A new method to determine the characteristic lengths of composite joints without testing

A numerical method is presented to determine the characteristic lengths for the failure analysis of composite joints without characteristic length tests. In the conventional methods, compressive characteristic length was determined from the result of a combined bearing test and finite element analysis. The present study, however, shows that the same compressive characteristic length can be obtained by numerical calculation without the bearing test. A new method to define the tensile characteristic length is also introduced so that the tensile characteristic length is numerically determined without the tensile test. Failure loads and modes based on the numerically calculated characteristic lengths are validated by the test results for composite joints with nine different geometries. Good agreement was found among the results of the present numerical method, the conventional method, and the test.     

A 3-DOF testing machine for in-plane behaviour of structures

A testing machine designed for studying the in-plane behaviour of structures controlling independently (either in load and in displacement) three degrees of freedom is presented. The main topics herein discussed are (i) limits of application, either due to the capability of jacks, transducers and rollers, and to local and global equilibrium; (ii) behaviour in the case of brittle failure, with particular regard to energy and displacement restitution due to the elastic behaviour of the machine itself and to oil deformability; and (iii) the control system, which is based on the classic scheme for feedback control systems and has a separate sub-system to keep under control the security devices.     

Optimized butterfly specimen for the fracture testing of sheet materials under combined normal and shear loading

The present paper is concerned with multi-axial ductile fracture experiments on sheet metals. Different stress-states are achieved within a flat specimen by applying different combinations of normal and transverse loads to the specimen boundaries. The specimen geometry is optimized such that fracture initiates remote from the free specimen boundaries. Fracture experiments are carried out on TRIP780 steel for four different loading conditions, varying from pure shear to transverse plane strain tension. Hybrid experimental–numerical analyses are performed to determine the stress and strain fields within the specimen gage section. The results show that strain localization cannot be avoided prior to the onset of fracture. Through-thickness necking prevails under tension-dominated loading while the deformation localizes along a band crossing the entire gage section under shear-dominated loading. Both experimental and simulation results demonstrate that the proposed fracture testing method is very sensitive to imperfections in the specimen machining. The loading paths to fracture are determined in terms of stress triaxiality, Lode angle parameter and equivalent plastic strain. The experimental data indicates that the relationship between the stress triaxiality and the equivalent plastic strain at the onset of ductile fracture is not unique.     

A probabilistic solid-porous model to determine the shear strength of unsaturated soils

A probabilistic solid-porous model has been developed to determine the shear strength of unsaturated soils. The probabilistic model was built by analyzing the probability of a certain pore or group of pores of a network to fill or remain filled with water during a wetting or drying process, respectively. This model is used to determine the equivalent stress which represents the stress supported by the solid skeleton of an unsaturated soil and is related to the strength of the material. The probabilistic model is an alternative to the use of computational models and shows some important advantages. The theoretical results of the model are compared with a series of triaxial tests performed at constant suction and constant volume. These comparisons demonstrate that this model is adequate to establish the strength of unsaturated materials.     

The effect of hydrostatic pressure on the shear yield behaviour of polymers

The torsional stress-strain behaviour of isotropic poly(methylmethacrylate) (P M M A), poly(ethylene terephthalate) (P E T) and polyethylene has been studied under hydrostatic pressures up to 7 kbar. In P M M A the following important features were observed. First, there is a monotonic increase in the initial slope of the stress-strain curve with increasing pressure. Secondly, there is a substantial increase in the yield stress and the strain to yield as pressure is raised. Thirdly, there is a transition in the mode of failure at elevated pressure, the specimens fracturing in the high pressure region before a drop in stress occurs. Finally, in the high pressure region the fracture stress increases with increasing pressure but the strain at fracture decreases.The observed yield behaviour can be represented formally in a number of ways, and the results will therefore be discussed accordingly, in an attempt to give a general yield criterion for P M M A. The fracture behaviour has been analysed in terms of the Griffith ideas for fracture of glassy materials, and this will also be discussed.The results for poly(ethylene terephthalate) (Arnite) differ significantly from those for P M M A. Specimens of Arnite as received from the manufacturers were ductile in torsion at atmospheric pressure, and the torsional yield stress rose monotonically with increasing hydrostatic pressure. Annealing the specimens produced embrittlement at atmospheric pressure, but on testing under conditions where there is no tensile component of stress (i.e. at very low hydrostatic pressures) ductile behaviour was observed.The contrast between P M M A and Arnite suggests that in the former case there aresurface flaws which are penetrated by the hydraulic fluid at high pressures, whereas in the latter caseinternal flaws are produced by annealing.Polyethylene remained ductile over the complete pressure range, with a pressure dependence of the tensile yield stress which was similar to that shown by polyethylene terephthalate.1 bar=10⁵ N/m^–2     

The behaviour of iron and aluminium during the diffusion welding of carbon steel to aluminium

The diffusion welding of carbon steel to aluminium may be achieved when the temperature and compression of the weld metal allows diffusion at the interface between both components; the degree of interdiffusion largely determines the quality of the welded joint. Such diffusion processes can take place under vacuum. However, in this paper, we report that high quality welds can be achieved without the use of vacuum or reducing atmospheres by applying sufficient compression to give very high levels of plastic deformation of an Al layer sandwiched between layers of carbon steel at 550 °C in laboratory air. The resulting structures were characterised using scanning electron microscopy, transmission electron microscopy and microanalysis, which showed that, under these conditions, Fe diffused across the interface into the Al layer.     

Solid solubility of silicon and germanium in aluminium under pressure

The solid solubility of silicon and germanium in aluminium under pressure are investigated using the microscopic electronic theory based on pseudopotentials and using the virtual crystal approximation. Obtained results for the lattice constant and the solid solubility under pressure in the Al-Si and Al-Ge systems are in good agreement with the few available experimental data, in spite of our not introducing any adjustable parameter except for the lattice constant of pure aluminium crystal. The heat of solution and the pressure-volume relation in the Al-Si and Al-Ge systems are presented theoretically.     

Failure behaviour of particulate-reinforced aluminium alloy composites under uniaxial tension

Tensile tests were carried out at room temperature on 6061-aluminium alloy reinforced with SiC and Al₂O₃ particulates. Although a significant increase in strength could be achieved by introducing ceramic reinforcements into the aluminium alloy matrix, it is associated with a substantial decrease in fracture strain. In order to understand the reason for the inferior ductility of such composites, analytical solutions were obtained using a simple composite model. SEM studies were carried out on the side surfaces of the fractured specimens to verify the proposed failure behaviour. Failure modes observed to operate in such composites under uniaxial tension are described.     

中等应变率下泡沫铝的吸能特性

进行了不同密度、高度和压缩方向下泡沫铝的准静态压缩试验和中等应变率下(＜100 s-1)的冲击试验,研究了具有不同密度的闭孔泡沫铝在准静态压缩和冲击工况下的吸能特性.结果表明,泡沫铝是一种近似的各向同性结构,具有较高的单位质量吸能特性,是一种较好的吸能材料.在准静态和中等应变率冲击条件下,泡沫铝对应变率不敏感,其应力应变关系与应变率关系不大.不同的泡沫铝,其平台应力与密度之间的关系不同,在研究其性能时,必须测量应力-应变关系.泡沫铝的致密区对其吸能特性有很大的影响.     

A machine for fatigue testing under the joint action of pure bending and torque moment

This paper describes a machine for fatigue testing of specimens of materials and structural elements under the joint action of pure bending and torque moment. We discuss the loading setup of the machine and the design and kinematic features of the mechanisms, which eliminate the mutual influence of the loading components. Performance specifications of the machine are given. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 132–135, July–August, 1999.     

The mechanical behaviour of polystyrene under pressure

Tensile deformation of polystyrene carried out under pressure up to 4 kbar has shown that the pressure-transmitting fluid (silicon oil) acts as a stress crazing and cracking agent. Unsealed specimens showed a brittle-to-ductile transition at 2.95 kbar, while specimens sealed with Teflon tape and rubber showed the same transition at only 0.35 kbar. Analysis of the stress-strain curves for the sealed specimens indicated that the pressure dependency of the craze initiation stress differs from that of shear band initiation stress. The brittle-to-ductile transition occurs when the initiation stresses of both processes become equal. The principal stress for craze initiation showed almost no pressure dependency, suggesting that crazes initiate when the principal stress level of the tensile specimen reaches a critical value irrespective of the applied hydrostatic pressure. Similarly, no pressure dependency was observed for the principal ductile fracture stress. The pressure dependency of yield stress agreed well with a non-linear pressure dependent von Mises yield criterion.     

Shear-thickening behaviour of concentrated polymer dispersions under steady and oscillatory shear

The rheological behaviour of a 58 vol.% dispersion of styrene/acrylate particles in ethylene glycol was investigated using a plate-on-plate rheometer. Experimental results showed that the concentrated polymer dispersion exhibited a strong shear-thickening transition under both steady shear and dynamic oscillatory conditions. The low-frequency dynamic oscillatory behaviour could be reasonably interpreted in terms of the steady shear behaviour. Accordingly, the critical dynamic shear rate [(g)\dot]_{\textc_d} , \dot{\gamma }_{{{\text{c\_d}}}} , agreed well with the critical shear rate obtained in steady flow [(g)\dot]_{\textc_s} , \dot{\gamma }_{{{\text{c\_s}}}} , where [(g)\dot]_{\textc_d} \dot{\gamma }_{{{\text{c\_d}}}} was calculated as the maximum shear rate by the critical dynamic shear strain γ _c and the frequency ω, i.e. [(g)\dot]_{\textc_d} = wg_\textc . \dot{\gamma }_{{{\text{c\_d}}}} = \omega \gamma_{\text{c}} . However, during high-frequency dynamic oscillation, it was observed that the shear thickening occurred only when an apparent critical shear strain was reached, which could not be fully explained by the wall-slipping effect. Based on freeze fracture microscopic observations, the effect of the micro-sized flocculation of particles on the rheology of concentrated dispersions was also discussed.     

Buckling and fracture behaviour of cracked thin plates under shear loading

Thin-walled structural components are widely used in several engineering applications such as in aerospace, naval, nuclear power plant, pressure vessel, mechanical and civil fields. Since they are frequently characterised by a high slenderness, the safety assessment of such structural components requires to carefully consider the buckling collapse which can heavily limit their allowable bearing capacity. For very thin plates, buckling collapse can occur under compression, shear, or even under tension. In the present paper, the buckling and fracture collapse mechanisms in an elastic rectangular thin-plate with a central straight crack under shear loading are analysed. Different boundary conditions, crack length and orientation are considered. Through a parametric finite elements (FE) numerical analysis, the crack sensitivity of the collapse load of such a structural component is examined. The obtained results are discussed, and some interesting and useful conclusions are drawn. The collapse mechanism occurring earlier (buckling or fracture) is found by varying the fracture toughness of the material, and some failure-type maps depending on the geometrical parameters of the crack are determined.     

Mechanical testing of disks under gaseous pressure

Numerical calculations are becoming more and more efficient in estimating the lifetime of structures under thermomechanical loading. However, these life estimations cannot be reliable if the necessary parameters have not been correctly identified and measured and if all the causes of damage have not been considered. Disk testing under gas pressure is similar to oil bulging testing. However, disk testing can easily be used for the mechanical characterization of materials subject to more varied solicitations: monotone loading (biaxial rupture tests at strain rates from 10⁻⁶ to 10⁰ s⁻¹), constant loading under high stresses (sustained load) at elevated temperature (creep tests), cyclic loading (mechanical slow fatigue tests); the temperature may be chosen between 20 and 900 °C and the environment may be studied by comparing the results obtained with either an inert gas or reactive gas. Moreover, disk testing reveals light damage since crossing cracks through the thin membrane create leakages detected by a mass spectrometer. In this paper, we present an original method of calculation developed to determine the true mechanical properties of the pressurized disk; the method of calculation is validated because its numerical results are identical to the measured tensile properties. In addition, the range of uniform deformation is correctly determined; this property is needed to establish sheet formability which is not clearly determined by oil bulging. Of course, the mechanical behaviour can be determined within the whole ranges of temperature and strain rates; such wide ranges cannot be tested by other techniques such as tensile testing or oil bulging. As disk edges are not stressed during testing, the results are very reproducible at any temperature and at any strain rate while the machining or cutting defects initiate very scattered ruptures of tensile specimens tested at high temperature or at high strain rate. The disk and its loading simulate real applications with thin walls embedded by thick parts such as thermal exchangers or spatial engines. The analytical method of calculation may be used for identifying the needed parameters of thermomechanical modelling; it will be optimized by finite elements methods and it would allow a rational quantification of hydrogen embrittlement.