Chapter 1. INTRODUCTION
1. GENERAL. Ultrasonic inspection of aircraft plays an important role from both a safety and economic aspect. In order to accomplish the prescribed ultrasonic inspection, only minor aircraft open-up for accessibility is usually required. For example, in the case of the aircraft engine, inspections are performed "on the wing" with the engine still installed in the aircraft. For the airframe, ultrasonics required a minimum of disassembly and removal of interfering equipment. Aircraft inspections with ultrasonics can be performed at the ramp" or "on the line" under circumstances entirely different from total disassembly. It is an extremely sensitive method of nondestructing testing. The limitations are few and when direct coupling (or contact) with the part being checked can be established, the resolving capabilities are excellent. However, interpretation by trained personnel is required.
2. USE OF ULTRASONICS. Ultrasonics employ electronically produced, high-frequency sound waves that will penetrate metals, liquids, composites, and other materials at speeds of several thousand feet per second. This technique can be used to:
d. Detect laps, seams, laminations, inclusions, cracks, corrosion, and other defects in installed
b. Locate porosity, cupping, and nonmetallic inclusions in bar stock.
c. Locate cracks, blow holes, insufficient penetration, lack of fusion, and other discontinuities in
d. Evaluate bond quality in brazed joints and honeycomb composites assemblies.
e. Inspect forgings such as turbine engine shafts, turbine engine discs, and landing gear structural members.
3. LIMITS ON APPLICATION OF ULTRASONIC TESTING. Among the factors which may limit the application of ultrasonic testing are:
a. Sensitivity. The ability of the instrument to detect the small amount of energy reflected from a discontinuity.
b. Resolution. The ability of the instrumentation to detect flaws lying close to the test surface or to separate and distinguish the indications from several defects occurring close together in the specimen.
c. Noise discrimination. The capacity of the instrumentation for differentiating between the signals from defects and the unwanted noise of either electrical or acoustics nature.
d. These factors are affected by others, such as frequency and pulse energy. For example, when frequency is increased, the sensitivity increases.
With the increase in sensitivity, smaller inhomogeneties within the material will become detectable.
This will increase the noise level, thus hindering signal discrimination. With an increase in pulse energy, material noise will increase and resolution
4. ULTRASONIC INSPECTION FOR CORROSION DETECTION. Although ultrasonic inspections have been employed by the aviation industry for several years, it was not until recently that ultrasonics have been used as a means of corrosion detection. Presently, this method of corrosion detection is still in the early stages and certainly not infallible; but it has been demonstrated that, within limitations, ultrasonics can provide a fairly reliable indication of corrosion attack. Highly trained personnel must conduct the examination if any useful information is to be derived from the indicating devices. This is compounded by the fact that the results obtained vary, depending on the model and make of equipment used, and on the techniques used by the individual performing the examination.
5.-9. RESERVED.
Chapter 2. TRANSDUCERS (SEARCH UNITS, PROBES, CRYSTALS)
10. GENERAL. To understand how inaudible sound is used to reveal certain conditions which are not perceptible in the normal hearing range, it is first necessary to know how ultrasound is transmitted and received.
may be of any conducting material such as aluminum, silver, gold, or chromium. However, coatings are difficult to deposit on lithium sulphate crystals so thin metallic foils are often cemented to the crystal.
11. TRANSDUCER MATERIALS. Three transducer materials which can be used in the manufacture of ultrasonic search units are natural quartz crystals, lithium sulfate, and polarized crystalline ceramics.
a. Quartz. Principal advantages are: electrical and thermal stability, insolubility in most liquids, high mechanical strength, wear resistance, excellent uniformity and resistance to aging. A limitation of quartz is its comparatively low electromechanical conversion efficiency.
b. Lithium Sulfate. Principal advantages are:
ease of obtaining optimum acoustic damping for best resolution, intermediate conversion efficiency, and negligible mode interaction.
c. Polarized Ceramics. Principal advantages are: high conversion efficiency which yields high search unit sensitivity. Because of lower mechanical strength and relatively high electrical capaci.
tance, their use is generally restricted to frequencies below 15 Mega-Hertz (MHz). Another limitation is some interaction between various modes of vibration.
In order for a crystal to utilize its piezoelectric characteristics, it is placed in a circuit much like a condenser. That is, both faces are coated with a conducting material with no contact between the two faces. (See Figure 1.) Coatings for crystals 5 COAXIAL CABLE CONNECTION
12. PIEZOELECTRIC EFFECT. Ultrasonic testing may use the piezoelectric effect to generate ultrasonic vibrations. Crystals, when subjected to an alternating electric charge, expand and contract under the influence of these charges. Conversely, it was found that these materials when subjected to alternating compression and tension developed alternating electric charges on their faces. (See Figure 2.) This was named the piezoelectric effect.
The heart of an ultrasonic testing system is this method of converting electrical energy into mechanical vibrations, and converting the mechancial vibrations back into electrical energy.
ALTERNATING VOLTAGE APPLIED •
TO AN X-CUT CRYSTAL
CRYSTAL EXPANDS CRYSTAL CONTRACTS FIGURE 2.--Generation of Ultrasonic Vibration Generation of the ultrasonic pulse is usually accomplished by producing a radio-frequency wave train of the desired frequency at a precise time and converting this into vibrations by means of piezoelectric transducers. Some ultraconsic instruments do not use a radio-frequency wave train, but instead, use a shock pulse and allow the search unit to select the frequency of operation.
13. TRANSDUCER TYPE SEARCH UNITS. The PLATED BACK search unit consists of a shell for mechanical pro- CRYSTAL tection, a means to conveniently handle or mount PLATED FACE the unit for use, the transducer element, electrical connections, and a backing material to dampen the backward directed energy that is transmitted FIGURE 1.-Angle Beam Search Unit by the crystal. (See Figure 3.)
ELECT. 14. TRANSDUCER GROUPS. There are three gen- ELECT. CONNECTION eral groups of transmitter-receiver search units:
LEAD TO CRYSTAL SHELL a. Common Transmitter-Receiver (T-R). These search units employ a single crystal and have com- BACKING mon connections to the transmitter and receiver
- MATERIAL amplifier units. (See Figure 5.)
SPECIMEN PIEZOELECTRIC CRYSTAL 2 CRYSTAL FIGURE 3.-Diagram of an Ultrasonic Search Unit — LUCITE BLOCK Transducers are available in a wide variety of types which include:
a. X-Cut crystals for longitudinal-wave gen- T SPECIMEN eration. (See Figure 4.)
b. Y-Cut crystals for shear-wave generation.
(See Figure 4.)
FIGURE 5.-Common Transmitter-Receiver Search Unit
c. Dual crystals with common holder.
d. Mosaics three or more crystals.
e. High frequency, 50 MHz or more.
f. Alternate crystal materials.
g. Sandwich and tandem arrangements.
h. Curved crysals to fit the specimen.
i. Wheel search units.
¡. Focused search units.
k. Temperature search units (for measuring wall thickness at temperature up to 1,100°F).
/ AXIS Since the search unit acts as both transmitter and receiver, it transmits a pulse of 1 to 4 microseconds duration; then acts as a receiver for a period up to several thousand microseconds. This cycle of transmitting and receiving is repeated at a rate of 50 to 5,000 times per second, or higher if required for high-speed automatic scanning.
b. C o m b in e d Transmitter-Receiver (T-R).
These search units have two transducers mounted on a single head and insulated acoustically from each other. One transducer is connected to the pulser and the other is connected to the receiver.
The combined T-R search unit is used for testing close to the entry surface and for thickness measurements from .040" to 2.0" when the opposite side is rough or corroded. The transmitting search unit projects a beam of vibrations into the material; the vibrations travel through the material and are re-
X-CUT X AXIS IRANSMITTING RECEIVING
- Y AXIS CRYSTAL CRYSTAL
FRONTAL
PARENT MEMBERS
SPECIM (LUCITE)
CRYSTAL Y-CUT Z AXIS • FLAW FIGURE 4.-Natural Quartz Crystal (X and Y Cut) Transducers are available that are smaller than ⅛" diameter and larger than 1" x 4". However, for most ultrasonic testing, standard diameters of -BACK SURFACE ¼", ½", and 1.0" are used. FICURE 6.--Combined Transmitter-Receiver Search Unit
fleeted back to the receiving search unit from any discontinuities or from the opposite boundary if Ly parallel to the entrant surface. --(See Figure 6.)
- c. Separate Transmitter-Receiver U-RI or Pitch-Catch, Search Units. Two heads are employed in these units having separate electrical connections to the transmitter and the receiving units.
One head is used as the transmitting unit while the other head is the receiving unit. (See Figure 7.)
. Materials which are coarse grained tend to scatter the ultrasonic sound beam; these materials can be effectively inspected using separate T-R search units that are mounted on individual wedges of a suitable plastic solid. When separate wedges are used, the angle of incidence may be varied according to the section thickness to be examined.
FIGURE 7.--Separate Transmitter-Receiver Search Unit
15.-20. RESERVED.
Chapter 3. WAVE PROPAGATION
21. ULTRASONIC WAVES. Ultrasonic waves can be propagated to some extent in any elastic material. This propagation occurs as a displacement of the successive elements of the material. If one part of a solid is distributed or displaced in some manner, molecules in other parts of the solid will be affected, but not instantaneously. As successive molecules in the medium are displaced, the disturbance is propagated away from its point of origin. (See Figure 8.) Since the lattice structure of all materials is elastic, a restoring force exists which tends to return each molecule to its original position. Because of inertia, these particles will tend to oscillate about their original undisturbed position until they come to rest. This molecule-tomolecule propagation results in a continuous train of disturbances called a compression-rarefaction wave. If the frequency of motion is above 20,000 cycles per second (cps) the waves are referred to as ultrasonic compressional waves. A small group of these waves, which occur together and are not preceded or followed by other waves, is called a wave train or a pulse. A pulse may have one of several different forms depending upon the individual wave amplitude, and the way the waves build up and decay. The most used modes of vibration are longitudinal, shear, surface, and plate waves.
IMPACI The longitudinal mode of wave transmission is probably the most widely used in ultrasonic testing and is also the easiest to see with respect to the method of propagation. This wave is easily generated, detected, and has a high velocity of travel in most media.
Longitudinal waves are used for the detection and location of defects that present a reasonably large frontal area to the surface from which the test is being made.
2 PARTICLE VIBRATION
DIRECTION OF PROPAGATION FIGURE 9.-Longitudinal Waves
23. SHEAR WAVES. The wave is said to be shear (transverse) when the movement of the particles are perpendicular to the direction of the wave motion. (See Figure 10.) These waves have a lower velocity than do longitudinal waves (in steel and other metals, about half). Because of their slower speed, shear waves have shorter wave lengths than those of longitudinal waves of the same frequency. This shorter wave length makes shear waves more sensitive to small inclusions, and consequently they are more easily scattered within the specimen.
PARTICLE VIBRATION
REFLECTION FIcURE 8.-Mechanical Analogy of Wave Propagation
22. LONGITUDINAL WAVES. The wave is said to be longitudinal (compressional) when the movement of the particles is parallel to the direction of the wave motion. (See Figure 9.)
DIRECTION OF PROPAGATION FIGURE 10.-Shear Waves The principal advantage of these waves is in applications that require an ultrasonic beam to be transmitted into the test object at a small angle to the surface. (See Figure 11.)
SEARCH UNIT
CRYSTAL
PLASTIC WEDGE
SPECIMEN
DIRECTION OF SOUND
VIBRATION DIRECTION FIGURE 11.-Transmitting A Wave at a Small Angle
24. SURFACE (RAYLEIGH) WAVES. Surface waves travel with little attenuation in the direction of propagation, but their energy decreases rapidly as the wave penetrates below the surface. The particle displacement of the wave motion follows an elliptical orbit consisting of both the longitudinal and shear wave motion. (See Figure 12.)
PARTICLE VIBRATION FOLLOWS
AN ELLIPTICAL ORBIT
DIRECTION OF PROPAGATION FIGURE 12.-Surface Waves Velocity of surface waves depends upon the material and is about nine-tenths of the shear-wave velocity. Surface waves are likely to be affected, in their propagation, by variations in hardness, plated coatings, shot peening, or surface cracks, and are easily dampened by dirt or grease on the surface of the specimen. Surface waves can often be produced as an unwanted effect, especially when the contact surface is rough.
The waves are not limited to flat surfaces. They will travel around curves and surface contours.
Sharp corners, such as the boundaries of plates or flaws, will reflect these waves. In fact, it is this characteristic or traveling around contours which makes surface waves so useful in surface flaw detection. (See Figure 13.)
SEARCH UNIT CRYSTAL FIGURE 13.-Surface Wave Technique
25. PLATE (LAMB) WAVES. When ultrasonic vibrations are introduced into a relatively thin sheet, the energy propagates in the form of plate waves.
Unlike the longitudinal, shear, and surface modes, the plate wave velocity is dependent both on frequency and plate thickness. A complex particle motion exists somewhat like the elliptical orbits described for surface waves. Greatly simplified, they can be divided into two basic types: The symmetrical type are known as dilational waves and the asymmetrical type as bending waves.
Examples of these two modes are illustrated in Figure 14.
ASYMMETRIC SYMMETRIC FIGURE 14.-Plate Waves Plate waves can be used for detecting nonbonded areas in laminated structures such as sandwich panels. By sending waves along the outer surface, any areas of the top sheet that are not 'securely bonded in place can be made to vibrate in one of these modes. By sensing such local areas of vibration, lack of proper bonding can be detected.
26.-30. RESERVED.
Chapter 4. ULTRASONIC VIBRATIONS
31. TESTING METHODS. Ultrasonics employ lowamplitude stresses which do not permanently affect a specimen because of the short wave lengths and relatively low energy of the ultrasonic vibrations which are transmitted through the material. On the other hand, destructive mechanical tests, such as static physical tests and forced vibration fatigue testing involve high-amplitude stresses. These highamplitude stresses may cause heating, nonlinear effects, permanent deformation, and eventual rupture of the specimen.
Sound waves beyond the hearing range of the human ear are referred to as ultrasonic vibrations.
The term embraces all vibrational waves of a frequency greater than approximately 20,000 Hz.
Ultrasonic vibrations of the lower frequencies act in essentially the same manner as do audible waves.
Most commercial ultrasonic testing is done at frequencies from 200 KHz to 25 MHz; however, application exists for frequencies as low as 2 KHz and as high as 200 MHz.
In practical testing, selection of frequency depends on sensitivity desired and sound penetration required; high frequency for sensitivity and low frequency for penetration. In general, sound waves of all frequencies will penetrate fine-grain material.
However, as the grain structure becomes more coarse, interference in the form of scattering may be expected when using higher frequencies (shorter wave lengths) and greater depth of penetration will be obtained by going to lower frequencies (longer wave lengths). The resonant frequency of a crystal is determined primarily by its thickness, the higher the frequency the thinner the crystal. For this reason, crystals with frequencies above 10 MHz are generally considered too fragile for contact testing. All testing frequencies, however, can be used in immersion testing.
Ultrasonic vibrations have several important characteristics:
a. They travel relatively long distances in solid materials.
b. They travel in well-defined sound beams.
c. The velocity of a specific vibrational wave mode is constant through a given homogenous material.
d. Most of the energy contained in one vibrational wave train is dissipated before the succeeding pulse is introduced to avoid confusing indications.
e. Vibrational waves will be reflected at boundaries of different elastic and physical properties.
f. Vibrational waves may change their mode of vibration when passing between materials having different elastic and physical properties or when reflected from boundaries at certain angles.
32. REFLECTION OF ULTRASONIC WAVES. Reflection of the ultrasonic vibrations will occur at the boundary between two different materials if a mismatch of acoustic impedance is encountered. The ultrasonic beam will be reflected at the interface exactly like light waves. For instance, the silvered back of a mirror has a high impedance mismatch to air, and light reflects from it at a very high efficiency. A plain piece of polished crystal has a very low impedance mismatch with air, and consequently, reflects very little.
When ultrasonic energy impinges upon the interface between two different mediums, part will
ANGLE OF REFLECTIONS = TO
ANGLE OF INCIDENCE
SOUND REFLECTION
FROM BACK
SURFACE
SOUND REFLECTED BACK
TO SEARCH UNIT FIGURE 15.-Reflection of Ultrasound 9
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.