Compressive behaviour of aluminium foams at low and medium strain rates

Compressive behaviour of CYMAT aluminium foams with relative densities ranged from 5% to 20% has been studied experimentally in this paper. An MTS machine is employed to apply a compressive load at strain rates of 10⁻³–10⁺¹ s⁻¹ to these closed-cell aluminium foams. It has been found that the plateau stress is insensitive to the strain rate and is related to the relative density by a power law. Deformation is not uniform over the whole sample: it first occurs in the weakest band, followed by the next weakest bands after the first one has been completely crushed.     

Steel-fibre-reinforcement and hydration coupled effects on concrete tensile behaviour

This paper presents the effect of hydration development on the tensile behaviour of steel-fibre-reinforced concrete. Tensile tests were performed on plain and fibre-reinforced concretes at 2, 7 and 28 days in order to determine the response of the composites, in particular to establish the post-peak behaviour and the evolution of the residual post-peak strength with hydration development and the associated improvement in the fibre–matrix bond.From the laws governing the tensile behaviour of plain concrete on the one hand and the residual strength capacity due to fibre reinforcement on the other, parameters fitting an analytical model of the uniaxial tensile response of fibre-reinforced concrete were determined. The proposed model takes into account the tensile damage to the concrete and the development of hydration. An original aspect of the model is that it also integrates the damage to the fibre–matrix bond.     

Ab initio tensile testing simulation of aluminum and aluminum nitride ceramics based on density functional theory

The ideal strength and mechanical behavior of aluminum, aluminum nitride, and their composites at zero temperature are investigated using ab initio norm conserving pseudopotential methods based on local density approximation. The total energy and stress of supercells which contain these atomistic models are calculated under uniaxial tensile strain. Estimated various physical properties of aluminum and aluminum nitride at equilibrium state are in good agreement with the experiment. The tensile test of the composite model shows a failure of the composite in the aluminum matrix. The ideal strength of these models is also estimated.     

Compressive behavior of closed-cell aluminum alloy foams at medium strain rates

Compressive behavior of closed-cell aluminum alloy foams at strain rates of 10⁻³-450 s⁻¹ has been studied experimentally. The fully stress-strain curves of specimens at medium strain rates were obtained using the High Rate Instron Test System, which can maintain a constant loading rate. The experimental results show that plateau stress and energy absorption capacity are remarkable dependent on strain rate, while the densification strain has a negligible dependence.     

Compressive properties of closed-cell aluminum foams with different contents of ceramic microspheres

In this paper, closed-cell aluminum foams with different kinds and contents of ceramic microspheres are obtained using melt-foaming method. The distribution and the effects of the ceramic microspheres on the mechanical properties of aluminum foams are investigated. The results show that both kinds of ceramic microspheres distribute in the foams uniformly with part in the cell wall matrix, some in adhere to the cell wall surface and part embed in the cell wall with portion surface exposed to the pores. Ceramic microspheres have an important effect on the yield strength, mean plateau stress, densification strain and energy absorption capacities of aluminum foams. Meanwhile, the content of ceramic microsphere in aluminum foams should be controlled in order to obtain good combination of compressive strength and energy absorption capacity. The reasons are discussed.     

Microstructure and tensile properties of friction welded aluminum alloy AA7075-T6

Solid-state welding processes like friction welding and friction stir welding are now being actively considered for welding aluminum alloy AA7075. In this work, friction welding of AA7075-T6 rods of 13 mm diameter was investigated with an aim to understand the effects of process parameters on weld microstructure and tensile properties. Welds made with various process parameter combinations (incorporating Taguchi methods) were subjected to tensile tests. Microstructural studies and hardness tests were also conducted. The results show that sound joints in AA7075-T6 can be achieved using friction welding, with a joint efficiency of 89% in as-welded condition with careful selection of process parameters. The effects of process parameters are discussed in detail based on microstructural observations.     

Defects and tensile properties of 6013 aluminum alloy T-joints by friction stir welding

In this paper, 6013-T4 T-joints were successfully fabricated with different welding parameters by friction stir welding in two different combination modes of skins and stringers. The distribution features and formation mechanisms of defects in T-joints were observed and analyzed. The effect of defects and welding parameters on tensile properties of T-joints was investigated. The result shows that the T-joint without tunnel defect only can be obtained with the traverse speed of 100 mm/min in this experiment, and the welding parameters influence the features and sizes of kissing bond defects. The fracture of T-joints along the shin is attributed to the kissing bond defect and the tunnel defect is the main factor affecting the tensile properties along the stringer.     

Effect of nitrogen on tensile flow behaviour of type 316 LN austenitic stainless steel

Abstract

The influence of nitrogen content on the tensile flow behaviour of type 316 LN austenitic stainless steel has been studied. Nitrogen content in the steel has been varied in the range 0·07 to 0·22 wt-%. Tensile tests were carried out over the temperature range of 300–1123 K at a nominal strain rate of 3×10^?3 s^?1. The tensile flow behaviour of the steels has been analysed based on the constitutive equation proposed by Voce. The Voce’s parameters of initial stress (σ_i) and saturation stress (σ_s) were found to increase linearly with increase in nitrogen content at all the test temperatures. Tensile properties of the steels were predicted from Voce constitutive equation parameters.     

A study to evaluate and understand the response of aluminum alloy 2026 subjected to tensile deformation

The strain concentration factors were determined for aluminum alloy 2026 in the T3511 temper using multi-hole structural coupon specimens. Samples of the alloy were evaluated for both the 6.25 mm (0.25 in.) thick and 10 mm (0.4 in.) thick specimens and having widths of 50 mm (2 in.) and 100 mm (4 in.), respectively. For the case of the specimens that were 50 mm in width the mechanical tests were conducted for both the open hole and filled hole conditions and the corresponding strain concentration value was determined. Threaded fasteners having collars were used for the case of the filled hole specimens. The fasteners posses a shank diameter that was slightly larger than the nominal hole size in order to provide for some interference. The strain concentration values were evaluated at both the failure strain (ε_f) and the strain at maximum load (ε_max). The average strain concentration value was then used to predict the results for the stack-up tests.     

Effects of sensitisation-induced martensitic transformation on the tensile behaviour of 304 austenitic stainless steel

The effects of sensitisation-induced martensitic transformation on the tensile behaviour of 304 austenitic stainless steel have been investigated. Yield strength is reduced by sensitisation, but ultimate tensile strength is nearly unaffected. Strain-hardening behaviour is changed by sensitisation, too. Although sensitisation may induce martensite formation near grain boundary, twin boundary, and austenite/martensite interface, the sensitisation-induced martensite does not exert a great influence on tensile behaviour in the 304 steel. In the unsensitised condition, martensitic transformation in the steel bulk induced by prior deformation and liquid-nitrogen immersion also does not change strain-hardening behaviour in the present steel.     

Effect of temperature and strain rate on tensile behavior of ultrafine-grained aluminum alloys

Ultrafine-grained Al–4Y–4Ni and Al–4Y–4Ni–0.9Fe (at.%) alloys were synthesized by the consolidation of atomized powders and subsequent hot extrusion. The mechanical behavior of these two alloys has been studied by performing uniaxial tension tests ranging from room temperature to 350 °C. These alloys, with high volume fraction of second-phase particles, exhibited ambient temperature tensile strength ranging from 473 to 608 MPa and plastic elongation ranging from 6.7 to 9.6% at an initial strain rate of 1 × 10⁻³ s⁻¹. However, lower ductility was observed with decreasing strain rate at the intermediate temperature ranging from 150 to 250 °C for Al–Y–Ni–Fe alloys due to limited work hardening.     

Deformation behaviour of self-compacting concrete under tensile loading

The deformation behaviour of self-compacting concretes of the compressive strength classes C30/37, C45/55 and C60/75 according to the European Standard EN 206-1 under sustained tensile loading was investigated up to 2.5 years. The long-term tensile strength was estimated to be 69% of the short-term tensile strength, determined at an age of 28 days. Load-free shrinkage has been measured on companion specimens. The usual way to determine creep is to subtract the measured shrinkage strain and the elastic or initial strain from the measured total strain. In doing so, a phenomenon was discovered which is called stress-induced shrinkage. It turned out that the drying shrinkage was larger for loaded specimens than for load-free specimens. Similar results have been found earlier. It seemed that the stress-induced shrinkage tends to increase with decreasing compressive strength. An important practical consequence is arising from stress-induced shrinkage: structural elements subjected to tensile␣load, tend to dry and to shrink faster than one␣would expect based on the assumption of load-free shrinkage. In the case of sustained deformation, this would raise the risk of cracking and would have a negative effect on the durability of a concrete structure. An erratum to this article can be found at     

Ageing effect on tensile and shrinkage behaviour of new green hybrid fibre-reinforced cementitious composites

Ageing effects on both uniaxial tensile and shrinkage behaviour of new green cementitious composites reinforced with bagasse fibre and steel fibre with ultra-high volume of fly ash are investigated in this paper. The tensile behaviour of the composites is investigated at the age of 28 days, 3 months, 6 months and 10 months after curing in weather conditions. Restrained shrinkage behaviour of the composites after curing for 3 months is tested on ring specimens. SEM tests are also conducted to study the influence of the ageing on the microstructure of the new composites. The test results show that the tensile strength of the composites at all ages increases with the decrease of the content of the fly ash and the bagasse fibre, and that the increasing application of fly ash and bagasse fibre decreases the steel ring strain and restrains the development of crack. It is also found that the tensile strength and the shrinkage of the composites such as crack width ascend greatly with time.     

Compressive and tensile behaviour of carbon fibres

Attempts have been made to discuss the fibre axial tensile and compressive behaviour of several carbon fibres prepared from different precursors, and surface- and/or sizing-treatments. With all fibres, the number of breaks increased with increasing tensile or compressive strain, and remained constant beyond a certain strain. The constant number of breakages based on the precursor differed remarkably in compression, whereas those based on the surface- and/or sizing-treatments differed remarkably in tension. In tension, the PAN-based fibre could be broken with a somewhat greater ease than the pitch-based fibre, while in compression, the pitch-based fibre could be broken with somewhat greater ease than the PAN-based fibre. © 1998 Chapman & Hall     

Melt flow and solidification during infiltration in making steel matrix syntactic foams

Steel matrix syntactic foam is a promising lightweight and energy absorbing material. In this study, an infiltration casting technology is developed to prepare syntactic foams, which brings in alumina hollow spheres with an average size of around 3.97?mm to make the pores and enhance the properties of the foams. During the process, melt flow along with heat transfer during the infiltration is investigated. The study shows that melt flow and velocity distribution of molten steel are significantly unstable; the critical solid fraction of the steel is 0.67; and the alumina ceramic mould in which the foams are prepared must be preheated to at least 1000°C to prevent incomplete infiltration. Finally, the microstructure of syntactic foams is studied.     

Elastic and electric properties of closed-cell aluminum foams: Cross-property connection

Foamed aluminum (AlMg1Si0.6) in the porosity range 0.45–0.85 produced by the powder metallurgy method is analyzed with regard to its elastic and electric properties. Various predictive models for the electrical conductivity and Young's modulus of closed-cell metal foam are assessed based on the experimental measurements. It is shown that the differential scheme provides the best predictions of the electrical conductivity in the porosity range 0.7–0.85, while Mori–Tanaka's scheme gives the best results for the Young's modulus. Comparing the two sets of the experimental data, cross-property coefficient that connects changes in the Young's modulus and electrical conductivity of a material due to pores was determined. A non-trivial finding is that the best prediction of the cross-property coefficient is obtained in the framework of non-interaction approximation.     

Compressive behaviour of closed-cell aluminium foams at high strain rates

It has been well established that ALPORAS® foams is a strain rate sensitive material. However, the strain rate effect is not well quantified as it is not unusual for strain rate to vary during high speed compression. Moreover, according to previous research, aluminium foams, especially ALPORAS® foams, behave differently at low and high strain rates. Therefore, different plastic deformation mechanisms are expected for low and high strain rate loadings as a result of micro-inertia of cell walls. In this paper, the strain rate effect on the energy dissipation capacity of ALPORAS® foam was investigated experimentally by using a High Rate Instron Test System, with cross-head speed up to 10 m/s. The compressive tests were conducted over strain rates in the range of 1 × 10^?3 to 2.2 × 10² s^?1, with each test being at a fairly constant strain rate. An energy efficiency method was adopted to obtain the densification strain and plateau stress. The effect of strain rate and the foam density was well presented by empirical constitutive models. The experimental data were also discussed with reference to the recent results by other researchers but with different range of strain rates. An attempt has been made to qualitatively explain the observed decrease of densification strain with strain rate.     

Effect of pore size on the tensile behavior of open-cell Ti foams: Experimental results

Open-cell Ti foams having an average pore size of 50 and 150 µm, respectively have been subjected to room temperature tensile tests to explore their tensile properties. Since extensometry is difficult in foams due to problems such as localized surface deformation and attachment of clip gauges, the foams' mechanical properties have been measured in this work accurately by the help of a camera which records the resulting dimensional changes during the loading. The camera acts as a non-contact extensometer and thus it helps in avoiding any small amounts of pre-deformation that might be caused by the attachment of conventional extensometer to the sample's surface prior to testing. Ti foam samples of smaller pores were found to experience higher strength and elongation than those with larger ones. The cell walls in both foam samples failed suddenly in the 45° maximum shear plane direction.     

The reactive stabilization of Al-Zn foams using a powder metallurgy approach

The Al-Zn binary system was chosen in order to study the possibility of generating a reactive foam system within the semi-solid region. The idea is to create foam at lower temperatures than the melting point of pure aluminum using a transient liquid phase that softens the matrix prior to bulk expansion. This minimizes crack formation, collapse, drainage and deformation generated during processing. The Al-Zn foams were fabricated via the powder metallurgy route by hot compaction and subsequently foamed using TiH₂ as a blowing agent. The investigated systems consist of low, medium and high concentrations of Zn (10 wt%, 33 wt% and 50 wt%) in an Al based matrix containing 0.8 wt% TiH₂. High temperature in situ confocal microscopy was used to study the formation of the transient liquid phase of the compacted elemental powders. As the percentage of Zn was increased, the liquidus temperature of the melt was lowered along with an increase in the volume of transient liquid phase. This reduces the mismatch between the hydrogen release temperature of the blowing agent and the liquidus temperature of the melt, thus increasing foaming stability. Reasonable foam structures near 300 vol% expansion and fair pore distributions were achieved at low concentrations of Zn (10 wt%) only above the alloy liquidus point. The mechanical compressive strength properties of the alloyed foam systems were also assessed.     

Tensile behaviour of 316LN stainless steel at elevated temperatures

Abstract

The tensile deformation behaviour of 316LN stainless steel was investigated from ambient temperature up to 1000°C. The hardness and microstructure of area near tensile fracture were characterised. The results show that the engineering stress increases smoothly with engineering strain when the tensile temperature is at 400°C or below, while the plastic deformation stage displays a serrated/jerky flow at 600°C. At tensile temperatures of 800°C or above, the plastic deformation stage is dramatically prolonged. The deformation mechanisms of 316LN stainless steel are proposed to be sliding and twinning at 400°C or below, tangle dislocations due to cross-slipping at 600°C, dynamic recovery at 700°C, and dynamic recrystallisation at 800°C or above. The finding provides useful guidelines for the processing and service of 316LN stainless steel components at high temperatures.