This research aims in characterizing modified cement mortar with carbon nanotubes (CNTs) that act as nanoreinforcements
leading to the development of innovative materials possessing multi-functionality and smartness. Such
multifunctional properties include enhanced mechanical behavior, electrical and thermal conductivity, and piezo-electric
characteristics. The effective thermal properties of the modified nano-composites were evaluated using IR
Thermography. The electrical resistivity was measured with a contact test method using a custom made apparatus and
applying a known D.C. voltage. To eliminate any polarization effects the specimens were dried in an oven before testing.
In this work, the thermal and electrical properties of the nano-modified materials were studied by nondestructively
monitoring their structural integrity in real time using the intrinsic multi-functional properties of the material as damage
sensors.
The introduction of nanoscaled reinforcement in otherwise conventional fiber reinforced composite materials has opened
an exciting new area in composites research. The unique properties of these materials combined with the design
versatility of fibrous composites may offer both enhanced mechanical properties and multiple functionalities which has
been a focus area of the aerospace technology on the last decades. Due to unique properties of carbon nanofillers such as
huge aspect ratio, extremely large specific surface area as well as high electrical and thermal conductivity, Carbon
Nanotubes have benn investigated as multifunvtional materials for electrical, thermal and mechanical applications.
In this study, MWCNTs were incorporated in a typical epoxy system using a sonicator. The volume of the
nanoreinforcement was 0.5 % by weight. Two different levels of sonication amplitude were used, 50% and 100%
respectively. After the sonication, the hardener was introduced in the epoxy, and the system was cured according to the
recommended cycle. For comparison purposes, specimens from neat epoxy system were prepared. The
thermomechanical properties of the materials manufactured were investigated using a Dynamic Mechanical Analyser.
The exposed specimens were subjected to thermal shock. Thermal cycles from +30 °C to -30 °C were carried out and
each cycle lasted 24 hours. The thermomechanical properties were studied after 30 cycles .
Furthermore, the epoxy systems prepared during the first stage of the study were used for the manufacturing of 16 plies
quasi isotropic laminates CFRPs. The modified CFRPs were subjected to thermal shock. For comparison reasons
unmodified CFRPs were manufactured and subjected to the same conditions. In addition, the interlaminar shear strength
of the specimens was studied using 3-point bending tests before and after the thermal shock. The effect of the
nanoreinforcement on the environmental degradation is critically assessed.
The remarkable mechanical and electrical properties exhibited by carbon nanotubes (CNTs) have encouraged efforts to
develop mass production techniques. As a result, CNTs are becoming increasingly available, and more attention from
both the academic world and industry has focused on the applications of CNTs in bulk quantities. These opportunities
include the use of CNTs as conductive filler in insulating polymer matrices and as reinforcement in structural materials.
The use of composites made from an insulating matrix and highly conductive fillers is becoming more and more
important due to their ability to electromagnetically shield and prevent electrostatic charging of electronic devices. In
recent years, different models have been proposed to explain the formation of the conductive filler network. Moreover,
intrinsic difficulties and unresolved issues related to the incorporation of carbon nanotubes as conductive fillers in an
epoxy matrix and the interpretation of the processing behavior have not yet been resolved. In this sense, a further
challenge is becoming more and more important in composite processing: cure monitoring and optimization. This paper
considers the potential for real-time control of cure cycle and dispersion of a modified epoxy resin system commonly
utilized in aerospace composite parts. It shows how cure cycle and dispersion control may become possible through realtime
in-situ acquisition of dielectric signal from the curing resin, analysis of its main components and identification of the
significant features.
The attainment of structural integrity of the reinforcing matrix in composite materials is of primary importance for the
final properties of the composite structure. The detailed monitoring of the curing process on the other hand is paramount
(i) in defining the optimal conditions for the impregnation of the reinforcement by the matrix (ii) in limiting the effects of
the exotherm produced by the polymerization reaction which create unwanted thermal stresses and (iii) in securing
optimal behavior in matrix controlled properties, such as off axis or shear properties and in general the durability of the
composite. Dielectric curing monitoring is a well known technique for distinguishing between the different stages of the
polymerization of a typical epoxy system. The technique successfully predicts the gelation and the vitrification of the
epoxy and has been extended for the monitoring of prepregs. Recent work has shown that distinct changes in the
properties of the propagated sound in the epoxy which undergoes polymerization is as well directly related to the gelation
and vitrification of the resin, as well as to the attainment of the final properties of the resin system.
In this work, a typical epoxy is simultaneously monitored using acoustic and dielectric methods. The system is
isothermally cured in an oven to avoid effects from the polymerization exotherm. Typical broadband sensors are
employed for the acoustic monitoring, while flat interdigital sensors are employed for the dielectric scans. All stages of
the polymerization process were successfully monitored and the validity of both methods was cross checked and verified.
The increasing use of composite materials in aerostructures has prompted the development of an effective structural
health monitoring system. A safe and economical way of inspection is needed in order for composite materials to be used
more extensively. Critical defects may be induced during the scheduled repair which may degrade severely the
mechanical properties of the structure. Low velocity impact LVI damage is one of the most dangerous and very difficult
to detect types of structural deterioration as delaminations and flaws are generated and propagated during the life of the
structure. In that sense large areas need to be scanned rapidly and efficiently without removal of the particular
components. For that purpose, an electrical potential mapping was employed for the identification of damage and the
structural degradation of aerospace materials. Electric current was internally injected and the potential difference was
measured in order to identify induced damage in Carbon Fiber Reinforced Polymer (CFRP) structures. The experimental
results of the method were compared with conventional C-scan imaging and evaluated.
Infrared Thermography (IrT) has been shown to be capable of detecting and monitoring service induced damage of
repair composite structures. Full-field imaging, along with portability are the primary benefits of the thermographic
technique. On-line lock-in thermography has been reported to successfully monitor damage propagation or/and stress
concentration in composite coupons, as mechanical stresses in structures induce heat concentration phenomena around
flaws. During mechanical fatigue, cyclic loading plays the role of the heating source and this allows for critical and
subcritical damage identification and monitoring using thermography. The Electrical Potential Change Technique
(EPCT) is a new method for damage identification and monitoring during loading. The measurement of electrical
potential changes at specific points of Carbon Fiber Reinforced Polymers (CFRPs) under load are reported to enable the
monitoring of strain or/and damage accumulation. Along with the aforementioned techniques Finally, Acoustic Emission
(AE) method is well known to provide information about the location and type of damage. Damage accumulation due to
cyclic loading imposes differentiation of certain parameters of AE like duration and energy. Within the scope of this
study, infrared thermography is employed along with AE and EPCT methods in order to assess the integrity of bonded
repair patches on composite substrates and to monitor critical and subcritical damage induced by the mechanical loading.
The combined methodologies were effective in identifying damage initiation and propagation of bonded composite
repairs.
Composite materials are widely used especially in the aerospace structures and systems. Therefore, inexpensive and
efficient damage identification is crucial for the safe use and function of these structures. In these structures low-velocity
impact is frequently the cause of damage, as it may even be induced during scheduled repair. Flaws caused by lowvelocity
impact are dangerous as they may further develop to extended delaminations. For that purpose an effective
inspection of defects and delaminations is necessary during the service life of the aerospace structures. Within the scope
of this work, an innovative technique is developed based on current stimulating thermography. Electric current is
injected to Carbon Fiber Reinforced Composite and aluminium (Al) plates with concurrent thermographic monitoring.
For reference, both damaged and undamaged plates are inspected. Low-velocity impact damaged composite laminates at
different energy levels are interrogated employing the novel technique. Live and pulse phase infrared thermography is
employed for the identification of low-velocity impact damage at various energy levels while the electric current induces
the transient thermal field in the vicinity of the defect. In all cases conventional ultrasonics (C-scan) were performed for
the validation and assessment of the results of the innovative thermographic method.
KEYWORDS: Composites, Acoustic emission, Sensors, Nondestructive evaluation, Wave propagation, Manufacturing, Control systems, Acoustics, Picture Archiving and Communication System, Signal processing
This study deals with the investigation of cross ply composites failure by acoustic emission (AE). Broadband AE sensors
monitor the different sources of failure in coupons of this material during a tensile loading-unloading test. The
cumulative number of AE activity, and other qualitative indices based on the shape of the waves, were well correlated to
the sustained load. AE parameters indicate the shift of failure mechanisms within the composite as the load increases.
The ultimate goal is a methodology based on NDT techniques for real time characterization of the degradation and
identification of the fracture stage of advanced composite materials.
Thermographic techniques offer distinct advantages over other techniques usually employed to assess damage
accumulation and propagation. Among the advantages of these techniques are the fully remote-non contact monitoring
and their ability for full field imaging. Due to the transient nature of the heat transfer phenomenon, phase and lock-in
techniques are of particular interest in order to increase the resolution of the signal or provide depth discrimination. Last
but not least, when a structure is subjected to load, these techniques can be used in order to monitor the irreversible
damage phenomena, as manifested by the local heat accumulation in the vicinity of the defect. This eliminates the need
for external heat source, as any cyclic loading can induce the heat gradient necessary to pinpoint the defect accumulation
and propagation.
In the aforementioned context, lock-in thermography has been employed to monitor the delamination propagation in
composites and the critical failure of bonded repairs when the materials are subjected to fatigue loading. Lock-in
thermography proved successful in identifying debonding initiation and propagation as well in depicting the
thermoelastic stress field around purposely induced discontinuities.
The role of coating in preserving the bonding between steel fibers and concrete is investigated in this paper. Straight
types of fibers with and without chemical coating are used in steel fiber reinforced concrete mixes. The specimens are
tested in bending with concurrent monitoring of their acoustic emission activity throughout the failure process using two
broadband sensors. The different stages of fracture (before, during and after main crack formation) exhibit different
acoustic fingerprints, depending on the mechanisms that are active during failure (concrete matrix micro-cracking,
macro-cracking and fiber pull out). Additionally, it was seen that the acoustic emission behaviour exhibits distinct
characteristics between coated and uncoated fiber specimens. Specifically, the frequency of the emitted waves is much
lower for uncoated fiber specimens, especially after the main fracture incident, during the fiber pull out stage of failure.
Additionally, the duration and the rise time of the acquired waveforms are much higher for uncoated specimens. These
indices are used to distinguish between tensile and shear fracture in concrete and suggest that friction is much stronger
for the uncoated fibers. On the other hand, specimens with coated fibers exhibit more tensile characteristics, more likely
due to the fact that the bond between fibers and concrete matrix is stronger. The fibers therefore, are not simply pulled
out but also detach a small volume of the brittle concrete matrix surrounding them. It seems that the effect of chemical
coating can be assessed by acoustic emission parameters additionally to the macroscopic measurements of ultimate
toughness.
The variation of the electrical properties of fiber reinforced polymers when subjected to load offer the ability of strain
and damage monitoring. This is performed via electrical resistance and electrical potential measurements. On the other
hand Carbon Nanotubes (CNTs) have proved to be an efficient additive to polymers and matrices of composites with
respect to structural enhancement and improvement of the electrical properties. The induction of CNTs increases the
conductivity of the matrix, transforming it to an antistatic or a conducting phase. The key issue of the structural and
electrical properties optimization is the dispersion quality of the nano-scale in the polymer phase. Well dispersed CNTs
provide an electrical network within the insulating matrix. If the fibers are conductive, the CNT network mediates the
electrical anisotropy and reduces the critical flaw size that is detectable by the change in conductivity. Thus, the network
performs as an inherent sensor in the composite structure, since every invisible crack or delamination is manifested as an
increase in the electrical resistance. The scope of this work is to further exploit the information provided by the electrical
properties with a view to identify strain variation and global damage via bulk resistance measurements. The
aforementioned techniques were employed to monitor, strain and damage in fiber reinforced composite laminates both
with and without conductive nanofillers.
Bonded repair offers significant advantages over mechanically fastened repair schemes as it eliminates
local stress concentrations and seals the interface between the mother structure and the patch.
However, it is particularly difficult to assess the efficiency of the bonded repair as well as its
performance during service loads. Thermography is a particularly attractive technique for the particular
application as it is a non-contact, wide field non destructive method. Phase thermography is also
offering the advantage of depth discrimination in layered structures such as in typical patch repairs particularly in the case where composites are used. Lock-in thermography offers the additional advantage of on line monitoring of the loaded structure and subsequently the real time evolution of any progressive debonding which may lead to critical failure of the patched repair. In this study composite systems (CFRP plates) with artificially introduced defects (PTFE) were manufactured. The aforementioned methods were employed in order to assess the efficiency of the thermographic technique. The obtained results were compared with typical C-scans.
The acoustic emission (AE) behaviour of steel fibre reinforced concrete is studied in this paper. The experiments were
conducted in four-point bending with concurrent monitoring of AE signals. The sensors used, were of broadband
response in order to capture a wide range of fracturing phenomena. The results indicate that AE parameters undergo
significant changes much earlier than the final fracture of the specimens, even if the AE hit rate seems approximately
constant. Specifically, the Ib-value which takes into account the amplitude distribution of the recent AE hits decreases
when the load reaches about 60-70 % of its maximum value. Additionally, the average frequency of the signals decreases
abruptly when a fracture incident occurs, indicating that matrix cracking events produce higher frequencies than fibre
pull-out events. It is concluded that proper study of AE parameters enables the characterization of structural health of large structures in cases where remote monitoring is applied.
Acoustic emission is a powerful technique for identifying and monitoring the evolution of service induced degradation in
structural components and localising damage. The present study is dedicated to the investigation of model composite
systems in order to identify, locate and quantify service induced damage. These systems are cross ply translucent glass
fibre reinforced composite materials. In cross ply composites, service induced primary damage is manifested in the form
of matrix cracking of the off-axis layers. For the purposes of this study, the cross ply composite were subjected to step
loading with the concurrent recording of the acoustic activity. At specific intervals of the loading process the propagation
characteristics of ultrasonic waves were also recorded using the acoustic emission sensors in a pulser-receiver setup. The
acoustic emission activity has been successfully correlated to damage accumulation of the cross ply laminates, while
specific acoustic emission indices proved sensitive to the various modes that evolve during the loading.
This work deals with the AE behavior of concrete under four-point bending. Different contents of steel fibers were
included to investigate their influence on the load-bearing capacity and on the fracture mechanisms. The AE waveform
characteristics revealed that, although tension was the dominant mechanism of fracture for the plain material, the
increase in the fiber content resulted in extension of the shear failure due to improvement of the weak tensile properties
of concrete. Appropriate AE indices employed for early warning prior to macroscopic failure can lead to more suitable
design of the reinforcement, in order to withstand the specific stresses.
This study deals with new generation composite systems which apart from the primary reinforcement at the typical fiber
scale (~10 μm) are also reinforced at the nanoscale. This is performed via incorporation of nano-scale additives in typical
aerospace matrix systems, such as epoxies. Carbon Nanotubes (CNTs) are ideal candidates as their extremely high aspect
ratio and mechanical properties render them advantageous to other nanoscale materials. The result is the significant
increase in the damage tolerance of the novel composite systems even at very low CNT loadings. By monitoring the
resistance change of the CNT network, information both on the real time deformation state of the composite is obtained
as a reversible change in the bulk resistance of the material, and the damage state of the material as an irreversible
change in the bulk resistance of the material. The irreversible monotonic increase of the electrical resistance can be
related to internal damage in the hybrid composite system and may be used as an index of the remaining lifetime of a
structural component.
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