Rate dependence of the tensile and flexural strengths of glass–ceramic Macor

Understanding of the tensile and flexural strengths of the glass–ceramic Macor bears important applications in materials science, aerospace, defense, and other engineering disciplines. In this article, we systematically investigate the rate dependence of the tensile strength and the flexural strength of Macor utilizing two methods: the Brazilian disk (BD) test and semi-circular bend (SCB) test. Both static tests and dynamic tests are conducted to explore the rate dependence of tensile and flexural strengths of Macor. The static measurement is conducted with a servo-controlled material testing machine, and the dynamic experiment is carried out with a 6.35-mm diameter split Hopkinson pressure bar (SHPB) system. The pulse-shaping technique is used to achieve dynamic force balance, and thus eliminates the loading inertial effect and enables quasi-static stress analysis. The experimental results show that both the tensile strength and the flexural strength of Macor are loading rate dependent. The flexural strength is observed to be consistently higher than the tensile strength.     

The effects of strain rate and CCVC interfacial layer on mechanical behaviours of CALL

In this paper, the loading and loading-unloading tests of CALL and CALL (CCVC) under tensile impact have been carried out by a self-designed Rotating Circular Disk Tensile impact Apparatus. The quasi-static tension and short beam bending tests are performed on the Shimadzu-5000 testing apparatus. Experiment results show that both CALL and CALL (CCVC) have positive hybrid effect. Under quasi-static tension, the two composites have no obvious yielding until fracture, but have an obvious yielding point on the dynamic tensile stress-strain curves. The dynamic unstable fracture strain is about three times the static unstable fracture strain. The interlaminar shear strength (ISS) of CALL (CCVC) is 10 more than that of CALL. At the same time, the tensile strength and unstable fracture strain of CALL (CCVC) are also higher than that of CALL. In this paper, some conclusions are also drawn from the SEM observation of the fracture specimen surfaces.     

应变率及CCVC界面层对CALL材料拉伸性能的影响

用自行研制的旋转盘式杆杆型冲击拉伸装置实施了ＣＦＲＰ／Ａｌ超混杂复合材料（ＣＡＬＬ）以及带有ＣＣＶＣ界面层的ＣＡＬＬ冲击拉伸加载及加卸载试验；用岛津试验机测定了它们的准静态拉伸及层间剪切性能材料在准静态拉伸时，应力－应变曲线无明显的屈服点，呈脆性断裂；而冲击拉伸时，应力－应变曲线有明显屈服点，呈脆－韧性断裂，动态失稳应变是准静态的３—４倍无论是准静态拉伸还是冲击拉伸，ＣＡＬＬ均存在明显的正混杂效应；ＣＡＬＬ（ＣＣＶＣ）层间剪切强度比ＣＡＬＬ约高１０％；在同一应变率下，ＣＡＬＬ（ＣＣＶ）的拉伸强度及失稳应变均比ＣＡＬＬ高．文中详细讨论以上试验现象，得出一些有意义的结论     

Sintering enhancement in dynamically compacted commercial iron powders

Powder metallurgy compacts of near theoretical density have been made from commercial sponge iron and atomized iron powders, the latter with and without admixed lubricant. Equivalent compacts were made by conventional quasi-static die pressing and by the dynamic powder compaction method to allow for comparative testing of mechanical properties. The compacts were sintered over a range of temperatures from 700 to 1120°C. Test specimens were cut from the compacts and tested to produce data on tensile strength, ductility (area reduction) and cantilever beam (Izod) impact strength.

Compacts made dynamically from both the sponge iron and atomized iron powders exhibited higher tensile strengths and ductilities than those made quasi-statically and sintered to the same temperature. However, there were marked differences in the impact strength. With the sponge iron powder, dynamic compacts had lower impact strength than equivalent quasi-static compacts, but the reverse result was obtained with the atomized powder. The atomized powder was of much higher purity than the sponge iron and the microstructural evidence indicated that the inferior impact strength of the dynamically compacted sponge iron was due to interaction between the shock waves used for compaction and the numerous brittle inclusions present in this material.

The results with the lubricated powder showed no sintering enhancement attributable to dynamic powder compaction. This suggests that the mechanism for the sintering enhancement that can be achieved with this consolidation technique is related to the friction processes at the particle boundaries, possibly coupled with the elevated temperatures present at these boundaries during dynamic compaction.     

Mechanical characterization of hierarchical carbon fiber/nanofiber composite laminates

Susceptibility to matrix driven failure is one of the major weaknesses of continuous-fiber composites. In this study, helical-ribbon carbon nanofibers (CNF) were dispersed in the matrix phase of a continuous carbon fiber-reinforced composite. Along with an unreinforced control, the resulting hierarchical composites were tested to failure in several modes of quasi-static testing designed to assess matrix-dominated mechanical properties and fracture characteristics. Results indicated CNF addition offered simultaneous increases in tensile stiffness, strength and toughness while also enhancing both compressive and flexural strengths. Short-beam strength testing resulted in no apparent improvement while the fracture energy required for the onset of mode I interlaminar delamination was enhanced by 35%. Extrinsic toughening mechanisms, e.g., intralaminar fiber bridging and trans-ply cracking, significantly affected steady-state crack propagation values. Scanning electron microscopy of delaminated fracture surfaces revealed improved primary fiber–matrix adhesion and indications of CNF-induced matrix toughening.     

Deformation behavior and mechanisms of a nanocrystalline multi-phase aluminum alloy

A nanocrystalline (nc) Al–Fe–Cr–Ti alloy containing 30 vol.% nc intermetallic particles has been used to investigate deformation behavior and mechanisms of nc multi-phase alloys. High compressive strengths at room and elevated temperatures have been demonstrated. However, tensile fracture strengths below 300 °C are lower than the corresponding maximum strengths in compression. Creep flow of the nc fcc-Al grains is suppressed even though rapid dynamic recovery has occurred. It is argued that the compressive strength at ambient temperature is controlled by propagation of dislocations into nc fcc-Al grains, whereas the compressive strength at elevated temperature is determined by dislocation propagation as well as dynamic recovery. The low tensile fracture strengths and lack of ductility at temperatures below 300 °C are attributed to the limited dislocation storage capacity of nanoscale grains. Since the deformation of the nc Al-alloy is controlled by dislocation propagation into nc fcc-Al grains, the smaller the grain size, the higher the strength. This new microstructural design methodology coupled with ductility-improving approaches could present opportunities for exploiting nc materials in structural applications at both ambient and elevated temperatures.     

Weibull master curves and fracture toughness testing Part I Master curves for quasi-static uniaxial tensile and bend tests

Three types of specimen-size-independent Weibull master curves, characterizing strength and failure of macroscopically homogeneous, brittle materials have been derived. These Weibull master curves are significant if an uniaxial tensile stress is applied to the investigated specimens, as, for example, in the case of quasi-static uniaxial tensile tests or, under some restrictions, in the case of quasi-static three- or four-point bend tests. In addition, the existence of three types of apparent fracture toughness master curves, which can be applied to any material undergoing brittle cleavage fracture such as ceramics, intermetallics, or structural steels at low homologous temperatures, has been established. Furthermore, the same is also valid for the specimen-size-independent Weibull master curves. The apparent fracture toughness master curves can be obtained, by performing fracture toughness tests, or simply by applying a mathematical transformation to the corresponding Weibull master curves, which have been evaluated from quasi-static uniaxial tensile or bend tests.     

预加载下岩石的动态力学性能研究

动态和静态载荷共同作用下的岩石力学特性是深部地下岩石工程的关键问题。设计了用于测试静态预加载下岩石动态力学性能的分离式霍普金森压杆系统,并详细介绍了具有预加载装置的分离式霍普金森压杆系统的原理、数据分析和应力波的传播过程。通过具有预加载装置的分离式霍普金森压杆系统研究了岩石在不同预拉伸应力下的拉伸强度。结果表明:动态拉伸强度和总拉伸强度随着加载率的增加而增加,同时,在相同加载率下,动态拉伸强度随着预拉伸载荷的增加而减小,而总拉伸应力与预拉伸载荷的大小无关。此外,对不同预加载条件下岩石的动态断裂韧度也进行了研究,实验结果说明岩石的动态断裂韧度和总断裂韧度随着加载率的增加而增加。在相同加载率下,动态断裂韧度随着预加载荷的增加而减小,而总断裂韧度随着加载率的增加而增加。     

Modelling the quasi-static behaviour of bituminous material using a cohesive zone model

This paper investigates the applicability of a cohesive zone model for simulating the performance of bituminous material subjected to quasi-static loading. The Dugdale traction law was implemented within a finite volume code in order to simulate the binder course mortar material response when subjected to indirect tensile loading. A uniaxial tensile test and a three-point bend test were employed to determine initial stress-strain curves at different test rates and the cohesive zone parameters (specifically, fracture energy and cohesive strength). Numerical results agree well with the experimental data up to the peak load and onset of fracture, demonstrating the value of the cohesive zone modelling technique in successfully predicting fracture initiation and maximum material strength.     

Silicon carbide (NicalonTM) fibre-reinforced borosilicate glass composites: mechanical properties

The objective of this study was to assess the applicability of an extrinsic carbon coating to tailor the interface in a unidirectional NicalonTM–borosilicate glass composite for maximum strength. Three unidirectional NicalonTM fibre-reinforced borosilicate glass composites were fabricated with different interfaces by using (1) uncoated (2) 25 nm thick carbon-coated and (3) 140 nm thick carbon coated Nicalon fibres. The tensile behaviours of the three systems differed significantly. Damage developments during tensile loading were recorded by a replica technique. Fibre–matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and elastic modulus. Tensile specimen design was varied to obtain desirable failure mode. Tensile strengths of NicalonTM fibres in all three types of composites were measured by the fracture mirror method. Weibull analysis of the fibre strength data was performed. Fibre strength data obtained from the fracture mirror method were compared with strength data obtained by single fibre tensile testing of as-received fibres and fibres extracted from the composites. The fibre strength data were used in various composite strength models to predict strengths. Nicalon–borosilicate glass composites with ultimate tensile strength values as high as 585 MPa were produced using extrinsic carbon coatings on the fibres. Fibre strength measurements indicated fibre strength degradation during processing. Fracture mirror analysis gave higher fibre strengths than extracted single fibre tensile testing for all three types of composites. The fibre bundle model gave reasonable composite ultimate tensile strength predictions using fracture mirror based fibre strength data. Characterization and analysis suggest that the full reinforcing potential of the fibres was not realized and the composite strength can be further increased by optimizing the fibre coating thickness and processing parameters. The use of microcrack density measurements, indentation–frictional stress measurements and shear lag modelling have been demonstrated for assessing whether the full reinforcing and toughening potential of the fibres has been realized. This revised version was published online in November 2006 with corrections to the Cover Date.     

Simulation of porosity by microballoon dispersion in epoxy and urethane: mechanical measurements and models

Effective mechanical properties of microballoon-dispersed epoxy and urethane are studied under quasi-static and dynamic loading conditions. Elastic modulus measurements of these mixtures over a volume fraction range of 0–0.45 are in good agreement with Hashin-Shtrikman lower-bound predictions for two-phase mixtures comprising of randomly distributed spherical pores in an elastic matrix. The measurements have also been predicted accurately by a LEFM based pore-flaw model for a selected flaw size to pore size ratio. These imply that the microballoons offer negligible reinforcement due to extremely small wall thickness to diameter ratio. Accordingly, feasibility of using these materials to simulate controlled porosity for tensile strength and fracture toughness modeling is explored. Measured tensile strength and fracture toughness values decrease monotonically similar to the Young's modulus variation with volume fraction of microballoons. Guided by the measurements linear elastic models for porous materials that predict tensile strength and fracture toughness of these mixtures are proposed and validated. The tensile strength predictions are in very good agreement with measurements for both epoxy and urethane compositions. The quasi-static crack initiation toughness prediction captures the measurement trends rather well in both cases. The agreement between the measurements and predictions are modest for epoxy matrix while they are good for urethane compositions. Based on fracture surface micrography, an empirical corrective procedure is advanced to improve the agreement between the measurements and the model. The dynamic crack initiation toughness measurements for epoxy, on the other hand, are in excellent agreement with the predictions.     

超细晶铜力学和阻尼性能及微观结构研究

在众多阻尼材料中,金属阻尼材料既能满足高阻尼减振降噪性能,又具有较高的强度,是理想的阻尼材料.为了提高商业纯铜的力学性能,分析晶粒细化程度对纯铜力学性能和阻尼性能的影响,在室温下对商业纯铜棒进行12道次BC路径等通道转角挤压(ECAP)实验.对挤压后样品进行单轴微拉伸试验和高循环拉伸疲劳试验研究其力学性能;通过动态力学...     

二维C/SiC复合材料准静态和动态拉伸力学性能

采用分离式Hopkinson拉杆装置和电子万能试验机研究了二维C/SiC复合材料在4种应变率（0.001、0.010、90.000和350.000 s-1）下的拉伸力学性能,计算并验证了动态试验中的应力平衡状态;采用SEM分析了复合材料在不同应变率下的破坏断口和失效机制;建立了复合材料包含损伤和应变率相关的本构方程。结果表明:二维C/SiC复合材料的应力-应变曲线都表现出非线性的特征。随着应变率的增加,二维C/SiC复合材料的拉伸强度从204 MPa增加到270 MPa,增加了33%,这表明复合材料的拉伸强度具有较强的应变率敏感性。复合材料在准静态和动态加载下表现出不同的破坏模式是由材料内部界面行为的应变率效应造成的。      

The influence of alloy composition on microstructure and tensile behaviour of copper-lead alloys

Lead is a soft metal that possesses excellent antifriction and lubricating characteristics and is a desired addition to alloys which find use in friction-critical and low-load-bearing applications. The influence of alloy composition on microstructure, tensile properties and quasi-static fracture behaviour has been studied. Alloy composition, that is, lead content, was observed to have an influence on the size and distribution of lead globules in the copper matrix. The yield strength, ultimate tensile strength and elastic modulus of the alloy decreased with increase in lead content. The ductility of the alloys showed an improvement with increase in lead content. The influence of lead content on quasi-static fracture is discussed in detail.     

Impact Tensile Stress–strain Characteristics of Wrought Magnesium Alloys

Abstract:  Impact tensile properties of three different wrought magnesium alloys (AZ31B-F, AZ61A-F and ZK60A-T5) are evaluated using the split Hopkinson bar. Reliable tensile stress–strain data up to fracture in the extrusion direction at strain rates of nearly 1000 s⁻¹ are presented and compared with those at quasi-static and medium rates of strain obtained on an Instron testing machine. The effect of strain rate on the tensile strength, elongation at fracture and absorbed energy is examined in detail. It is demonstrated that the tensile strength increases with increasing strain rate, and the strain-rate dependence of elongation at fracture and absorbed energy varies, depending on the magnesium alloys tested. The limitations of the test technique are discussed.     

Development and validation of an analytical method for tensile strength determination of fibrous bulk solids

The increasing significance of products consisting of elongated particles or fibres along with a lack of understanding flow properties of fibrous bulk solids in processes urge for appropriate test procedures.Therefore, a tensile tester was designed with respect to the special needs in terms of test techniques of those bulk solids. The procedure of tensile strength determination was tested with regard to several possible influencing factors. Following from that, the manner of filling and filling height were identified to have the greatest influence on results. Furthermore, it is shown that the developed load system is capable of improving the repeatability of test results for fibrous bulk solids.Based on the derived standard procedure, systematic tests were carried out on beech and spruce chips in three different fractions each as well as model materials such as polypropylene fibres and differently sized bunches of glass fibres. The biomass materials have been characterised by dynamic image analysis prior to and after experiments resulting in particle size and shape factor distributions. It is found that tensile strength is affected by particle shape, size, roughness and interparticle contact area.     

Processing, microstructure and fracture behaviour of a spray atomized and deposited aluminium–silicon alloy

In this study a hypereutectic aluminium–silicon alloy was synthesized by spray atomization and deposition technique. Microstructure characterization studies were performed to provide an understanding of the influence of spray processing on microstructure of the hypereutectic alloy. Ambient and elevated temperature tensile tests reveal the spray-processed alloy to have better strength and ductility than a conventional ingot metallurgy processed alloy having the same chemical composition. The quasi-static fracture characteristics of the spray-processed alloy is presented and discussed in light of processing and intrinsic microstructural effects. This revised version was published online in November 2006 with corrections to the Cover Date.     

Studies on mechanical,thermal and dynamic mechanical properties of untreated (raw) and treated coconut sheath fiber reinforced epoxy composites

The untreated (raw) coconut sheath fiber reinforced epoxy (UTCSE) composite and treated coconut sheath fiber reinforced epoxy (TCSE) composite have been fabricated using hand layup followed by compression molding technique. The prepared specimens were characterized by Fourier transform infrared spectroscopy (FTIR), dynamic mechanical analysis (DMA), thermo gravimetric analysis (TGA) and scanning electron microscopy (SEM) techniques. The prepared specimens are cut as per ASTM Standards to measure tensile, flexural and impact strengths by using universal testing machine and izod impact tester respectively. The treated coconut sheath fiber reinforced epoxy composite (TCSE) posses higher mechanical strength and thermal stability compared to untreated (raw) coconut sheath fiber reinforced epoxy composite (UTCSE). In the SEM fracture analysis, TCSE composite showed better fiber–matrix bonding and absence of voids compared to UTCSE composite.