Evaluation of NDI methods

Nationaal Lucht- en Ruimtevaartlaboratorium – National Aerospace Laboratory NLR

Evaluation of In-service NDI Methods for Composite Aerospace Structures

Jaap H. Heida and Derk J. Platenkamp

Aerospace Vehicles Division

6th Int. Workshop NDT in Progress, Prague, 10-12 October 2011

6th International Workshop NDT in Progress, Prague, 10-12 October 2011 2

National Aerospace Laboratory NLR

Staff of 700

Turnover ~ 80 MEuro/yr

Amsterdam Flevoland


Contents

National Technology Project in NL on NDI/repair for composite aerospace structures

� Composite benchmark

� Base-line ultrasonic C-scan inspection

� Selection of NDI methods

� Evaluation of NDI methods

� Conclusions


Composite benchmark- structural details -

Damage type/size

• Range of impact damage relative to BVID (initial dent depth of 1.0 mm)

• Interply delaminations in skin, diameter range 0.25 – 1.0 inch

• Skin-to-stiffener disbonds, diameter range 0.25 – 2.0 inch


Composite benchmark- structural details -

Damage type/size• Range of impact damage relative to BVID (initial dent depth of 1.0 mm)

• Interply delaminations in outer skin, diameter range 0.25 – 1.0 inch

• Outer skin-to-honeycomb core disbonds, diameter range 0.25 – 2.0 inch

• Skin-to-rib/frame disbonds, diameter range 0.25 – 2.0 inch


Composite benchmark- specimen manufacture -

Panel NTP-D

Material based on carbon fabric- HTA carbon fibres- HexPly M18-1 resin- HRH-10 Nomex honeycomb


Composite benchmark- application of paint system -

Paint system� Aerodur 37047 CF primer� PUR-Declack topcoat (PU Finish 03-66207 RAL 6031 F9), colour bronze green

� Standard paint system used on military weapon systems such as Cougar, Chinook and Apache helicopters

� Two cross layers with total paint thickness of ~30 micron


Composite benchmark- infliction of low-velocity impact damage -

Instrumented impact tester

• Guided drop weight device

• Impactor with hemispherical steel tupwith diameter 0.5 or 1.0 inch

• Specimens clamped in frame

• Strain gauge bridge mounted near top of impactor to provide a force-time plot

• Range of impact damage varying from non-visible to clearly visible damage


Base-line ultrasonic C-scan inspection

Scan window 4.0 x 2.5 x 2.5 mImmersion and squirter inspectionPulse-echo and through-transmission Complex scan profilesTurntable for circular components


Base-line ultrasonic C-scan inspection

Immersion inspection, pulse-echo backwall skin reflection

NTP-B


Selection of in-service NDI methods

Selection criteria for NDI methods

� Practical in use:

- portable

- one-sided accessibility

- limited safety precautions

� Proven applicability for the in-service detection of the following defect types in composite structures:

- impact damage

- interply delaminations

- disbonds (skin-honeycomb, skin-stiffener)

� Cost-effective


Selection of in-service NDI methods

Down-selection of:� Laser-ultrasonics (costs)� Local UT with conventional handheld transducers (no 2-dimensional plan view of defects)

� Portable UT C-scan system (no specific advantage over phased array UT)

� Single element dry-coupling UT roller probes (multiple element roller probes will be included)

� Air coupled UT (through-transmission mode)� Low kV radiography (two-sided access, safety precautions)� Edge of Light and D-Sight optical techniques (no through-the-thickness damage information)

� Some speckle techniques (sensitive to vibration, no through-the-thickness damage information)

� Eddy current (only applicable to conducting materials)


Selection of inspection methods- visual inspection and NDI -

Tap hammer AcoustoCam UT cameraBondmaster 1000e+

RapidScan roller probe Thermography

Phased array UT Shearography


Evaluation of NDI methods- visual inspection -

•General visual� Close-proximity, intense visual examination of relatively

localised areas of internal and/or external structure for

indications of impact damage or other structural anomaly

•Results of general visual inspection� Primary technique for in-service inspection of composites

� Low-cost, relatively fast with a large field of view

� Detectable: all impacts with initial dent depth > 0.5 mm

(½ BVID-size)

� Delaminations and disbonds not detectable


Evaluation of NDI methods- automated tap test -

Mitsui Woodpecker WP-632• Light-weight device (~ 0.5 kg): battery-driven solenoid hammer with force sensor built in the hammer tip

• Measurement of the contact time (increases in areas with defects that lower the local contact stiffness of the part)

• Instrument settings� Four Average modes (1/2/4/8)

� Four frequencies for the tapping speed (2/4/8/16 Hz)


Evaluation of NDI methods- BondMaster inspection -

BondMaster 1000e+ (Olympus NDT)• Light-weight device (2 kg) for mechanical impedance and sonic testing

• Three inspection modes: MIA, pitch-catch and resonance• NTP: pitch-catch mode

� Dual-element, point-contact UT probe (transmitter-receiver) that measures amplitude and phase changes

� Three inspection methods: RF, impulse and swept frequency

Pitch-catch technique


Evaluation of NDI methods- handheld UT camera -

AcoustoCam i500 (Imperium, Inc.)• Handheld camera for fast and real-time UT inspection• C-scan image by array transducer (120x120 elements)• Field of view 1 inch2, frequency range 1 – 7.5 MHz• Couplant necessary

i600 i500



Presentation of C-scan image and A-scan



Examples of AcoustoCam images on panel NTP-A1



Examples of AcoustoCam images panel NTP-A1


Evaluation of NDI methods- phased array UT -

Omniscan (Olympus NDT)with a 128-element PA transducer

Multiple element transducer for electronic scanning, steering and focusing of the UT beam

Real-time imaging of defects during scanning



Omniscan (Olympus NDT)- 5 MHz, 128-element PA transducer- wedge + encoder- UT coupling by application of water film



NTP-B

BW skin BW stiffener Time-of-flight


Evaluation of NDI methods- UT dry coupling roller probe -

RapidScan (Sonatest Ltd.)• UT phased array probe housed within in a rubber coupled and water-filled wheel probe (50 or 100 mm active array)

• Almost couplant-free (fine water spray on test part provides adequate coupling)

• Manual scan or 3D scan with 7-axis Faro scanning arm

50 mm probe, 64 PA elements


Evaluation of NDI methods- UT dry coupling roller probe -

NTP-B

Ampl. wide gate BW skin Time-of-flight


Evaluation of NDI methods- shearography inspection -

ISISmobile 3100mobile equipment(Steinbichler)

- Thermal loading (3 kW heating lamps)- Vacuum loading- FOV 220x160 mm

Optical method based on speckle interferometry for measurement of out-of-plane deformations



Mobile systemthermal load technique

impact locations

NTP-B



Stationary systemvacuum load technique

NTP-C


Evaluation of NDI methods- thermography inspection -

External heat source- Lockin (low-freq. modulated heating)- Transient (pulse of long t: > 10 s)- Flash (pulse of short t: << 1 s)

Theolt, NL

FLIR Systems IR camera ThermoCAM SC6000

Measurement of infrared radiation after thermal stimulation of an object with a heat source


Evaluation of NDI methods- thermography inspection -

NTP-D

Lockin technique Transient technique


Summary of evaluation of NDI methods

Defect detection/sizing/depth estimation and portability0 not, - with limitation, + yes, ++ very well (primary technique)

Yellow – positive, white – with limitation, grey - negative


Conclusions

•Visual inspection� Primary method for in-service inspection of composites� Low-cost, relatively fast and capable of detecting relevant impact damages (initial dent depth > 0.5 mm)

•Automated tap tester Woodpecker� Low-cost, couplant-free inspection unit for smaller areas where damage is suspected

� Impact damages generally well detectable� Detectability for delaminations and disbonds is varying and not always consistent

•BondMaster 1000e+� Relatively low-cost, couplant-free instrument for local inspection of structures with the pitch-catch technique

� Limited detection performance for in-service defects


Conclusions (continued)

•Ultrasonic inspection� Primary method for in-service inspection of composites, regarding its capability for defect characterization

� Limitation can be the requirement to use couplant

AcoustoCam� Handheld ultrasonic imaging camera that produces a C-scan image real-time

� Limited field of view (~1 inch2)

Ultrasonic phased-array inspection� Best capabilities for in-service inspection of composites� Position encoder is required to produce a C-scan image

RapidScan� Phased array, handheld roller probe, almost couplant-free� With a multi-axis scanning arm: fast and real-time UT inspection of relatively large areas


Conclusions (continued)

•Shearography and thermography� Relatively fast, non-contact methods that require no coupling

or complex scanning equipment

� Impact damages are readily detectable but detectability for

delaminations and disbonds is poor/moderate when

compared to UT

� Detectable defect size decreases with increasing defect depth

� Both techniques not suited for defect depth estimation

Options

� Shearography may be promising for inspection of honeycomb

sandwich structures

� Thermography may be promising for fast inspections and

inspection of water ingress in composite structures


Recommendations

• Evaluate other inspection configurations

� Curved panels (curvature can limit the application of e.g.

large phased array probes)

� Panels with other defect types (e.g. water ingress in

honeycomb structures)

• Investigate the NDI of repaired composite structure

� Delaminations within a repair patch

� Disbond (or poor bonding) between patch and sub-structure

� Delaminations in the sub-structure underneath the repair

� Impact damage at/near the repair location


Questions?

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Evaluation of NDI methods

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