Introduction to NDT of Composites Stephen G. LaRiviere Boeing Commercial Aircraft Manufacturing Research & Development
Reasons for non-destructive tests: • Ensure product reliability • Prevent accidents and save human life • Make a profit for the user by: – Ensuring customer satisfaction and to maintaining the manufacturer’s good name – Aiding in better product design – Controlling the manufacturing processes – Lowering manufacturing costs – Maintaining a uniform quality level McMasters 1959
The Role of NDT • NDT is an essential part of manufacturing and requires trained and experienced inspectors. • NDT methods are used to: – Supplement visual inspection – Avoid costly teardown in gaining access to hidden structure – Provide early detection of defects before they reach critical size – Obtain additional information
NDT Utilization Throughout the Airplane Product Life Cycle Pre-production for process development
7J7 skin panel
NDT Utilization Throughout the Airplane Product Life Cycle During production to provide process information and ensure part conformance
NDT Utilization Throughout the Airplane Product Life Cycle In service to maintain a safe fleet Ultrasonic Mechanical Impedance
Acoustic Flaw Detector
Detects skin -toand
NDT Methods Traditional Methods • Eddy current • Radiography • Ultrasonic • Penetrant • Magnetic particle
Non-traditional Methods • Thermography • Shearography • Bond Testing
Eddy Current – Principle of Operation
• Electrical currents are generated in a conductive material by an induced alternating magnetic field • Instrumentation detects interruptions in the flow of eddy currents, caused by imperfections, dimensional changes or changes in the material's conductivity and permeability
Eddy Current Advantages
Disadvantages
• • • •
• Suitable for inspection of conductive materials only • Surface must be accessible to probe • Usually no permanent record • Manual tests require high degree of operator skill to properly use test equipment
Highly portable Moderate cost Immediate results Sensitive -to small imperfections • Minimum part preparation required
Defect Types: Cracks, corrosion, conductivity, coating measurement Seldom used for composite inspection
Radiography – Principle of Operation • Penetrating radiation (Xray or gamma ray) passes through a test piece to produce an image on a radiographic film or non film media • Variations in test part produce changes in the amount of radiation absorbed • Changes in absorption are captured on the detecting media
Radiation Source
X-ray beam
Discontinuity
Test object Detector
⇓
Detector Image Plan view ⇓
Image density varies with amount of X-rays reaching the film through the test object
Radiographic Methods • Film
• Real-time (a.k.a. radioscopy) • Digital Radiography – Storage Phosphor – Linear Diode Array – Flat Panel
Radiography - Film Advantages
Disadvantages
• Well proven
• Radiation safety hazard • Moderately expensive • Wet-chemical processing required to develop • Proper interpretation of results may require high degree of operator skill
• Permanent record of test results • Minimum part preparation required
Defect Types: Voids, foreign material, internal damage, core conditions, water ingestion
Non-film Radiography Advantages
Disadvantages
• Lower recurring costs
• Radiation safety hazard (reduced from film) • Higher initial cost • Proper interpretation of results may require high degree of operator skill
• Shorter cycle time • Elimination of chemical processing • Portability
Defect Types: Voids, foreign material, internal damage, core conditions, water ingestion
Non-Film Radiography • Real-time (a.k.a. radioscopy) • Digital Radiography – Storage Phosphor: Use a flexible “imaging plate” to produce a latent image that must be subsequently scanned by a laser and converted into a video image
– Linear Diode Array: Uses an x-ray sensitive photodiode array in place of film.
– Flat Panel Detectors: Consist of a coated Thin Film Transistor (TFT) array that captures and converts XRay energy into electronic signals
Real Time Radiography Principle of Operation Image Intensifier used in place of film Incoming - rays X
Cathode Side
Anode Side Output Phosphor Screen
ns lectro e o t o ph
Test object
Camera
Input Window Input Phosphor Photocathode
Evacuated Housing
Electron Focus Plates
To Display
Real Time Radiography Advantages
Disadvantages
• Inexpensive • Well established • Instantaneous “real- time” (dynamic) imaging
• Analog device • Detector blooming (washout) • Limited bit depth (Typically 4- 5bits usable) = limited sensitivity • Most applications require magnification
Real Time Radiography
Linear Diode Arrays - Principle of Operation Uses an x-ray sensitive photodiode array in place of film. X-rays penetrate the part as a conveyor belt moves it through the “inspection tunnel”
The LDA reads and converts x-ray energy to usable data
Conveyor X-ray Generator
Final image is reconstructed by a computer and displayed on video monitor
Linear Diode Arrays Advantages
Disadvantages
• • • • • •
• Requires movement between part and detector • Resolution can be limited • Low kV applications only
Robust design Fast High dynamic range No blooming Digital imaging Can be tailored to application
Linear Diode Arrays
Linear Diode Arrays
- ray LDA image of honeycomb structure with excess resin X
Storage Phosphor Detectors – Principle of Operation A direct film replacement method, storage phosphor systems use a flexible “imaging plate” to produce a latent image that must be subsequently scanned by a laser and converted into a video image
Image plate readout
Optical Optical Scanner Scanner
Photomultiplier Photomultiplier Tube Tube
Laser Laser Beam Beam A/D A/D Converter Converter
Imaging Imaging Plate Plate
110010010010110 110010010010110
Motor
Storage Phosphors Advantages
Disadvantages
• • •
• Latent image process – no real time • Dynamic range not quite as good as latest flat-panel detectors
• • •
Flexible Good dynamic range Large area format- does not require movement between part/detector Digital Proven – widely used in medical community Not environmentally sensitive
Storage Phosphor Detectors
Imaging plate
Reader/Display
Storage Phosphors
Flat panel Detectors – Principle of Operation • Detectors consist of a coated Thin Film Transistor (TFT) array that captures and converts X-Ray energy into electronic signals • Electronic signals are “read-out” as digital values by the system electronics and software • Digitized data is then presented on a video display for interpretation
Flat panel Detectors Magnified View of TFT Matrix from an Amorphous Selenium -X ray Detector Element •Arrow points to the center portion of a single detector element (139x139 microns) •Detector element is shown in comparison to the width of a human hair (80 microns) •A 14” x 17” detector array contains more than seven million elements
Flat Panel Detectors Advantages
Disadvantages
• High dynamic range – up to 16 bit • High contrast sensitivity • Easily incorporated with robotics • Some capable of real-time (dynamic) imaging • High energy capable
• • • • •
Expensive Fragile Temperature sensitive “Bad” pixel issues Image latency (ghosting) may occur
Digital Radiography - Flat panel Detectors Sample Flat-Panel Detectors
Flat panel Detectors Amorphous silicon images
Honeycomb graphite composite with entrapped bagging and excess resin
Honeycomb graphite composite with excess resin
Thermography
Thermography Methods • Liquid Crystal and Electronic Thermography • Pulsed Thermography (Thermal Wave Imaging) • Ultrasonic Thermography (ThermoSonix)
Liquid Crystal Thermography Principle of Operation Inspector
Vacuum Frame Part
Lamp
Hot Air Gun
Hot Good Area Cold
Water in Honeycomb
Liquid Crystal Sheet (Heat Sensitive- Changes color)
Defect detected by differential heat diffusion into part
Electronic Thermography - Principle of Operation Infrared Camera Part Lamp Hot Air Gun
Hot Good Area Cold
Water in Honeycomb
Defect detected by differential heat diffusion into part
Liquid Crystal and Electronic Thermography Advantages
Disadvantages
• • • •
• Not effective for panels with metal skins • Liquid crystal method requires intimate contact
Real time imaging Non contact (electronic) Remote inspection Low non-recurring costs
Defect Types: Water in honeycomb structure
Liquid Crystal and Electronic Thermography
Liquid Crystal Method
Electronic Method
Liquid Crystal and Electronic Thermography
Liquid Crystal Thermography
Electronic Thermography
Radiography
Pulsed Thermography -Principle of Operation HEAT IMPULSE FROM PHOTGRAPHIC FLASHLAMPS > 1 MEGAWATT OF POWER
INFRARED CAMERA SHROUD
Process 1. 2. 3. 4. 5.
HEAT DIFFUSES INTO STRUCTURE DEFECTS BLOCK HEAT FLOW Data Acquisition CAUSE SURFACE HOT SPOTS Image Processing DETECTED WITH THE INFRARED CAMERA PROCESSED TO INCREASE SENSITIVITY
Pulsed Thermography Advantages
Disadvantages
• Rapid, wide area technique • Real time imaging, Noncontact, remote inspection • Versatile - New applications being approved and developed
• High equipment cost • Application base still limited
Defect Types: Moisture in composites, metal skin to metal doubler disbonds
Pulsed Thermography
Pulsed Thermography Metal to metal disbond in 737 skin Part
Pulsed Thermography Image
Ultrasonic Thermography Principle of Operation Ultrasonic Excitation Source - 20 kHz 400 Watt Infrared Camera
Crack
•Differential motion across crack face produces friction •Single point excitation covers large inspection area •Real Time / Wide area inspection
Cracks
Disbond
Decreased stiffness at in-plane defect sites cause greater localized bending above defects Heat from: Acoustic\Hysteresis damping
Disbonds/Delaminations
Lateral heat flow in conductive materials require image processing for defect detection
Ultrasonic Thermography Advantages
Disadvantages
• Rapid, wide area technique • Real time imaging • Versatile - New applications being approved and developed • Allows simultaneous detection of multiple defect types
• High equipment cost • Application base still limited
Defect Types: Cracks, Corrosion and Disbonds
Ultrasonic Thermography Pneumatic Press
IR Camera
Ultrasonic Horn UT Welder Power Supply
ThermoSonix System for Laboratory Testing
Ultrasonic Thermography Images of Defect
Lightning Strike Graphite Composite Disbonded Doubler Fuselage Skin Broken Fibers Graphite Laminate
Shearography – Principle of Operation Process •Pre-stress image taken •Stress applied – (heat, vacuum , pressure…) •Stress image taken •Image difference (pre-stress – stress) produces shearogram
Defects cause surface Displacement under stress
Shearing Optics
Laser interferometry technique
Part
Shearography Advantages
Disadvantages
• Rapid, wide area technique – locates and sizes defects • Real time imaging • Non-contact
• High equipment cost • Application base still limited
Defect Types: Metal skin to metal doubler disbonds, moisture in composites
Shearography
Shearography Image of Defect
Shearography of disbond
Ultrasonics – Principle of Operation • Ultrasonic inspection involves sending high frequency sound waves into parts of structures to interact with internal discontinuities or part features • Typical frequency range for ultrasonics is between 500,000 Hertz (500Khz) to 20,000,000 Hertz (20Mhz) – Bondtest methods may use sound down into audible frequencies • Ultrasonic systems typically use the following components – Instrument: Generates and receives the electronic pulse to from the transducer and analyzes and displays signal – Transducer: induces sound waves into the part and/or listens for returning sound – Couplant: provides for transfer of sound to & from part – Reference Standard: Provides calibration of test parameters
Ultrasonic Methods Three primary methods • Pulse echo • Thru transmission • Bond testers
Pulse Echo Ultrasonics – Principle of Operation
An ultrasonic beam is transmitted in a part and is reflected from a flaw or the back surface and returned to the transducer
Pulse Echo Ultrasonics Advantages
Disadvantages
• Single sided inspection
• May be difficult to resolve defects near surface defects • May be ineffective with porosity • May not be sensitive to off angle defects • May not be effective with sandwich structures.
• Identifies defect depth relative size of defect • Reasonably high resolution depending on equipment • Sensitive to a wide range of foreign materials
Defect Types: Delaminations, foreign material, porosity (loss of back)
Pulse Echo Ultrasonics
Automated Pulse Echo Scanner for 777 empennage skins
Through Transmission Ultrasonics – Principle of Operation •Uses two transducers, one to transmit the sound beam and the other to receive it •Through-transmission can be performed using the “contact” or “immersion” inspection method
Immersion through transmission
Contact through-transmission
Through Transmission Ultrasonics Advantages
Disadvantages
• Complete part inspection with single scan • Large dynamic range available • Relatively fast inspection • Irregular part contours
• Requires access to both sides of part • Does not identify defect type or depth • Not as sensitive to foreign materials as other methods
Defect Types: Delaminations and voids, porosity (equipment dependant) foreign materials (equipment dependant)
Through Transmission Ultrasonics
Automated Thru-transmission Ultrasonic Scanner for 737 Engine Cowls
Through Transmission Ultrasonics
Through Transmission Scan Image
Bondtesters • Bondtesters operate in the acoustic- m echanical mode and are used for single- sided bond inspections • Bondtesters are classified by their operating frequency – High frequency (100 - 500 kHz) • Ultrasonic resonant transducer
– Low frequency (7 - 80 kHz) • Mechanical impedance • Eddy Sonic • Velocimetric methods
– Audible • Tap test (resonance)
Resonant Transducer Bondtesters – Principle of Operation
Resonant transducer
Resonant Transducer Bondtesters Advantages
Disadvantages
• Single sided inspection
• Limited defect types • Limited sizing capability
• Inspect an area of a part at one time • Fast data acquisition
Defect Types: Delaminations
Resonant Transducer Bondtesters
Velocimetric Bondtesters – Principle of Operation Velocimetric bondtesters measure stiffness changes of the structure through ultrasonic mechanical vibrations. The transducer produces a flexural (bending) wave mode
Velocimetric Bondtesters Advantages
Disadvantages
• Single sided inspection
• Limited defect types • Poor sizing accuracy
• Inspect an area of a part at one time • Fast data acquisition
Defect Types: Skin to core disbonds, interply delaminations
Velocimetric Bondtesters
Velocimetric Bondtester
Tap Testers – Principle of Operation Tap testing is a process of evaluating a structure by lightly tapping (structure vibrated) with a metallic disk, rod, or an automatic tap tester instrument. The taping excites structural resonances that will vary with how the energy is transferred into the part. Aluminum head Adhesive
Adhesive
Stem (piano wire)
Handle (aluminum)
Tap Testers Advantages
Disadvantages
• Low Cost • Relatively simple to use
• Somewhat unreliable • Limited to defects 3 to 4 plies deep • Limited to defects larger than 1 inch diameter • Results affected by environmental noise • Results affected by hearing ability of inspector
Defect Types: Skin to core disbonds, interply delaminations
Tap Testers
Automated Tap Test Instrument
Other NDT Methods Available • • • • • • • •
Acoustic Emission Testing Computed Tomography Magnetic Resonance Imaging Neutron Radiography Positron Emission Tomography Compton Backscatter Embedded sensors DiffractoSite
NDT Summary Method Usage Matrix NDT Method Ultrasonics Pulse Echo Ultrasonics Through Transmisson Ultrasonics Bondtesters
Composite Laminate Delaminations Porosity Foreign Material
Shearography Key
Radiography Thermography
Honeycomb Structure Delaminations Porosity Foreign Crushed Material Core
Works well May be used under certain conditions
Entrapped Moisture
NDT Summary • Current NDT technology adequate for composite inspection • New emerging NDT techniques show additional promise • NDI a valuable tool to detect & characterize composite defects in all stages of product lifecycle • Trained inspectors are required for successful inspection
