Contributi on to Crack Sizing by Phas ed Array U trasonic Techniques
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Contributi on to Crack Sizing by Phas ed Array U trasonic Techniques

Part 2 Comparison with Optical Mag netic Particles Fracture Mechani cs and Metallography for Last Si gnificant Crack Tip Ciorau P Ontario Power Generation Canada peterciorauopgcom Abstract The paper presents pha sed array re sults for 1D linear a

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Contributi on to Crack Sizing by Phas ed Array U trasonic Techniques

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Contributi on to Crack Sizing by Phas ed Array U trasonic Techniques. Part 2: Comparison with Optical, Mag netic Particles, Fracture Mechani cs and Metallography for Last Si gnificant Crack Tip. Ciorau, P. – Ontario Power Generation – Canada Abstract : The paper presents pha sed array re sults for 1-D linear array probes of high frequency (7-10 MHz) i L-, and S-waves for d tecting the crack shap e and the last signi ficant tip. Fa tigue and str ss-corrosio cracks wit height ranging from 1.6 mm to 20.4 mm were detected in weld ed samples, piping weld s and

straight bar s with thickness between 6 mm to 38 mm. Th e results of S-scan display are compared with different methods: optical, magnetic particles, fracture mechanics and metallography. The experimental results concluded the undersizing trend of PAUT in detecting th e last crack tip or closure, in spite of using dynamic depth focu sing, and/or focusin o crack tip. The average un dersizing er ror is – 0.4 mm. This error increase for cracks with depth > 12 mm. The la rgest errors occur when the crack is sized from outer surface cou led with initiation fro the outside surface with propag ation

towards the inside surf ace. These errors were reduced by a combination of shear and lo ngitudinal w ves and by increasing t he angular r solution. 1.0 Introduction Our pre ious published papers compared conventional and phased array ultrasonic technology [ 1- 4 ] . Two conclu sions were PAUT pro ided more a ccurate sizing and produced an image of crack. These image patterns (see Figure 1 ) were more easi y interpreted than the A-Scan f conventional instruments. Small cracks (h crac < 2 mm) co uld be sized due to redu ndancy in angles of S-scan. Figure 1: Example of a stress-corro sion crack

tip displa y by S-scan of 1 -MHz longitudinal waves 1- d change th e width, the angle and th e closure a pect (see Fi gure 2 ). D linear array probe (left) and comparison with crack sha (stereo microscope, 2 (right). 1
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Examples of thermal / corrosion fatigu e crack morphology and crack closu e of last significant tip. Generally, a transition of 0.2 to 0.4 mm is noticed from a “wider” crack ( 15- accuracy and cra ck pattern display in S-scan. These result s are discussed below. al Program Table 1 . f r sizing ev aluation Sample ID [ mm ] [ mm ] 25 P m) to a “narrower” crack

(0.5-2 P m). ase of o r investigation focused on The samples with cracks are presen Table 1: Sam Crack type Crack heigh t Crack heigh Optical; MP Optical; MP 13.6 OHR-20 20 8.5-9.5 3B 38 fatigue 4.2-5.3 Optical; MP; metallographic 9B 38 fatigue 8.3-8.9 Optical; MP; fracture mechanics 3E 36 fatigue 15.7 Optical; MP; metallographic The crack ctual” heig t was difficult to assess by optical or MP from the side of the sample. An example is given in Fig re 3, for sa mple 9B. 2
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Figure 3: after the cra ck was broken open (bo ttom left). Difference be tween 0.2 mm to 0.6 mm in height

measurement wa s found. ce of t he sample. The crack h ight the sample width, with the maximum height in the middle where PAUT was ap plied. Table 2: 1-D phased array probes used for crack height evaluation. Probe ID Frequency Pitch [mm ] / Example of crack height measureme t by th ree methods: optical (top le ft ); MP (right) and ight was oversized by optical and MP. This may be e ained by magnetic part cle on the cra ck tip and b Due to the importance of these sa mples for tr aining and p ocedure validation, on ly one sample was broken open to measure the crack height via fract ography. The

remai nder of the measureme ts were performed on the side of the samples by optical and MP meth ods. Welded samples were etched and a metallographic examination added for crack ev aluation. The probes used for evaluation are p esented in Table 2 . [ MHz ] nr. elements Rem rks 2J 10 0.31 / 20 LW between -65 q to 65 q ; variable focus depths 30 10 0.31 / 32 mm mm – 30 52+70T 8 0.33 / 10 SW with 70 q wedge for OD e aluation 43+60T 7 0.6 / 16 SW with 60 q wedge; angles: 28 q to 8 q 2J + 60T 10 0.31 / 20 SW with 60 q wedge for OD e aluation 18+52TW 10 0.5 / 64 q and DDF for specifi cr 18 10 0.5 /

64 LW / DDF for thickness > 18 mm 42 10 0.5/ 32 LW / DDF for thickness > 18 mm The probes were used xi mum crack heigh angles. Each scan was repeated five times. Scr een shots a nd UT data were analyzed using S- B-A directions ( Figure 4) a ed positive 3
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layouts. Fin l crack height was measured in S scan (true depth). Both OmniSca MX 32/32 and Focus LT 64 / 128 instru ments were used to acq ire data. The sizing principle w s based on back-scattering tip echo diffr acted techn ques and AATT (absolute ar rival time technique) ap plied to a tr ial scan display Figure 5 -as

princip e, and Figure 6 -as real d ta related t specimen and probe). Figure 4 ves Figure 5: r shear waves (left) and Figure 6: Crack height sizing for she r waves (left) and L-waves (right). Note the SCC display with branched tip sized by L-waves (right). : Examples of crack height evaluation by: Azim uth l S-wa ves (left), by Azi uthal L-wa (middle) and by lateral L-waves (right). Principle of cr ack sizing b sed on back-scattering diffraction fo longitudinal waves (right). 4
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5 3.0 Data Evalu tion Exa ples of crack heigh t measurements are pre ented in Fi gure 7 to Figure 16

. Figure 7: Crack sizing comparison between MP (left) and PAUT (right) on sample 3E. Figure 8: Crack height p otting for sa mple B12 (left – OD sizing; right - ID sizing) Figure 9: Crack height sizing in sample C1 by OmniScan 32/32 and probe 30 (L-waves). A systematic undersizing o 0.3 mm wa s found by all three tech niques com pared to optical/MP.
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t has (courtesy of OlympusNDT-Waltham-USA) Figure 10: Stress Corrosion crack in sample OHR-20 at different refracted angles. Crack heigh a minimum response nor mal incidence. See values from Table 3. Table 3: Crack height measured

at different a ngles for sample #B18 using a 10 MHz L-wave probe. Angle [ q ] 46 32 17 0 -17 -32 -46 Height [ mm ] 14.2 13.6 12 11.5 11.7 13.7 14 Figure 11 : Comparison for sample 3B with three cracks. PAUT undersized by 0.5 mm. The best sizing angle ra nge is betw een 30 – 35 , and symmetrical negative (-30 to -35 ). 6
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Crack sizing using L-wa ves on sample C2. The crack was measured optically as 20.4 mm. Note the undersizin of OD technique. Crack sizing using S-waves on sample C2. ID negative came very close to the optical Figure 12: Figure 13: value (20.3 mm vs. 20.4

mm-optical). Note the same trend of undersizin for OD technique. 7
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Figure 15: Crack sizing by two syst ems on sample B18: left OmniScan with 7MHz p obe of 16- elements, sampling at 0.5 degree (data converted for Tomovi w analysis); right : Focu LT, 10MHz with optimized wedge + Dyna mic Depth Focussing (DDF) + sampling at 0.2 degrees. Figure 14: Exa ples of maxi mum SCC evaluati on on sample OHR-20. and crack o ientation pe rformed from outer surface on sample C1. Figure 16: d the back- scattering fro the last significant t p depend on crack openin , , load, oxide presence,

probe access and optimiz ing the focus beam alo ng the crack height [6-10 . An example of t he influence of phased a rray set-up on crack d splay is presented in Fig re 17 . 8
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Phased array set-up influence on SCC height measurement for 10-MHz probe, pitch 0.5 mm, 64 ele ents, L-waves mode. When the full probe aper ture was used with F=20 mm i.e. fo cused on th e inside surf ace, the cra ck was not detected. The b st PAUT response was with a 20-element aperture, DDF and a focal len th of 15 mm. The over-all performance of sizing f r this experiment is presented in Fi gure 18 .

Figure 18: Over-all sizing performance on cracks from Table 1. An undersizing tren d was found. This degree of undersizing increase with crack height, espe cially for the OD technique. 4.0 Conclusio s Stress corro sion crac ks were m re accurately siz ed by L-waves The cracks presented significant var ations (bet ween 0.2 to 0.6 mm) alo ng the sample width 1. The crack t p or closure i.e. the last 0.2-0.4 mm was difficult to detect an d size. 3. Fatigue cracks were more accurately sized by S-waves 4. A very narrow and energetic bea m supplemented by DDF and a fine angular resolution resulted

in t he m st acc rate better sizing, e pe cially on co ressed cracks. 9
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1 6. Only one sample was compared with fractogra phy. The PAUT results presented in Figure 19 , were very similar to the fractogra phy measurements for the middle of the sample. 7. The results were consistent with our previous results i.e. t he last significant tip wit PAUT undersized by 0.2 to 0.6 mm. 8. Under field conditions, these techni ques were expected to undersize by 0.5 mm for cracks with height < 8 mm and 0.8 - 1.0 mm for cracks with h > 8 Recent field inspec tions of T- welds (ID) [4 and on re

pairs of out er surface-b eaking cra cks o outl t welds ( Fi gure 20 ) confirmed these result s. Figure 19: PAUT results on sample 9B before it was broken open. The material thickne ss at the crack lo cation was 3 .2 mm. Th e PAUT crack height = 8.7 mm. Th e actual cra k height after breaking op en the sample and fracto graphy mea urements ( Figure 3), is between 8.8 – 8.9 mm. This measurement was from the centre of the sa mple. in outlet weld using OmniScan and P52+60T. The crack wa s ight was Exa confirmed by MP (top left) and sized as h=7.3 mm (top right). Slag inclu detected (se confirmed

as h grinding = 7.6 mm. The results of this experiment are used as a conf idence boun dary for ECA (see ref. 4).
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Acknow ledgements x ”Advances in Phased Array Ultrasonic Technolo y Applications x Wence Daks - CAD WI RE – Markh m, Ontario, Canada - for plotting ph ased array data into 3-D specimens and for crack height measur ements by optical a nd MP meth ods. x Dick Gray - OPG-Thermal Productio – Nanticoke for suppor ting this pro ect and usin g PAUT as valuable ECA tool. Reference 1 Ciorau, P.: “Contribution to Detection and Sizing Linear Defe cts by Con entional and Phased

Array Ultras onic Techn iques Proceedings 16 th WCNDT Montreal, Sept. 2004, p aper 233. Ciorau, P.: ontribution to Detectio n and Sizin Linear De fects by Ph ased Array Ultrasonic The author wants to thank the follow ng organiza tions and pe ople: x Senior Management – for approving publication of this paper m (USA) – for allowing publication of some figures from their book: , , v. 10, no. 11, Nov. 2005 3 Ciorau, P.: Critical Comments on Detection and Sizing Linear Defects by Con Welds” – 4 th PA seminar - EPRI-Miami – Dec 2005, 4 Ciorau, P., Gray, D., Daks, W.: “Phased Array

Ultra onic Te ch nology Contribution t Engineering Critical Assessment (ECA) of Economizer Piping Welds” - – vol. 11, no. 5, May 2006 5 OlympusNDT: “Ad an ces in Phased Array Ultrasonic Technology Application ”, March 2007, Walth m, USA. 6. Virkkunen, Pitkänen, Kemppainen, M : Effect of cra ck opening on UT response Proceedings 9 th ECNDT – Berlin - Oct. 2006, pap er Th.4.4.2 7. Poidevin, C. , Bredif, P., Dupond, O.: A phased array techn que for crack character zation Proceedings 9 th ECNDT - Berlin - Oct. 2006, pap er Th.1.1.2 8. Saka, M., Salam Akand a, M. A., 2 004, “Ultrasonic

measurement of t he crack depth and crack opening stress in tensity factor under a n load condition”, Journ l of Nonde structive Evaluation, vol. 23 (2004), no. 2, pp . 49 – 63 main stea m pipework”, Insight , 9. Pitkänen, J. , Kemppain en, M., Virk kunen, I., Laukkanen, A.: response in detection a nd sizing of cracks”, Pr oceedings 9 th ECNDT , R.:” Practical limitations of vol. 48, no. 9 (Sept. 2006), pp. 559-563. 1