外文原文-利用超聲波確定鐵素體鋼焊縫內(nèi)部的縱向裂紋的大小

Ultrasonic sizing of embedded vertical cracks in ferritic steel welds Sony Baby a, T. Balasubramaniana, R.J. Pardikarb,* a Department of Physics, Regional Engineering College, Tiruchirappalli 620015, India b NDTL, Bharat Heavy Electricals Limited, Tiruchirappalli 620014, Tamil Nadu, India Abstract Embedded vertical cracks are very often encountered in structural components necessitating reliable procedure for determining their size. Specialized ultrasonic techniques are required for estimation of through thickness height of these cracks Mater. Eval. (1970) 28; J. NDI 26 (1977) 320; NND (1975) 146. The conventional pulse-echo technique has serious limitations for quantitative estimation of vertical cracks because it is subject to variations due to refl ectivity, coupling factors etc. The present study aims to improve the accuracy of embedded crack height measurement based on transit time rather than the refl ected amplitude NDT Int. (1982) 315; J. JSNDT 27 (1978) 118. A unique method has been developed for generating known sizes of fatigue cracks ranging from 2.5 to 18 mm in height and 15 mm in length. These cracks were generated in rectangular steel specimens and subsequently embedded at predetermined locations using welding. These welded specimens containing the embedded vertical cracks were examined using ultrasonic techniques based on transit time like (a) fl aw tip echo method and (b) mode conversion method. Using both the techniques excellent agreement was obtained between the estimated crack height and actual crack height with an ac- curacy of ?1 mm. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Embedment of cracks; Crack height; Fracture mechanics parameter 1. Introduction Welded structures can fail catastrophically if they contain cracks above a certain critical size for the load applied 6. Using the concepts of fracture mechanics, it is possible to determine the extent to which a preexisting crack might propagate to an unacceptable level during service. For computa- tion of fracture mechanics parameters like stress intensity factor, it is essential to know the exact location, confi guration and the size of the cracks. There are many methods for detecting surface breaking cracks while only a few techniques pre- vail for detecting embedded cracks 1,35. Recent improvements have been made in the performance of ultrasonic fl aw detectors and transducers. One of the most promising lines of de- velopment is the use of computers and micropro- cessors along with the digital fl aw detectors. This has not only reduced the cost of inspection but also helped to perform calculations to determine the projection distances, skip distances, depth of the *Corresponding author. Fax: +91-431-2520710/2520730. E-mail address: rjpbheltry.co.in (R.J. Pardikar). 0167-8442/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0167-8442(03)00042-9 Theoretical and Applied Fracture Mechanics 40 (2003) 145151 /locate/tafmec defect, fl aw sizes and attenuation co-effi cient in an easy way. The echo height, beam path length, lo- cation and size of the fl aw are digitally displayed on the CRT or LC panel at real time entirely elimi- nating possibility of human errors. As a result various methods utilizing ultrasonic propagating time could be developed for accurate sizing of de- fects. Several techniques have been proposed using the bulk waves 2,7,8. However only two such methods used for crack height estimation are out- lined in this paper. 2. Specimen preparation The preparation of the specimen mainly in- volves the generation of fatigue cracks in steel specimens 9 and welding of the specimens to submerge these cracks at predetermined location. The carbon steel blocks of width 90 mm and thickness 16 mm containing edge notch of 11 mm (Fig. 1) were subjected to fatigue loading using INSTRON-1276. The fatigue load parameters were selected such that the operating stress inten- sity factor at the beginning of the fatigue test would be above threshold stress intensity factor for the material and that the crack propagation would be in the lower part of the stage II regime. The calculations of the stress intensity factor range DK can be made from DK Y Drpa1=21 Note that r is stress amplitude, a the crack length and Y the compliance factor. Few specimens contain two parallel cracks emanating from starter notches. After reaching the required crack length that has grown from the notch, the specimen were cut to size of 300 mm in length and machined at the notched edge to re- move the notch and leave behind the crack of re- quired size. The length measurement of the crack was done using a profi le projector. The thickness of the specimen after machining and the crack length, were measured precisely before welding, as these dimensions would give the size of crack that would eventually be obtained as submerged crack. The sizes of the edge cracks obtained are shown in Table 1. 3. Embedment of crack In order to get the vertical cracks at the prede- termined locations in the weld for detection and sizing by ultrasonic testing, it was decided to embed the edge cracks in 32 mm plate approximately at mid-thickness and normal to the test surface. MAG welding process was used for this purpose. The steps involved are shown in Fig. 2. The cracked Fig. 1. Specimen for crack generation. Table 1 Sizes of edge cracks SpecimenNo. of edge cracks Length of I crack (mm) Length of II crack (mm) A15.1 B19.35 C211.75.4 D218.32.5 E27.54.7 146S. Baby et al. / Theoretical and Applied Fracture Mechanics 40 (2003) 145151 specimen (16 mm plate) was welded with another plate of the same thickness in order to embed crack on one face as shown in Fig. 2(a) and (b). To embed the crack on the remaining faces, a slice of 32 mm width containing the crack was extracted and wel- ded between the two plates of 32 mm as shown in Fig. 2(c) and (d). During welding a ?k? type geo- metry was selected to avoid any edge preparation on the plate containing cracks. This geometry was found to be useful in overcoming the distortion problems. After burying the cracks at the required locations in 32 mm plates, the blocks were radio- graphed to confi rm the locations. Table 2 shows the details of blocks containing vertical cracks. The test blocks containing embedded cracks were examined for estimating the crack height by using two techniques based on transit time. They are the fl aw tip echo method and mode conversion method. 4. Flaw tip echo method When an ultrasonic beam is directed with an angle to a defect, an echo is obtained from the defect edge as shown in Fig. 3. In this method, the height of the defect is determined geometrically from the beam path distance at the position where the peak value of echo is obtained from the end of the defect and the angle of refraction of the probe used. Better accuracy of measurements and applica- bility to this method is obtained when the follow- ing conditions are satisfi ed: The ultrasonic beam is directed with an angle of about 1015? to the plane of hidden defect. The surface should be smooth (about 1/15 to 1/30th of the wavelength or better). Beam path length is relatively short (generally within 100 mm). Fig. 2. Welding for embedding the crackfi rst and second stage. Table 2 Actual height of the cracks after embedment Block no.Height of I crackHeight of II crack 15.1 29.35 311.75.4 418.32.5 57.54.7 S. Baby et al. / Theoretical and Applied Fracture Mechanics 40 (2003) 145151147 The defect is not tightly closed and has an ap- preciable height (2 mm or higher). A low frequency is applied (about 24 MHz is adequate). A focused probe is used to converge the beam. 5. Experimental set-up A digital fl aw detector with a touch panel sen- sor was used for this purpose 10. Four megahertz miniature probes with 45?, 60? and 70? shear wave angles were used. The equipment was calibrated using 0.5 mm wide machined notch. The gain was adjusted for optimum detection of the tip of the notch. The probe was placed on the surface of the test block to aim at the upper tip of the embedded crack. A clearly resolved tip echo at 40% of CRT screen height was recorded. Then the probe was moved back and forth to get clear echo from the bottom tip of the crack. The sound path, projected distance and the depth of the tips from the test surface and the A-scan displays are shown in Fig. 4. The sound path diff erence between the ultra- sonic beams refl ected from the tips account for the height of the crack. Changes in the direction of the penetration of crack or misplacement of probes can, of course, cause errors by increasing the ob- served time delay 11. Table 3 and Fig. 5 show the relationship between the true crack height and the ultrasonically estimated height. 6. Mode conversion method When the shear wave generated from the angle probe impinges on the crack like defect as shown in Fig. 6(a) some of the shear waves are refl ected directly at the upper tip of the defect and travel back to the probe (S echo). At the same time some of the shear wave are mode converted to surface wave and propagated along both side of the defect 12. When the mode converted surface waves come down to the lower tip of the defect, some of them are refl ected and some of them are bent around the tip. These surface waves are propa- gated along the defect and at the upper tip of the Fig. 3. Flaw tip echo method. Fig. 4. CRT display: (a) echo from top tip of the crack; (b) echo from bottom tip of the crack. 148S. Baby et al. / Theoretical and Applied Fracture Mechanics 40 (2003) 145151 defect get reconverted to the shear wave and travel back to the probe (R echo). As width of the crack like defects are usually small, the time required to propagate along the defect for surface wave be- comes almost same. Therefore only one echo is obtained as R echo. Therefore, in CRT screen the S echoes and R echoes are displayed as shown in Fig. 6(b) in which R echo delay is compared with S echo. Then adjusting the time base of CRT as the velocity of shear wave, H (height of crack like defect) is determined by Eq. (2) 12. H CR=CS ? DX2 Here, CR and CS are the velocity of the surface wave and the shear waves respectively. Better accuracy of measurement are provided for this method when the following conditions are satisfi ed: In the case of planar defect the ultrasonic beam is directed to the plane of defect with an inci- dent angle of 25? or more. It is preferable that the beam be directed as nearly parallel as possi- ble to the plane of the defect. The defect is isolated with no other defect in vi- cinity. A focused probe is used to converge the beam. The defect is not tightly closed and has an ap- preciable height (above 1 mm). The defect is linear than wavy. Fig. 5. Estimation of crack height by fl aw tip echo method. Fig. 6. Mode conversion method. Table 3 Estimation of crack height by fl aw tip echo method SpecimenCrackActual crack height (mm) Estimated crack height (mm) Error (mm) A1a5.14.8)0.3 B1a9.359.9+0.55 C1a11.711.5)0.2 2a5.45.6+0.2 D1a18.318.5+0.2 2a2.52.431)0.069 E1a7.58.4+0.9 2a4.75.3+0.6 Note that afatigue crack, mean error0.235 mm and standard deviation0.377 mm. S. Baby et al. / Theoretical and Applied Fracture Mechanics 40 (2003) 145151149 The method is applicable not only to planar de- fects but also to a defect with volume. Accuracy of the measurement is approximately ?1 mm. 7. Experimental work Experiments were carried out for determination of height of fatigue cracks employing mode con- version method. In case of mode conversion technique, 2 MHz, 70? shear wave transducer was used along with the digital fl aw detector having software for searching mode-converted echo. The equipment was calibrated and the sensitivity was set using a 1.5 mm side drilled hole. The probe parameters and the job parameters were fed to the microprocessor. The probe was then scanned on the test object so that the shear wave strike the upper tip of the crack and Rayleigh waves are produced which go around the crack and get re- converted into shear waves before being received by the probe. The CRT then displays two echoes one due to direct refl ection (S echo) and the other due to converted refl ection (R echo). The echo height, the beam path length and the location were recorded to estimate the height of the crack using Eq. (2). The observations are shown in Table 4. The relationship between the true slit height and the ultrasonically estimated height are shown in Fig. 7. 8. Conclusions It has been demonstrated that fatigue cracks of known size could be generated and embedded in weldments at predetermined locations for sizing of cracks by ultrasonic techniques. Two ultrasonic techniques based on transit time approach, bulk wave timing and surface wave timing were studied for their applicability to estimation of height of embedded cracks. It can be seen from the results that for both the methods there is a good agree- ment for the estimated crack height and the actual crack height. Both the methods used in the study also executed the uniform high levels of accuracy within ?1 mm. Using fl aw tip echo method 45?/ 60?/70? probes could measure the crack height satisfactorily. However 70? gave high degree of Fig. 7. Estimation of crack height by mode conversion method. Table 4 Estimation of crack height by mode conversion technique SpecimenCrackActual crack height (mm) Estimated crack height (mm) Error (mm) A1a5.14.74)0.36 B1a9.359.49+0.14 C1a11.710.35)1.35 2a5.45.175)0.225 D1a18.318.54+0.24 2a2.53.450+0.95 E1a7.57.33)0.17 2a4.75.175+0.475 Note that afatigue crack, mean error0.0375 mm and standard deviation0.4032 mm. 150S. Baby et al. / Theoretical and Applied Fracture Mechanics 40 (2003) 145151 accuracy because most of the cracks were embed- ded at a comparatively shallow depth. This sug- gests that the accuracy of crack height measured was also infl uenced by the angle of incidence on the refl ector surface of the crack. Acknowledgements Sincere thanks are due to Council of Scientifi c and Industrial Research (CSIR), New Delhi, India for awarding the Senior Research Fellowship to Ms. Sony Baby sanction no. 1(64) 010-2k2/1 and fi nancially supporting her in the pursuance of her research study. Authors wish to thank the man- agement of Bharat Heavy Electricals Limited, Tiruchirappalli for providing all the necessary fa- cilities in carrying out this study. References 1 G. Diagiacomot, An ultrasonic method of measuring crack depth in structural weldments, Materials Evaluation (1970) 2829. 2 I. Katoh, Y. Sakurai, T. Matsumura, The measurement of fl aw height by ul