Impact-crater ejecta on Bennu indicate a surface with very low strength

A planetary surface’s resistance to change is generally described as its “strength” (units of stress). The surface strength of small, rubble-pile asteroids, which consist of fragments of larger bodies that were collisionally disrupted, is poorly constrained due to their wide departure from terrestrial analogs. Here, we report the observation of an ejecta deposit surrounding an impact crater that limits the maximum surface strength of the near-Earth rubble-pile asteroid (101955) Bennu. The presence of this deposit implies that ejecta were mobilized with velocities less than the escape velocity of Bennu, 20 cm/s. Because ejecta velocities increase with surface strength, the ejecta deposit can only be explained if the effective strength of the surface material near the crater is exceedingly low, ≤100 Pa. This is three orders of magnitude below values commonly used for asteroid surfaces, but is supported by previous observations of an artificial impact crater on a similar asteroid, Ryugu. Our findings indicate a mobile surface that has likely been renewed multiple times since Bennu’s initial assembly and have far-reaching implications for interpreting observations of Bennu and other rubble piles.

Experimental Study on the Flexural Behavior of Reinforced Polystyrene Blocks in Concrete Beams

A new type of lightweight beam system was recently proposed by embedding polystyrene in beams to improve structural efficiency. This removes the non-performing concrete in the neutral axis and tension region to provide a comparable strength as a solid beam. There are, however, limited studies conducted to investigate the structural behavior of such beams. Therefore, this research presents an experimental investigation to assess the effect of polystyrene shapes in the beams. This involved testing a solid beam and five lightweight beams under flexural load using a four-point load test. The inclusion of polystyrene was estimated to have reduced the self-weight of beams by 8.6% to 11.8% when compared with the solid beam. The results also showed the ellipse polystyrene with a width of 70 mm and height of 50 mm produced the highest effective strength to weight ratio (sw) of 1.12 and performed 12% better than the solid beam. Moreover, the lightweight beams have more weight reduced than the strength, and those with ellipse polystyrene were found to have performed better than circular ones based on first crack load, ultimate load, and effective strength to weight ratio (sw). The beams with ellipse polystyrene allowed better stress distribution and this gave them a higher strength than sphere shape. For industry application, the polystyrene content is recommended to be greater than 10% while the effective strength to weight ratio (sw) of the beam is greater than 1. The successful reduction of the weight without affecting the structural performance has the ability to help in reducing construction costs.

Molecular Dynamics Simulations and Theoretical Model for Engineering Tensile Properties of Single-and Multi-Walled Carbon Nanotubes

To apply carbon nanotubes (CNTs) as reinforcing agents in next-generation composites, it is essential to improve their nominal strength. However, since it is difficult to completely remove the defects, the synthesis guideline for improving nominal strength is still unclear, i.e., the effective strength and the number of nanotube layers required to improve the nominal strength has been undermined. In this study, molecular dynamics simulations were used to elucidate the effects of vacancies on the mechanical properties of CNTs. Additionally, the relationships between the number of layers and effective and nominal strengths of CNTs were discussed theoretically. The presence of extensive vacancies provides a possible explanation for the low nominal strengths obtained in previous experimental measurements of CNTs. This study indicates that the nominal strength can be increased from the experimentally obtained values of 10 GPa to approximately 20 GPa by using six to nine nanotube layers, even if the increase in effective strength of each layer is small. This has advantages over double-walled CNTs, because the effective strength of such CNTs must be approximately 60 GPa to achieve a nominal strength of 20 GPa.

Performance of Reinforced Concrete Beam with Polystyrene Blocks at Various Regions

Lightweight materials, such as polystyrene, can be embedded in reinforced concrete (RC) beams to reduce its weight. However, this may, to some extent, affect the performance of the structure. This research investigates the behaviour of the lightweight beams under load and determines the best position of polystyrene blocks in beams. Nine specimens with a size of 175 mm x 300 mm x 1600 mm were tested under four-point load test. The number and position of polystyrene blocks in the beams were varied. The specimens were evaluated for effectiveness in terms of effective strength to weight ratio (s-w ratio). The lightweight beam was effective when the polystyrene blocks were placed at the neutral and tensile region, offering an s-w ratio of greater than 1. The beam lost 3.8% strength with 8.4% reduction of weight.

Repeated Contrast Adaptation Does Not Cause Habituation of the Adapter

Adaptation can optimize information processing by allowing the visual system to always adjust to the environment. However, only a few studies have investigated how the visual system makes adjustments to repeatedly occurring changes in the input, still less about the related neural mechanism. Our previous study found that contrast adaptation attenuated after multiple daily sessions of repeated adaptation, which was explained by the habituation of either the adapter’s effective strength or the adaptation mechanisms. To examine the former hypothesis, in the present study we used the frequency tagging technique to measure the adapter-elicited steady-state visual evoked potential (SSVEP) amplitudes. Participants repeatedly adapted to the same contrast adapter in a top-up manner for six continuous days, which was called training of adaptation. The behavioral adaptation effect and SSVEP response to the trained adapter and an untrained control adapter were measured before and after training. The psychophysical results showed that the effect of adaptation in the trained condition significantly reduced after training, replicating our previous finding. Contradicting the prediction of the hypothesis that repeated adaptation attenuated the effective strength of the adapter, the SSVEP amplitude was unchanged after training, which was further confirmed by an equivalence test. Taken together, the results challenge the account of habituation of adapter in repeated adaptation, while leaving the account of habituation of adaptation mechanism to be tested.

Physico-chemical nature of clayey soils strength

The principles of soil mechanics based on Mohr–Coulomb strength theory and Terzaghi effective stresses theory give us the idea about the nature of clay strength. However, these theories consider the clay as a solid body and do not take into account their internal structure. In the present paper, the problems in determining the strength of clayey soils are considered from the standpoint of the physicochemical theory of effective stresses. This theory is based on the ideas of the internal structure of clays. The main idea implies that the structure of clays depends on the contacts formed between mineral particles, where the external stresses transmitted to the soil are concentrating. Water in clayey soils not only fills the pore space but also interacts with the mineral particles forming hydrate films around them. The prevailing type of structural contacts in clays determines the properties of latter. Strength is an intrinsic property of soil determined by its composition and internal structure, independent of the conditions of load application to it and characterized by the actual effective strength value. The value of actual effective strength in clays is determined by the total strength of individual contacts. This is the maximal stress transferred to the soil contacts, the structure being ruined upon exceeding this value. The actual effective strength in soils with the same predominant type of contacts depends on the number of contacts. The numerical strength characteristics of contact types are determined for different clay soil varieties to characterize the actual effective strength. The parameters of relationship between the actual effective strength in clays and their strength characteristics are obtained from the soil testing by standard methods.