1,721,108 research outputs found
Influence of dynamic load and temperature on guided wave ultrasonic damage detection in thin plates
Full text linkStarr, Andrew - Associate SupervisorLong-thin metallic materials are essentially used in constructing structures of high
economic importance, but their service life is shortened by damage such as
cracks, corrosion, cavities, notches, and dents. Damage is an inevitable condition
of metallic structures over time and, when not detected, could result in a
catastrophic breakdown. In the past decades, high interest has been developed
in using the guided wave ultrasonic technique (GWUT) to monitor the health of
structures and detect damage due to its long-distance coverage potential with
little attenuation and cost-effectiveness. Most guided wave ultrasonic studies
have focused on detecting and characterising empty cracks or notches. Limited
literature is available to explain the behaviour of guided waves while travelling in
thin plates exposed to damage filled with debris, which is more likely possible in
long-thin structures such as pipelines for oil, water or gas transportation. Debris-
filled damage leads to corrosion processes, particularly inducing pitting corrosion.
This form of corrosion is localised and difficult to detect. It has contributed to many
structural failures, particularly in oil and gas pipelines. Hence, early detection and
characterisation of this form of damage is vital to avert catastrophic failure. This
study explored the detection of damage filled with different proportions of debris
in thin plates using guided wave ultrasonic techniques. The captured response
signals underwent analysis through various signal-processing methods in
MATLAB. Additionally, the research examined how temperature variations and
low-frequency vibrations impact the guided wave responses, aiming to simulate
the effect of environmental operation conditions. Through the analysis, an
empirical model was developed to predict debris-filled damage and differentiate
it from empty damage and the health state of the structure. The predictive model
has an average error of about 1.34. Also, the analysis revealed that cross-
correlation of the detrended response and reference signals could demonstrate
a quick way to visualise and spot debris-filled damage in the structure.
Additionally, a model called Olisa-Khan low-vibration mitigation architecture
(Olisa-Khan LMA) was created to counteract the severe effects of varying low-
frequency vibrations and improve the performance of the damage detection
technique. The average percentage deviation of the model response signal and
static response signal was about 1.64 %, suggesting the two signals are very
close. The slight deviation could be attributed to the signal loss due to clipping
and imperfection in the system. In characterising debris that filled the damage,
an excitation signal with a central frequency of 80KHz was found optimal because
the deviation of each state of damage differs from the other and decreases from
an empty case to a debris-filled case and continues as fluid-filled viscosity
increases. The study's merit cannot be overemphasised as it establishes models,
especially for predicting novel damage of debris-filled and characterising different
debris that filled the damage even in severe environmental operation conditions.
Hence, the study would be useful for continuously monitoring long-thin structures
of high economic values for possible damage detection and characterisation.PhD in Manufacturin
Structural dynamics and crack propagation behaviour under uniform and non-uniform temperature conditions.
Full text linkThe robustness and stability of machinery depend on structural integrity. This
stability is, however, compromised by aging, wear and tear, overloads, and
environmental factors. A study of vibration and fatigue crack growth for
structural health monitoring is one of the core research areas in recent times.
The research is yet to input sufficient explanations about the dynamic behaviour
of the structure under distributed temperature. The structural dynamics can be
influenced by material microstructure, temperature distribution, and duration of
exposure to the thermal environment. The applied temperature can cause
significant variations in the modal response. The existing studies are limited
concerning temperature change and compel extensive investigation in a crack
and uncracked condition. In this research, the structural dynamics and fatigue
crack propagation behaviour when subjected to thermal and mechanical loads
have been studied. It investigates the modal parameters of uncracked and
various cracked specimens under uniform and non-uniform temperature
conditions. An analytical model considering the effective length of the beam is
developed to analyse the modal response of the beam. Then, the model is
modified to enumerate the modal behaviour of the beam in the presence of
crack. The model is validated by experimental and numerical approaches. The
experimental evaluation is conducted by considering three heating rates to
attain the required temperatures. In the first case, ramping at 2°C/min is
assumed as a slow heating rate. While ramping at 5°C/min and 8°C/min are
assumed as moderate and rapid heating rates respectively. The heating rates
are considered to compare the structural response changes. A small variation
on modal parameters is noticed for different heating rates and when the applied
uniform temperatures are changed to non-uniform temperatures, especially at
elevated temperatures. This signifies heating at different rates has a slight effect
while measuring the dynamic response of any mechanical system. The results
showed that changes in modal parameters of the beam are associated with the
change in temperatures and heating rate.
Furthermore, this research substantiates the fatigue crack propagation
behaviour of pre-seeded cracks. The propagated crack depths are measured
based on pixels contains in the crack. It is found that propagated crack depends
on applied temperatures and associated mass. The appearance of double crack
fronts and multiple cracks are observed. The multiple crack appearance seems
due to the selection of pre-seeded crack shapes. Hence, the real crack and pre-
seeded crack are distinct and need careful consideration in crack propagation
evaluation.PhD in Manufacturin
Enhancing mechanical properties of concrete material with fibres of different materials
Full text linkStarr, Andrew - Associate SupervisorFibre reinforced cementitious composites are highly effective for construction due
to their enhanced concrete properties. Materials such as steel fibre have been
used extensively to reinforce concrete because of their excellent mechanical
properties. Academic researchers have comprehensively discussed the impact
and challenges of fibre reinforcement to obtain optimal properties in the resultant
concrete. Most researchers have reported the mechanical performance of fibre-
reinforced concrete (FRC) under static loads. Concrete with fibre reinforcement
is stronger and more ductile than concrete without reinforcement. Significant
efforts have been made to demonstrate the properties and enhancements of
concrete after reinforcing it with different types and shapes of fibres. However,
the optimization in the reinforcement process is still unanswered. No academic
study in the literature now available can pinpoint the ideal fibre type, quantity,
shape, and, more crucially, the overall technical viability of the reinforcement.
After performing the optimization, researchers considered how these
optimizations could affect the crack resistance or properties under dynamic loads
with different temperatures. However, a comprehensive analysis is still missing
that can explain the crack resistance performance of FRC under dynamic loads
at relatively high temperatures. The main aim of this thesis is to investigate the
mechanical behaviour of concrete structures under thermo-mechanical dynamic
loads about reinforcing fibres of different weight ratios. This study uses
parametric analysis in accordance with extensive mechanical tests to identify the
optimal shape, size, and percentage of fibres. The design variables for
optimization are divided into input and output parameters. The input parameters
are the influences of the type, length, and percentage of fibres on concrete
performance, including samples of fresh and mechanical concrete properties, to
search for the most effective relation of fibre dose and dimension to optimize the
combined responses of workability, splitting tensile strength, flexural strength,
and compressive strength. The current work also proposes the Khan Khalel
model, which can predict the desirable compressive and flexural strengths for any
given values of key fibre parameters. Statistical tools are used to develop and
validate the model with numerical results. The proposed model is easy to use but
predicts compressive and flexural strengths with errors under 6% and 15%,
respectively. This error primarily represents the assumption made for the input of
fibre material during model development. It is based on the elastic modulus of the
material and hence neglects the plastic behaviour of the fibre. A possible
modification in the model for considering the plastic behaviour of fibre will be
considered as future work. Finally, this study analyses the efficacy of FRC beams
for crack resistance under coupled loads, i.e., dynamic loads at relatively high
temperatures. Cantilever FRC beams are tested on a modal exciter in a band
heater to expose the beams to bending loads at different temperature values. The
variation in the dynamic response parameters of the beam, including modal
amplitude and frequency, is discussed and compared with experimental results
for regular and reinforced concrete beams. The stress intensity factor and
displacement amplitude characteristics show that the steel FRC specimens have
excellent ductile behaviour and higher crack resistance than ordinary concrete
samples.PhD in Aerospac
Wear and airborne noise interdependency at asperitical level: analytical modelling and experimental validation.
Full text linkStarr, Andrew - Associate SupervisorGeneration of wear and airborne sound is inevitable during friction processes.
Most correlation between the wear and the sound generated during a sliding
process have been experimental. Analytical models do exist, but they remain
scarce and do not fully account for the wear and the airborne sound generation
especially at asperitical level. The model developed in this research attempts to
fill the gap by providing a quantifiable relationship between the wear generated
and the sound emitted in a simple pin-on-disc setup. It provides a relationship
between the wear and the sound from an asperitical level. This is done by
examining the conditions at which wear would occur on an asperity distribution.
The asperity distribution is considered to be exponential, although a Gaussian
distribution was also considered. Impact forces are calculated on a per-asperity
basis and the wear and vibrational displacement is calculated as a result. This
leads to the quantification of wear and acoustic noise. The model is validated
using a pin-on disc setup for three varied materials (iron (4% carbon content),
mild steel (0.18% carbon content) and aluminium T351) under two loads (10 N
and 20 N) at 300 RPM. The loads and speeds were chosen so as to observe a
range of wear behaviour while remaining within the constraints of the lab
limitations and the safety of the force sensors on the tribometer. Temperatures
are also examined, and a second set of validation experiment is performed at
temperatures of 40 °C and 60 °C. The model computes the predicted wear and
sound pressure, and it is compared with the experimental sound pressure
measured by the microphone and the wear measured by the tribometer
sensors. Sound pressure is chosen as a measure over frequency because it is
easier to analyse and compare. The theoretical model agrees with the
experimental results with a varying error of 10 to 15 % error in iron and
aluminium. However, a larger error is observed in the case of mild steel. The
model could be refined to improve the accuracy as it assumes point impacts on
the asperities where a distributed impact would be more suitable. Furthermore,
the pin is assumed a single asperity to simplify the model at the expense of
accuracy. Overall, the experimental results are in fair correlation with the
theoretical results and this model provides the first step in quantifying wear
using only the recorded sound pressure.PhD in Manufacturin
Dynamic response of 3d-printed acrylonitrile butadiene styrene (abs) damaged structure under thermo-mechanical loads.
Full text linkStarr, Andrew - Associate SupervisorFused deposition modelling (FDM), as the most widely used additive
manufacturing (AM) process, has great potential for various applications. The
structures manufactured with the FDM technique has the potential to be used in
a variety of complex working environments, such as the coupled thermo-
mechanical loads. The coupled thermo-mechanical loads can likely lead to
fatigue cracking swiftly in structures till the catastrophic failure. Therefore, it is
critical to research the fatigue crack behaviour in FDM structures. This
behaviour is mainly responsible for the change of structural stiffness and hence
can influence the dynamic response of the structure under the mentioned loads.
The measurement of the structural dynamic response can give us an idea of the
severity due to crack growth in an in-situ manner. This thesis mainly aims to
investigate the dynamic response of the cracked FDM structures under thermo-
mechanical loads. The relationship between the coupled loads, crack
propagation and dynamic response is developed analytically and later validated
experimentally. This research has improved the existing torsional spring model,
which can represent the crack depth more accurately and hence estimated the
fundamental frequency of the selected structure with an up to around 20% to
120% reduced error in the case of deep cracks. Furthermore, the analytical
relationship between the structural displacement amplitude and crack depth and
location was modelled for the very first time in the presence of the crack
breathing effect. Extensive experimentation is performed to validate the
developed analytical relationship and its related theory. The fatigue crack
growth of FDM ABS beams under thermo-mechanical loads with varying
printing parameters is also investigated. The optimal printing parameters
combination (X raster orientaion, 0.8 mm nozzle size, 0.15 mm layer thickness)
is determined. The underlying reasons behind the experimental data are
analysed. The outcome of this optimisation can help manufacturers to print
long-life and crack resistant printed structures.PhD in Manufacturin
Machine learning (ml) approaches to model interdependencies between dynamic loads and crack propagation
Full text linkStarr, Andrew - Associate SupervisorThe application of machine learning in structural health and crack prediction is of
paramount importance, as it offers the potential to enhance the accuracy,
efficiency, and reliability of detecting and predicting damage in various materials
and structures. This research presents an in-depth exploration of machine
learning (ML) applications in the field of Structural Health Monitoring (SHM)
across various materials, including composites, metals, and polymers. The study
identifies the current challenges in implementing ML in SHM, such as data
sparsity, interpretability of ML models, overfitting, and the absence of general
guidelines for ML model selection.
The research analyses the dynamic response data of different materials and
establishes significant crack depth predictors for materials such as aluminum,
concrete, and 3D-printed Acrylonitrile Butadiene Styrene (ABS). It further
investigates and validates selected ML models to predict crack depth in different
materials. The models' performance is evaluated using Mean Squared Error
(MSE) on both training and test sets, demonstrating their ability to capture
meaningful patterns within the data and make reasonably accurate predictions.
A significant contribution of this study is the proposal of an automated model
utilizing the H2O library for crack propagation prediction in ABS materials. This
model demonstrates the potential of automation in SHM, offering substantial
benefits for structural integrity assessment, maintenance strategies, and
materials design in various industries. This research concludes with
recommendations for future research, including the exploration of advanced ML
algorithms, investigation of additional predictive features, and evaluation of the
models in different real-world scenarios.PhD in Manufacturin
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Living in bio-climatic layers: An investigation of Cappadocian caves in relation to today’s design and its futures
Full text linkThis exploratory paper discusses a primary study on Cappadocian caves’ bioclimatic performance, speculating on its application to today’s and future Performance Oriented Architecture (Hensel 2010; Hensel 2011; Hensel 2013). It is a rough initial pre-study to future broader research, claiming the need and relevance for in depth investigations. As too little has been done in this field, the project seeks to demonstrate how layering of spaces in relation to material and building techniques may manipulate different peals of exterior, semi-interior and interior spaces’ climates in onion principle in respect to its use (Davidová 2016a; Davidová 2016b) and different species’ habitation (Davidová 2016b), discussing its contemporary and future potentials for architectural practice on the work of Collaborative Collective’s examples (Collaborative Collective 2012; Collaborative Collective 2016). The paper argues for fully adaptable architecture, that is full part of and in constant coexistence with its surrounding ecosystem. Thematic GIGA-mapping (Sevaldson 2011; Sevaldson 2012; Sevaldson 2015) was used as an analysing tool for systemic relations of collected registered data, as well as existing information, merging hard data with tacit knowledge (see Figure 1). The map shows air flow passing through different layers of spaces as the most important factor of the climatic conditions, depth and height location as a second one. This is all interrelated in co-existence to the use of the spaces. Here it seems that symbiosis of humans and other species can play a crucial role in climate comfort and both mentioned vary over time. Therefore, we believe, that due to recent fast climate and society change, with expected weather extremes (Czech Republic Ministry of the Environment and Czech Hydrometeorological Institute 2015; Republic of Turkey Ministry of Environment and Urbanization 2012; Flæte et al. 2010; Richardson 2010), transformative adaptive architecture should be investigated with the use of biology: reconfiguration as a new form of recycling
Physics-based modelling of cyclic deformation and microstructure-sensitive fatigue crack propagation from shallow scribes
Full text linkFace-centered cubic (FCC) metals with low to medium stacking fault energy
(SFE) develop similar mesoscale substructures under cyclic loading. The
formation of these substructures is controlled by dislocation interactions and
loading conditions. For instance, cross slip facilitates cell formation and Hirth
locks define the labyrinth structure. In the case of aluminium (high SFE metal),
cross slip is easily activated and a cell structure is often observed. However, it is
not always recognised that aluminium can also form PSBs at low temperatures.
This highlights that the underlying mechanism controlling the cyclic response in
aluminium is not different from other FCC metals.
This work proposes the role of mesoscale substructure as a material-invariant
among FCC metals to predict the cyclic response of aluminium. The effect of
number of cycles on modelling dislocation substructures is explored, which is
found to trigger a change in dislocation structures in aluminium at 298K. A crystal
plasticity framework based on mesoscale substructures is developed to study the
cyclic response of aluminium under different crystal orientations, strain
amplitudes, number of cycles, and temperatures.
Finally, this work implemented the crystal plasticity model to study the
microstructure-sensitive crack propagation from shallow scribes in pure
aluminium. The gradient of fatigue indicator parameters (FIPs) is estimated as
crack extends inside a grain with explicit microstructure simulations, which
followed the same decaying trend predicted by experiments. Thereby, an
engineering solution is proposed to couple microstructural and geometric
gradients at the crack tip independently. The model predicted the transgranular
fatigue life with independently coupled gradients that agree well with
experiments.PhD in Manufacturin
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