& 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.
Chapter 9. DELAY LINE
76. DESCRIPTION. By using a delay unit to increase resolution power, "close-to-surface" defects, such as those normally found in spot welds, may be investigated and evaluated.
The basic operation of a delay line depends on the fact that acoustic signals travel much slower than electrical signals. At the input end of the line, the electrical signal to be delayed is converted to an acoustical signal by a transducer.
The signal travels along the delay line as an acoustic wave, requiring a specific time to travel to
INPUT OUTPUT
• COIL COIL O
WAVE GUIDE
DAMPER DAMPER FIGURE 46.-Ultrasonic Delay Time the output end. At the output end, the signal is converted back to its electrical form by another transducer. Figure, 46 illustrates one of a number of methods which may be used to delay or slow down the electrical signal by converting it to the slower traveling sound waves.
The ultrasonic delay line shown employs an input and an output transducer coil with a magnetostrictive core which is attached to a sonic wave guide.
At the input end, the flux changes in the coil caused by the electrical input signal set up mechanical stress (vibrations) in the score. These vibrations travel down the line to the output coil. Acoustic absorbers (dampers) are used at both ends of the line to prevent reflections along the wave guide, which would introduce a form of distortion. A given amount of delay can be accomplished by using different lengths of line.
77.-80. RESERVED.
Chapter 10. COUPLANT
81. GENERAL. A couplant is a liquid having good wetting properties to transmit ultrasonic vibrations from the transducer to test surfaces.
82. DESCRIPTION. Ultrasound generated by the piezoelectric effect, for all practical purposes, will propagate in air due to its high frequency and short wave length. The actual movement of a piezoelectrical crystal vibration is in the micron range;
it is often described as acceleration without motion.
To transmit such energy into a material, use a fluid couplant between the search unit face and the material surface. A film of oil, glycerine, or water is generally used. (See Figures 47 and 48.) When water is used as the couplant, a wetting agent should be used to eliminate surface tension and facilitate "wetting" of the surface. When it is either impracticable or undesirable to use oil or water, a couplant paste is used.
WATER
COUPLANT
FILM OF OIL
- CRYSTAL FIGURE 48.-Couplant (Immersion Tesing) TEST MATERIAL.
FIGURE 47.-Couplant (Contact Testing) 83.-85. RESERVED.
Chapter 11. ULTRASONIC REFERENCE BLOCKS
86. INTRODUCTION. Ultrasonic testing should not be accomplished without first calibrating the equipment to the proper reference standard. Standard Reference Blocks are used to standardize the ultrasonic equipment, set the sensitivity, and evaluate the discontinuities in the material being inspected.
These blocks provide the comparison for any combination of crystal size, frequency, or test instrument used to inspect materials. The evaluation of the discontinuities within the material is accomplished by comparing the ultrasonic response from the discontinuity with the known artificial defects (flat-bottomed holes or notches) in the Standard Reference Blocks. The test instrument should be adjusted to indicate the hole representing the smallest defect it is desired to pick up. When making a test set-up, care should be taken to assure that the test block is of the same formulation alloy with similar surface conditions as the material under test. This precaution is necessary to assure that distance calibration is correct. If any adjustments are made on the apparatus, or if the probes are changed during the test, the probe and apparatus should be checked again on the reference blocks for sensitivity and proper functioning before resuming test.
87. DISTANCE-AMPLITUDE CO M PARIS ON BLOCKS.
a. Hitt Blocks. Ultrasonic Standard Reference Blocks designed to be used for distance amplitude comparisons have a specific size flat-bottomed hole placed at varying depths below the surface of the material. The three sets have 64", 64", or %4" size flat-bottomed holes. The metal distance of the blocks normally used varies from 16" or 6.0".
(See Figure 49.)
b. Calibration of ultrasonic thickness meas urement equipment may be accomplished by using a "step-block." For exact calibration, it is always best to have a block whose thickness will give five FIGURE 49.-Distance-Amplitude Comparison 29
signals on the screen for the range selected. (See Figure 50.)
.1" FIGURE 50.-Step Block
88. AREA-AMPLITUDE COMPARISON BLOCKS (ALCOA BLOCKS). A set consists of eight Ultrasonic Standard Reference Blocks. Each set contains one block with the following flat-bottomed holes:
64", 64", 364", 61", 5/64", G4", 64", and 64".
The holes are placed at a depth of 3.0" below the surface of the material. Area-amplitude relationships at the 3-inch metal distance are obtained by intercomparison of the blocks within the set. (See Figure 51.)
89. I.I.W. WELDING BLOCK. The I.I.W. Welding block was developed through the International Institute of Welding (I.I.W.). The block has gained wide acceptance as a reference standard in distance calibration and in the calibration of the angle of sound beam propagation from angle-beam search units. (See Figure 52.)
90. ASME WELD REFERENCE PLATE. This standard ultrasonic reference plate is required to check the performance and sensitivity of the ultrasonic equipment and ultrasonic search units used in the inspection of welded joints under the ASME code requirements. This block provides three reproducible levels of instrument sensitivity for both longitudinal and shear-wave inspections and insures re-examination at the same level of sensitivity.
(See Figure 53.)
91. SPECIAL REFERENCE STANDARDS. Many applications in ultrasonic inspections require special reference standards or blocks. The application may involve the use of standard sizes of blocks but employing a special type material or it may involve the use of special shaped blocks. A convex or concave entry surface as applicable is required when the test specimen has a curved entry surface. Many users of ultrasonic equipment prepare special reference blocks from a sample of the material under test that contains a known defect to represent the standard that is acceptable.
FIGURE 51.-Area Amplitude Comparison Blocks 30
FIGURE 52.-I.I.W. Weld Block FIGURE 53.-ASME Weld Reference Plate