Nondestructively Inspect Material Integrity With An Airborne Ultrasonic Flaw Detector

TABLE OF CONTENTS How &  Why it Works The  Ultrasonic Trek Energy and  Amplitude CURLIN AIR, Airborne  Ultrasonic Flaw Detector  Compensation Application Notes: Composites Urethane  Foams AN-1 Fiberglass  AN-2 Construction  & Building Materials AN-3 Honeycomb  Structures AN-6 Tires  AN-4

Curlin Air Tech Note
Technical Note by NDT Systems
 Huntington Beach, CA,  USA

HOW & WHY IT WORKS

The CURLIN AIR, Airborne Ultrasonic Flaw Detector was initially developed to ultrasonically inspect an  ever growing class of materials/products that are too attenuative to inspect  with conventional (Megahertz frequency) ultrasonic flaw detectors. However,  because of its non-contact airborne beam feature, the CURLIN AIR, Airborne  Ultrasonic Flaw Detector, also, offers  applications advantages for certain materials/products (Samples) that are  routinely inspected with conventional equipment. Refer to the Application  Notes (AN) which are published on an ongoing basis for the  CURLIN AIR, Airborne  Ultrasonic Flaw Detector.

The unique performance of the CURLIN AIR, Airborne Ultrasonic Flaw Detector sometimes may seem amazing, especially  to those who have had experience with conventional ultrasonic flaw detectors,  as well as those who could not find a practical means to nondestructively  inspect their particular product. Neglecting actual performance  levels/details, it almost becomes an issue of what CURLIN AIR, Airborne  Ultrasonic Flaw Detector's ultrasonic  beam can't "punch through", rather than what it can punch through.

The object of this Tech Note is to briefly explain, in simple and general  terms, how & why it works.

No "Magic"

In order for the CURLIN AIR, Airborne Ultrasonic Flaw Detector to work, the following major technical issues  needed to be addressed:

Test Frequency: A frequency of 50KHz (having a wavelength in air of  1/4") was chosen because it was:

• low enough so ultrasonic attenuation (scatter and absorption) is  greatly reduced to levels which permit ultrasound to readily propagate  through both air and the categories of materials targeted for inspection,  yet
   
• high enough so satisfactorily small-diameter, well-collimated  ultrasonic beams can be generated by acceptably small-sized probes  (transducers). The 1.9" OD Model ATI Probe has an effective beam diameter of  about 1 3/8" at the probe face, with a beamspread of only about 91/2°  (half-angle at - 20dB).

Tone Bursts: Generate pulsed ultrasonic energy in the form of  rapidly reoccurring tone bursts (cycle packets at the 50KHz test frequency)  which possess the necessary duration (pulse length) and amplitude to deliver  the desired "penetration power", yet eliminate/minimize standing wave  interference.

Sensitivity: Provide the high levels of low-noise adjustable  amplification needed (up to 100dB or 100,000X) to compensate for the large  amount of ultrasonic energy lost by reflection at both material surfaces  (airborne ultrasound experiences an exceptionally large acoustic impedance  mismatch at air-solid interfaces - far greater than the liquid couplant-solid  interfaces experienced with conventional flaw detectors).

Special Probes (Transducers): Provide exceptionally effective energy  coupling to transfer satisfactory levels of ultrasound into and back out of  the air.

An easy way to initially visualize  what's happening ultrasonically and why flaws are detectable is to take a  "trek" along with an ultrasonic tone burst (longitudinal wave mode) as it  travels from the transmitter probe, through air to the material being  inspected, through the material and eventually through air again to the  receiver probe.

The Ultrasonic Trek

• Transmission Into Air: Initially, a portion of ultrasonic energy  is lost (due to the transmitter probe/air impedance mismatch) as the tone  burst is intially launched into the air.
• Airpath To Material: Once launched, the tone burst travels  through the air and loses further energy due to attenuation (absorption) and  beamspread (diffraction). For example, about 3dB of tone burst amplitude is  lost by traveling across an airpath of 6". (There is also a "complex"  radiation field, called the Near Field, which extends about 1.9" in front of  the Model ATI transmitter probe).
• Entry Into Material: A large amount (over 99%) of the incident  tone burst energy is lost due to reflection at the material surface, with  only a small amount of the ultrasound actually entering the material. This  reflection loss is caused by the extremely large acoustic impedance mismatch  at the air-material interface. The acoustic impedance mismatch can be  thought of as a "valve or shutters" which determine how much ultrasound is  permitted to cross the interface (a perfect impedance match allows all the  ultrasound to pass-no reflection).
• Travel Through Material: As this weakened tone burst travels  through the flawless solid material, it experiences further energy loss  caused by attenuation (both absorption and scattering, in the case of a  solid), as well as by additional beamspreading.
• Exit From Material: The remaining burst now experiences another  huge energy reflection loss (again over 99%) due to the same excessive  material-air acoustic impedance mismatch as was experienced at the  above-mentioned entry surface. At this point, the tone burst has lost more  than 99.9% of its energy due to only the two surface reflections.
• Airpath To Receiver Probe: After exiting the material, the now  extremely low energy tone burst experiences further attenuation and  beamspread loss as it travels along the airpath to the receiver probe.
• Reception From Air: Finally, when this low-level tone burst  impinges on the receiver probe, an additional energy loss is experienced  during its ultrasonic transfer to (ultrasonic activation of) the probe.
• Impact Of Material Flaw: Ironically, while impedance mismatches  at the two material surfaces can be consid-ered energy-stealing enemies, the  large air-material mismatch caused by a flaw (delamination, split or cavity)  becomes a great ally. Basically, the tone burst experiences another similar  99%-plus energy loss when it impinges on a flaw (assuming the flaw is large  enough to intersect the complete beam). Even"pressed together" delaminations  at this test frequency (50KHz) will still reflect 99%-plus of the signal  energy (because there is a microscopically thin layer of air still present  at the flaw site). Thus, the flaw "blocks" a huge percentage of the normal  (flawless) tone burst energy - making the flaw readily detectable. 

ENERGY AND AMPLITUDE

The above-mentioned ultrasonic energy (energy intensity) losses are  electronically detected in terms of overall signal amplitude losses. Signal  amplitude is related to the square root of the energy intensity.

CURLIN AIR, Airborne Ultrasonic Flaw Detector COMPENSATION

In the "real world", many inspectable products create actual energy  intensity losses which require compensatory signal amplitude boosts by the  receiver amplifier that are in the range of 1,000x to 10,000x (60 to 90dB).  The gain required mainly depends upon the product's acoustic impedance, attenuation and thickness.

APPLICATION NOTES

Ultrasonic Airborne of Graphite Composites

Material:
Graphite Fiber in  Polymeric (Plastic) Matrix/Laminate Layups or Filament Wound Structures

Application:
Detection of  Delaminations and Impact Damage

Customers:
Manufacturers and  end-users

Industries:
Aerospace, Aircraft  (Fixed Wing And Rotary/Helicopters), Airline (In-service Inspection),  Military, Construction, Infrastructure (Bridges), Petrochemical, Marine

Products:
Sheets, Tubes, Cases,  Helicopter Rotor Blades, Fuselage, Tail And Wing Sections For Aircraft,  Panels With Graphite Composite Face Sheets Having Foam Cores or Honeycomb  Cores

Curlin Air Performance:
General:
Conventional ultrasonic pulse-echo,  thru-transmission flaw detectors (manual or auto-scanning) and ultrasonic  bond testers are the major NDT approaches for inspecting these "advanced"  graphite composite structures. The ultrasonic flaw detectors require liquid  couplant (either liquid film contact or auto-scan water squirter coupling),  while the bond testers require contact with either dry or liquid film  coupling. CURLIN AIR, Airborne  Ultrasonic Flaw Detector features non-contact, air-coupled ultrasonic inspection  for delaminations, disbonds or in-service impact damage, provided there is  access to both surfaces for use of the thru-transmission mode of testing.  CURLIN AIR, Airborne  Ultrasonic Flaw Detector can penetrate even thick sections of graphite or foam cored  composites and is capable of detecting smaller-sized flaws.

Example:

The tables below  show CURLIN AIR, Airborne  Ultrasonic Flaw Detector penetration levels through various thicknesses of graphite  composites and the sensitivity to smaller-sized (0.3" diameter) simulated  delaminations.

EXAMPLES OF PENETRATION:

Gain Level To Produce 50% Signal Amplitude
 

EXAMPLES OF FLAW SENSITIVITY (SMALL FLAWS)

Flaw Simulated By Peripherally Bonding a 0.3" Diameter Paper Disk On Test  Surface

IMPORTANT NOTES:
1. Placing the Miniprobe receiver close  to the composite surface produces increased flaw sensitivity. For example,  placing the Miniprobe about 1/8" from the surface produced a signal  amplitude change from 75 to 25 for the 0.3" diameter flaw in the 0.045"  thick sample. 2. Large-sized delaminations can produce much greater signal  amplitude reductions-approaching

Ultrasonic Airborne Inspection of Fiberglass

Material:
Fiberglass Reinforced  Plastic (FRP) composites manufactured by laminate layup, chopped layup or  filament wound

Application:
Detection of  delaminations, foreign inclusions and major deviations in fiber-resin ratio

Customers:
Fiberglass manufacturers  and end-users

Industries:
Petrochem, Aerospace,  Transportation, Marine/Shipbuilding, Chemical, Constructions/ Architectural,  Electronic, Mining, Furniture, Military and Others

Products:
Tanks, Pressure Vessels,  Honeycomb Panels, Auto/Truck/Railcar Panels, Electronic Enclo-sures, Beams,  Tubes, Radomes, Sonar Domes, Antennas, Tubs/Spas/Shower Stalls,  Circuit-board Stock, Satellite Dishes, Covers, Ducts, Scrubber Towers,  Rocket Motor Cases, Structural Domes

Curlin Air Performance:
General:
Unlike standard flaw detectors, CURLIN AIR, Airborne  Ultrasonic Flaw Detector can  readily penetrate even thick sections of FRP and detect delaminations,  certain foreign inclusions and major resin-rich or resin-starved  condi-tions. The background noise is low compared to standard flaw  detectors. This performance, combined with CURLIN AIR, Airborne  Ultrasonic Flaw Detector's noncontact airborne  ultrasonic beam feature, opens up many new opportunities for the inspection  of FRP structures.

Example:
Using thru-transmition  Systems

Penetration Power:

Delamination Detection:

FRP* Thickness (Inches)	Gain (dB)	Signal Amplitude
		Unflawed Area	Large Delamination
2	81	80	2
3	85	80	2

*Note: Graphite and  Kevlar fiber reinforced plastics show similar results.

Ultrasonic Airborne Inspection of Urethane Foams

Material:
Rigid foam panels and  foam core laminates that are either cast or molded.

Application:
Detecting splits,  cavities, delaminations, foreign inclusions and major density deviations

Customers:
Foam manufacturers and  end-users

Industries:
Construction/Architectural,  Aerospace, Furniture, Marine/Shipbuilding, Door Manufacturers, Military,  Refrigeration, and Transportation (Truck/Rail)

Products:
Plate, Sheet, Tubes,  Bonded Laminates/Panels, Foam Core Doors, Foam-Core Refrigeration  Panels/Enclosures, Structural Shapes, Aerospace Panels (FRP or Graphite  Composite Face Sheets)

Curlin Air Performance:
General:
Conventional ultrasonic flaw detectors cannot  penetrate these highly attenuative foams and, furthermore,  manufacturers/end-users usually require non-contact scanning and no liquid  couplants. CURLIN AIR, Airborne  Ultrasonic Flaw Detector readily penetrates structural urethane foams and  features non-contact, air-coupled scanning. Splits, gas cavities,  delaminating, many foreign inclusions and major density deviations are  readily detectable. Urethane foam panels were the driving force behind the  development of CURLIN AIR, Airborne  Ultrasonic Flaw Detector and still remains one of its best applications.

Penetrating Power:
Very thick  sections of urethane foams can be penetrated (and, thus, inspected for  flaws). For example, a 9" thick section was penetrated using a gain level of  only 73 dB (near full-scale signal). Sectional thicknesses approaching 24"  may be inspectable (depending upon material properties).

Example:
Rigid Urethane Panel, 5  1/2" Thick, Molded
Test Setup:
CURLIN AIR, Airborne  Ultrasonic Flaw Detector, Thru-transmission  Mode
Model ATI Probes
Gain = 64dB.
 

Ultrasonic Airborne Inspection of Construction Materials Such As Drywall etc.
Construction / Building  Materials

Material:
Drywall, Plywood,  Particleboard, Flakeboard, Fiber-Reinforced Cement and Structural Urethane  Foam

Application:
Detection of  delaminations, splits, and blows

Customers:
Manufacturers and  end-users

Industries:
Construction/Building

Products:
Structural/Decorative  Members, Panels and Doors

Curlin Air Performance:
General:
The construction products listed above are  typically impractical to inspect with conventional ultrasonic flaw detectors  because such materials are extremely attenuative and some can possess  significant (although acceptable) material property variations. Also,  manufacturers and end-users of these products typically desire non-contact  testing and no use of liquid coupling. The CURLIN AIR, Airborne  Ultrasonic Flaw Detector can penetrate these  materials and detect the mentioned flaws using non-contact, airborne  ultrasonic testing. Plywood can cause a noisy background for the ultrasonic  signal, but larger-type blows are typically quite detectable.

Example:
Test Setup:  CURLIN AIR, Airborne Ultrasonic Flaw Detector
Thru-transmission
Model ATI Probes
Response: See  materials table.

Ultrasonic Airborne Inspection of Honeycomb Structures

Material:
Bonded Honeycomb  Structures Composed of a Wide Variety of Different Metallic and Nonmetallic  Materials (Aluminum, Stainless, Nomex, Phenolic, Graphite/FRP Composites,  Structural Paper, Balsa, Foam, Kevlar, Etc.)

Application:
Detecting  delaminations, voids, fractured core, crushed core, impact damage

Customers:
Manufacturers, end-users  and inspection labs

Industries:
Aerospace, Aircraft,  Airlines (In-Service Aircraft) Helicopters (Including Rotor Blades),  Military, NASA, Automotive/Truck, Marine (Shipbuilding/Boatbuilding),  Furniture, Construction Flooring, Cabinetry, Decks, Tabletops, Acoustical  Paneling), Radomes/Antennas, Gangplanks (Industrial and Marine), Electronic  Enclosures, Cargo Holders (Auto).

Curlin Air Performance:
General:
Aerospace honeycomb bonded structures have  been effectively inspected for many years using standard ultrasonic flaw  detectors and bond testers. Both single-surface and thru-transmission modes  are in common use. The use of contact inspection with liquid film coupling  or automatic scanning with water squirter coupling is commonplace. Some bond  testers feature "dry" contact testing. Single-side test methods (bond tester  and ultrasonic echo-mode setups) usually need to separately scan both  surfaces to detect face sheet delaminations/ voids and frequently cannot  detect internal crushed/fractured core or bond lines at internal septums in  multi-layered honeycomb core structures. Furthermore, some of the  non-metallic core material or heavier face sheets hinder conventional  single-sided testing. The use of couplants and contact are generally not  desired and time-consuming. In contrast, CURLIN AIR, Airborne  Ultrasonic Flaw Detector features simplistic  non-contact, airborne ultrasonic thru-transmission testing. While access is  required to both sides of the honeycomb structures, all of the flaws  mentioned delaminations, voids, impact damage and crushed/fractured core)  are detectable - even in thick multi-layered non-metallic honeycomb  structures and perforated face sheet noise abatement honeycomb structures  (e.g. jet engine nacelles).

Examples:
 

Ultrasonic Airborne Inspection of New & Re-Tread Tires Tires

Material:
Passenger, Truck,  Off-Road And Aircraft Tires

Application:
Delamination detection

Customers:
Tire Manufacturers and  Re-treaders

Industries:
Automotive / Heavy truck  / Aerospace

Curlin Air Performance:
General:
CURLIN AIR, Airborne  Ultrasonic Flaw Detector has shown applicability for  detecting delaminations in tires. Its major attractive feature is  non-contact, airborne scanning. Testing can be portable or by using a  fixture that rotates the tire.

Example:
Passenger Tire, 3/4"  Maximum Wall Thickness In Tread Area

Test Setup:
CURLIN AIR, Airborne  Ultrasonic Flaw Detector  Thru-transmission mode with Model ATI Probes Gain = 71dB  Response:
Average signal amplitude for good area =  80
Average signal amplitude for delamination = 2