Mahfuz, Hassan

This research studied the effects of mooring line pretension, spring safe working load, and spring response curve on peak loads and platform surge. The maximum tension load from the optimized assembly was applied to a modelled section of 8-strand multiplait rope to study stress concentrations. The analyses yielded a mooring line pretensioned at 1250 kN with a 4500 kN safe working load degressive spring was optimal. Fatigue damage from 12-hour duration of 50-year storm conditions was 8.04 × 10−6. Infinite life is predicted at annual average conditions. The peak tension from 50-year storm conditions of 3671 kN and annual average conditions of 1388 kN was applied to the section model, yielding a maximum stress of 3.70 × 108 Pa and 2.01 × 108 Pa, respectively, from friction and longitudinal compression of the rope’s cross section. The maximum stress from the static structural analysis was 33.5% of polyester’s ultimate strength, based on the maximum stress failure criterion.

Nano-reinforced polymeric systems have demonstrated a great deal of interest
within academia and industry, due to the intrinsic properties of the graphene nanofillers,
having excellent mechanical, thermal and electrical properties. The reinforcement of multiwall
carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) were introduced
into a low cost, non-carcinogenic, high temperature PMR type polyimide resin. The effects
of the interfacial interaction and dispersion quality resulted in improvement in the glass
transition temperature (Tg), elastic modulus and thermal stability by, 31°C, 63% and 16°C,
respectively. In fine, this study presents a simple but effective high temperature polyimide
(HTPI) nanocomposites manufacturing procedure and established that nanoparticle
reinforcement can be used to improve both thermal and mechanical properties.

We report a method of increasing fracture toughness (KIc) and strain energy release rate (GIc) of vinyl-ester (VE) matrix by adopting a hybrid (dual) reinforcement strategy. The idea of using this strategy was to trigger intrinsic polymer-nanoparticle interaction such as carbon nanotube (CNT) pull-out and interface sliding to enhance energy absorption during fracture. Additionally, we included a second reinforcement, graphene nanoplatelets (GNP), to promote crack-deflection, crack bridging and cross-linking density. Both reinforcements were dispersed into the polymer in three states: non-functionalized (nf>); functionalized with COOH (f>); surface-treated with Triton X-100 (TX100). We embarked on numerous experiments with many combinations of these variables. We measured KIc and GIc using ASTM D5045-14. We conducted an exhaustive iterative investigation with three systems (f>CNT-VE; f>GNP-VE; f>CNT-f>GNP-VE) to determine the best weight-percentage for the nanocomposite system that produced the highest KIc and GIc values when compared to neat-VE. We found that 0.5wt% f>CNT with 0.25wt% f>GNP in the VE matrix resulted in the highest fracture toughness values and was termed the optimized hybrid nanocomposites (OHN) system. Subsequently, we explored further increasing the KIc and GIc of OHN through altering the nanoparticle surface characteristics, which led to four OHN groups: f>CNT-f>GNP-VE; f>CNT-f>GNP-TX100-VE; nf>CNT-nf>GNP-TX100-VE; nf>CNT-nf>GNP-VE. We discovered that the OHN group with non-functionalized nanofillers that were TX100 surface treated (0.5wt%nf>CNT-0.25wt%nf>GNP-TX100-VE) generated the greatest improvements in KIc and GIc.
Ultimately, we observed that the KIc of neat-VE increased by 65%, from 1.14 to 1.88 MPa*(m½). The improvement in GIc was even greater with an increase of 166%, from 370 to 985 J/(m2). Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) studies showed a minor shift in glass transition temperature (Tg) by up to 8°C when comparing neat-VE specimens to OHN specimens. A similar increase in maximum thermal decomposition temperature (Tp) of up to 8°C was observed through thermogravimetric analysis (TGA) and derivative TGA (DTG). Scanning electron microscope (SEM) studies revealed that the source of improvements in fracture toughness and thermal properties was primarily the three-dimensional hybrid nanostructures (3DHN) that formed by binding CNT and GNP together, which caused an increase in nanoparticle surface area and inhibited agglomerations.

Sandwich composites provide excellent structural integrity for a variety of
applications. In this study pristine and functionalized 30 nrn Silicon Carbide
nanoparticles are infused into a low density polyurethane foam used for the inner core
of the sandwich structure. The mechanical properties are characterized using
compressive, tensile, and flexural tests. A plane-strain fracture test and a TSD (Tilted
Sandwich Debond) test characterize the fracture properties of the foam and the coreskin
interface. Thermal characterization is carried out using Dynamic Mechanical
Analysis (DMA) and Thermo-Gravimetric Analysis (TGA). FTIR spectral analysis
reveals changes in molecular bonding due to pristine and functionalized nanoparticle
infusion. The fracture resistance of the foam is improved and the delamination
strength of the sandwich construction with nanophased cores is dramatically
improved. The TSD testing indicated that the G1c value rose from 0.14 kJ/m^2 in the
neat foam to 0.56 kJ/m^2 with just 0.1 wt% of SiC nanoparticle inclusion reflecting an
enhancement of almost 300%.

The degradation of polymer composites in moist environments is a limiting
factor in the advancement of composite technology. The key to mitigate this
degradation is to maintain the integrity of the fiber/matrix (F/M) interface. In this
study, the F/M interface of carbon/vinyl ester composites has been modified by
treating the carbon fiber with polyhedral oligomeric silsesquioxane (POSS). Two
POSS systems, namely octaisobutyl and trisilanolphenyl, have been
investigated. A set of chemical and mechanical procedures has been developed
to coat carbon fibers with POSS, and fabricate layered composites using vinyl
ester resin. lnterlaminar shear, transverse tension, and low velocity impact tests
on composites have indicated around 10-38% improvement in mechanical
properties with respect to control samples. Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) tests have also shown
significant improvement in glass transition temperature (T9). Hygrothermal tests,
under various environments, have demonstrated that POSS reduces water
absorption by 20-30%.

Nanoscale silica particles are functionalized and ultrasonically dispersed into a
mixture of polyethylene glycol and ethanol, and then reinforced with Kevlar. The stab or
puncture resistance of the flexible nanophased materials system supersedes recent
advances made in this area. Through SEM scans, thermal and chemical analysis, it is
evident that the functionalized nanoparticles offer multiple facets of resistance to
penetration of a sharp impactor. The improvement in protection is traced to the
formation of siloxane bonds during functionalization. The framework for a theoretical
model is established to estimate penetration depth under low velocity impact of a sharp
object through the flexible composite. For comparison ofthese novel fabric composites,
a method is also introduced to evaluate penetration resistance quantitatively. The method
is capable of showing subtle changes that would otherwise be missed.

A comprehensive study was performed to overcome the design issues related to
Ocean Current Turbine (OCT) blades. Statistical ocean current models were developed in
terms of the probability density function, the vertical profile of mean velocity, and the
power spectral density. The models accounted for randomness in ocean currents, tidal
effect, and ocean depth. The proposed models gave a good prediction of the velocity
variations at the Florida Straits of the Gulf Stream.
A novel procedure was developed to couple Fluid-Structure Interaction (FSI) with
blade element momentum theory. The FSI effect was included by considering changes in
inflow velocity, lift and drag coefficients of blade elements. Geometric non-linearity was
also considered to account for large deflection. The proposed FSI analysis predicted a
power loss of 3.1 % due to large deflection of the OCT blade. The method contributed to
saving extensive computational cost and time compared to a CFD-based FSI analysis. The random ocean current loadings were calculated by considering the ocean
current turbulence, the wake flow behind the support structure, and the velocity shear. The
random ocean current loadings had large probability of high stress ratio. Fatigue tests of
GFRP coupons and composite sandwich panels under such random loading were
performed. Fatigue life increased by a power function for GFRP coupons and by a linearlog
function for composite sandwich panels as the mean velocity decreased. To accurately
predict the fatigue life, a new fatigue model based on the stiffness degradation was
proposed. Fatigue life of GFRP coupons was predicted using the proposed model, and a
comparison was made with experimental results.
As a summary, a set of new design procedures for OCT blades has been introduced
and verified with various case studies of experimental turbines.

Several modifications have been implemented to numerical simulation codes based on
blade element momentum theory (BEMT), for application to the design of ocean current
turbine (OCT) blades. The modifications were applied in terms of section modulus and
include adjustments due to core inclusion, buoyancy, and added mass. Hydrodynamic loads
and mode shapes were calculated using the modified BEMT based analysis tools. A 3D
model of the blade was developed using SolidWorks. The model was integrated with
ANSYS and several loading scenarios, calculated from the modified simulation tools, were
applied. A complete stress and failure analysis was then performed. Additionally, the
rainflow counting method was used on ocean current velocity data to determine the loading
histogram for fatigue analysis. A constant life diagram and cumulative fatigue damage
model were used to predict the OCT blade life. Due to a critical area of fatigue failure being
found in the blade adhesive joint, a statistical analysis was performed on experimental
adhesive joint data.

In fabrication of nanoparticle-reinforced polymers, two critical factors need to be
taken into account to control properties of the final product; nanoparticle
dispersion/distribution in the matrix; and interfacial interactions between nanoparticles and
their surrounding matrix. The focus of this thesis was to examine the role of these two
factors through experimental methodologies and molecular-level simulations. Carbon
nanotubes (CNTs) and vinyl ester (VE) resin were used as nanoparticles and matrix,
respectively.
In a parametric study, a series of CNT/VE nanocomposites with different CNT
dispersion conditions were fabricated using the ultrasonication mixing method. Thermomechanical
properties of nanocomposites and quality of CNT dispersion were evaluated.
By correlation between nanocomposite behavior and CNT dispersion, a thermomechanical
model was suggested; at a certain threshold level of sonication energy, CNT dispersion would be optimal and result in maximum enhancement in properties. This
threshold energy level is also related to particle concentration. Sonication above this
threshold level, leads to destruction of nanotubes and renders a negative effect on the
properties of nanocomposites.
In an attempt to examine the interface condition, a novel process was developed to
modify CNT surface with polyhedral oligomeric silsesquioxane (POSS). In this process, a
chemical reaction was allowed to occur between CNTs and POSS in the presence of an
effective catalyst. The functionalized CNTs were characterized using TEM, SEM-EDS,
AFM, TGA, FTIR and Raman spectroscopy techniques. Formation of amide bonds
between POSS and nanotubes was established and verified. Surface modification of CNTs
with POSS resulted in significant improvement in nanotube dispersion. In-depth SEM
analysis revealed formation of a 3D network of well-dispersed CNTs with POSS
connections to the polymer. In parallel, molecular dynamics simulation of CNT-POSS/VE
system showed an effective load transfer from polymer chains to the CNT due to POSS
linkages at the interface. The rigid and flexible network of CNTs is found to be responsible
for enhancement in elastic modulus, strength, fracture toughness and glass transition
temperature (Tg) of the final nanocomposites.

A finite element tool has been developed to design and investigate a multi-hull
composite ship structure, and a hybrid hull of identical length and beam. Hybrid hull
structure is assembled by Titanium alloy (Ti-6Al-4V) frame and sandwich composite
panels. Wave loads and slamming loads acting on both hull structures have been
calculated according to ABS rules at sea state 5 with a ship velocity of 40 knots.
Comparisons of deformations and stresses between two sets of loadings demonstrate that
slamming loads have more detrimental effects on ship structure. Deformation under
slamming is almost one order higher than that caused by wave loads. Also, Titanium
frame in hybrid hull significantly reduces both deformation and stresses when compared
to composite hull due to enhancement of in plane strength and stiffness of the hull.
A 73m long hybrid hull has also been investigated under wave and slamming loads in time
domain for dynamic analysis.