Modelling ductile fracture behaviour from deformation parameters in HSLA steels

In this work, an attempt is made to model the ductile fracture behaviour of two Cu‐strengthened high strength low alloy (HSLA) steels through the understanding of their deformation behaviour. The variations in deformation behaviour are imparted by prior deformation of steels to various predetermined strains. The variations in parameters such as yield strength and true uniform elongation with prior deformation is studied and was found to be analogous to that of initiation fracture toughness determined by independent method. A unique method is used to measure the crack tip deformation characterized by stretch zone depth that also depicted a similar trend. Fracture toughness values derived from the stretch zone depth measurements were found to vary in the same fashion as the experimental values. A semiempirical relationship for obtaining ductile fracture toughness from basic deformation parameters is derived and model is demonstrated to estimate initiation ductile fracture toughness accurately.     

The deformation and fracture behaviour of glass-filled ABS

The tensile and fracture characteristics of pigmented ABS containing 30 wt % rubber reinforced with 30 wt % glass fibre have been examined over a range of strain-rates extending from approximately 10⁻⁴ to 10⁻¹ sec⁻¹ within the temperature range 293 to 353 K. The glass fibre-reinforced composite had significantly increased fracture strength compared with the base polymer but possessed decreased ductility. The marked yield point which is characteristic of the ABS base polymer was absent from the reinforced material. Two different regions were found to exist on the fracture surfaces of composite specimens. One region possessed the characteristics of a weak interfacial bond while the other showed evidence of strong, interfacial bonding. In both regions extensive fibre pull-out was observed. The variation in fracture strength and morphology with strain-rate and temperature of testing is explained in terms of the properties of an interfacial region adjacent to the fibres which possesses viscoelastic properties different from those of the bulk polymer. The effect of adiabatic heating at the crack tip is also taken into account in the high temperature—high strain rate regime.     

Transformation characteristics and related deformation behaviour in orthodontic NiTi wire

Several orthodontic NiTi wires have been investigated by differential scanning calorimetry measurements and uniaxial tensile tests. An attempt is made to relate pseudoelastic deformation behaviour to characteristics of diffusionless martensitic transformation. Pseudoelasticity is explained by the respective importance of each parameter for use in orthodontics and experimental results are presented in terms of classification.     

High-temperature deformation and fracture behaviour of Cu-SiO2 bicrystals

Bicrystals of CU-SiO₂ dispersion-hardened alloys and of pure copper were tensile tested at various temperatures between 450 and 1050 K at a strain rate of 1.5 x 10^–4 sec^–1. In the case of pure copper bicrystals, elongation to fracture did not depend significantly on temperature and the fracture mode was invariably transgranular up to 850 K. On the other hand, the ductility of CU-SiO₂ bicrystals decreased with increase in temperature and the transition in the fracture mode from transgranular to intergranular occurred at around 450 K. SiO₂ particles on grain boundaries play an important role on intergranular fracture by suppressing grain-boundary sliding and also on the retardation of recrystallization during deformation. Two types of Cu-SiO₂ bicrystals having different crystal orientation relationships show quite different deformation and fracture behaviour. This can be explained in terms of the contribution of lattice dislocations to the grain-boundary sliding.     

Deformation and fracture behaviour of wheat starch plasticized with glucose and water

The mechanical properties of wheat starch and the effects of plasticizers upon them are studied in flexure at 293 K. For compositions low in water and glucose the material is glassy, with a flexural modulus between 0.7 and 5.0 GPa. The addition of water and glucose to wheat starch plasticizes the material through its glass transition into a rubbery state. The flexural moduli of the rubbery samples are in the range 50 to 200 MPa, which is indicative of a partially crystalline polymer. For starch-water mixtures the glass transition occur in the water content range 18 to 20%. The addition of glucose progressively shifts the glass transition to lower water contents. At strains below 0.04 brittle failure is only observed in the glassy samples. The surface morphology of the fractured samples shows features typical of pure synthetic glassy polymers.     

The deformation and failure behaviour of wheat starch plasticized with water and polyols

The mechanical properties of starch plasticized with water and glycerol or xylitol have been studied in flexure at room temperature. The addition of a polyol lowers the glassy modulus and the glass transition temperature of the starch. With the addition of polyols, the fall in modulus with increasing water content at the glass transition becomes less sharp and shifts to lower water contents. The high strain deformation shows that all samples fracture at a water content below 10% (wet weight basis), a proportion of which tears at water contents in the range 9%–10% and no fracture occurs in samples above about 10% water. Microscopy shows that there is evidence of permanent plastic deformation in fractured samples as the water content is increased to 10% water, although there is a negligible change in fracture stress and strain.     

Strength,deformation, fracture behaviour and ductility of aluminium-lithium alloys

The ever increasing need for high strength, improved performance, lightweight and cost-effective materials has resulted in significant improvements and development of new aluminium alloys for structural applications. Lithium additions to aluminium have the potential for providing a class of high strength alloys with exceptional properties suitable for weight-critical applications. In this paper, published studies of composition-processing-microstructure relationships are discussed. Contributions to strength of the solid solution are discussed with reference to the presence of lithium in solid solution, the presence of coherent, ordered precipitates in the matrix and the co-precipitation of binary, ternary and more complex strengthening phases. Microstructural influences on strength are discussed with reference to metallurgical variables. These variables include the intrinsic microstructural features; the presence of dispersoids, the nature and type of matrix strengthening precipitates and the presence of denuded zones adjacent to grain boundaries. The extrinsic and intrinsic micromechanisms governing the deformation characteristics and fracture behaviour are critically examined with specific reference to ageing condition of the alloy, the matrix slip characteristics, and the nature, volume fraction and distribution of strengthening precipitates. The deleterious effects of strain localization and the exacerbating effect of precipitate-free zones are also highlighted. The micromechanics governing the fracture processes are examined and the sequence of events in the fracture process is reviewed in light of the specific role of several concurrent factors involving nature and volume fraction of second-phase particles, deformation mode, and dislocation-microstructure interactions. Past attempts made to improve the tensile ductility and mechanical response of these alloys are also examined so as to provide a better basis for understanding processing-microstructure-deformation interactions.     

Microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055

The microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055 were studied. Detailed optical and electron microscopy observations were made to analyse the as-received microstructure of the alloy. Detailed transmission electron microscopy observations revealed the principal strengthening precipitates to be the hexagonal disc-shaped η′ phase of size 2 mm×20 mm and fully coherent with the aluminium alloy matrix, the presence of spheroidal dispersoids, equilibrium grain-boundary η precipitates and narrow precipitate-free zones adjacent to grain-boundary regions. It is shown that microstructural characteristics have a profound influence on tensile deformation and fracture behaviour. Tensile test results reveal the alloy to have uniform strength and ductility in the longitudinal and transverse orientations. Strength marginally decreased with an increase in test temperature but with a concomitant improvement in elongation and reduction in area. No change in macroscopic fracture mode was observed with sample orientation. Fracture, on a microscopic scale, was predominantly ductile comprising microvoid nucleation, growth and coalescence. The tensile deformation and fracture process are discussed in the light of the competing influences of intrinsic microstructural effects, matrix deformation characteristics, test temperature and grain-boundary failure. This revised version was published online in November 2006 with corrections to the Cover Date.     

Determining the stress-strain behaviour at large strains from high strain rate tensile and shear experiments

To characterise the high strain rate mechanical behaviour of metals, split Hopkinson bar experiments are frequently used. These experiments basically yield the force and elongation history of the specimen, reflecting not only the specimen material behaviour but also the specimen structural behaviour. Calculation of the real material behaviour from this global response is not straightforward, certainly for materials such as Ti6Al4V where due to low strain hardening, the specimen deformation is very inhomogeneous. However, for fundamental material research and constitutive material modelling, knowledge of the true effective stress versus plastic strain, strain rate and temperature is essential.In this contribution, a combined experimental-numerical approach for extraction of the strain rate and temperature dependent mechanical behaviour from high strain rate experiments is presented. The method involves the identification of the material model parameters used for the finite element simulations. The technique is applied to determine the stress-strain behaviour of Ti6Al4V using both high strain rate in-plane shear and tensile test results. For the tensile tests, even stress-strain data beyond diffuse necking are retrieved. A comparison is made between the material behaviour extracted from the tensile and the shear experiments. The material behaviour is modelled with the Johnson-Cook constitutive relation. It is found that the simultaneous use of tensile and shear tests to identify the model parameters gives a more generally applicable model. Validation of the material model and the finite element simulations is done by local strain measurements in the shear and tensile test by means of digital image correlation.     

Asymptotic behaviour of thin interphases within the large deformation case

A rigid solid material and an hyperelastic body are brought together into contact, within a large displacement situation. We suppose that a thin interphase appears between these two bodies. The limit problem and the contact law between the two bodies are derived, when the thinness of the interphase goes to 0, using appropriate test functions. The case of Saint Venant-Kirchhoff materials is first considered. Then, we consider the case of Mooney-Rivlin hyperelastic materials. Epi-convergence arguments are used in order to derive the limit problem and the limit contact law.     

Buckling and large deformation behaviour of composite domes compressed between rigid platens

This paper aims at describing the large deformation behaviour of transversely loaded composite spherical domes. Two distinctive glass fabrics, woven and knitted, embedded in the same thermoplastic matrix, polypropylene, were utilised to produce the composite domes. The doubly curved shells with different number of plies and orientations (radius to thickness ratio in the range of 22–53) were compressed between two rigid flat platens. Load–displacement responses and deformed patterns were examined using experiments and numerical simulations. Because of the large deformation involved, a non-linear stress analysis technique has been adopted to simulate the responses. The loaded domes, both woven and knitted, display rolling-plastic hinge post-buckling response after initial buckling. Some interesting results due to orthotropy have been presented along with comprehensive material characterisations. The numerical simulations have produced very promising matching results.     

Behaviour of low and medium carbon free cutting steels during deformation to large strains

Abstract

Low and medium carbon free cutting steels were deformed by cold rolling to reductions of up to 98%. The resultant microstructures were observed and characterised using optical microscopy, SEM, and TEM. Deformation of the pearlite grains and manganese sulphide inclusions was quantified in terms of their relative plasticity (compared to that of the steel). The evolution of the ferrite microstructure in the steels was seen to be dependent on the volume fraction of pearlite present. The ferrite grains in the low carbon steels underwent structural subdivision characterised by the formation of dense dislocation walls and microbands. At intermediate rolling deformations much of the strain was accommodated inhomogeneously in narrow bands of shear (S bands). Strain in the pro-eutectoid ferrite of the medium carbon steel occurred in a more homogeneous manner owing to the constraints imposed by the pearlite. The manganese sulphides and pearlite in the steels also acted as fiducial markers of the surrounding ferrite flow. Plasticity of the sulphides was generally found to be less than the overall rolling strain. However, within certain narrow strain ranges, sulphide plasticities greater than that of the steel were measured.     

Determination of fatigue fracture toughness by conventional rotary bending specimen

Determination of fatigue fracture toughness,K _fc, is made by rotary bending specimen considering partial contact of fatigue cracked surfaces in the compression side of the beam specimen. It is shown thatK _fc is a material constant independent of the nominal stress at the notch section, the specimen geometry, and the shape of the final fracture area.

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Résumé On détermine la ténacité à la rupture par fatigueK _fc à l'aide d'éprouvetes soumises à flexion rotative, en considérant le contact partiel entre les surfaces fissurées par fatigue, au cours de la phase de compression. On montre queK _fc est une constante du matériau, indépendante de la tension nominale au droit de la section entaillée, de la géométrie de l'éprouvette, et de la forme de la surface finale de rupture.

     

Modelling the large strain solid phase deformation behaviour of polymer nanoclay composites

This work concerns the solid phase deformation processing of polypropylene/nanoclay composites, for which the materials are stretched to large tensile deformations at elevated temperatures. Under these conditions the polymer matrix is nonlinearly dependent on time and strain rate. A constitutive model that is a combination of an Eyring process and physically-based molecular chain models has been shown to give a good representation of the polymer behavior, which includes strain-rate dependent yielding and stress relaxation. In order to model the nanocomposite, platelike regions that are relatively stiff are introduced into a continuum of model polymer material. This is done using a Monte Carlo approach that sequentially places non-overlapping platelets in the matrix. The process for introducing the platelets has the potential to produce platelet orientation distributions that conform with prescribed statistics, such as may be deduced from observations on real nanocomposite.     

Modelling deformation behaviour of polyelectrolyte gels under chemo‐electro‐mechanical coupling effects

In this paper, the deformation behaviour of polyelectrolyte gels induced by surrounding chemical and electrical stimulations is investigated. The polyelectrolyte gel is composed of a solid polymer‐based network attached with electric charged groups and an interstitial fluid, and it will induce various deformations such as swelling, de‐swelling as well as bending due to the variation of ambient chemical and electrical field. In order to study these mechanical behaviours of the polyelectrolyte gels, a multiphysics mathematical model is developed based on the multiphase mixture theory. In the present equilibrium study, the neglect of electro‐osmosis phenomenon is assumed and the numerical simulations are carried out for a two‐dimensional gel strip with the coupling effect of the external salt solution and electric field. The bending of the gel strip is concluded from the numerical results, which agrees with the previous experimental observation. The influence of several parameters on the deformation behaviour of the gel strip has also been discussed. Copyright © 2006 John Wiley & Sons, Ltd.     

Characterization and thermal behaviour of lanthanum tartrate crystals grown from silica gels

Results obtained on characterization of lanthanum tartrate crystals, grown by the gel method, using chemical analysis, x-ray and electron diffraction, infra-red and mass spectroscopy are reported. The thermal behaviour is studied using DTA, TGA and DTG. The decomposition pattern is reported to be typical of a hydrated metal tartrate. Kinetic parameters like order of reaction, frequency factor and activation energy are evaluated. Contracting cylinder kinetic model is found to be the best fit for the decomposition processes involved. Magnetic susceptibility measurements indicate the material to be diamagnetic.     

Influence of aggregate deformation and contact behaviour on discrete particle modelling of fracture of concrete

The discrete element method, DEM, has been used in fracture studies of non-homogeneous continuous media adopting circular or spherical particles. A 2D circular rigid DEM formulation developed with the purpose of modelling concrete is described and evaluated in uniaxial tensile and compression tests. According to this model, the aggregate can be modelled either as a rigid macro-particle or as a deformable group of particles. The inter-particle contacts can either be assumed as brittle or follow a given bilinear softening curve. It is shown that aggregate deformability, together with the consideration of pure friction contacts working under compression, increases the fracture energy in compression, leading to a better agreement with concrete tests. The softening contact model, by adding a higher capability of load redistribution, is shown to give a better agreement than the brittle model under tensile loading. The recognized crack mechanisms of the brittle model (tensile splitting, branching, bridging) are also present with softening.