- w i : ..:D + 1 1 . . : . . ' N TOF I ORNL P 2725 ! 1 .... . . * į * 8 1.0 EEEEEEEE II III 01.25 1.4 LLC a MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU OF STANDARDS - 1963 - ORNE 2 27.25 1965 (Papur to be presented at the Fifth International Conference on Non- destructive Testing to be held in Montreal, Canada, May 21-25, 1967.) ORNI - AEC - OFFICIAL 6- DEC 6 Corfa 670505 - CESTI PRICES NL-AEC - OFFICIAL ASTI PRICSS . . RELEASED FOR ANNOUNCEMENT 44 893.09 MN_65 MASTER IN NUCLEAR SCIENCE ABSTRACTS :02 J. OPTICAL METHODS FOR STUDYING ULTRASONIC ;. PROPAGATION IN TRANSPARENT MEDIA* H. L. Whaley, K. V. Cook, R. W. McClung, and L. S. Snyders* Metals and Ceramics Division Oak Ridge National Laboratory : Dak Ridge, Tennessee - - * . : - - 1 - LEGAL NOTICE This report was prepared as an account of Government sponsored work. Neither the United States, aor the Commission, nor any forson acting on behalf of the Commission: A. Makes any warranty or representation, expressed or implied, with respect to the accu- racy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not Infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report. As used in the above, "person acting on behalf of the Commission" includes any em- ployee or contractor of ihe Commission, or employee oi sucb contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, disseminales, or provides access to any information pursuant to his employment or contract with the Commission, or his employment witis such contractor. 1 + . " *. minit. L...i t t *Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. **Visitor from the South African Atomic Energy Board. ORNL - AEC - OFFICIAL ORNL - AEC - OFFICIAL INTRODUCTION Ultrasonic testing is a major nondestructive testlug technique; however, much of the basic theory concerning the behavior of ultrasound is. based on light-wave or optic theories and experimental observatioks. In general, the correlation of light-wave and ultrasonic phenomena is ex. cel.lent, and !899,efforts to visualize ultrasonic propagation in trančarent media have 'not been pursued with a great amount of effort. . Since there are optical methods, such as schlieren and the photo- elastic effect, for observing ultrasonic propagation, we at the Oak Ridge National Laboratory felt that much could be learned by observing basic . ultrasonic phenomena such as reflection, refraction, and the effects of surface waves and Lamb-wave:8. Observation of ultrasonic transducer beam symmetry, collination and focusing, and modes of propagation for actual de applications, such as for tubing inspection, could be of much practical i benefit and should lead to much improved techniques. The possibility: of using a visual technique in flaw detection is also intriguing. :: . . ORNL - AEC - OFFICIAL 20 -* .mo pa impopo ini... ... 3 ORNL - AFC - OFFICIAL ORNL - AEC - OFFICIAL For these reasons we have developed systems for making visual observe- tians of ultrasound by the schlieren and photoelastic methods. These methode have been used by many others in the past for a number of applications including aerodynamics? and ultrasonics. 2,3,4 Very few of the publications, . however, discuss the experimental setups in detail, and this imposes a hard- ship on those who wish to utilize the techniques. Cur studies of the neces- . sary equipment covered different conhinations of light sources, collimeting lenses, ultrasonic generator, etc. From this effort we have developed .. w . advanced systems which are optimized for the desired results. It ie hoped -.: .. that this pape' will aid those desiring to set up similar systems, and the -.-.- . . . equipment and developmental procedures we have employed az'e discussed in detail . . to that end. NE TIL SA - .... D. W. Holder and R. J. North, Schlieren Methods, Notes on Applied Science No. 31, National Physical Lab., Her Majesty's Stationery Office, London 1963, p. 56. . SE. E. Aldridge, A Numerical Study of the Mode of working of the ultrasonic Micrometer Used in Thickness Measw'ements of Stainless Steel Tubing, UKABA, Harwell, England, April 1966, AERE M-1742. 3C. E. Fitch, Jr. and B. W. Chettle, Critical Angle Ultrasonic Tests, Hanford Atomic Products Operation, Richland, Wash., AW-79928 (Dec. si, 1963). *K. B. Hobbs, et al., Display of Ultrasonic Waves by_Schlieren Photograyby Battelle Memorial Institute, Columbus Laboratories, paper presented at Spring National Merting SNT, March, 1964. ; ORNL - AEC - OFFICIAL ORNL - AEC - OFFICIAL Twili tu . 11 EQUIPMENT · ORNL - AEC - OFFICIAL SIL" The 2-meter (78.7 in.) optical bench used for both the schlieren and photoelastic methods or viewing ultrasound is shown in Fig. 1 holding all the schlicren equipment. The following components are mounted on the bench; from left to right: 1) An argon jet s park light source which can supply a pulse of light with ! duration of 0.2 hsec from an electric discharge of 3 joules and also contains an incandescent pilot light for convenience in alignment. 2) An achromatic lens for collimating the light from the source. . . 3) A water-filled tank into which ultrasound 18' introduced. 4) A second lens for refocusing the parallel light rays. . ; 5) A vertically adjustable slit which blocks the refocused parallel light rays and passes diverted rays. 6) A Calumet view camera which has a ground glass screen for viewing the images produced by ultrasonic propagation in the water-filled tünk, and a 4 x 5-in. (10.16 cm * 12.70 cm) Polaroid attachment for photographing the images. ORNL - AEC - OFFICIAL ORNL-AEC - OFFICIAL 1121310-331-INIO Photograpk-No.-823664 м , .. . . . Fig. 1. Optical Bench with Schlieren Equipment. ORNL - AEC - OFFICIAL 。 . 地华​, , 了省 ​事事​, . 青 ​- - " - 事 ​- : 1 重量 ​: , : , .PHP 在對 ​男子 ​: LA, : . | 14 一事 ​已 ​" , …. * , . } 工学 ​。 其中​, 1 「 在产​。 _ if _ . " th : } 在 ​14: , ; - t; 比 ​. "T产 ​TI- 青 ​- Y” 事​” ·收N- v - - 。 在 ​- 。 ar. - - w - 举重​, . TL . " ", . 在中 ​: 1. 人 ​- - .. , 量 ​. . : , . . . : : 十 ​.. 中 ​拿 ​重時事 ​“ 1 “ 1. . 「. . T “ 1: . .PT “非L 1 . 上 ​- H . 应 ​, - 中美 ​'- - 非 ​1. …. 41 - , 是 ​」 量 ​7- 1 . + | 中国 ​. 化 ​2 Th 目 ​」 # 中 ​, , 1.1自我 ​,Ill ": TF, - ""; 事事集 ​# 出 ​中一中学 ​ht: * 1 一 ​: .4 T等​, ul 11 Pr=, Plus1. * 本 ​. * , . 业 ​中 ​, : . 中年 ​- 是​。 1 - (六学​, * , 事 ​, 在 ​, “ ”, ” , 中共​, * 名人 ​* * 中 ​时 ​. 1 - - “性 ​, * . .. 17 + 1 宇 ​佳 ​f 是 ​, r . 里 ​| 。 1| 2 - - 。 . 4 “是 ​.... ( , 「 . , , th 者 ​-4 。 - . ::: . , -- 「了了 ​: 41 4 1 - , - --L , , , 14 「 , 是 ​.. 1. … f . . 也 ​1. 「 十 ​是​。 1%,中 ​国 ​。 * 。 * = 事​,作出 ​f. .. - 1 " 1 4 . ", : { 才畫 ​- , , , - , 产 ​" P 身 ​11 : - .… 「... “非重 ​" |- | | 1 基 ​” | 。 量 ​了​! 」 , 11 | . 11, . 事​, 4 ' , [ 事 ​事 ​- 中一 ​于是 ​. *. . 1 : .. . 1 , 相 ​| 青 ​了 ​; A . 到​, * - - 于是​, . - Tai . - if “ 三 ​. . 6 ORNI - AFC - OFFICIAL ORNL - AEC - OFFICIAL ... The Arenberg 2.f. generator, shown on the right in Fig. 1, supplies the ultrasonic transducer with an electrical driving signal having a frequency adjustable from 350 kc to 25 Mc ip elther a continuous Have or pulsed mode of operation. A delay line and pulse generator are also used to trigger the jet spark. light source in proper sequence with the generater pulse of wi.trasound. : All of tie above mentioned equipment, with the excepcion of the water- filled tank and the movable slit, 18 19ed in the photoelastic method as well as the schlieren method. In addition, polarizing plates are required for the photoelastic method. Additional information on equipment used is given in Appendix 1. SCHLIEPEN METHOD The "schlieren" optical system for viewing ultrasound derives its name LT from the fact that it was originally used in Germany for the detection of inhomogeneous regions in glass which occur often in the form of streaks or "schliere: 16 i $D. Holder and R. J. North, Schlieren Methods, 1st Edition, der Majesty's Stationery Office, London 1963, p. l. : -.. . . OPNI - Arnarria ORNL - AEC - OFFICIAL FASI 7. . General Principles of the Schlieren Technique RNL - AEC - OFFICIAL A number of theories have been advanced to explain how an ultrasonic : beam or pulse is made visible by the schlieren optical system. G. W. Willard has discussed the manner in which Raman and Nath treat the optical diffraction by ultrasound as a result of the phase modulation of the wave front by the sound beam. Debye and Sears' rejected the phase grating explanation of the effect and developed a theory "based on the assumption of a volune scattering, in which every volume of the liquid contributes to the total scattering in ac- cordance with Maxwell's equations." Neither of these theories appear to tell . the whole truth. Recently such scientists as W. G. Mayer, M. A. Breazeale and E. A. Hiedemann, 9 and many others have demonstrated through the direction of their work that interest in the theoretical aspects of als problem still exists. 8G. W. Willard, J. Acoust. Soc. Am., 21, 101 (1949). 'P. Debye and F. W. Sears, Proc. Natl. Acad. Sci. U.S. 18, 410 (1932). 8w. G. Mayer, Ultrasonic News (Spring, 1961). : : M. A. Breazeale and E. A. Hledemann, --Naturwissensch, 45, 157.(1958). :ORNL - AEC - OFFICIAL ORN - AEC - OFFICIAL For our purposes we shall assume that ultrasonic propagation diverts ORNL - AEC - OFFICIAL or diffracts parallel light rays thereby allowing its visualization by : ht..Tome.no, the schlieren optical system as illustrated in Fig. 2. Light from a bright source is focused on slit s. A lens In, at a distance from S, equal to its focal length, 1, collimates the light from the source. If there le no.. . . . . . . air optical disturbance of the light between Ly and IR, the second lens, 1o, will refocus it in the plane of se, a vertically adjustable slit. Sa is set to block out essentially all the undisturbed light with one of its edges and thus produce a dark field at the screen or photographic plate at the far 2 2 . 1 . . 15 right of Fig. 2. If there is some optical disturbance between I, and Is, some 11 light rays will be altered from their parallel alignment so that light will . miss Sz and appear at the plane of "ihe screen. For example, if the disturbance SCHODISK is è speck of dust, an image of the speck will appear in the image plane of 6 the second. įens as illustrated in Fig. 2. The screen was '80 placed that a disturbance midway between 1 and Le would be in sharp focus at the screen. Rasmussen by making the distances p and q in Fig. 2 conform to the well-known formula of geometrical optics, ORNI - AEC - OFFICIAL + ORNL - AEC - OFFICIAL . 0 i. “ GROUND GLASS SCREEN OR PHOTOGRAPHIC PLATE ii. . ' I : - - . - SOURCE FOCUSED HERE :- MOVABLE SLIT (JUST OFF CENTER OF OPTIC AXIS) - FOCAL POINT OF . UNOISTURBED PARALLEL PLAYS - PARALLEL RAYS---- - 11 L'.: OPTIC AXIS :::::: : . . . . 1 . CISTURBANCE IN WATER MEDIUM: (DUST SPECK, COMPRESSION, ETC.) Figure 2. Schlieren optical system. ORNL IWC 66-3810 : ... ORNE-AEC -Orricidi ..QINL:- AEC - OFFICIAL 1 .. ? . . i ....... .. . .. **. *. is pH. -- . .. 19 . ORNI - AEC - OFFICIAL Such an optical system will render a sound beam in water visible in the manner described below. A glass-walled tank is filled with water and placed midway between Is and le. An ultrasonic crystal is submerged in the water and activated :: by an r.f.'oscillator, sending ultrasonic energy through the water per-:;! pendicular to the direction of travel of the parallež light rays. The ultrasonic waves traversing the water are pressure waves traveling at the speed of sound in water. A single complete vibration of the transducer produces a region of increased density followed by a region of decreased density of the water as compared to the density of undisturbed water. These areas of density change disturb the rays of parallel light by refraction, and images of these regions appear on the screen... If the screen is placed so that the object and image size are the same, the Linage on the scöeen in stop motion consists of sets of narrow horizontal bright and dark lines. The width of a bright line-dark line pair is equal to the wavelength of the sound in the water. Development of the Schlieren Optical System 'The direction taken in the experimental program was to first establish ORNL - AEC - OFFICIAL ORNL - AEC - OFFICIAL a sensitive optical schlieren system without using ultrasound, and then ... 11 determine optimum components and manner of operation for both pulsed and C.W. (continuous Wave) modes of ultrasonic propagation. For convenience, before actual introduction of ultrasonics, the schlieren image for comparing these components was provided by introducing thermal gradients into the water in the tank. Most schlieren systems described in the literature incorporate a pinhole as the light source and a circular opaque obstacle to block out the undeviated light. However, we found that a pair of slits proved to be a far superior arrangement with better image contrast and ease of adjustment. The apparent reason for this is that the deviation of the light beams from their original path by stress waves is always extreme.ly slight, so only very small solid angles of deviated light are available and these can be readily admitted by a narrow slit. Any other scattering mechanism, such as particles in the water or on the glass walls of the water tank that scatter the parallel beams through a large solid angle, will not show up as brightly with the slits, resulting in better contrast in the image. The best results were obtained with a pair of slits made by electro-discharge-inachining. The slits were machined in TTTTTTO 0.007-in. (0.018 cm) thick metal sheets and were 0.01-in. (0.025 cm) wide Oestha . 12 . . x 0.5 in. (1.27 cm) long. With the installation of the best slits and the accurate alignment of the achromats, the optical system was sufficiently '. . sensitive that convection currents in air rising from a human hand could be ' seen when a high intensity mercury arc light source was used. After the sensitivity of the optical system had been maximized, an .. ultrasonic transducer, which was driven by a poyer oscillator, provided the :.... disturbance of the collimated light necessary to produce a schlieren Image. .... There are several ways to improve the schlieren image of the ultrasound. The .. : windows of the water tank should be as nearly strain-free as possible and kept free of fingerprints, dust, and air bubbles during use. Initially we used a fused quartz glass tank but later found that a Plexiglas tank (made from 1/4-in. (0.635 cm) sheet] was adequate. The precision achromatic lenses should be cleaned only when necessary and then with extreme care. . . Although the use of distilled water in the tank is recommended in some references, tap water was found to work equally well. Excessive dust in the water will result in scattering unwanted light into the schlieren field. The importance of such factors is greater for systems with poor sensitivity. 13 Our improved tolerance for scattering centers such as dust is due to the ORNL - AEC - OFFICIAL 11i use of slits as previously discussed. One extremely important consideration in the quality of the resultant": schlieren image is the perpendicularity of the direction of the collimated light and ultrasound beam. For an ideal diffraction grating, the thickness of the grating is assumed to be zero and to have no effect on the diffraction even if the incidence of the light on the grating is not normal. If the 11ght lo not Incident normally, the diffraction grating formula sliply changes from d sin = na (2) where d is the grating spacing, n is the order number, 1 is the wavelength of the light and is the diffraction angle d(sin @ + sin 1) = nl, · (3) where i 1s the angle by which the direction of the light incident on the -- grating differs from the normal. However, the ultrasound beam always hels - .-- .- considerable thickness, and if the ultrasound beam 18 inclined enough with .-.-.- - -.--. OİNL - AEC - OFFICIAL . . . 14 the parallel light rays that a light ray.crosses both compressions and ORNL - AEC - OFFICIAL rarefactions, its net deflection may be canceled, and the schlieren Image will not occur. In our experimental setup, the transducer was mounted in. the end of a search tube normally used for immersed ultrasonic testing. The tube was clamped in a manipulator (for angular adjustment), which was in turn mounted on a 6-in. (15.24 cm) lathe bed. This arrangement allowed for convenient alignment of the sound beam approximately normal to the direction of the light. The aligrment was completed by furthor adjustment of the manipulator while viewing the schlieren image on the ground glass screen. For many transducers, the beam image was visible when oft normal by as much as 10°. This was affected by both beam diameter and ultrasonic frequency. Refinements for Viewing C.W. Ultrasound Continuous ultrasonic waves (produced by applying a continuous r.f. sine wave to the ultrasonic transducer) were observed using the Arenberg oscillator in the cow. mode of operation. A number of light sources were tried in an effort to develop the most sensitive c.W. system. A 10-watt zirconium arc lamp, 10 a 150-watt projector lamp of a common variety, a 10 George W. Gates Co., Inc. ORNL - AEC - OFFICIAL 15 100-watt mercury arc lamp, 11 and the pilot light in the jet spark light ORNL - AEC - OFFICIAL source were compared. Use of the mercury arc source always resulted in a more sensitive system; however, because of the convenience of using the pilot lamp in the jet spark light source, it was generally used. The continuous wave system is sensitive enough that a dim image of the ultrasonic beam'. was observed using a common flashlight as the light source. The intensity of the schlieren images 18 obviously rela ted to the lotensity of the ultrasonic energy which in turn depends on the voltage applied to the transducer. For the best images the highest operating voltages are desirable, but extreme care must be taken since higher voltages will shorten the transducer life. For continuous wave operation, a possibility of transducer damage was encountered since heat 18 a by-product of the trans- ducer operation. In some cazes this could be observed in the schlieren image as a thermal gradient aujacent to the transducer. To prevent the overloading of a transducer, ei rof..current meter was used in the output circuit. If the transducer became warm during operation, the meter reading was noted, and . 2. General Electric Company ORNL - AEC - OFFICIAL o saa . . . cosas .com . 16 the current was kept less than that value in subsequent operations. With ORNL - AEC - OFFICIAL the Arenberg oscillator, the range of frequencies over which a good schlieren image of the ultrasound occurs for a given transducer is usually large. For example, a crystal with a resonant frequency of 5 Mc gave a good image at .. 2.25 Mc, and a 10 Mc transducer was photographed working at 3 Mc. However, for the best image, the crystal should be operated near its resonant frequency. 'i "Refinements for Viewing Pulsed Ultrasound Use of the schlieren system with pulse i ultrasound allows the tracing. of ultrasound through transparent media and the isolation of reflections, refractions, etc. from the transmitted pulse. This is accomplished by synchronizing a pulsed light source with a pulsed rof. oscillator using an adjustable delay of the signal triggering the light flash. Since a rather bright light source is needed for good schlieren work, finding an apropriate pulsed light source was not easy. We wanted a source with sufficient energy per flash and a high enough repetition rate that continuous adjustment of the delay time would make the pulse appear to travel in an unbroken path. The first light source used was a General Radio Strobotac 3 12 General Radio Model 1531-A Strobotac. : ORN - AEC - OFFICIAL which we happened to have on hand. Figure 3 is a block diagram of the OXNL - AEC - OFFICIAL puised system used with the Strobotac. A triggering signal taken from the pulsed oscillator was fed through an appropriate delay circuit to activate. the Strobotac whose light pulse has a 'duration of approx 1. Usec. · The stroboscopic effect caused the pulse to appear to stand still at a variable distance from the transducer, dependent upon the amount of delay. Although the intensity of the image was not as great as we desired, visualization of pulses was possible with this system. We were able to resolve individual cycles within a pulse as high as 2.25 Mc using this light source. The ability to operate at high repetition rates, allows the strobotac to be used with com- mercial ultrasonic equipment having repetition rates as high as 600 cycles/sec. An argon jet spark13 was considered and selected as one of the better available pulsed light sources. Figure 1 shows the argon jet spark source in place at the left end of the optical bench. The instrument has banks of large capacitors in which an electrical charge is stored before being discharged in a brilliant spark. A laninar jet of argon gas flows steadily between the two 13 Model 7, Argon Jet Light Source, Lunartron Electronics.. ORNL - AEC - OFFICIAL 17121310-3Y - INYO .. ....... . . .. *0.-33V-inio VIII330-33V - TN80 .. . . ..... . . . . HIGH POWER PULSER - (PULSED OSCILLATOR) ... . . . :.:.:. SEARCH TUBE AND TRANSDUCER - . . PUL SE DELAYING ... CIRCUIT 11 PULSED LIGHT SOURCE - SOUND PULSE COLLIMATING LENS SINGLE BURST OF LIGHT- Figure - . system, usta i Block diagram cl the polised system, USED wizS TROBOTA - . 1 1. CORAL DW 6 66-3814 . . Linksmi LAVRA Fig. 3. Block Diagram of the Pulsed System Used with the ORNL - AEC - OFFICIAL - . . · ORNI - AEC - OFFICIAL * 1 . 10 19 electrodes to channel the spark when it occurs so that it will follow exactly ORNL-AIC - Officiat. the same path each time. This permits accurate focusing of the spærk on a slit. A pilot lamp whose position is optically equivalent to that of the spark is used for convenient focusing of the spark on a slit. The pulse of light from OU the spark is extremely brilliant and of short duration, three joules of energy being dissipated in only 0.2 x 10*° seconds. The repetition rate is low (on. the order of'olte a second at maximum), but it can be used for visual as well. as photographic work. Due to this low repetition rate, triggering was ac- complished through the use or an auxiliary pulser designed and l'abricated at ORNL. The output pulse was fed to both the Arenberg oscillator and through the delay and amplifier circuitry to the light source. Figure 4 is a block diagram of the pulsed system with the jet spark light source. Individual cycles were resolved, within pulses, at frequencies up to 4 Mc with this system before the compressions became so close together that resolution was lost due to pulse movement. Results and Applications (Schlieren Method) With the C.W. system for viewing ultrasound, beam images have been obtained using a variety of quartz and ceramic transducers for a range of ORNI - AEC - OFFICIAL A . Binti: SLI . TIM 22 2 . 0 ORNL - AEGLASE FEC - OFFICIAL..i ORNL - AEC - OFFICIAL RNL-NEC - OFFICIAL: . . . . . . n : * ope Triggering Pulse 1 R. f. pulse O- HOOV Adqustable 225usec long Adausta 17 le systemet ving? h e maio.o. 1 Pulsed Oscillator - U ' 1 1 . . : go And 7rAviso ucee . 1 . . r ii! . . III . . zama What . 1 - - K. . LI mne nisi O 3. . 11H . . INI ER. ";.n . - .... . . 1 Delayms. + Amplifier Cicero Ligh . донесе . 1 1 s! . . 1 ime ..iiii l . C ' I I . . i . . i : . ,. igure 4, Block Diagram of the Pulsed System used with the Argon Jet Spark Light Sour . CSSICIAL 08NI-A? *Toplos; TAL OFFICIALS NL - AEC - OFFICIAL :ORNL - AEC - Oisicin www.1- . , . 21 frequencies from 2.25 Mc through 25 Mc. The C.W. method has been used to · ORNL - AEC - OFFICIAL make cursory examinations of a few of our transducers, and one was found . that had a blistered epoxy coating on its face which reduced its active area considerably. The c.w. nethod has also been applied to viewing beam cola.. :: limation, the location of focal points of focused 'transducers, and the ... reflection of beams from flat and curved reflectors. Figure 5 shows a con- tinuous 5 Mc been reflected from a piece of aluminum with a curved reflecting surface. Note how the reflected beam 18 focused. The pulsed method of viewing ultrasound with our system is more sensitive than the c.W. method, although both are useful. The Arenberg oscillator can deliver greater r.f. voltages to the transducer in pulses than in continuous r.f. current, due to its necessarily limited power output. The higher r.f. voltages applied to the transducer increase the amplitude of its vibrations and, thus, of the local density changes in the water. These more pronounced density changes increase the contrast of the schlieren image by making the ' density gradj.ent between compressions steeper. The fact that sharper Images of pulses can be obtained than for the corresponding cow. beam allows ultrasonic ORNL - AEC - OFFICIAL NINI - AFC-NEFICII * .::..... , *. Fig. 5. of Aluminum with a Curved Surface. Continuous 5 Mc Beam Reflected from A Piece . . ...ve .. . www ORNL - AEC - OFFICIAL OINAEC - OBSICIAL . tipi i .. a n smission and maintained victorian en die - anderen is timmiten menim own ** 6 Chak Ridge National Lintxoratory Metals and Cercnics Division Ook Ridge, Tennessee CYN. 942 Emulsion £92340 Dota C-3/-0) - - - | : 14 “ " . - ๆ | 11 - " - 11 4. - - น. เ ! * - | : - 2 . 11 1. 1 - เก1 น. - - { } s " - } 1 - - พ. 12 To เจ " - .45A- - : : " F1 เT ) ! * 1 - - * . - * A * * * ) : .. ป * 1 * 1 " - - ลนคร It' 1 - - 2 I * " ร % * \ น 4 1 s : เ : T น . : 14 41 51 * | : | .. “ :.* : -- + + ไป :".. เ 44.7"/ " tit1 " 41 + 44" น *- - - .. ร . 1. 1 " - " 1 " | - 2 . - - 4 il - . - - | * - . - ศ - เE " 4 - 1 4 * " พ - " " " - เ ) 11, P - 1 - เรา 14 * . ++ . . " ": เS4=; * * " ** t * 1. “ เC น “ ** : . A. 4 " " . 3 2 ลวง -- . 1 " 4.ห !" | + | " 5 ) 1 | เ " 1. " | - | ") , " * U " . - 10 " " “ " x - " " เจ" " - " " . . 1 - . " * 11 น. * r4ๆ : " | | + + " : ะ ดี - . ! N " จ 0 - * • " . : ค 2 . | 7 " โ - \ 1 " 1 \ , ! 1 , , " \ “เC: . IA - "1 คน 1 / บน ( น " 1. \ - I ) - - ! A A " - ." 1 1 1 ) - " : - . - น : , - ว + สมพร | | * เพศ . . - + 1 " ใน 1 คน . 2 " - 4 ” } 1 . "" h - *เ 2) " เมย ( เ: 14 . - * 14 I :: 17" " - " | แH 61 หา - เ “ .34 , 16: " ปี 1 - - “ 4 น จะ ! " 1 " * . - จาน - - า - - 1 1 " - - \ * ... I - " } -- เ A " - 11: : เพ: 1- : "" * - * 1 I -.แบบ : : คน * : : " " * * T. " - - * + “ ": + 3) 1.* " ** " | | * * - * , | ( | * , | *. ( . : * 114 I . เ 1 " - " | | . - - - : - - - - - - 23 pulses to be observed in some situations where the use of c.w. ultrasound DEMI - AES-OFFISIAI ORKL - AEC - OFFICIAL alone would not suffice. One case in which this proved possible 18 shown in Fig. 6. It was desired to photograph the ultrasound in a Plexiglas block, but only negative results were obtained with the c.w. system. The attempt j with the pulsed system was, however, successful. Outside the block, inter- ference patterns can be seen from the interaction of compressions that have Worthi' etc. been reflected, and others starting later in time. With the pulsed system, we were able to view images over a frequency range from 0.8 Mo to 25 Mc. The tele progress of the pulses could be followed in detall both by eye and by making 4- a series of photographs. Reflections and scattering at interfaces were ob- served by proper adjustment of the delay time. In one instance, the puised . . . ... system was used to get an estimate of ultrasonic velocity for a low velocity ! rubber material. This estimate was made by observing the time in microseconds .. . required to delay a pulse through a known thickness of the material. t' - - - .. For the work with ultrasound in solids, ordinary glass and transparent . * .. metry . - epoxy blocks were employed as well as the Plexiglas already mentioned. Results .... . . A W obtained with the epoxy were much poorer than for the Plexiglas at all frequencies. T. ORNL - AEC - OFFICIAL al ORNL-AEC - OFFICIAL .. " " , " : ) * . . . . . - 28/ Q709ๆ 0621) Fig. 6. 2.25 Me Pulse Incident on the Corner of a Plexiglas Block. ORNL - AEC - OFFICIAL 25 . QRNI - AIC - OFFICIAL The glass had small attenuation but such large acoustic impedance that most of the incident sound was reflected from its surface. Contact crystals gave about the same results as the immersed type. The Plexiglas was by far ti best material that we found for both the cow. and pulsed schlierea work with transparent solids. In the Plexiglas the ultrasonic Indication is very bright near the surface where it enters; however, due to its high attenuation, we have been able to observe penetration to only about 2.0 in. (5.08 cm). Ob- servations have also been made of ultrasonic behavior around drilled boles in Plexiglas. Further work is needed on observations of internal reflection, refraction, mode conversion, and reflection from drilled holes, notches, or other discontinuities. Selected materials will need low acoustic impedance . and attenuation. PHOTO ELASTIC METHOD We have also used another method to view ultrasonic propagation in transparent solids. This method is based on the photoelastic effect and uses monochromatic light and a polariscope to observe the stress patterns introduced. by the ultrasonic propagation.14 ORNI - ASC-ne 14 L. S. Snyders, M.S. Thesis, The University of Tennessee" (1966). . 26 General Principles of the Photoelastic Method The photoelastic methodl6 as adapted for viewing ultrasound employs a polariscope, which in its simplest form 18 nothing more than an optical polarizer, an analyzer, and a light source. Our polariscope was assembled . by altering the schlieren system already available (Fig. 1). The water tank and second slit were removed and a polarizer, a quarter wave plate, an un- stressed transparent epoxy solid, another quarter wave plate, and an analyzer in that order were set up on the optical bench between the collimating and refocusing lenses. For this approach, pulsed ultrasound was introduced by a transducer attached directly to the 'epoxy solid. The Jet spark light source wau triggered as before with the auxiliary pulser. If polarized light 18 passed through an unstressed transparent plate, through an analyzer whose plane of polarization is normal to that of the polarizer, and then projected on a ground glass screen, a darkened light field will be observed. Some solid materials have the optical property of being able to split an incident light ray into two components which are polarized CIAL... in planes at right angles to each other (a phenomenon called birefringence). 15R. B. Heywood, Designing_by Photoelasticity, Chapman and Halls, London (1952). OPNL - AEC - OFFICIAL .- 27 27 . . .*** apie.. Almost any solid material becomes birefringent when stresses are introduced. ORNI – AEC - OFFICIAL At each point of the stressed solid, the planes into which an Incident light ray is split are coincident with the planes of maximum and minimum stress. . The polariscope arrangement causes an interference pattern to occur which 18 characteristic of the stress-caused birefringence. These are bright areas on the darkened field at the ground glass screen. If the stresses introduced are traveling ultrasonic pulses, they can be viewed and photographed with the aid of the polariscope. Results and Applications (Photoelastic Method) We have used the photoelastic method to observe ultrasound in transparent solids and to measure quantitatively the intensity of the ultrasound in the material. By successive measurements as the sound propagated, attenuation could be determined. A 1/4-in. (0.635 cm) plate of transparent epoxy was molded with attention to produring a model free of permanent stresses. Ultra- sonic pulses in a frequency range of 0.5-1.0 Mc were introduced into the solid, and the photoelastic imge of the resulting stress was photographed. Measure- ments on the fringe orders allowed evaluation of the principal stress magnitudes as a function of ultrasonic penetration. Strain gauge measurements compared favorably ORNL - AFC-CEFICIAL ORNI -AEC - OFFICIAL 28 with results of the photoelastic method. The frequency range was restricted ORNL - AEC - OFFICIAL on the pulse intensity measurements since our Arenberg oscillator became unstable below 0.5 Mc, and the amount of movement of the pulse during the 20 VaR .. duration of the light flash became too large compared to the ultrasonic . wavelength above 1.0 Mc. The upper frequency limit for the photoelastic method should be comparable to that for the schlieren technique where resolution of individual cycles is not a necessity. This has not, however, been determined for our setup. Of course, the photoelastic method is suitable for continuous wave as well as pulsed observation, although, again, frequency limits for the equipment in our setup have not been determined. Preliminary indications are that the schlieren method affords greater sensitivity for visual observation, while the photoelastic method may be better adapted to quantitative determinations. Since birefringence does not occur for liquids, the photoelastic method cannot be used to view interface effects at liquid-solid interfaces for submerged solids, a use for which the schlieren technique works very well. 1.- A S - - Epoxy was the only solid to which the photoelastic technique was applied. - Although it was chosen for its high photoelastic response, there may be other ORNL - AEC - OFFICIAL .T ...14 .T. . T . : . . ..... . . - T- 29 ORNL - AEC - OFFICIAL materials that would allow better photoelastic observation of ultrasonic : ORNL - AEC - OFFICIA: phenomena in solids. SUMMARY, AND CONCLUSIONS • Visual means of observing ultrasonic phenomena shows promise of being'. ' a valuable tool in nondestructive testing. After studying many different combinations of equipment, we have developed a highly sensitive optical system with which we can observe both c.W.--and pulsed ultrasound in trans- 1: . parent solids and 11quids by a schlieren technique and in transparent solids by a photoelastic technique. The schlieren technique seems especially promising for visual study of ultrasonic phenomena at water-solid interfaces and inside some transparent solids while the photoelastic method is useful for quantitative study of stress-strain mechanics in transparent solids. There are good reasons for believing that the schlieren method can be applied to flaw detection although work on this problem is still in the early stages. 24 2 TIETO ORNL - AEC - OFFICIAL 2 . . : 30, APPENDIX 1 O RNL - AEC - OFFICIAL EQUIPMENT (1) Optical Bench 2 meter (78.7 in.) The Ealing Corporation 225 Massachusetts Ave. Cambridge, Mass. C Pulsed Oscillator Model PG-650-C with Modification No. 2 Arenberg Ultrasonic Laborat Jamaica Plain 30, Mass... -- -- .- Built at ORIL: (3) Foun, Volt Pulser (4): Pulse Amplifier (5) Delay Line Built at ORNL Model 202 E.S.C. Electronics Corporation Palisades Park, N, J. Pulsed Light Source Model 7 Argon Jet Light Source ... Lunartron Electronics Luton, England (7) View Camera Calumet Manufacturing Co. 6650 N. Clark St. Chicago, Ill. San (8) Achromatic Lenses Kodak Aero-Ektars, f/2.5, 7 in. focal length Free Style Sales Co. Los Angeles, Calif. (9) Zirconium Arc Lamp 10 watts George W. Gates Co., Inc. Franklin Square, L.I., N. Y. 100 watts General Electric Co..! (10) Mercury Arc Lamp (12) Strobotac Model 1531-A General Radio Co... Concord, Mass. 1 . . 2 . :. -, . 141 79 1! 9 . : E. " #311 + 1 " END . . DATE FILMED 3 / 1 / 167 ,,VT Y . nc . 24