High-strength Metal Research Articles

Processing and Development of a New High Strength Metal Foam

ABSTRACTThe research sited in this paper involves the development of new closed cell metal foam composite materials using powder metallurgy (PM) and gravity casting techniques. The foam is comprised of steel hollow spheres packed into a random dense arrangement, with the interstitial space between spheres occupied with a solid metal matrix. Using a casting technique, an aluminum alloy infiltrates the interstitial spaces between steel spheres. In a powder metallurgy method, steel spheres and iron powder are sintered to form a solid, closed cell structure. The measured densities of the Al-Fe composite foam and iron foam are 2.4 g/cm3 and 3.2 g/cm3, with relative densities of 42% and 41% respectively.The hollow sphere metal foam composite materials developed in this study displayed superior compressive strength as compared to hollow sphere foams currently being produced. The compressive strength of the cast Al-Fe foam averaged 67 MPa over a region of 10 to 50% strain, while the steel PM foam averaged 45 MPa over the same strain region. Densification began at approximately 50% for the cast foam and 55% for the PM foam.

The effect of plasticity on incipient mixed‐mode fatigue crack growth

ABSTRACTSeveral criteria for the prediction of incipient crack path direction of non‐proportional mixed‐mode fatigue cracks, immediately after a change of load from steady mode I conditions are investigated. The analysis is based on two‐dimensional plane strain FE‐simulations in which the actual elasto‐plastic stress distribution is used for the MTS, MTSR and MEPSR criteria. The purpose of the analysis is to compare the numerical results with experimental results as well as with previous predictions based on solely elastic stress analyses, taken from the literature. It is shown that the influence from elasto‐plastic deformation on crack branching direction is of utmost importance. It is found that the incipient crack growth of metals falls into two categories: high strength metals follow the MTSp criterion whereas more ductile metals follow the MTSRp criterion. The subscript p indicates that the elasto‐plastic evaluation of the respective criterion should be used.

High damping capacity of ultra-fine grained aluminum produced by accumulative roll bonding

Accumulative rolling bonding (ARB) is a kind of severe plastic deformation process which can produce high strength metals with ultrafine (sub-micron) grained microstructure. Metals produced by this process are also expected to show excellent damping capacity since they contain significant amounts of lattice defects which give rise to internal friction. We examined the damping capacity of high purity (99.99% Al) and commercial purity (99% Al) aluminum sheets ARB processed to an equivalent strain of 4. Internal friction values ( Q −1) of the 99.99% Al and the 99% Al were as high as 12×10 −3 and 6×10 −3, respectively. Microstructures were observed to clarify the damping mechanism. The mean grain thickness of the pancake-like shaped ultrafine grains were 640 and 250 nm in 99.99% Al and 99% Al, respectively. A significant amount of dislocations was observed within the ultrafine grains in both of the ARB-processed aluminum sheets. The high damping capacity was attributed to vibration of the dislocations whose ends were tightly bound with the grain boundaries.

Al/APRP 적층재의 수지혼합비가 인장 및 티-필(T-peel) 강도 특성에 미치는 영향

Aluminum alloy/aramid tiber reinforced plastic(AI/AFRP) laminates consists of high strength metal(AI5052) and laminated aramid tiber with structural adhesive bond. The mixture ratio effect of epoxy resin. curing agent. accelerator on the tensile strength and T-peel strength characteristic in A1/AFRP laminates were investigated in this study. The epoxy, diglycidylether of bisphenol A(DGEBA), was cured by methylene dianiline(MDA) with or without an accelerator(K-54). Eight different kinds of resin mixture ratios were selected for the test; tive kinds of AI/AFRP laminates were named as AUAFRP( I) and three others of Al/AFRP laminates were named as AI/AFRP(2). The comparison of tensile strength and T-peel strength with variation of resin mixture ratio were studied. Respectively, A1/AFRP( I) and A1/AFRP(2) indicated approximately 6.0 times and 7.0 times more improved maximum tensile strength in comparison with those of monolithic A15052. AI/AFRP(2) indicated approximately 1.5 times more impoved maximum T-peel strengths in comparison with those of Al/AFRP(I). As results. A1/AFRP(2) turned out to have more effective characteristics on the tensile strength and T-peel strength than those of A1/AFRP(I).

Characterization of melanoidins from a glucose-glycine model system

The properties of melanoidins prepared from glucose and glycine (GG) were investigated by a three step purification protocol consisting of dialysis, gel filtration at high ionic strength and ion metal affinity chromatography. The high molecular weight fraction obtained in the GG system is responsible for 80% of the total brown colour and its antioxidative ability was about 1/4 of that of Trolox measured by the inhibition of linoleic acid oxidation. GG melanoidins have good affinity towards Cu (II) (32% bound to the resin) while it is much lower towards Pb (II) (10%) and Fe (II) (5%). Capillary zone electrophoresis analysis suggests that GG melanoidins are positively charged, although no signal was observed analysing melanoidins by matrix-assisted laser desorption-ionisation time-of-flight mass spectrometry (MALDI-TOF/MS).

New method to prepare iron particles with different morphologies: a way to get high green strength metal compacts

Metaliron powders of well controlled size and morphology were synthesised by thermal decomposition under hydrogen of precipitated ferrous oxalates. Green compacts were prepared by uniaxial pressing of metal powders at 290 MPa. The bending green strengths of compacts were measured.The precipitation of β-FeC2O4.2H2O oxalate from ammonium oxalate gives rise to the formation of spherical particles by aggregation ofelongated grains. Thermal decomposition of this oxalate from 400 to 500°C under hydrogen permits metal iron particles with a rough surface to be obtained. Decomposition occurring above 500°C induces a smoothness of the particle surface. Metal particles synthesised at 500°C show both surface roughness and micrometer sized primary grains.This specific microstructure has allowed the highest value ofcompact green strength (31·7 MPa) to be obtained.Acicular shaping of the β-FeC2O4.2H2O particles precipitated from oxalic acid involves, after decomposition, an increase in the surface roughness and shape irregularity of the metal particles, owing to an entanglement of the elementary grains. An exceptional value (about 60 MPa) for the metal compact green strength was thus obtained for this type of powder.

The Mixture Ratio Effect of Epoxy Resin, Curing Agent and Accelerator on the Fatigue Behavior of FRMLs

Fiber reinforced metal laminates(FRMLs) are new types of hybrid materials. FRMLs consists of high strength metal(Al 5052-H34) and laminated fiber with structural adhesive bond. The mixture ratio effect of epoxy resincuring agentaccelerator on the fatigue behavior of FRMLs was investigated in this study. The epoxy, diglycidylether of bisphenol A(DGEBA), was cured by methylene dianiline(MDA) with or without an accelerator(K-54). Eight different kinds of resin mixture ratios were selected for the test ; five kinds of FRMLs(1) and three others of FRMLs(2). The relationship between da/dN and ΔK with variation of resin mixture ratio was studied. FRMLs(1) and FRMLs(2) indicated approximately 2 times and 2.2 times more improved maximum bending strengths in comparison with those of Al 5052-H34. The resin mixture ratio in case of FRMLs(1) indicated the maximum fatigue life, while the resin mixture ratio in case of FRMLs(2) indicated the maximum fatigue life. As results, FRMLs(2) turned out to have more effective characteristics on the fatigue properties and the bending strength than those of FRMLs(1).

Tensile fracture of fibre reinforced metal laminates containing a circular hole

Fibre reinforced metal laminates (FRMLs) are a new family of aerospace structural materials developed for fatigue critical applications. These materials are laminated sheets of thin and high strength metal layers, usually aluminium, and alternating plies of fibre reinforced polymer composite layers, viz, aramid/epoxy, carbon/epoxy, glass /epoxy, etc. Afaghi-Khatabi and Ye have considered the fracture data existing on various ARALL (aramid reinforced aluminium) laminates containing a circular hole for evaluation of notched tensile strength, using the effective crack growth model (ECGM), point stress criterion (PSC) of Whitney and Nuismer and damage zone criterion (DZC) of Eriksson and Aronsson. It is noted that ECGM overestimates, whereas PSC and DZC underestimate the notched tensile strength of FRMLs. This note highlights briefly the two different criteria of Whitney and Nuismer known as ‘point stress criterion (PSC)’ and ‘average stress criterion (ASC)’, and suggests a modification in the criteria for accurate evaluation of the notched tensile strength.

Shear Localization with an Arrhenius Flow Law

A flow law with Arrhenius dependence on temperature is used to model shear localization and shear-band phenomena in thermoviscoplastic materials. Arrhenius dependence is suggested by microstructural arguments for some high-strength metals. Whereas this form has been used in numerical studies, this paper offers the first comprehensive analytical study. Results are presented for the one-dimensional problem governing the unidirectional shearing of a slab. A nonlinear analysis reveals the existence of multiple steady states whose stability is determined. The steady Arrhenius model is discussed and compared to similar models in combustion and chemical kinetics. Steady solutions are found to depend on a parameter related to both the stress applied at the boundary and to the competition between diffusion and heat generation in the problem. Varying this parameter results in an S-shaped response curve (or bifurcation diagram), which is new to the shear-band literature. The response curve is constructed asymptotically and verified numerically for a steady model in which stress is absent from the flow law but not from the problem. Stability analysis shows that both the lower and upper branches of the curve are stable. The lowerbranch corresponds to a low-temperature steady state similar to those found in earlier studies. The upper branch, not previously observed, is likely to represent a high-temperature state with fully formed shear bands. Finally, the effects of reinstating full stress dependence are analyzed.

Flexural stiffness envelopes for planar multilayered systems containing two dissimilar materials

The flexural stiffness of multilayer systems, using two dissimilar materials, namely a high-strength titanium alloy and titanium metal matrix alloy, has been determined. The development of different types of symmetric and asymmetric layered systems is presented and stiffness envelopes are determined for the systems. It is demonstrated that there are several layered structures, which can provide the same flexural stiffness. The results are discussed in the context of other mechanical properties such as the strength, impact toughness and damping capacity.

Realistic problems in the physics of plasticity and hardness for single crystals and polycrystalline materials

The basic results from investigations of certain real problems in the physics of plasticity for single crystals and polycrystalline metal alloys carried out under the direction of the authors are given. The microdeformation patterns and formation of the flow limit in polycrystalline material are treated; the features of the mechanisms of deformation, deformational hardening, and the defect substructure in high-strength metal alloys are characterized. Analyses are carried out for phenomena involving activation of grain boundaries by grain boundary flows of impurity atoms, and experimentally based features of deformation on different structural levels under active extension, creep, and sign-alternating loading conditions. The main attention is given to the development of collective deformation modes. A discussion of some structural aspects of the realization of meso-level plastic flow with different deformation conditions is presented.

Fracture behaviour of diffusion bonded bimaterial Ti–Al joints

Failure modes of constrained metal foils between two elastic solids are rather different from those in the unconstrained condition. If the interface adhesion is strong between materials, a lower strength thin metal (plastic) foil between two much higher strength metals (elastic) can undergo substantial plastic deformation and fail with high triaxiality induced ductile fracture. Experiments have been conducted to explore the modes of failure and the factors governing fracture in such a constrained metal interlayer. In the present work, the effects of soft inter layer thickness and brittle reaction layer on the fracture behaviour of four point bend specimens have been investigated. A series of solid state diffusion bonds were produced between 25 × 25 mm section titanium bars using pure aluminium foils of different thickness (50, 457, 914, and 2000 μm) as the soft constrained inter layer. All four point bend specimens containing an ∼ 2 μm thick intermetallic reaction layer TiAl3 between the titanium and aluminium failed in ductile fracture mode within the soft aluminium interlayer next to the interface. A number of void formations were observed ahead of the crack tip next to the interface. No evidence of interface debonding was observed. However, the specimens containing an 8 μm thick TiAl3 layer failed by brittle fracture along the interface between the titanium substrate and the TiAl3 layer. It was found that decreasing the soft interlayer thickness from 2000 to 457 μm increased the load carrying capacity and decreased the fracture toughness caused by constrained plastic deformation (high triaxiality) of the interlayer.

New test method and facilities for investigating short fatigue crack propagation: Pan, J.Z. Exp. Techn. (1995) 19 (2), 19–23

It is confirmed that the fatigue strength of high-strength metals would not continuously increase as the tensile strength enhances. In order to find out the optimal fatigue strength of maraging steel, the tensile and cyclic pull-push (R = − 1) loading tests of 18Ni maraging steel (250 grade) aged under different conditions were carried out in the present study. Meanwhile, the fracture surface and microstructural differences were also investigated. It is detected that decreasing the ageing temperature can bring about the decrement in interparticle spacing and austenite fraction, which would further improve the tensile strength and change the tensile fracture mode. Increasing tensile strength can effectively improve the fatigue strength in the low-strength region. However, as the strength further increases, the intergranular fracture would happen, which means intense deformation inhomogeneity at grain boundaries, contrarily causing the decrement in fatigue strength. Finally, it is believed that the optimal fatigue strength can be obtained in the high-strength region where there is a transition in the fracture mode. Moreover, it is found that the specimen aged at 550 °C for 5 h with an ultimate tensile strength of 1838 MPa exhibiting the transgranular fracture possesses the highest fatigue strength of 685 MPa.

Micromechanics of coalescence—I. Synergistic effects of elasticity, plastic yielding and multi-size-scale voids

Void growth and coalescence under physical states similar to those found in highly stressed regions ahead of a crack is investigated. The analysis introduces a representative material volume containing several large voids and a population of microvoids present from the very beginning, all of which are modeled as discrete entities. Plastic yielding has pervaded the material volume of interest. The underlying micromechanics of final rupture is dominated by a succession of rapidly growing microvoids. This involves the synergistic interaction between elasticity associated with high stress triaxiality, stiffness softening caused by plastic yielding and a rich supply of length scales arising from voids of vastly different sizes. A primary feature of the coalescence phase is an unstable deformation mode whereby a minute, benign void rapidly enlarges reaching a size set by the characteristic length of the locally elevated stress field. The process begins with a large void growing in concert with the plastic strain. Simultaneously, a local zone of high stress concentration emanates from the large void and spreads across the material raising the stresses at nearby microvoids. As a result, the hydrostatic stress surrounding one or more microvoids is raised to a level that activates an unstable deformation mode in which the stored elastic energy drives the plastic expansion of the microvoid. Although the overall stress decreases rapidly, small zones of high stress concentration are generated near growing voids—causing even smaller nearby microvoids to grow rapidly. This process continues until the submicron ligament fails by microcleavage or by shearing along crystallographic planes. Plastic yielding plays a crucial role in the above process by lowering the stress level required for the unstable-like growth mode of microvoids. The process outlined above appears to be the main operative mechanism in several observed failure modes in metal alloys. The morphologies of fracture surfaces dominated by flat dimpled rupture and voidsheet formation can be elucidated by the present work. For high-strength metals, our studies suggest that microvoid cavitation and link-up will increasingly dominate the failure process resulting in a brittle-like ductile rupture mode in which very little energy is expended.

Fatigue Life of Welded Bellows. Effect of Configuration of Corrugated Plate.

Using three kinds of double-ply welded bellows made of Ni base alloy A and / or B, the effect of the configuration of corrugated plate on the fatigue life of bellows has been investigated. Ni base alloy A is a higher-strength metal and Ni base alloy B is a highly corrosion-resistant metal. Sample 1 is a conventional (same-metal welded) bellows made of alloy A and sample 2 is a dissimilar-metal welded bellows made of alloys A and B. These two bellows are constructed from corrugated plates of the same shape. On the other hand, sample 3, which is a dissimilar-metal welded bellows made of alloys A and B, is constructed from two differently shaped corrugated plates arranged alternately. From the fatigue test, it was shown that sample 3 has a longer fatigue life compared with samples 2 and 1. The advantage of bellows constructed from differently shaped corrugated plates was also confirmed by FEM stress analysis.

Biochemical studies in marine species—II. NADP +-dependent isocitrate dehydrogenase from turbot ( Scophthalmus maximus L.) larvae

The isocitrate dehydrogenase activity specific for NADP + has been studied in early turbot larvae and compared with that of their adult counterparts. The activity showed a broad optimum pH between 7.9 and 9.0, and an optimum temperature of 40°C (with an E a of 23.3 kJ/mol) and was inhibited by high ionic strength and heavy metals. The apparent K m values for the substrates were 16.7 μM for isocitrate, 3.5 for NADP +, 29.6 μM for Mg 2+ and 3.3 μM for the Mg-isocitrate complex. The effect of the absence of Mg 2+ on the enzyme affinity for its substrates is also reported. The enzyme stability was decreased by iodoacetic acid and this effect reverted by isocitrate. The enzyme activity was inhibited by metabolites such as citrate, oxoglutarate and adenine dinucleotides. It was also fully inhibited by the concerted action of glyoxylate and oxaloacetate.

Fabrication of a rotor for a field based, self-excited compulsator power supply for a 9 MJ railgun demonstrator

Fabrication of a high-speed, self-excited air-core rotor for a field based compulsator is well under way. The compulsator is to be used as a power supply for firing 9 MJ projectiles from a skid mounted electromagnetic launcher (railgun). The rotor flywheel is made of fiber reinforced epoxy composite and is supported with high-strength metal stub shafts which are shielded from the excitation field by water-cooled copper sleeves. The rotor features separate windings for self-excitation and railgun firing, and a two-pole configuration for pulse length considerations. The rotor is designed to store 210 MJ at 8600 r/min, with a rating of 3.2 MA peak current and 20 GW peak power into a 9 MJ railgun load. The concentric fiber-reinforced epoxy composite rings that make up the rotor flywheel are filament wound on steel mandrels, cured, and interference fit together. The main and excitation armatures and output conductors leading from the armatures to the shaft conductors are constructed of epoxy-impregnated aluminum and copper Litz wire.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An improved method for compressive stress-strain measurements at very high strain rates

This paper describes a modification of the split Hopkinson pressure bar, to allow compression testing of high strength metals at a strain rate of up to about 10 5 s –1 . All dimensions are minimized to reduce effects of dispersion and inertia, with specimens of the order of 1 mm diameter. Strain is calculated from the stress record and calibrated with high-speed photography. Particular attention has been paid to the accuracy of the technique, and errors arising from nonlinearity in the instrumentation, dispersion, frictional restraint and inertia have all been quantitatively assessed. Stress–strain results are presented of Ti 6A14V alloy, a high strength tungsten alloy, and pure copper.

R &amp; D Trends of Advanced Metal Matrix Composites and Fracture Mechanics Characterization.

Various advanced structural composite materials have recently been researched and developed in the field of aeronautics and aerospace technologies. One of the most important and urgent problems is to characterize the damage tolerance behavior of these advanced materials for broad use in primary structural applications. The final goal of this study is to establish new concepts of material design for high performance (high strength, high toughness and high durability) metal matrix composites. In this paper, current R & D trends of advanced structural composite materials in Japan were first outlined. Secondly, some of the author's current research projects, especially fracture mechanics of discontinuous and continuous fiber reinforced metal matrix composites were briefly introduced relating to the testing and evaluation of damage tolerance behavior. Some technical problems were also discussed in the development and standardization of fracture mechanics testing and evaluation methods. Finally, the current status of testing and evaluation of metal matrix composites was introduced by reporting on the round robin test (RRT) done for pre-standardization.

Yarns and tows comprising high strength metal coated fibres, process for their production, and articles made therefrom: Morin, L.G. (American Cyanamid, ME, USA) US Pat 4 909 910 (20 March 1990)

Yarns and tows comprising high strength metal coated fibres, process for their production, and articles made therefrom: Morin, L.G. (American Cyanamid, ME, USA) US Pat 4 909 910 (20 March 1990)