Free-field Tests Research Articles

Blast Wave Simulator for Laminated Glass Panels Experimental Evaluation

The study of blast loads on structures is important due to the potential of significant consequences in various scenarios. From terrorist attacks to industrial accidents, comprehending how structures respond to blast waves is critical for ensuring public safety and designing resilient structures. Studying these effects typically involves two main methods: free-field tests with live explosives and shock tube tests. Although shock tube testing offers certain advantages, both approaches are costly and demand significant space. This research aims to develop a cost-effective and straightforward technique for generating stress waves that closely replicate the progressive and spatial characteristics of free-field or shock tube blast waves. This method was designed to evaluate the dynamic response of laminated glass panels. The stress wave was generated by impacting a piston on the fluid inside a tube, which was connected to a fluid chamber. This setup produced impulsive loads that were distributed across a laminated glass test panel. Moreover, it was used to simulate the shock near filed explosions for a certain part of a structure. High-speed cameras were utilized to analyze the initial velocity of flying glass fragments. The apparatus successfully produced various blast waves and impulsive profiles for different drop weight heights. The initial velocities of randomly selected flying shards ranged from 3 m/s to 4 m/s.

Experimental investigation of blast mitigation of pre-fractal obstacles

Obstacles arranged into a pre-fractal shape (Sierpinski carpet) were tested for their blast attenuation abilities using 250 g PE4 at three different scaled distances ( Z = 1.87, 2.24, 2.99 m/kg1/3). Three pre-fractal iterations were tested, as well as free-field tests for comparative purposes. Reductions in peak overpressure up to 26% and peak specific impulse up to 19% were observed, attributed to a mechanism known as ‘trapping’. This mechanism is characterised by a reduction in the ability of a blast wave to advect downstream, with corresponding increases in pressure observed within the bounds of the pre-fractal obstacle. Attenuation magnitudes and areas of reduced pressure and impulse were found to be drastically different with each pre-fractal iteration, with a transition from shadowing to wave trapping as the obstacles more closely resembled true fractals. A linear dependence on a newly-defined obstruction factor ( OF) was found for arrival time, overpressure and impulse at the sensor locations, suggesting that the attenuation of a pre-fractal obstacle is inherently determinable. The results indicate that the mechanism of blast mitigation of pre-fractal obstacles is fundamentally different from singular or arrays of regular obstacles, and could be exploited further to develop novel protective structures with enhanced blast attenuation.

Turbine power loss during yaw-misaligned free field tests at different atmospheric conditions

This paper presents an analysis of the power loss of the upstream turbine for yaw misalignment depending on the inflow condition. High-quality experimental field data, obtained from a lidar wind profiler, met mast, GPS based yaw sensor, turbine data, and load data is used. To reduce the spreading of the power loss during yaw misalignment, the rotor effective wind speed is estimated using load data. Furthermore, the so-called power loss coefficient currently used in most engineering models, for yaw misalignment, is assessed at various inflow conditions. As main finding, the power loss coefficient increases for stronger vertical wind shear. In addition, there is an asymmetry with respect to the direction of the yaw misalignment that is enhanced by the wind shear exponent and the wind veer. Neglecting these two effects could result in in-conservative wake-steering decisions at stable stratification

Experimental Performance Assessment of Thermobaric Explosives in Free Field and Internal Blast Tests

Experimental Performance Assessment of Thermobaric Explosives in Free Field and Internal Blast Tests

Electrically Evoked Auditory Late Responses as a Novel Method for Cochlear Implant Programming.

Electrically evoked auditory late responses (eALR) are useful as an objective cochlear implant (CI) fitting method. Different objective and behavioral methods are used for CI fitting. However, there is no objective method that indicates that the electrical signal reaches the auditory cortex. eALR is an indicator that electrical signals reach the auditory cortex, so our aim was to investigate the use of eALR as an objective method for CI programming. Two different programs were created for 21 unilateral Med-El CI users. In the first program, the most comfortable level (MCL) was adjusted with the electrical stapes reflex threshold (eSRT), and the threshold levels (THR) were behaviorally adjusted according to the user's feedback. In the second program, the MCL level was adjusted to a level where all the components of the eALR were clearly seen, and the user did not feel uncomfortable; the THR levels were adjusted to the lowest level where the eALR P1 wave could be seen. The results of the MCL and THR levels of the two programs and the free field tests conducted with both programs were compared. While MCL levels did not differ significantly between the two programs, a significant difference was observed between THR levels. In addition, no significant difference was found between hearing and speech tests with CI in the free field. The results revealed no significant performance difference between the two programs and that eALR could be preferred as an objective method for MCL determination.

Experimental Performance Assessment of Thermobaric Explosives in Free Field and Internal Blast Tests

Experimental Performance Assessment of Thermobaric Explosives in Free Field and Internal Blast Tests

Initial outcomes of the cochlear implant surgery at 108 Military Central Hospital

Objective: To assess the initial outcomes of cochlear implant (CI) surgery and the auditory recovery of the patients underwent CI surgery at the Ear-Nose-Throat Department, 108 Military Central Hospital from January 2016 to June 2020. Subject and method: Twenty-two CI recipients were included in this study. Result: 50% of the included subjects was younger than six years old, had prelingual and severe to profound hearing loss bilaterally. No severe complications was observed except the two cases with minor ones. Postoperatively, 80 - 90% of the subjects was able to recognize and discriminate the environmental sounds and the six ling sounds. Postoperative free field tests gained to normal or mild hearing loss. Conclusion: CI surgery is an effective approach for managing cases with severe to profound hearing loss.

Prediction method for ground shock parameters of explosion in concrete

The ground shock caused by the explosion is the main load causing the destruction of underground engineering. To study the propagation law of ground shock parameters in concrete, free-field tests of explosions at seven different scaled depth of burial (SDOB) have been carried out by using self-constructed concrete blocks with large-sizes. The results indicate that when the SDOB ranges from −0.25 to 1.2 m/kg1/3, the pressure and acceleration at the same scaled distance from the explosion center increase with the SDOB. When the SDOB reaches 1.2 m/kg1/3, the explosion energy is completely enclosed into underground, and the coupling coefficient reaches 1.0. Combined with the dimensional analysis theory, it is found that when the SDOB of an explosion is determined, the propagation law of pressure and acceleration presents a power function attenuation with the increase of the scaled distance from the explosion center. For the three different strength grades of C30, C50, C70 concrete under semi-buried explosion, the attenuation gradient of the pressure or acceleration is inconsistent with an increase in the scaled distance from the explosion center. The higher the concrete strength, the slower the gradient changes. By introducing the depth of burial (DOB) coefficient of equivalent energy, the prediction method of ground shock parameters at different SDOB of explosion and explosions with different mass of charge can be obtained, and it has high accuracy for calculating the ground shock parameters of explosion with large mass charge which verified by numerical simulation.

Experiments of an atrium-style metro station under harmonic excitation

The atrium-style station is one of the favourite choices in underground railway systems, as it can provide a larger and clearer space for passengers and commercial clients at the hall level of the station. With such style, as hall slabs are replaced with flat-beams, the station's capabilities to withstand lateral action like seismic action become one of the main concerns. Shaking table tests were conducted to investigate the dynamic response of soil-structure interaction of atrium-style station model under harmonic excitations covering the full frequency range, with comparison to free-field tests. The experimental results confirm that the influence of the structure on the dynamic response of the ground surface begins when the frequency of input motions is greater than 10 Hz. The predominant frequencies of the ground corresponding to the second and third modes both show an increase of 10% from the FF model to the SS model due to the presence of an atrium-style metro station. For atrium-style station: (i) the discontinuous structural elements such as beams and columns are more severely strained than the continuous ones like slabs and side walls, (ii) the peak dynamic soil normal stresses along the side wall follow an approximate L-shaped distribution and the maximum stress occurs at the side wall bottom, and (iii) the spectral characteristics of both dynamic structural strain and dynamic soil normal stress reflect the amplification function of the ground.

Macro-mechanical modelling of blast wave mitigation in foams. Part I: review of available experiments and models

Multiphase flows, which involve compressible or incompressible fluids with linear or nonlinear dynamics, are found in all areas of technology at all length scales and flow regimes. In this contribution, we discuss application of aqueous-foam barriers against blast wave impact. The first experiments demonstrating this behaviour were conducted in the early 1980s in free-field tests. Based on structural requirements, various foams with different blast energy contents were tested with the aim of characterizing the time history of the blast pressure reduction. A number of consistent methodologies for calculating this pressure reduction in foam are based on the effective gas flow model. For estimating the uncertainties of these methodologies, we briefly demonstrate their comparison with existing experimental data. Thereafter, we present various modifications of modelling approaches and their comparison with new results of blast wave experiments.

Influence of a wind turbine service life on the mechanical properties of the material and the blade

ABSTRACTThis paper presents results out of investigations of the DEBRA‐25 wind turbine blades. Almost unique in the history of modern wind energy, these blades were in operation for 18 years next to a weather station and were investigated afterward. Therefore, the loads experienced in the operational life could be post‐processed accurately with the measured data of the weather station and the turbine. The blades are made of materials that are similar with today's wind turbines. Furthermore, intensive laboratory tests and free field tests have been carried out, and all load assumptions and data and results are still available today. The results include experimental investigations on the moisture content of the load‐carrying material, static and fatigue behavior of the material, the relaxation of the coupling joints, the natural frequencies of the blade and a full scale static blade test. It is shown that the structural performance of the DEBRA‐25 service blades is comparable with modern wind turbine blades. Although some damage was found by visual inspection, the service blade of the DEBRA‐25 showed excellent mechanical behavior in the full scale blade test. Only small changes of the edgewise eigenfrequencies were detected. The pre‐tensioning forces of the IKEA bolts, where the two blade parts are connected, were measured and were still adequate. Copyright © 2012 John Wiley & Sons, Ltd.

Laboratory scale tests for the assessment of solid explosive blast effects. Part II: Reflected blast series of tests

An experimental set-up has been developed in order to detonate small explosive charges and to measure their induced blast on reduced scale structures. On the basis of Crank Hopkinson’s Law, the results obtained at reduced scale are used to derive full scale predictive models. A previous paper summarized the development of the reduced scale experimental set-up as well as the first free-field tests performed. The main features of the table, the instrumentation and the pyrotechnics were given.The present paper summarizes a second series of tests investigating the validity of models designed to predict reflected blast waves characteristics. Overpressure maxima, arrival time of the shock wave and impulse have been measured and are presented in this paper as non-dimensional characteristics of the pressure-time history. The results obtained from the experiments are in good agreement in comparison with data from the open literature and may thus be considered as reference results for further numerical or analytical model developments.The series of tests reported in this paper is in addition the final series of tests dedicated to validate the experimental set-up and the experimental procedures associated.

A Prospective Multi-Centre Study of the Benefits of Bilateral Hearing Aids

In this prospective study the benefits of a second hearing aid were evaluated objectively (functional tests) and subjectively (questionnaires). In addition, a battery of diagnostic tests (by headphones) was applied to determine whether the bilateral benefit could be predicted from a priori tests. Diagnostic tests by headphones (binaural masking level difference, interaural time difference, and speech reception thresholds per ear) were conducted to focus on binaural functioning, and free-field tests were used for unilateral and bilateral hearing aids in the same participants to analyze differences in speech intelligibility and horizontal localization. The participants were recruited from the regular patients for hearing aid fitting in eight Audiological Centers. Eventually, 214 participants participated in this study. Each of these participants was fitted with two new hearing aids and started a trial period. Before the trial period the headphone tests were conducted, after the trial period the free-field tests were conducted with one and two hearing aids, and the participants completed a questionnaire. In a number of respects bilaterally fitted hearing aids offered a benefit relative to unilaterally fitted hearing aids (the so-called bilateral benefit), both subjectively (questionnaire) and objectively (speech perception in noise and localization). However, we found large interindividual differences and not all differences were clinically relevant. The results of the diagnostic tests showed that it was not possible to predict the bilateral benefit from a priori information based on headphone tests. At the end of the trial period 93% of the participants preferred a bilateral fitting.

Shaking Table Dynamic Tests Of AGranular Deposit

CEDEX has a six degrees of freedom seismic simulator formed by 3 m long square table, able to move 10 t mass with up to 1-g accelerations. On that simulator a rigid box was placed and filled with some 6 t of Hostun RF sand to study the free field response of a granular deposit subjected to dynamic loading. During the filling of the box, some accelerometers were installed inside the sand to measure accelerations suffered by the granular deposit, and two displacement transducers were placed on the deposit surface. With this equipment, ten tests were conducted in which the sand deposit, with different initial relative densities, was subjected to sinusoidal dynamic loadings, in terms of accelerations, with different amplitudes imposed at the bottom of the box. During those free field tests, accelerations inside the sand and horizontal and vertical movements on the top surface of the granular deposit were measured. The results deduced from the signals recorded by the different accelerometers and from the measurements of vertical and horizontal movements of the sand surface are discussed and compared to real cases and values deduced from theories published in the literature.

Speech recognition in background noise: monaural versus binaural listening conditions in normal-hearing patients.

Patients with unilateral conductive hearing impairment often experience a great advantage in improved hearing thresholds after reconstructive ear surgery because the binaural hearing leads to increased speech recognition in competing noise. The aim of this study was to develop a clinical audiometric test to quantify the ability of speech recognition in background noise preoperatively and postoperatively. A model consisting of phonetically balanced (PB) monosyllabic words and/or a just-follow-conversation test (JFC) was used in 16 normal-hearing listeners. In 8 of the participants, JFC at two levels of background noise (60 and 40 dB sound pressure level) was used as the only method. The tasks were performed in an anechoic chamber with varying spatial separations of speech and noise sources under free field conditions. The tests were made binaurally and monaurally; the latter condition was achieved by occluding one ear with a hearing protector. In PB word tests, the percent of correctly repeated words was scored. In the JFC task, the speech signal was adjusted by the listener to a level where he or she could just understand what was being said. This was expressed as a signal-to-noise (S/N) ratio. In both types of tests, the subjects achieved significantly better speech recognition scores in the binaural situations. In summary, compared with monaural conditions, the subjects achieved 17% to 18% better speech recognition in PB tests, and in JFC tests a 2-to 3-dB lower S/N ratio in binaural conditions. These free field tests can be developed further as a clinical tool preoperatively and postoperatively to evaluate the effect of binaural hearing after ear surgery.

Mechanisms of Resistance to the Potato Leafhopper (Homoptera: Cicadellidae) in Selected Alfalfa Clones

Selected clones of alfalfa, Medicago sativa L., from three populations (‘Arc’, B16, and Indiana Syn C) were subjected to free and no-choice laboratory and field tests to identify specific mechanisms of resistance to potato leafhopper, Empoasca fabae (Harris). Initial selection of clones used in this study was based on seedling survival under heavy potato leafhopper adult feeding pressure. In addition to those mechanisms of resistance associated with dense pubescence in B16 clones (feeding and oviposition nonpreference and nymphal antibiosis), this study identified specific clones in ‘Arc’ and Indiana Syn C populations demonstrating feeding or oviposition nonpreference or both. A small but significant level of nymphal antibiosis also was identified in several ‘Arc’ clones. No clones that had an adverse effect on adult survival or development were identified. The identification of individual clones with moderate levels of feeding and oviposition nonpreference and nymphal antibiosis in laboratory tests under controlled conditions suggests that greater progress would be achieved in identifying resistant clones if initial and recurrent selection for these resistance mechanisms were stressed using individual plant selection rather than seedling selection.

Clinical role of free-field voice tests in children

Clinical voice testing forms part of the routine assessment of children's hearing. However, its role in paediatric practice has not been defined. In particular, the sensitivity and specificity of the test in detecting the presence of a hearing impairment is not known. The value of the test as a screen was assessed by comparison of the results of free-field clinical voice testing with pure-tone audiometric thresholds in 141 children aged 5-12 years. In order to pass the screen, a child had to respond to a whispered voice 2 feet (60 cm) from the test ear. The mean pure-tone average (average at 0.5, 1.0 and 2.0 kHz) of those ears that passed was 14 dB HL compared with 31 dB HL for those that failed (P less than 0.001). The test was 80% sensitive (95% specific) in detecting a hearing impairment greater than or equal to 25 dB HL, and 89% sensitive (90% specific) in detecting a hearing impairment greater than or equal to 30 dB HL. Therefore, clinical voice tests will fail to detect a significant proportion of children with a hearing impairment and cannot be advocated as a screening method. The test would appear to be of limited value in a specialist situation where facilities for pure-tone audiometry exist.

Localization of tones by horses: Use of binaural cues and the role of the superior olivary complex.

The ability of horses to use binaural time and intensity difference cues to localize sound was assessed in free-field localization tests by using pure tones. The animals were required to discriminate the locus of a single tone pip ranging in frequency from 250 Hz to 25 kHz emitted by loudspeakers located 30 degrees to the left and right of the animals' midline (60 degrees total separation). Three animals were tested with a two-choice procedure; 2 additional animals were tested with a conditioned avoidance procedure. All 5 animals were able to localize 250 Hz, 500 Hz, and 1 kHz but were completely unable to localize 2 kHz and above. Because the frequency of ambiguity for the binaural phase cue delta phi for horses in this test was calculated to be 1.5 kHz, these results indicate that horses can use binaural time differences in the form of delta phi but are unable to use binaural intensity differences. This finding was supported by an unconditioned orientation test involving 4 additional horses, which showed that horses correctly orient to a 500-Hz tone pip but not to an 8-kHz tone pip. Analysis of the superior olivary complex, the brain stem nucleus at which binaural interactions first take place, reveals that the lateral superior olive (LSO) is relatively small in the horse and lacks the laminar arrangement of bipolar cells characteristic of the LSO of most mammals that can use binaural delta I.

Some imperfections of the free-field voice tests.

Some imperfections of the free-field voice tests.