NAT'L INST. OF STAND &.T|CH NBS REFERENCE Publi- cations AlllDS TbfibM3 o \ NBS TECHNICAL NOTE 1075 U.S. DEPARTMENT OF COMMERCE National Bureau of Standards / 193^ NATIONAL BUREAU OF STANDARDS The National Bureau ofStandards' was established by an act ofCongress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides; (1) a basis for the Nation's physical measurement system, (2)scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau'stechnical work isper- formed by theNational Measurement Laboratory, theNational Engineering Laboratory, and the Institute for Computer Sciences and Technology. THE NATIONAL MEASUREMENT LABORATORY provides the national system of physical and chemical and materials measurement; coordinates the system with measurement systems of other nations and furnishes essential services leading to accurate and uniform physical and chemical measurement throughout the Nation's scientific community, industry, and commerce; conducts materials research leading to improved methods of measurement, standards, and data on the propertiesofmaterials needed byindustry,commerce,educational institutions, and Government; provides advisory and research services to other Government agencies; develops, produces, and distributes Standard Reference Materials; and provides calibration services. The Laboratory consists of the following centers: — — — Absolute Physical Quantities^ Radiation Research Chemical Physics — Analytical Chemistry Materials Science THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities; provides engineering measurement traceability services; develops test methods and proposes engineering standards and code changes; develops and proposes new engineering practices; and develops and improves mechanismsto transfer results ofitsresearch to the ultimate user. The Laboratory consists of the following centers: — — Applied Mathematics Electronics and Electrical Engineering^ Manufacturing — — — Engineering Building Technology Fire Research Chemical Engineering- THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides scientific and technical services to aid Federal agencies in theselection, acquisition, application, and use of computer technology to improve effectiveness and economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory servicesand assistanceto Federal agencies; and provides the technical foundation for computer-related policies ofthe Federal Government. The Institute consists of the following centers: — Programming Science and Technology Computer Systems Engineering. 'Headquartersand LaboratoriesatGaithersburg, MD,unlessotherwisenoted; mailing address Washington, DC 20234. 'Some divisions within the center are located at Boulder, CO 80303. 1 OF STAi^Di'J'.D ^ LIBRARY QC oo i Electromagnetic-Acoustic-Transducer/ ^^^^ System Synthetic-Aperture for ^^^^ Thick-Weld Inspection C. M. Fortunko"^ Schramm R. E. J. C. Moulder J. D. McColskey Fracture and Deformation Division Center for Materials Science National Bureau of Standards U.S. Department of Commerce Boulder, Colorado 80303 ^Present address: Aerojet Ordnance, Tustin, California 92680 Funded by: Ames Laboratory Energy and Mineral Resources Research Institute Iowa State University Ames, Iowa 5001 ¥ / % e U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director Issued May 1984 National Bureau of Standards Technical Note 1075 Natl. Bur. Stand. (U.S.), Tech Note 1075, 130 pages (May 1984) CODEN: NBTNAE U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1984 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402 CONTENTS Chapter Page 1 INTRODUCTION 1 References 5 2 INSPECTION CONCEPT 7 Ultrasonic Principles 7 Signal Processing 10 Summary 17 References 19 3 SYSTEM CONCEPT - OVERVIEW 21 Reference 27 4 DESIGN OF EFFECTIVE SH-WAVE EMATS 29 Operational Principles 29 Practical Configurations 31 Final Design 35 References 43 5 TRANSDUCER ELECTRONICS 45 Design of Low-Noise Receiver Amplifiers for EMATs 45 Design of Efficient Power Amplifiers for EMAT Applications 49 References 55 6 DIGITAL PROCESSING 59 Signal Averaging Method 59 Synthetic Aperture Method 59 Scanner Control and Ultrasonic Data Collection and Processing 62 References 68 7 MECHANICAL APPARATUS 69 Gimbal -Mounted Transducer Housing 69 Two-Axis Positioner 72 8 MEASUREMENTS 75 Preparation of Calibration Specimens 75 Experimental Procedure 77 Evaluation of Surface Slot Scattering Measurements 79 References 89 9 SUMMARY AND RECOMMENDATIONS 91 References 96 m Appendices Page I SOFTWARE LISTING 97 II SOFTWARE SAMPLE RUN 109 III TECHNICAL PAPERS RESULTING FROM THIS PROGRAM 115 IV ACKNOWLEDGMENTS 117 IV LIST OF FIGURES Figure Page 2-1 SH-wave detection of planar flaws in a weld. 8 2-2 Ultrasonic scattering by two-dimensional defects. (a) Short wavelength, (b) Long wavelength. 10 2-3 Ultrasonic signal paths, (a) Semi-infinite half-space. (b) Multiple paths in finite thickness plate. 12 2-4 SH-wave reflection signals, (a) EMAT-flaw distance about 55 mm. Signal interference, (b) EMAT-flaw distance about 65 mm. Signal separation. 13 2-5 Multipath propagation of ultrasonic pulses in a plate and the equivalent path length, (a) Even number of surface reflections, (b) Odd number of surface reflections. 16 2-6 Synthetic aperture processing of a reflection echo from a saw cut in the surface of a 25-mm thick plate. Signals #1 to #10 are raw data taken at incremental increases of 2.1 mm between EMATs and cut. The processed waveform is at the bottom. 18 3-1 Block-diagram of SH-wave-EMAT weld inspection system. 22 3-2 Photograph of laboratory SH-wave-EMAT weld inspection system. 23 3-3 Block-diagram of the analog electronics section. 24 3-4 Concept for detecting and characterizing weld flaws. (a) Longitudinal, (b) Transverse. 26 4-1 Primitive EMAT element. The eddy current J produced by I in a wire at the surface interacts with the external mcignetic field Ho to produce traction force T. 30 4-2 The radiation pattern of the simple EMAT in Fig. 4-1 produces a stronger reflection from the plate back surface than does a flaw. 31 4-3 Schematic of a periodic-permanent-magnet (PPM) EMAT con- taining M pairs of magnets. 32 4-4 Aluminum test block and transducer configuration to measure the angular distribution of ultrasonic energy. 34 4-5 Simple EMAT (M=l) with maximum sensitivity to SH waves propagating normal to the surface. 34 Figure Page 4-6 Angular radiation pattern of EMAT in Fig. 4-5 (M=l). Maximum amplitude is normal to the surface. 36 4-7 Angular radiation pattern of EMAT in Fig. 4-3 (M=16). Maximum amplitude grazes the surface. 36 4-8 Improved EMAT design. The two coils are counter-wound to minimize pickup of EMI. 37 4-9 Radiation patterns of EMAT in Fig. 4-8. Three cycle tone burst, (a) 400 kHz. (b) 441 kHz. (c) 500 kHz. (d) 700 kHz. 39 4-10 Radiation patterns of EMAT in Fig. 4-8. 441 kHz. (a) Four cycle tone burst, (b) Eight cycle tone burst. 42 4-11 Transverse radiation distribution of M=l EMAT (Fig. 4-5). 42 5-1 Idealized schematic of a receiver-amplifier system. 46 5-2 Schematic of the transmission-line transformers and J-FET preamplifier used in the current work with a transmission line < one meter long. 48 5-3 Preamplifier schematic. Useful for longer transmission lines. 48 5-4 (a) Basic capacitive discharge circuit used in most commercial pulser designs for driving piezoelectric transducer elements, (b) Voltage waveform seen by transducer T. 51 5-5 (a) Basic schematic of a power amplifier for a trans- mitter EMAT. (b) Voltage and current waveforms produced by this amplifier. 53 5-6 Schematic of amplifier for 300-600 kHz used in this work. 56 5-7 Construction of output power transformers used in the amplifier of Fig. 5-6. 56 5-8 Oscilloscope traces of the output of the amplifier in Fig. 5-6. (a) Voltage, (b) Current. 57 6-1 By repeated mathematical processing, the same data set can be brought to focus at multiple points. 60 6-2 Phase shift of Fourier coefficients in the real- imaginary plane. 61 VI . Figure Page 6-3 Flow chart of computer program used to collect, store, process, and display synthetic aperture data. 63 6-4 Geometrical parameters required for synthetic aperture processing. 63 6-5 (a) Three sets (out of ten) of raw signal data. (b) Signal after synthetic aperture processing. 66 6-6 Typical Fourier coefficients for some raw data sets and after processing. All the information is centered about the operational frequency of 450 kHz. (a) Real, (b) Imaginary. 67 6-7 Power distribution of processed signal. 68 7-1 Gimbal -mounted transducer housing for both transmitter and receiver EMATs. The two axes of rotational freedom allow the probe surface to comform to the plane of the plate under inspection. 70 7-2 Transducer housing attached to pneumatic piston and suspended from the two-axis positioner operated under computer control 71 8-1 Calibration plate with EDM notches. 76 8-2 Detailed scan pattern used for data collection. 78 8-3 Scan pattern used when the slots were on the same side of the plate as the EMATs. 78 8-4 Peak-to-peak signal voltage from scan of slots on same side of plate as EMATs. (a) Flaws A-E. (b) Flaws F-J. 81 8-5 Power signal from scan of slots on same side of plate as EMATs. (a) Flaws A-E. (b) Flaws F-J. 82 8-6 Flaw signal as a function of flaw depth for a constant length. The results remain the same for flaws up or down, (a) Peak-to-peak voltage, (b) Power. 85 8-7 Flaw signal as a function of flaw length for a constant depth. The results remain the same for flaws up or down, (a) Peak-to-peak voltage, (b) Power. 86 . 8-8 Depth and length measurement capabilities and limitations of the present EMAT system. The letters A through J - identify the points indicating the dimensions of the calibration notches listed in Table 8-1. 88 vn LIST OF TABLES Table Page 8-1 Dimensions of EDM notches in calibration plate. 76 VI 1 1