development of lightweight structural health monitoring systems for aerospace applications

297 Pages·2013·12.68 MB·English

DEVELOPMENT OF LIGHTWEIGHT
STRUCTURAL HEALTH MONITORING
SYSTEMS FOR AEROSPACE APPLICATIONS

PhD Thesis
February 2013

Matthew Pearson (MEng)

Cardiff School of Engineering
Cardiff University
Cardiff
UK
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currently submitted in candidature for any degree.
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This thesis is being submitted in partial fulfilment of the requirements for the degree
of PhD
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This thesis is the result of my own independent work/investigation, except where
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Prifysgol Caerdydd SUMMARY OF THESIS Cardiff University
Candidate’s Surname: Pearson Institute at which study pursued:
Candidate’s Forenames: Matthew Robert Cardiff University
Candidate for Degree of: PhD
Full Title of Thesis: Development of Lightweight Structural Health Monitoring Systems for
Aerospace Applications
Summary:
This thesis investigates the development of structural health monitoring systems (SHM) for
aerospace applications. The work focuses on each aspect of a SHM system covering novel
transducer technologies and damage detection techniques to detect and locate damage in
metallic and composite structures. Secondly the potential of energy harvesting and power
management methodologies to provide a stable power source is assessed. Finally
culminating in the realisation of smart SHM structures.
1. Transducer Technology
A thorough experimental study of low profile, low weight novel transducers not normally used
for acoustic emission (AE) and acousto-ultrasonics (AU) damage detection was conducted.
This included assessment of their performance when exposed to aircraft environments and
feasibility of embedding these transducers in composites specimens in order to realise smart
structures.
2. Damage Detection
An extensive experimental programme into damage detection utilising AE and AU were
conducted in both composites and metallic structures. These techniques were used to
assess different damage mechanism within these materials. The same transducers were
used for novel AE location techniques coupled with AU similarity assessment to successfully
detect and locate damage in a variety of structures.
3. Energy Harvesting and Power Management
Experimental investigations and numerical simulations were undertaken to assess the power
generation levels of piezoelectric and thermoelectric generators for typical vibration and
temperature differentials which exist in the aerospace environment. Furthermore a power
management system was assessed to demonstrate the ability of the system to take the
varying nature of the input power and condition it to a stable power source for a system.
4. Smart Structures
The research conducted is brought together into a smart carbon fibre wing showcasing the
novel embedded transducers for AE and AU damage detection and location, as well as
vibration energy harvesting. A study into impact damage detection using the techniques
showed the successful detection and location of damage. Also the feasibility of the
embedded transducers for power generation was assessed.
Key words: Structural health monitoring, damage detection, energy harvesting, power
management,
Acknowledgements
I would like to take this opportunity to express a great debt of gratitude to my
academic supervisors Prof. Karen Holford, Dr Carol Featherston and Dr. Rhys Pullin for their
guidance, immense support and patience throughout my work. I would also thank Dr. Mark
Eaton for his advice, help and endless answering of questions. My thanks also to the
technical staff at Cardiff School of Engineering especially Steve Mead, Paul Farrugia, Denley
Slade, Richard Rogers and Harry Lane for their help, support and technical advice
throughout my experimental investigations.
I am also grateful to Tim Bradshaw of Mistras and Christophe Paget of Airbus for the
technical feedback and advice
Finally thanks to my parents, sister and the rest of my family as well as girlfriend,
Mayada for supporting me though out the years and to my friends who have often reminded
to keep smiling and to have fun.
Glossary
Terms relating to AE (ASTM 1982)
Hit - indication that a given AE channel has detected and processed an acoustic emission
transient.
Event –A group of AE hits that was received from a single source.
Source - A mechanical mechanism that produces AE signals.
Terms relating to the detection of the signal:
Acoustic emission signal - The electrical signal obtained through the detection of acoustic
emission.
Couplant - Substance providing an acoustic coupling between the propagation medium and
the transducer.
Transducer - Device that converts the physical parameters of the wave into an electrical
signal.
Terms relating to the processing of the signal:
Threshold - A preset voltage level, which has to be exceeded before an AE signal is
detected, and processed. The following terms are made with reference to the threshold
(Figure i)

Figure i. AE waveform features
Duration - The interval between the first and last time the threshold was exceeded by the
signal.
Peak Amplitude - Maximum signal amplitude within the duration of the signal.
Counts - Number of times the signal amplitude exceeds the threshold.
Rise Time - The interval between the first threshold crossing and the maximum amplitude of
the signal.
Initiation Frequency - The average frequency of the waveform from the initial threshold
crossing to the peak of the AE waveform.
Energy (Absolute) - The integral of the squared voltage signal divided by the reference
resistance (10kOhm) over the duration of the AE waveform packet.
Terms relating to wave propagation:
Dispersion - The phenomenon whereby wave velocity varies with frequency.
Group wave velocity - The perceived velocity at which a packet of energy (or wave packet)
travels.
Phase wave velocity - Velocity of individual waves within a packet of energy (or wave
packet), each wave may travel at a different velocity (see dispersion). Phase velocity does
not have to equal group velocity.
Attenuation - The rate at which signal amplitude is reduces with distance of propagation.
S mode – Symmetrical or extensional fundamental Lamb wave mode that propagates in
0
plate like materials and are sensitive to in-plane damage.
A mode - Asymmetric or flexural fundamental Lamb wave mode that propagates in plate
0
like materials and are sensitive to out of place damage.
Hsu-Nielson (H-N) Source – An artificial source of AE (Hsu and Breckenbridge 1981)
Time of Arrival (TOA) – Conventional AE source location algorithm used to located AE
sources in structure. Using the difference in arrival times between transducers and a known
wave velocity to estimate the location of an AE event by minimising the error between the
measured and calculated different in arrival times.
Delta-T mapping technique – Advanced AE location technique, an area of interest is
identified on a structure. Artificial AE source is used at node positions within the interest area
and resulting time of arrival recorded at the transducers. This enables the generation of
different in arrival time contour maps for each transducer pairing. These are used to indentify
contours when trying to located actual AE events, the maps are overlaid at the intersection
of the contours corresponds to an estimate source location.
AIC delta-T mapping technique – Uses the same technique as above but uses the Akiakie
Information Criteria (AIC) (Maeda 1985) to determine the onset of an AE wave. The AIC
uses the entropy of the signal to determine between when the structures has structure and
when it has not i.e. between signal and noise.
Wavestream – Raw AE activity recorded for a set time period irrespective of the threshold
used.
Wavelet Transform – Signal processing technique which decomposes a transient signal in
order to release a time frequency representation of a wave.
Acousto-ultrasonics – An active monitoring technique which utilises the fundamentals of
guided Lamb waves(GLW) to detect damage in structure. An actuation signal is used to
generate GLW in a structure by using surface mounted or embedded piezoelectric
transducers. The resulting waves are recorded at receivers, where changes in the signal are
observed due to scattering and mode conversion due to the presence of damage in the
structure.
Cross Correlation – Signal processing technique which can be utilised in AU applications
to determine whether damage is present in a structure. Uses the integral of the products of
two signals to determine the similarity of the shape of two signals with respect to each other.
Ultrasonic C-scanner – An ultrasonic technique which is used to detect, measure and
characterise a range of manufacturing and in-service defects in composites materials.
Thermoelastic stress analysis - The TSA system detects the small temperature changes
occurring in a material subject to cyclic transient loading, where the change in temperature
observed is directly related to the stress experienced by the material. The system provides a
full-field measurement; hence any redistribution of the stress field resulting from damage
onset can be detected.
Kernal Density Estimation - This is a statistical analysis of the location data which assess
the probability of where a location is most likely to occur based on a statistical analysis of the
resulting located events.
Digital Image Correlation - An optical technique that allows for the full field measurement of
contour deformation, vibration and strain by tracking the pixels of a black and white speckle
pattern applied to the specimen from a baseline image to those under load.
Energy harvesting – The process of scavenging energy from the envrioment using a
variety of methods for example piezoelectric and thermoelectric energy harvesting.
Piezoelectric effect – The ability of certain crystals to become electrically polarised when
they were subjected to a mechanical strain, the amount of this polarisation is proportional to
the mechanical strain.
d mode – Mode of operation of a piezoelectric material where the applied force and the
33
poling direction are parallel
d mode - Mode of operation of a piezoelectric material where the applied force and the
31
poling direction are perpendicular
Permittivity – Or the dielectric constant is defined as the dielectric displacement per unit
electric field.
Compliance – Is defined as the strain produced per unit stress.
Piezoelectric charge constant – Defined as the electric polarisation per unit mechanical
stress applied, or the mechanical strain experience by the material per unit electric field
applied.
Piezoelectric voltage constant – Defined as the electric field generated in a material per
unit mechanical stress applied to it, or the mechanical strain experience by the material per
unit electric displacement applied.
Coupling factor – A measure of the effectiveness with which electrical energy is converted
to mechanical energy and vice versa.
Polarisation – The process of making a ceramic piezoelectric in a given direction by
applying a strong electric field at a temperature slightly below the Curie point.
Curie point - The temperature at which spontaneous polarisation is lost.
Seebeck effect - Promotes a thermoelectric electromagnetic field across two different semi-
conductors when their junctions are placed across a temperature gradient.
Thermoelectric generator (TEG) – Uses the principle of the Seebeck effect to generate a
voltage from an applied temperature differential
Power Management - A power management system is therefore required to regulate the
dynamic input power from an energy harvesting and provide a stable output power for a load
device
Smart structure – A structure which has sensing devices embedded within the structure
which are used for SHM techniques.
Nomenclature
Δt Difference in arrival times between transducer pairs (s)
C Wave velocity in the medium (ms-1)
D Distance between transducers (m)
s
d Distance from source to first hit transducers (m)
2
d Distance from source to seconds hit transducer (m)
1
h Distance between the source and the line connecting transducers (m)
Δt Difference in observed arrival time between transducers pairs (s)
i,obs
Δt Difference in calculated arrival time between transducer pairs (s)
i,calc
(X ,Y ) X and Y positions of the source (m)
s s
(X ,Y ) X and Y location of the first hit transducer (m)
1 1
(X,Y) X location and Y location of transducer i (m)
i i
t Current sample point
T Final sample point
Var Variance (V2)
x[1;t] Waveform from first sample to current sample (V)
x[1;T] Waveform from current sample to final sample (V)
C Coherence between two waveforms
xy
P Power spectrum density of a time series x(t) (V2.Hz-1)
xx
P Power spectrum density of a time series y(t) (V2.Hz-1)
yy
P Power spectrum density the function x(t)*y(t) (V2.Hz-1)
xy
Y Young‘s Modulus (Pa)
ν Poisson‘s Ratio
ζ Tensile yield strength (Pa)
y
ζ Ultimate tensile strength (Pa)
UTS
f Force (N)
ρ Density (Kg.m-3)
δ Mechanical strain
ζ Mechanical stress (Pa)
d Piezoelectric charge constant (C.N-1)
E Electric field (N.C-1)
D Electric displacement/charge density (C.m-2)
ε Permittivity (F.m-1)
s Compliance (m2.N-1)
g Piezoelectric voltage constant (V.m.N-1)
k Coupling factor