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15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.
[ID29] 1
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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15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.
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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
15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.
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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
30m per operator per 8 working hours
45m
per operator per 8 working hours
Magnetic Particle Inspection
90m per operator per 8 working hours
120m
per operator per 8 working hours
Penetrant Testing
30m per operator per 8 working hours
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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