Tension perpendicular to grain strength of wood, Laminated Veneer Lumber (LVL), and Cross-Banded LVL (LVL-C)

Recent experimental tests carried out on structural timber members have highlighted the importance of tension perpendicular to grain strength, particularly in beams with holes and notches, in connection regions, in curved beams, and in post-tensioned timber frames. Innovative engineered wood products such as Cross Banded Laminated Veneer Lumber (LVL-C) have been introduced into the market specifically to improve the perpendicular to grain properties of normal Laminated Veneer Lumber (LVL). This paper reports on tests that were performed at the University of Canterbury using specimens of sawn timber Radiata Pine, LVL and LVL-C. The perpendicular to grain tension strengths of LVL was generally lower than those for sawn timber, but the LVL-C showed a significantly improved strength. The paper also compares the experimental results with strengths predicted using both coupled elastic Finite Element Method (FEM) and Linear Elastic Fracture Mechanics (LEFM) models. These models were found to predict the average strength with reasonable accuracy. © 2011 Taylor & Francis Group, London

Statistical mechanics of permanent random atomic and molecular networks: Structure and heterogeneity of the amorphous solid state

Under sufficient permanent random covalent bonding, a fluid of atoms or small molecules is transformed into an amorphous solid network. Being amorphous, local structural properties in such networks vary across the sample. A natural order parameter, resulting from a statistical-mechanical approach, captures information concerning this heterogeneity via a certain joint probability distribution. This joint probability distribution describes the variations in the positional and orientational localization of the particles, reflecting the random environments experienced by them, as well as further information characterizing the thermal motion of particles. A complete solution, valid in the vicinity of the amorphous solidification transition, is constructed essentially analytically for the amorphous solid order parameter, in the context of the random network model and approach introduced by Goldbart and Zippelius [Europhys. Lett. 27, 599 (1994)]. Knowledge of this order parameter allows us to draw certain conclusions about the stucture and heterogeneity of randomly covalently bonded atomic or molecular network solids in the vicinity of the amorphous solidification transition. Inter alia, the positional aspects of particle localization are established to have precisely the structure obtained perviously in the context of vulcanized media, and results are found for the analogue of the spin glass order parameter describing the orientational freezing of the bonds between particles.Comment: 31 pages, 5 figure

Solution of the local field equations for self-generated glasses

We present a self-consistent local approach to self generated glassiness which is based on the concept of the dynamical mean field theory to many body systems. Using a replica approach to self generated glassiness, we map the problem onto an effective local problem which can be solved exactly. Applying the approach to the Brazovskii-model, relevant to a large class of systems with frustrated micro-phase separation, we are able to solve the self-consistent local theory without using additional approximations. We demonstrate that a glassy state found earlier in this model is generic and does not arise from the use of perturbative approximations. In addition we demonstrate that the glassy state depends strongly on the strength of the frustrated phase separation in that model.Comment: 11 pages, 3 figure

‘‘Lozenge’’ contour plots in scattering from polymer networks

We present a consistent explanation for the appearance of “lozenge” shapes in contour plots of the two dimensional scattering intensity from stretched polymer networks. By explicitly averaging over quenched variables in a tube model, we show that lozenge patterns arise as a result of chain material that is not directly deformed by the stretch. We obtain excellent agreement with experimental data

Random solids and random solidification: What can be learned by exploring systems obeying permanent random constraints?

In many interesting physical settings, such as the vulcanization of rubber, the introduction of permanent random constraints between the constituents of a homogeneous fluid can cause a phase transition to a random solid state. In this random solid state, particles are permanently but randomly localized in space, and a rigidity to shear deformations emerges. Owing to the permanence of the random constraints, this phase transition is an equilibrium transition, which confers on it a simplicity (at least relative to the conventional glass transition) in the sense that it is amenable to established techniques of equilibrium statistical mechanics. In this Paper I shall review recent developments in the theory of random solidification for systems obeying permanent random constraints, with the aim of bringing to the fore the similarities and differences between such systems and those exhibiting the conventional glass transition. I shall also report new results, obtained in collaboration with Weiqun Peng, on equilibrium correlations and susceptibilities that signal the approach of the random solidification transition, discussing the physical interpretation and values of these quantities both at the Gaussian level of approximation and, via a renormalization-group approach, beyond.Comment: Paper presented at the "Unifying Concepts in Glass Physics" workshop, International Centre for Theoretical Physics, Trieste, Italy (September 15-18, 1999

Elasticity of Gaussian and nearly-Gaussian phantom networks

We study the elastic properties of phantom networks of Gaussian and nearly-Gaussian springs. We show that the stress tensor of a Gaussian network coincides with the conductivity tensor of an equivalent resistor network, while its elastic constants vanish. We use a perturbation theory to analyze the elastic behavior of networks of slightly non-Gaussian springs. We show that the elastic constants of phantom percolation networks of nearly-Gaussian springs have a power low dependence on the distance of the system from the percolation threshold, and derive bounds on the exponents.Comment: submitted to Phys. Rev. E, 10 pages, 1 figur

Elasticity near the vulcanization transition

Signatures of the vulcanization transition--amorphous solidification induced by the random crosslinking of macromolecules--include the random localization of a fraction of the particles and the emergence of a nonzero static shear modulus. A semi-microscopic statistical-mechanical theory is presented of the latter signature that accounts for both thermal fluctuations and quenched disorder. It is found (i) that the shear modulus grows continuously from zero at the transition, and does so with the classical exponent, i.e., with the third power of the excess cross-link density and, quite surprisingly, (ii) that near the transition the external stresses do not spoil the spherical symmetry of the localization clouds of the particles.Comment: REVTEX, 5 pages. Minor change

Tube Models for Rubber-Elastic Systems

In the first part of the paper we show that the constraining potentials introduced to mimic entanglement effects in Edwards' tube model and Flory's constrained junction model are diagonal in the generalized Rouse modes of the corresponding phantom network. As a consequence, both models can formally be solved exactly for arbitrary connectivity using the recently introduced constrained mode model. In the second part, we solve a double tube model for the confinement of long paths in polymer networks which is partially due to crosslinking and partially due to entanglements. Our model describes a non-trivial crossover between the Warner-Edwards and the Heinrich-Straube tube models. We present results for the macroscopic elastic properties as well as for the microscopic deformations including structure factors.Comment: 15 pages, 8 figures, Macromolecules in pres

Determination of Acceptable Structural Irregularity Limits for the Use of Simplified Seismic Design Methods

Paper 14Approximations to the exact response of a structure under design level excitation may be obtained by conducting a number of 3-D inelastic dynamic time history analyses considering all relevant effects and using the best information available. Factors that should be considered include foundation effects, floor diaphragm effects, and the likely variation of earthquake demand and structural capacity. In general, this type of approximate analysis is too complex for design engineers, so simpler, and hence more approximate analysis methods are commonly used. These have been calibrated based on the response of regular structures. When it is felt that a structure is too irregular, some approximate analysis methods are not permitted to be used. The amount of irregularity permitted has generally been selected based on engineering judgment. This paper describes a quantitative method to determine limits for structural irregularity for structures designed using different analysis procedures. The method considers both the design approach estimations and the response from inelastic dynamic time history analysis. Irregularity limits based on a specified level of confidence are selected and these are then proposed for the use in codes. The method is illustrated by an example considering mass irregularity