Mechanical relation for porous metal foams under complex loads of triaxial tension and compression

The mechanical relation of isotropic three-dimensional reticulated porous metal foams with stochastic pores is investigated under complex loads of triaxial tension and compression. From the simplified structural model, the mathematical relationship between three nominal main stresses and porosity has been derived for this class of porous materials at failure under the above-mentioned complex loads, covering three loading conditions of biaxial tension with monoaxial compression, of biaxial compression with monoaxial tension, and of triaxial compression. Through the relevant expression from the deduction, the criterion of strength design can be further obtained for these porous materials under these multiaxial complex loadings.     

Microstructural and mechanical properties of biodegradable iron foam prepared by powder metallurgy

Research into biodegradable porous materials has been increasingly focused on iron-based materials because such materials possess suitable properties for orthopedic applications. In this study, we prepared porous iron with porosities of 32–82 vol.% by powder metallurgy using ammonium bicarbonate as a space-holder material. We studied the influence of initial powder size and compacting pressure on sample microstructure, contamination and mechanical characteristics. The experimental results were analyzed as well, using Gibson–Ashby model and this analysis showed a good agreement in theoretical and experimental data. Whereas increasing compression pressure decreased porosity, the use of finer iron powder led to an increase in porosity. Increasing the amount of space-holder material in the initial mixture increased the total porosity, improved compressibility and consequently decreased the number of pores originating from imperfect compaction. A higher compacting pressure and the use of finer powder enhanced both the flexural and compressive properties. Even the most porous samples prepared from the fine iron powder possessed mechanical properties comparable to human cancellous bone. Based on these results, we can claim that the use of fine initial iron powder is necessary to obtain highly porous iron, which appears to be suitable for orthopedic applications.     

高气孔率莫来石制备、性能及其非线性导热模型

以高铝矾土、硅灰为原料, 玉米淀粉为造孔剂制备高气孔率莫来石, 通过XRD、SEM等对产物物相、形貌进行表征, 研究淀粉含量对显气孔率、体积密度和抗折强度的影响, 及不同显气孔率的莫来石随温度变化的导热系数, 建立体积密度、抗折强度与气孔率关系模型及非线性导热模型。结果表明: 体积密度、抗折强度随气孔率增加而减小, 并符合指数函数关系。导热系数随温度的升高而增大, 实测值与非线性导热模型计算值吻合较好, 非线性导热模型能够准确地反映高气孔率莫来石导热系数与温度、气孔率、平均孔径和热辐射等之间的关系。     

Structural nanocrystalline materials: an overview

This paper presents a brief overview of the field of structural nanocrystalline materials. These are materials in either bulk, coating, or thin film form whose function is for structural applications. The major processing methods for production of bulk nanocrystalline materials are reviewed. These methods include inert gas condensation, chemical reaction methods, electrodeposition, mechanical attrition, and severe plastic deformation. The stability of the nanocrystalline microstructure is discussed in terms of strategies for retardation of grain growth. Selected mechanical properties of nanocrystalline materials are described; specifically strength and ductility. Corrosion resistance is briefly addressed. Examples of present or potential applications for structural nanocrystalline materials are given.     

多孔材料性能模型研究3:数理推演

基于三维网状多孔材料的八面体结构模型,本文介绍了多孔材料基本物理、力学性能数理关系的推演过程。此过程覆盖了多孔材料的单向拉伸、多向拉压、传导性和疲劳性能等方面。重点描述了多孔材料内部构成的等效电路、多孔材料单向拉伸的准刚体结构受力模型和变形体结构受力模型,在此基础上讨论了压缩强度问题,并对双向拉伸和三向拉压的有关数理关系展开推演和分析。根据本八面体模型,多孔材料在弯曲等非直接拉压受力形式下的力学性能数理关系,同样可由单向拉压的推演而获得。     

纳米羟基磷灰石/聚合物多孔复合支架材料

为提高骨组织工程支架材料的力学性能,改善其生物活性,综合天然与合成高分子的优点,采用溶液共混相分离法制备出聚己内酯(PCL)-壳聚糖(CS)多孔支架材料, 并进一步采用离心注浆法填充具有生物活性的纳米羟基磷灰石(HA)-聚乙烯醇(PVA)复合浆料, 制备了n-HA-PVA/PCL-CS复合多孔支架材料, 改善了PCL-CS支架材料力学性能。采用扫描电子显微镜、红外光谱、元素分析、孔隙率和抗压强度试验对材料进行了表征。结果表明, PCL-CS支架材料的内部具有蜂窝状的相互贯通的孔隙结构,孔隙率可以达到60%～80%。CS含量越大,孔隙率越大,而抗压强度越小。填充后的n-HA-PVA/PCL-CS复合多孔支架材料,孔隙率有所下降,但仍大于60%,而其弹性模量可提高至25.71 MPa。      

Preparation of a polyurethane scaffold for tissue engineering made by a combination of salt leaching and freeze-drying of dioxane

In the past several types of synthetic porous materials have been made as meniscus reconstruction materials. Most of these materials lacked a good combination between suitable mechanical properties and a high interconnectivity. In this work porous scaffolds were made from the polyurethane Estane 5701-F1 by freeze drying of a dioxane and water solution in combination with salt leaching. It was possible to obtain very porous scaffolds with a very high interconnectivity. Porosity, pore size and interconnectivity can be independently adjusted by varying the amount of water, porogen size and the amount of porogen used. The obtained compression moduli of the scaffolds were between 40 kPa and 400 kPa with a variation in porosity between 72 and 87%. These scaffolds are very suitable for the use as meniscus replacement materials.     

电沉积法制备块体纳米晶材料的研究进展

综述了电沉积法制备块体纳米晶材料的原理;阐述了电流密度、电流波形、有机添加剂等工艺参数对沉积层晶粒尺寸的影响;介绍了直流电沉积、脉冲电沉积、喷射电沉积和复合电沉积等几种常见的电沉积方法;概述了电沉积法制备块体纳米晶材料的国内外研究现状;探讨了电沉积块体纳米晶材料的力学性能、磁学性能、耐蚀性能、热稳定性及其应用前景.     

Physical characterization of polycaprolactone scaffolds

Films and sponges were prepared from a solution of Poly(ε-caprolactone) (PCL) in tetrahydrofuran (THF). The porosity, crystallinity, and mechanical properties of the samples were studied. Porosity of around 15% was obtained for the films produced by evaporation of THF at room temperature. A much more porous structure (50–70%) was found for the sponges obtained by cooling the solution at −30 °C and subsequently eliminating the solvent by freeze drying. The porosity of the samples was also observed by scanning electron microscopy (SEM). The crystallinity of the samples was studied by the calorimetric technique (DSC) before and after the compression scans. The mechanical properties of the different samples were determined by compression test, and were compared to those corresponding to the PCL in bulk. The compression scans did not affect the crystallinity of the samples. The variations observed in the results of the different scans were attributed to the differences in porosities and crystallinity.     

含孔隙混凝土复合材料有效力学性能研究

混凝土、岩石等工程材料是典型的多孔介质材料,孔隙或微裂纹的存在对材料的弹性模量及强度等力学参数产生很大影响。该文基于三相球模型确定了含孔隙复合材料的有效体积模量,提出采用空心圆柱形杆模型推导得到了含孔隙复合材料有效剪切模量的理论公式,并在各向同性材料的假设条件下确定了材料的有效弹性模量及泊松比;推导并得到了含孔隙材料的有效抗拉、抗压强度及有效抗剪强度与孔隙率之间的定量关系公式,并进一步得到了含孔基质在达到有效强度时的临界应变与孔隙率之间的定量关系。结果表明该文方法能较好的预测含孔混凝土材料的有效力学性能,且公式简单,易于应用。     

Plaster of Paris as a model material for brittle porous solids

Plaster of Paris is a brittle, porous solid, easy to shape, which has potential as a model material for the study of brittle, porous, solids such as ceramics, rocks and cement. This paper describes the mechanical properties of plaster of Paris — modulus, strength, fracture toughness, etc. — as a function of porosity. The material is then used to study the initiation and propagation of cracks in compression, as a function of porosity, stress state and stress concentration.     

Investigation on compressive behavior of Cu-35Ni-15Al alloy at high temperatures

Microstructures and mechanical properties of Cu-35Ni-15Al alloy in cast and porous states were studied by scanning electron microscopy and compression tests. The influence of porosity, deformation temperature and loading rate on mechanical properties of the two kinds of alloys was investigated. The results show that the as cast alloy and porous alloys have almost the same phase constitution: Cu rich phase, Ni rich phase and K intermetallics. The yield strength of porous alloys increases continuously with decreasing porosity, the relationship between porosity and yield stress follows Gibson-Ashby equation. With decreasing deformation temperature, the yield strength of as cast alloy and porous alloy increase. With the increase of loading rate, the yield strength of these alloys shows an increasing trend. After compression, the microstructure of as cast alloy is more uniform, and porous alloys are more prone to have localized deformations.     

Failure by buckling mode of the pore-strut for isotropic three-dimensional reticulated porous metal foams under different compressive loads

Compression is one of the most basic loading modes for engineering materials, and the failure of lost stability is possibly resulted from buckling for the bar under compression. The pore-strut within porous metal foams under compression may be similar to the compression bar, so the strut is possible to buckle when the porous body is under compressive loads. With the analytical property model of the simplified structure, the pore-strut buckling behavior is analyzed for isotropic three-dimensional reticulated porous metal foams, and the failure modes resulting from this buckling are investigated for these materials under compressive loadings. These loading modes cover all of three loading conditions, including uniaxial compression, biaxial compression and triaxial compression. The treating ways of the pore-strut are relative to three slenderness-ratios, including three conditions of thin-long bar, middle-long bar and stocky bar. Based on these works, the mathematical relationships between nominal main stresses and porosity are found for this buckling failure of these materials under compression. Through the relevant expression, the relevant strength criterion and the relevant loading condition resulting in the strut buckling are further achieved for these porous metal foams under compression.     

碳酸钙对磷渣-煤矸石烧结多孔微晶玻璃结构和性能的影响

多孔微晶玻璃具有导热系数低、力学性能好、抗腐蚀性强等优点,是一种隔热、吸声的轻质功能材料.为拓展多孔微晶玻璃的制备原料范围,同时大宗量、高附加值利用工业废渣-磷渣和煤矸石.本文以磷渣和煤矸石为原料,采用烧结法制备多孔微晶玻璃.利用X射线衍射、扫描电子显微镜及阿基米德法,对制备的试样进行表征,研究发泡剂碳酸钙掺量对体系析...     

Preparation of nanocrystalline Ni–Fe strip via mechanical alloying–compaction–sintering–hot rolling route

Nanocrystalline structures offer opportunity for the development of soft magnetic materials, such as 80 wt% Ni–20 wt% Fe, with superior properties. In recent years, nanocrystalline 80Ni–20Fe (wt%) alloy has been prepared by mechanical alloying of elemental powders. However, retention of nanocrystallinity during consolidation of powder is the key issue to take advantage of improved magnetic properties. In the present work, it has been shown that near-full density bulk nanocrystalline 80Ni–20Fe strip can be prepared via a route consisting of mechanical alloying, cold compaction, sintering, and multi-step unsheathed hot rolling. A crack-free strip of nanocrystalline 80Ni–20Fe, having 99% theoretical density and a grain size of approximately 55 nm, was successfully prepared by sintering and hot rolling of mechanically alloyed powder preforms at 1140 °C. The bulk nanocrystalline 80Ni–20Fe material resulted in a very narrow hysteresis loop indicating a very small hysteresis loss. The present study shows that mechanical alloying–sintering–hot rolling route can be a promising method for producing bulk nanocrystalline materials.     

纳米材料微阵列超塑微成形机理与尺度效应

微成形技术是未来批量制造高精密微小零件的关键技术,但是,微小尺度下材料的塑性变形行为不仅表现出明显的尺度效应,而且零件尺度已经接近常规材料的晶粒尺寸,每个晶粒的形状、取向、变形特征对整体变形产生复杂的影响,难以保证微成形的工艺稳定性。本项目采用纳米材料进行微成形,制造微阵列,零件内部包含大量的晶粒,可以排除晶粒复杂性的影响,而且纳米材料具有超塑性,在超塑状态下,变形抗力和摩擦力都明显降低,从而显著降低微成形工艺对模具性能的苛刻要求,提高工艺稳定性和成形精度。目前,纳米材料超塑性微成形技术方面的研究极少,变形时纳米材料的力学行为、变形机理、尺度效应、位错演化、力学模型等关键问题还有待研究。采用电沉积技术制备晶粒尺寸可控的纳米材料,将工艺实验研究、性能测试、组织分析、力学性能表征、数值模拟相结合,深入探究了纳米材料微阵列超塑性微成形机理和成形规律,以促进该技术的广泛应用。     

Crystallization of 4He in Aerogels

Static behaviors of crystallization of ⁴He in porous materials, such as the increase of melting pressure, have been studied extensively, but nonequilibrium dynamics of the phase transition is hardly known. Our interest was in how ⁴He crystals grow in a 90.4% porosity aerogel. Aerogels are transparent and the dynamics in them can be studied visually. A Pomeranchuk-type variable-volume cell was used to study crystallization at a fixed temperature with a blocked capillary condition. By continuously compressing a chamber, the pressure rose above the bulk melting pressure and at a pressure 1.7 bar above the bulk melting pressure these crystals began to invade the aerogel. A clear crystal-superfluid interface was moved smoothly by the steady compression. No macroscopic facets were observed in the aerogel well below the bulk roughening transition temperature.      

Acoustical and Poromechanical Characterisation of Titanium Scaffolds for Biomedical Applications

Abstract:  Biocompatible materials are designed so as to mimic biological materials such as bone as closely as possible. As regards the mechanical aspect of bone replacement materials, a certain stiffness and strength are mandatory to effectively carry the loads imposed on the skeleton. In this paper, porous titanium with different porosities, produced on the basis of metal powder and space holder components, is investigated as bone replacement material. For the determination of mechanical properties, i.e. strength of dense and porous titanium samples, two kinds of experiments were performed – uniaxial and triaxial tests. The triaxial tests were of poromechanical nature, i.e. oil was employed to induce the same pressure both at the lateral surfaces of the cylindrical samples and inside the pores. The stiffness properties were revealed by acoustic (ultrasonic) tests. Different frequencies give access to different stiffness components (stiffness tensor components related to high-frequency-induced bulk waves versus Young's moduli related to low-frequency-induced bar waves), at different observation scales; namely, the observation scale the dense titanium with around 100  μ m characteristic length (characterised through the high frequencies) versus that of the porous material with a few millimetres of characteristic length (characterised through the low frequencies). Finally, the experimental results were used to develop and validate a poro-micromechanical model for porous titanium, which quantifies material stiffness and strength from its porosity and (in the case of the aforementioned triaxial tests) its pore pressurisation state.     

A coupled discrete element‐finite difference approach for modeling mechanical response of fluid‐saturated porous materials

A model of fluid‐saturated poroelastic medium was developed based on a combination of the discrete element method and grid method. The developed model adequately accounts for the deformation, fracture, and multiscale internal structure of a porous solid skeleton. The multiscale porous structure is taken into account implicitly by assigning the porosity and permeability values for the enclosing skeleton, which determine the rate of filtration of a fluid. Macroscopic pores and voids are taken into account explicitly by specifying the computational domain geometry. The relationship between the stress–strain state of the solid skeleton and pore fluid pressure is described in the approximations of simply deformable discrete element and Biot's model of poroelasticity. The developed model was applied to study the mechanical response of fluid‐saturated samples of brittle material. Based on simulation results, we constructed a generalized logistic dependence of uniaxial compressive strength on loading rate, mechanical properties of fluid and enclosing skeleton, and on sample dimensions. The logistic form of the generalized dependence of strength of fluid‐saturated elastic–brittle porous materials is due to the competition of two interrelated processes, such as pore fluid pressure increase under solid skeleton compression and fluid outflow from the enclosing skeleton to the environment. Copyright © 2015 John Wiley & Sons, Ltd.