Classification of unarmed/armed personnel using the NetRAD multistatic radar for micro-Doppler and singular value decomposition features

In this letter, we present the use of experimental human micro-Doppler signature data gathered by a multistatic radar system to discriminate between unarmed and potentially armed personnel walking along different trajectories. Different ways of extracting suitable features from the spectrograms of the micro-Doppler signatures are discussed, particularly empirical features such as Doppler bandwidth, periodicity, and others, and features extracted from singular value decomposition (SVD) vectors. High classification accuracy of armed versus unarmed personnel (between 90% and 97% depending on the walking trajectory of the people) can be achieved with a single SVD-based feature, in comparison with using four empirical features. The impact on classification performance of different aspect angles and the benefit of combining multistatic information is also evaluated in this letter

Corrosion Behaviour of Additively Manufactured High Entropy Alloys

Additive manufacturing (AM) is a modern manufacturing technique that facilitates the production of components layer by layer from CAD files, with more recent developments in the field leading to the ability to create these components from metal. Laser powder bed fusion (LPBF) is one of the many techniques used to manufacture metallic components and has drawn significant attention for its ability to create parts with high degrees of complexity, exceptional strength-to-weight ratios and internal structures. However, parts produced by AM are documented to suffer from build defects such as porosity, which can negatively affect not only its mechanical properties but its corrosion resistance,particularly its resistance to pitting corrosion. Whilst the mechanical properties of components produced through metal AM have been well documented since the technology’s inception, there are significant knowledge gaps in understanding the corrosion behaviour of metals produced in this way.This thesis aims to expand upon the current understanding of this manufacturing method with a particular focus on its corrosion resistance. High Entropy Alloys (HEAs) are a class of advanced materials that differ from conventional alloys in composition. Traditional alloys usually consist of one or two principal elements with smaller amounts of additional elements to impart specific properties. In contrast, HEAs are characterised by the presence of multiple principal elements in roughly equal proportions. HEAs' complex and disordered structure can result in unique mechanical, thermal, and magnetic properties. HEAs have shown promise in exhibiting high strength, hardness, and corrosion resistance, making them attractive for various engineering applications. Studies of HEAs have been increasing over recent years; however, significant knowledge gaps are still associated with this classification of materials, especially concerning their corrosion resistance. This lack of knowledge is intensified when discussing the properties of these alloys when manufactured by AM methods.LPBF was used to produce parts in 316L with process induced porosity by manipulating the process parameters to investigate the effect density has on the corrosion resistance of AM parts. The corrosion resistance of these parts were compared to their wrought counterpart using potentiodynamic polarisation. It was observed that increasing the porosity in the AM parts resulted in poorer corrosion resistance, both by weaker performance across key metrics and a greater degree of unreliability. It was also found that the AM parts proved to have a greater corrosion resistance than the wrought material. However, the decreased consistency in this resistance is often cited as a barrier these components must overcome to supplace conventionally manufactured components.316L was also produced through induction casting as well as a schedule more representative of industry that consisted of a solution anneal at 1080 °C followed by water quenching followed by a cold rolling reduction by 70 %, and a final anneal at 900 °C. The microstructures and corrosion resistance of these were investigated using SEM-EDS, XRD and potentiodynamic polarisation, and whilst the corrosion resistance of the cold rolled sample had increased, it was less than expected due to the formation of detrimental chromium carbides.A Swansea University developed AlCrFeMnNi HEA was put through the same 3 manufacturing processes to investigate their effect on the microstructure and corrosion resistance. It was found that, unlike 316L, the HEA suffered less from pitting corrosion and more from a generalised corrosion attack. Very similar corrosion results were seen across the manufacturing methods; however, the cast sample was observed to have the most consistent display of corrosion resistance.Based on the pitting resistance equivalent number, which relates the amount of Cr and Mo by wt.% in a stainless steel to its corrosion resistance, it was theorised that the addition of Mo to this HEA could also increase its corrosion resistance. The results were inconclusive; however, better corrosion resistance was seen in the AM sample of the HEA with the addition than in the AM sample without

Fatigue Behavior of FRP-Reinforced Douglas-Fir Glued Laminated Bridge Girders

The use of composites in engineered wood products has recently led to the use of fiber-reinforced polymers (FRP) as a reinforcing for glued-laminated (glulam) beams. Bridge girders are among the more common applications of FRP-reinforced glulam beams and therefore the beam is subject to millions of load cycles as well as moisture fluctuations. Significant flexural strength can be gained through the use of such reinforcing, however, the behavior of the wood composite when subject to repeated load cycles and hygrothemal effects is not well understood. In this study, eighteen glulam beams were reinforced in tension with 1.93% E- glass/epoxy reinforcing (defined as the volume of reinforcing fiber divided by the volume of wood) and tested in flexural fatigue at stress levels corresponding to 1 .OFb and 1 .3Fb, where Fb is the allowable flexural capacity. Both full length and partial length reinforced specimens were tested. The FRP sheet was terminated at the theoretical cut off point (or the point at which the reinforcing is no longer needed to sustain the applied loads) with the partial length reinforcing and was explored with and without end restraints on the reinforcing. Unrestrained terminations were beveled to alleviate peeling stresses while restrained terminations were confined by a steel plate and lag screws. Fatigue testing of all specimens cycled the beams in four-point bending for a total of two million cycles with static bending tests performed periodically to track changes in stiffness. Specimens were then broken in static bending to determine residual strength. Loading at 1.OFb fatigued the specimens at a stress ratio of R=0.333 while loading at 1.3Fb produced a stress ratio of R=0.255. Load heads were spaced to produce flexural stress-to-shear stress ratios consistent with those seen by typical in-service timber bridge girders. However, the flexural capacity of the reinforced beams was over-estimated due to lower than expected lamstock properties and the use of a transformed section modulus where the wood section modulus was required. The cumulative effect of this resulted in a conservative testing program where the specimens fatigued at 1.OFb were actually stressed to 1 .52Fb and the specimens fatigued at 1 .3Fb were actually stressed at 1 .98Fb. The results of these tests showed that the full length reinforced beams fatigued at l.OFb were not prone to fatigue failures. At the higher stress level of 1 .3Fb, specimens failed prematurely and exhibited fatigue failures causing bending stiffness losses. The results also showed that with adequate confinement of the FRP terminations, partial length reinforcing may be structurally feasible. Beams with unconfined terminations fared poorly in fatigue. In addition, the effects of hygrothermal stresses in combination with mechanical fatigue are of particular concern. To better understand the effect, both finite difference and finite element modeling was done to quantify the stresses due to hygrothermal fluctuations that are typical over the life span of a timber bridge girder. A kiln schedule was designed to subject beams to extreme high and low moisture contents to reproduce the cumulative damage occurring over a 50 year life span of a timber bridge girder in a New England environment

Monostatic and Bistatic Radar Measurements of Birds and Micro-drone

This paper analyses the experimental results from recent monostatic and bistatic radar measurements of multiple birds as well as a quadcopter micro-drone. The radar system deployed for these measurements was the UCL developed NetRAD system. The aim of this work is to evaluate the key differences observed by a radar system between different birds and a micro-drone. Measurements are presented from simultaneous monostatic co/cross polarized data as well as co-polar bistatic data. The results obtained show comparable signature within the time domain and a marked difference in the Doppler domain, from the various birds in comparison to the micro-drone. The wing beat properties of the birds are shown for some cases which is a stark contrast to the rotor blade micro-Doppler signatures of the drone

Feature diversity for optimized human micro-doppler classification using multistatic radar

This paper investigates the selection of different combinations of features at different multistatic radar nodes, depending on scenario parameters, such as aspect angle to the target and signal-to-noise ratio, and radar parameters, such as dwell time, polarisation, and frequency band. Two sets of experimental data collected with the multistatic radar system NetRAD are analysed for two separate problems, namely the classification of unarmed vs potentially armed multiple personnel, and the personnel recognition of individuals based on walking gait. The results show that the overall classification accuracy can be significantly improved by taking into account feature diversity at each radar node depending on the environmental parameters and target behaviour, in comparison with the conventional approach of selecting the same features for all nodes

Gait Classification Based on Micro-Doppler Features

This paper focuses on the classification of human gaits based on micro-Doppler signatures. The micro-Doppler signatures can represent detailed information about the human gaits, which helps in judging the threat of a personnel target. The proposed method consists of three major steps. Firstly, the micro-Doppler signatures are obtained by performing time-frequency analysis on the radar data. Then two micro-Doppler features are extracted from the time-frequency domain. Finally, the one-versus-one support vector machine (SVM) is used to realize multi-class classification. Experiments on real data show that, with the selected features, high classification accuracy of the human gaits of interest can be achieved

Dynamic Hand Gesture Classification Based on Radar Micro-Doppler Signatures

Dynamic hand gesture recognition is of great importance for human-computer interaction. In this paper, we present a method to discriminate the four kinds of dynamic hand gestures, snapping fingers, flipping fingers, hand rotation and calling, using a radar micro-Doppler sensor. Two micro-Doppler features are extracted from the time-frequency spectrum and the support vector machine is used to classify these four kinds of gestures. The experimental results on measured data demonstrate that the proposed method can produce a classification accuracy higher than 88.56%

Multistatic Radar Synchronisation Using COTS GPS Disciplined Oscillators

The benefits of multistatic radar have been well understood for decades, though the challenges of implementing such systems have limited their development and subsequent operational use. Multistatic radar’s performance enhancements, over standard monostatic radar, result from the cooperation and data-fusion between spatially separated radar nodes, however, to enable cooperation and data-fusion, some degree of node-to-node time and frequency synchronisation is critical. In this work, the use of commercial off-the-shelf (COTS) GPS Disciplined Oscillators (GPSDO) as a source of indirect radar synchronisation is evaluated and the development, and subsequent testing, of a GPSDO based radar synchronisation system is presented

Copolar Calibration of Multistatic Radar in the Presence of Multipath

This paper addresses the Polarimetrie calibration of the nodes of a multistatic radar system, by using a reference object with known scattering matrix, such as a metallic sphere. A calibration technique is proposed and its experimental validation performed in a realistic scenario, by accounting also for the multipath effect. The intensity of the signal scattered by a metallic sphere and received by the monostatic and bistatic nodes of the NetRAD system is measured, by varying the antenna height, the object range and the bistatic angle. The adopted calibration technique shows a quite good accuracy, as the calibrated values of the radar cross section of the reference object are close to the theoretical ones, after the compensation of the multipath effect