The effects of curing temperature on fracture energy and cohesive parameters for the adhesive Araldite 2015

This paper attempts to investigate the effects of curing temperature on the fracture energy, the glass transition temperature (T_g) and cohesive parameters for the adhesive Araldite 2015. Relationship between curing temperature and the glass transition temperature was taken into account. Tensile tests were performed on the dogbone-shaped bulk specimens to evaluate the effect of curing temperature on the mechanical properties of the adhesive. DCB test results were used to obtain the cohesive laws of the adhesive Araldite 2015. The exponential and PPR cohesive zone models were used to obtain some of the fracture properties. Inverse analyses were also performed, if the experimental softening curves are incompatible with the numerical ones. It was seen that softening behavior of the adhesive can be easily controlled by the shape parameters available in the PPR cohesive zone model. It is seen from the DCB test results that curing the adhesive about the temperature at which the T_g∞ is obtained caused the adhesive to have more ductility, higher load-carrying capacity and higher fracture energy than curing it below or above the temperature at which T_g∞ is attained. Here, the T_g∞ is the T_g of the fully cured network. Experimental and numerical R curves were obtained to account for deviations between experiments and simulations. A good agreement between the numerical and experimental load-displacement curves was achieved showing the adequacy of the cohesive model used.     

Studying the thermoviscoelastic response of the Redux 312 adhesive—Part 1: Experimental characterization

The aim of this study is to investigate the thermoviscoelastic behaviour of the Redux 312 structural adhesive. Two types of tests are performed on bulk specimens: dynamic tests over a wide range of temperature and stress-relaxation tests for five temperatures ranging between −50 and 70 °C. Dynamic tests clearly show that the shear modulus of the adhesive decreases with temperature. The parameters governing the Maxwell model used to model the viscoelastic properties of the adhesive are identified with the curves obtained with the stress-relaxation tests for the five different temperatures. The procedure used to identify these parameters takes into account the evolution of these parameters with temperature. Results obtained are presented and discussed throughout the paper.     

The effect of constant loading on the mechanical behavior of bulk adhesive

The aim of this research was to develop an experimental–numerical approach to characterize the effect of constant loading coupled with elevated temperature on epoxy bulk adhesive and to predict the stress degradation of bulk adhesive specimen under 15% and 25% tensile failure loads for the automotive industry. A power-law creep model was built to simulate the effect of temperature and loading on adhesive mechanical behavior, and the related strength degradation simulation has also been implemented using a creep strain-dependent ductile damage model. Experiments were conducted on bulk adhesive specimens under constant temperature coupled with mechanical load, and the corresponding experimental results provided creep parameters for the simulation procedure as well as effective validation with the numerical results in this study. The results obtained from experiments and numerical simulations were also in good agreement.     

Yield behavior of rubber-modified epoxy adhesives under multiaxial stress conditions

In rubber-modified epoxy resins, a damage zone is generated in the vicinity of the crack tip due to the cavitation of rubber particles, which improves fracture toughness dramatically. Hence, in evaluating the stress distribution in adhesive joints with rubber-modified adhesives, the void formation and growth should be taken into account. In most studies, however, the adhesive layer is still considered as a continuum material governed by the von Mises yield criterion. For many ductile materials, Gurson's model is used for the stress analysis, in which the void formation and growth is taken into account. In a previous study, using adhesively bonded scarf and torsional butt joints, the effect of stress triaxiality on the yield stress in the adhesive layer was investigated. In this study, these experimentally-obtained yield stresses were compared with those obtained by a finite element method, where Gurson's constitutive equations were applied to the adhesive layer. As a result, the calculated yield stresses agreed well with the experimentally-obtained yield stresses. This indicates that Gurson's model is a useful tool for estimating stress distributions in adhesive joints with rubbermodified adhesives.     

Modeling of Drying in Moving Bed

The aim of this work is to study the simultaneous heat and mass transfer between air and soybean seeds in moving bed dryers with parallel flow (concurrent and countercurrent), by means of an experimental and simulation work, verifying the validity of classical assumptions. The numerical solution of a one-dimensional boundary value problem was obtained by means of a computational code based on axial integration through DASSL code. Deviations from flat air velocity profile were taken into account considering empirical and mechanistic equations found in the literature that describes air profile as function of radius. The experimental data of air humidity, temperature, and seed moisture content and temperature at the dryer outlet were compared to the simulated values.     

Modeling of Drying in Moving Bed

The aim of this work is to study the simultaneous heat and mass transfer between air and soybean seeds in moving bed dryers with parallel flow (concurrent and countercurrent), by means of an experimental and simulation work, verifying the validity of classical assumptions. The numerical solution of a one-dimensional boundary value problem was obtained by means of a computational code based on axial integration through DASSL code. Deviations from flat air velocity profile were taken into account considering empirical and mechanistic equations found in the literature that describes air profile as function of radius. The experimental data of air humidity, temperature, and seed moisture content and temperature at the dryer outlet were compared to the simulated values.     

Numerical modeling of thermally induced microcracking in porous ceramics: An approach using cohesive elements

A numerical framework is developed to study the hysteresis of elastic properties of porous ceramics as a function of temperature. The developed numerical model is capable of employing experimentally measured crystallographic orientation distribution and coefficient of thermal expansion values. For realistic modeling of the microstructure, Voronoi polygons are used to generate polycrystalline grains. Some grains are considered as voids, to simulate the material porosity. To model intercrystalline cracking, cohesive elements are inserted along grain boundaries. Crack healing (recovery of the initial properties) upon closure is taken into account with special cohesive elements implemented in the commercial code ABAQUS. The numerical model can be used to estimate fracture properties governing the cohesive behavior through inverse analysis procedure. The model is applied to a porous cordierite ceramic. The obtained fracture properties are further used to successfully simulate general non-linear macroscopic stress-strain curves of cordierite, thereby validating the model.     

THE DISSOLUTION OR GROWTH OF A SPHERE

The problem of the dissolution or growth of an isolated, stationary, sphere in a large fluid body is analyzed. The motion of the boundary as well as the resulting motion in the liquid are properly taken into account. The governing equations are solved using a recently developed technique (Subramanian and Weinberg, 1981) which employs an asymptotic expansion in time. Results for the radius of the sphere as a function of time are calculated. The range of utility of the present solution is established by comparison with a numerical solution of the governing equations obtained by the method of finite differences.     

Modeling the temperature-dependent mode I fracture behavior of adhesively bonded joints

An ethylene propylene diene monomer (EPDM) rubber film has been used as an inhibitor and insulation in solid rocket motors (SRMs) due to its excellent heat-insulating property. EPDM is wrapped on the surface of the grain layer-by-layer via an adhesive; thus, the adhesive property between EPDM films is one of the key factors that influence the structural integrity of an SRM. The adhesive properties are largely temperature dependent, therefore, it is essential to study the effect of temperature on the properties of the bonding interface between EPDM films. In this article, double cantilever sandwich beam (DCSB) and uniaxial tensile experiments were performed to study the temperature-dependent mode I fracture of the bonding interface, in the service temperature range of the SRMs. A comparison of experimental and numerical results obtained using experimental parameters indicates that the fracture parameters determined by the simple beam theory (SBT) and the compliance-based beam method (CBBM) are not accurate. Next, we obtained accurate parameters using an inverse analysis method. Moreover, we made an initial attempt to establish a temperature-dependent cohesive zone model to predict the temperature-dependent fracture behavior of adhesively bonded joints. Good agreement between experimental and numerical results demonstrates that this temperature-dependent model is applicable.     

THE DISSOLUTION OR GROWTH OF A SPHERE

The problem of the dissolution or growth of an isolated, stationary, sphere in a large fluid body is analyzed. The motion of the boundary as well as the resulting motion in the liquid are properly taken into account. The governing equations are solved using a recently developed technique (Subramanian and Weinberg, 1981) which employs an asymptotic expansion in time. Results for the radius of the sphere as a function of time are calculated. The range of utility of the present solution is established by comparison with a numerical solution of the governing equations obtained by the method of finite differences.     

Monodisperse monocomponent fuel droplet heating and evaporation

The results of numerical and experimental studies of heating and evaporation of monodisperse acetone, ethanol, 3-pentanone, n-heptane, n-decane and n-dodecane droplets in an ambient air of fixed temperature and atmospheric pressure are reported. The numerical model took into account the finite thermal conductivity of droplets and recirculation inside them based on the effective thermal conductivity model and the analytical solution to the heat conduction equation inside droplets. The effects of interaction between droplets are taken into account based on the experimentally determined corrections to Nusselt and Sherwood numbers. It is pointed out that the interactions between droplets lead to noticeable reduction of their heating in the case of ethanol, 3-pentanone, n-heptane, n-decane and n-dodecane droplets, and reduction of their cooling in the case of acetone. Although the trends of experimentally observed droplet temperatures and radii are the same as predicted by the model taking into account the interaction between droplets, the actual values of the predicted droplet temperatures can differ from the observed ones by up to about 8 K, and the actual values of the predicted droplet radii can differ from the observed ones by up to about 2%. It is concluded that the effective thermal conductivity model, based on the analytical solution to the heat conduction equation inside droplets, can predict the observed average temperature of droplets with possible errors not exceeding several K, and observed droplet radii with possible errors not exceeding 2% in most cases. These results allow us to recommend the implementation of this model into CFD codes and to use it for multidimensional modelling of spray heating and evaporation based on these codes.     

Effect of temperature on the shear strength of aluminium single lap bonded joints for high temperature applications

An experimental and numerical investigation into the shear strength behaviour of adhesive single lap joints (SLJs) was carried out in order to understand the effect of temperature on the joint strength. The adherend material used for the experimental tests was an aluminium alloy in the form of thin sheets, and the adhesive used was a high-strength high temperature epoxy. Tensile tests as a function of temperature were performed and numerical predictions based on the use of a bilinear cohesive damage model were obtained. It is shown that at temperatures below T_g, the lap shear strength of SLJs increased, while at temperatures above T_g, a drastic drop in the lap shear strength was observed. Comparison between the experimental and numerical maximum loads representing the strength of the joints shows a reasonably good agreement.     

Post extrusion heat and solvent transfer from polymeric film

The problem of heat and solvent transfer from plasticized film is considered. The transport equations are solved by a numerical method. The formulation of the model includes the temperature dependence of diffusivity, the dependence of diffusivity on decreasing solvent concentration, as solvent leaves the film, and the latent heat of vaporization of the solvent. The Flory-Huggins theory is used as a model for vaporliquid equilibrium. Heat and mass transfer coefficients are taken either as constants (to simulate extrusion with blowing at the film surface) or from analytical solutions to the appropriate boundary layer equations (to simulate extrusion into a stationary medium.) The boundary layer theory takes into account the effect of rapid vaporization on heat and mass transfer coefficients. Several numerical solutions were obtained for cases corresponding to extrusion of polyvinylacetate, plasticized with acetone, extruded into air.     

Instability of a combustion model with surface vaporization and overheat in the condensed phase

Using numerical modeling, we have revealed the instability of a steady-state combustion regime which was previously obtained using analytical methods in the substance-combustion model with surface vaporization and with exothermic reactions in the condensed phase which are intense enough to form a maximum of temperature underneath the surface. The instability has been studied analytically using the method of small perturbations to eliminate the version of its nonphysical (numerical) character. Steady-state combustion regimes with maxima on the condensed-phase temperature profile are shown to be actually unsteady. It is suggested that convection in a liquid-subsurface layer owing to bubble motion caused by the Marangoni effect should be taken into account to describe correctly the experimentally observed steady-state regimes with a leading role of the condensed phase. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 1, pp. 43–50, January–February, 1997.     

Generalized analytical solution for compressive forces in adhesively-bonded-joint assembling

Normal forces exerted by the adhesive to the substrate during the squeeze flow occurring in compaction of bonded joints are analyzed using theoretical, numerical and experimental techniques. An analytical solution, derived from the squeeze-flow theory of a viscoplastic material, is generalized to be valid for any initial shape of the adhesive cord. The rheology of the material is modeled according to the Herschel–Bulkley model and is fitted with experimental data available from the characterization of an epoxy-based adhesive. The analytical law is compared with a numerical model, where the two-phase problem for the squeeze-flow test is solved by finite-volume methods using a commercial CFD solver. The results obtained with these two approaches show excellent agreement with experimental forces obtained for a wedge-shaped specimen. The proposed methodology can therefore be useful for the optimization of the bond lines in assembling processes.     

Viscoelastic Analysis of an Adhesive Tubular Joint

The investigations so far available with regard to stress analysis of adhesive joints assume that the adhesive is elastic. In the present analysis the time dependent properties of the adhesive are taken into account by assuming that the adhesive is viscoelastic. The viscoelastic analysis of a tubular joint has been attempted using a prony series fitting for the relaxation modulus of two adhesives. The long term redistribution of the stresses in the adhesive is evaluated using the finite element method.

Viscoelastic analysis of an adhesive tubular joint has been performed for the first time, using the finite element method with a prony series fitting for the relaxation modulus of the adhesive. For a typical epoxy it has been found that not only the elastic stresses are different at different levels but also the viscoelastic response shows considerable variation from one level to another. As large a reduction as 57 % is noticed in the normal stress and an even larger reduction of 62% is noticed in the shear stress over three decades of time.

The technique used in this report for the solution of the long term response can be employed for a study of the short-term response also.     

Predicting the Thermal Behaviour of Wood During Linear Welding Using the Finite Element Method

A numerical model to simulate the temperature behaviour of wood welding samples during the welding process was developed to understand the influence of material parameters on the welding temperature. A finite element method and the CAST3M software were used to simulate and model the temperature changes during welding of beech wood. This model takes into account the different properties of the wood welded bondline, the geometry of the sample and the external conditions. The energy produced by the friction welding of the wood samples was determined from infrared thermography measurements for the welding process and inputted into the model. The comparison between the predicted and experimental results shows that the model is reliable. The applied pressure, the vibration, the extrusion of material and the chemical reactions, particularly exothermic reactions, are not taken into account in this model and thus probably explain the differences existing between actual and simulated values. However, this numerical simulation gives information on the distribution of the temperature in the sample. The model predicted that the temperature difference between the centre and the side of the sample is not higher than 4°C. This means that the border effects are negligible. The model was tested for different welding times. According to the model a heat flow about 70 kW/m² is necessary at the welding line to ensure a satisfactory bonding for the chosen sample geometry. Welding of large wood pieces has also been simulated in this study.     

Radial liquid velocity distribution in an external-loop airlift column with a tapered riser

Radial and axial liquid velocity distributions in the tapered riser were investigated theoretically and experimentally. The liquid velocity distributions were computed by solving the Navier-Stokes equation numerically based on a modified mixing-length theory. Both radial and axial components of liquid velocity were taken into account. As a result, we found that the radial velocity component was much smaller than the axial velocity component. For a cylindrical column, which means no tapered section, a simplified solution was obtained. The simplified solution was found to agree well with the rigorous numerical solution even in the tapered riser. To confirm the validity of the present hydrodynamic model, the velocity distributions in the tapered riser were measured by an electric probe method using KCl solution as a tracer. The measured velocity distributions agreed with the computed ones, except in the vicinity of the bottom of the tapered riser at high gas flow rates.     

Characterization and modelling of the viscous behaviour of adhesives using the modified Arcan device

The non-linear behaviour of an epoxy adhesive has been studied thanks to the use of a modified Arcan test device. Multilevel creep tests have been performed in order to characterize the time-dependent behaviour. A visco-elastic model has been chosen to describe the behaviour of the adhesive. This model is based on the decomposition of the visco-elastic strain into elementary viscous mechanisms. The non-linear behaviour of the visco-elasticity is taken into account thanks to a non-linear function which depends on the stress. A specific identification procedure based on a single test has been proposed. Finally, the model has been validated using tests that have not been used for the identification. The model proposed in this paper enables the time-dependent non-linear behaviour of adhesives to be reproduced in a correct manner.