Wednesday, 12 October 2016 17:43

A comparative analysis of European and American...

“A comparative analysis of European and American quality requirements for evaluating welded joints using a manual ultrasonic inspection.”


There are more and more projects done in Poland for the American market, where the technical and quality requirements concerning steel constructions differ vastly from the European standards.

There are more and more projects done in Poland for the American market, where the technical and quality requirements concerning steel constructions differ vastly from the European standards. In Poland, NDT inspectors, who use European norms and standards on everyday  basis, are often found in a situation where given welded joints have to be evaluated according to American standards. The non-destructive laboratory NDT Reports Sp. z o.o [Ltd.] Sp.K. [L.P.] has also come across this problem during a UT inspection of welded joints on a container for an offshore project, where equipment for works on an oil platform was to be installed. Namely, there was a revision for the UT procedure concerning the sensitivity of the inspection. The revised instruction for setting the sensitivity on a calibration block said to use a 1.5mm side drilled hole instead of a 3mm one – in consequence, the laboratory was asked to consider the problem and present the results as a comparative analysis. In the following analysis there are compared acceptance criteria for the manual UT inspection, where a DAC curve was used for the 3mm side drilled hole according to the EN 1712, and another DAC curve was used for the 1.5mm hole according to the customer’s requirements as seen in the American standard. Posted analysis has been submitted to the client and has been accepted without any reservations.

COMPERATIVE ANALYSIS

Acceptance criteria in accordance
with EN 1712/acceptance level 2
(Quality level B in accordance with EN ISO 5817)
Acceptance criteria in accordance
with Total Standard GS ED STR 3011
(AWS D1.1/Class R)
General

Setting of sensitivity

The setting of sensitivity shall be performed by  3mm diameter side drilled hole.
The reference level is a distance-amplitude-curve (DAC) for a 3mm diameter side drilled hole.

Setting of sensitivity

The setting of sensitivity  shall be performed by  1.5mm diameter side drilled hole.
The reference level is a distance-amplitude-curve (DAC) for a 1.5mm diameter side drilled hole.
 
Note:  If the diameter of a cylindrical reflector increases (from 1.5 mm to 3 mm), the height of the echo increases by 3dB.
 
Reference level Setting of sensitivity
The reference level is a distance-amplitude-curve (DAC) for a 3mm diameter side drilled hole. The reference level is a distance- amplitude-curve (DAC) for a 1.5mm diameter side drilled hole.
Recording level Recording level
For recording level the following value shall be used:
Reference level -6dB (50% DAC)

For recording level the following value shall be used:
Reference level -6dB (50% DAC)

Evaluation level Evaluation level

All indication equal or exceeding the following value shall be evaluated: Reference level -10dB (33% DAC)

All indication equal or exceeding  the following value shall be evaluated: Reference level -6dB (50% DAC)
All indications having half (-6dB) the amplitude or less than the reference level shall be disregarded.
All imperfections from which the reflected  echo amplitude exceeds the evaluation level shall be characterized, and all that are characterized as planar (e.g. cracks, lack of fusion, incomplete penetration) shall be rejected. Linear or planar reflectors whose lengths (extent) exceed the limits of Figure 6.7 shall be rejected. Additionally, root reflectors shall not exceed the limits of Class X.
 
Note: Standard EN 1712 (for acceptance level 2) is more restrictive than AWS 1.1 (for class R). AWS 1.1 allows traces of planar defects contrary to EN 1712 (all planar defects whose echo exceeds the evaluation level are not acceptable) .
   
Linearly aligned and grouped indication
Linearly aligned indications shall be considered as continuous if they are separated by a distance, dx, of less than twice the length of the longest indication.
Linearly aligned and grouped indication
Adjacent reflectors separated by less than their average length shall be treated as continuous.
   

Example:

wykres 2

  Standard EN 1712 is more restrictive and orders to combine indications in this case contrary to AWS 1.1.
Measurement of indication length
The length of the indication shall be determined by measuring the distance between the point where echo exceeds  the evaluation level with the angle probe giving the maximum response.
Pomiar długości wskazania
Długość wskazania musi być określona poprzez zmierzenie odległości pomiędzy miejscami, w których echo amplitudy wskazania spada o 50% (6dB) poniżej stosowanej klasyfikacji wskazań. Miejsce spadku określane jest poprzez środek głowicy/przetwornika.
 

Note:  Measurement of the length indication in accordance with EN 1712 is more accurate. The length of an indication measured this way is larger.

   

Standards comparison based on the examples (max. permitted linear indications length)

wykres en 1

 

Weld thickness – 10 mm
Echo amplitude in relation
to the curve DAC
(reference level)
Max. permitted
indication length
Echo amplitude in relation
to the curve DAC
(reference level)
Max. permitted
indication length
 -2dB 10 mm +1B  6 mm
 +1dB  not acceptable  +4dB  6 mm
 -4dB  10 mm  -1dB 20 mm
Weld thickness – 15 mm
 Echo amplitude in relation
to the curve DAC
(reference level)
 Max. permitted
indication length
Echo amplitude in relation
to the curve DAC
(reference level)
Max. permitted
indication length
-2dB 10 mm +1dB 8 mm
-1dB 7.5 mm +2dB 8 mm
+5dB not acceptable +8dB 8 mm
-4dB 15 mm -1dB 25 mm
  Weld thickness – 20 mm
Echo amplitude in relation
to the curve DAC
(reference level)
 Max. permitted
indication length
Echo amplitude in relation
to the curve DAC
(reference level)
Max. permitted
indication length
 -2dB  20 mm  +1dB 9 mm
-1dB 10 mm +2dB 9 mm
+5dB not acceptable +8dB 9 mm
-4dB 20 mm -1db 27 mm
Weld thickness – 25 mm   
Echo amplitude in relation
to the curve DAC
(reference level)
Max. permitted
indication length
Echo amplitude in relation
to the curve DAC
(reference level)
Max. permitted
indication length
 -2dB  25 mm +1dB  12 mm
-1dB 12,5 +2dB 12 mm
+5dB not acceptable +8dB 12 mm
 -4dB 25 mm  -1dB 25 mm
Summary:
For linear indications having an echo larger than +2dB above the DAC curve (for testing in accordance with GS UT STR ED 301) and what follows larger than -1dB below the DAC curve (for testing in accordance with EN 1712), the maximum acceptable lengths of discontinuities are similar to each other and with a significant value of the amplitude above the curve, standard EN 1712 is more restrictive. This analysis shows that indications which give an echo at the level of +1dB above DAC curve (for UT testing in accordance with GS ED STR 301) and what follows -2dB below the DAC curve (for testing in accordance with EN 1712), standard GS ED STR 301 is more restrictive. Planar imperfections (e.g. lack of fusion, incomplete penetration - undesirable from the safety of the structure) give a relatively higher echo which would likely be acceptable when tested in accordance with EN 1712 if it reached 0dB, it would be acceptable according to GS STR 301 ED even if it exceeded the curve. I recommend standard EN 1712 to assess the quality of welded joints, based on the above analysis.

 

Author: Laboratory Manager/ NDT Inspector: VT, MT, PT, UT, RT level 2
Bibliography: PN-EN 1712, EN ISO 5817, TOTAL STANDARD GS ED STR 301 (AWS D1.1/CLASS R)

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