Modelling cell wall growth using a fibre-reinforced hyperelastic–viscoplastic constitutive law

AbstractA fibre-reinforced hyperelastic–viscoplastic model using a finite strain Finite Element (FE) analysis is presented to study the expansive growth of cell walls. Based on the connections between biological concepts and plasticity theory, e.g. wall-loosening and plastic yield, wall-stiffening and plastic hardening, the modelling of cell wall growth is established within a framework of anisotropic viscoplasticity aiming to represent the corresponding biology-controlled behaviour of a cell wall. In order to model in vivo growth, special attention is paid to the differences between a living cell and an isolated wall. The proposed hyperelastic–viscoplastic theory provides a unique framework to clarify the interplay between cellulose microfibrils and cell wall matrix and how this interplay regulates sustainable growth in a particular direction while maintaining the mechanical strength of the cell walls by new material deposition. Moreover, the effect of temperature is taken into account. A numerical scheme is suggested and FE case studies are presented and compared with experimental data

Elastic–plastic analysis of offset indentations on unpressurised pipes

AbstractThe results of investigations to determine the elastic–plastic behaviour of unpressurised pipes with long offset indentations and unsymmetric support conditions are presented in this paper. They include the results of experimental tests, FE analyses and analytical methods. Three different materials and five different geometries are used to investigate their effects on the behaviour. A comparison of the experimental results, FE and analytical solutions indicates that the general analytical formulation developed in this paper for predicting the peak indenter loads in offset indented pipes, is reasonably accurate. Also, the analyses presented in this paper indicate that using a representative nominal flow stress, which is the average of yield and ultimate tensile stresses, in the analytical method, is appropriate for predicting the peak indenter loads

A two-material miniature specimen test method and the sssociated inverse approach for high temperature applications

A miniature two-material test specimen has been developed for material property characterization. Elastic-plastic and creep damage models have been applied for a semi-analytical model and a finite element (FE) model to simulate the miniature specimen tensile and creep tests on a two-material system. An inverse optimization algorithm has been developed, which can be used to extract the creep properties of the unknown material in the two-material system. Single material miniature specimen tensile tests have been performed for an aluminum alloy at room temperature and 400°C and full stage creep tests have been performed for a P91 steel at 650°C. All the tensile and creep test specimens are brought to fracture failure. The experimental results have been compared to the results from the corresponding results from conventional uniaxial tensile and creep tests on the same materials and under the same test conditions. This miniature specimen testing technique and the developed inverse method have the potential to become a new approach for determining the elastic-plastic and the full-stage creep properties until rupture for a two-material system at high temperature, e.g. a coating-substrate system

A two-material miniature specimen test method and the sssociated inverse approach for high temperature applications

A miniature two-material test specimen has been developed for material property characterization. Elastic-plastic and creep damage models have been applied for a semi-analytical model and a finite element (FE) model to simulate the miniature specimen tensile and creep tests on a two-material system. An inverse optimization algorithm has been developed, which can be used to extract the creep properties of the unknown material in the two-material system. Single material miniature specimen tensile tests have been performed for an aluminum alloy at room temperature and 400°C and full stage creep tests have been performed for a P91 steel at 650°C. All the tensile and creep test specimens are brought to fracture failure. The experimental results have been compared to the results from the corresponding results from conventional uniaxial tensile and creep tests on the same materials and under the same test conditions. This miniature specimen testing technique and the developed inverse method have the potential to become a new approach for determining the elastic-plastic and the full-stage creep properties until rupture for a two-material system at high temperature, e.g. a coating-substrate system

A New Model for Void Coalescence by Internal Necking

A micromechanical model for predicting the strain increment required to bring a damaged material element from the onset of void coalescence up to final fracture is developed based on simple kinematics arguments. This strain increment controls the unloading slope and the energy dissipated during the final step of material failure. Proper prediction of the final drop of the load carrying capacity is an important ingredient of any ductile fracture model, especially at high stress triaxiality. The model has been motivated and verified by comparison to a large set of finite element void cell calculations.

On membrane interaction in matrix theory

We compute the interaction potential between two parallel transversely boosted wrapped membranes (with fixed momentum $p_-$) in D=11 supergravity with compact light-like direction. We show that the supergravity result is in exact agreement with the potential following from the all-order Born-Infeld-type action conjectured to be the leading planar infra-red part of the quantum super Yang-Mills effective action. This provides a non-trivial test of consistency of the arguments relating Matrix theory to a special limit of type II string theory. We also find the potential between two (2+0) D-brane bound states in D=10 supergravity (corresponding to the case of boosted membrane configuration in 11-dimensional theory compactified on a space-like direction). We demonstrate that the result reduces to the SYM expression for the potential in the special low-energy (\a'\to 0) limit, in agreement with previous suggestions. In appendix we derive the action obtained from the D=11 membrane action by the world-volume duality transformation of the light-like coordinate $x^-$ into a 3-vector.Comment: 18 pages, latex. v2: Some clarifying remarks and references added. v3: Further minor corrections and reference

Noise-Driven Mechanism for Pattern Formation

We extend the mechanism for noise-induced phase transitions proposed by Ibanes et al. [Phys. Rev. Lett. 87, 020601-1 (2001)] to pattern formation phenomena. In contrast with known mechanisms for pure noise-induced pattern formation, this mechanism is not driven by a short-time instability amplified by collective effects. The phenomenon is analyzed by means of a modulated mean field approximation and numerical simulations

The prevalence of diabetes mellitus and impaired glucose tolerance in a group of urban South African blacks

The prevalence of diabetes mellitus and impaired glucose tolerance (IGT) was determined in 479 urbanised South African blacks (141 men and 338 women) of Zulu descent selected by cluster sampling in a suburb of Durban.All subjects underwent a modified glucose tolerance test whereby fasting and 2-hour postglucose (75 g) plasma glucose levels were measured. On the basis of the revised World Health Organisation criteria, the overall prevalence of diabetes was 4,2% and of IGT 6,9%; the age- and sex-adjusted prevalences were 5,3% and 7,7% respectively.Diabetes mellitus was more common in women (5,2% v. 2,3%), while the reverse was true of IGT (5,5% v. 11,5%). The mean age-adjusted body mass indices (BMIs) of diabetic (31,3 ± 1,9) and IGT (29,7 ± 1,9) subjects were significantly higher than those of the group with normal glucose tolerance (28 ± 0,5). Female subjects with all types of glucose tolerance had significantly higher mean BMIs than men.There was a significant correlation between BMI and both fasting glucose (r = 0,16; P = 0,0039) and 2-hour plasma levels (r =0,15; P =0,0065) in the women, while in men only the fasting levels showed such a relationship (r =0,21; P = 0,01719)

Kahler moduli double inflation

We show that double inflation is naturally realized in K\"ahler moduli inflation, which is caused by moduli associated with string compactification. We find that there is a small coupling between the two inflatons which leads to amplification of perturbations through parametric resonance in the intermediate stage of double inflation. This results in the appearance of a peak in the power spectrum of the primordial curvature perturbation. We numerically calculate the power spectrum and show that the power spectrum can have a peak on observationally interesing scales. We also compute the TT-spectrum of CMB based on the power spectrum with a peak and see that it better fits WMAP 7-years data.Comment: 21 pages, 8 figure

NMR Detection of Temperature-Dependent Magnetic Inhomogeneities in URu2Si2

We present 29Si-NMR relaxation and spectral data in URu2Si2. Our echo-decay experiments detect slowly fluctuating magnetic field gradients. In addition, we find that the echo-decay shape (time dependence) varies with temperature T and its rate behaves critically near the Neel temperature TN, indicating a correlation between the gradient fluctuations and the transition to small-moment order. T-dependent broadening contributions become visible below 100 Kelvin and saturate somewhat above TN, remaining saturated at lower temperatures. Together, the line width and shift suggest partial lattice distortions below TN. We propose an intrinsic minority phase below $T_{\rm N}$ and compare our results with one of the current theoretical models.Comment: 2 pages RevTeX, 1 figure, SCES 99-Japan, to appear in Physica