be reflected, and the rest will be allowed to pass CRYSTAL ANGLE OF BEAM IN into the second medium, depending upon the im- WEDGE the path traveled by the vibrations and whether pedance ratio between the two materials. Also, [SURFACE WAVES they return to their source depends upon the angles at which the beam impinges upon the reflecting - LONGITUDINAL surfaces, as well as the number and location of WAVES these surfaces. (See Figure 15.)
33. REFRACTION AND MODE CONVERSION OF ULTRASONIC WAVES. Ultrasonic beams introduced at an angle into a specimen are refracted.
The velocities in the wedge material and the metal are different, therefore, the longitudinal vibrations will be refracted when passing into the metal. (See Figure 16.) At certain angles, conversion to other
ANGLE OF BEAM IN
WEDGE MATERIAL
• WEDGE MATERIAL
ANGLE OF BEAM
IN METAL FIGURE 18.-Generating Surface Waves
34. BEAM DIVERGENCE. Beam divergence varies with frequency and crystal diameter. The higher frequencies give more directivity to the sound beam.
Also, a large diameter crystal is more directive than one of a smaller diameter when operated at the same frequency. (See Figure 19 and 20.)
3 M.C. 2% M.C. 1 M.C. ½ M.C.
QUART QUARTZ
ANGLE OF BEAM
IN TEST MATERIAL 56"/ FT 1½"/FT 3'/FT FIGURE 16.-Refraction of the Ultrasonic Beam modes of vibration, such as shear (see Figure 17) FIGURE 19.-Beam Divergence of Sound Waves in Steel and surface waves (see Figure 18) occurs.
WHERE: «= angle normal to the beam in the wedge. QUARTZ ß= angle of refracted beam in -LONGITUDINAL specimen. ..13 € = 229,000 368 C1 = velocity of incident vibra- 29* SURFACE = 115,000 tions in the wedge (usually the longitudinal velocity). 90° c2 = velocity of vibration in the material under inspection TRANSVERSE OR for the desired wave mode. 1: 126,000 SILAR STEEL CRYSTAL \ ANGLE OF BEAM IN WEDGE FIGURE 20.-Diagram of Longitudinal Sound Striking a Surface
- LONGITUDINAL WAVES
35.-40. RESERVED.
2SHEAR WAVES
ANGLE OF BEAM
IN METAL FIGURE 17.-Generating Shear Waves 10
Chapter 5. ULTRASONIC SYSTEMS
41. GENERAL. There are two basic ultrasonic systems: pulsed and resonance.
42. PULSED. The pulsed system may be either echo or through transmission. The pulse-echo is the most versatile of the two pulse systems and suitable for service and overhaul use.
a. Echo. Flaws are detected by measuring the amplitude of signals reflected and the time required for these signals to travel between specific surfaces and discontinuity. (See Figure 21).
RF PULSER AMPLIFIER tated, the ultrasound will be reflected back to the transducer when it reaches the internal flaw and the opposite surface of the specimen. The time interval between the transmission of the initial impulse and the reception of the signals from within the specimen is measured by the timing circuits. The reflected pulse that is received by the transducer is amplified, then transmitted to the oscilloscope where the pulse received from the flaw is displayed on the CRT screen in the same relationship to the front and back pulse as the flaw is in relation to the front and back surface of the specimen. (See Figure
22.)
TRANSDUCER TRANSDUCER FLAW FLAW RATE
GENERATOR
SPECIMEN
- B
CIRCUIT TIMING FLAW CATHODE RAY
SPECIMEN TUBE FIGURE 22.-Oscilloscope Display in Relationship to Flaw Detection
CATHODE RAY OSCILLOSCOPE FIGURE 21.-Block Diagram of Basic Pulse-Echo System The time base, which is triggered simultaneously with each transmission pulse, causes a spot to sweep across the screen of the CRT. The spot sweeps from left to right across the face of the scope 50 to 5,000 times per second, or higher if required for high-speed automated scanning. Due to the speed of the cycle of transmitting and receiving, the picture on the oscilloscope appears to be stationary.
A few microseconds after the sweep is initiated, the rate generator electrically excites the pulser and the pulser in turn emits an electrical pulse.
The transducer converts this pulse into a short train of ultrasonic sound waves. If the interfaces of the transducer and the specimen are properly orien-
only amplitude information and operates on the principle that certain specific changes in the sample will produce significant changes in the intensity of an ultrasonic beam passing through it. This system requires two transducers placed on opposite sides of the specimen. (See Figure 23.) One transducer transmits the wave through the piece and the other picks up the signal. If there is a defect in the path of the wave, the received indication is reduced in size to the degree that the signal is blocked. This system is used extensively for bond testing of laminated and clad materials where the area of a laminar-type defect is of interest, and its probable depth is either known or of no importance.
This system is also useful in testing for metallurgical changes due to heat, pressure, stress, and tatigue.
/ TRANSMITTER C.R.T TRANSMITTER
RECEIVER
RECEIVER DEFECT FIGURE 23.-Through-Transmission Technique
43. RESONANCE. This system differs from the pulse method in that the frequency of transmission is, or can be, continuously varied. The resonance method is principally used for thickness measurements when the two sides of the material under test are smooth and parallel. The point at which the frequency matches the resonance point of the material under test is the thickness determining factor.
TRANSDUCER
INCIDENT WAVE REFLECTED WAVE It is necessary that the frequency of the ultrasonic waves, corresponding to a particular dial setting.
should be accurately known. Checks should be made with standard test blocks to guard against possible drift of frequency. If the frequency of an ultrasonic wave is such that its wave length is just twice the thickness of a specimen, then the reflected wave will arrive back at the transducer in the same phase as the original transmission so that strengthening of the signal, or a resonance, will occur. If the frequency is increased so that three times the wave length equals four times the thickness, then the reflected signal will return completely out of phase with the transmitted signal and cancellation will occur. Further increase of the frequency, so that the wave length is equal to the thickness again, gives a reflected signal in phase with the transmitted signal and resonance occurs once more. By starting at the fundamental frequency, where the wave length equals twice the thickness and gradually increasing the frequency, the successive cancellations and reinforcements can be noted and the readings used to check the original or fundamental, frequency reading. (See Figure
24.)
In some instruments, the oscillator circuit contains a motor-driven capacitor which changes the frequency of the oscillator. (See Figure 25.) In other instruments, the frequency is changed by REFLECTING electronic means.
SURFACE
WAVELENGTH OSCI 1 = 2 PULSE F = F, (FUNDAMENTAL FREQUENCY) AMPLIFIER CRT T = W F = 2F, (2ND. HARMONIC)
MATERIAL
UNDER TEST 14½W F = 3F, (3RD. HARMONIC) T = 2W F = 4F, (4TH HARMONIC) FICURE 24.- -Conditions of Ultrasonic Resonance in Metal Plate
MOTOR ' TUNING /CAPACITOR
TRANSDUCER
HORIZONTAL
TIME-BASE MATERIAL
GENERATOR CONTACTS FIGURE 25.--Block Diagram of Resonance Thickness Measuring System The change in frequency is synchronized with the horizontal sweep of a CRT. The horizontal axis thus represents a frequency range. If the frequency range contains resonances, the circuitry is arranged to present these vertically. Calibrated transparent scales are then placed in front of the tube and the thickness can be read directly. The instruments normally operate between 0.25 MHz and 10 MHz in four or five bands.
The resonant thickness instrument can be used to corrosion on fuel tanks fuselage and wing skins can test the thickness of such materials as steel, cast be located and evaluated.
iron, brass, nickel, copper, silver, lead, plastics, aluminum and magnesium. In addition, areas of 44.-50. RESERVED.
Chapter 6. PRESENTATION
51. OBSERVING AND RECORDING RESPONSE PATTERNS. There are several methods of observing and recording ultrasonic response patterns such as a CRT, indicating lights, alarm lights, alarm devices (bells, buzzers, etc.), paint-spray markers, strip-chart and facsimile recorders, photographic representations, go/no-go monitors, and others.
These methods may be used in combination to suit a particular need.
52. CATHODE-RAY TUBE (CRT).
a. Size: The screen sizes may vary from 3" to , however, there is usually no need for providing a signal larger than that which can be presented by a 5" tube. The large screens do not provide any more picture information. Usually a small tube will have better contrast and definition. The primary purpose in using a large screen is in automated systems where the scanning transducer must be positioned some distance from the viewing screen of the test instrument.
b. Signal Trace. Figure 26 shows the two most common trace presentations. The Radio-Frequency (RF) presentation exhibits signals both above and below the sweep line. This type of presentation provides maximum resolution for locating defects close to the surface or for separating signals following closely upon one another. The video trace presentation is a cleaner and less cluttered signal than the RF presentation, however, it provides for less signal resolution because of its broader pulse characteristics. The video presentation is, in fact, an RF presentation with the bottom half electronically off and only the outline of the original pulses, added on to the top half, is shown. Therefore, larger signals are indicated, but flaw definition is more limited. Of the two traces, the video is most commonly used because it is easier to read.
c. Range Markers. To provide a means of measuring the depth of a flaw indication, square waves are electronically superimposed on, or beneath, the regular sweep line of the test presentation. The lengths of these waves can be adjusted to represent inches or feet of the material being tested. Figure 27 shows three of the most commonly used types of wave markers.
VIDEO FIGURE 26.-Radio-Frequency Trace and Video Trace The square-wave markers shown at the top are easy to distinguish from the spike-like echo signals.
They generally provide the most precise means of measuring flaw depth. The pyramidal markers (center) are especially useful in an angle-beam test where the sonic energy is sent into a piece at an angle and bounces back and forth between the walls as it travels on through the part under test.
These markers simulate the actual path of the search beam passing through the piece, making it easier to determine the flaw depth with respect to the top and bottom surfaces of he specimen. The bottom marker illustrated differs from the other two types in that it does not have equal positive and negative values. These markers are a compressed negative wave and deflect in the opposite direction to the echo signals.
& SEARCH! UNIT -SPECIMEN CATHODE RAY TUBE FIGURE 27.-Types of Marker Systems
53. SCAN PRESENTATIONS. The ultrasonic echoes are electronically translated into visual presentations on the CRT. There are three different presentations available; the A-scan; B-scan; and C- scan.
a. A-scan. In the A-scan presentation, the horizontal base line on the screen indicates elapsed time (from left to right) and represents the depth of the test specimen. The vertical deflection shows response amplitude. The signal amplitude represents the intensities of transmitted or reflected beams. This may be related to flaw size, sample attenuation, or other factors. (See Figure 28.)
FIGURE 29.-B-Scan Presentation Since the C-scan presentation is a true-to-scale reproduction of the flaw, the CRT is used only for calibration, tuning, and visual monitoring of the test. The use of facsimile-type paper recorders are currently supplementing the CRT.
§ SEARCH UNIT FLAW
- FLAW
SEARCH UNIT T F
• (FLAW)
CATHODE RAY
SPECIMEN TUBE B FIGURE 28.-A-Scan Presentation
b. B-scan. The B-scan takes the same signal received by the A-scan and presents them in a different pattern. The location and depth of the flaw can be determined as in the A-scan, but, in the case of small defects, only a rough estimation of the flaw size can be obtained. (See Figure 29.)
The B-scan presentation has been used in the testing of plate stock for laminations.
c. C-scan. The C-scan presentation is a plan view of the part under test; a flaw is indicated in a manner similar to that given when a radiograph is made of a part. The electronic beam follows the movement of the probe and traces a true-to-scale reproduction on the CRT. However, accurate flaw depth cannot be determined from the C-scan presentation. (See Figure 30.)
SPECIMEN CATHODE RAY TUBE FIGURE 30.-C-Scan Presentation The recorders provide excellent resolution, a permanent record, and are available with paper widths up to 36" by several hundred feet in length. By varying intensities ranging from black to light grey, the depth of a defect can be approximated on the facsimile recorder.
54. INDICATIONS. It is possible to illustrate in this circular every variation of displays presented by the CRT. Fortunately, the great majority of patterns will fall into one or another of ten basic types which are illustrated and explained in the following paragraphs. Before the ultrasonic system can give results, as outlined in the following figures, it must, of course, have been properly set up and adjusted using standard reference blocks (see Figures 31 through 40).
a. Figure 31 represents a smooth-surfaced rectangular or round block with parallel surfaces placed directly under the search tube and at right angles to it. In such a case, clear pips representing the front and back surfaces of the material will be seen, and any flaw pips which are present will appear between these two.
The echo from the front surface of the metal returns first and is located at the left-hand side of the screen. The echo from the back surface of the sample arrives last and so is located at the extreme right of the screen. A pip which is located twothirds of the distance from the front echo to the back echo indicates a flaw which is two-thirds of the thickness of the sample below its upper surface.
For instance, if the sample is 3" thick, and the pip appears on the right-hand side of the screen at twothirds of the distance between the pips, measured from left to right, then the flaw is located 2" below the surface. If the flaw is sufficiently large, all of the remaining sound may be reflected from it. In this case, the back echo will disappear, and the flaw will cast an ultrasonic shadow on the back surface.
2 PARTS 1 PART will not return to the crystal. This means, of course, that the pattern on the screen will not have a right-hand pip, which represents an echo from the back surface. If a second pip does appear, it probably represents a flaw. If any question exists as to whether this second pip is a flaw or an echo from the back surface, a distance measurement will indicate which it is. Any flaws will, of course, appear on the screen at a distance less than the actual thickness of the material being tested.
c. Figure 33 indicates a situation where the depth of the metal through which the sound waves have to pass is so great and the nature of the metal is such that all of the sound waves are absorbed so that the echo, if any, from the back surface of the material is unable to get through. Again, of course, any pip to the right of the one representing the front surface will represent a flaw. In such a case, a distance measurement will clearly indicate that the flaw is close to the front surface.
< FRONT SURFACE [FLAW < BACK SURFACE 2 IN. SURFACE ' FLAWLOSS OF BACK SURFACE - FRONT 3 IN.
FLAW- FIGURE 31.-Immersion Crystal Focused on Test Block FLAW' and Indications to be Expected
b. Figure 32 illustrates a situation in which the back surface of the sample to be tested is not parallel to the front surface. Under these circumstances, when the sound waves reach the back surface they will be reflected at an angle in the same manner as a beam of light would be reflected, and so they SURFACE / FLAW FRONT • NO BACK SURFACE FLAW FIGURE 32.-Immersion Crystal Focused on Block with Defect and Non-parallel Surface FIGURE 33.-Immersion Crystal Focused on Shaft too Long for Back Reflection of Return
d. Figure 34, the item being tested is a seam weld on a piece of thin-walled tubing. Behavior characteristics of ultrasound at an acute angle of approach are such that some of the sound waves will be reflected from the surface and be lost. The rest of the waves will become trapped in the circular path of the metal, and very little echo will be received from either the front surface or the back surface. Those waves which are proceeding through the wall of the tubing will strike any obstruction in the wall at approximately right angles, and will return an echo from such an obstruction. If the tube has been inspected prior to welding and found to be free of flaws, then any flaw which shows up after welding must be in the weld or the weld bead.
e. Figure 35 represents the inspection of a welded tube with a wall thickness in excess of
0.125". Refraction conditions are such that pips 17
< FLAW IN WELD SEAM WELD .1 IN. MAX.
FIGURE 34.-Angle Beam Penetrating a Weld Bead may be obtained representing both the outer and inner surfaces of the tube together with pips from any flaws which may be in the tube. The differences between the results received in Figures 34 and 35 situations are due to the beam angles.
dinal waves will result. Above 33°, longitudinal waves will disappear. Shear waves will produce the same indications on the screen as longitudinal waves. To produce shear waves in steel, set the transducer to some angle above 33°.
In shear-wave inspection, a rather small and poorly defined echo is obtained from the front surface of the specimen. However, a strong echo will be returned from vertical fissures, or cracks, in the material being inspected. If the searching tube is moved horizontally without changing its angle, the flaw pip will move toward or away from the surface pip. This is because the acoustic path between the front surface and the flaw is made stronger or longer by the motion of the screening tube.
LFRONT - FLAW
SURFACE
L SEAM FRONT FLAW
WELD SURFACE - BACK
SURFACE .125 MIN.
FIcURE 35.-Angle Beam Penetrating a Weld Bead
f. Figure 36 represents the inspection of a weld in a heavy plate. The rough surface of the weld bead prevents a direct overhead inspection of the weld zone because the ultrasound beam cannot enter near the surface of the rough bead. Therefore, the transducer must be directed so the sound beam will be refracted to strike the flaw at right angles. The ultrasound, when it leaves the couplant and enters the metal, will refract in such a way that the beam will be bouncing between the surfaces of the material.
When an ultrasonic beam enters metal at some angle other than normal to the surface, the internal angle will change rapidly with small changes in the external angle. If the internal deviation angle exceeds approximately 15°, both shear and longitu-
- FLAW FIGURE 36.-Angle Beam Penetrating a Flat Plate
g. Figure 37 represents a situation in which the front surface of the metal is so rough that a complete scattering of the sound waves results. In such a situation, no appreciable penetration into the metal will occur and any echoes that ordinarily appear on the screen will also be dissipated by the roughness and will not be received. The only possible solution to such a problem is either to have the
POOR INFORMATION 100%- 90% 1OSS in 9:
FIGURE 37.-Results of a Rough Front and Back Surface 18
surface smoothed sufficiently, or to conduct the test through a smoother surface from another angle. It is also suggested that the lowest frequency possible be used.
h. Figure 38 represents a typical reading from two different materials that have been brazed or welded face-to-face. Where a good uniform bonding has been obtained, a clear reading will be received with normal front and back pips, and a series of very small ridges. These ridges represent the uneveness of the bonded surfaces. If, however, the bonding is faulty and a void exists between the two surfaces, the sound waves will be unable to penetrate the void, and the pip from the bottom plate will be lost entirely, and a series of pips like tall grass will appear after the front surface indication, which represents reverberations of the sound waves within the top plate.
of sufficient size to be recognized. Ultrasound will not, however, penetrate deeply into exceptionally porous material. As shown in Figure 37, lower frequencies penetrate more readily than higher frequencies. As shown in Figure 32, distance measuring will ascertain that pips are not actually an echo fron the back surface.
It may be necessary to inspect a sample involving complex curved surfaces which do not fit any of the preceding illustrations. In such cases, the sound should be directed to hit the flash-line zone or grain flow at right angles since flaws would be oriented along the flash line or the grain flow direction. As shown in Figure 40, the probes are improperly positioned, therefore, any indication received would be erroneous.
GOOD BRAZE FRONT / NO 1.OSS OF BACK SURFACE SURFACE:
INDICATION
LARGE ENOUGH
TO BE SHOW'N
FRONT - SLIGHT SLIGHT BACK AS A MAJOR FLAW
SURFACE INDICATION SURFACE
BAD BRAYA OF INTERFERFACE
BAI BRAZE
TOOL STEFL FIGURE 39.-Indication Received from Porous Material
FRONT SURFACE -FLAW FIGURE 38.-Evaluating Braze of Carbide Tip to Steel ?
i. Figure 39 represents an inspection of relatively porous material. The front pip will be clear, but the back pip will be either very small or nonexistent because the ultrasonic waves have either been absorbed or dissipated by the porous nature FLASH LINE of the material. The clear pip of the front surface will probably be followed by a long series of bumps, POSSIBILITY OF FLANS or very short "grass", which represents tiny echoes received from the porous structure itself and do not FIGURE 40.-Irregular Part actually represent flaws. If a significant flaw is located within the material, it should create a pip 55.-60. RESERVED.
Chapter 7. RECORDERS
61. TYPES. The recording of flaws may be either by deflection modulation or intensity modulation.
62. DEFLECTION MODULATION, CON VEN - TIONAL PEN-CHART RECORDERS. In the deflection modulation method of recording, a pen draws a continuous line on a chart. (See Figure 41.)
When a flaw is detected, the line is deflected from its true path in proportion to the flaw amplitude.
63. INTENSITY MODULATION, C-SCAN METHOD OF FACSIMILE RECORDING. A true-to-scale reproduction of defects or a change in material density is indicated by various intensities ranging from black to light grey. (See Figure 42.) When testing plate or other relatively flat specimens, the recorder prints a plan view of the test piece showing defect location and relative defect size. When testing material on a rotating mechanism, a true position along the X axis of defects detected is shown. When testing cylindrical components on a
RECTILINEAR CHART
STRIP CHART CIRCULAR CHART FIGURE 41.-Various Recording Charts turntable, the true position along the Z axis of flaws detected is shown. When conducting a spiral scan of a disc which is mounted on the testing turntable, the recording chart is also mounted on a turntable that rotates in synchronism with the testing turntable.
64.-70. RESERVED.
FIGURE 42.-Ultrasonic Recording of Brazed Honeycomb Panel 21
Chapter 8. ELECTRONIC GATING
71. FUNCTIONS. One of the most important functions required in a high-speed ultrasonic testing system with automatic readout or control, is the selection and processing of only those signals from discontinuities which occur within particular zones or areas of the test piece. This is accomplished The second class of gate, called the switching or logical gate, is defined as one in which the ouput is a pulse which may have no resemblance to any of the inputs, except that the pulse occurs during the interval selected by the control voltage.
SIGNAL AMPLITUDE LEVEL SET • WATER PATH →0
PERMANENT
SIGNAL DISCONTINUITY
DISCONTINUITY GATE LENGTH -GATE- SIGNAL
- GATE START -INTERFACE SIGNAL -INTERFACE FIGURE 43.-Controllable Gated Zone in Test Piece by establishing a specific controllable gated zone within the test piece. (See Figure 43.) Signals above a pre-test amplitude occurring within the gate, can be monitored automatically and used to SIGNAL operate visual or aural alarms, sorting deflectors, INPUT R or paint-spray markers.
72. BASIC OPERATING PRINCIPLES OF GATES.
A gate is a device having several inputs and one output. Some of the inputs may be called signal inputs and others may be designated as control or selector inputs, although often the inputs are indistinguishable from one another.
There are two general classes of gates. The first, CONTROL GATE OR called the transmission or linear gate, is defined as INPUT one in which the output is approximately a replica of one of the inputs, but the output occurs only during times selected by the control inputs. Thus - E the gate transmits the signal from input to output in a linear manner during selected times. FIGURE 44.-Threshold Gate Circuit Diagram 23
The gate of Figure 44 is suitable for a positive- + 5x going input signal. The gate signal (also called a control pulse, a selector pulse, or an enabling pulse) is a rectangular wave form which makes abrupt SIGNAL transitions between the negative levels - E1 and
- 5v - - — Ег• In Figure 45, - Ez is assigned a value of minus 5 volts, and for a 10-volt input pulse, a 5-volt out-
- 15v GATE put pulse appears. When used in this manner, the circuit is referred to as a threshold gate.
FICURE 45.-Threshold Gate Wave Form 73.-75. RESERVED.