Processing of aromatic thermosetting copolyesters into foams and bulk parts: characterization and mechanical properties

Fully dense sheets of aromatic thermosetting copolyester (ATSP) have been produced by blending ATSP oligomers, curing the blend to produce foam, grinding the cured material to a powder, followed by sintering of the cured powers via hot press. The resulting product possesses an excellent combination of mechanical strength and high‐temperature performance to help improve part functionality, gain long‐term reliability, and cost savings. Dynamic mechanical analysis of this system featured a glass transition higher than most available performance thermosets and thermoplastic polymers or reversible bonding polymer system described in literature. Compression and tensile properties of the foam and fully dense ATSP structures exceed those of most engineering polymers. Copyright © 2016 John Wiley & Sons, Ltd.     

基于三相球模型确定泡沫塑料有效模量

通过复合材料三相球模型确定泡沫塑料的体积模量、剪切模量、杨氏模量和泊松比等材料等参数与材料孔隙比的关系，并将该文结果同微分法得到的结果进行了对比；结果表明：三相球模型确定的材料参数在相同孔隙比情况下，一般都高于微分法确定的结果，两种方法得到的有效剪切模量和杨氏模量差异较小，而体积模量和泊松比则差异较大，然而，由三相球模型确定的泡沫塑料杨氏模量仍可与实验较好地符合。此外，该文结果也同现有的理论及实验     

非均匀泡沫金属材料在冲击载荷下的变形模拟

提出一种二维非线性弹塑性质量-弹簧-连杆模型，该模型将泡沫金属材料离散成许多质量块，质量块在受载方向由非线性弹塑性弹簧连接，垂直于受载方向由可延伸的弹性连杆铰接。采用该模型模拟并分析了层非均匀泡沫金属材料及局部不均匀泡沫金属材料在冲击载荷下的变形特性，说明了非均匀性对泡沫金属材料冲击变形的影响。     

High-strain-rate compressive behavior of a rigid polyurethane foam with various densities

The dynamic compressive stress-strain behavior of a rigid polyurethane foam with four values of density (78, 154, 299, and 445 kg/m³) has been determined in the strain-rate range of 1000–5000 s⁻¹. A pulse shaping technique was used with a split Hopkinson pressure bar to ensure homogeneous deformation in the foam specimens under dynamic compression. Dynamic stress equilibrium in the specimen was monitored during each experiment using piezoelectric force transducers mounted close to the specimen end-faces. Quasi-static experiments were also performed to demonstrate rate effects. Experimental results show that both the quasistatic and the dynamic stress-strain curves of the foam exhibit linear elasticity at small strains until a peak is reached. After the peak, the stress-strain curves have a plateau region followed by a densification region. The peak stress is strain-rate sensitive and depends on the square of the foam density.     

Mechanical characterization of the role of defects in sintered FeCrAlY foams

Open celled metal foams fabricated through metal sintering are a new class of material that offers novel mechanical and acoustic properties. Previously, polymer foams have been widely used as a means of absorbing acoustic energy. However, the structural applications of these foams are limited. The metal sintering approach offers a cost- effective means for the mass-production of open-cell foams from a range of materials, including high-temperature steel alloys. In this first part of two-paper series, the mechanical properties of open-celled steel alloy (FeCrAlY) foams were characterized under uniaxial compression and shear loading. Compared to predictions from established models, a significant knockdown in material properties was observed. This knockdown was attributed to the presence of defects throughout the microstructure that result from the unique fabrication process. Further in situ tests were carried out in a SEM (scanning electronic microscope) in order to investigate the effects of defects on the properties of the foams. Typically, the onset of plastic yielding was observed to occur at defect locations within the microstructure. At lower relative densities, ligament bending dominates, with the deformation initializing at defects. At higher relative densities, an additional deformation mechanism associated with membrane elements was observed. In the follow-up of this paper, a finite element model will be constructed to quantify the effects of defects on the mechanical performance of the open-cell foam. The project supported by the US Office of Naval Research (N000140210117), the National Basic Research Program of China (2006CB601202), the National Natural Science Foundation of China (10328203, 10572111, 10632060), and the National 111 Project of China (B06024).     

SiO2/炭泡沫和SiC/炭泡沫复合材料的制备及表征

对炭泡沫为支撑骨架的氧化硅气凝胶(SiO₂/炭泡沫)和碳化硅(SiC/炭泡沫)复合材料分别采用XRD、SEM、激光导热仪、万能力学试验机进行物相、微观结构、热学及力学性能方面的表征。结果表明:所制备的SiO₂/炭泡沫与原炭泡沫相比,具备更高的抗压强度(14.95MPa)和更低的室温热导率(0.44W·m^-1·K^-1)。SiC/炭泡沫材料则保持了较高的抗压强度值(14.66MPa),其在1200℃下具备极低的高温热导率(2.18W·m^-1·K^-1)。热重分析表明,SiC/炭泡沫在氧化氛围中到610℃才发生质量的损失,而内部炭发生完全烧蚀的温度高达844℃,这表明该材料的抗氧化性能远好于纯的炭泡沫材料。     

Development of a portable shear test fixture for low modulus porous (foam) materials

The shear properties of brittle and highly porous carbon (graphitic) foam cannot be measured reliably with most standard test methods, such as single rail, double rail, Iosipescu shear, etc. A new testing device has been developed to accurately measure the shear stiffness and strength of carbon foam or other porous materials. Specimens of cylindrical cross section are used to reduce the high stress concentration that normally occurs in the vicinity of the grip section. Since strain gages could not be installed on the specimen surface (due to porosity), the shear strain is determined from the specimen end rotation. A high resolution in the rotational measurement is achieved by using a stepper motor with multiple gear reduction. In view of testing low modulus material, the load cell of the fixture was mounted onto an axial roller to relieve the axial constraint while twisting the specimens. The accuracy of the measurement and calibration of the test fixture has been demonstrated by measuring the shear modulus of two plastic (polyvinyl chloride (PVC) and urethane).     

Kelvin结构开孔泡沫材料的弹性性能研究

采用一修正的十四面体结构模型(Kelvin结构模型)对开孔泡沫金属的弹性性能进行研究,对低密度开孔泡沫材料表现出不可压的特性进行了分析。该模型考虑作用在泡沫筋条上的弯矩、剪力和轴向力,以及轴向力的平衡。修正模型的数值计算结果与实验结果及其他模型的结果进行了对比,结果表明修正模型计算的杨氏模量比原有模型的略有提高,筋条截面为星形的修正模型计算的结果与实验比较符合。在密度等同的条件下,筋条截面惯性矩越大的开孔泡沫材料,其弹性模量也越大,而泊松比则越小。Kelvin结构的开孔泡沫材料的泊松比随相对密度的减小而趋于0.5。     

爆炸载荷下聚氨酯泡沫材料中冲击波压力特性

采用间接测压法,对爆炸载荷作用下聚氨酯泡沫材料中的冲击波压力特性进行了研究。结果表明,在装药特性相同的爆炸载荷作用下,聚氨酯泡沫材料中的初始冲击波压力随初始孔隙度增大而显著下降;冲击波传播衰减效应受初始孔隙度影响同样显著,但不呈单一加快或减慢趋势;当初始孔隙度在０．２５左右时,聚氨酯泡沫材料对爆炸载荷具有较强的抗冲击减压效能。     

关于泡沫塑料各向异性模型的修正

本文研究了开孔泡沫塑料的力学各向异性问题。在已有模型的基础上，结合扫描电镜的分析结果提出了对Ｋａｎａｋｋａｎａｔ模型和Ｈｕｂｅｒ－Ｇｉｂｓｏｎ模型的修正。这种修正反映了开孔泡沫塑料支柱的横向尺寸比对泡沫塑料各向异性性质的影响