Thermo-mechanical Environments Research Articles

Large-scale assembly of isotropic nanofiber aerogels based on columnar-equiaxed crystal transition

Ice-templating technology holds great potential to construct industrial porous materials from nanometers to the macroscopic scale for tailoring thermal, electronic, or acoustic transport. Herein, we describe a general ice-templating technology through freezing the material on a rotating cryogenic drum surface, crushing it, and then re-casting the nanofiber slurry. Through decoupling the ice nucleation and growth processes, we achieved the columnar-equiaxed crystal transition in the freezing procedure. The highly random stacking and integrating of equiaxed ice crystals can organize nanofibers into thousands of repeating microscale units with a tortuous channel topology. Owing to the spatially well-defined isotropic structure, the obtained Al2O3·SiO2 nanofiber aerogels exhibit ultralow thermal conductivity, superelasticity, good damage tolerance, and fatigue resistance. These features, together with their natural stability up to 1200 °C, make them highly robust for thermal insulation under extreme thermomechanical environments. Cascading thermal runaway propagation in a high-capacity lithium-ion battery module consisting of LiNi0.8Co0.1Mn0.1O2 cathode, with ultrahigh thermal shock power of 215 kW, can be completely prevented by a thin nanofiber aerogel layer. These findings not only establish a general production route for nanomaterial assemblies that is conventionally challenging, but also demonstrate a high-energy-density battery module configuration with a high safety standard that is critical for practical applications.

Improving the Heat and Ablation Resistance of Silicone Rubber Composites by Incorporating Hollow Microspheres.

For thermal protection materials (TPMs) which are used to protect space vehicles from extreme thermomechanical environments, the thermal conductivity of the original material and the char layer that has formed during ablation plays a significant role in determining the ablation performance. In order to investigate this, in this study, we introduced glass hollow microspheres (GHMs), phenolic hollow microspheres (PHMs), and acrylonitrile-methyl methacrylate copolymer hollow microspheres (AMHMs) into silicone rubber (SR), and the ablation performance of these composites was systematically studied. The thermogravimetric results showed that the residue yield of the SR composites was increased with the incorporation of the hollow microspheres. Compared to the SR composites without the hollow microspheres, the residue weight values under 800 °C (R800) of the SR composites with the 30 parts of fumed silica per hundred of the SR (phr) addition of GHMs, PHMs, and AMHMs were increased from 10.11% to 21.70%, 18.31%, and 20.83%, respectively. The ablation tests showed that the addition of the AMHMs enhanced the ablation performance of the SR composites because the linear ablation rates and the backplane temperature were clearly decreased when compared to the SR composites without the hollow microspheres. This work provides an effective and potential method for preparing thermal protection materials with an improved ablation performance.

Static, stability and dynamic characteristics of asymmetric bi-directional functionally graded sandwich tapered elastic arches in thermo-mechanical environments

This research paper investigates the in-plane static, stability and dynamic characteristics of tapered Timoshenko bi-directional functionally graded (BDFG) sandwich curved elastic arches (symmetric and asymmetric) in thermo-mechanical environments using the differential quadrature element method (DQEM). Physical properties such as Young's modulus, the mass density, Poisson's ratio and the thermal expansion coefficient are defined to vary both with respect to temperature and position while shear deformation, axial deformation, and rotary inertia effects are incorporated in the analysis. Different types of arches namely circular, parabolic, catenary, elliptic and sinusoidal models are modelled by having non-uniform cross-section and the BDFG properties. General functions are used to vary the physical properties through the thickness and length of the curved. Incorporating the influence of having a thermal environment for simplified models, the current methodology for studying such structures is verified by comparing the results with previously published literature and finite element software simulations. The effect of the temperature rise, physical properties variation and geometrical parameters (such as the ratio of thickness to length, the opening angle and non-uniform cross-section) on the static deformation, buckling load and free vibration characteristics of BDFG arches with metal/ceramics in the inner surface and ceramics/metal in the outer surface of the arch is investigated. In addition, the mechanical behaviour of sandwich arches for various combinations such as homogeneous or FG for outer skins and the inner core is presented under thermo-mechanical loadings.

Accelerated design of architectured ceramics with tunable thermal resistance via a hybrid machine learning and finite element approach

Topologically interlocked architectures can transform brittle ceramics into tougher materials, while making the material design procedure a cumbersome task since modeling the whole architectural design space is not efficient and, to a degree, is not viable. We propose an approach to design architectured ceramics using machine learning (ML), trained by finite element analysis data and together with a self-learning algorithm, to discover high-performance architectured ceramics in thermomechanical environments. First, topologically interlocked panels are parametrically generated. Then, a limited number of designed architectured ceramics subjected to a thermal load is studied. Finally, the multilinear perceptron is employed to train the ML model in order to predict the thermomechanical performance of architectured panels with varied interlocking angles and number of blocks. The developed feed-forward artificial neural network framework can boost the architectured ceramic design efficiency and open up new avenues for controllability of the functionality for various high-temperature applications. This study demonstrates that the architectured ceramic panels with the ML-assisted engineered patterns show improvement up to 30% in frictional energy dissipation and 7% in the sliding distance of the tiles and 80% reduction in the strain energy, leading to a higher safety factor and the structural failure delay compared to the plain ceramics.

Quantifying thermal barrier coating delamination through luminescence modeling

Thermal barrier coatings (TBCs) are widely used in the hot sections of gas turbine systems as they are remarkably efficient in insulating the underlying superalloys, leading to higher operating temperatures and therefore enhancing the performance of the engines. However, this benefit is only possible if the integrity of the TBC under aggressive thermo-mechanical environments is ensured. Delamination is a common but hard to detect failure mode. We present, in this work, supported by experimentation, the implementation of a modeling approach applying the Kubelka-Munk theory to provide numerical quantification of luminescence contrast and intensity due to top coat delamination in TBCs. The method relies on the drastic change in reflectivity when a delamination forms, exploiting it for high-contrast luminescence mapping. Two distinct TBC configurations containing an erbium-doped yttria-stabilized zirconia (YSZ:Er3+) layer for delamination sensing were used to validate this model. A delamination zone induced by Rockwell indentation was successfully tracked by measuring an increase of the intensity of the erbium emission line at 562 nm. Luminescence-based methods for delamination detection can provide a revolutionary non-invasive technique, with potential for both off-line and on-line engine monitoring.

Numerical study of hyperstoichiometric fuel creep (UO2+x) in fuel clad interaction of WWER1000

Nuclear fuel is faced with extreme thermomechanical environments during its operating time. In this period, fuel mechanical response is deeply influenced by inherent heterogeneous microstructures, dependent on stress and temperature. Under the reactions occurring inside the reactor, the fuel crystal will change, and the UO2 will be UO2±x. Creep as a slow deformation process, strongly affected by this irradiation. Indeed, the hyperstoichiometric crystals effect the creep rate and mechanical stresses. In this article, the thermomechanical behavior of WWER1000 fuel pellets considering the hyperstoichiometric fuel (UO2+x) is studied. Fuel swelling, densification, thermal stress and creep are numerically considered and pellet clad interactions (PCIs) are evaluated. For validating the model, the results for pellet clad interactions are compared with experimental data. The hyperstoichiometric study of fuel cladding mechanical interaction in WWER1000 shows advancement of mechanical interaction during fuel burnups. It is found while the fuel and clad interaction occurs in 50.8 MWd/kgU for UO2, by increasing the amount of x and hyperstoichiometric fuel in the pellet, the pellet clad interactions will occur in lower burnups. These advancements for several hyperstoichiometric fuels are calculated and results are discussed.

Evolution of Thermal Microcracking in Refractory ZrO₂-SiO₂ after Application of External Loads at High Temperatures.

Zirconia-based cast refractories are widely used for glass furnace applications. Since they have to withstand harsh chemical as well as thermo-mechanical environments, internal stresses and microcracking are often present in such materials under operating conditions (sometimes in excess of 1700 °C). We studied the evolution of thermal (CTE) and mechanical (Young’s modulus) properties as a function of temperature in a fused-cast refractory containing 94 wt.% of monoclinic ZrO2 and 6 wt.% of a silicate glassy phase. With the aid of X-ray refraction techniques (yielding the internal specific surface in materials), we also monitored the evolution of microcracking as a function of thermal cycles (crossing the martensitic phase transformation around 1000 °C) under externally applied stress. We found that external compressive stress leads to a strong decrease of the internal surface per unit volume, but a tensile load has a similar (though not so strong) effect. In agreement with existing literature on β-eucryptite microcracked ceramics, we could explain these phenomena by microcrack closure in the load direction in the compression case, and by microcrack propagation (rather than microcrack nucleation) under tensile conditions.

Lightweight, flaw-tolerant, and ultrastrong nanoarchitected carbon

It has been a long-standing challenge in modern material design to create low-density, lightweight materials that are simultaneously robust against defects and can withstand extreme thermomechanical environments, as these properties are often mutually exclusive: The lower the density, the weaker and more fragile the material. Here, we develop a process to create nanoarchitected carbon that can attain specific strength (strength-to-density ratio) up to one to three orders of magnitude above that of existing micro- and nanoarchitected materials. We use two-photon lithography followed by pyrolysis in a vacuum at 900 °C to fabricate pyrolytic carbon in two topologies, octet- and iso-truss, with unit-cell dimensions of ∼2 μm, beam diameters between 261 nm and 679 nm, and densities of 0.24 to 1.0 g/cm3 Experiments and simulations demonstrate that for densities higher than 0.95 g/cm3 the nanolattices become insensitive to fabrication-induced defects, allowing them to attain nearly theoretical strength of the constituent material. The combination of high specific strength, low density, and extensive deformability before failure lends such nanoarchitected carbon to being a particularly promising candidate for applications under harsh thermomechanical environments.

Hazard Assessments of Energetic Systems, A Field Still in Development

Hazard Assessments of Energetic Systems, A Field Still in Development

A dislocation-based crystal viscoplasticity model with application to micro-engineered plasma-facing materials

Materials developed with special surface architecture are shown here to be more resilient to the transient thermomechanical environments imposed by intermittent exposures to high heat flux thermal loading typical of long-pulse plasma transients. In an accompanying article, we present experimental results that show the relaxation of residual thermal stresses in micro-engineered W surfaces. A dislocation-based model is extended here within the framework of large deformation crystal plasticity. The model is applied to the deformation of single crystals, polycrystals, and micro-engineered surfaces composed of a uniform density of micro-pillars. The model is utilized to design tapered surface micro-pillar architecture, composed of a Re core and W coatings. Residual stresses generated by cyclic thermomechanical loading of these architectures show that the surface can be in a compressive stress state, following a short shakedown plasma exposure, thus mitigating surface fracture.

Viscoplastic characterization and modeling of hybrid carbon fiber/carbon nanotubes reinforced composites

Abstract This investigation illuminates the effect of surface grown carbon nanotubes (CNTs) on the viscoplastic behavior of fiber reinforced plastics (FRPs). The study demonstrates the feasibility of growing CNTs on conventional woven carbon fibers without degrading the fibers structural properties. The subsequent hierarchal reinforcement was utilized to fabricate laminated FRPs. The viscoplastic behaviors of the hybrid composites were investigated through series of stress relaxation and creep tests at different thermomechanical environments. The results of the experimental tests were utilized to both construct and validate a phenomenological viscoplastic constitutive model of the hybrid FRPs. The experiments and the model predictions established that surface grown CNTs could enhance the composite resistance to viscoplastic deformation.

Nanocharacterization of the negative stiffness of ferroelectric materials

Phase changing materials such as ferroelectric materials could exhibit negative stiffness under certain thermomechanical environments. This negative stiffness is embodied by a deflection along the opposite direction of the applied load. So far negative stiffness materials were investigated with the specific morphology of embedded inclusions in stiff matrices then the resulting composite is studied to measure the behavior of each constituent indirectly. In this study, a modified nonisothermal nanoindentation method is developed to measure the negative stiffness of triglycine sulfate single crystal directly. This in-situ method is intended to first demonstrate the feasibility of detecting the negative stiffness via nanoindentation and nanocreep of a ferroelectric material at its Curie point and then to quantify the negative stiffness without the need for embedding the crystal within a stiffer matrix.

Effect of Temperature-Dependent Material Properties on Nonlinear Flexural Response and Thermal Postbuckling of Shear Flexible FGM Beams: A Study Using FEM

The effect of temperature-dependent material properties on the geometric nonlinear flexural response and thermal postbuckling behavior of shear flexible Functionally Graded Material (FGM) beams is investigated under various thermal and thermo-mechanical environments. The important aspects of the thermal and thermo-mechanical bending and thermal post-buckling of FGM beams are studied. The temperature variation across the thickness is obtained analytically and the finite element method (FEM) is used to predict the transverse deflections and stresses in the flexural analysis and the load-deflection paths for the thermal postbuckling analysis. The through thickness continuous variation of the material properties of the FGM beams is considered using the standard power law distribution. The von-Karman-type strain-displacement relations are used to account for the moderately large deflections. The FGM beams, with the classical hinged and clamped boundary conditions, are analyzed considering the axially immovable ends. The numerical results are provided to clearly bring out the importance of including the temperature dependency of the material properties to evaluate the realistic flexural response and thermal postbuckling behavior of the FGM beams subjected to thermal and thermo-mechanical loadings.

Multifunctional coating interlayers for thermal-barrier systems

Abstract

A calibrated Monte Carlo approach to quantify the impacts of misorientation and different driving forces on texture development

A calibrated Monte Carlo (cMC) approach, which quantifies grain boundary kinetics within a generic setting, is presented. The influence of misorientation is captured by adding a scaling coefficient in the spin flipping probability equation, while the contribution of different driving forces is weighted using a partition function. The calibration process relies on the established parametric links between Monte Carlo (MC) and sharp-interface models. The cMC algorithm quantifies microstructural evolution under complex thermomechanical environments and remedies some of the difficulties associated with conventional MC models. After validation, the cMC approach is applied to quantify the texture development of polycrystalline materials with influences of misorientation and inhomogeneous bulk energy across grain boundaries. The results are in good agreement with theory and experiments.

C/C–SiC composites for space applications and advanced friction systems

Ceramic matrix composite materials are being considered the primary materials for hot structures of future launch vehicles. Melt infiltrated composites based on the liquid silicon infiltration process have proven their suitability under extreme thermo-mechanical environments in different structural parts like nose caps, nozzle jet vanes and engine flaps. Considerable progress has been achieved within the last few years to mature the manufacture technology and to tailor the properties of the materials. Among low densities and high damage tolerance behaviour C/C–SiC composites show superior tribological properties predestining these materials for advanced friction systems.

A miniature extrusion line for small scale processing studies

A miniature extrusion line using a micro-extruder was developed and used to produce extrudates with a simple cross-section from a few grams of raw polymer material. Given the difficulty in applying standard design scale-up rules, the definition of the geometry of the screws was based on the predictions provided by plasticating extrusion software developed previously by the authors. The aim was to induce thermo-mechanical environments comparable to those found in bigger machines. Several materials were processed with the device, model predictions were assessed experimentally and the quality of the dispersive mixing achieved was estimated for two more complex polymer systems.

Development of the skin layer in injection moulding: phenomenological model

This work explores the conditions affecting the development of the skin layer in injection moulding. The thermomechanical environment is varied by systematic changes on the operative processing parameters for three moulding geometries: an axi-symmetric tensile bar, a lateral gated disc and a two-point central gated box. The thermomechanical environments are evaluated by computer simulations, by the local bulk temperature and shear stress level. The calculation of dimensionless numbers (Cameron, Brinkman and Nahme) allows the interpretation of the main physical phenomena occurring in the distinct moulding geometries. The relationships between the imposed thermomechanical environment and the thickness of the skin layer are established. For all the mouldings the skin thickness decreases with the increase in the thermal and shear stress levels, although with distinct contributions for the different moulding geometries. The development of the skin layer is interpreted in the light of a phenomenological model involving two time variables: the time allowed for relaxation of the highly oriented melt until the crystallization temperature is reached and the relaxation time of the material. The influence of the operative processing parameters on both time variables are discussed and used to interpret the experimental variations of the thickness of skin layer.

Truncated-Newton training algorithm for neurocomputational viscoplastic model

We present an estimate approach to compute the viscoplastic behavior of a polymer matrix composite under different thermomechanical environments. This investigation incorporates computational neural network as the tool for determining the creep behavior of the composite. We propose a new second-order learning algorithm for training the multilayer networks. Training in the neural network is generally specified as the minimization of an appropriate error function with respect to parameters of the network (weights and learning rates) corresponding to excitory and inhibitory connections. We propose here a technique for error minimization based on the use of the truncated Newton (TN) large-scale unconstrained minimization technique with quadratic convergence rate. This technique offers a more sophisticated use of the gradient information compared to simple steepest descent or conjugate gradient methods. In this work we briefly specify the necessary details for implementing the TN method for training the neural networks that predicts the viscoplastic behavior of the polymeric composite. We provide comparative experimental results and explicit model results to verify the effectiveness of the neural networks-based model. These results verify the superiority of the present approach compared to the explicit modeling scheme. Moreover, the present study demonstrates for the first time the feasibility of introducing the TN method, with quadratic convergence rate, to the field of neural networks.

Thermomechanical properties of TiC particle-reinforced tungsten composites for high temperature applications

Tungsten and tungsten alloys are widely used in high temperature environments where arc ablation or mechanical deformation and damage are the main sources of materials failure. For high temperature critical applications in thermomechanical environments, however, the low strength limits the use of tungsten and tungsten alloys. Hence, new tungsten based materials with good high temperature thermomechanical properties need to be developed in order to extend the use of tungsten. TiC particle-reinforced tungsten based composites (TiC p/W) were fabricated by hot pressing at 2000 °C, 20 MPa in a vacuum of 1.3×10 −3 Pa. The composites were examined with respect to their thermophysical and mechanical properties at room temperature and at elevated temperature. Vickers hardness and elastic modulus increased with increasing TiC content from 0 to 40 vol.%. The highest flexural strength, 843 MPa, and the highest toughness, 10.1 MPa m 1/2, of the composites at room temperature were all obtained when 20 vol.% TiC particle were added. As the test temperature rose, the flexural strength of the TiC p/W composites firstly increased and then decreased, except in the monolithic tungsten. The highest strength of 1155 MPa was measured at 1000 °C in the composite containing 30 vol.% TiC particles. The strengthening effect of TiC particles on the tungsten matrix is more significant at high temperatures. With the addition of TiC particles, the thermal conduction of tungsten composites was drastically decreased from 153 W m −1 K −1 for monolithic W to 27.9 W m −1 K −1 for 40 vol.% TiC p/W composites, and the thermal expansion was also increased. The new composites are successfully used to make high temperature grips and moulds.