNIQUE



The CSA W59 committee has proposed an alternative technique based on a Time Corrected Gain (TCG) for the sensitivity calibration.

This would be used in parallel or in substitution of the fixed attenuation technique.


The alternative technique would allow for the use of a wide variety of probes and the ability to match the inspection angle to the weld geometry.

New acceptance criteria are developed for equivalence to the long-standing acceptance tables.


Since use of a TCG is integral to phased array, it also allows for manual PAUT inspection. Manual inspection requires probe rastering and oscillation, allowing the user to peak the signal. This is integral to workmanship criteria based primarily on indication amplitude.

Most importantly, it puts us on equal footing with the rest of the world and makes adoption of newer technologies possible.

ALTERNATIVE TECHNIQUE


Sensitivity calibration is performed basically the same as it would be for a pressure vessel inspection.

A number of side-drilled holes at varying depths are used for reference.


The signals are then normalized with a TCG at 80% screen height.


Use of a TCG also allows for zero degree inspection

Zero degree inspection is extremely effective in detecting lamellar tearing, which is common on thick tee-joints.


The alternative technique developed for CSA W59 (the TCG Technique) is well known and commonly practiced elsewhere in the industry.

This is not new… it’s just new to us!

ADAPTING THE CRITERIA



In order to adapt the current criteria for the TCG technique, the influence of the 2 dB/inch model needed to be quantified and extracted from the current acceptance tables.


Using the current, “large, low frequency” probes, actual attenuation was modeled in CIVA and compared to the 2 dB/inch model.


Every weld thickness category was considered as a unique scenario along with each weld inspection procedure. As well, every range of sound paths for each procedure had its own error value.

These were assembled into a large array of data to tabulate discrete correction factors for each weld category.


For illustration, the existingacceptance criteria are first converted to chart form as shown here…


In North America, indication ratings in structural UT are counter-intuitive: the lower the number, the bigger the indication.

Here the traditional table of acceptance levels for the 70 degree angle is displayed as bar chart. The black coloured bars are the most severe indications (Large, or Class A), and are seen at the bottom.


The criteria for the TCG technique is intuitive, and follows the model most of the world uses: higher = more severe.

Here the values in the 70 degree table have been inverted, where now the bad stuff is on top (i.e. as the indication rating increases, so does the severity).


Then the correction factors are introduced, and the ratings in each weld category adjusted accordingly.

-0.4 -2.0 -2.1 -2.2 -2.0


The tables for 70, 60 and 45 degrees are then averaged to make one table for all angles.

Separate tables are still derived for statically and cyclically loaded structures.

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D -8 & below -7 & below -7 & below -6 & below -6 & below

C -7 -6 -6 -3 to -5 -1 to -5

B -6 -5 -4 to -5 -1 to -2 +1 to 0

A -5 & above -4 & above -3 & above 0 & above +2 & above

Class (⁵/₁₆ to ³/₄ in) ( > ³/₄ to 1¹/₂ in) ( > 1¹⁄₂ to 2¹⁄₂ in) ( > 2¹⁄₂ to 4 in) ( > 4 to 8 in)


8 to 20 mm > 20 to 38 mm > 38 to 65 mm > 65 to 100 mm > 100 to 200 mm

The resulting values are presented in both a table form and the chart format.

VALIDATION



CIVA modeling was used to run simulations. With the mathematical models that were derived from the simulation data, the TCG technique and the fixed attenuation technique were compared side-by-side.


Here we see current Table 11.3 for statically loaded structures for use with the fixed attenuation (2dB) technique.

The blue overlays represent the error that using the 2 dB/inch model produces, simply due to the fact that attenuation is not a linear phenomenon.


A properly constructed TCG effectively creates an accurate model of real attenuation.

Using a TCG eliminates the mathematical error altogether.

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TCG technique

COMPARISON TO AWS


CSA W59 and AWS D1.1 currently share the same basic acceptance criteria for the 2 dB/inch technique which have remained virtually unchanged in 50 years. The tables are identical in both form and function.

Recently, AWS D1.1 and D1.5 have been revised to offer the possibility of performing DAC or TCG sensitivity calibrations for alternative UT inspections.


However:

One is manual UT which permits probe oscillation and rastering to obtain the highest signal, which is currently standard practice in conventional testing.

AWS D1.1-2015 : Annex QConventional UT with DAC calibrationTransducers from ¼ in. diameter and up to 6 MHz


The other is encoded phased array, with no oscillation or rastering possible.

AWS D1.5-2015: Annex KEncoded PAUT with TCG calibration


Despite the fact that one permits probe movement to maximize indications and the other is bound to the data acquired at the fixed offsets and angles of the PAUT scan plan, both AWS techniques share the same alternative acceptance criteria.


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AWSD1.1 / D1.5(Annexes)

CSAW59 TCG


As shown, there is over 10 dB difference between the proposed W59 TCG criteria and the AWS criteria for use with alternate UT.

As well, the AWS alternate criteria has the same acceptance levels regardless of weld thickness. This is a departure from the standard practices where thicker welds are permitted to withstand discontinuities with higher amplitudes.

The W59 criteria was designed for equivalence with the existing tables and does not intend to chang the long-standing quality levels that the construction, manufacturing, and inspection industries have come to depend on.

CONCLUSIONS


• Current technique unchanged since 1969, and remains prohibitive to adoption of new technologies or practices

• The TCG technique has been proposed for W59-2018 to bring the code in line with modern technology and best practices

• The acceptance levels developed for use with the TCG technique provide equivalent quality levels to the those used with the 2dB/inch approximation.

ADAPTING CSA W59 ULTRASONIC INSPECTIONS FOR USE WITH ... · ADAPTING CSA W59 ULTRASONIC INSPECTIONS...

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