Influence of curvature geometry of laminated FRP composite panels on delamination damage in adhesively bonded lap shear joints

Three dimensional non-linear finite element analyses of Lap Shear Joints (LSJs) made with curved laminated FRP composite panels having pre-existing delaminations between the first and second plies of the strap adherend have been carried out using contact and Multi-Point Constraint elements (MPC). Progressive growth of delamination has been simulated by sequential release of the MPC elements. Strain Energy Release Rate (SERR), being an indicative parameter has been computed using Virtual Crack Closure Technique (VCCT) for assessing the growth and propagation of the delamination damage fronts. The inter-laminar stresses and the SERRs at the two fronts of the pre-embedded delamination are found to be significantly influenced by the delamination size. The three individual modes of SERR on the two delamination fronts are found to be much different from each other, indicating dissimilar rates of propagation. The curvature geometry of adherends significantly influences the SERR values. It is seen that decrease of radius of curvature of adherend panels, keeping their widths unchanged, increases the SERR values. Flatter FRP composite adherends have superior resistance to delamination damage propagation as compared to LSJs made with curved composite laminated panels.     

Measurement of Bulk Mechanical Properties and Modeling the Load-Response of Rootzone Sands. Part 1: Round and Angular Monosize and Binary Mixtures

The bulk mechanical properties of two different types of rootzone sands (round and angular) were measured using a cubical triaxial tester. Two monosize sands (d 50 = 0.375 mm and 0.675 mm) and their 50:50 binary mixtures (d 50 = 0.500 mm) were studied. The compression, shear, and failure responses of the above-mentioned six compositions were analyzed, compared, and modeled. Two elastic parameters (bulk and shear moduli) and two elastoplastic parameters (swelling and consolidation indices) of the six sand compositions were also calculated and compared. The angular sand was more compressible than round sand during isotropic compression. In addition, the angular sands tended to have lower initial bulk density and high porosity values. Among the three different size fractions, the 0.375 mm mixture was least compressible for both sand shapes. The failure strength and shear modulus of the angular sand were higher than the round sands. In addition, due to their simplicity, phenomenological models were developed to predict the compression and shear behavior of the sands. The prediction models were validated using subangular and subround sands. Average relative difference values were calculated to determine the effectiveness of the prediction models. The mean average relative difference values for compression profiles, i.e., volumetric stress vs. volumetric strain, were from 16 % to 39 %, except for the initial load-response portion (< 1 % volumetric strain). The predictive models were effective in reproducing the failure responses: at 17.2 kPa confining pressure, the mean of average relative difference was 23 %; at 34.5 kPa , the mean difference was 24 %.     

Shear and bending performance of carbon fiber composite sandwich panels with pyramidal truss cores

Structural performance in direct (pure) shear and three-point bending was investigated for sandwich panels with a carbon fiber pyramidal truss core. Analytical estimates for sandwich panel strength for each loading condition were presented for possible competing failure modes. In the experimental part of the study, pyramidal truss cores were made using the hot press molding technique and then attached to flat carbon fiber composite face sheets to build all-composite sandwich panels. Panels with different configurations (e.g., core relative density and face sheet thickness) were tested to probe different failure modes and investigate the mechanical properties. In general, measured failure loads showed good agreement with the analytical predictions. Failure mechanism maps illustrate the controlling failure mechanisms in various regions of parameter space.     

Predicting Shear Adhesion of Water-Based Pressure-Sensitive Adhesives via Transient Rheological Measurements

The relation between shear holding power and pressure-sensitive adhesive (PSA) rheological properties is investigated. Two constitutive models of PSA rheology, i.e., Newtonian and Power Law, are considered and experimentally tested for their ability to predict shear holding power. A Power Law model is found to be suitable for this purpose and is used to predict shear holding power with dynamic viscosity and steady shear viscosity measured by means of a transient creep experiment. Dynamic viscosity is shown to fail in predicting shear adhesion of high-viscosity PSA, whereas steady shear produces good agreement. It is demonstrated that the Cox-Merz rule does not hold for high-viscosity PSAs, explaining the need for steady shear viscosity data in modeling shear holding power.     

Rheokinetic model to characterize the maturation process of gelatin solutions under shear flow

The characterization of maturing gelatin solutions in shear flow is required for an appropriate formulation of food products. Under static conditions, the maturation process of gelatin solutions may be described through a basic structural parameter evolving to the percolation value (gel point). Within the rheological framework, two asymptotic viscosities are well identified for the maturing process of gelatin solutions at the limit zero shear rate. One involves the initial solution viscosity that may be associated with the null structural parameter (the microstructure is not formed yet). The other is the percolation zero shear rate viscosity (assuming an infinite value when approaching the gel time) and corresponds to the maximum value of the structural parameter. Under flow, thixotropic theories combined with the knowledge of suspension rheology allow one to convert directly experimental data obtained as shear stress versus time for a given shear rate into the time evolution of the structural parameter. Consequently rheometric experimental data available places the search for a rheokinetic model of the structural parameter. Here, different expressions for this model are investigated, mainly those involving both the rates of structure breakdown and buildup, where the average cluster size is affected by the shear rate. The rate equation thus obtained may be then applied to arbitrary shear rate histories. Numerical results of the rheokinetic model proposed in this work fit well experimental rheometric data obtained in shear flow for the maturing of different gelatin solutions. Experimental data acquired in this work are presented and discussed in relation to those reported previously in the literature.     

Shear viscosity to entropy density ratio in BUU transport model

Shear viscosity (η) is a basic transport coefficient of the medium.In this work,we calculate shear viscosity to entropy density ratio (η/S) of an equilibrated system in intermediate energy heavy ion collisions within the framework of the Boltzmann-Uehling-Uhlenbeck model (BUU) model.After the equilibration of Au + Au system at central collision in a fixed volume is reached,temperature,pressure and energy density are extracted by the phase space information and then η/S is calculated using the Green-Kubo for...     

An All‐Natural Adhesive for Bonding Wood

A novel adhesive that is solely based on natural materials of defatted soy flour (SF) and magnesium oxide (MgO) has been investigated for preparation of five‐ply plywood panels. The resulting plywood panels met the industrial water‐resistant requirement for interior plywood. In this study, mechanisms by which an aqueous mixture of SF and MgO served as a strong and water‐resistant adhesive for bonding wood were investigated. SF was first fractionated into soy protein isolates (SPI), a water‐soluble fraction, and insoluble carbohydrates (ICs) that were mixed with MgO, respectively, for preparation of maple laminates. The water resistance of the resulting maple laminates was evaluated by a three‐cycle water‐soaking‐and‐drying (WSAD) test and a two‐cycle boiling‐water test (BWT). The mixture of MgO and the soluble fraction was not able to bond maple veneers together. The shear strengths of the resulting maple laminates before and after WSAD and BWT all had the following order: MgO–SPI > MgO–SF > SF only > MgO–IC. The water solubility of SF in the heat‐cured SF–MgO mixture was much lower than that of the heat‐cured SF. We believe that the low water solubility of SF–MgO and close interactions between MgO and soy proteins instead of soy carbohydrates were responsible for the superior strengths and high water resistance of the soy‐MgO adhesive.     

Scherfließen und Wandgleiten von Massen für den Pulverspritzguss

This contribution presents coupling of laws for shear flow and wall slipping by the shear stress at the slipping interface. It includes the special case of Coulombian friction postulated by Uhland as well as the assumption of a constant sliding velocity along the flow channel according to Mooney. As an example, Ostwald and de Waele's law of shear thinning flow is combined with a shear stress of sliding depending on internal pressure by a power law. Examined feedstocks for metal injection molding showed a rheological behavior according to the model presented.     

Inverse ductile–brittle transition in metallic glasses?

There is evidence that metallic glasses can show increased plasticity as the temperature is lowered. This behaviour is the opposite to what would be expected from phenomena such as the ductile–brittle transition in conventional alloys. Data collected for the plasticity of different metallic–glass compositions tested at room temperature and below, and at strain rates from rate 10^?5 to 10³ s^?1, are reviewed. The analogous effects of low temperature and high strain rate, as observed in conventional alloys, are examined for metallic glasses. The relevant plastic flow in metallic glasses is inhomogeneous, sharply localised in thin shear bands. The enhanced plasticity at lower temperature is attributed principally to a transition from shear on a single dominant band to shear on multiple bands. The origins of this transition and its links to shear bands operating ‘hot’ or ‘cold’ are explored. The stress drop on a shear band after initial yielding is found to be a useful parameter for analysing mechanical behaviour. Schematic failure mode maps are proposed for metallic glasses under compression and tension. Outstanding issues are identified, and design rules are considered for metallic glasses of improved plasticity.     

Coupled analysis of bifurcation and shear band in saturated soils

The focus of this study is the analysis of the onset of a shear band in saturated soils under biaxial stress conditions taking into account bifurcation in both pore pressure and soil skeleton. By considering the saturated soil as a two-phase medium, the inhomogeneous displacement and pore pressure fields are assumed to be nonlinear and satisfy the continuity conditions of displacements, displacement gradients and excess pore pressure on the boundaries of the band. It is shown that bifurcation may take place in loose, contractive soils in the form of two-phase diffuse mode or localized mode of soil skeleton deformation. For low permeability saturated soils, the onset of a shear band occurs at peak friction, with the shear band parallel to Coulomb's failure plane. Numerical examples using a simple stress–strain model are given to demonstrate the occurrence of the onset of a shear band and its inclination, as a function of the void ratio and the initial consolidation pressure.