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15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.Productivity & Reliability Study of Magnetic Particle testing & Eddy current testing forInspection of construction welds.More info about this article: http://www.ndt.net/?id 22086S.K.Babu†1, W.T.Chan2 and Alan Chan3†1SEEM Dept, City University, Hong Kong†E-mail:23Systemsskbabu@astarconsult.comVice President, Hong Kong Institute of Steel ConstructionEngineering & Engineering Management, City University, Hong KongAbstractThe aim of this study is to evaluate the productivity & reliability of magnetic particle testing used forsurface inspection in Hong Kong construction industry. A set of fillet welds with artifacts in differentorientation is prepared with a round robin test with several set of qualified operators in determining theproductivity and probability of detection of test results. Eddy current test were also used to determinethe equivalent productivity and probability of detection of discontinuities in various structural weldsmade predominantly from carbon steel for structural welds employed in the Hong Kong ConstructionIndustry. The study aims to set new criteria for testing & inspection practices in Hong Kong to raiseproductivity & reliability of Inspection techniques.Keywords: Structural steel, Magnetic Particle, Eddy Current, Productivity, Reliability, POD1 IntroductionThere is more than a century's history of using structural steel as a method of building construction. InHong Kong, the use of structural steel in construction has a history of more than 80 years. Buildingslike the old headquarters building of Hong Kong and Shanghai Banking Corporation and the old Bankof China Building are examples of buildings in structural steel, which were constructed from the 30'sto 50's. Recent examples of such buildings in Hong Kong are the Kowloon Commercial Centre inTsim 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 haveways 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 temporarystructures 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.)[ID29]1
15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.Non-destructive test techniques are being advanced more and more and are used as methods toevaluate engineering structures and systems, as much during the construction phase as during theirservice life. Special attention has been given to welds used for this construction due to the seriousconsequences that could occur with the structural failure, such as loss of human lives & economiclosses. However, one of the parameters that should be taken into consideration on selecting the mostadequate non-destructive test technique to be used is its reliability, which is evaluated usingprobability detection curves (PoD) that represent the probability of detection of a defect with aparticular size. For the surface inspection of structural welds one technique is outstanding due to theirportability and ease of operation which is Magnetic particle testing. Eddy current testing is sparinglyused 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 theinspection of structural welds. Two different techniques were considered Magnetic Particle Testingand Eddy current testing.1.1 Test Specimen1.1.1 Production of Test SpecimenTo 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 inunpredictable locations such as toe, heat affected zone, centre line. All the discontinuities wereinserted more than 5mm.S.K. Burke and R.J. Ditchburn explains in a literature review “Review of Literature on Probability ofDetection (POD) for Magnetic Particle Non-destructive Testing”. POD determination using Magneticparticle testing [16], in the below figure 1 we could understand the POD is always 100% over 3.5mmin terms of discontinuity size. Hence, in order to achieve the reliable results in trial all artefactsinserted during the production of specimens were over 5mm. Figure 2a & 2b below shows thedevelopment of artefacts & conducting tests on the master data sample.The specimens prepared were to meet the local construction needs and various type of joints werechosen to cover the varying difficulties. T- Joint, Lap Joint, Cruciform Joint[ID29]2
15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.Figure 1: Notional POD curve for fluorescent MPT generated by pooling the hit/miss data from all six selectedPOD data sets. The total number of inspection results is n 1329Figure 2a: Development of Test Coupons with cracks at precise location[ID29]3
15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.Figure 2b: MPI Indication of Crack Location tested during the time of developing defects2.1 Evaluation of Test SpecimenInitial Magnetic Particle testing evaluation was carried out by certified personnel from an authorizedqualification body of a certification body accredited to ISO 17024:2012 and tabulated the results. Allthe defects were detected with 100% Probability of Detection (POD). The summary results frommagnetic particle examination are provided in Table 1 below;Sample Tag NoTypeNo of DefectsDefects DetectToleranceD1 / L1D2/ L2D3 / L3D4 / L4D5 / L5D6 / L6D7 / L7D8 / L8D9 / L9D10 / L10D11 / L11Data SetLength 5mm22182524187242020232411S1T Joint1111Location Type 207HAZ246HAZ11Length 5mm242725331727202218242111S2T Joint1111Location Type 0HAZ180Toe11Length 5mm1222252533252125259S3Lap Joint99Location Type CC9Length 5mm2117284425252524179S4Lap Joint99Location Type CC9Length 5mm25242622262825S5Cruciform77Location Type 10mm38CC185CC251HAZ27HAZ66CC203HAZ246HAZ77Table 1: Primary master evaluation using magnetic particle testing by test centreTwo parameters were considered for the reliability study, which is the location of defect from datumand length of defect. The defect length shall be able to detectable within a tolerance of /-5mm asrecommended 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 andLC1, LC2 . from Lab C respectively. D1 is the defect location and L1 is the length of the defect.[ID29]4
15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.3 Productivity & Comparison of Magnetic Particle Testing & Eddy Current testingThe NDT methods used in Hong Kong for construction testing of materials and welded joints in aremainly ultrasonic and magnetic particle testing, but to some extent, also radiography and penetranttesting 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 ofdetecting all kinds of discontinuities. In accordance to structural use of steel 2013 [18], Structural steelin 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 currenttesting 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 asmethod in lieu of Magnetic particle testing to study the productivity.3.1 Magnetic Particle Testing TrialsThe Hong Kong HOKLAS Supplementary Criteria 36 [11] recommends the maximum productivity ofthe Non Destructive Testing methods as detailed in Table 2 below for manual inspectionTest MethodRecommended MaximumWork Capacity for Tests conductedon Site(Notes 1 & 3)Recommended MaximumWork Capacity for Tests conductedin Workshop (Notes 2 & 3)Visual Examination120mper operator per 8 working hours170mper operator per 8 working hoursUltrasonic Testing30mper operator per 8 working hours45mper operator per 8 working hoursMagnetic Particle Inspection90mper operator per 8 working hours120mper operator per 8 working hoursPenetrant Testing30m50mper operator per 8 working hoursper operator per 8 working hoursTable 2: Recommended Maximum Work Capacities for Non-destructive Tests on WeldsNotes:(1) The “working hours” refers to normal working hours ON SITE, including the time for record filling and all the preparationworks 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 thepreparation works prior to testing, but excluding the time for meals, etc.[ID29]5
15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.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 aninspection mediumThe 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, thetest set up is identical for 5 plates, 600mm -800mm in length, & no changes in setting is required. Allthe 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 450minutesfor all 5 specimens for examination condition. However, they have used much lesser time as done inproduction and the maximum time was 98minSample IDType of JointSample LengthOperator 3400Total 3400S2T-Joint600S3Lap Joint800S4Lap Joint800S5Cruciform600Time S17507807206509007839001200Time S27357207806709006279001080Time S36067207806209007449601020Time S47607807806509007349601080Time 001155Total Timein eragetime inminute for1mProductivity inm per hour18.153.322.792.625.04Average2.42.78Table 3: Results of Productivity trials for Magnetic Particle TestingFigure 3: MPI Productivity trial results of 3 laboratories[ID29]6
15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.According to HOKLAS table [11] the average expected productivity is 15m/hr under the workshopconditions, although the experimental productivity is very low compared to the published table in theindustry, the results could be skewed due to laboratory A as shown in figure 3, which was highercompared to other two laboratory. The industrial average of discontinuities is far below 8% (throughinterviews 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 averageproductivity resulted for 40% defective welds is 2.78m/hour and it is establishing a productivity ofnearly 14m/hr as published in Industrial standard for Hong Kong.3.2 Eddy Current Testing TrialsEddy 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 inconductive weld material connection. The selection of the appropriate probe and eddy currentequipment parameters setting is paramount to obtaining precise and valid inspection results. The eddycurrent signal from the inspection is used to characterize the defect profile. [19]. Eddy Current trialswere 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 wereundergone in house training which includes theory and practical Training. The testing was conductedusing Ether NDT Equipment with 100kHz Single Frequency probe, Gain used 75dB and thesensitivity 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 laboratoriesperformance in productivity.Sample IDType of JointSample LengthOperator IDLA1LA2LA3LA4LB1LB2LB3LB4LC1LC2LC3LC43400Total 3400S1T-Joint600Time 2T-Joint600Time S3Lap Joint800Time p Joint800Time Cruciform600Time otal Timein Averagetime inminute for1mProductivity inm per hour30.042.024.382.529.38Average2.02.17Table 4: Results of Productivity trials for Eddy Current Testing[ID29]7
15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.Figure 4: ET Productivity trial results of 3 laboratoriesAs there were no evidence during literature review, this experimental study proven it is stillproductive to use conventional magnetic particle testing and two laboratories provide consistencyresults on the productivity data. Eddy current testing resulted in 22% lower productivity compared toMagnetic Particle Testing.4. Reliability study of Magnetic Particle Testing & Eddy Current Testing for Carbon SteelStructural weldsMagnetic particle testing is a mature non-destructive inspection method for the detection of surfacebreaking 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 methodsfor detecting surface-breaking cracks in metallic parts. [16]Magnetic particle testing was used to study the reliability trials of testing for the detection and sizingof the length of defects, the specimen was inspected by twelve (12) qualified inspectors. The sizing ofthe length of the defect was carried with EN ISO 9934-1:2016. All Yokes were satisfactory lifted therelevant block and black ink is used for the visible method. The details of test settings are provided allinspectors. 4 operators from 3 laboratories repeated the test. The operators are qualified to magneticparticle testing level 2 in testing weld certified by an UKAS accredited certification body to ISO / IEC17024:2012 in accordance with EN ISO 9712. The results were tabulated in Table 5. Table 5summarizes the average POD for the magnetic particle testing was determined as 99.29%.[ID29]8
15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.Operator TagNo.No of defectspresent in 5specimenNo of Table 5: Summary of Magnetic Particle Test ResultsPOD Total number of positive calls (Rejects) / Total number of opportunities for RejectsPOFA 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 MTCertified operator carries out magnetic Particle test, every data obtained for a particular sample isconsidered valid and tabulated for defect location (DX) and defect length (DL). A statisticalevaluation is carried out on the set of data to determine the outliers. An outlier is defined as anobservation that "appears" to be inconsistent with other observations in the data set [6]. An outlier hasa low probability that it originates from the same statistical distribution as the other observations inthe data set. On the other hand, an extreme value is an observation that might have a low probabilityof occurrence but cannot be statistically shown to originate from a different distribution than the restof the data. A box plot is a graphical representation of dispersion of the data. The graphic representsthe lower quartile (Q1) and upper quartile (Q3) along with the median. The median is the 50thpercentile of the data. A lower quartile is the 25th percentile, and the upper quartile is the 75thpercentile. 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 notnormally distributed, a box plot can be used because it depends on the median and not the mean of thedata.[ID29]9
15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.Figure 5: Typical Box Plot for Sample 1 for defect locationA summary of outliers within 12 operators were tabled below for all the five defect samplesincluding those missed to identify defects.SampleNumberNo ofoperatorsdeemed asoutlierNo 33%0S5466.67%056.67%31.67%AverageTable 6: Summary of outliers in Magnetic Particle test results.4.1 Reliability study & Statistical Evaluation including defect sizing and defect location for ETThe set of 12 operators were briefed with ET principles by the Level 3, the sensitivity setting has beenused identical for all the operators including the frequency of probe. Table 7 below summarizes theidentification 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%.[ID29]10
15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017), Singapore.Figure 6: Eddy current testing for fillet-welded place & locating discontinuitiesOperator Tag No.No of defects present in 5specimenNo of ble 7: Summary of Eddy Current test results.Statistical evaluation is carried out in a similar way as explained in clause 4.1 for ET, thesummary table below list the number of outliers within 12 operators and none of theoperators result revealed falls call however some defects were missed. The number of outliersby ET were equally similar as compared to Magnetic Particle Testing (MT).[ID29]11
15th Asia Pacific Conference for Non-Destructive Testing (APCNDT2017),
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
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