Multi-stable metastructure with multi-layer and multi-degree of freedom: A numerical and experimental investigation

This paper proposes a family of multi-stable metastructures with multiple layers, which possess the capability of multi-degree of freedom deformations. In its single layer, four preshaped beams connecting two frames are employed as the main component for the design of multi-stable metastructures. Compared with the traditional flat state obtained by axial compression when all beams snap through, four inclined stable states are easy to trigger by lateral compression at a local position when two adjacent beams snap through. The transitions between these states are studied by both experiments and numerical simulation. The transition to inclined states requires less energy than the transition to the flat state. Different trends of load–displacement responses are associated with loading positions and transitions. A parametric analysis is performed to illustrate the relationship between the stability of inclined states and critical parameters, such as span, apex height, and thickness. Two types of hourglass double-layer units are designed and studied through experiments. The continuous transitions in two steps or three steps are observed, and the load–displacement response is the accumulation of responses from each single layer. At last, two multi-layer structures with multi-stability have been developed to demonstrate their deformation capability in multiple directions through multiple steps

The controlling mechanisms of horizontal flame spread over thick rods in upward cross flow

This work studies the flame spread over horizontal thick PMMA (polymethyl methacrylate) rods with three radii under different oxygen concentrations and upward cross flow velocities. The flame spread rate and the limit oxygen concentration are measured. Far away from the extinction limit, the flame spread rate increases with the flow velocity but decreases with radius. Near the extinction limit, the flame spread rate is insensitive to the flow velocity and radius. The flame spread rate can be correlated by the stretch rate, and it is found that the flame spread rates for different radii are close at the same stretch rate. A scaling analysis shows that the flame spread rates are approximately square-root dependent on the stretch rate. Prior to the extinction, the flame has entered the regressive burning regime where the flame leading edge will not spread forwardly but continuously retreat. The flame extinction is dependent on the local stretch rate and in-depth heat conduction. For a given radius, the limit oxygen concentration increases with the stretch rate. For a given stretch rate, the smaller cylinder can sustain at lower oxygen concentration due to the less solid-phase heat loss

Opposed flame spread over thick solid fuels under influence of sub-atmospheric pressure and low-velocity flow

The future generation of inhabited spacecraft will have a significantly different cabin environment from the present ones, characterized by low pressure and elevated oxygen concentration. This new atmosphere and the low-velocity gas flows in microgravity provide distinct conditions for the combustion of the solid materials used, and their influence on material flammability is of particular interest in the fire safety of spacecraft. Experiments have been conducted to investigate the effects of sub-atmospheric pressure and low flow velocity on the opposed flame spread and extinction behaviors over a thick PMMA. A flammability map was constructed that delineates the uniform regime, the flamelet regime, and extinction limits for thick PMMA under sub-atmospheric pressures. The limiting oxygen concentration increases with the reduced ambient pressure at a fixed opposed flow, while the flamelet regime becomes wider. Under low ambient pressure, the flame spread rate increases with the flow velocity, but the increasing rate slows down. At a constant oxygen concentration, the flame spread rate increases with the ambient pressure and gas-phase conduction dominates flame spread. At a constant oxygen partial pressure, the higher ignition temperature and less gas-phase conduction reduce the flame spread rate synchronously with the increased pressure

Horizontal flame spread over thin solids in reduced buoyancy environments

A Horizontal Channel Apparatus (HCA) has been employed to investigate opposed flame spread over solid fuels in reduced buoyancy environments, which would be encountered at the diminished gravitational levels on extraterrestrial surfaces (e.g., the partial gravity on the Moon and Mars) as well. With a scaling analysis, the characteristic buoyant flow velocity in HCA is well correlated to the channel height and the Rayleigh ( Ra ) number. The experimental results show that, without forced flow, the flame spread rate and flammability in HCA can be correlated with those at different gravity levels by using Ra number, in-dicating HCA may have the ability to simulate the burning characters in partial gravity. Together with the Reynolds ( Re ) and Ra numbers, which signify the role of forced and buoyant flow, respectively, the flame spread rates and the map of flammability of the solid fuel obtained in the current HCA experiments are determined. Furthermore, four distinctively different flame spread patterns are found at various channel heights and opposed-flow velocities once the width of the cellulosic sample is extended. Likewise, the map of flame spread patterns is governed by the Re and Ra. It is further found through the examinations of the time average of the mass-loss rate that as long as Ra is below a critical value, the opposed flow in terms of Re mainly determines the oxygen transportation, while the reduced buoyancy in terms of Ra mainly contributes to the bifurcations of the potential hydro/thermal instabilities.(c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Microgravity combustion of polyethylene droplet in drop tower

Microgravity experiments of polyethylene (PE) droplet combustion were conducted by a 3.6-s drop tower with the gravity level of 10(-3)similar to 10(-4) g to investigate the burning behaviors and fire hazards of molten thermoplastics in the spacecraft. Pre-ignited droplets with a diameter of about 3 mm were continually generated and detached from burning PE tubes. Once the drop capsule started free-fall, droplets entered the microgravity environment with an initial velocity of 10-35 cm/s (Stage I). A comet-shape flame with an intense bubbling and ejecting process of the moving droplet was observed, and the burning-rate constant (K) was found around 2.6 +/- 0.3 mm(2)/s. After the droplet landed on the floor, it could rebound with a near-zero velocity, showing as a spherical flame (Stage II). The combustion of PE droplet followed the classical d-square law with K = 1.3 +/- 0.1 mm(2)/s. The measured large burning-rate constant (or the volume shrinkage rate) of the moving droplet was caused by the robust bubbling process, which reduced the bulk density of molten PE and ejected unburnt fuel (about 25% of total mass loss). However, the actual mass burning rate of the PE droplet should be smaller than most hydrocarbon liquids because of a smaller mass-transfer number (B approximate to 2). The flame burning rate of PE droplet is 4 +/- 1 g/m(2)-s per unit flame-sheet area that may be used to estimate the fuel mass-loss rate and fire heat release rate in microgravity. This novel microgravity combustion experiment on the thermoplastic droplet could expand the physical understanding of fire risk and hazard of plastic material in the spacecraft environment. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Simulation Methods for MEMS S&A Devices for 2D Fuze Overload Loading

An experimental testing system for the two-dimensional (2D) fuze overload loading process was designed to address the loading issues of recoil overload and centrifugal overload in fuze safety and arming (S&A) device. By incorporating centrifuge rotation energy storage, impact acceleration simulation, and equivalent centrifugal rotation simulation, a block equipped with a fuze S&A device accelerated instantly upon having impact from a centrifuge-driven impact hammer, simulating recoil overload loading. The impact hammer was retracted instantaneously by adopting an electromagnetic brake, which resulted in the centrifugal rotation of the block around its track, to simulate the centrifugal overload loading. The dynamic equations of the experimental testing system and the equations of impact hammer motions were established, whereby the rotation speed of the centrifuge and the braking force of the electromagnetic brake were calculated and selected. A dynamic model of the collision between the impact hammer and block was established using ANSYS/LS-DYNA software for simulation analysis. The acceleration curves of the recoil overload and centrifugal overload with variations in the centrifuge speed, cushion material, and buffer thickness were obtained, which verified the feasibility of the proposed loading simulation method. Two-dimensional overload loading simulation tests were performed using the developed experimental testing system, and the acceleration curves of the recoil overload and centrifugal overload were measured. The test results indicated that the proposed system can accomplish 2D overload loading simulations for a recoil overload of several 10,000× g and centrifugal overload of several 1000× g

Structural, Physicochemical and Digestive Property Changes of Potato Starch after Continuous and Repeated Dry Heat Modification and Its Comparative Study

To investigate the effects of repeated dry heat treatment (RDH) and continuous dry heat treatment (CDH) on the structure and physicochemical and digestive properties of potato starch, potato starch was treated continuously and repeatedly at 130 °C for 3–18 h. The results showed that the crystalline form of starch was consistent with the original type B. Still, its physicochemical properties, such as swelling power, transparency, peak viscosity (PV), final viscosity (FV), breakdown (BD) and thermal properties (To, Tp, Tc, ΔT), tended to decrease. At the same time, solubility and RS increased after dry heat treatment. Moreover, RDH-treated starches were higher than CDH-treated ones in terms of molecular weight, crystallinity, swelling power, transparency and final viscosity for the same treatment time. Still, there was no significant difference between the thermal properties of the two. Meanwhile, the resistant starch (RS) content showed a downward trend after the peak value of 9 h of CDH treatment and five cycles of RDH treatment with increasing treatment time and the number of cycles, indicating a decrease in the overall digestibility of the starch. Overall, RDH had a more significant effect on potato starch’s structure and physicochemical properties than CDH

Na2CaV4O12: A low-temperature-firing dielectric with lightweight, low relative permittivity, and dielectric anomaly around 515 C

A low temperature fired Na2CaV4O12 ceramic was synthesized via a solid-state reaction route at a temperature range of 350–550 °C. The Thermal analysis confirmed the densification and melting temperature of Na2CaV4O12 to be 530 °C and 580 °C, respectively. Dielectric properties together with the electrical conductivity were characterized at a broad frequency and temperature range. A super-low relative permittivity of εr = 7.72 and loss tangent of tanδ = 0.06 were obtained at 1 MHz at room temperature. A dielectric anomaly peak took place around 515 °C, which was associated with the phase transition from P4/nbm to P 4‾ b2. Ac impedance spectrum coupled with complex modulus plots unveiled the electrical conduction mechanism, which was dominated by the short-range movement of the charge carriers at low temperatures (T ≤ 220 °C) however long-range migration of charge carriers emerged at higher temperatures