6th NDT in Progress Evaluation of metal weld-zone by using Poisson s ratio distribution

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1 6th NDT in Progress 2011 International Workshop of NDT Experts, Prague, Oct 2011 Evaluation of metal weld-zone by using Poisson s ratio distribution Jong Yeon Lee *1, Jeong-Hoon Moon 2, Hyunbyuk Kim 1, Young H. Kim 1 1 Applied Acoustics Lab, Korea Science Academy of KAIST; Busan, Korea Phone: , Fax: ; yhkim627@kaist.ac.kr 2 Dept. of Mechanical Eng., Suwon Science College, Korea Abstract Poisson s ratio has not received much attention because of its narrow range of measured values and difficulties in measurement. In previous work, genuine non-destructive evaluation method of characterizing weld-zone using Poisson s ratio distribution was proposed. The method was to use immersion ultrasonic testing for measuring longitudinal and transverse wave speeds simultaneously. In present work, scanning results by focused transducer and normal transducer are compared. Focused transducer shows improved performance. We also compare the results by Poisson s ratio scanning and direct Vickers hardness test. Keywords: Poisson s ratio, weld zone evaluation, micro-hardness 1. Introduction Heat affected zone (HAZ) is one of fracture sensitive areas which is weaker than both the base metal and weld-zone, and it should be inspected for structural integrity. Typical material characterization of weldment involves microscopic analysis of material texture and microhardness test. One of these inspection methods is ultrasonic technique which has been widely employed for the non-destructive testing of materials to characterize material properties and to detect its flaws. In this technique, wave speeds and attenuation are measured to determine material properties such as hardness and microstructures [1]. Poisson s ratio is one of material parameters. However its variation is so small that makes its application less attractive. Maess showed that Poisson s ratio is closely related with the bonding forces and sound wave speeds in a certain material [2]. Especially, it was found that shear wave velocity is highly related to Poisson's ratio. Several applications of Poisson's ratio have been reported also. High-strength steel that had gone through cyclic-softening process was found to have Poisson's ratio close to 0.5 [3, 4]. Pore fraction in silicon nitride disks was mapped by using ultrasonic contact pulse-echo method [5]. In that work Poisson s ratio maps for the disks were constructed from longitudinal and shear-wave velocity images. Non-destructive measurement of Poisson's ratio has been a challenging job, since bulk and shear moduli, or longitudinal and transverse wave velocities of material should be measured at the same time. Possibility of Poisson s ratio mapping ('µ-scanning') using longitudinal wave mode transducer immersed in water has been suggested through a series of works. It was shown that transverse wave can be generated by mode conversion of normal incident longitudinal wave [6]. And simultaneous measurement of longitudinal and transverse wave velocities by using immersion ultrasonic testing was reported [7]. Poisson s ratio mapping without information on specimen thickness was also demonstrated [8]. The feasibility of µ- scanning over weldment has been studied [9]. In present work, we concentrate on evaluation of weldment by µ-scanning as well as microhardness measurements. Ultrasonic pulse-echo signals of immersion testing are acquired and * Current address: California Institute of Technology

2 mode converted signals are identified. Poisson's ratios are evaluated from time of flight of mode converted echoes and µ-scanning is constructed from data over the specimen. 2. Theoretical Background Poisson's ratio is defined as the ratio of lateral contraction to longitudinal extension. It can be expressed with Lamé constants as follows [10]: µ ε ε λ 2 λ G where µ, ε and ε are Poisson's ratio, axial and transverse strains respectively. λ and G are Lamé constants. Longitudinal and transverse wave speeds in the medium of density ρ are c λ 2G /ρ and c G/ρ. Then we can rewrite formula for Poisson s ratio: µ 1 2 c 2c 1 2α c c 2 2α where α c /c is the ratio of transverse and longitudinal wave velocity. When the paths of both longitudinal and transverse waves are identical, then α τ /τ where τ and τ are the transit times of longitudinal and transverse waves respectively. 3. Experiments 3.1 Specimen preparation Double V-groove welded steel plates (base metal: SM490) were fabricated by CO 2 welding method. As a welding material, flux cored welding wire 1.6 (AWS A5.20 E71T) was used. The specimen is of 20 mm thickness and of 75 mm width. Specimen was cut out from the weldment with water cooling to avoid thermal effects during cutting the specimen. By etching process, different zones in weldment were identified. Micro-structure photos were taken. The grain size and grain pattern were observed and fracture like structure was identified in HAZ. Figure 1. Weld specimen used in the present work.

3 (a) (b) (c) Figure 2. Microstructures of (a) base metal, (b) HAZ and (c) weld zone. 3.2 Ultrasonic testing system The echo signals were captured using normal beam transducer of which diameter and a center frequency were 9.5 mm and 10 MHz and focused transducer of which diameter and a center frequency were 12.7 mm and 10 MHz. Focal length of focused transducer was 50.8mm. Ultrasonic pulser/receiver (Panametrics 5800) and A/D converter module (NI PXI-5124) were used for generation, reception and acquisition of signals. Sampling rate was 200MHz/s, so that the time resolution was 5 ns. Cross-section of specimen was scanned in water tank. Scan area was 40 mm x 30 mm with 0.2 mm step in both width and length. 3.3 Micro-hardness measurement Figure 3. Ultrasonic testing system. For measuring the hardness of the sample, sample was etching processed to classify HAZ and other characteristic areas. After that, Vickers hardness testing was performed 5 times in each area including base material, HAZ, upper welding zone and lower welding zone. Load used in the testing was 10 kgf and duration was 10 s. Deformation was observed and Vickers hardness of materials, which is directly related to yield strength of material, was obtained.

4 4. Results and discussion 4.1 Comparison between normal transducer and focused transducer Fig. 4 shows the measured pulse-echo signals by normal and focused transducer. As similar to previous work, for normal transducer, it is hard to identify 1P1S echo in the signal while 3P1S echo is relatively easy to identify [9]. The mode converted signals using focused transducer are much more clearly detected than signals using normal transducer. For focused transducer, even 2S signals can be distinguished. Because of these contrasting performances, different sets of signals were selected to calculate the Poisson's ratios from normal transducer and focused transducer. 2P, 4P and 3P1S signals for normal transducer were used for calculating Poisson s ratio while first reflected signal, 2P and 1P1S signals were used for focused transducer. Fig. 5 shows results of µ-scanning using normal and focused transducers. It is obvious that µ-scanning using focused transducers offers more precise image by virtue of strong echo of mode conversion. (a) Figure 4. RF waveforms of ultrasonic pulse-echo signals obtained by (a) normal and (b) focused transducers. (b) (a) (b)

5 Figure 5. Results of µ-scanning using (a) normal and (b) focused transducer. 4.2 Poisson's ratio mapping Fig. 6 shows comparison of 2D Poisson's ratio mapping data and the photo of etchingprocessed specimen. Base metal, HAZ and weld-zones can be clearly distinguished in both figures. For two selected lines in Fig. 6, the Poisson's ratio distributions along those lines are shown in Figure 7 and 8. Marked areas in Fig. 7 and Fig. 8 show decrease of Poisson's ratio at HAZ. Figure 6. Poisson's ratio mapping and the photo of specimen

6 Figure 7. Poisson's ratio distribution along the line 1 in Fig. 6. Figure 8. Poisson's ratio distribution along the line 2 in Fig Comparison of Poisson's ratio and micro-hardness Table 1 shows the measured Vickers Hardness and average Poisson s ratio of each area. HAZ has smaller Vickers Hardness than base metal, which means HAZ has lower yield strength. In previous work, lower Poisson s ratio was analyzed as the proof of hardened structure [11]. That analysis is contradicting to Vickers hardness data in Table 1, which shows weakened structure of HAZ. Generally, low Poisson's ratio is thought to be hard characteristic of material, but in this case, HAZ have both low Poisson's ratio and low Vickers hardness. Because Vickers hardness is a non-linear physical property and Poisson's ratio is a linear physical property, it is hard to generalize definite relationship between these two variables examined in here. Table 1. Comparison of micro-hardness and Poisson s ratio. Sample 1 Base metal HAZ Upper weld zone Lower weld zone Vickers Hardness

7 Poisson s ratio Sample 2 Base metal HAZ Upper weld zone Lower weld zone Vickers Hardness Poisson s ratio Conclusions In present work, HAZ of welded steel specimen is characterized by Poisson s ratio distribution. It is shown that focused transducer has the advantage of strong mode-conversion signals, and µ-scanning using focused transducers offers more precise image than normal transducer. Micro-hardness is measured by Vickers hardness test and those values are compared to the Poisson s ratios. It is confirmed that HAZ has lower Poisson s ratio and lower Vickers hardness for the test specimen. References 1. J. Maess, Attenuation models for materials characterization, MS thesis, Georgia Institute of Technology, A. Kumar, T. Jayakumar, B. Raj and K. K. Ray, Correlation between ultrasonic shear wave velocity and Poisson s ratio for isotropic solid materials, Acta Materialia 51, pp , S. Matsuoka, M. Yuyama and S. Nishijima, Young's modulus and Poisson's ratio on low-cycle fatigue (in Japanese), Trans. Japan Soc. Mech. Eng. A. 488, J. T. Fong, R. J. Fields, R. P. Wei and R. P. Gangloff, Basic questions in fatigue, American Soc. for Testing and Materials, pp.84-87, D. J. Roth, J. D. Kiser, S. M. Swickard, S. A. Szatmary and D. P. Kerwin, Quantitative mapping of pore fraction variations in silicon nitride using an ultrasonic contact scan technique, NASA technical paper 3377, Y. H. Kim, S. -J. Song, J. K. Lee and H. C. Kim, Transverse-wave modes in the pulseecho signal of a normal-beam longitudinal-wave mode transducer, Journal of the Korean Physical Society, Vol. 42, No. 1. January Y. Shin, Y. H. Yoon and Y. H. Kim, Measurement of longitudinal and transverse wave speed in solid materials using immersion ultrasonic testing (in Korean), Journal of the Korean Society for Nondestructive Testing Vol. 28, No. 1. February 2008.

8 8. S. Y. Oh, Y. H. Kim, Y. Shin and H. J. Cho, Poisson s ratio scanning using immersion ultrasonic testing, Journal of the Korean Society for Nondestructive Testing Vol. 28, No. 6. December J. H. Park, S. Y. Oh, Y. H. Kim and S. S. Lee, Poisson s ratio scanning using immersion ultrasonic testing, Proceedings of 5th NDT in Progress, Prague, Czech Republic, Oct , 2009, H. F. Pollard, Sound waves in solids, Pion Ltd, London, pp J. H. Park, Y. H. Kim, S. S. Lee and Y. G. Kim, Material characterization of weld-zone using Poisson's ratio distribution, Journal of the Korean Society for Nondestructive Testing 29, No.6. December 2009.