th asia pacific conference for non-destructive testing … · 2018-02-21 · 15th asia pacific...

15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.

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Productivity & Reliability Study of Magnetic Particle testing & Eddy current testing for

Inspection of construction welds.

S.K.Babu†1, W.T.Chan2 and Alan Chan3

†1 SEEM Dept, City University, Hong Kong †E-mail: [email protected]

2 Vice President, Hong Kong Institute of Steel Construction

3Systems Engineering & Engineering Management, City University, Hong Kong

Abstract

The aim of this study is to evaluate the productivity & reliability of magnetic particle testing used for

surface inspection in Hong Kong construction industry. A set of fillet welds with artifacts in different

orientation is prepared with a round robin test with several set of qualified operators in determining the

productivity and probability of detection of test results. Eddy current test were also used to determine

the equivalent productivity and probability of detection of discontinuities in various structural welds

made predominantly from carbon steel for structural welds employed in the Hong Kong Construction

Industry. The study aims to set new criteria for testing & inspection practices in Hong Kong to raise

productivity & reliability of Inspection techniques.

Keywords: Structural steel, Magnetic Particle, Eddy Current, Productivity, Reliability, POD

1 Introduction

There is more than a century's history of using structural steel as a method of building construction. In

Hong Kong, the use of structural steel in construction has a history of more than 80 years. Buildings

like the old headquarters building of Hong Kong and Shanghai Banking Corporation and the old Bank

of China Building are examples of buildings in structural steel, which were constructed from the 30's

to 50's. Recent examples of such buildings in Hong Kong are the Kowloon Commercial Centre in

Tsim Sha Tsui, International financial centre in Central, or the Convention Centre in Chep Lap kok.

Structural steel is heavily used in Hong Kong construction industry, therefore it is necessary to have

ways to monitor, evaluate and assure the integrity of the welding used to join the structural steel.

Structural steels are also used on foundation members such as socketed H-Piles or temporary

structures during excavation. In an average 516.7M tonnes of structural steel especially H Sections &

Sheet Piles is used in Hong Kong in the year 2014 (Source: HONG KONG TRADE STATISTICS,

CENSUS & STATISTICS DEPT.)

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Non-destructive test techniques are being advanced more and more and are used as methods to

evaluate engineering structures and systems, as much during the construction phase as during their

service life. Special attention has been given to welds used for this construction due to the serious

consequences that could occur with the structural failure, such as loss of human lives & economic

losses. However, one of the parameters that should be taken into consideration on selecting the most

adequate non-destructive test technique to be used is its reliability, which is evaluated using

probability detection curves (PoD) that represent the probability of detection of a defect with a

particular size. For the surface inspection of structural welds one technique is outstanding due to their

portability and ease of operation which is Magnetic particle testing. Eddy current testing is sparingly

used in construction to test welds. There we not much of expertise in Eddy current testing. However,

it has various advantages over the Magnetic particle test method.

The aim of this work is to evaluate the productivity & reliability of non-destructive tests in the

inspection of structural welds. Two different techniques were considered Magnetic Particle Testing

and Eddy current testing.

1.1 Test Specimen

1.1.1 Production of Test Specimen

To carry out the study, 5 specimen was manufactured from an S355JR plate with a length of 300 mm,

thickness of 6mm fillet welds were made with defects artificially inserted on laying the weld bead,

Predominantly cracks were produced with various orientation. The cracks were oriented in

unpredictable locations such as toe, heat affected zone, centre line. All the discontinuities were

inserted more than 5mm.

S.K. Burke and R.J. Ditchburn explains in a literature review “Review of Literature on Probability of

Detection (POD) for Magnetic Particle Non-destructive Testing”. POD determination using Magnetic

particle testing [16], in the below figure 1 we could understand the POD is always 100% over 3.5mm

in terms of discontinuity size. Hence, in order to achieve the reliable results in trial all artefacts

inserted during the production of specimens were over 5mm. Figure 2a & 2b below shows the

development of artefacts & conducting tests on the master data sample.

The specimens prepared were to meet the local construction needs and various type of joints were

chosen to cover the varying difficulties. T- Joint, Lap Joint, Cruciform Joint


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Figure 1: Notional POD curve for fluorescent MPT generated by pooling the hit/miss data from all six selected

POD data sets. The total number of inspection results is n = 1329

Figure 2a: Development of Test Coupons with cracks at precise location


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Figure 2b: MPI Indication of Crack Location tested during the time of developing defects

2.1 Evaluation of Test Specimen

Initial Magnetic Particle testing evaluation was carried out by certified personnel from an authorized

qualification body of a certification body accredited to ISO 17024:2012 and tabulated the results. All

the defects were detected with 100% Probability of Detection (POD). The summary results from

magnetic particle examination are provided in Table 1 below;

Table 1: Primary master evaluation using magnetic particle testing by test centre Two parameters were considered for the reliability study, which is the location of defect from datum

and length of defect. The defect length shall be able to detectable within a tolerance of +/-5mm as

recommended by the manufacturer. All the specimens were given masked identification from S1 to S5.

The Lab Inspectors were provided with LA1, LA2…. from Lab A and LB1, LB2…. from Lab B and

LC1, LC2…. from Lab C respectively. D1 is the defect location and L1 is the length of the defect.

Sample Tag No

Type

No of Defects

Defects Detect

Length Location Type Length Location Type Length Location Type Length Location Type Length Location Type

Tolerance ± 5mm ± 10mm ± 5mm ± 10mm ± 5mm ± 10mm ± 5mm ± 10mm ± 5mm ± 10mm

D1 / L1 22 41 Toe 24 31 Toe 12 153 CC 21 143 CC 25 38 CC

D2/ L2 18 45 HAZ 27 65 HAZ 22 200 Toe 17 202 Toe 24 185 CC

D3 / L3 25 143 CC 25 97 CC 25 75 HAZ 28 71 Toe 26 251 HAZ

D4 / L4 24 198 HAZ 33 160 Toe 25 135 Toe 44 127 CC 22 27 HAZ

D5 / L5 18 226 Toe 17 207 HAZ 33 194 CC 25 200 HAZ 26 66 CC

D6 / L6 7 55 CC 27 241 CC 25 75 HAZ 25 71 HAZ 28 203 HAZ

D7 / L7 24 60 HAZ 20 7 Toe 21 145 CC 25 145 CC 25 246 HAZ

D8 / L8 20 92 HAZ 22 39 HAZ 25 200 HAZ 24 202 HAZ

D9 / L9 20 177 CC 18 87 CC 25 188 CC 17 156 CC

D10 / L10 23 207 HAZ 24 180 HAZ

D11 / L11 24 246 HAZ 21 180 Toe

Data Set 11 11 11 11 9 9 9 9 7 7

S4

Lap Joint

9

9

S5

Cruciform

7

7

S2

T Joint

11

11

S3

Lap Joint

9

9

S1

T Joint

11

11


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3 Productivity & Comparison of Magnetic Particle Testing & Eddy Current testing

The NDT methods used in Hong Kong for construction testing of materials and welded joints in are

mainly ultrasonic and magnetic particle testing, but to some extent, also radiography and penetrant

testing are used. Ultrasonic and radiography are used for inspecting volumes (depth). Magnetic particle,

and penetrants are used for inspection of surfaces. All NDT methods are not physically capable of

detecting all kinds of discontinuities. In accordance to structural use of steel 2013 [18], Structural steel

in Hong Kong uses predominantly carbon steel, Penetrant test is used only for stainless steel material.

According to Damhuji Rifai[19] For fast assessment of defects in conductive materials, Eddy current

testing is a most widely non-destructive testing (NDT) evaluation methods utilized in industry,

especially in oil and gas, aircraft, nuclear and coating industries. Hence, eddy current was choosen as

method in lieu of Magnetic particle testing to study the productivity.

3.1 Magnetic Particle Testing Trials

The Hong Kong HOKLAS Supplementary Criteria 36 [11] recommends the maximum productivity of

the Non Destructive Testing methods as detailed in Table 2 below for manual inspection

Test Method

Recommended Maximum Work Capacity for Tests conducted

on Site(Notes 1 & 3)

Recommended Maximum Work Capacity for Tests conducted

in Workshop (Notes 2 & 3)

Visual Examination

120m per operator per 8 working hours

170m

per operator per 8 working hours

Ultrasonic Testing


45m


Magnetic Particle Inspection


120m


Penetrant Testing


50m

per operator per 8 working hours Table 2: Recommended Maximum Work Capacities for Non-destructive Tests on Welds

Notes:

(1) The “working hours” refers to normal working hours ON SITE, including the time for record filling and all the preparation

works prior to testing, but excluding the time for travelling to sites, meals, etc.

(2) The “working hours” refers to normal working hours in WORKSHOP, including the time for record filling and all the

preparation works prior to testing, but excluding the time for meals, etc.


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Test operators were given a set of instructions to use AC Electromagnetic Yoke to carry out the testing;

the yoke were verified with lifting weights and field indicators. Wet visible particles were used as an

inspection medium

The results of 12 operators for testing 3400mm length of welds with were tabulated in Table 3 below.

Overall, the plates were with discontinuities of over 40% of the scanned weld length. In general, the

test set up is identical for 5 plates, 600mm -800mm in length, & no changes in setting is required. All

the operators were certified personnel and hence I allowed sufficient time to conduct their inspection.

The time allowed for practical is 1.5 hours per specimen; they could utilize a maximum of 450minutes

for all 5 specimens for examination condition. However, they have used much lesser time as done in

production and the maximum time was 98min

Table 3: Results of Productivity trials for Magnetic Particle Testing

Figure 3: MPI Productivity trial results of 3 laboratories

Sample ID S1 S2 S3 S4 S5

Type of Joint T-Joint T-Joint Lap Joint Lap Joint Cruciform

Sample Length 3400 600 600 800 800 600

Operator ID Total Length Time S1 Time S2 Time S3 Time S4 Time S5

Total Time

in sec

Productivity

s/m

Average

time in

minute for

1m

Productivity in

m per hour

LA1 3400 750 735 606 760 801 3652 1074.12

LA2 3400 780 720 720 780 1200 4200 1166.67

LA3 3400 720 780 780 780 1140 4200 1166.67 18.15 3.3

LA4 3400 650 670 620 650 824 3414 948.33

LB1 3400 900 900 900 900 1200 4800 1333.33

LB2 3400 783 627 744 734 1260 4148 1152.22

LB3 3400 900 900 960 960 1500 5220 1450.00 22.79 2.6

LB4 3400 1200 1080 1020 1080 1140 5520 1533.33

LC1 3400 1200 1200 1050 1050 1400 5900 1638.89

LC2 3400 900 900 1200 1150 1350 5500 1527.78

LC3 3400 900 900 1050 1100 1200 5150 1430.56 25.04 2.4

LC4 3400 850 900 1100 1080 1155 5085 1412.5 Average 2.78


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According to HOKLAS table [11] the average expected productivity is 15m/hr under the workshop

conditions, although the experimental productivity is very low compared to the published table in the

industry, the results could be skewed due to laboratory A as shown in figure 3, which was higher

compared to other two laboratory. The industrial average of discontinuities is far below 8% (through

interviews with laboratories) compared to the specimens provided with 40% defect.

The lab result could be extrapolated to 13.9/hr which (2.78/hr * 5times). Referring table 3 the average

productivity resulted for 40% defective welds is 2.78m/hour and it is establishing a productivity of

nearly 14m/hr as published in Industrial standard for Hong Kong.

3.2 Eddy Current Testing Trials

Eddy current testing is used in many applications for defect inspection and thin coating measurement.

The basic use of the non-destructive eddy current method is to detect the defect and crack in

conductive weld material connection. The selection of the appropriate probe and eddy current

equipment parameters setting is paramount to obtaining precise and valid inspection results. The eddy

current signal from the inspection is used to characterize the defect profile. [19]. Eddy Current trials

were conducted using the same set of operators, which includes data collection and interpreting &

sizing. The operators were not certified to ISO 9712 Level 2 in ET however, all of the testers were

undergone in house training which includes theory and practical Training. The testing was conducted

using Ether NDT Equipment with 100kHz Single Frequency probe, Gain used 75dB and the

sensitivity was established using a 0.5mm notched reference at 40%.

The results of the trial are tabled in Table 4 and the figure 4 depicts the pie chart of 3 laboratories

performance in productivity.

Table 4: Results of Productivity trials for Eddy Current Testing

Sample ID S1 S2 S3 S4 S5

Type of Joint T-Joint T-Joint Lap Joint Lap Joint Cruciform

Sample Length 3400 600 600 800 800 600

Operator ID Total Length Time S1 Time S2 Time S3 Time S4 Time S5

Total Time

in sec

Productivity

s/m

Average

time in

minute for

1m

Productivity in

m per hour

LA1 3400 1200 1200 900 900 1050 5250 1544.12

LA2 3400 1400 1400 1200 1200 1100 6300 1750.00

LA3 3400 1500 1500 1400 1400 1300 7100 1972.22 30.04 2.0

LA4 3400 1500 1400 1350 1350 1400 7000 1944.44

LB1 3400 900 1200 720 720 840 4380 1216.67

LB2 3400 1200 1200 1050 1050 1350 5850 1625.00

LB3 3400 1200 1200 900 1000 1250 5550 1541.67 24.38 2.5

LB4 3400 1020 1080 960 1200 1020 5280 1466.67

LC1 3400 1400 1400 1200 1200 1400 6600 1833.33

LC2 3400 1200 1300 1300 1400 1500 6700 1861.11

LC3 3400 1300 1200 1250 1150 1300 6200 1722.22 29.38 2.0

LC4 3400 1200 1200 1100 1100 1280 5880 1633.33 Average 2.17


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Figure 4: ET Productivity trial results of 3 laboratories

As there were no evidence during literature review, this experimental study proven it is still

productive to use conventional magnetic particle testing and two laboratories provide consistency

results on the productivity data. Eddy current testing resulted in 22% lower productivity compared to

Magnetic Particle Testing.

4. Reliability study of Magnetic Particle Testing & Eddy Current Testing for Carbon Steel

Structural welds

Magnetic particle testing is a mature non-destructive inspection method for the detection of surface-

breaking or near-surface discontinuities in ferromagnetic steels and has been in use since the 1940s.

Along with visual inspection and liquid penetrant testing, MPT is one of the most common methods

for detecting surface-breaking cracks in metallic parts. [16]

Magnetic particle testing was used to study the reliability trials of testing for the detection and sizing

of the length of defects, the specimen was inspected by twelve (12) qualified inspectors. The sizing of

the length of the defect was carried with EN ISO 9934-1:2016. All Yokes were satisfactory lifted the

relevant block and black ink is used for the visible method. The details of test settings are provided all

inspectors. 4 operators from 3 laboratories repeated the test. The operators are qualified to magnetic

particle testing level 2 in testing weld certified by an UKAS accredited certification body to ISO / IEC

17024:2012 in accordance with EN ISO 9712. The results were tabulated in Table 5. Table 5

summarizes the average POD for the magnetic particle testing was determined as 99.29%.


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Operator Tag No.

No of defects present in 5 specimen

No of Defects POD POFA

Detected False Call

% %

LA-1 47 47 0 100.00% 0.00%

LA-2 47 47 1 100.00% 1.06%

LA-3 47 47 0 100.00% 0.00%

LA-4 47 47 0 100.00% 0.00%

LB-1 47 47 1 100.00% 1.06%

LB-2 47 47 4 100.00% 4.26%

LB-3 47 47 4 100.00% 4.26%

LB-4 47 47 4 100.00% 4.26%

LC-1 47 46 4 97.87% 4.26%

LC-2 47 47 0 100.00% 0.00%

LC-3 47 46 0 97.87% 0.00%

LC-4 47 45 4 95.74% 4.26%

Average 99.29% 1.95%

Table 5: Summary of Magnetic Particle Test Results

POD = Total number of positive calls (Rejects) / Total number of opportunities for Rejects

POFA = Total number of false positive (false alarms) / Total number of opportunities for Acceptance [6]

4.1 Statistical Evaluation including defect sizing and defect location for MT

Certified operator carries out magnetic Particle test, every data obtained for a particular sample is

considered valid and tabulated for defect location (DX) and defect length (DL). A statistical

evaluation is carried out on the set of data to determine the outliers. An outlier is defined as an

observation that "appears" to be inconsistent with other observations in the data set [6]. An outlier has

a low probability that it originates from the same statistical distribution as the other observations in

the data set. On the other hand, an extreme value is an observation that might have a low probability

of occurrence but cannot be statistically shown to originate from a different distribution than the rest

of the data. A box plot is a graphical representation of dispersion of the data. The graphic represents

the lower quartile (Q1) and upper quartile (Q3) along with the median. The median is the 50th

percentile of the data. A lower quartile is the 25th percentile, and the upper quartile is the 75th

percentile. The upper and lower fences usually are set a fixed distance from the interquartile range

(Q3 – Q1). Figure 5 shows the upper and lower fences to be set at 1.5 times the interquartile range.

Any observation outside these fences is considered a potential outlier. Even when data are not

normally distributed, a box plot can be used because it depends on the median and not the mean of the

data.


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Figure 5: Typical Box Plot for Sample 1 for defect location

A summary of outliers within 12 operators were tabled below for all the five defect samples including those missed to identify defects.

Sample Number

No of operators deemed as

outlier

No of operators Reliability

of Detection

Fall Calls

Falls calls

S1 5 58.33% 0

S2 6 4 50.00% 66.67%

S3 6 1 50.00% 91.67%

S4 5 58.33% 0

S5 4 66.67% 0

Average 56.67% 31.67%

Table 6: Summary of outliers in Magnetic Particle test results. 4.1 Reliability study & Statistical Evaluation including defect sizing and defect location for ET

The set of 12 operators were briefed with ET principles by the Level 3, the sensitivity setting has been

used identical for all the operators including the frequency of probe. Table 7 below summarizes the

identification of discontinuities and the average POD for the Eddy Current test is achieved at 93.79%.

One lab as an average POD of 81% and other two laboratory performed with 100%.


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Figure 6: Eddy current testing for fillet-welded place & locating discontinuities

Operator Tag No. No of defects present in 5 specimen

No of Defects POD POFA

Detected False Call

% %

LA-1 47 47 0 100.00% 0.00%

LA-2 47 47 2 100.00% 4.26%

LA-3 47 47 2 100.00% 4.26%

LA-4 47 47 0 100.00% 0.00%

LB-1 47 46 0 97.87% 0.00%

LB-2 47 47 0 100.00% 0.00%

LB-3 47 47 1 100.00% 2.13%

LB-4 47 47 0 100.00% 0.00%

LC-1 47 39 1 82.98% 2.13%

LC-2 47 36 1 76.60% 2.13%

LC-3 47 39 1 82.98% 2.13%

LC-4 47 39 1 82.98% 2.13%

Average 93.62% 1.60%

Table 7: Summary of Eddy Current test results. Statistical evaluation is carried out in a similar way as explained in clause 4.1 for ET, the

summary table below list the number of outliers within 12 operators and none of the

operators result revealed falls call however some defects were missed. The number of outliers

by ET were equally similar as compared to Magnetic Particle Testing (MT).


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Sample Number

No of operators deemed as

outlier

No of operators Reliability

of Detection

Fall Calls

Falls calls

S1 6 50.00% 0

S2 6 50.00%

S3 6 1 50.00%

S4 4 66.67% 0

S5 3 75.00% 0

Average 58.33%

Table 8: Summary of outliers in Eddy Current test results.

5 Conclusion

This paper concludes the detectability of defects by Magnetic Particle Testing & Eddy current test for

structural steel welds, Magnetic Particle Testing is still as proven superior productivity to 22% more

compared to Eddy Current testing (ET). The reliability of ET is 93.62% compared to 99.29% with

magnetic particle testing, however with the inclusion of defect sizing and tolerance the reliability of ET

is dropped to 58.33%, which implies there is a chance of 41.7% of improper sizing of discontinuities

and magnetic particle testing exhibits reliability of 56.67% and falls calls of 31.7%. The study has

proven ET is more reliable in sizing of the discontinuities in structural steel however; ET still exhibited

lower in productivity. The author recommends Magnetic particle testing proven to be more productive

with better reliability in defect identification. Further study on Eddy current array vs Magnetic particle

testing would aid better understanding on productivity.

Acknowledgements

The author wish to acknowledge 3 laboratories & their NDT Managers in Hong Kong who had provided

necessary resources & participated in the study

Mr. Wong Tsz King of ASTAR NDT Consultants Ltd

Mr. B. Vinodh Kumar of Fugro Technical Services Ltd.

Mr. Eric Tam of ETS Test consult Ltd.


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References

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(NDE), August 2001, NTIAC

[6] V. Barnett and T. Lewis, Outliers in Statistical Data (John Wiley & Sons, 2d ed., New York, NY,

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[8] The unicast Ontology of Ethical Intelligence, Peter Belohlavek

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[11] HOKLAS Supplementary Criteria No.36 – HKAS Publication, Construction Material Test

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[13] Code of Practice for Structural Use of Steel – Buildings Department Publication

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[18] Code of Practice for the structural use of steel, 2013

[19] Damhuji Rifai, Ahmed N. Abdalla, Noraznafulsima, Kharudin and Ruzlaini, “Defect Signal Analysis for Non-Destructive Testing Assessment”, ARPN Journal of Engineering and Applied Sciences, VOL. 11, NO. 4, FEBRUARY 2016

[20] Babu, Sajeesh Kumar & W.T. CHAN & Chan, Alan. (2016). Productivity & Reliability Study of Non-Destructive testing techniques for Inspection of structural welds in Construction.

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