Potential Analysis of High-g Shock Experiment Technology for Heavy Specimens Based on Air Cannon

According to the technical requirements of harsh shock environment test, this paper presents the study on the pneumatic vertical test technology with large load and high-g value. The inspiration of this paper comes from the fact that a compressed air cannon can produce instantaneous and powerful air jets that can be used to drive the tested object to achieve a high initial collision velocity. Then, the principle of shock test technology based on an air cannon and an impact cylinder was put forward, and the idea gas mechanics model was established to theoretically analyze the laws that how the parameters of the air cannon and cylinder influence the initial impact velocities. The test system was built, and the test research was carried out. When the air cannon pressure is 0.5 MPa and 0.65 MPa, respectively, under no-load, the impact acceleration measured is 1990 g (pulse width, 1.26 ms) (1g = 9.8 m/s2) and 4429 g (pulse width, 1.20 ms). It preliminarily validated the effectiveness and feasibility.

Experimental constraints on the ordinary chondrite shock darkening caused by asteroid collisions

<p><strong>Introduction</strong></p><p>Shock-induced changes in planetary materials related to impacts or planetary collisions are known to be capable of altering their optical properties. One such example is observed in ordinary chondrite meteorites. The highly shocked silicate-rich ordinary chondrite material is optically darkened and its typical S-complex-like asteroid spectrum is altered toward a darker, featureless spectrum resembling the C/X complex asteroids. Thus, one can hypothesize that a significant portion of the ordinary chondrite material may be hidden within the observed C/X asteroid population.</p><p>The exact pressure-temperature conditions of the shock-induced darkening are, however, not well constrained and due to this gap in knowledge, it is not possible to correctly assess the significance of the shock darkening within the asteroid population. In order to address this shortcoming, we experimentally investigate the gradual changes in the chondrite material optical properties together with the associated mineral and textural features as a function of the shock pressure. For this purpose, we use a Chelyabinsk meteorite (LL5 chondrite), which is subjected to a spherical shock experiment. The spherical shock experiment geometry allows for a gradual increase in the shock pressure within a single spherically shaped sample from 15 GPa at its rim toward hundreds of gigapascals in the center.</p><p><strong>Results</strong></p><p>Four distinct zones were observed with an increasing shock load (Fig. 1). We number the zones in the direction of increasing shock from the outside toward the center as zones I–IV The optical changes in zone I are minimal up to ~50 GPa. In the region of ~50–60 GPa corresponding to zone II, shock darkening occurs due to the troilite melt infusion into silicates. This process abruptly ceases at pressures of ~60 GPa in zone III due to an onset of silicate melting and immiscibility of troilite and silicate melts. Silicate melt coats residual silicate grains and prevents troilite from further penetration into cracks. At pressures higher than ~150 GPa (zone IV), complete recrystallization occurs and is associated with a second-stage shock darkening due to fine troilite-metal eutectic grains.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.369960f7c0fe58218382951/sdaolpUECMynit/0202CSPE&app=m&a=0&c=65ce9691abaaf54f5e7768045027f7ea&ct=x&pn=gnp.elif" alt="" width="777" height="639"></p><p>The order of the spectral curves in the UV-VIS-NIR (ultraviolet – visible – near-infrared) region follows the visual brightness in which zone I is the brightest, followed by zones III and II, and zone IV is the darkest one (Fig. 2). The MIR reflectance (Fig. 3) follows the same albedo order as UV-VIS-NIR up to the primary Christiansen feature at 8.7 µm. At higher wavelengths in the Si-O reststrahlen bands region, the reflectance order changes with zones II and III, which are brighter than zones I and IV. The comparison of the powdered sample spectra to the one obtained from the rough saw-cut surface reveals the following trends. The overall reflectance of the powdered sample is an order of magnitude lower compared to the rough surface one. The reststrahlen bands in both samples show similar positions at approximately 9.1, 9.5–9.6, 10.3, 10.8, 11.3, and 11.8–12 µm. They are dominated by olivine with possible presence of orthopyroxene. The amplitudes of the reststrahlen bands are higher in the rough surface sample. The transparency feature at 12.7 µm is only observed in the powdered sample. The primary Christiansen feature at 8.7 µm is more pronounced in the powdered sample, while the secondary one at 15.6 µm is of a low amplitude in both samples.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.ad38963ac0fe50178382951/sdaolpUECMynit/0202CSPE&app=m&a=0&c=1cbf38a911d6e0bef4cff605b284362f&ct=x&pn=gnp.elif" alt=""></p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.4ff3f9a9c0fe59658382951/sdaolpUECMynit/0202CSPE&app=m&a=0&c=9e7ff0973b952ce0eed7a0fbfc5b24cc&ct=x&pn=gnp.elif" alt=""></p><p><strong>Conclusions</strong></p><p>The important finding is the presence of the two distinct shock darkening mechanisms in ordinary chondrite material with characteristic material fabric and distinct pressure regions. These two regions are separated by a pressure interval where no darkening occurs. Thus, the volume of the darkened material produced during asteroid collisions may be somewhat lower than assumed from a continuous darkening process. While the darkening mainly affects the UV-VIS-NIR region and 1 and 2-µm silicate absorption bands, it does not significantly affect the silicate spectral features in the MIR region. These are more affected by material roughness. MIR observations have the potential to identify darkened ordinary chondrite material with an otherwise featureless UV-VIS-NIR spectrum.</p>

Experimental constraints on the ordinary chondrite shock darkening caused by asteroid collisions

Context. Shock-induced changes in ordinary chondrite meteorites related to impacts or planetary collisions are known to be capable of altering their optical properties. Thus, one can hypothesize that a significant portion of the ordinary chondrite material may be hidden within the observed dark C/X asteroid population. Aims. The exact pressure-temperature conditions of the shock-induced darkening are not well constrained. Thus, we experimentally investigate the gradual changes in the chondrite material optical properties as a function of the shock pressure. Methods. A spherical shock experiment with Chelyabinsk LL5 was performed in order to study the changes in its optical properties. The spherical shock experiment geometry allows for a gradual increase of shock pressure from ~15 GPa at a rim toward hundreds of gigapascals in the center. Results. Four distinct zones were observed with an increasing shock load. The optical changes are minimal up to ~50 GPa. In the region of ~50–60 GPa, shock darkening occurs due to the troilite melt infusion into silicates. This process abruptly ceases at pressures of ~60 GPa due to an onset of silicate melting. At pressures higher than ~150 GPa, recrystallization occurs and is associated with a second-stage shock darkening due to fine troilite-metal eutectic grains. The shock darkening affects the ultraviolet, visible, and near-infrared region while changes to the MIR spectrum are minimal. Conclusions. Shock darkening is caused by two distinct mechanisms with characteristic pressure regions, which are separated by an interval where the darkening ceases. This implies a reduced amount of shock-darkened material produced during the asteroid collisions.

Shock-wave experiment with the Chelyabinsk LL5 meteorite: experimental parameters and the texture of the shock-affected material)

The shock experiment with Chelyabinsk LL5 light lithology material was performed as a spherical geometry shock. The material experienced shock and thermal metamorphism from the initial S3–4 up to complete melt stage. Temperature and pressure realized were estimated above 2000°С and 90 GPa. Textural shock effects were studied by the means of optical and electron microscopy. By the only experimental impact, all the range of the shock pressures and temperatures was realized. Four zones were revealed from the petrographic analysis: 1 – melt zone, 2 – melted silicates zone, 3 – black ring zone, 4 – weakly shocked initial material. Several features of the material texture were noted: displacement of the metal and troilite phases from the central melt zone; mixed lithology zone formation (light-colored chondrules within the silicate melt); dark-colored lithology ring formation; generation of radial-oriented shock veins. Thus, at the experimental fragment, four texture zones were formed. These zones correspond to the different lithology types of the Chelyabinsk LL5 meteorite, which could be found in different fragments of the meteoritic shower from UrFU collection. The results obtained prove that the shock wave loading experiment could be used for space shock modeling. Therefore, the processes of the small bodies of the Solar system could be experimentally modeled at the laboratory conditions.

Imagine avoidance: Induction of conditioned avoidance via mental imagery

Excessive avoidance is a key diagnostic criterion across mental disorders. Theoretical modelsargue that such avoidance is acquired via direct experience, instructions, or social observation.Here, we investigated whether avoidance can also be acquired via mental imagery. Participantslearned to associate a neutral stimulus (A+) with a shock and two other neutral stimuli (B-, C-)without shock. Afterwards, they learned to avoid A+ but not C- Then, they imagined that Bwouldbe followed by the shock (Experiment 1; ! = 66) or they imagined a shock while B- waspresented (Experiment 2; ! = 60). Results showed that when participants were afterwardspresented with unreinforced presentations of A+, B-, or C-, they tended to avoid B- if they wereinstructed to imagine the B- together with the shock but not when they imagined the shock alone.We extend on how our findings could explain the acquisition of excessive avoidance.