Variation Of Thermomechanical Properties Research Articles

Effects of Chemical Composition and Cross-Linking Degree on the Thermo-Mechanical Properties of Bio-Based Thermosetting Resins: A Molecular Dynamics Simulation Study.

Bio-based epoxy resins have received significant attention in terms of concerns regarding carbon emission. Epoxidized soybean oil (ESO) derived from sustainable feedstock has been widely used to blend with traditional diglycidyl ether of bisphenol-A (DGEBA) to replace some of the petroleum-based components. In this work, molecular dynamics (MD) simulations were applied to track the network formation and predict the performance of methyl hexahydrophthalic anhydride (MHHPA)-cured ESO/DGEBA blend systems. The effects of ESO content and cross-linking degree on the mass density, volumetric shrinkage, glass transition temperature (Tg), coefficient of thermal expansion (CTE), Young's modulus, yield strength, and Poisson's ratio of the epoxy resin were systematically investigated. The results show that systems with high ESO content achieve gelation at low cross-linking degree. The Tg value, Young's modulus, and yield strength increase with the increase in cross-linking degree, but the CTE at the glassy state and Poisson's ratio decrease. The comparison results between the simulated and experimental data demonstrated that the MD simulations can accurately predict the thermal and mechanical properties of ESO-based thermosets. This study gains insight into the variation in thermo-mechanical properties of anhydride-cured ESO/DGEBA-based epoxy resins during the cross-linking process and provides a rational strategy for optimizing bio-based epoxy resins.

Fiber Orientation and Concentration in an Injection-Molded Ethylene-Propylene Copolymer Reinforced by Hemp.

This paper characterizes and analyzes the microstructures of injection-molded polypropylene parts reinforced with 20 wt% of hemp fibers in order to understand the process induced variations in thermomechanical properties. In-thickness fiber orientation and fiber content were determined by X-ray tomography along the flow. The fiber content along the flow path was also determined by direct fiber content measurements after matrix dissolution, showing an increase of 2%/100 mm for a 2.2 mm-thick plate due to fiber migration during the filling stage. A typical shell/core structure for fiber orientation in injection molding was observed, but with a very clear transition between the layer solidified under high shear rates and the core in which the fiber content was reduced by more than 50%. The orientation of hemp fibers is lower than the one of glass fibers, especially in thickness direction. However, the overall fiber orientation in the injection direction induces significant anisotropic thermomechanical properties, which cannot be explained by simple micromechanical models that consider isotropic mechanical properties for hemp fibers. These phenomena must be taken into account in process simulation codes for injection molding to better predict thermomechanical properties as well as part shrinkage and warpage to design molds.

Minimise thermo-mechanical batch variations when processing medical grade lactide based copolymers in additive manufacturing

Additive manufacturing is suitable for producing complex geometries; however, variation in thermo-mechanical properties is observed during one batch cycle when degradable aliphatic polyesters are used in melt extrusion-based methods. This is one important reason for why additive manufacturing has not yet been fully utilised to produce degradable medical implants. Herein, the internal variation has been minimised during one batch cycle by assessing the effect of different processing parameters when using commercially available medical grade copolymers.To minimise the molar mass, thermal and mechanical variation within one batch cycle, the rheological fingerprint of the commercially available medical grade poly(L-lactide-co-ε- caprolactone) and poly(L-lactide-co-trimethylene carbonate) has been correlated to the process parameters of the ARBURG Plastic Freeforming. An increase in the temperature up to 220°C and the associated increase in pressure are beneficial for the viscoelastic and thermally stable poly(L-lactide-co-ε-caprolactone). In contrast, a temperature below 220°C should be used for the poly(L-lactide-co-trimethylene carbonate) to reduce the variation in strain at break during one batch cycle. The residence time is decreased through the increase of the discharge parameter. An increase in temperature is however required to reduce the viscosity of the polymer and allow the pressure to stay within the machine limitations at higher discharge parameters.The results are highly relevant to the development of additive manufacturing for the production of degradable medical devices with identical properties. In fact, Food and Drug Administration guidelines for additive manufacturing of medical implants specify the need to control changes in for example material properties during the process.

Multiscale structural characterization of biocompatible poly(trimethylene carbonate) networks photo-cross-linked in a solvent

Poly(trimethylene carbonate) (PTMC) polymeric networks are biocompatible materials with potential biomedical applications. By combining Dynamic Mechanical Analysis (DMA), Solid State Nuclear Magnetic Resonance (NMR), and tensile testing, it was possible to fully characterize the inner structure and its relationship with the macroscopic properties of photo-crosslinked PTMC materials in a solvent medium. PTMC prepared from macromer with various molecular weights (3 kg/mol, 18 kg/mol, and 32 kg/mol) and with various polymer concentrations within the reactive media were analyzed, with the variation of thermomechanical properties and NMR signal decay characterized as a function of both aforementioned synthesis parameters. DMA and solid state Double Quantum (DQ) 1H NMR investigations demonstrated that the network crosslink density is directly related to the macromer molar mass and polymer concentration. More interestingly, tensile tests confirmed that mechanical behavior depended on the materials’ inner structure, notably their crosslink density. Specifically for the 18 kg/mol PTMC networks, dangling and free chains reinforced the network, exemplified by higher Young’s modulus E values. This multiscale investigation provides a promising and precise approach to tailor the macroscopic behavior of PTMC materials, by controlling their specific inner network structural morphologies during synthesis.

Influence of phase composition on thermal aging behavior of plasma sprayed ytterbium silicate coatings

In this study, ytterbium silicate coatings with different phase compositions were designed by controlling Yb2O3/SiO2 ratio and fabricated by atmospheric plasma spray (APS) technique. Thermal aging behaviors such as microstructure evolution and thermomechanical properties were focused, and the influence of phase composition was analyzed. Results showed that obvious grain growth and defect reduction occurred, and the thermomechanical properties were significantly modified after thermal aging. The larger reduction in defects and greater increase in elastic modulus, Vickers hardness and coefficient of thermal expansion took place in the Yb2O3-rich coatings containing Yb2O3 and Yb2SiO5 phases. The bigger grains and transgranular cracks were formed and the greater increase in thermal conductivity took place in the SiO2-rich coatings containing Yb2SiO5 and Yb2Si2O7 phases. The relationship among microstructure evolution and variation of thermomechanical properties was analyzed. The results of this study may provide a reference for design and application of rare-earth silicates as environmental barrier coatings (EBCs).

Modes of basin (de)formation, lithospheric strength and vertical motions in the Pannonian-Carpathian system: inferences from thermo-mechanical modelling

AbstractAfter a rapid multiphase evolution and a transition from passive to active rifting during late Early Miocene to Pliocene times, the Pannonian Basin has been subjected to compressional stresses leading to gradual basin inversion during Quaternary times. Stress modelling demonstrates the significance of the interaction of external plate-boundary forces and the effect of gravitational stresses caused by continental topography and crustal thickness variation. Flexural modelling and fission-track studies have elucidated the complex interplay of flexural downloading during collision, followed by rapid unroofing by unflexing and isostatic rebound of the lithosphere. The stretching and subsidence history of the Pannonian Basin, the temporal and spatial evolution of the flexure of the Carpathian lithosphere, and the lithospheric strength of the region reflect a complex history of this segment of the Eurasia-Africa collision zone. The polyphase evolution of the Pannonian-Carpathian system has resulted in strong lateral and temporal variation in thermo-mechanical properties in the area. Modelling results suggest that, as a whole, the Pannonian Basin has been an area of pronounced lithospheric weakness since Cretaceous time, shedding light on the high degree of strain localization in this region. This basin, the hottest in continental Europe, has a lithosphere of extremely low rigidity, making it prone to multiple tectonic reactivations. Another feature is the noticeable absence of lithospheric strength in the mantle lithosphere of the Pannonian Basin. Modelling studies suggest pronounced lateral variations in lithospheric strength along the Carpathians and their foreland, which have influenced the thrust load kinematics and post-collisional tectonic history. The inferences and models discussed in this paper are constrained by a large geophysical database, including seismic profiles, gravity and heat-flow data.

Thermal expansion coefficient of polycrystalline silicon and silicon dioxide thin films at high temperatures

The rapid growth of microelectromechanical systems (MEMS) industry has introduced a need for the characterization of thin film properties at all temperatures encountered during fabrication and application of the devices. A technique was developed to use MEMS test structures for the determination of the difference in thermal expansion coefficients (α) between poly-Si and SiO2 thin films at high temperatures. The test structure consists of multilayered cantilever beams, fabricated using standard photolithography techniques. An apparatus was developed to measure the thermally induced curvature of beams at high temperatures using imaging techniques. The curvatures measured were compared to the numerical model for multilayered beam curvature. The model accounts for the variation in thermomechanical properties with temperature. The beams were designed so that the values of Young’s moduli had negligible effect on beam curvature; therefore, values from literature were used for ESi and ESiO2 without introducing significant error in curvature analysis. Applying this approximation, the difference in thermal expansion coefficients between αSi and αSiO2 was found to increase from 2.9×10−6 to 5.8×10−6 °C−1 between room temperature and 900 °C. These results suggest that the α for poly-Si thin films may be significantly higher than values for bulk, crystalline Si.

Measurement of thermal expansion coefficients of composites using strain gages

The measurement of the coefficients of thermal expansion (CTEs) of composite materials using electrical resistance strain gages is addressed. Analytical expressions for the CTEs of an orthotropic lamina are derived, accounting for the effects of transverse sensitivity and possible misalignment of the gages. Experiments are performed for the characterization of the thermal expansion behavior of a fiber-glass-reinforced epoxy unidirectional lamina using an invar specimen as reference material. Preliminary training cycles are performed for the determination of an optimal heating rate for the measurements, which ensures thermal equilibrium conditions. Three measurement cycles yield the principal CTEs of the lamina α1, α2 and α12 with repeatability within ±0.34×10−6, ±0.85×10−6 and ±2.8×10−6/°C, respectively. It is noted that inhomogeneity of the specimen and variation in thermomechanical properties of the gages can cause a noticeable spead in the measurements.

Analysis of Residual Stresses and Metallurgical Phase Changes in Centrifugally Cast Bi-Metal Grinding Rolls

A mathematical model was developed for computing the metallurgical phase transformations and residual stresses in centrifugal casting of bi-metal grinding rolls used for coal pulverizing. The model considers variation of thermo-mechanical properties of material with temperature, effects of phase changes and metallurgical phase transformations. Rolls idealized as having infinite cylindrical geometry and the actual finite conical geometry are analysed for cases of air and water jet cooling of the centrifuging die. As expected an increase in the martensitic fraction is obtained on the grinding surface due to water jet cooling. The study indicates that the large size rolls analysed herein have higher martensitic fraction as compared to small size roll. Simplified analysis model is developed for usage in parametric studies involving various sizes for initial sizing of the rolls.

Lithosphere dynamics and tectono-stratigraphic evolution of the Mesozoic Betic rifted margin (southeastern Spain)

Quantitative subsidence analysis demonstrates that Mesozoic sediments in the eastern Betic Cordilleras have been deposited on a thermally subsiding, rifted margin. The dynamics of rifting are related to regional trans-tension in the Atlantic realm. Modelling of this phase of rifting at the southern Iberian margin shows that the associated stretching of the lithosphere significantly altered its geometrical and mechanical structure and exerted a major control on subsequent convergence tectonics. The Mesozoic stretching led to the formation of a ramp at the margin due to strong lateral variations in the effective elastic thickness (EET) of the lithosphere and the thickness of the crust, in a direction perpendicular to the margin. The existence of a ramp structure in the Late Mesozoic at the Iberian margin, which is also supported by stratigraphic and structural data, is important for the style of subsequent deformation of the lithosphere in Late Cretaceous-Tertiary times. The lateral variations in EET inferred from forward modelling of rifted margin stratigraphy could explain the observed multiple collapse of the Betic margin and the partitioning of deformation between the converging units involved in the Betic orogeny. Modelling of depth-dependent palaeo-rheological profiles of the margin shows that the mechanical properties of the lithosphere after stretching differed distinctly from those before stretching. Lithospheric deformation in the Betic Cordilleras during the Tertiary orogeny seems, therefore, to be largely controlled by the previous rifting phase. A comparison of the palaeo and present-day rheologies in terms of estimates for EET during Late Cretaceous and present times indicates that the mechanical properties of the lithosphère underlying the Betic orogen were also affected by rifting during Late Oligocene-Aquitanian times. This phase of rifting is coeval with extension in the Valencia trough, the Gulf of Lions and the West European rift system, which supports a causal relation with regional lithospheric processes. The lateral and spatial variations in thermo-mechanical properties of the lithosphere underlying the Betics, therefore, reflect the dynamics of lithospheric extension on a larger scale.