The development of a new generation of high temperature ceramic materials for aerospace applications, reinforced at a scale closer to the molecular level and three orders of magnitude less than conventional fibrous reinforcements, by embedded carbon nanotubes, has recently emerged as a uniquely challenging scientific effort. The properties of such materials depend strongly on two main factors: i) the homogeneity of the dispersion of the hydrophobic medium throughout the ceramic volume and ii) the ultimate density of the resultant product after sintering of the green body at the high-temperatures and pressures required for ceramic consolidation. The present works reports the establishment of two independent experimental strategies which ensure achievement of near perfect levels of tube dispersion homogeneity and fully dense final products. The proposed methodologies are validated across non-destructive evaluation data of materials performance.
The amount of construction and demolition waste has increased considerably over the last few years, making desirable the reuse of this waste in the concrete industry. In the present study concrete specimens are subjected at the age of 28 days to four-point bending with concurrent monitoring of their acoustic emission (AE) activity. Several concrete mixtures prepared using recycled aggregates at various percentages of the total coarse aggregate and also a reference mix using natural aggregates, were included to investigate their influence of the recycled aggregates on the load bearing capacity, as well as on the fracture mechanisms. The results reveal that for low levels of substitution the influence of using recycled aggregates on the flexural strength is negligible while higher levels of substitution lead into its deterioration. The total AE activity, as well as the AE signals emitted during failure, was related to flexural strength. The results obtained during test processing were found to be in agreement with visual observation.
Nowadays, construction and demolition waste constitutes a major portion of the total solid waste production in the world. Due to both environmental and economical reasons, an increasing interest concerning the use of recycled aggregate to replace aggregate from natural sources is generated. This paper presents an investigation on the properties of recycled aggregate concrete. Concrete mixes are prepared using recycled aggregates at a substitution level between 0 and 100% of the total coarse aggregate. The influence of this replacement on strengthened concrete’s properties is being investigated. The properties estimated are: density and dynamic modulus of elasticity at the age of both 7 and 28 days. Also, flexural strength of 28 days specimens is estimated. The determination of the dynamic elastic modulus was made using the ultrasonic pulse velocity method. The results reveal that the existence of recycled aggregates affects the properties of concrete negatively; however, in low levels of substitution the influence of using recycled aggregates is almost negligible. Concluding, the controlled use of recycled aggregates in concrete production may help solve a vital environmental issue apart from being a solution to the problem of inadequate concrete aggregates.
The addition of a conductive admixture in a cement-based material could lead to innovative products with multifunctional features. These materials are designed to possess enhanced properties, such as improved mechanical properties, electrical and thermal conductivity, and piezo-electric characteristics. Carbon nanotubes (CNTs) can be used as nano-reinforcement in cement-based materials because of their huge aspect ratio as well as their extremely large specific surface area. For cement-based composites, one of the major types of environmental attack is the chloride ingress, which leads to corrosion of the material and, subsequently, to the reduction of strength and serviceability of the structure. A common method of preventing such deterioration is to avert chlorides from penetrating the structure. The penetration of the concrete by chloride ions is a slow process. It cannot be determined directly in a time frame that would be useful as a quality control measure. Therefore, in order to assess chloride penetration, a test method that accelerates the process is needed, to allow the determination of diffusion values in a reasonable time. In the present research, nanomodified mortars with various concentrations of multi-wall carbon nanotubes (0.2% wt. cement CNTs - 0.6% wt. cement CNTs) were used. The chloride penetration in these materials was monitored according to ASTM C1202 standard. This is known as the Coulomb test or Rapid Chloride Permeability Test (RCPT).
Cement-based materials have in general low electrical conductivity. Electrical conductivity is the measure of the ability of the material to resist the passage of electrical current. The addition of a conductive admixture such as Multi-Walled Carbon Nanotubes (MWCNTs) in a cement-based material increases the conductivity of the structure. This research aims to characterize nano-modified cement mortars with MWCNT reinforcements. Such nano-composites would possess smartness and multi-functionality. Multifunctional properties include electrical, thermal and piezo-electric characteristics. One of these properties, the electrical conductivity, was measured using a custom made apparatus that allows application of known D.C. voltage on the nano-composite. In this study, the influence of different surfactants/plasticizers on CNT nano-modified cement mortar specimens with various concentrations of CNTs (0.2% wt. cement CNTs - 0.8% wt. cement CNTs) on the electrical conductivity is assessed.
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.
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.
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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