Strain Rate Jumps Research Articles

Microplasticity of biomorphic SiC/Al composite under uniaxial compression

Inhomogeneity of the microplastic strain rate (deformation jumps) of a biomorphic SiC/Al composite under uniaxial compression has been studied by laser interferometry on the nanometer level. The value of strain rate jumps has been calculated from the deviation of the form of separate beats in the interferogram of a deformation from the standard form corresponding to a constant strain rate within one beat. In addition to strain rate oscillations extended by 100–180 nm along the displacement (the variation in the length of the specimen), peaks of small width and amplitude with a distance of 10–20 nm between them are observed, as well as peaks with a width of ∼ 50 nm. These peaks may be associated with the sizes of structural formations of an aluminum alloy (grains, subgrains, precipitates, etc.) or with the sizes of SiC nano- and microcrystals situated separately from large-grain crystals and surrounded by residual carbon. The results of this work offer hope to the possibility of enhancing plasticity and strength of biomorphic composites by increasing the fraction of fine-grain elements (< 1.5 µm) in their structure.

High-temperature mechanical behavior of semi-crystalline polymers and relationship to a rubber-like “relaxed” state

This work addresses the general issue of the mechanical behavior of the confined amorphous phase in rubbery semi-crystalline polymers. The tensile behavior of four semi-crystalline polymers (with various degrees of crystallinity and transition temperatures) was characterized and analyzed at high-temperature (i.e. a few tens degrees below the melting point) and at low strain-rates to try to approach a “relaxed” mechanical behavior. Monotonic loadings followed by different unloading/relaxation/creep stages, and strain-rate jumps experiments were performed on two testing machines equipped with different extensometers. Despite significant differences of micro-structure between the tested materials, a general phenomenon was observed: a non strain-rate sensitive regime was reached both at loading and unloading. However, a “relaxed” equilibrium response cannot account for this regime because of concomitant contradictory features among which (i) the creep and relaxation kinetics which involve the same strain-rates as observed in the non strain-rate sensitive loading regime or (ii) the unusual strain-rate sensitivity observed in strain-rate jump tests. The whole set of experiments has consequences for mechanical modeling. They are discussed as a conclusion of this experimental mechanics work.

Homogeneous deformation of bulk metallic glasses in the super-cooled liquid state

Abstract Different Zr–Ti–Al–Ni–Cu bulk metallic glasses have been processed. They have been characterized by neutron and X-ray diffraction in the glassy state and after DSC heating up to crystallization. The homogeneous visco-plasticity of the glasses has been studied in the under-cooled liquid state by compression tests at constant strain-rate and by differential mechanical tests with strain-rate jumps at different temperatures. The flow stress appears to be strongly thermally activated. Another characteristic feature of the deformation of the fully amorphous state is that the flow stress is independent of the strain-rate path followed to reach a given level of strain. The stress–strain curves and their dependence on temperature and strain-rate are then discussed in terms of Newtonian and non-Newtonian viscosity. It is shown that a reduced viscosity can be scaled on a master curve, independently of the strain-rate and temperature. A second analysis of the results is made, in terms of free volume. The temperature and stress variation of the plastic flow are compared with Spaepen model. A constitutive equation of the deformation is proposed. Finally, the effect of crystallization on mechanical behaviour of metallic glasses is treated trough a model involving backstress due to a composite behaviour.

A Visco-Hypoplastic Constitutive Relation for Soft Soils

ABSTRACT Soils with soft angular particles are markedly viscoplastic. For constitutive modelling with hypoplasticity, the granulate hardness is replaced by a rate-dependent (argotropic) solid hardness hs. Use is made of the viscosity index IV. The state limits of isotropic compression, stationary shearing (critical state) and cyclic shearing are thus argotropic. They are asymptotic solutions of the proposed visco-hypoplastic relation, the core of which is a viscosity factor fv depending on an overconsolidation ratio pe/p’. It resembles a theory by Niemunis (1996), but has a wider variety of asymptotic solutions (attractors). For constant effective stress, the familiar volumetric creep is obtained, and a dependency of strain rate ratio on stress ratio as observed. The response to strain rate jumps is obtained realistically, and also the relaxation. The material parameters can be easily determined from standard tests and estimated from index tests. Further explanation is given with a back analysis of Henkel’s (1956) triaxial test results. Deviations are due to non-uniform sample deformations. Peak stress conditions are derived from instantaneous state values including strain rate. Extensions are briefly outlined: tensorial formulation, intergranular strain and polar terms. The microphysical background is explained with Persson’s (2000) theory of thermally activated shear melting. Realistic dislocation energies are thus obtained from observed Iv values.

Modeling of AlMg Sheet Forming at Elevated Temperatures

The process limits of aluminum sheet forming processes can be improved by control-ling local flow behavior by means of elevated temperatures and temperature gradients. In order to accurately model the deep drawing or stretching of aluminum sheet at elevated temperatures, a model is required that incorporates the temperature and strain-rate dependency of the ma-terial. In this paper two models are compared: a phenomenological material model in which the parameters of a Ludwik–Nadai hardening curve and a power law strain-rate influence are made temperature dependent and a physically-based model according to Bergstra om. The model incorporates the influence of the temperature on the flow stress and on the hardening rate and includes dynamic recovery aspects. Although both models can be fitted quite well to monotonic tensile tests of an AA 5754-O alloy, large differences appear if strain rate jumps are applied. Subsequent simulation of cylindrical cup deep drawing shows a large influence of friction and the shape of the yield surface.

Measurement and modeling of back stress at intermediate to high homologous temperatures

Unified creep-plasticity models often require a number of internal state variables to accurately capture the path dependence of rate- and temperature-dependent alloys especially at intermediate to high homologous temperatures. However, it is impossible to fully measure the internal state variables directly. Consequently, assumptions on the relative importance of the various state variables must be made when determining the material parameters and their evolution must be inferred. Any enhancements designed to better capture the evolution of the state variables are dependent on these assumptions and are generally limited. The strain transient dip test and the rapid load/unload test are two indirect experimental methods used to obtain an approximate measure of the evolutionary nature of the back stress in 60Sn–40Pb, a common rate- and temperature-dependent solder alloy. The rapid load/unload test gives a better measure of back stress because there is less time for the internal state to evolve during data acquisition. Modifications to the back stress evolution equations in the McDowell unified creep-plasticity model are proposed using these new measurements as guidance. The modified model can better capture the transients under cyclic loading and following strain-rate jumps.

An Assessment of the Method Used to Determine Activation Parameters in L12 Alloys

ABSTRACTThe characterisation of dislocation mechanisms in connection with macroscopic mechanical properties are usually performed through transient tests, such as strain-rate jumps, load relaxations or creep experiments. The present paper includes a careful and complete theoretical analysis of the relaxation and the creep kinetics. We experimentally show that the plastic strain-rate is continuous at the transition between constant strain-rate conditions and both load relaxation and creep test. The product of the plastic strain-rate at the onset of the transient test () with the characteristic time (tk) of the transient is found to be independent of , as theoretically expected. This is a clear indication that the assumptions underlying the theoretical analysis are relevant.

Strengthening at intermediate temperatures in iron aluminides

Abstract The strength peak found at intermediate temperatures for Fe3Al and FeAl alloys is of interest both from the fundamental point of view and also because such strengthening increases the temperature range where good strength can be obtained. Several theories have been proposed to explain the strengthening involved, but no clear agreement has been reached as to the true mechanisms involved. After reviewing likely strengthening mechanisms, some recent results on mechanical behaviour and dislocation microstructures will be presented. In particular the temperature and strain-rate dependencies of mechanical behaviour in a Fe3Al based alloy will be considered, as well as the influence of strain-rate jumps. Dislocations found in deformed samples, both in the as-deformed condition as well as after subsequent annealing treatments, will be discussed. Observations on deformed FeAl samples are shown to suggest somewhat different conclusions than for Fe3Al as to the important strengthening mechanisms. Processes such as APB relaxation, local climb, and superdislocation reactions and decompositions may all play roles in determining strength.

The mechanical properties of single phase γ Ti 47Al 51Mn 2 polycrystals

The mechanical properties of polycrystalline samples of the single phase γ-Ti 47Al 51Mn 2 alloy have been studied during compression tests in a wide range of temperatures (120–1270 K). The flow stress and the work hardening rate are measured during imposed strain rate tests, while the strain rate sensitivity of the stress is examined using both strain rate jumps and stress relaxation experiments. From the temperature, strain and stress dependence of these parameters, it is shown that the investigated temperature domain can be divided into three régimes corresponding to different deformation mechanisms. The results are compared to the data available in the literature and are found to be in good agreement with the dislocation structures and dislocation motion mechanisms that we have previously reported.

A model of creep at intermediate temperatures in aluminium

Abstract A new model of creep at intermediate temperatures in Al is presented, based on a set of previous results of in situ experiments. This model explains the origin of the high forward stresses necessary to extract dislocations from sub-boundaries, and the strong correlation between the speed of sub-boundary migration and the creep rate, asevidenced at different temperatures and by means of in situ strain-rate jumps. High stresses originate from incompatibilities of glide which occur during migration. Under such conditions, it is shown that the creep rate is controlled by a geometrically necessary glide of screw dislocation segments in {100} planes, i.e. by nucleation and propagation of kinks in these non-compact. As the kink energy is estimated from an experimental determination of the ‘dissociation’ energy in Al, all the parameters of the creep law are in good agreement with experimental values.