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How NDT will underpin the certification
of greener aircraft
Robert A Smith
Professor of NDT and High Value manufacturing,
University of Bristol.
President – British Institute of NDT
Acknowledgements
• Prof Stephen Hallett
• Dr Luke Nelson
• Dr Martin Mienczakowski
• Dr Rostand Tayong
• Miss Ningbo Xie (PhD student)
• Acknowledgements
• The Green Challenge
• Composite materials
• Non-destructive Testing (NDT) and aircraft design
• Ultrasound
• Microscopic 3D imaging
• NDT-based prediction of strength
• Conclusions - Will Green Aircraft Fly?
Content
Environmental targets:
• Advisory Council for Aeronautical Research in Europe (ACARE) target:
• 50% reduction in CO2 per passenger-km by 2020
• ‘FlightPath 2050’ (EU) targets:
• 75% reduction in CO2 per passenger-km by 2050
• 65% reduction in noise by 2050
The Green Challenge
• Transport-mode CO2 trends projected to 2050
The Green Challenge
Source: Road & Rail – TREMOVE v3.3.1 (EU-27 + 3 countries) Air – TRENDS (EU-15 countries)
• Air CO2 trends projected to 2050, with targets
The Green Challenge
Source: TRENDS (EU-15 countries)
The Green Challenge
AIRBUS Concept Plane Fuel-efficient (green) design
Longer, slimmer wings reduce drag.
Engines at rear and semi-embedded. Reliable, quiet and fuel-efficient.
U-shaped tail shields noise.
Images: Airbus
The Green Challenge
• Environmentally-friendly Aircraft
• Low CO2 emissions
• Low noise
• Low fuel burn
• High speed
• Low hassle
• Low-cost flights
• Perfectly safe
• Let’s look at safety…
Image: Airbus
The amazing safety of air travel
Source: Bureau of Aircraft Accidents Archive Source: ICAO http://www.theguardian.com/world/ng-interactive/2014/aviation-100-years
The amazing safety of air travel
Source: Bureau of Aircraft Accidents Archive Source: ICAO
1/8bn
http://www.theguardian.com/world/ng-interactive/2014/aviation-100-years
The amazing safety of air travel
Source: Bureau of Aircraft Accidents Archive Source: ICAO
NDT is not solely responsible for this increase in safety but it is largely the reason why air travel is still affordable and (reasonably) delay-free whilst also being safe.
The amazing safety of air travel
• Safety is improving consistently due to:
• advanced modelling and simulation techniques
• rigorous testing of specimens, components and full-scale tests such as on wings…
The amazing safety of air travel
• Safety is paramount
• The absolute level of safety demanded by certifying authorities will not decrease.
• Manufacturing variations / deviations
• do not reduce safety because:
• designs are made stronger to maintain safety levels
• NDT will be key to the achievable level of:
• environmental impact (CO2, noise, fuel burn, etc)
• cost (manufacture, maintenance, fuel, etc)
• reliability / hassle (take-off delays, cancellations)
- whilst achieving certification for safe flight.
• Better NDT opens up the design envelope by:
• increasing confidence, and
• reducing uncertainty in performance.
Why Wouldn’t Green Aircraft Fly?
Composite materials
Increased strength compared with the same weight of metal, allowing weight reduction.
• Carbon-fibre polymer-matrix (plastic) composites.
• Long fibres made of carbon, embedded in…
• Matrix – epoxy (eg Araldite®)
Composite materials
1 mm
• Composites use a stack of crossed fibres 45°/0°/-45°/90°
• Deviations from straight fibres reduce the strength.
Composite materials
0°
45°
90°
-45°
1 mm
6 mm
• Effect of fibre waviness.
Mukhopadhyay, Jones & Hallett (2013)
Composite materials
TENSION
COMPRESSION
6 mm
6 mm
Just prior to damage Fibre failure in axial ply
Delamination & further fibre failure
Full compressive failure
• How wrinkles are avoided
• Mould-shape optimisation
• Rigorous testing and sectioning of specimens
• Optimum sequence of plies, especially at corners
• Any remaining wrinkles are compensated by:
• adding extra plies (thickness) to maintain safety levels
Composite materials
• Metals are only replaced by composites if:
• Appropriate properties are considerably improved
• Other materials
• Used for a reason
• Lower cost, or
• Harder, or
• Heat tolerant
Composite materials
Material breakdown for an Airbus A350 (Stansbury, 2014)
Image: Airbus
• Few options for more composite components.
The future for composites?
Co
mp
osi
tes
Stru
ctu
ral W
eig
ht
[%]
80
70
60
50
40
30
20
10
0
1970 1980 1990 2000 2010 2020 2030 2040 2050
Maximum number of composite components
Non-destructive Testing (NDT)
and the design envelope
The more we can guarantee that performance of composite will be as expected, the happier the certifying authority will be to certify the lighter designs.
• Many of the visionary objectives are in conflict.
• Most of the technical challenges are expensive.
• Low emissions conflict with low noise in engines.
Why Wouldn’t Green Aircraft Fly?
Low CO2
High cost High noise
Low cost High CO2
High noise
• Design goals met by thinner components
• Less weight
• Less drag (friction)
• Less raw material
• Less fuel
• Less CO2
• Less noise
• At what cost?
NDT and the design envelope
Image: Rolls Royce. Composite fan test-bed.
• Manufacturing goals for thinner components
• High quality
• Confirmed quality
• High yield (success)
• High throughput
• Low cost
• NDT can help!
NDT and the design envelope
Image: Boeing
NDT and the design envelope
• The aim:
• To help meet both design AND manufacturing goals.
• Thinner components, made faster and cheaper.
Transmitting Transducer
Receiving Transducer
Loudspeaker Microphone
Ultrasound Transducers
Ultrasound Transducers
Audible sound Transducers
• Defects cause echoes that can be detected.
Ultrasonic NDT
Transducer
Specimen
F B
Time delay
F
B
Defect
Send pulse
• Plot the ‘brightness’ of the echo at every point.
• Standard medical image is a B-scan
B-scan Presentation
• Ultrasonic vibrations of a composite
• Composite plies are stacked like resonant tubes.
• Plies of fibres – like tubes.
• Resin layers – like air gaps.
Microscopic 3D imaging
• Ultrasonic vibrations of a composite
• Measure resonant frequency, calculate ply thickness
Microscopic 3D imaging
• ‘Ply drops’ can be shown to have a significant impact on laminate strength
Ply tracking through ply drops
3D tracking of resin inter-ply layers
• Ply-drop coupons
• X-ray CT: 49 kV, 20 μm voxel size, 4 shots
48
• In-plane fibre orientation
In-plane slice shows
Echoes at one time
Delay.
Variations due to the
Fibre bundles.
Microscopic 3D imaging
52
In-plane slice
• In-plane fibre orientation
In-plane slice shows
Echoes at one time
Delay.
Variations due to the
Fibre bundles.
Microscopic 3D imaging
53
In-plane slice
• 3D map of
• Fibre bunches
• 3D orientation
Microscopic 3D imaging
Dr Luke Nelson, Research Associate
• Predict performance of component.
• Materials model with actual 3D NDT data inputs
• Create a volume-element mesh using ply interface heights
• Populate with fibre directions
z y
x
NDT-based prediction of strength
Miss Ningbo Xie, PhD student
NDT-based prediction of strength
Modelled failure modes
• Delaminations
• Matrix crack (split wood)
• Fibre kink
Wrinkle Shape Parameters Analysis
62
Ultrasonics and NDT Group
𝐷𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 = 𝐴𝑒−𝑥2 𝑊1
2 cos2𝜋𝑥
𝐿
Wrinkle shape along x-direction: Wrinkle shape along y-direction:
X (fibre-direction)
(thickness-direction) Z
(width-direction) y
A: amplitude L: wavelength θ: maximum angle
x
Gaussian envelope: governs wrinkle volume
Cosine phase: defines wrinkle shape
Wrinkle Shape Parameters Analysis
63
Ultrasonics and NDT Group
The depiction of two amplitude distributions:
𝐴𝑖 = 𝐴𝑒− 𝑖−𝑖𝑚𝑖𝑑2/𝑛2
4-Interface Group reduction:
1.0:0.63:0.39:0.0
1.0
0.63
0.39
0.0
Gaussian reduction:
decrease continuously
0.63
0.39
0.0
decrease continuously
Wrinkle Shape Parameters Analysis
64
Ultrasonics and NDT Group
Knock-down of failure stress:
4-Interface Group reduction:
1.0:0.63:0.39:0.0
Co
mp
ress
ion
Fai
lure
Str
ess
Kn
ock
-do
wn
(M
Pa)
For a wrinkle region with fixed volume, maximum angle is the major parameter for determining compression strength.
Knock-down = tested pristine value (643.5 MPa ) - model value
Gaussian reduction:
𝐴𝑖 = 𝐴𝑒− 𝑖−𝑖𝑚𝑖𝑑2/𝑛2
• The future –
• 3D mapping and modelling of Porosity
• Further gains in weight due to extra confidence
Composite materials
• Increased confidence at the design stage allows:
• better understanding of the effect of defects
• designs that reduce weight, fuel and CO2
• design for low in-service maintenance
Reducing weight
• Increased confidence at manufacture allows:
• confirmation of conformance to design
• less need for extra thickness/weight to mitigate risk
• process-control feedback to increase manufacturing yield
Reducing weight