134 research outputs found
Dynamic response of 3d-printed acrylonitrile butadiene styrene (abs) damaged structure under thermo-mechanical loads.
Starr, 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
Field-aware Calibration: A Simple and Empirically Strong Method for Reliable Probabilistic Predictions
It is often observed that the probabilistic predictions given by a machine
learning model can disagree with averaged actual outcomes on specific subsets
of data, which is also known as the issue of miscalibration. It is responsible
for the unreliability of practical machine learning systems. For example, in
online advertising, an ad can receive a click-through rate prediction of 0.1
over some population of users where its actual click rate is 0.15. In such
cases, the probabilistic predictions have to be fixed before the system can be
deployed.
In this paper, we first introduce a new evaluation metric named field-level
calibration error that measures the bias in predictions over the sensitive
input field that the decision-maker concerns. We show that existing post-hoc
calibration methods have limited improvements in the new field-level metric and
other non-calibration metrics such as the AUC score. To this end, we propose
Neural Calibration, a simple yet powerful post-hoc calibration method that
learns to calibrate by making full use of the field-aware information over the
validation set. We present extensive experiments on five large-scale datasets.
The results showed that Neural Calibration significantly improves against
uncalibrated predictions in common metrics such as the negative log-likelihood,
Brier score and AUC, as well as the proposed field-level calibration error.Comment: WWW 202
Effects of printing parameters on the fatigue behaviour of 3D-printed ABS under dynamic thermo-mechanical loads
Fused deposition modelling (FDM) is the most widely used additive manufacturing process in customised and low-volume production industries due to its safe, fast, effective operation, freedom of customisation, and cost-effectiveness. Many different thermoplastic polymer materials are used in FDM. Acrylonitrile butadiene styrene (ABS) is one of the most commonly used plastics owing to its low cost, high strength and temperature resistance. The fabricated FDM ABS parts commonly work under thermo-mechanical loads in actual practice. For producing FDM ABS components that show high fatigue performance, the 3D printing parameters must be effectively optimized. Hence, this study evaluated the bending fatigue performance for FDM ABS beams under different thermo-mechanical loading conditions with varying printing parameters, including building orientations, nozzle size, and layer thickness. The combination of three building orientations (0°, ±45°, and 90°), three nozzle sizes (0.4, 0.6, and 0.8 mm) and three-layer thicknesses (0.05, 0.1, and 0.15 mm) were tested at different environmental temperatures ranging from 50 to 70 °C. The study attempted to find the optimal combination of the printing parameters to achieve the best fatigue behaviour of the FDM ABS specimen. The experiential results showed that the specimen with 0° building orientation, 0.8 mm filament width, and 0.15 mm layer thickness vibrated for the longest time before the fracture at each temperature. Both a larger nozzle size and thicker layer height can increase the fatigue life. It was concluded that printing defects significantly decreased the fatigue life of the 3D-printed ABS beam
Learn Continuously, Act Discretely: Hybrid Action-Space Reinforcement Learning For Optimal Execution
Optimal execution is a sequential decision-making problem for cost-saving in
algorithmic trading. Studies have found that reinforcement learning (RL) can
help decide the order-splitting sizes. However, a problem remains unsolved: how
to place limit orders at appropriate limit prices? The key challenge lies in
the "continuous-discrete duality" of the action space. On the one hand, the
continuous action space using percentage changes in prices is preferred for
generalization. On the other hand, the trader eventually needs to choose limit
prices discretely due to the existence of the tick size, which requires
specialization for every single stock with different characteristics (e.g., the
liquidity and the price range). So we need continuous control for
generalization and discrete control for specialization. To this end, we propose
a hybrid RL method to combine the advantages of both of them. We first use a
continuous control agent to scope an action subset, then deploy a fine-grained
agent to choose a specific limit price. Extensive experiments show that our
method has higher sample efficiency and better training stability than existing
RL algorithms and significantly outperforms previous learning-based methods for
order execution
Generating Synergistic Formulaic Alpha Collections via Reinforcement Learning
In the field of quantitative trading, it is common practice to transform raw
historical stock data into indicative signals for the market trend. Such
signals are called alpha factors. Alphas in formula forms are more
interpretable and thus favored by practitioners concerned with risk. In
practice, a set of formulaic alphas is often used together for better modeling
precision, so we need to find synergistic formulaic alpha sets that work well
together. However, most traditional alpha generators mine alphas one by one
separately, overlooking the fact that the alphas would be combined later. In
this paper, we propose a new alpha-mining framework that prioritizes mining a
synergistic set of alphas, i.e., it directly uses the performance of the
downstream combination model to optimize the alpha generator. Our framework
also leverages the strong exploratory capabilities of reinforcement
learning~(RL) to better explore the vast search space of formulaic alphas. The
contribution to the combination models' performance is assigned to be the
return used in the RL process, driving the alpha generator to find better
alphas that improve upon the current set. Experimental evaluations on
real-world stock market data demonstrate both the effectiveness and the
efficiency of our framework for stock trend forecasting. The investment
simulation results show that our framework is able to achieve higher returns
compared to previous approaches.Comment: Accepted by KDD '23, ADS trac
Interdependencies between dynamic response and crack growth in a 3D-printed acrylonitrile butadiene styrene (ABS) cantilever beam under thermo-mechanical loads
Acrylonitrile butadiene styrene (ABS) is the most commonly used thermoplastic printing material for fused deposition modelling (FDM). FDM ABS can be used in a variety of complex working environments. Notably, the thermo-mechanical coupled loads under complex operating conditions may lead to cracking and ultimately catastrophic structural failure. Therefore, it is crucial to determine the crack depth and location before a structural fracture occurs. As these parameters affect the dynamic response of the structure, in this study, the fundamental frequency and displacement amplitude response of a cracked 3D-printed ABS cantilever beam in a thermal environment were analytically and experimentally investigated. The existing analytical model, specifically the torsional spring model used to calculate the fundamental frequency change to determine the crack depth and location was enhanced by the proposed Khan-He model. The analytical relationship between the displacement amplitude and crack was established in Khan-He model and validated for the first time for FDM ABS. The results show that a reduced crack depth and location farther from the fixed end correspond to a higher fundamental frequency and displacement amplitude. An elevated ambient temperature decreases the global elastic modulus of the cracked beam and results in a lower fundamental frequency. Moreover, a non-monotonic relationship exists between the displacement amplitude and ambient temperature. The displacement amplitude is more sensitive to the crack change than the fundamental frequency in the initial stages of crack growth
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