Insight of Blind Test Qualification Process of AUT Mitesh Panchal, Niyant J Mehta and Pemmaraju Raghavendra

NDE Department, Heavy Engineering Division, Larsen and Toubro Limited, Hazira, Surat,India-

394510.

niyant.mehta@larsentoubro.com

Abstract:

The competency of personnel performing Non Destructive Examination (NDE) is most important in

all industries to have reliable NDE results. The Competency of NDE personnel is highly dependent

on proper training, examinations and experience to perform NDE tasks using the applicable test

method or technique. Mainly two types of certification programmes are available worldwide i.e

“Employer based” and “Central” certification programme such as SNT-TC-1A of ASNT as well as

PCN/ISO 9712 of Bi NDT.However for Advanced Ultrasonic Testing (AUT) techniques need

stringent requirements of qualification and certification. “Blind Test” qualifications are introduced in

addition to ASME requirements by many international clients to have more competent personnel

with respect to AUT techniques. “Blind Test” are test of personnel, procedure and equipment as well

and very tough to clear due to stringent passing criteria.

Since there is no proven/time tested database of reliability of AUT is available, every time we apply it

to new product and the same technique requires to be validated on a welded mockup block that is

similar to actual weld joint. This mockup can be either weld joint with known induced defects or

unknown blind blocks with weld joints. Code allows this validation on known mockup, however

certain international clients prefer to verify the competence of NDE personnel vide a blind test block

.i.e a weld test block (with similar weld joint configuration that of actual weld joint on the job) is

supplied to the vendor with induced defects at various zones of welds without disclosing

fingerprinting report of the block. The real challenge for NDE personnel is to detect , size and

analyze all the flaws induced. Several times in our experience it was observed that these flaws are

induced keeping in view of inherent limitations of AUT (TOFD and /or PAUT). The examinations

based on these blind test blocks enhance our technique sheet, procedure & personal to be more

sensitive towards near surface /close to surface detection as well as through wall height sizing.

To meet the above challenges we have framed our training, qualification, certification program in

such a way that we can address stringent requirements of Blind Test qualification. This includes

rigorous theory and practical training followed by mock up examination.We have successfully

cleared all the “Blind Test “examination conducted by international clients (such as

KBR,Jacobs,Exxon Mobil, Shell Global etc..) gaining customer confidence and full satisfaction.

Keywords: AUT, ASNT, Blind Test.

1.0 Introduction:

ASME requirements for Advanced Ultrasonic Testing (AUT) are generic and provide guideline to

have AUT setup including personnel and procedure qualification. Many international client are

introducing “blind Test” in addition to ASME code requirements. Blind tests is a true test of

competency for a NDE dept. involves in preparing a procedure/scan plan and applying the same on

the block/specimen provided.Hence,it encompasses the procedure preparation capabilities, capability

of individual with respect to code requirements and hands on practicals including capability of flaw

evaluation. Blind Test blocks are made keeping in view of inherent limitation of AUT techniques

such as flaws induced at dead zones, multiple flaws induced in same depth plane etc makes the test

very tough to clear. They are supplied by the customer in a sealed container to the Vessel fabricator

and opened only in presence of customer representative. We understand the technique limitations

and introduces additional checks to overcome the same.We develop rigorous training programme

National Seminar & Exhibition on Non-Destructive Evaluation, NDE 2014, Pune, December 4-6, 2014 (NDE-India 2014)

Vol.20 No.6 (June 2015) - The e-Journal of Nondestructive Testing - ISSN 1435-4934www.ndt.net/?id=17869

which addresses such limitation of techniques including accurate and reliable flaw sizing methods to

match code as well as blind test requirements.

2.0 ASME Code and stringent Blind Test requirements regarding AUT:

As per ASME only qualified UT personnel trained in the use of the equipment and who have

demonstrated the ability to properly acquire examination data, shall conduct production scans.

Personnel who analyze and interpret the collected data shall be a Level II or III who have

documented training in the use of the equipment and software used.The training and demonstration

requirements shall be addressed in the employer’s written practice.

Written Practice to be made based on SNT-TC-1A 2006 Edition guidelines however the same is

silent on requirements for AUT Personnel qualification and Certification.

Well defined and structured training programme is mandatory before the blind test qualification.

Personnel performing blind test shall have adequate training, experience and certification in the

particular test techniques including A scan UT as a prerequisite.

The Mandatory appendix IX, article 4 of ASME Sec V provides the requirements of qualification of

AUT Procedures. This includes the requirements of demonstration blocks preparation, Qualification

data,Evaluation and demonstration records. A Demonstration block shall be prepared by welding or

provided the acoustic properties are similar, The hot isostatic process (HIP) may be used. The

thickness of the demonstration block shall be within 25% of the thickness to be examined.For weld

joining of Two different thickness of material, demonstration block shall be made as per thinner of

the two thicknesses. The Demonstration block’s weld joint geometry shall be representative of the

production joint’s detail. The demonstration block shall contain minimum three actual planer flaws or

three EDM Notches oriented to simulated flaws parallel to the production weld’s axis and major

groove faces. The flaws shall be located at or adjecant to the block’s groove faces as follows:

• One surface flaw on the side of the block representing the component O.D. surface

• One surface flaw on the side of the block representing the component I.D. surface

• One subsurface flaw

When the scan plan to be utilized subdivides a weld into multiple examination zones, a minimum of

one flaw per zone is required.

In addition to the above ASME requirement blind test block contains challenging flaws such as:

• Flaws oriented transverse to the weld center line

• Flaws kept in blind zones of TOFD UT

• Flaws located at HAZ area near to both ID and OD surface

• Multiple flaws at various depths and locations.

Also joint having geometry such as dissimilar thickness weld joints, Nozzle joints with less lip

distance etc. requires specific scan plans.

Acceptable performance as per ASME requires that the flaws are sized as being equal to or greater

than their actual size (i.e., both length and height).However in blind test stringent tolerances are

applied.

As per ASME all the flaws in the demonstration block are required to be detected however during

blind test we come across False Call Rate (FCR) which is marking acceptable indication as repair

and vice versa.

The above requirements shall be addressed properly during blind test to avoid missing of flaws,

improper sizing and characterization of flaws, wrong indication marking (FCR) etc. thus

disqualifying the Blind Test.

3.0 Learnings and its implementation based on Blind Test:

We study,learn and implement many learnings from the blind test and some of them are listed

below.We done brainstorming and lots of experiments and its validation before implementing the

same.

3.1 Training, qualification and certification of NDE personnel:

Based on experience of blind test, we have made specific training programme for our Level II

operators which include theory and Practical of particular AUT technique. We have library of

qualification block made for thickness range between 22 mm to 450 mm with various joint

geometries and material. We also developed a library of flaw images which can be referred while

characterization of a flaw. A trainee is subjected to mock up AUT examination on various

qualification blocks.Level III is comparing the result with fingerprinting report of the block and

giving feedback to the trainee regarding selection of scan plan,equipment and apparatus, parameter

setting,data acquisition, image evaluation and Report preperation.Particularly for flaw evaluation, a

trainee is subjected to many reference images having artificial flaws, conditions like improper

parameter settings , images showing mode converted signals, images showing multiple flaws etc.

Trainee also learn usage of advance software tools such as lateral wave removal, backwall removal,

SAFT (synthetic Focusing Aperture Technique), mode converted parabola etc. In this process

trainee gains experience and confidence to face the blind test examination.

Our Written Practice is made incorporating SNT TC 1A 2006 as well as 2011 Edition although 2006

edition is only applicalble.Detailed syllabus for AUT techniques are tailor made and mentioned in our

Written Practice.We specified training hours and annual practical verification of our Level I/II

personnel performing AUT as per SNT TC 1A 2011 edition.

3.2 Specific scan plan to get length of transverse flaws:

Transverse flaws may go undetected in D scan (non-parallel scan) hence we perform TOFD

transverse B scan (parallel scan) on a weld to detect such flaws as shown in Fig.1 Fracture

mechanics based acceptance criteria require indication length and height. While doing TOFD,

any indication oriented transverse to the weld will give height dimension however length

dimension cannot be captured. To overcome such limitation we find out lenght of such

indication by applying a pulse echo A scan, PAUT or by transverse D-scan of TOFD. API 934

Appendix A can be followed as a guideline to detect the flaws oriented transverse to the weld.

A sketch of TOFD transverse D scan is shown in Fig.1.

Fig.1: TOFD UT image of transverse indication, Schematic of transverse B scan and transverse

D scan.

3.3 Specific probe selection for improved near surface detection and resolution:

Flaws kept in blind zones of TOFD UT may be missed if selection of probe is not appropriate. After

carrying out various trials with combinations of different probes with different diameter and

frequencies we finalized 10 Mhz, 3 mm Dia TOFD probe for improved near surface detection. 70

deg. 5 Mhz with 6 mm dia probe having dead zone of 5-6 mm (refer Fig 2 a) whereas 70 deg 10

Mhz, 3 mm dia probe is having dead zone of only 2-3 mm. Due to the same we could find out

bottom tip of the surface indications as shown in fig 2b.

Fig 2a: 70 deg, 5 Mhz 6 mm dia Fig 2b: 70 deg, 10 Mhz 3 mm dia

We also incorporated use of creep wave and PAUT techniques to detect surface flaws which are

open to surface or 2-3 mm below the surface in blind zones while performing TOFD UT.(Refer Fig

3a and 3b)

3.4 Advanced simulation software and its use on dissimilar thickness joints:

We have improved our procedures and scan plans considering the requirements of blind test.We use

advance AUT simulation softwares i.e Eclipse Scientific beam tool USA,BSP by MJ inspection

Malaysia etc.For dissimilar thickness weld joints ,based on scan plan software input, we use separate

PCS for each probe as well as separate angle as per the taper angle with offset scanning to cover the

full weld with adequate sensitivity at each zone (Refer Fig 4a).

Fig 3a: Scan plan including Creepwave and PAUT Fig 3b: Image showing Creepwave and PAUT alingwith TOFD

3.5 Scan plan with Focus scan and Offset scans:

We added focus scans and offset scans in our scan plan to address accurate detection and sizing of

the flaws which are located at HAZ area. (Refer Fig 4b)

It also helps to separate out multiple indications. We utilize B scan to find out location of indications

thus finalizing that they are in same depth plane or in different depth plane.(Refer fig. 5a & b)

Fig 4a: Scan plan for dissimilar thickness weld joint. Fig 4b: Focus scan

Fig 5a: Schematic showing B scan Fig 5 b: TOFD UT image of B scan

3.6 Control on index (Y axis) offset errors in TOFD/PAUT:

Index Offset (Y axis) errors in PAUT may lead to missing of indications and the same is addressed

by usage of circular rings with magnet. Similarly for TOFD UT offset during scanning may result in

missing indications. The same is addressed by implementing center line marking and tracking the

same by laser tracker.(Refer Fig 8a,6a and b respectively)

45 deg scan showing No indication

60 deg focused scan showing indication

3.7 Control on oversizing of surface flaws in PAUT:

There is a tendency of oversizing surface flaws in PAUT which are found in full skip scanning. To

address this issue we implement maximum amplitude technique in stead of 6 db Drop method for

sizing the indications.(Refer Fig. 7)

3.8 Control on Scan (X axis) error due to encoder:

Encoder which records the scanning distance is also very important with respect to blind test

qualification. Each indication in the blind test block is located at distinct location from datum point

or reference mark. If the scan surface distance (X value) from datum point is wrongly marked,

indication may be considered as missed. To avoid such situation, encoder shall be always calibrated

before starting AUT blind test examination.(Refer Fig 8 a and b)

Fig 8a: Schematic showing scan directions Fig.8b: TOFD UT Image showing Shift

in X value due to encoder error.

Fig 6a : PAUT scanning using circular Rings Fig 6b: TOFD/PAUT Auto scanner with Laser Tracker

Fig 7: Scan plan of PAUT showing surface indication and its image on PAUT screen

We design our qualification blocks keeping in view of the entire above blind test requirements.

We keep stringent performance evaluation criteria to match the requirements of blind tests. We allow

stringent tolerances in sizing of defects.Undersizing of indications are not acceptable. Also

identifying and reporting Falls Indication i.e. Falls Call Rate (FCR) are not acceptable.

Conclusion:

The paper has attempted to bring to the notice that the study and experience gained from blind test

can be helpful to enhance the competency of personnel performing AUT and continual improvement

in AUT procedures. We could successfully clear Blind test examinations more than 7 times, which

was conducted by different international clients.

References:

[1] American Society of Mechanical Engineers, Boiler and Pressure Vessel Code, Section

VIII & Sec V,2010 Edition 2011a addenda.

[2] American Society of Mechanical Engineers, Boiler and Pressure Vessel Code, Section

VIII, Division 2, Part 7, 2011a addenda.

[3] P. Raghavendra, Niyant Mehta, G.D.Acharya, Ajit Anupam, “Replacement of

Radiography Testing of Nozzle to Shell Butt Joints of Pressure Vessels by Phased Array

Ultrasonic Testing Technique”, NDE Conference, Chennai, 2011.

[4] P.Raghavendra,Niyant Mehta,Girish Namboodiri,M.M.Makwana „ Limitation of AUT’s

Mandatory Requirements of Codes“, NDE Conference,Delhi,2012.