Mechanical Behaviour of Materials

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Authors: Hubert Pollock
Abstract: Several of the authors of this collection of papers presented at the international meeting on the mechanical behaviour of materials have been working continuously in that field for several decades. In contrast, in this instance we have an example of an author who, having some experience in nanoindentation and surface - mechanical research, now pursues interdisciplinary studies of nanoscale properties in a different field. This paper discusses how a near-field version of infrared microspectroscopy, together with multivariate data analysis points a way towards a new method for identifying biomarkers for use in biomedical evaluation procedures. We also outline some details of a non-statistical method of classification, employing fuzzy logic.
Authors: L.M. Brown
Abstract: The flow of material out from under regions in compression must occur by the operation of many slip systems, which together produce rotational flow. Such flow requires the accumulation of geometrically necessary dislocations, and leads to the indentation size effect: smaller indents produce higher hardness, a component of the hardness being inversely proportional to the square-root of the indenter size. A pattern of flow in polycrystals which satisfies both continuity of normal stress and continuity of matter at boundaries can be achieved by rotational flow, and it leads to a grain-size effect. Under most circumstances, the flow stress has a component which is inversely proportional to the square-root of the grain size, the Hall-Petch law. The flow is accompanied by the build-up of internal stress which can be relieved by intercrystalline cracking, thereby limiting the cohesive strength of polycrystals. The relationship between these ideas and traditional views is briefly explained, and an analysis is given of recent experimental results.
Authors: Alan D. Roberts
Abstract: This review describes early work on rubber wiper blades and subsequent investigations. Observations on model wipers pressed against glass showed that the dry contact width was greater than that predicted by the classical Hertz equation, due to adhesion. That led to the establishment of the JKR equation to account for the adhesion. Although originally intended for ‘soft’ contacts, the equation now finds wide use. Surprisingly, the boundary lubrication aspects of the work revealed the action of repulsive forces that could be directly measured between rubber and glass in the presence of water. The quality of lubrication is subject to the acidity/alkalinity of the water.
Authors: Keith Fuller
Abstract: The work presented started with the aim of trying to explain why clean smooth surfaces of materials such as metals brought gently into contact show no adhesion. The observation by Johnson Kendall & Roberts of adhesion between smooth surfaces of an elastomeric hemisphere and a rigid substrate suggested a model contact system with which the effect of surface roughness could be investigated experimentally. Moreover the Johnson-Kendal-Roberts (JKR) contact theory could be used to predict the effect of roughness on adhesion. The observations and predictions obtained are compared, and the implications for the contact of two rigid materials outlined. The limitations of the validity of the predictions are mentioned. Further studies of the adhesion to rough surfaces, in particular the phenomenon of enhanced adhesion at low roughness, will be discussed. Finally, more recent theoretical work is briefly referred to.
Authors: Feodor M. Borodich
Abstract: An overview of development of indentation techniques and connections between contact mechanics and methods of extracting mechanical characteristics from indentation data is given. Observed disagreements between the experimental observations and the models of indentation are discussed. It is shown that this disagreement is often caused by violation of hypotheses that are used in the formulation of the appropriate boundary-value contact problems and strictly speaking one cannot apply directly the solutions of Hertz type contact problems to indentation tests employing the sharp indenters. It is shown that commonly used experimental test involving sharp pyramidal and conical indenters may be applied to study plastic properties of materials while this approach is not very accurate for estimations of elastic modulus of the test solid. The recently proposed by Borodich and Galanov non-direct method that employs data of elastically loading of a spherical indenter is described. It is argued that the non-direct method can be used for determination of both the work of adhesion and elastic modulus of the tested material.
Authors: Eric Le Bourhis
Abstract: Contact mechanics is widely used to explore the mechanical properties of brittle materials while it rules the mechanical performance of industrial pieces. Elastic, plastic and brittle responses can be investigated and are highlighted. The indentation plastic zone can be decomposed into a central highly strained zone and rosette arms. The evolution of the central zone extension can be predicted by Johnson’s cavity model obtained under full load while on the other hand rosette arm extension is to be modeled considering the residual stresses from the indentation. Upon a post heat treatment, further development is obtained both experimentally and theoretically. Using large loads, elimination of cracks can be achieved using higher temperature to pass the brittle-ductile transition.
Authors: Sergey Dub, Igor Zasimchuk, Leonid Matvienko
Abstract: Mechanical properties of (001) Mo and (001) Mo – 1.5 at.% Ir single crystals have been studied by nanoindentation. It has been found that the iridium addition to molybdenum leads to an increase in both hardness and elastic modulus. An abrupt elasto-plastic transition (pop-in) at a depth of about 20 - 40 nm caused by dislocation nucleation in previously dislocation-free volume has been observed in the initial portion of the loading curve. It has shown that the Ir addition essentially affects the dislocation nucleation. Mean shear stress required for the dislocation nucleation increased from 10.8 GPa (G/12) for a Mo single crystal to 18.2 GPa (G/8) for the Mo – 1.5 at% Ir solid solution. Thus, the Ir solution in a Mo single crystal affects not only the resistance to the motion of dislocations (hardness) but the nucleation of them as well. The latter is likely to occur as a result of an increase in the structure perfection of the Mo – 1.5 at% Ir solid solution as compared to the pure Mo single crystal.
Authors: M. Munawar Chaudhri
Abstract: High-speed framing photography in conjunction with circularly polarised light has been employed to monitor qualitatively the state of residual stress in Prince Rupert’s drops of soda-lime glass undergoing disintegration by a self-sustained fracture wave in the glass drops. It is revealed that the fracture wave through a Prince Rupert’s drop is driven by the residual stress in the drop, with the propagation speed of the fracture wave being (1700 ± 100) ms-1, which is close to the terminal speed of individual cracks in the soda-lime glass, but is much smaller than the longitudinal wave speed of 5300 ms-1 in the glass. These observations support our recently reported observations and also give support to our conclusions that the fracture wave speed of a self-sustained fracture wave is equal to the terminal speed of individual cracks in the glass. Some preliminary observations from fracture waves in Prince Rupert’s drops of a lead oxide glass are also described, which show that in Prince Rupert’s drops of the lead oxide glass the fracture wave is also self-sustained and it travels through the drop at a steady and stable speed of (1300 ± 100) ms-1, which is also considerably smaller than the longitudinal wave speed of 4800 ms-1 in the lead glass. A brief comment is also made on the fracture waves observed by other workers in brittle oxide glasses and solids generated by plate impacts and shock waves.

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