Mechanical degradation of foam-cored sandwich materials exposed to high moisture

Sandwich specimens composed of E-glass/polyester face sheets bonded to a PVC foam core were exposed to high moisture (95% RH) and immersed in sea-water for extended periods of time. Degradation of mechanical properties of the face sheets, foam core and face/core interface were progressively evaluated using flexural testing of the laminates, through-thickness tension of the foam core and interfacial sandwich DCB fracture testing. Testing reveals substantial flexural stiffness and strength reductions for the laminated composites, and only minor reduction in the tensile stiffness and strength of the foam. Degradation of the interfacial face/core fracture toughness is weak for specimens subjected to elevated moisture and more pronounced for sandwich specimens immersed in sea-water. After 30 days of exposure to high moisture, foam damage is visible in the form of cracks and pits on the cell walls. Optical examinations of expansional strains show that moisture absorbed by the foam penetrates only about to 2–3 mm from the core free surface for the 95% RH condition, while penetrates deeply for the immersed condition.     

Mechanical properties of nanocrystalline materials

The mechanical properties of nanocrystalline materials are reviewed, with emphasis on their constitutive response and on the fundamental physical mechanisms. In a brief introduction, the most important synthesis methods are presented. A number of aspects of mechanical behavior are discussed, including the deviation from the Hall-Petch slope and possible negative slope, the effect of porosity, the difference between tensile and compressive strength, the limited ductility, the tendency for shear localization, the fatigue and creep responses. The strain-rate sensitivity of FCC metals is increased due to the decrease in activation volume in the nanocrystalline regime; for BCC metals this trend is not observed, since the activation volume is already low in the conventional polycrystalline regime. In fatigue, it seems that the S-N curves show improvement due to the increase in strength, whereas the da/dN curve shows increased growth velocity (possibly due to the smoother fracture requiring less energy to propagate). The creep results are conflicting: while some results indicate a decreased creep resistance consistent with the small grain size, other experimental results show that the creep resistance is not negatively affected. Several mechanisms that quantitatively predict the strength of nanocrystalline metals in terms of basic defects (dislocations, stacking faults, etc.) are discussed: break-up of dislocation pile-ups, core-and-mantle, grain-boundary sliding, grain-boundary dislocation emission and annihilation, grain coalescence, and gradient approach. Although this classification is broad, it incorporates the major mechanisms proposed to this date. The increased tendency for twinning, a direct consequence of the increased separation between partial dislocations, is discussed. The fracture of nanocrystalline metals consists of a mixture of ductile dimples and shear regions; the dimple size, while much smaller than that of conventional polycrystalline metals, is several times larger than the grain size. The shear regions are a direct consequence of the increased tendency of the nanocrystalline metals to undergo shear localization.The major computational approaches to the modeling of the mechanical processes in nanocrystalline metals are reviewed with emphasis on molecular dynamics simulations, which are revealing the emission of partial dislocations at grain boundaries and their annihilation after crossing them.     

Mechanical properties of chemically-treated hemp fibre reinforced sandwich composites

In this study, hemp fabrics were used as reinforcements with polyester resin to form composite skins while short hemp fibres with polyester as a core for making composite sandwich structures. To improve the fibre matrix adhesion properties, alkalisation, silane and acetylation treatments on the fibres surface were carried out. Examinations through fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were conducted to investigate the physical and thermal properties of the fibres. Mechanical properties such as flexural and compressive strengths of the sandwich structures made by treated and untreated hemp fibres were studied. Based on the results obtained from the experiments, it was found that the fibre treated with alkalic solution and post-soaked by 8% NaOH exhibited better mechanical strength as compared with other treated and untreated fibre composite samples. Besides, DSC and TGA analysis showed that the thermal stability of all treated fibre was enhanced as compared with untreated samples.     

Mechanical properties of dental investment materials

Measurement of the elastic modulus (E) of investment materials has been difficult because of their low strength. However, these values are essential for engineering simulation and there are many methods available to assess the elasticity of materials. The present study compared two different methods with one of the methods being non-destructive in nature and can be used for specimens prepared for other tests. Two different types of investment materials were selected, gypsum-and phosphate-bonded. Method 1 is a traditional three-point bending test. Twelve rectangular bars with dimension of (70 x 9 x 3 mm) were prepared and placed on supports 56.8 mm apart. The test was conducted at a cross-head speed of 1 mm/min by use of a universal testing machine. The load applied to the test specimen and the corresponding deflection were measured until the specimen fractured. The E value was calculated from a linear part of the stress-strain plot. Method 2 is an ultra micro-indentation system to determine near surface properties of materials with nanometer resolution. The measurement procedure was programd such that the specimens were indented with an initial contact force of 5 mN then followed by a maximum force of 500 mN. Measurement consisted of 10 indentations conducted with a spherical stainless steel indenter (R = 250 m) that were equally spaced (500 m). The E value rose asymptotically with depth of penetration and would approach the three-point bending test value at approximately four times maximum contact depth for both materials. Both methods are practical ways of measuring the E of investment materials.     

Mechanical properties of some plant materials

A study of mechanical properties of some plant materials, particularly vegetable flesh and cultural plant stalks is reported. It is shown that the tensile (compressive) strength, _m, of these and other plant materials is controlled by a relatively close exponential regression relation _m=0.2E ^0.75, where E is Young's modulus (r=0.975). More significant deviations from this relation are explained by the participation of buckling strength in the deformation by compression of the materials consisting of large thin-walled cells filled with air. A marked dependence of Young's modulus and strength of plant tissues on the crude fibre content is also demonstrated.     

Z向增强泡沫夹芯阻燃复合材料力学性能

研制了一种Z向玻璃纤维增强酚醛泡沫的高阻燃性复合材料, 并试验分析了承力柱高度、 分布密度、 排布方式及缝编纱细度、 缝合面板层数等结构参数对复合材料力学性能的影响。结果表明: 与普通泡沫夹芯复合材料相比, Z向增强泡沫夹芯复合材料的力学性能得到了大幅度提升; 在承力柱分布密度相同的条件下, Z向增强泡沫夹芯复合材料的力学性能基本不随承力柱排布方式而变化; 承力柱高度、 分布密度及缝编纱细度、 缝合面板层数等结构参数对Z向增强泡沫夹芯复合材料的力学性能有重要影响。     

Mechanical properties and microstructure of polyetheretherketone-hydroxyapatite nanocomposite materials

For the first time the polyetheretherketone (PEEK)-hydroxyapatite (HA) nanocomposite materials were successfully prepared, and their microstructure and mechanical properties, such as tensile strength, load-displacement and Young's modulus, were examined. The specimens laminated by the PEEK-HA composite layers with 5 vol.% and 15 vol.% HA were also successfully made, which gave a promising mechanical strength and a high HA content on the specimen surface. This novel approach should be of significance in manufacturing PEEK-HA biomaterials with both satisfactory mechanical properties and high bioactivity.     

Kevlar纤维缝纫泡沫芯材复合材料力学性能研究

基于热压罐成型工艺,制备了kevlar纤维缝纫泡沫芯材复合材料夹层板,并通过扫描电镜观察了胶膜中的树脂在kevlar纤维束之间的浸润状态,为工程化应用提供参考.选取未缝纫泡沫夹芯复合材料和碳纤维预浸料缝线缝纫泡沫芯材复合材料夹层板为对比试样,实验研究了kevlar纤维缝纫泡沫芯材复合材料夹层板的平压、剪切和侧压力学性能,并考察了缝纫针距、行距的变化对其力学性能和破坏模式的影响.研究表明:在真空压力下,胶膜中的树脂与kevlar纤维浸润良好;对泡沫芯材进行kevlar纤维缝纫增强后,其力学性能显著提高,并改变了夹层板的破坏机理.实验范围内,随着缝纫密度的提高,平压强度和模量增大;夹层板剪切性能和侧压性能受缝纫密度的影响较大,在缝纫参数(缝纫行距×针距)为10 mm×10 mm时,增强效果较佳,其剪切强度和侧压强度分别提高了44%和21%,剪切模量和侧压模量分别提高了34%和127%.     

Experimental analysis of the flexural properties of sandwich panels with cellular core materials

This paper addresses the flexural properties of sandwich structures with cellular core materials. Experimental three point bending tests are conducted in order to determine the flexural stiffness and the load‐carrying capacity of these advanced composites. In addition, the significant failure modes after exceeding the load‐carrying capacity are identified. The results of these analyses are compared for sandwich structures containing various core materials. These core materials comprise two aluminium foams, namely M‐Pore® and Alporas®, honeycomb structures and novel metallic hollow sphere structures (MHSS).     

Mechanical properties and durability characteristics of polymer- and cement-based repair materials

This study was conducted to evaluate the mechanical properties and durability characteristics of nine polymer- and cement-based repair mortars. Mechanical properties, such as compressive, tensile and flexural strength, elastic modulus, shrinkage and thermal expansion were studied. The durability characteristics of the repair materials were evaluated by measuring: (i) chloride permeability, (ii) electrical resistivity and (iii) carbonation depth. The mechanical properties of the selected repair mortars did not vary very significantly from each other. The elastic modulus of the polymer-based repair mortars was less than that of the cement-based repair mortars. This will lead to a reduced drying shrinkage cracking in the former repair mortars compared to the latter. The electrical resistivity of polymer-based repair mortars was more than that of cement-based repair mortars. Such a trend was not noted in the chloride permeability data. The chloride permeability in all the repair materials was very low according to ASTM C 1202 criteria. Enhanced carbonation was noted in some of the polymer-based repair mortars.     

Mechanical properties of hot isostatically pressed zirconium nitride materials

The influence of powder properties on the sintering behaviour, the microstructural development and the mechanical properties of hot isostatically pressed (HIPed) zirconium nitrides were investigated. The results show that the densification behaviour is dependent on the powder characteristics, more precisely the grain morphology and size, and oxygen, carbon and metallic impurity contents. The mechanical properties are controlled mainly by the amount of porosity and the presence of a complex intergranular phase in direct relation to the purity of the starting powder. The differences in the fracture strength and toughness between the two grades are perceptible. On the other hand, the thermal shock resistance to fracture initiation, as well as the elastic parameters of both dense materials, are similar.     

重晶石/橡胶复合材料的力学性能研究

采用不同偶联剂对重晶石表面活化改性,通过直接共混法制备重晶石/橡胶(NR)复合材料。研究了不同偶联剂改性重晶石后复合材料的拉伸性能、硬度、耐磨性能变化规律。结果表明:当活化重晶石含量在20%～30%时,能获得拉伸强度、断裂伸长率及耐磨性能优异的复合材料。硬脂酸改性后重晶石与橡胶复合材料的拉伸性能、硬度及耐磨性最佳。活化后重晶石具有优异的补强效果及理想的加工特性,可替代炭黑实现其在橡胶中的应用。     

Mechanical properties of bio-mimetic energy-absorbing materials under impact loading

The elytra can protect the body and hind wings of the beetle by absorbing the impact energy and resisting damage from outside loading. In this paper, we firstly observed the microstructures of hollow column and pole canal in the ladybird beetle elytra and revealed the relationship between them. A bionic energy-absorbing structure inspired by ladybird beetle elytra was proposed, and a micron-scale finite element model was built. The mechanical characteristics of bionic structures with and without poles under axial impact loading were investigated by numerical simulations. It could be obtained that the poles could absorb the impact energy by its deformation. Then the parameter studies including the different impact velocities, the different column diameters, and the different thickness of cuticle were carried out. This parameter study shows that geometric variations and impact velocity have a significant influence on mechanical properties.