Closed-Cell Rigid Polyimide Foams for High-Temperature Applications: The Effect of Structure on Combined Properties

Closed-cell rigid polyimide foams with excellent thermal stability and combined properties were prepared by thermal foaming of a reactive end-capped polyimide precursor powder in a closed mold. The precursor powder was obtained by thermal treatment of a polyester-amine salt (PEAS) solution derived from the reaction of the diethyl ester of 2,3,3′,4′-biphenyl tetracarboxylic dianhydride (α-BPDE) with an aromatic diamine mixture of p-phenylenediamine (PDA) and 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) in the presence of an end-capping agent (mono-ethyl ester of nadic acid anhydride, NE) in an aliphatic alcohol. The effect of polymer mainchain structures on the foaming processability and combined properties of the closed-cell rigid polyimide foams were systematically investigated. The polyimide foams (100–300 kg/m3) with closed-cell rates of 91–95% show an outstanding thermal stability with an initial thermal decomposition temperature of ≥490 °C and a glass transition temperature of 395 °C. Polyimide foams with density of 250 kg/m3 exhibited compression creep deformation as low as 1.6% after thermal aging at 320 °C/0.4 MPa for 2 h.

Transversely Isotropic Creep Characteristics and Damage Mechanism of Layered Phyllite Under Uniaxial Compression Creep Test

The layered surrounding rocks of deep tunnels undergo large creep deformation due to the presence of planes of weakness and the presence of prolonged high in-situ stress, thereby the deformation severely endangers the safety of tunnels. This study conducts uniaxial compression creep tests to experimentally investigate the transversely isotropic creep characteristics and the damage mechanism of layered phyllite samples having bedding angles of 0°, 22.5°, 45°, 67.5°, and 90°. The results indicate that the creep deformation of the specimens takes place in four stages: the instantaneous elastic deformation stage, the deceleration creep stage, the steady-state creep stage, and the accelerated creep stage. The cumulative creep deformation and the creep time during the steady-state creep stage of the specimens initially decrease and then increase as the bedding angle changes from 0° to 90°, thereby, corresponding to the initial increase and subsequent decrease in creep rate during the deceleration creep stage. Based on the existing viscoelastic-plastic damage creep model, the creep parameters E1, E2, η2, and η3 are observed to initially decrease and then increase with the increase in bedding angle, hence demonstrating that the creep characteristics and damage mechanism of the layered rock mass are controlled by the effect of the natural weakness planes and show significant transversely isotropic characteristics.

High Temperature Behavior of Al-7Si-0.4Mg Alloy with Er and Zr Additions

In recent years, many efforts have been devoted to the development of innovative Al-based casting alloys with improved high temperature strength. Research is often oriented to the investigation of the effects of minor element additions to widely diffused casting alloys. The present study focuses on Al-7Si-0.4Mg (A356) alloy with small additions of Er and Zr. Following previous scientific works on the optimization of heat treatment and on tensile strength, creep tests were carried out at 300 °C under applied stress of 30 MPa, a reference condition for creep characterization of innovative high-temperature Al alloys. The alloys containing both Er and Zr displayed a lower minimum creep strain rate and a longer time to rupture. Fractographic and microstructural analyses on crept and aged specimens were performed to understand the role played by eutectic silicon, by the coarse intermetallics and by α-Al matrix ductility. The creep behavior in tension of the three alloys has been discussed by comparing them to tension and compression creep curves available in the literature for Al-7Si-0.4Mg improved by minor elemental additions.

Anisotropy in Compression Creep-Ageing Behavior of 2219-T3 Aluminum Alloy

Creep age forming technology (CAF) has been widely used to manufacture large integral panels in aerospace industry. However, due to the bending of the sheet metal, the stress states usually changes along the thickness direction during the CAF process, resulting in a complex distribution of stress. In addition, deformation texture is introduced when the sheet has a large pre-deformation, which also greatly affects the shape and performance of the component after aging. In this paper, the anisotropy in compression creep-ageing behavior of 2219-T3 aluminum alloy was studied. It was found that there is obvious anisotropy of compressive creep strains, the creep strain is the largest when the applied stress is along the rolling direction (RD) and the smallest when the applied stress is along the transverse direction (TD). The results of room temperature (25 ° C) and high temperature (165 ° C) tensile property test shows that the as-received material properties has obvious in-planar anisotropy, and the yield strength in the RD is the largest, but the 45° and TD are basically the same. Interestingly, the anisotropy of yield strength after SFA and compressive stress creep aging has basically disappeared, that is,the material properties tended to be isotropic after ageing.

Properties of Green Mortar Containing Granite Sawmill

The cement industry is one of the world’s largest CO2 emitters. The need to minimize these emissions, and assimilate by substitution and different types of waste, are challenges faced in the European Union. The use of granite sawmill from the ornamental stone industry allows for the manufacturing of pozzolanic cements, in which 10% and 20% of ordinary Portland cement (OPC) has been replaced by waste. In the present paper, properties of cements and mortars have been tested (when fresh and once set), such as workability, setting, retraction, mechanical resistance to bending and compression, elastic modulus to compression, creep, retraction, and durability. In all cases and substitution proportions, the results have been as satisfactory as those achieved with OPC, even better, allowing a second life to the waste, and participating in the principles of the circular economy. Bot substitutions are very resistant and have great durability for the gelifraction processes from the new green cements. Using this waste—granite sawmill—its volume is minimized and transfer to a landfill is avoided.

Seepage Evolution Model of the Fractured Rock Mass under High Seepage Pressure in Dam Foundation

The seepage of the fractured rock mass in dam foundations involves complex fluid-structure coupling behavior, due to practical hydrogeological conditions. In this work, the seepage characteristics of the fractured rock mass and their correlations with the structural permeable mediums are experimentally explored to reveal the cracking effect on the hydromechanical properties firstly. Subsequently, the tangential and the compression creep damage constitutive models are, respectively, established by introducing a nonlinear viscoplastic body to improve the Nishihara model. Afterwards, an innovative evolution equation of the permeability coefficient considering the creep damage is proposed. It can indicate the time effect of the porosity, the permeability, and damage variables of the fractured rock mass under the long-term infiltration action of the hydraulic pressures. Ultimately, the proposed methods are applied to the seepage simulation on the dam foundation of the Longyangxia hydropower station and the significantly increased leakage is in good agreement with the measured values during the storage period. It was further confirmed that the crack expansion and penetration in the rock masses can be constantly intensified by the seepage pressures. The research results can provide a reference for engineering repair and supervision through controlling the permeability performance for long-term operations.