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
92.-95. RESERVED.
Chapter 12. TESTING METHODS
96. DESCRIPTION. Contact and immersion are the immersion tested with a manually operated scanning two basic methods used for ultrasonic nondestruc- system. Inspection is accomplished by:
tive inspection. The instrumentation of one method can be adapted, in a limited way, to the other.
FIGURE 54.-Contact Angle-Beam Test into Hidden Weld Region of a Landing Gear Oleo Strut
97. CONTACT TESTING. This method of testing can be used when the aircraft is on the line or ramp since the test equipment can be handcarried directly to the work area. (See Figures 54, 55, and
56.) The principle of contact testing is illustrated in Figure 57.
Scanning of the test specimen is accomplished by manually moving the search unit over the test surface. A couplant, such as oil, is required between the face of the probe and the entrant surface of the specimen to exclude the air gap and couple the sound waves into a specimen.
98. IMMERSION TESTING. This method transmits sound into an inspection specimen through a water path or column. The immersion search unit is usually of waterproof construction. Repetitive inspections provide a pre-programmed fully automated system. Forgings, castings, and non-symmetrical parts with fillets, rough surfaces. radii, etc., that cannot be tested by the contact mehod may be FIGURE 55.- asonic Inspection of a Wing Front Spar FIGURE 56.-Ultrasonic Inspection of a Pylon Structural Member 33
-SEARCH UNIT CRYSTAL S TEST PIECE between the face of the transducer and the surface of the specimen. (See Figure 59.)
c. By a wheel-search unit in which the transducer is mounted in a liquid-filled tire. The tire is operated in direct contact with the material.
(See Figure 60.)
FRONT BACK SURFACE
SURFACE - DISCONTINUITY
• OVERFLOW
- SEARCH UNIT
COUPLANT SUPPLY * TO ULTRASONIC INSTR.
CRT PRESENTATION FIGURE 59.-Bubbler Angle-Beam Testing (Pipe) -BACK REFLECTION -DISCONTINUITY INDICATION LIQUID FILLED TIRE
• INITIAL PULSE (FRONT SURFACE)
AXLE STATIONARY FIGURE 57.-Principle of Ultrasonic Testing (Contact) _ CRYSTAL
WATER PATH FIGURE 60.-Wheel Scanning Method
99. IMMERSED SCANNING TECHNIQUE. Both BACK SURFACE the search unit and the part to be tested are totally SEARCH UNIT - DISCONTINUITY immersed in water. The sound beam is directed
FRONT SURFACE into the material normally by a straight-beam search unit. Angle-beam techniques such as shear LENGTH OF WATER PATH or plate waves are accomplished through control and direction of the sound beam. (See Figures 61, 62, and 63.)
CRT PRESENTATION
- BACK REFLECTION
- DISCONTINUITY INDICATION
- INTERFACE SIGNAL
INITIAL PULSE FiGURE 58.-Principle of Ultrasonic Testing (Immersion) SEARCH UNIT
a. Immersing both the search unit and the material in a liquid couplant, normally water.
(See Figure 58.)
b. By a squirter or bubbler which directs a FIGURE 61.-Immersed Angle-Beam Technique column of flowing water to form a couplant (Pipe or Tube) 34
focused through acoustic lens structures in a manner similar to that achieved by glass lenses in optics.
The acoustic lens is an integral of the focused
- SEARCH UNIT search unit assembly and is designed to provide a well-defined and directed sound beam pattern. The energy propagated from the search unit can be concentrated through spherical lens structure into coneshaped beams with high intensity. (See Figure 65.)
FIGURE 62.-Immersed Angle-Beam Technique (Plate or Sheet)
• LENS TRANSMITTING S.U.
RECEIVING S.U W SOUND FOCUSED
DEFECT ENERGY REDUCED
BY DEFECT IN PATH OF BEAM FIGURE 63.-Immersed Through-Transmission Technique In immersion testing by the straight-beam technique, the water path distance (search unit to front surface of the test piece) is generally set longer in time than the length of scan (front surface to back surface) so that the first multiple of the interface signal will appear further along the CRT sweep than the back reflection. This is done to clear the test area of signals which may cause misinterpretations. This is particularly important when the test area is gated for automatic signalling and recording operations. (See Figure 64.)
a. Focused Search Unit. The high-frequency mechanical vibrations of the sound beam can be
THIN PLATE FIGURE 65.-Focused Search Unit
b. Beam Collimator. Beam divergence varies with frequency and crystal diameter. The higher frequencies give more directivity to the sound beam and a 1" diameter crystal at any frequency is more directive than a ½" diameter crystal operating at the same frequency. Therefore, to obtain more directivity (which results in a better definition of the defect) the frequency must be increased or a larger diameter crystal must be used.
Increasing the frequency to obtain a better definition of the defect often defeats the purpose of the test, since the increase in frequency is also an increase in sensitivity. The higher sensitivity discloses irrelevant factors such as surface scratches, nicks, and dents which make the defect evaluation difficult.
Therefore, to obtain more directivity of the sound beam without increasing the frequency, a larger diameter crystal is used. Since the defect under examination may occupy only a small portion of the
SBACK FACE
OF TEST PIECE 5 TRANSDUCER
WATER PATH
LENGTH OF WATER PATH — BEAM
INTERFACE (ENTRANT SIGNAL SURFACE) COLLIMATOR
BACK REFLECTION & SPECIMEN TEST
DEFECT SIGNAL
INITIAL PULSE DEFECT FIGURE 64.-Removing First Multiple of Interface Indication From Test Area FIGURE 66.-Beam Collimator 35
total area of the beam spread, a beam collimator is mobile fixture which runs over the material. (See used to reduce the area of the sound beam entering Figure 70.)
the test piece to cover only the area under inspection.
WHEEL WATER FILLED TIRE The sound-carrying shaft in the center of the STATIONARY collimator conducts the straight center rays of the CRYSTAL transducer into the specimen. The remainder of the beam is prevented from reaching the test piece or returning a signal to the transducer by the acous- MATERIAL MOVES tical block. (See Figure 66.)
100. WATER COLUMN TECHNIQUE (BUBBLER, SQUIRTER). The water column scanning technique operates on the immersion principle where a beam FIGURE 69. Wheel Search Unit in a Fixed Position of high-frequency sound is projected through a water path into the test material. This technique is adapted to high-speed scanning of plate, sheet, WATER FILLED TIRE strip, cylindrical forms and other regularly shaped WHEEL MOVES parts. The sound beam is projected into the ma- OVER MATERIAL terial through a column of flowing water. The • CRYSTAL sound beam can be directed into the material either perpendicular to the test surface (Figure 67) or at an angle (Figure 68).
TEST PIECE
OVERFLOW - SEARCH UNIT TO ULTRASONIC INSTR. WATER COUPLANT FIGURE 67.-Bubbler Scanning Method
OVERFLOW
- SEARCH UNIT
COUPLANT SUPPLY TO ULTRASONIC INSTR.
FIcURE 68.-Bubbler Angle-Beam Technique (Plate)
101. WHEEL SEARCH UNIT. The wheel search unit operates on the immersion principle in that a beam of high-frequency sound is projected through a liquid path into the test material. The search unit, mounted on the axle of a wheel with a liquidfilled rubber tire, is held in a fixed position relative to the test surface while the wheel rotates freely.
The wheel search unit may be mounted on a stationary fixture, in which case the material moves past it (see Figure 69) or it may be mounted on a FIGURE 70.-Wheel Search Unit over the Material The wheel is used in a wide variety of applications, and is especially useful for high-speed automated ultrasonic flaw detection systems since it combines advantages of both contact and immersion testing methods. The wheel eliminates the need for immersion tanks by coupling the transducer to the specimen through a water path and the resilient material of the tire-shaped casing.
Rusty, scaled, pitted, rough machined and asforged are successfully scanned with the wheel search unit.
The position and angle of the crystal mounting on the axle are determined by the test method and technique to be used; i.e., straight beam or angle beam.
102. ULTRASONIC IMAGE CONVERTER. The ultrasonic image converter is an evacuated tube having a piezoelectric quartz window, and contains an electron gun and multiplier. (See Figure 71.)
When ultrasonic energy is impressed upon the quartz window, a piezoelectric voltage is produced whose magnitude at any one point corresponds approximately to the ultrasonic intensity at that point.
These voltage variations influence the secondary emission generated by the scanning electron beam.
The produced signal is then amplified and displayed using conventional closed-circuit television techniques.
IMAGE OF FLAW AMPLIFIER 2 VIDEO MOLTIPLIER ELECTRON TO ULTRASONIC GENERATOR 7
• SPECIMEN WATER T.V. MONITOR S TANK
TRANSMITT ING
- CRYSTAL
FOCUSING AND
DEFLECTION
CIRCUITS PIEZOELECTRIC QUARTZ WINDOW
ELECTRON GUN FIGURE 71.-Schemaic-Ultrasonic Image Converter System
103.-110. RESERVED.
Chapter 13. INSPECTION OF INTEGRAL FUEL TANKS FOR CORROSION
111. GENERAL. The existence of micro organisms in the fuel used in turbine-powered aircraft, results in failure of integral fuel tank coatings and corrosion of the wing skins.
Ultrasonic nondestructive testing provides a method of inspection for the detection and degree of corrosion in integral wing fuel tanks.
112. METHOD. Ultrasonic inspection techniques are used for determining and recording the condition of the interior surfaces of wet-wings. Longitudinal waves should be used since shear and plate waves will not accurately establish the definition or extent of corrosion pits.
113. EQUIPMENT. To scan the underneath surface of an aircraft wing, the following equipment is
a. A wheel search unit. To assure a satisfactory coupling, the surface to be scanned is
b. An oscilloscope with a fast pulse repetition This is necessary to conduct a rapid scan of 500 square inches or more in 15
d. An automatic and manually controlled scanning bridge and carriage.
e. A positioning mechanism, scanner support structure, and lift platform. This equipment places the sheel search unit in a position to scan the lower surface of the wing and provides the apparatus necessary to relate the motion of the search unit to the facsimile recorder. This equipment also permits a rapid engagement of the scanner with the aircraft and requires no jacking or other handling of the aircraft, nor is it necessary to drain the
114. INDICATIONS. Intergranular corrosion appears on the C-scan recording as a solid area projecting outward from fastener holes or other locations where there is a transverse cut exposing the edge of the plate. Large or small areas of pit type corrosion may appear anywhere on the recording with high-density areas giving a mottled appearance to the recording. Thus, the extent and the kind of corrosion can be determined from the recording.
c. A C-scan facsimile recorder. 115.-120. RESERVED.