Multi-level analysis of localisation problems

Localisation processes, such as shear banding and necking, have been investigated following a macroscopic and a microscopic approach. Both approaches have been formulated within a finite deformation plasticity framework. Additional terms have been used to regularise the problem and solve mesh dependency. In the macroscopic model viscosity is introduced as a means to control the thickness of the shear band, while in the microscopic model the nonlocal interaction of dislocations acts as a stabiliser. The micro-mechanical model is formulated in a crystal plasticity framework. A diffusion term that represents cross-slip of dislocations is included in the evolution equations for dislocation densities. The effect of the viscous term (macro-model) and the diffusion-like term (micro-model) in the constitutive relation on the resulting formation of localised shear modes is studied. An analysis of a strip in tension oriented for multiple slip is presented for both models

Thermal stability of the microstructure of severely deformed copper

Copper specimens were deformed by equal channel angular pressing (ECAP) up to 8 passes. The microstructure was studied by X-ray line profile analysis. The crystallite size is reduced to a few tens of nanometers even after the first ECAP pass and it does not change significantly during further deformation. At the same time, the dislocation density increases gradually up to 4 ECAP passes. The thermal stability of the microstructure is examined by differential scanning calorimetry (DSC). The temperature of the DSC peak decreases whereas the stored energy increases with increasing strain. At the beginning of the heat release a bimodal grain structure develops indicated by a special double-peak shape of the diffraction line profiles

The effect of pre-processing and grain structure on the bio-corrosion and fatigue resistance of magnesium alloy AZ31

Magnesium alloys are broadly used for structural applications in the aerospace and automotive industries as well as in consumer electronics. While a high specific strength is the forte of magnesium alloys, one serious limitation for Mg alloys is their corrosion performance. Unlike aluminium, it does not form a stable passive film to provide long-term protection from further corrosion. The poor corrosion resistance of magnesium and magnesium alloys is regarded as a major drawback, and significant effort has been focused on improving this.[1-3] However, the high reactivity of magnesium alloys in corrosive media can be used to advantage in biomedical applications, particularly in temporary implants where the capacity of a material for bio-degradation is one of the most sought after properties. Indeed, permanent implant materials, such as stainless steel, titanium alloys or Nitinol (55Ni-45Ti), are the only choices currently available for hard tissue implantation. They can cause permanent physical irritation, long-term endothelial dysfunction and chronic inflammatory local reaction. Sometimes a second operation is needed for the implant to be removed. Given the ability of the human body to gradually recover and regenerate damaged tissue, the ideal solution would thus be a degradable implant, which would offer a physiologically less invasive repair and temporary support during tissue recovery. After fulfilling its function, this implant would be obliterated, being absorbed by the body. This philosophy of implant surgery would also be of particular interest for endovascular stent

Mechanisms Of Fracturing In Structures Built From Topologically Interlocked Blocks

Failure of materials is in many cases associated with initiation and subsequent propagation of macroscopic fractures. Consequently, in order to increase the strength, one needs to inhibit either crack initiation or propagation. The principle of topological interlocking provides a unique opportunity to construct materials and structures in which both routes of the strength increase can be realised. Materials and structures built on the basis of this principle consist of many elements which are hold together by the special geometry of their shape, together with an external constrain. The absence of the binder phase between the elements allows the interfaces to arrest macroscopic crack propagation. In addition, with sufficiently small size of the elements an increase in local strength and, possibly, in the stress for crack initiation can be achieved by capitalising on the size effect. Furthermore, the ability of some interlocking structures to tolerate missing elements can serve to prevent the avalanche-type failure initiated by failure of one of the elements. In this paper, experimental results and a theoretical analysis with regard to this possibility are presented

Irreversible thermodynamics approach to plasticity: Dislocation density based constitutive modelling

© 2015 Institute of Materials, Minerals and Mining. In this paper, we describe a simple methodology that offers a familiar constitutive description of plasticity in terms of the dislocation density evolution as an outcome of the approach based on the thermodynamics of irreversible processes. We further demonstrate that the dislocation density evolution approach can organically predict the critical strain corresponding to the Considère instability point. Finally, we show that the fractal dimension (FD) of the dislocation population of a deforming material can be integrated in the proposed modelling framework and, consequently, the FD behaviour can be traced, providing insights in the evolution of the dislocation structure in the course of deformation

Yield strength of a material pre-processed by simple shear

Modern techniques of severe plastic deformation used as a means for grain refinement in metallic materials rely on simple shear as the main deformation mode. Prediction of the mechanical properties of the processed materials under tensile loading is a formidable task as commonly no universal, strain path independent constitutive laws hold. In this paper we derive an analytical relation that makes it possible to predict the mechanical response to uniaxial tensile loading for a material that has been pre-processed by simple shear and presents a linear strain gradient in it. A facile recipe for mechanical tests on solid bars required for this prediction to be made is proposed. As a trial, it has been exercised for the case of commercial purity copper rods. The results of the derivation of the true stress-strain curve for large tensile deformation of copper are presented. The method proposed is recommended for design with metallic materials that underwent preprocessing by simple shear.Для прогнозу властивостей субмікрокристалічних металів, отриманих методами інтенсивних пластичних деформацій, необхідно знати напруження при одновісному пластичному розтягуванні після обробки великим простим зсувом (деформація зсуву більше 10). Механічні властивості матеріалів при таких непропорційних шляхах деформування вивчають в основному за допомогою трубчастих зразків. Через втрату стійкості трубок при крученні такі експерименти можливі лише для малих пружньопластичних деформацій, що не перевищують кількох відсотків. У статті запропоновано й обґрунтовано метод визначення напруги течії матеріалу, попередньо обробленого великим простим зсувом. Метод заснований на двох стандартних випробуваннях: крученні з вільними торцями й одновісному розтягуванні. Отримано співвідношення, що дозволяє по напрузі течії неоднорідного зразка, попередньо підданого крученню, знайти напругу пластичного розтягування його поверхневого шару з певною деформацією простого зсуву. Шляхом простих аналітичних оцінок показано, що пружні залишкові напруги першого роду, які виникають після розвантаження зразка, попередньо підданого крученню, практично не впливають на межу тікучості при розтягуванні.Для прогноза свойств субмикрокристаллических металлов, полученных методами интенсивных пластических деформаций, необходимо знать напряжение их течения при одноосном растяжении после обработки простым сдвигом большой величины (деформация сдвига более 10). Механические свойства материалов при таких непропорциональных путях деформирования изучают в основном с помощью трубчатых образцов. Из-за потери устойчивости трубок при кручении такие эксперименты возможны лишь для малых упругопластических деформаций, не превышающих нескольких процентов. В статье предложен и обоснован метод определения напряжения течения материала, предварительно обработанного большим простым сдвигом. Метод основан на двух стандартных испытаниях: кручении со свободными торцами и одноосном растяжении. Получено соотношение, позволяющее по напряжению течения неоднородного образца, предварительно подвергнутого кручению, найти напряжение пластического растяжения его поверхностного слоя с определенной деформацией простого сдвига. Путем простых аналитических оценок показано, что упругие остаточные напряжения первого рода, возникающие после разгрузки образца, предварительно подвергнутого кручению, практически не влияют на предел текучести при растяжении

Geometrically non-linear modeling of the Portevin-Le Chatelier effect

In this work we investigate the plastic instabilities associated with the Portevin-Le Chatelier (PLC) effect in Al alloy 2024. A semiphenomenological approach is taken. A simple geometrically non-linear elastic-viscoplastic constitutive model is proposed for simulation of material response under various applied strain rates. Using the model we determine numerically the relation between the critical strain for the onset of discontinuous yielding and the applied strain rate. The results obtained are in very good quantitative agreement with the available experimental data and cover both the normal and the inverse behavior of the critical strain. The simulations are performed using non-linear finite element method. Additional verification of the proposed constitutive framework was carried out using statistical analysis of the simulated stress-time series. A transition from a non-linear chaotic regime to self-organized critical behaviour of the localized strain bands were predicted in terms of the temporal two-point correlation function of the stress-time series. Finally we investigated the influence of different factors, such as the geometry of the specimen, its orientation with respect to the rolling direction and loading conditions (strain rate), on the type of PLC instabilities and the critical conditions for their onset