Dynamic Mechanical Analysis Apparatus Research Articles

Shape memory thin films of Polyurethane: Does graphene content affect the recovery behavior of Polyurethane nanocomposites?

AbstractThin nanocomposite films of polyurethane have received remarkable attention due to their shape memory properties. As most of the reports focus on the beneficial aspects of the presence of nanofillers such as graphene nanoplatelets (GNPs) introduced into shape memory polymers, some research results reveal the opposite trend. The polyether/polyester‐based polyurethane was synthesized through a condensation polymerization and the nanocomposite films containing 0.1, 0.25, 0.5, and 1 wt.% of GNPs were prepared by the solvent casting method. The Fourier transformed infrared (FTIR) spectroscopy was carried out to investigate the chemical structure of nanocomposite films. The properties of nanocomposite films were verified by the differential scanning calorimetry (DSC), X‐ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), tensile test, and the dynamic mechanical analysis (DMA). The shape memory properties were studied through various thermomechanical cycles applied by the DMA apparatus. All the nanocomposite films showed larger glassy storage moduli compared with those of the neat PU films. The nanocomposite film containing 0.25 wt.% GNPs showed the highest shape recovery ratio (92.4%) since the net‐point effect of GNPs dominates its slipping effect. While for the other nanocomposite films the slippage effect overcomes the net‐point effect.

Viscoelastic behavior of an epoxy resin during cure below the glass transition temperature: Characterization and modeling

This paper presents a viscoelastic temperature- and degree-of-cure-dependent constitutive model for an epoxy resin. Multi-temperature relaxation tests on fully and partially cured rectangular epoxy specimens were conducted in a dynamic mechanical analysis apparatus with a three-point bending clamp. Master curves were constructed from the relaxation test results based on the time–temperature superposition hypothesis. The influence of the degree of cure was included through the cure-dependent glass transition temperature which was used as reference temperature for the shift factors. The model parameters were optimized by minimization of the differences between the model predictions and the experimental data. The model predictions were successfully validated against an independent creep-like strain history over which the temperature varied.

On the stability efficiency of anchorage self-tapping screws: Ex vivo experiments on miniscrew implants used in orthodontics.

The clinical success of orthodontic miniscrews is strictly related to primary stability, which depends on bone viscoelastic properties too. In this study, we evaluated the short time mechanical response of native bone to miniscrews, by a laboratory test based on dynamic loading. Thirty-six segments of porcine ribs were first scanned by cone-beam computerized tomography to obtain insertion-site cortical thickness, cortical and marrow bone density. Twelve different types of miniscrews were implanted in the bone samples to evaluate the elastic compliance of the implants in response to a point force applied at the screw head normally to the screw axis. The compliance was measured dynamically in a Dynamic Mechanical Analysis apparatus as the Fourier Response Function between the signals of displacement and force. The measurements were repeated in five days successive to the insertion of the miniscrew. The elastic compliance was positively related to observation timepoints, but it was not related neither to the screw type nor to the value of the insertion torque. Stability behavior is significantly related to the short time response of native bone rather than to the screw design or the insertion torque values.

Influence of Building Parameters on the Dynamic Mechanical Properties of Polycarbonate Fused Deposition Modeling Parts

Abstract Fused deposition modeling (FDM) is one of the most important additive manufacturing technologies nowadays. However, there is a need to get more insight in the relationship between the process parameters and the final performance. Several studies have already identified some of these relationships, considering only the mechanical behavior of uniaxial tensile specimen under static loading. Yet, FDM technology is also designed to produce final parts that might be used in machinery or transportation applications. In such cases, dynamic loading is the most common situation and should be considered. The present article focuses on understanding the influence of three process parameters (nozzle diameter, number of contours, and raster-to-raster air gap) on the mechanical behavior under dynamic loading at specified conditions. A dynamic mechanical analysis apparatus has been used to characterize the polycarbonate mechanical behavior. On the other hand, a Taguchi approach and an analysis of variance have b...

Effect of post-cure on the glass transition temperature and mechanical properties of epoxy adhesives

The effects of post-curing and cure temperature on the glass transition temperature, T g, and the mechanical properties of epoxy adhesives were studied. T g was measured by a dynamic mechanical analysis apparatus developed in-house and the mechanical properties of the adhesives (yield strength, Young’s modulus and failure strain) were measured by a tensile machine. The relationships between T g and mechanical performance under various post-cure conditions were investigated. The curing process was the same for all tests, consisting of an initial stage performed at different temperatures followed by cooling at room temperature. Three sets of specimens were considered, sharing the same initial cure process, but with a different post-curing procedure. In the first set, the specimens were only subjected to a curing process; in the second set, the specimens were subjected to a curing process followed by a post-cure performed at a temperature below the T g of the fully cured network, T g∞; and in the third set, the specimens were subjected to a curing process followed by a post-cure performed at a temperature above the T g∞. When post-cured at a temperature above T g∞, the mechanical and physical properties tend to have a constant value for any cure temperature.

Study on the Mechanism of the Soybean Adhesive Improved by Micro/Nano Fibrils and the Bond Strength of the Poplar Plywood with it

The micro/nano fibrils was used to modify the soybean glue, which was used as the adhesive of poplar plywood. The poplar plywood bond strength were evaluated. To explain the improvement in mechanism, Differential Scanning Calorimetry(DSC) and Dynamic Mechanical Analysis apparatus(DMA) were applied to analyze the influence of poplar macro/nano fibrils on soybean glue. The test of bonding strength results show that the bond strength of poplar plywood was enhanced as micro/nano poplar fibrils was added. And the DMA figures of soybean glue illustrate that poplar macro/nano fibrils has improved the deformation resistence of soybean glue plywood and the heat resistance of soybean glue.

Is Dynamic Mechanical Analysis (DMA) a non-resonance technique?

Dynamic Mechanical Analysis is an experimental technique commonly used to study the viscoelastic behaviour of materials over a range of temperatures and frequencies. Although considered as a forced non-resonance technique, the warned users perfectly know that resonant frequencies are inevitably encountered on the typical frequency range of DMA apparatus. In consequences, the measured properties in the area of the resonant frequencies are completely perturbed. This work proposes an analysis of the dynamic behaviour of a Bose Electroforce DMA. The resonant and perturbed frequencies have been identified in different experimental configurations using several materials. A straightforward analytical model was developed to simulate and predict the dynamic behaviour of our apparatus. The model parameters are identified using different experimental configurations. The simulations delivered by the model allow the user to predict the perturbed frequency areas and by this way authorise a strategic definition of the experimental protocol according to the sample characteristics.

Influence of load and elastic properties on the rolling and sliding friction of lubricated compliant contacts

Lubricated “soft” contacts, where one or both contacting solids have a low elastic modulus, are present in many practical engineering and biological applications including windscreen wipers, wet tyres, elastomeric seals, contact lenses and the tongue/palate system. In such contacts, the prevailing lubrication mode is “isoviscous EHL” (elastohydrodynamic lubrication). Unlike in steel–steel contacts, rolling friction can be considerable and this originates in part from the viscoelastic properties of the compliant surfaces.In this paper the influence on friction of both applied load and the elastic properties of the solids is studied using a mini traction machine. In this machine, the rolling and sliding friction can be separately determined. The viscoelastic properties of the polymers employed are measured using a dynamic mechanical analysis apparatus. The measured friction is compared to theoretical models for soft EHL and the viscoelastic energy losses arising from the contact deformation. Consideration of the frequency dependence of the substrate viscoelasticity enables reasonably accurate predictions of the rolling friction coefficient, especially within the mixed and boundary lubrication regimes.

Experimental Studies on the Loss Factor of Alternately Laminated Anisotropic Damped Thin Plates

Optimization designs on the parameters of seven-layers of alternately laminated thin plate structures have been carried out through two orthogonal experiments and a dynamic mechanical analysis apparatus (DMA Q800), including the variations of damping material species, the fiber orientation angle in constrained layers and the thickness of the damping layer. The effects of these parameters on the loss factor of the structures have been investigated. It may provide some experimental basis for new anisotropic design system of the alternately laminated damped structures. Firstly, the experimental results show that the second and the sixth damping layers are respectively the best-influencing layers on the frequency range and the temperature range of the loss factor. As for the second damping layer, the greater the thickness, the better the frequency characteristics of internal friction. Secondly, the effects of the second outer constrained layer on the loss factor are the highest, and the structural damping characteristics are best when the orientation angle is close to 90. Finally, temperature and frequency characteristics of the loss factor can be optimized with different damped materials in different damping layers.

Molecular mobility in a thermoset as seen by TSR and DMA near Tg

The conformational mobility of a polyester thermoset in the glass transition region is studied by dynamic mechanical analysis (DMA) and thermally stimulated recovery (TSR). All experiments were carried out in a commercial DMA apparatus, in the flexural mode. With this equipment thermally stimulated creep (TSCr) and thermal sampling (TS) experiments were also performed. Each TS curve was analysed according to a simple Voigt-Kelvin model where the retardation time follows the Arrhenius equation. Both the activation energy and the pre-exponential factor show the usual trends with temperature and present the well-known compensation phenomenon. The activation energy increases with temperature in the glassy state and even for lower temperatures the isolated processes are found to be highly cooperative. Two master curves for the storage modulus and the storage compliance were successfully constructed from the isothermal DMA experiments. The Vogel-Fulcher-Tamman-Hesse curve plot was directly obtained from the WLF parameters that were calculated without any non-linear fitting. The complementary character of the TSR and DMA techniques can be seen in an Arrhenius plot. The equivalent frequency of the TSR elementary processes increases with increasing temperature, with values around ∼10-2.8 Hz. Some results obtained by simulation were shown, allowing to conclude that this increase of frequency is a consequence of the increase of the activation energy with temperature in a system showing a compensation behaviour.