1,721,135 research outputs found
On the structure of the solution of continuous-time algebraic Riccati equations with closed-loop eigenvalues on the imaginary axis
Get PDFEffect of post-cure temperature and different reinforcements in adhesive bonded repair for damaged glass/epoxy composites under multiple quasi-static indentation loading
No full textThis paper investigates the individual and associated effects of post-cure temperature and reinforcement phases on the local bending response of adhesive reinforced repairs in damaged glass/epoxy composite laminates. Multiple quasi-static indentation tests were performed on repaired composite laminates: damage propagation, absorbed energy and residual deflection data assisted in the evaluation of phase changes, which cause the transition from brittle to ductile nature and control the visco-elastic behavior the material is undergoing under the influence of temperature. Three phases of fiber reinforcements, namely particulate glass fibers, chopped short glass fibers and continuous glass fibers, were tested to study the fiber reinforcement’s effect on indentation strength and damage mechanism. The effects of post-cure temperature on the local bending response of the repaired glass/epoxy specimens were investigated by performing indentation tests on laminates exposed to post-cure at ambient temperature (30 °C) and elevated temperatures up to glass transition temperature of epoxy resin (50, 70 and 90 °C). Multiple quasi-static indentation test results indicated that the post-cure temperature and fiber reinforcements have considerable effect on the indentation response of the repaired specimens. Damaged glass/epoxy specimens repaired using chopped short fibers and exposed to a post-cure temperature of 50 °C showed the most favorable indentation behavior
Effect of temperature on low velocity impact damage and post-impact flexural strength of cfrp assessed using ultrasonic c-scan and micro-focus computed tomography
No full textThe effect of temperature on the low velocity impact resistance properties and on the post-impact flexural performance of CFRP laminates were studied. With this aim, 150x75 mm cross-ply carbon fibre/epoxy laminates with a [0/90/90/0]2s layup, therefore with a total of sixteen layers, were impacted at ambient temperature (30°C) and at elevated temperatures (55, 75 and 90?C) at a velocity of 2m/s using a drop weight impact tower. This was followed by flexural tests carried out at ambient temperature using a three-point bending rig. Damage assessment of impact and post-impact behaviour were carried out using ultrasonic C-scan and microfocus X-ray computed tomography (?CT). Interrupted flexural tests using ?CT allowed delamination propagation to be observed. In general, lower projected damage was observed at elevated temperatures, which resulted also in a possible hindrance to delamination and shear cracks propagation during impact and in a greater amount of retained flexural strength after impact
Effect of temperature on low velocity impact damage and post-impact flexural strength of CFRP assessed using ultrasonic c-scan and micro-focus computed tomography
No full textThe effect of temperature on the low velocity impact resistance properties and on the post-impact flexural
performance of CFRP laminates were studied. With this aim, 150 75 mm cross-ply carbon fibre/
epoxy laminates with a [0/90/90/0]2s layup, therefore with a total of sixteen layers, were impacted at
ambient temperature (30 C) and at elevated temperatures (55, 75 and 90 C) at a velocity of 2 m/s using
a drop weight impact tower. This was followed by flexural tests carried out at ambient temperature using
a three-point bending rig. Damage assessment of impact and post-impact behaviour were carried out
using ultrasonic C-scan and microfocus X-ray computed tomography (lCT). Interrupted flexural tests
using lCT allowed delamination propagation to be observed. In general, lower projected damage was
observed at elevated temperatures, which resulted also in a possible hindrance to delamination and shear
cracks propagation during impact and in a greater amount of retained flexural strength after impact
Porosity effect on residual flexural strength following low energy impact of carbon fibre composites
No full textStudies of the combined effects of the presence of porosity (as it may result from partially effective cure cycles)
and of low-energy impact damage on the residual properties of CFRP laminates have led so far to controversial
results. In particular, it is not clear from the literature whether the presence of voids would blunt crack propagation
following impact or rather would promote damage development. These effects would respectively either increase
or reduce post-impact residual strength, relative to that of the laminate with virtually no voids, as the result
of an optimal manufacturing procedure. With this in mind, different cure cycles have been applied to produce
carbon fibre-reinforced polymer (CFRP) composites with various levels of void content, which were subjected
to low energy impact damage (3, 4.5 and 6 J) and then to post-impact flexural strength measurement. Damage
assessment using micro-focus computed tomography (μCT) was used to complement traditional ultrasonic
C-scans, which proved ineffective on the high-porosity samples. Three cure-cycles were investigated: one which
led to high porosity (average void content 4 vol%) and two conventional low-porosity cure cycles, only one
of which included a post-cure cycle. This study has found that, despite a lower initial flexural strength, higher
residual flexural strength was retained after impact in the high-porosity material than in the low-porosity one.
This is explained by the lower extent of impact damage observed in the high porosity material, where voids had
the effect of suppressing delamination propagation
Porosity effect on residual flexural strength following low energy impact of carbon fibre composites
No full textStudies of the combined effects of the presence of porosity (as it may result from partially effective cure cycles) and of low-energy impact damage on the residual properties of CFRP laminates have led so far to controversial results. In particular, it is not clear from the literature whether the presence of voids would blunt crack propagation following impact or rather would promote damage development. These effects would respectively either increase or reduce post-impact residual strength, relative to that of the laminate with virtually no voids, as the result of an optimal manufacturing procedure. With this in mind, different cure cycles have been applied to produce carbon fibre-reinforced polymer (CFRP) composites with various levels of void content, which were subjected to low energy impact damage (3, 4.5 and 6 J) and then to post-impact flexural strength measurement. Damage assessment using micro-focus computed tomography (μCT) was used to complement traditional ultrasonic C-scans, which proved ineffective on the high-porosity samples. Three cure-cycles were investigated: one which led to high porosity (average void content 4 vol%) and two conventional low-porosity cure cycles, only one of which included a post-cure cycle. This study has found that, despite a lower initial flexural strength, higher residual flexural strength was retained after impact in the high-porosity material than in the low-porosity one. This is explained by the lower extent of impact damage observed in the high porosity material, where voids had the effect of suppressing delamination propagation
Characterization of failure modes in compression-after impact of glass–epoxy composite laminates using acoustic emission monitoring
No full textThis paper investigates the effect of impact velocity on compression-after impact (CAI) strength, while the different failure modes arising during CAI test are characterized using the acoustic emission (AE) technique. For this purpose, CAI GFRP laminates manufactured as per ASTM D7137 were subjected to impact velocities such as 3.5, 5 and 6 m/s at room temperature. Impacted laminates were then subjected to compression tests using a specially fabricated fixture: AE activity was monitored during loading. Frequency analysis and location analysis were performed on the AE data obtained during CAI test of GFRP specimens impacted at different velocities to investigate the nature and extent of damage
Effect of fiber orientation in uni-directional glass epoxy laminate using acoustic emission monitoring
No full textAcoustic emission is one of the powerful techniques that can be used for in situ structural health monitoring
of composite laminates. One of the main issues of AE is to characterize the different damage mechanisms
from the detected AE signals. In this present work, pure resin and GFRP composites laminates with different
stacking sequences such as 0o
, 90o
, angle ply[±45o
], crossply [0o
/90o
] are used to trigger different failure
mechanisms when subjected to tensile test with AE monitoring. The study of failure mechanisms is
facilitated by the choice of different oriented specimens in which one or two such mechanisms predominate.
Range of Peak frequencies in each orientation is investigated using FFT analysis. Fast Fourier Transform
(FFT) enabled calculating the frequency content of each damage mechanism. Randomly chosen hits from
each range of peak frequencies for the specimens with different orientations subjected to tensile test with
acoustic emission monitoring are analyzed using STFFT analysis. STFFT analysis is used to highlight the
possible failure mechanism associated with each signal. The predominance of failure modes in each
orientation is useful in the study of discrimination of failure modes in composite laminates from acoustic emission data
Failure modes characterization of impacted carbon fiber reinforced plastic laminates under compression loading using acoustic emission
No full textComposite laminates have low resistance under dynamic loading, particularly impact loading. A low-velocity impact on laminated composites causes various types of damage, such as delamination, fibre breakage, matrix cracking and fibre matrix interfacial debonding. Post-impact compressive strength is one of the greatest weaknesses in carbon fibre reinforced plastics laminates. After impact, due to the delaminations present in the laminates, local instability is triggered, which ultimately reduces considerably their residual strength. In this work, symmetric cross ply carbon fibre reinforced plastics laminates [(0°/90°)2]12 were subjected to falling weight impact at two different velocities, 2.5 and 3.5 m/s. Compression after impact studies showed substantial differences in failure mode between the two cases, passing from end crushing to crack propagation with higher impact energy. Acoustic emission technique was able to confirm this result and characterize the different types of failure modes during compression after impact test, in particular by frequency distribution
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