Eddy Current and Ultrasonic Inspection

10-21 Principles of Operations Eddy currents are induced in a test article when an AC is applied to a test coil (probe). The AC in the coil induces an alternating magnetic field in the article, causing eddy currents to flow in the article. [Figure 10-11] Flaws in or thickness changes of the test-piece influence the flow of eddy currents and change the impedance of the coil accordingly. [Figure 10-12] Instruments display the impedance changes either by impedance plane plots or by needle deflection. [Figure 10-13] The specimen is either placed in or passed through the field of an electromagnetic induction coil, and its effect on the impedance of the coil or on the voltage output of one or more test coils is observed. The process that involves electric fields made to explore a test piece for various conditions involves the transmission of energy through the specimen much like the transmission of x-rays, heat, or ultrasound.

Eddy current inspection can frequently be performed without removing the surface coatings, such as primer, paint, and anodized films. It can be effective in detecting surface and subsurface corrosion, pots, and heat-treat condition. Eddy Current Instruments A wide variety of eddy current test instruments are available. The eddy current test instrument performs three basic functions: generating, receiving, and displaying. The generating portion of the unit provides an alternating current to the test coil. The receiving section processes the signal from the test coil to the required form and amplitude for display. Instrument outputs or displays consist of a variety of visual, audible, storage, or transfer techniques utilizing meters, video displays, chart recorders, alarms, magnetic tape, computers, and electrical or electronic relays.

A reference standard is required for the calibration of eddy current test equipment. A reference standard is made from the same material as the item is to be tested. A reference standard contains known flaws or cracks and could include items, such as a flat surface notch, a fastener head, a fastener hole, or a countersink hole. Figures 10-14, 10-15, and 10-16 show typical surface cracks, subsurface cracks, and structural corrosion that can be detected with eddy current techniques.

Ultrasonic Inspection

Ultrasonic inspection is an NDI technique that uses sound energy moving through the test specimen to detect flaws. The sound energy passing through the specimen is displayed on a cathode ray tube (CRT), a liquid crystal display (LCD) computer data program, or video/camera medium. Indications of the front and back surface and internal/external conditions appear as vertical signals on the CRT screen or nodes of data in the computer test program. [Figure 10-17] There are three types of display patterns: “A” scan, “B” scan, and “C” scan. Each scan provides a different picture or view of the specimen being tested. [Figure 10-18] Ultrasonic detection equipment makes it possible to locate defects in all types of materials. Minute cracks, checks, and voids too small to be seen by x-ray can be located by ultrasonic inspection. An ultrasonic test instrument requires access to only one surface of the material to be inspected and can be used with either straight line or angle beam testing techniques.

Two basic methods are used for ultrasonic inspection. The first of these methods is immersion testing. In this method of inspection, the part under examination and the search unit are completely immersed in a liquid couplant, such as water or other suitable fluids. The second method is called contact testing. It is readily adapted to field use and is the method discussed in this chapter. In this method, the part under examination and the search unit are coupled with a viscous material, liquid, or a paste that wets both the face of the search unit and the material under examination. 10-22 ConductorProbe coil Alternating current Primary magnetic field Eddy current e Eddy current inspection on crankshaft Eddy current inspection on fan blade There are three basic ultrasonic inspection methods: pulse echo, through-transmission, and resonance. Through- transmission and pulse echo are shown in Figure 10-19.

Pulse Echo Flaws are detected by measuring the amplitude of signals reflected and the time required for these signals to travel between specific surfaces and the discontinuity. [Figure 10-20] The time base, triggered simultaneously with each transmission pulse, causes a spot to sweep across the screen of the CRT or LCD. 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 10-23 Conductor P S The alternating current flowing through the coil at a chosen frequency generates a magnetic field around the coil.

A When the coil is placed close to an electrically conductive material, eddy current is included in the material. B If a flaw in the conductive material disturbs the eddy current circulation, the magnetic coupling with the probe is changed and a defect signal can be read by measuring the coil impedance variation. C Probe coil Alternating current Primary magnetic field Secondary magnetic field Eddy current of the transducer and the specimen are properly oriented, the ultrasound is 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 are measured by the timing circuits. The reflected pulse received by the transducer is amplified, transmitted to, and displayed on the instrument screen. The pulse is displayed in the same relationship to the front and back pulses as the flaw is in relation to the front and back surfaces of the specimen. [Figure 10-21] Pulse-echo instruments may also be used to detect flaws not directly underneath the probe by use of the angle beam testing method. Angle beam testing differs from straight beam testing only in the manner that the ultrasonic waves pass through the material being tested. As shown in Figure 10-22, the beam is projected into the material at an acute angle to the surface by means of a crystal cut at an angle and mounted in plastic. The beam, or a portion thereof, reflects successively from the surfaces of the material or any other discontinuity, including the edge of the piece. In straight beam testing, the horizontal distance on the screen between the initial pulse and the first back reflection represents the thickness of the piece; while in angle beam testing, this distance represents the width of the material between the searching unit and the opposite edge of the piece.

Through-Transmission Through-transmission inspection uses two transducers, one to generate the pulse and another placed on the opposite surface 10-24 Skin Chord Angle Crack Skin SkinSplice plate Chord Skin gap Upper member crack Second or deeper member crack Fastener hole crack, fastener in place Crack Fastener removed to receive it. A disruption in the sound path indicates a flaw and is displayed on the instrument screen. Through-transmission is less sensitive to small defects than the pulse-echo method. Resonance This system differs from the pulse method in that the frequency of transmission may be continuously varied.

The resonance method is used principally for thickness 10-25

Corrosion

WebStringer Skin Chord Bonded doubler Body skin lap splice Skin and bonded doubler Skin and chord web measurements when the two sides of the material being tested are smooth and parallel and the backside is inaccessible. The point where the frequency matches the resonance point of the material being tested is the thickness determining factor. It is necessary that the frequency of the ultrasonic waves corresponding to a particular dial setting be accurately known. Checks are made with standard test blocks to guard against possible drift of frequency. If the frequency of an ultrasonic wave is such that its wavelength is twice the thickness of a specimen (fundamental frequency), then the reflected wave arrives back at the transducer in the same phase as the original transmission so that strengthening of the signal occurs. This results from constructive interference or a resonance and is shown as a high amplitude value on the indicating screen. If the frequency is increased such that three times the wavelength equals four times the thickness, the reflected signal returns completely out of phase with the transmitted signal and cancellation occurs. Further increase of the frequency causes the wavelength to be equal to the thickness again and gives a reflected signal in phase with the transmitted signal and a resonance once more. By starting at the fundamental frequency and gradually increasing the frequency, the successive cancellations and resonances can be noted and the readings used to check the fundamental frequency reading.

[Figure 10-23] In some instruments, the oscillator circuit contains a motor- driven capacitor that changes the frequency of the oscillator. [Figure 10-24] In other instruments, the frequency is changed by electronic means. The change in frequency is synchronized with the horizontal sweep of a CRT. The horizontal axis 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 millicycle (mc) and 10 mc in four or five bands.

The resonance thickness instrument can be used to test the thickness of such metals as steel, cast iron, brass, nickel, copper, silver, lead, aluminum, and magnesium. In addition, areas of corrosion or wear on tanks, tubing, airplane wing skins, 10-26 TEST SPECIMEN Plan view C-scan Amplitude X B-scan PATH OF THE PROBE (FRONT TO BACK)X X (Material Thickness) Back Front 0 1 2 3 4 A-scan Front Back Signal amplitude Flaw and other structures or products can be located and evaluated. Direct reading dial-operated units are available that measure thickness between 0.025 inch and 3 inches with an accuracy of better than ±1 percent. Ultrasonic inspection requires a skilled operator who is familiar with the equipment being used, as well as the inspection method to be used for the many different parts being tested. [Figure 10-25] Ultrasonic Instruments A portable, battery-powered ultrasonic instrument is used for field inspection of airplane structure. The instrument generates an ultrasonic pulse, detects and amplifies the returning echo, and displays the detected signal on a CRT or similar display. Piezoelectric transducers produce longitudinal or shear waves, the most commonly used waveforms for aircraft structural inspection.

Reference Standards Reference standards are used to calibrate the ultrasonic instrument. Reference standards serve two purposes: to provide an ultrasonic response pattern that is related to the part being inspected and to establish the required inspection sensitivity. To obtain a representative response pattern, the reference standard configuration is the same as that of the test structure or is a configuration that provides an ultrasonic response pattern representative of the test structure. The reference standard contains a simulated defect (notch) that is positioned to provide a calibration signal representative of the expected defect. The notch size is chosen to establish inspection sensitivity (response to the expected defect size).

The inspection procedure gives a detailed description of the required reference standard. Couplants Inspection with ultrasonics is limited to the part in contact with the transducer. A layer of couplant is required to couple the transducer to the test piece, because ultrasonic energy does not travel through air. Some typical couplants used are water, glycerin, motor oils, and grease. Inspection of Bonded Structures Ultrasonic inspection is finding increasing application in aircraft bonded construction and repair. Many configurations and types of bonded structures are in use in aircraft. All of these variations complicate the application of ultrasonic inspections. An inspection method that works well on one part or one area of the part may not be applicable for different parts or areas of the same part. Some of the variables in the types of bonded structures are as follows: • Top skin material is made from different materials and thickness • Different types and thickness of adhesives are used in bonded structures 10-27 Amplifier RF pulser Rate generator Timing circuit Cathode ray oscilloscope Specimen Flaw Transducer 1 3 2 WATER YOKEHANDHELD Pulse Echo Through-transmission Ultrasonic (TTU) 0 1 2 3 4 5 6 7 8 9 10 10 9 8 7 6 5 4 3 2 1 0 DEPTH SIGNAL STRENGTH DELAMINATION 0 1 2 3 4 5 6 7 8 9 10 10 9 8 7 6 5 4 3 2 1 0 DEPTH SIGNAL STRENGTH NORMAL INSPECTION – NDI OVERVIEW Flaw FlawTransducer Specimen T F B Cathode ray tube 10-28 A B C D Transducer incident wave Reflective wave Reflective surface Material under test T = Wavelength 2 F = F1 (Fundamental frequency) T = W F= 2F1 (2nd Harmonic) T = 11/2 W F= 3F1 (3rd Harmonic) 8-23 It is necessary that the frequency of the ultrasonic waves corresponding to a particular dial setting 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 wavelength is twice the thickness of a specimen (funda- mental frequency), then the reflected wave will arrive back at the transducer in the same phase as the original transmission so that strengthening of the signal will occur. This results from constructive interference or a resonance and is shown as a high amplitude value on the indicating screen. If the frequency is increased such that three times the wavelength equals four times the thickness, the reflected signal will return completely out of phase with the transmitted signal and cancella- tion will occur. Further increase of the frequency causes the wavelength to be equal to the thickness again and gives a reflected signal in phase with the transmitted signal and a resonance once more.

By starting at the fundamental frequency and gradually increasing the frequency, the successive cancellations and resonances can be noted and the readings used to check the fundamental frequency reading. [Figure 8-10] In some instruments, the oscillator circuit contains a motor driven capacitor which changes the frequency of the oscillator. [Figure 8-11] In other instruments, the frequency is changed by electronic means. 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 pres- ent these vertically. Calibrated transparent scales are then placed in front of the tube, and the thickness can be read directly. The instruments normally operate Coaxial cable Quartz crystal






 
 
  A B D in a metal plate.

45° Coaxial cable Quartz crystal Defect Material • Underlying structures contain differences in core material, cell size, thickness, height, back skin material and thickness, doublers (material and thickness), closure member attachments, foam adhesive, steps in skins, internal ribs, and laminates (number of layers, layer thickness, and layer material) • The top only or top and bottom skin of a bonded structure may be accessible Types of Defects Defects can be separated into five general types to represent the various areas of bonded and laminate structures as follows: 1. Type I—disbonds or voids in an outer skin-to- adhesive interface.

2. Type II—disbonds or voids at the adhesive-to-core interface. 3. Type III—voids between layers of a laminate. 4. Type IV—voids in foam adhesive or disbonds between the adhesive and a closure member at core-to-closure member joints. 5. Type V—water in the core. Acoustic Emission Inspection Acoustic emission is an NDI technique that involves the placing of acoustic emission sensors at various locations on an aircraft structure and then applying a load or stress. The materials emit sound and stress waves that take the form of ultrasonic pulses. Cracks and areas of corrosion in the stressed airframe structure emit sound waves that are registered by the sensors. These acoustic emission bursts can be used to locate flaws and to evaluate their rate of growth as a function of applied stress. Acoustic emission testing has an advantage over other NDI methods in that it can detect and locate all of the activated flaws in a structure in one test. Because of the complexity of aircraft structures, T = 2W F= 4F1 (4th Harmonic)