Understanding Of Micromechanics Research Articles

Transformation-induced plasticity in an ultrafine-grained steel: An in situ neutron diffraction study

An ultrafine-grained steel with an average grain size of about 350nm was developed. The tensile testing at ambient temperature shows a threefold increase in the yield strength compared to its coarse-grained counterpart. Moreover, the increase in the strength was achieved without the sacrifice of the ductility due to strain-induced martensitic transformation. The evolution of lattice strains and phase fractions of the austenite and martensite phases during the deformation was investigated using in situ neutron diffraction to provide a micromechanical understanding of the transformation-induced plasticity responsible for the combination of high strength and ductility.

Molecular Changes during Tensile Deformation of Single Wood Fibers Followed by Raman Microscopy

Raman spectra were acquired in situ during tensile straining of mechanically isolated fibers of spruce latewood. Stress-strain curves were evaluated along with band positions and intensities to monitor molecular changes due to deformation. Strong correlations (r = 0.99) were found between the shift of the band at 1097 cm(-1) corresponding to the stretching of the cellulose ring structure and the applied stress and strain. High overall shifts (-6.5 cm(-1)) and shift rates (-6.1 cm(-1)/GPa) were observed. After the fiber failed, the band was found on its original position again, proving the elastic nature of the deformation. Additionally, a decrease in the band height ratio of the 1127 and 1097 cm(-1) bands was observed to go hand in hand with the straining of the fiber. This is assumed to reflect a widening of the torsion angle of the glycosidic C-O-C bonding. Thus, the 1097 cm(-1) band shift and the band height ratio enable one to follow the stretching of the cellulose at a molecular level, while the lignin bands are shown to be unaffected. Observed changes in the OH region are shown and interpreted as a weakening of the hydrogen-bonding network during straining. Future experiments on different native wood fibers with variable chemical composition and cellulose orientation and on chemically and enzymatically modified fibers will help to deepen the micromechanical understanding of plant cell walls and the associated macromolecules.

Micromechanical understanding of the cell-wall structure

For improving properties of pulp fibres, a better understanding of the relationships between its macroscopic mechanical properties, fibre ultrastructure, and properties of the wood polymers is important. This paper discusses such relations between elastic properties of fibres, their matrix structure and the wood polymer elastic constants. It is argued that an orientation of all of the wood polymers in the direction of the cellulose microfibrils is most likely. The elastic longitudinal modulus of cellulose is well described by the value of 134 GPa dominating the longitudinal fibre properties. In the transverse direction the amorphous polymers play a more important role. To cite this article: L. Salmén, C. R. Biologies 327 (2004).

Shear band evolution and accumulated microstructural development in Cosserat media

AbstractThis paper prepares the ground for the continuum analysis of shear band evolution using a Cosserat/micropolar constitutive equation derived from micromechanical considerations. The nature of the constitutive response offers two key advantages over other existing models. Firstly, its non‐local character obviates the mathematical difficulties of traditional analyses, and facilitates an investigation of the shear band evolution (i.e. the regime beyond the onset of localization). Secondly, the constitutive model parameters are physical properties of particles and their interactions (e.g. particle stiffness coefficients, coefficients of inter‐particle rolling friction and sliding friction), as opposed to poorly understood fitting parameters. In this regard, the model is based on the same material properties used as model inputs to a discrete element (DEM) analysis, therefore, the micromechanics approach provides the vehicle for incorporating results not only from physical experiments but also from DEM simulations. Although the capabilities of such constitutive models are still limited, much can be discerned from their general rate form. In this paper, an attempt is made to distinguish between those aspects of the continuum theory of localization that are independent of the constitutive model, and those that require significant advances in the understanding of micromechanics. Copyright © 2004 John Wiley & Sons, Ltd.

Towards a micromechanical understanding of biological surface devices

Abstract Interesting analogies exist between the science of materials in small dimensions and the mechanics of biological systems. In both instances, mechanical forces and displacements are measured and the results are interpreted in the light of theoretical expectations. The approaches are, however, fundamentally different: consideration of structural diversity and global performance in biology contrasts with more quantitative evaluation of local properties and emphasis on micromechanical modeling in materials science. We report on two examples of our first efforts to combine these approaches in the investigation of biological attachment systems in insects. Nanoindentation studies were performed on the head–neck joint and the wing-locking mechanism in a beetle, and the foot–substrate adhesion in flies and geckos has been examined from the perspective of theoretical contact mechanics. By aiming at a quantitative understanding of the interrelationship between local materaal properties and complex structure...

Numerical modelling of particle distribution effects on fatigue in Al–SiC p composites

Various reports in the literature have highlighted the effects of particle distribution on the fatigue behaviour of particulate reinforced metal matrix composites (PMMCs), although few attempts have been made at modelling such effects. A micromechanical understanding of the effects of clustering on short crack growth behaviour in Al–SiC p composites has been achieved via finite element modelling. Comparison of preliminary models with the literature has shown that shielding/anti-shielding effects were significantly affected by the relative sizes of the particle and the overall model such that, when edge effects were removed, a crack was predicted to be accelerated rather than decelerated as it propagated through closely spaced pairs of particles. Consistent differences were identified between models with homogeneous versus clustered particle arrangements in terms of crack path morphologies and local crack–tip stress intensity fluctuations. Furthermore, predicted influences of clustering on growth rates in the numerical models were found to be consistent with previous experimental results (i.e. growth rates rose with increased clustering), demonstrating that load transfer effects associated with changes in particle distribution may play a direct role in controlling the growth of short cracks in these materials.

Numerical Modelling of Particle Distribution Effects on Fatigue in Al-SiC<sub>p</sub> Composites

Various reports in the literature have highlighted the effects of particle distribution on the fatigue behaviour of particulate reinforced metal matrix composites (PMMCs), although few attempts have been made at modelling such effects. A micromechanical understanding of the effects of clustering on short crack growth behaviour in Al–SiCp composites has been achieved via finite element modelling. Comparison of preliminary models with the literature has shown that shielding/anti-shielding effects were significantly affected by the relative sizes of the particle and the overall model such that, when edge effects were removed, a crack was predicted to be accelerated rather than decelerated as it propagated through closely spaced pairs of particles. Consistent differences were identified between models with homogeneous versus clustered particle arrangements in terms of crack path morphologies and local crack–tip stress intensity fluctuations. Furthermore, predicted influences of clustering on growth rates in the numerical models were found to be consistent with previous experimental results (i.e. growth rates rose with increased clustering), demonstrating that load transfer effects associated with changes in particle distribution may play a direct role in controlling the growth of short cracks in these materials.

Experimental stress/strain behavior of SiC-matrix composites reinforced with Si–Ti–C–O fibers and estimation of matrix elastic modulus

NAL, Ube Industries Ltd and Shikibo Ltd have conducted a joint programme to develop and evaluate ceramic-matrix composites (CMCs) reinforced with Si–Ti–C-O (Tyranno®) fiber from Ube Ltd in an orthogonal 3-D fabric reinforcement configuration woven by Shikibo Ltd. For most of the composites, the polymer conversion (PC) technique was used for matrix consolidation, although chemical vapor infiltration (CVI) was also used for some composites. The polymer precursor processing route is considered to be more cost effective as compared to the conventional CVI route. These materials were characterized by excellent oxidation resistance compared to carbon/carbon composites. The room-temperature monotonic tensile strength exceeded 450MPa, and this was retained at 1200°C in vacuum. However, a micro-mechanical understanding of these composites has not been completed because the matrix properties were unknown. An attempt has been made to estimate the elastic modulus of these PC-based matrices in the initial purely elastic region and an estimated value range obtained. The model is based on a simple linkage of parallel and serial arrangements of elastic blocks.