Synchronized Full-Field Strain and Temperature Measurements of Commercially Pure Titanium under Tension at Elevated Temperatures and High Strain Rates

Understanding the mechanical behavior of materials at extreme conditions, such as high temperatures, high strain rates, and very large strains, is fundamental for applications where these conditions are possible. Although tensile testing has been used to investigate material behavior under high strain rates and elevated temperatures, it disregards the occurrence of localized strains and increasing temperatures during deformation. The objective of this work is to combine synchronized full-field techniques and an electrical resistive heating system to investigate the thermomechanical behavior of commercially pure titanium under tensile loading at high temperatures and high strain rates. An electrical resistive heating system was used to heat dog-bone samples up to 1120 °C, which were then tested with a tensile Split Hopkinson Pressure Bar at strain rates up to 1600 s−1. These tests were monitored by two high-speed optical cameras and an infrared camera to acquire synchronized full-field strain and temperature data. The displacement and strain noise floor, and the stereo reconstruction error increased with temperature, while the temperature noise floor decreased at elevated temperatures. A substantial decrease in mechanical strength and an increase in ductility were observed with an increase in testing temperature. The localized strains during necking were much higher at elevated temperatures, while adiabatic heating was much lower or non-existent at elevated temperatures.

Detection of Motor-Evoked Potentials Below the Noise Floor: Rethinking the Motor Threshold

Background: Motor-evoked potentials (MEP) are one of the most prominent responses to brain stimulation, such as supra-threshold transcranial magnetic stimulation (TMS) and electrical stimulation. Understanding of the neurophysiology and the determination of the lowest stimulation strength that evokes responses requires the detection of even smaller responses, e.g., from single motor units. However, available detection and quantization methods suffer from a large noise floor. Objective: This paper develops a detection method that extracts MEPs hidden below the noise floor. With this method, we aim to estimate excitatory activations of the corticospinal pathways well below the conventional detection level. Methods: The presented MEP detection method presents a self-learning matched-filter approach for improved robustness against noise. The filter is adaptively generated per subject through iterative learning. For responses that are reliably detected by conventional detection, the new approach is fully compatible with established peak-to-peak readings and provides the same results but extends the dynamic range below the conventional noise floor. Results: In contrast to the conventional peak-to-peak measure, the proposed method increases the signal-to-noise ratio by more than a factor of 5. The first detectable responses appear to be substantially lower than the conventional threshold definition of 50 μV median peak-to-peak amplitude. Conclusion: The proposed method shows that stimuli well below the conventional 50 μV threshold definition can consistently and repeatably evoke muscular responses and thus activate excitable neuron populations in the brain. As a consequence, the IO curve is extended at the lower end, and the noise cut-off is shifted. Importantly, the IO curve extends so far that the 50 μV point turns out to be closer to the center of the logarithmic sigmoid curve rather than close to the first detectable responses. The underlying method is applicable to a wide range of evoked potentials and other biosignals, such as in electroencephalography.

Thin-Film-Based SAW Magnetic Field Sensors

In this work, the first surface acoustic-wave-based magnetic field sensor using thin-film AlScN as piezoelectric material deposited on a silicon substrate is presented. The fabrication is based on standard semiconductor technology. The acoustically active area consists of an AlScN layer that can be excited with interdigital transducers, a smoothing SiO2 layer, and a magnetostrictive FeCoSiB film. The detection limit of this sensor is 2.4 nT/Hz at 10 Hz and 72 pT/Hz at 10 kHz at an input power of 20 dBm. The dynamic range was found to span from about ±1.7 mT to the corresponding limit of detection, leading to an interval of about 8 orders of magnitude. Fabrication, achieved sensitivity, and noise floor of the sensors are presented.

A towed magnetic gradiometer array for rapid, detailed imaging of utility, geological, and archaeological targets

Efficient and accurate acquisition of magnetic field and gradient data have applications over a large range of environmental, archaeological, engineering, and geologic investigations. Developments in new systems and improvements in existing platforms have progressed to the point where magnetic surveying is a heavily used and trusted technique. However, there is still ample room to improve accuracy and coverage efficiency and to include reliable vector information. We have developed a vector magnetic gradiometer array capable of recording high-resolution field and gradient data over tens of hectares per day at 50 cm sensor spacing. Towed by an all-terrain vehicle, the system consists of eight vertical gradiometer sensor packages and incorporates differential GPS and an inertial measurement system. With a noise floor of around 6 nT at 15 km/h towing speed and 230 Hz sample rates, large areas can be mapped efficiently and precisely. Data are processed using a straightforward workflow, using both standard and newly developed methodologies. The system described here has been used successfully in Denmark to efficiently map buried structures and objects. We give two examples from such applications highlighting the system's capabilities in archaeological and geological applications.

Robust Acoustic Scene Classification in thePresence of Active Foreground Speech

We present an iVector based Acoustic Scene Clas-sification (ASC) system suited for real life settings where activeforeground speech can be present. In the proposed system, eachrecording is represented by a fixed-length iVector that modelsthe recording’s important properties. A regularized Gaussianbackend classifier with class-specific covariance models is usedto extract the relevant acoustic scene information from theseiVectors. To alleviate the large performance degradation when aforeground speaker dominates the captured signal, we investigatethe use of the iVector framework on Mel-Frequency CepstralCoefficients (MFCCs) that are derived from an estimate of thenoise power spectral density. This noise-floor can be extracted in astatistical manner for single channel recordings. We show that theuse of noise-floor features is complementary to multi-conditiontraining in which foreground speech is added to training signalto reduce the mismatch between training and testing conditions.Experimental results on the DCASE 2016 Task 1 dataset showthat the noise-floor based features and multi-condition trainingrealize significant classification accuracy gains of up to more than25 percentage points (absolute) in the most adverse conditions.These promising results can further facilitate the integration ofASC in resource-constrained devices such as hearables.

Rotational Seismology with a Quartz Rotation Sensor

We describe the construction and performance of a new high-precision ground- or platform-rotation sensor called the Quartz Rotation Sensor (QRS). The QRS is a mechanical angular accelerometer that senses rotational torque with an inherently digital, load-sensitive resonant quartz crystal. The noise floor is measured to be ∼45 pico-radians/root (Hz) near 1 Hz, and the resonant period of the sensor is about 10 s, making it a broadband sensor. Among similarly sized broadband rotation sensors, this represents more than two orders of magnitude improvement in noise floor near 0.1 Hz. We present measurements of rotational components of teleseismic waves recorded with the sensor at a vault. The QRS is useful for rotational seismology and for improving low-frequency seismic isolation in demanding applications such as the Laser Interferometer Gravitational-Wave Observatories.

High-Resolution Multi-Channel Frequency Standard Comparator Using Digital Frequency Measurement

The rapid improvement accuracy of the atomic frequency standard puts forward higher requirements for the measurement resolution of the atomic frequency standard comparison system. To overcome the defect that the single zero-crossing point detection is sensitive to noise in the traditional dual mixer time difference measurement method, a digital frequency measurement method is proposed. This method combines sinusoidal beat technology, multi-channel synchronous acquisition technology, and digital frequency measurement technology, and uses differential compensation of system error to realize the precision measurement of atomic frequency standard. The frequency measurement accuracy is less than 2.5 × 10−14 and the noise floor is better than 6.5 × 10−15/τ = 1 s. The system has a high frequency measurement accuracy and a low noise floor, which can realize the precise measurement of a highly stable frequency source.