Experimental determination of torsion and shear properties of sandwich panels and laminated composites by the plate twist test

A recently developed sandwich plate twist test is employed here for determination of the transverse shear modulus of the core and twist stiffness (D₆₆) of a sandwich panel consisting of a soft (H45 PVC foam) core and glass/vinylester face sheets. The shear modulus of the H45 PVC foam core extracted from the twist test was in good agreement with shear modulus obtained from ASTM plate shear testing of the foam core. D₆₆ values obtained from the sandwich twist test were in good agreement with predictions from classical laminated plate theory. In addition, the twist test was used to determine the in-plane shear modulus of glass/vinylester laminates isolated and as face sheets in sandwich panels with a stiff (plywood) core. The in-plane shear modulus of the face sheets, isolated and as part of a sandwich panel, was in good agreement with shear modulus determined using the Iosipescu shear test. The results point to the potential of the twist test to determine both in-plane and out-of-plane shear moduli of the constituents of a sandwich structure, as well as D₆₆.     

Shear coupling effects on stress and strain distributions in wood subjected to transverse compression

The mechanical behaviour of a wood board subjected to transverse compression is relevant to the performance of glulam beams and solid wood structures. The wood material can be described as polar orthotropic, due to the annual ring structure and to the differences in moduli in different directions in the radial–tangential plane. Strain measurements are performed on single wood boards using a whole-field digital speckle photography technique. Finite element analysis is performed and compared with experimental data. Good agreement in terms of strain fields and apparent moduli is observed between predictions and data. The experimental data show strong variations in local strain due to the polar orthotropic behaviour of wood in this plane, and the extremely low value for shear modulus G_rt as compared with the other moduli. This leads to shear coupling effects resulting in large local shear deformation and correspondingly low effective stiffness under transverse global loading.     

Measurement of the elastic parameters of densified balsam fir wood in the radial-tangential plane using a digital image correlation (DIC) method

Thermo-hydro-mechanical densified balsam fir (Abies balsamea (L.) Mill.) wood used in this study was made by compressing wood in the radial (R) direction (ray direction) at a temperature of 230 °C and a compression ratio of 60 % for 20 min, which significantly increased its density and some mechanical properties. However, the elastic parameters of densified wood in the R and tangential (T) directions, [moduli of elasticity (E _R and E _T), shear modulus (G _RT), and Poisson’s ratios (μ _RT and μ _TR)], have seldom been reported because of the relatively small thickness of a densified wood specimen. This study measured the E _R, E _T, G _RT, μ _RT, and μ _TR of densified balsam fir wood using a digital image correlation method, which was used to measure the strain field of a specimen under compressive loading. The corresponding five elastic parameters of undensified balsam fir wood were also measured. The experimental results were: (1) E _R was 284 MPa, which was about 66 % lower than that of undensified fir; (2) E _T was 2551 MPa, which was about ten times larger than that of undensified fir; (3) G _RT was 21 MPa, which was about 50 % lower than that of undensified fir; and (4) μ _RT and μ _TR were 0.104 and 0.331, which were about 75 and 10 % lower than those of undensified fir, respectively.     

Nanoindentation of wood cell walls: Continuous stiffness and hardness measurements

The objective of this study was to measure the mechanical properties of individual, native wood fibers using the continuous nanoindentation measurement technique. The indentation depth profile exhibited a small length-scale effect, which was confirmed using the size-effect index derived from the indentation loading curve. The hardness (H_u) or stiffness (E_u) values determined from indentation unloading were also examined for 10 different annual rings of a loblolly pine, with microfibril angles (MFA) between 14° and 36°. A predictable pattern of E_u values was found as a function of MFA, and hence E_u can at least be considered a relative measure of the longitudinal stiffness properties of wood cell walls. For H_u values, a dependence on orientation was observed, and there is a preliminary indication that the dependence could be affected by cell-wall extractives. It is thus desirable, for cell-wall modification studies, to minimize any unintended variations by using samples that are from the same growth ring, so that any treatment-induced changes in the cell-wall hardness can be identified.     

A new meshless method based on MLPG for elastic dynamic problems

The basic concept and numerical implementation of a new local Petrov-Galerkin method for solving a dynamic problem are presented in this paper. It uses a radial basis function (RBF) coupled with a polynomial basis function as a trial function, and uses the Heaviside function as a test function of the weighted residual method. The shape function has the Kronecker Delta properties for the trial-function-interpolation, and so no additional treatment to impose essential boundary conditions. The method does not involve any domain and singular integrals to generate the global effective stiffness matrix except for the mass and damping matrice; it only involves a regular boundary integral. It possesses a great flexibility in dealing with the numerical model of the elastic dynamic problem under various boundary conditions with arbitrary shapes. The Newmark family of methods is adopted in computation. The numerical results also show that using a multiquadrics (MQ) function with the polynomial basis function as the interpolation function can give quite accurate numerical results. The a_Q and a_S are investigated which are parameters of the radii of the sub-domain and influence domain, respectively.     

Accurate comparative determination of the in-plane shear modulus of T300/914 by the iosipescu and 45° off-axis tests

This paper compares the initial shear modulus distributions obtained from [0]₂₀, [90]₂₀ and [0/90]_5s Iosipescu and [45]₂₀ off-axis T300/914 specimens. The data reduction processes for the two tests are presented together with a finite element analysis of the Iosipescu test that enables the determination of correction factors to account for the non-homogeneity of the stress and strain fields along the notch line. A preliminary 45° off-axis test is performed to determine a region of linear elastic behaviour within which the samples are each tested four times. Close examination of the test data leads to a detailed discussion of the quality of the Iosipescu specimens in terms of perpendicularity of the loading surfaces to the lateral faces. The degrees of scatter due to the mechanical loading conditions and to the sample to sample variations are compared.     

Application of Different Turbulence Models in Unsteady Flow Simulations of a Radial Diffuser Pump

In this paper, the unsteady flow in a radial pump has been investigated numerically by utilizing different turbulence models at both design and off-design conditions. The numerical results are analyzed and compared on the pressure, relative velocity, and turbulence fields with experimental results from LDV and PIV. The analysis of the results shows that the turbulence model does not show significant influences on the pressure field. At the design condition (Q _des), k-ε predicts generally the best result on the relative velocity magnitude, slightly better than k-ω. However, k-ω has an overwhelming predominance on predicting the velocity vector directions and also on the turbulence kinetic energy. Furthermore, CFD can provide much better agreement to the measurements in the radial gap region than in the impeller region for all examined turbulence models. At off-design condition (0.5Q _des), DES and SST turbulence models predict successfully a “two-channel” stall phenomenon in the impeller detected by the measurements, and other turbulence models failed to predict it.     

Mixed-mode crack-tip stress fields for orthotropic functionally graded materials

Quasi-static mixed mode stress fields for a crack in orthotropic inhomogeneous medium are developed using asymptotic analysis coupled with Westergaard stress function approach. In the problem formulation, the elastic constants E ₁₁, E ₂₂, G ₁₂, ν ₁₂ are replaced by an effective stiffness ${E=\sqrt {E_{11} E_{22}}}$ , a stiffness ratio ${\delta =\left({{E_{11}}\mathord{\left/ {\vphantom {{E_{11}} {E_{22}}}}\right. \kern-0em} {E_{22}}} \right)}$ , an effective Poisson’s ratio ${\nu =\sqrt {\nu_{12}\nu _{21}} }$ and a shear parameter ${k=\left({E \mathord{\left/ {\vphantom {E {2G_{12}}}}\right. \kern-0em} {2G_{12}}}\right)-\nu }$ . An assumption is made to vary the effective stiffness exponentially along one of the principal axes of orthotropy. The mode-mixity due to the crack orientation with respect to the property gradient is accommodated in the analysis through superposition of opening and shear modes. The expansion of stress fields consisting of the first four terms are derived to explicitly bring out the influence of nonhomogeneity on the structure of the mixed-mode stress field equations. Using the derived mixed-mode stress field equations, the isochromatic fringe contours are developed to understand the variation of stress field around the crack tip as a function of both orthotropic stiffness ratio and non-homogeneous coefficient.     

Short range ordering in amorphous In-Se films by wide-angle X-ray scattering

Wide-angle X-ray scattering studies were performed on In-Se amorphous films, obtained by thermal evaporation, with selenium content of 60 and 66 at.%. The intensities were recorded in the scattering vector range between 3 and 160 nm^–1. Structural information about the local structure of the amorphous In-Se films has been derived from the radial distribution function using the curve-fitting method. The experimental results have been compared with model based simulations. The obtained structural parameters indicate that for In₄₀Se₆₀ In-In, In-Se and Se-Se contributions are involved in the near-neighbour coordination sphere. As the Se content is increased, the number of In-In bonds is reduced to zero, within the precision of the method. For both amorphous films In is tetrahedrally coordinated while Se has three near neighbours on the average.     

Mode II fracture testing method for highly orthotropic materials like wood

In this paper a mode II fracture testing method has been developed for wood from analytical, experimental and numerical investigations. Analytical results obtained by other researchers showed that the specimen geometry and loading type used for the proposed mode II testing method results in only mode II stress intensity and no mode I stress intensity at the crack tip. Experiments have been carried out to determine mode II fracture toughness K _IIC and fracture energy G _IIF from the test data collected from both spruce (pice abies) and poplar (populus nigra) specimens. It was found that there existed a very good relation between fracture toughness K_IIC and fracture energy G _IIF when the influence of orthotropic stiffness E _II ^* in mode II was taken into account. It verified that for this mode II testing method the formula of LEFM can be employed for calculating mode II fracture toughness even for highly orthotropic materials like wood. In the numerical studies for the tested spruce specimen, the crack propagation process, stress and strain fields in front of crack tips and the stress distributions along the ligament have been investigated in detail. It can be seen that the simulated crack propagating process along the ligament is a typical shear cracking pattern and the development of cracks along the ligament is due to shear stress concentrations at the crack tips of the specimen. It has been shown that this mode II fracture testing method is suitable for measuring mode II fracture toughness K _IIC for highly orthotropic materials like wood.     

A new monitor set for the determination of neutron flux parameters in short-time k0-NAA

Multipurpose research reactors such as LVR-15 in ?e? require monitoring of the neutron flux parameters (f, α) in each batch of samples analyzed when k₀ standardization in NAA is to be used. The above parameters may change quite unpredictably, because experiments in channels adjacent to those used for NAA require an adjustment of the reactor operation parameters and/or active core configuration. For frequent monitoring of the neutron flux parameters the bare multi-monitor method is very convenient. The well-known Au-Zr tri-isotopic monitor set that provides a good tool for determining f and α after long-time irradiation is not optimal in case of short-time irradiation because only a low activity of the ⁹⁵Zr radionuclide is formed. Therefore, several elements forming radionuclides with suitable half-lives and Q₀ and ē_r parameters in a wide range of values were tested, namely ¹⁹⁸Au, ⁵⁶Mn, ⁸⁸Rb, ¹²⁸I, ¹³⁹Ba, and ²³⁹U. As a result, an optimal mixture was selected consisting of Au, Mn, and Rb to form a well suited monitor set for irradiation at a thermal neutron fluence rate of 3×10¹⁷ m⁻² s⁻¹. The procedure of short-time INAA with the new monitor set for k₀ standardization was successfully validated using the synthetic reference material SMELS 1 and several matrix reference materials (RMs) representing matrices of sample types frequently analyzed in our laboratory. The results were obtained using the Kayzero for Windows program.     

A study of stable crack growth using experimental methods,finite elements and fractography

Systematic analysis of the in-plane constraint influence on J-resistance curves is presented. J_R curves were also recorded and analyzed beyond the limits of crack extension inside which the stress field can be assumed to be dominated by J-integral. Three steels and four types of specimen: SEN(B), SEN(T), CCT and DENT have been tested. Along with the J_R curves the fracture mechanisms have been analyzed with the help of scanning microscopy. The numerical, finite element analysis has been adopted to compute the Q-stresses, as a measure of the in-plane constraint prior to the onset of crack growth. The analysis of the stress field in front of the crack has been performed to check whether the state of stress prior to the crack growth can predetermine the way the crack will grow. It turns out that characteristic features in the J_R curves runs can be predicted qualitatively from the Q(a/W) and Q(J) curves. However, there is a good correlation between Q-stress and voids diameters on fractured surfaces. Several patterns in J_R curves runs have been observed for tested specimens; e.g. no influence of specimen thickness on J_R curves runs was observed for side-grooved specimens. Strong influence of specimen thickness on J_R curve shape was observed for non-side-grooved specimens. J_R curve run higher for thinner specimens unless they are dominated by plane stress. For bent specimens J_R curves run higher for shorter cracks but they run lower for specimens in tension.     

Microcrystalline cellulose filled composites for wooden artwork consolidation: Application and physic-mechanical characterization

Two types of historical wood (18th century) (Juglans regia and Abies alba) presenting different degradation conditions were consolidated through acetone solutions of microcomposites consisting of a commercial polymer (Paraloid® B72) often used for wood consolidation and two different amounts (5 and 30 wt%) of microcrystalline cellulose (MCC). As a comparison, the same tests were also performed on modern samples of the same types of wood in a well preserved state.Rheological and mass variation measurements evidenced that the introduction of microfiller did not affect the viscosity and the water sorption of the neat resin to a significant extent. Moreover, mercury intrusion porosimetry highlighted how MCC filled composites were able to decrease the pore radius of treated wood samples, reaching values close to those of modern intact wood. Remarkably, the presence of MCC within Paraloid led to a positive enhancement of the stiffness and the flexural strength of treated damaged wood in both quasi-static and impact conditions, with the increase of the flexural modulus, the maximum stress and the rise of both initial and total impact absorbed energies. Additionally, an increase of the radial and tangential surface hardness due to the treatment with MCC filled composites was observed.     

Quality of beam in the integer resonances crossing in a cyclotron

It is shown that the crossing of the integer resonance Q_r = 2 in an isochronous cyclotron is accompanied by the distortion of the closed orbit only. The amplitude of free radial oscillations of a beam is attenuated adiabatically and the radial emittance is constant in the course of acceleration.     

Identification of the Orthotropic Elastic Stiffnesses of Composites with the Virtual Fields Method: Sensitivity Study and Experimental Validation

ABSTRACT:  This paper presents the use of a sensitivity study to find the best combinations of free length and fibre angle in an unnotched Iosipescu test processed with the virtual fields method. The sensitivity to noise coefficients arising from the special virtual fields procedure are used to build up a cost function. This function is aimed at balancing out the coefficients of the different orthotropic stiffnesses so that the same confidence level can be reached on all these parameters. Then, experimental validation was performed using a speckled interferometry (ESPI) system. Full-field strains were measured and stiffnesses identified and compared between the usual 0°, 30 mm configuration and an improved 25°, 40 mm configuration. The outcome of the optimisation was confirmed by testing the same specimen several times and comparing scatter between the two configurations. This is a first promising result on the way to the design of a new test for orthotropic stiffness identification on a single specimen from full-field measurements.     

Fabrication of smart coatings based on wood substrates with photoresponsive behavior and hydrophobic performance

A smart coating based on wood substrate with photochromic behavior and hydrophobic performance was successfully fabricated by depositing modified organic photochromic material (PM) on wood surface using a simple drop-coating method. The prepared samples were characterized by the scanning electron microscopy (SEM) and attenuated total reflectance–Fourier transformation infrared (ATR–FTIR) techniques. The colorimetric parameters L*, a*, b*, and ΔE* of the samples under ultraviolet (UV) irradiation and visible (VIS) irradiation were measured, respectively. The response time of samples was calculated and the water contact angles (CAs) were also measured. The ATR–FTIR spectra and the SEM analysis demonstrate that PM modified by polyvinyl alcohol and the dextrin deposited on the wood surface, and the resulted coating was firmly adhered to wood by adhesion test results. With the PM concentrations increasing from 0 to 3.0%, the ΔE* values of samples remarkably increased from 3.7 to 58.2. Furthermore, the coloration time and fading time in the cross section were longer than the radial and tangential sections. Through the octadecyltrichlorosilane (OTS) treatment, the sample surface exhibited hydrophobic property with a contact angle of 134°, broadening the applications of the photoresponsive coatings in a harsh environment such as prolonged exposure under UV light or rain.     

Finite element analysis of axial splits in composite Iosipescu specimens

In this paper, a finite element analysis of skew-symmetric splits along the fiber direction in unidirectional composite Iosipescu specimens is performed. The energy release rates G _I, G _II, and G _total associated with axial splits in cracked Iosipescu specimens under external biaxial loading conditions are computed by four different numerical schemes: displacement correlation, displacement extrapolation, J-integral, and the modified crack closure integral. Using beam theory analysis, an analytical solution for the energy release rates is also proposed. Axial splits in Iosipescu specimen propagate under mixed mode conditions, with G _I and G _II varying with the crack length a. For short and medium crack lengths G _I>G _II, while for long cracks, G _II is dominant. The energy release rates G _I, G _II, and G _total are strongly dependent on the biaxial type of loading. The G-estimates obtained by the modified crack closure integral schemes are found to be the most accurate among all the numerical schemes chosen in this study. In the analyses of axial splits in composite Iosipescu specimens, the displacement correlation and extrapolation techniques yielded poor results. For long crack lengths, the analytical results from the beam theory analysis are in fair agreement with those from the modified crack closure integral schemes; however, for short and medium crack lengths, there is a significant difference between the analytical and numerical results. In composite Iosipescu specimens, stable crack propagation (mode I dominant) can be achieved by increasing the tension/shear ratio in the external loading boundary conditions.     

Synthesis of low loss, thermally stable CexY1−xTiTaO6 microwave ceramics

Ce_xY_1−xTiTaO₆ ceramics were prepared through the solid-state ceramic route. The materials were sintered in the range 1520-1580 °C. The structure of the system was analyzed by X-ray diffraction and Raman spectroscopic methods. The cell parameters of solid solutions were calculated using the least square method. The microstructure was analyzed using scanning electron microscopy. The dielectric constant (?_r), temperature coefficient of resonant frequency (τ_f) and the unloaded quality factor (Q_u) are measured in the microwave frequency region using cavity resonator method. The dielectric constant increases with higher concentrations of Ce in the solid solutions. Nearly zero temperature coefficient of resonant frequency (τ_f) was obtained for Ce_0.24Y_0.76TiTaO₆. The samples are of high quality factor and are useful electronic materials for microwave applications.