On a hybrid method to characterize the mechanical behavior of thin hollow glass microspheres

Hollow glass microspheres possess niche markets and future research areas due to their high strength-to-density ratio and low thermal conductivity. For example, these qualities make them a viable alternative insulation medium. For this study, Scotchlite^™ k1 hollow glass microspheres (HGMs) manufactured by 3M^™ were used; however, one of the challenges in investigating their mechanical properties was due to the limited availability of published data. Most importantly, the thickness (or the range of thicknesses) of produced thin hollow microspheres is not measured by the manufacturer, so the authors needed to develop a hybrid characterization method that depended on both experiments and simulations to estimate the HGMs mechanical properties. The experimental method utilizes a nano-indenter with a spherical sapphire tip of 500 μm in diameter. Each HGM was uniaxialy compressed under the indenter to yield the stiffness, diameter, and the force-displacement at fracture of each HGM tested. ABAQUS^™ was used to numerically model an HGM of mean diameter and stiffness to obtain a relation between the mechanical and geometrical properties of each HGM to its thickness, and investigate the development of stresses on an HGM during its uniaxial compression. The data show large scatter in the measured stiffness of each HGM as a function of diameter. The authors conjecture this is due to a large fluctuation of the microsphere thickness. Finally, the work necessary to deform an HGM was successfully correlated to its radius-to-thickness ratio. This gives us a unique insight on the force-displacement behavior against the geometrical features of HGMs.     

Ductile crack growth behavior of welded plate

Crack growth behaviour in welded plate is studied from the viewpoint of the constraint effect. Three cases are studied. One is the crack growth perpendicular to the weld line. By the generation phase analyses, the effect of the weld line on the crack tip fields is studied. In the second case, the crack transversing the weld line grows perpendicular to it in a compact tension (CT) specimen, and in the third case, the surface crack growth problem is studied. In the second and the third cases, the three dimensional numerical analyses are conducted and the crack tip fields are studied. It is shown that the Q factor proposed by O'Dowd and Shih has large effect on crack growth behavior in the weld plate, especially on the local behavior near the weld line.Presented at the Far East Fracture Group (FEFG) International Symposium on Fracture and Strength of Solids, 4–7 July 1994 in Xi'an, China.     

Analytical approach of mechanical behavior of carpet yarn by mechanical models

The viscoelastic models for describing mechanical behavior of polypropylene carpet yarn are presented in this paper. Since carpet yarn is subjected to tensile and frictional forces during tufting, the mechanical properties are important material features especially in vibration analysis. In this paper, the standard linear model and the standard nonlinear model are developed to simulate the polypropylene yarn tensile, stress relaxation and creep behaviors. The obtained formulas are fitted with experimental data and the ideal model parameters are determined by applying least squares. The results showed that the standard linear model fits well the tensile, relaxation and creep experimental curves, and will be employed to analyze the vibration characteristics of yarn during the tufting.     

On the mechanical behavior of fiber-reinforced composites

In this paper, the derivation of the state potential is presented to model the mechanical behavior of fiber-reinforced composites. It allows matrixcracking, interfacial debonding and sliding to be accounted for in the framework of Continuum Damage Mechanics. An application is performed on a unidirectional SiC/SiC composite.     

On the mechanical behavior of carbon-carbon optic grids determined using a bi-axial optical extensometer

Ion engines accelerate electrically charged plasma through two optic grids and emit the ions as exhaust. The process facilitates propulsion without use of chemical propellants. Braided carbon fiber reinforced composite (C _(f)/C) optics are presently being considered for use as the accelerator and screen grids in ion propulsion engines. In this study the mechanical behavior of four candidate tow configurations proposed for the grids of NASA's Evolutionary Xenon Thruster (NEXT) were examined. A new bi-axial optical extensometer based on Digital Image Correlation (DIC) was developed and employed in determining the in-plane strain distribution resulting from uniaxial tension. The effective elastic modulus ranged from 4 GPa to 10 GPa at the onset of deformation. The stiffness either increased or decreased with further elongation as a result of bending of the axial tows and corresponding unit cell distortion. The transverse strain and Poisson's ratio of the panels were found to be a function of the tow dimensions and bonding between longitudinal and transverse tows.     

On the mechanical behavior of BFRP to aluminum AA6086 mixed joints

The aim of this work is to analyze the possibility to join aluminum alloy AA6086 and composite laminates reinforced with basalt fibers, an innovative material which use is growing in several applications as an alternative to glass fibers. To this goal, three joining techniques were investigated: mechanical by Self Piercing Riveting (in the next called SPR), adhesive by co-curing technique and mixed in which the joining techniques (i.e. adhesive and mechanical) were combined. Two manufacturing technologies (i.e. hand lay-up and vacuum bagging) were used both to produce composite substrates and to realize co-curing adhesion between the substrates to be joined. Mixed joints were realized by inserting the rivets in co-cured joints after 48 h than the initial phase of the curing process (i.e. the phase of mixing the resin with own hardener). Overall, six lots of joints were realized (two for each joining technique). All joints were characterized by performing single lap joint tests. The mechanical results were analyzed through a two way analysis of variance.The experimental results show that adhesive joints, realized by vacuum bagging method, show average failure load 22.9% higher and standard deviation 70.6% lower than those realized by hand lay-up, respectively. This means that the vacuum bagging technology allows to increase the adhesion strength of the interface between metal sheet and Basalt Fiber Reinforced Polymer (in the next BFRP), allowing the above failure load growth. Furthermore the failure mechanisms change from adhesive mode to partially cohesive one for the adhesive joints realized by hand lay-up and vacuum bagging, respectively. By comparing mixed joints, different results are obtained: i.e. the hand lay-up joints show both higher average failure load (+42.9%) and standard deviation (+208.3%) than those realized by vacuum bagging. The poor performances of the mixed joints realized by vacuum bagging can be considered due to the excessive value of the chosen riveting load. Statistically, two variables were investigated: i.e. joining technology (i.e. mechanical, adhesive and mixed) and manufacturing process (i.e. hand lay-up and vacuum bagging). These can influence the properties of the joints. In particular, the joining technology results a significant factor. Moreover, an interaction between the two variables exists.     

Thermohygrometric and mechanical behaviour of concrete using damage models

A numerical formulation to analyse thermohygrometrically and mechanically coupled fields in concrete is presented. One of the main applications concerns the creep and shrinkage of concrete, wherein the transient hygrothermal effects are related to the mechanical behaviour and take account of geometrical non-linearities and anisotropic damage coupled with viscosity. The stress-strain relationship is represented by an integral equation. This procedure is implemented in a finite element code in order to analyse 2-D and 3-D structures of any shape, and the results regarding the numerical simulation of the creep and shrinkage in a concrete cylinder are presented.     

A structural experimental technique to characterize the viscoelastic behavior of concrete under restrained deformations

An innovative variable restraint frame is proposed to characterize the viscoelastic behavior of concrete under tensile stresses induced by restraints to shrinkage deformations (mainly due to drying). Two concrete specimens with the same cross section are used, subjected to equal thermal and moisture conditions: one is made of plain concrete, to assess the “free” deformations due to shrinkage and temperature; the other is reinforced with two steel threaded rods, which induce a manually controlled axial restraint to shrinkage. The restrained specimen is installed on a reaction frame, being stretched in force control mode. The concrete and the rods are instrumented with strain gauges and temperature sensors, which allow separation of the different components of concrete strains with the aid of equations based on equilibrium and compatibility conditions. This permits identifying the elastic and tensile creep concrete strains, as well as the concrete tensile stresses induced by the restrained shrinkage. The device also allows assessing the concrete modulus of elasticity during the test and remains operational even upon concrete cracking, features of great interest for the intended material characterization.     

Comment on the homogeneous nanofluid models applied to convective heat transfer problems

In several recent papers, the heat transfer characteristics of nanofluids have been investigated by simply replacing the transport coefficients of the base fluid by the effective transport coefficients of the nanofluids. The present note emphasizes, however, that the governing equations of these homogeneous nanofluid models (in which the velocity-slip effects of the nanoparticles are neglected) can be reduced with the aid of elementary scaling transformations to the respective equations of the regular fluids. Thus, the corresponding nanofluid results can be recovered from the solutions of already solved regular problems by simple arithmetic operations, without any additional research effort. This feature is illustrated here by the specific examples of the classical Blasius and Sakiadis forced convection heat transfer problems.     

Comparison of spectral fluorescent signatures-based models to characterize DOM in treated water samples

Statistical procedures enable a multivariate analysis of the measurements to identify specific characteristics of the dissolved organic matter (DOM) fractions in raw natural water, including the concentrations. In this work, three already established models were used to predict the concentrations of fractions of DOM from spectral fluorescent signatures (SFSs): a general linear regression (GLR), loadings and scores of a principal components analysis (PCA), and a partial least squares regression (PLS). Details about the method undertaken to prepare the fractions were given. Water samples from surface water treatment plants in New Jersey were used for the testing. In all cases, PLS have shown much better biases and accuracies than GLR and PCA models. Hydrophilic neutral, however, showed poor performances (bias 33%) due to the isolation technique used. Recommendations were provided in order to improve the DOM characterization through SFS, which linked to PLS make a powerful and cost-effective surrogate parameter to characterize DOM.     

Fatigue crack growth behavior of cracked aluminum plate repaired with composite patch

In this study, we investigated the fatigue crack growth behavior of cracked aluminum plate repaired with bonded composite patch especially in thick plate. Adhesively bonded composite patch repair technique has been successfully applied to military aircraft repair and expanded its application to commercial aircraft industry recently. Also this technique has been expanded its application to the repair of load bearing primary structure from secondary structure repair. Therefore, a through understanding of crack growth behavior of thick panel repaired with bonded composite patch is needed. We investigated the fatigue crack growth behavior of thick panel repaired with bonded composite patch using the stress intensity factor range (ΔK) and fatigue crack growth rate (da/dN). The stress intensity factor of patched crack was determined from experimental result by comparing the crack growth behavior of specimens with and without repair. Also, by considering the three-dimensional (3D) stress state of patch crack, 3D finite element analyses were performed to obtain the stress intensity factor of crack repaired by bonded composite patch. Two types of crack front modeling, i.e. uniform crack front model and skew crack front model, were used. The stress intensity factor calculated using FEM was compared with the experimentally determined values.     

On the crystallization behavior of amorphous Fe-Cr-Mo-B-P-Si-C powder prepared by mechanical alloying

The crystallization behavior and thermal stability of Fe-Cr-Mo-B-P-Si-C amorphous alloy, prepared by mechanical alloying (MA), were investigated by using differential scanning calorimetry (DSC). One exothermic peak was observed on DSC traces, implying that the crystallization process undergoes only one stage. The crystallization kinetic parameters, including activation energy (E_a), Avrami exponent (n) were determined with non-isothermal analysis method based on the DSC data. The results suggest that the crystallization mechanism is governed dominantly by a three-dimensional diffusion-controlled growth. In addition a relatively high value of activation energy of crystallization (386.04 ± 10 kJ/mol) was found, indicating that this amorphous alloy has high thermal stability.     

Predictive mechanical performance evaluation of consumer food cans using stereolithography models

The development of new metal food containers can be a technologically challenging and costly process. Understanding the interplay between the major design characteristics and requirements affecting the product's final structural capability is paramount to achieving an optimum design proposition. As a result, computer‐based simulation has been employed by industry to assess a container's performance under a variety of load conditions, including axial load and panelling of cans. In this paper, the feasibility of a new approach for addressing the effects of design parameters on the structural performance of containers under development is investigated. The evaluation methodology is based on structural testing of stereolithography‐built physical prototypes of a rigid metal container used for coffee packaging. It is shown that the experimentally obtained findings are in accordance to those resulting from computational simulation. This method can be used to support the development of existing and new metal containers. Copyright ©2003 John Wiley & Sons, Ltd.     

On the relevance of linear elastic fracture mechanics to ice

A new criterion is proposed which allows one to estimate the minimum size for a brittle ice fracture specimen comprised of a small number of grains. According to this criterion, linear elastic fracture mechanics is a useful theory for fresh-water ice but may have limited use for saline ice.     

Nonlinear aero-thermoelastic analysis of homogeneous and functionally graded plates in supersonic airflow using coupled models

In this paper, the aeroelastic behavior of homogeneous and functionally graded two- and three-dimensional flat plates is studied under supersonic airflow. The effects of coupled modeling of the aerodynamic heating with flight conditions and the thermal degradation of the plate are investigated, too. The von-Karman nonlinear strains, piston theory and a combination of simple rule of mixtures and the Mori–Tanaka scheme are used to model the structure, aerodynamic and material, respectively. The derived equations of motion are solved using Galerkin’s procedure along with the Runge–Kutta numerical technique. Finally, some examples are solved using the developed program and some conclusions are made. It is shown that under real flight conditions and using coupled model, the aerodynamic heating is very severe and the type of instability is divergence.     

On the capability of micromechanics models to capture the auxetic behavior of fibers/particles reinforced composite materials

This work investigates the possibility to predict the auxetic behavior of composites consisting of non-auxetic phases by means of micromechanical models based on Eshelby’s inclusion concept. Two specific microstructures have been considered: (i) the three-layered hollow-cored fibers-reinforced composite and (ii) a microstructure imitating the re-entrant honeycomb micro-architecture. The micromechanical analysis is based on kinematic integral equations as a formal solution of the inhomogeneous material problem. The interaction tensors between the inhomogeneities are computed thanks to the Fourier’s transform. The material anisotropy due to the morphological and topological textures of the inhomogeneities was taken into account thanks to the multi-site approximation of these tensors. In both cases, the numerical results show that auxetic behavior cannot be captured by such models at least in the case of elastic and isotropic phases. This conclusion is supported by corresponding finite element investigations of the second microstructure that indicate that auxetic behavior can be recovered by introducing joints between inclusions. Otherwise, favorable issues are only expected with auxetic components.     

On the capability of micromechanics models to capture the auxetic behavior of fibers/particles reinforced composite materials

This work investigates the possibility to predict the auxetic behavior of composites consisting of non-auxetic phases by means of micromechanical models based on Eshelby’s inclusion concept. Two specific microstructures have been considered: (i) the three-layered hollow-cored fibers-reinforced composite and (ii) a microstructure imitating the re-entrant honeycomb micro-architecture. The micromechanical analysis is based on kinematic integral equations as a formal solution of the inhomogeneous material problem. The interaction tensors between the inhomogeneities are computed thanks to the Fourier’s transform. The material anisotropy due to the morphological and topological textures of the inhomogeneities was taken into account thanks to the multi-site approximation of these tensors. In both cases, the numerical results show that auxetic behavior cannot be captured by such models at least in the case of elastic and isotropic phases. This conclusion is supported by corresponding finite element investigations of the second microstructure that indicate that auxetic behavior can be recovered by introducing joints between inclusions. Otherwise, favorable issues are only expected with auxetic components.