Depth Sizing of Surface Breaking Flaw on Its Open Side by Short Path of Diffraction Technique

Transcription

1 17th Worl Conference on Nonestructive Testing, 5-8 Oct 008, Shanghai, China Depth Sizing of Surface Breaking Flaw on Its Open Sie by Short Path of Diffraction Technique Hiroyuki FUKUTOMI, Shan LIN an Takashi OGATA Central Research Institute of Electric Power Inustry Iwatokita, Komae, Tokyo , JAPAN Tel: , Fax: Web: Abstract Central Research Institute of Power Inustry has evelope a highly accurate an low cost flaw epth sizing technique which is easy-to-use in comparison to stanar techniques such as crack tip iffraction an TOFD techniques. This technique is calle the short path of iffraction (SPOD) technique. In this technique, an angle beam transucer an a 0-egree transucer are use to excite an receive waves. So far the applicability of SPOD to epth sizing on flaw open sie has not been iscusse. This paper eals with the effectiveness of SPOD in this application. Finite element moeling for wave propagation reveale that two echoes were obtaine which were obviously relate to the tip an opening of a surface breaking flaw in SPOD configuration. Base on this result, an equation was erive to calculate the flaw epth from the ifference of the relative beam paths. To emonstrate the applicability of SPOD to surface breaking flaws, epths of fatigue cracks introuce in stainless steel blocks were measure. Because the epths measure were in excellent agreement with actual ones, SPOD coul be applie to flaw epth sizing on its open sie. Keywors: Ultrasonic Testing, Surface Breaking Flaw, Flaw Depth Sizing, Short Path of Diffraction Techniques 1. Introuction For assessing fitness for uty, accuracy of sizing stress corrosion cracks (SCC) has been require to be improve in the wels of pipes in nuclear power stations [1]. Also, long-term operate fossil power stations have face problems with creep amage of high energy pipes. The pipe burst occurring from this amage has been reporte [, 3]. In orer to assure the integrity of structural components an avoi such a catastrophic failure, high accuracy nonestructive inspection is require for etection an sizing of flaws in wels of thick-walle pipes. Nonestructive inspection by ultrasonic testing is being use to evelop more accurate sizing techniques. The special attention is being pai to phase array transucers an the Time of Flight Diffraction (TOFD) technique [4, 5]. Difficulties are often encountere when performing ultrasonic testing of thick-walle austenitic stainless steel wels. The primary causes of these problems are known to be regular, crystalline atomic structure. An ultrasonic wave traveling through such an inhomogeneous an anisotropic material suffers

2 beam path bening, attenuation ue to scattering at grain bounaries, an beam profile istortion [6-9]. TOFD cannot be easily applie to such specimens with high accuracy of flaw epth measurement. The ifficulty to ientify the tip echo burie in unexpecte echoes from the wel nees the experience inspector [10]. Central Research Institute of Power Inustry has evelope a new pit-catch technique for accurate flaw epth to tackle the isavantage of techniques mentione above. The new pit-catch technique arranges an angle beam transucer an a 0-egree transucer to etect iffracte waves that travel in the shortest path for flaw epth sizing, an is calle as the Short Path of Diffraction (SPOD) technique [11]. Effectiveness of SPOD has been emonstrate by applying to epth sizing of surface breaking flaws on back sie. This paper eals with applicability of SPOD to front surface flaws.. Flaw epth sizing by SPOD Flaw epth sizing can be easily mae with a longituinal or shear angle beam transucer an the longituinal 0-egree transucer as shown in figure 1. When the 0-egree transucer is place irectly over a flaw, the iffracte wave with the shortest beam path can be observe. Therefore, the attenuation by scattering an beam spreaing woul be lowere an strong echoes from longituinal waves iffracte irectly at the flaw tip an waves traveling near the flaw surface woul be etecte. A irectly iffracte longituinal wave, L 1, an a surface longituinal wave traveling near the flaw surface before reflecting at the back wall, L, with beam paths of W 1 an W, respectively, can be etecte by a longituinal 0-egree transucer place over the flaw. With this transucer arrangement the flaw epth,, is simply half of ifference between W 1 an W, that is 1 = ( W W1 ). (1) Consier epth sizing of surface breaking flaws on their open sies as shown in figure. With this test configuration, flaw epth, s, can be calculate by ( ) 1 s = W S, W where W is the beam path of tip iffraction echo, an S is the transucer separation. Without calibration of beam path in an ultrasonic testing instrument, the wege elay of angle beam transucer has to be measure because the reaings inclue the wege elay. Assuming that the echo ue to a refracte at an flaw opening is observe, the ifference between echoes ue to the waves iffracte at a flaw tip an refracte at an flaw opening, W, is given as W = W S () (3) Figure 1 surface flaw Figure surface flaw W 1 W Depth sizing of back S W Depth sizing of front s

3 because the relative beam path is ientical to S when the 0-egree transucer locates over the flaw. Using equations () an (3), s can be obtaine as s W = without measuring the wege elay. + W, (4) ( W + S ) 3. Finite element moeling In orer to investigate the effectiveness of SPOD to epth sizing of surface breaking flaws on their open sies, finite element computations were carrie out. A computation moel was shown in figure 3, where two assumptions were mae. They were plane strain an free bounary conition for all bounaries in this figure. On the upper surface, there were transmitting an receiving omains. Vertical forces were loae at the transmitting omain to generate ultrasonic waves. The loaing time varie with lateral position on the surface to generate a longituinal refracte angle of 70 egrees, in this computation. The central frequency of the incient wave was 5 MHz. A-scan waveforms were calculate from isplacements at the receiving omain. Also, there were a slit opening to the upper surface. The material constants in computation were use, assuming the test specimen was stainless steel. Consiering computation efficiency an solution convergence, integration time step t an mesh size h are restricte as h 1 16 λ, t 0.3t h, (5) where is the wavelength of a shear wave an t h is T Excitation omain Slit 10 T: Transmitter R R: Receiver w s s s = 3, 5, 7 w s = Echo intensity (a.u) P=-5mm P= 0mm P=+5mm Figure 3 60 Computational moel Unit: mm Time (micro sec.) Transmission area Shear wave Reception area Slit 4. s Figure 5 Echoes ue to slit with ifferent 0-egree transucer positions Reflecte longituinal wave Longituinal wave Longituinal wave 4.6 s Echo intensity (a.u) s =3mm s =5mm s =7mm 5.0 s Refracte longituinal wave Time (micro sec.) Figure 4 Wave front aroun slit from angle beam transucer Figure 6 Echoes ue to slits with ifferent epths

4 the propagation time of a shear wave through istance h. Figure 4 shows wave propagation at 3.6, 4.6 an 5.0 µs. As shown in this figure, shear an longituinal waves were excite, an propagate towar the lower surface. After the longituinal wave was iffracte at the tip of slit, its iffracte wave was receive by the receiving omain. Preicte waveforms with a slit 5 mm eep are shown in figure 5 as only the 0-egree transucer was move to right an left. P = 0 mm, 5mm an -5 mm stan for positions where the transucer was locate over the slit an move to right an left. There echoes, L s, L o an L t, in this figure, arose from the lateral wave an waves reflecte at the slit opening an iffracte at the slit tip, respectively. Slit epths are 5.0 mm an 4.8 mm, calculate from equation (4) an the waveforms at P = 0 mm an -5 mm. Figure 6 shows waveforms corresponing to slits with epths of 3 mm, 5 mm, 7 mm at P = 0 mm. The beam path ifference between L o an L t varie accoring to the slit epth. The epth calculate from equation (4) was compare with the actual epth in figure 7. Because the calculate epth is in excellent agreement with the actual epth, equation (4) is expecte to be useful. s = 10 mm s = 5 mm s = mm Figure 8 B scan image of slits in SPOD L s L s L o L t 4. Experimental results In orer to verify the sizing accuracy of SPOD, a test was performe using a 0-egree transucer as a receiver an the longituinal angle beam transucer at a refraction angle of 70 egrees as a L o L t Deep crack L o L t Shallow crack Measure epth (mm) Figure 9 B scan image of fatigue cracks in SPOD s = 10 mm s = 5 mm s = mm Actual epth (mm) Figure 7 Comparison between actual epth an epth calculate from preicte Figure 10 B scan image of slits in TOFD

5 transmitter. The central frequency an piezoelectric element iameter of transucers were 5 MHz an 10 mm. Setting the transucer separation to 18 mm, line scans were taken on the fat an smooth surface of stainless steel block where slits with epths of, 5 an 10 mm an a with of 0.3 mm were machine an fatigue cracks were inuce. The ultrasonic measurement system consiste of a commercial ultrasonic instrument an a scanner. Glycerin paste was use as the couplant. B-scan images of the slit an fatigue crack are shown in figures 8 an 9. Clear inications ue to L o an L t were ientifie, an W of each slit or fatigue crack was almost kept constant against transucer position variation when the relative inications appeare. For comparison, TOFD measurement was mae uner conitions about the as the SPOD measurement. As shown in figure 10, it might be ifficult to etect slit mm if inspection surface is rough an the refractor is a natural flaw instea of a slit. In figure 11, epths measure by SPOD were compare with actual epths. The epth of fatigue crack was observe by a microscope on the surface of a polishe cut, by sectioning the stainless steel on scanning potions. SPOD shows excellent performance for sizing epths in stainless steel. SPOD can be also applie to other typical structural materials, such as carbon steel or chromium alloy steel. 15 ) m (m 10 th p e re s u 5 a e M 0 Figure 11 Slit Fatigue crack Actual epth (mm) Comparison between actual an measure epths of slit an fatigue crack 5. Conclusions Application of SPOD to epth sizing of surface breaking flaws on their open sies was iscusse in this stuy. After eriving a simple equation for calculating epth from beam path lengths from geometrical relationship between transucers an a flaw, finite element computations of wave propagation for SPOD were performe to investigate the possibility of epth sizing. Numerical results showe that echoes arising from waves reflecte at the slit opening an iffracte at the slit tip were observe, whose beam path ifference was relate to the slit epth. In orer to emonstrate effectiveness of SPOD, experimental measurements were conucte with machine slits an fatigue cracks. Their epths measure were in excellent agreement with the actual epths. Therefore SPOD is expecte to be useful for the nonestructive inspection of power generation components. References [1] C. D. Cowfer an O. F. Heen, Journal of Pressure Vessel Technology, Transactions of the ASME, Vol. 113, 1991, pp [] F.V. Ellis an R. Viswanathan, Proc. ASME PVP Conference, PVP-Vol.380, Fitness-for Service Evaluations in Petroleum an Fossil Power Plants, ASME, 1998, pp [3] J.F. Henry, G. Zhou an C.T. War, Proceeings of Conference on Avances in Life Assessment an

6 Optimization of Fossil Power Plants, March 00, (EPRI/DOE). [4] H. Fukutomi, S. Lin an T. Ogata, CRIEPI Report, Q04016, 005. [5] S. Lin, H. Fukutomi an T. Ogata, CRIEPI Report, Q04015, 005. [6] A.H.Haker, J.A.Ogilvy an J.A.G.Temple, Journal of Nonestructive Evaluation, Vol.9, No.13, 1990, p [7] S.Ahme an R.B.Thompson, Nonestrutive Testing an Evalution, Vol.8-9, 199, pp [8] J.A.Ogilvy,Ultrasonics, Vol.4, No.11, 1986, pp [9] J.A.Ogilvy, NDT International, Vol.18, No., 1985, pp [10] J.R.Tomlinson, A.R.Wagg an M.J.Whittle, British Journal of NDT, 1980, pp [11] S. Lin, H. Fukutomi an T. Ogata, Review of Progress in Quantitative Nonestructive Evaluation, 6, 006, pp