Modelling the viscoelastic response of ceramic materials by commercial finite elements codes

A very prominent patented technology permits to obtained bent porcelain stoneware tiles by a proper combination of machining and secondary firing in a kiln. During this firing, the ceramic materials show a viscoelastic behaviour. The viscoelasticity permits the tiles to be bent without further interventions: just using the gravity force in a so-called pyroplastic deformation. Both the viscoelastic response in general and pyroplastic deformation in particular, are complex aspects of material behaviour to be modelled with accuracy. In general, the theory of viscoelasticity can be considered extremely large and precise, but its application on real cases is extremely tricky. A time-depending problem, as viscoelasticity naturally is, has to be merged with a temperature-depending situation. It means that, even if the constitutive equations could be set as general formulation, all the fundamental parameters inside these formulas change in function of both entities. Finite Elements codes could help to pass by this impasse, permitting to discretize the variability on temperature. But several cautions have to be taken into account, especially considering that few commercial codes developed proper algorithms. This paper investigates how the viscoelastic response of ceramic materials can be modelled by commercial Finite Elements codes, defining limits and proposing solutions A very prominent patented technology permits to obtained bent porcelain stoneware tiles by a proper combination of machining and secondary firing in a kiln. During this firing, the ceramic materials show a viscoelastic behaviour. The viscoelasticity permits the tiles to be bent without further interventions: just using the gravity force in a so-called pyroplastic deformation. Both the viscoelastic response in general and pyroplastic deformation in particular, are complex aspects of material behaviour to be modelled with accuracy. In general, the theory of viscoelasticity can be considered extremely large and precise, but its application on real cases is extremely tricky. A time-depending problem, as viscoelasticity naturally is, has to be merged with a temperature-depending situation. It means that, even if the constitutive equations could be set as general formulation, all the fundamental parameters inside these formulas change in function of both entities. Finite Elements codes could help to pass by this impasse, permitting to discretize the variability on temperature. But several cautions have to be taken into account, especially considering that few commercial codes developed proper algorithms. This paper investigates how the viscoelastic response of ceramic materials can be modelled by commercial Finite Elements codes, defining limits and proposing solutions

LIGHTENING STRUCTURES BY METAL REPLACEMENT: FROM TRADITIONAL GYM EQUIPMENT TO AN ADVANCED FIBER-REINFORCED COMPOSITE EXOSKELETON

A redesign procedure used for introducing new functional properties in innovative gym equipment is here reported. It is based on a metal replacement action where a tempered steel was firstly replaced by an aluminum alloy and then by high strength-to-weight fiber-reinforced polymers. The effect of fiber properties (as strength and volume ratio) and plies stacking sequences (as thicknesses and orientation) were investigated. Numerical analyses, done by Ansys ACP, allowed evaluating the stress-strain behavior in realistic boundaries and quasi-static loads, comparing materials and layouts in terms of stiffness. The single-layered shell method with additional integration points was preferred as a technique for discretizing composite laminates. Maximum Principal Stress and Maximum Distortion Energy (Tsai-Hill) were applied as anisotropic failure criteria. Changes in geometry were also considered given their relevant effects on parts and processes. Specifically, this paper is focused on a representative component of the main kinematic chain (the ‘forearm’) and details the different redesign phases for that part. The chosen solution consisted of 14 layers of unidirectional and bidirectional carbon fiber-reinforced pre-pregs, offering a 68 % weight reduction with respect to a solid aluminum component with equal stiffness. The part was manufactured by hand lay-up and cured in autoclave. This redesign practice was extended to the rest of the equipment allowing its transformation into an exoskeleton

Push-pull fatigue tests on ductile and vermicular cast irons

This article aims at measuring and comparing the fatigue strength with fully reversed push-pull tests in the case of two different cast irons: ductile and vermicular. Spheroidal Graphite Iron (SGI), also known as ductile cast iron, is nowadays used in a very large variety of applications. It represents a valid option when strength and stiffness are required, namely, when high values of tensile strength and Young’s modulus are coupled with appreciable deformation before failure. By contrast, a different cast iron, known as Compacted Graphite Iron (CGI) or vermicular cast iron, presents its benefits in replacing SGI with respect to specific applications. In particular, with better castability, machinability and thermal resistance, SGI is ideal when components suffer simultaneous mechanical and thermal loadings, such as cylinder blocks and heads. While SGI benefits of a wide scientific literature, CGI is a relatively unknown material, especially referring to its response under fatigue loads

Multi-Objective Design Optimization of the Reinforced Composite Roof in a Solar Vehicle

Abstract: A multi-step and -objective design approach was used to optimize the photovoltaic roof in a multi-occupant racing vehicle. It permitted to select the best combination of design features (as shapes, widths, angles) in composite structures simultaneously balancing opposite requirements as static strength and dynamic stiffness. An attention to functional requirements, as weight, solar cells cooling and solar energy conversion, was also essential. Alternative carbon fiber-reinforced plastic structures were investigated by finite elements using static and modal analyses in the way to compare several design configurations in terms of natural frequencies, deformations, flexural stiffness, torsional stiffness, and heat exchange surfaces. A representative roof section was manufactured and tested for model validation. A significant improvement respect to the pre-existing solar roof was detected. The final configuration was manufactured and installed on the vehicle

Thermal Behavior of Monocrystalline Silicon Solar Cells: A Numerical and Experimental Investigation on the Module Encapsulation Materials

This research outlines the numerical predictions of the heat distribution in solar cells, accompanied by their empirical validation. Finite element thermal models of five laminated silicon solar photovoltaic cells were firstly established using a simulation software (ANSYS®). The flexible laminated solar cells under study are made of a highly transparent frontsheet, a silicon cell between two encapsulants, and a backsheet. Different combinations of layers (i.e., materials and thicknesses) were taken into account in order to analyze their effect on thermal behavior. Thermal properties of materials were derived in accordance with the literature. Similarly, boundary conditions, loads, and heat losses by reflection and convection were also specified. The solar cells were tested using solar lamps under standard conditions (irradiance: 1000W/m2; room-temperature: 25°C) with real-time temperatures measured by a thermal imager. This analysis offers an interpretation of how temperature evolves through the solar cell and, consequently, how the design choice can influence the cells’ efficiency

Acid aging effects on surfaces of PTFE gaskets investigated by thermal analysis

This paper investigates the effect of a prolonged acid attack on the surface of PTFE by Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). PTFE is very non-reactive, partly because of the strength of carbon\u2013fluorine bonds and for its high crystallinity, and, as a consequence, it is often used in containers and pipework with reactive and corrosive chemicals. The PTFE under analysis is commercialized by two alternative producers in form of Teflon tapes. These tapes are adopted, as gaskets, in process plants where tires moulds are cleaned by acid solutions inside a multistage ultrasonic process. In this case, PTFE shows, in a relatively short operation time, inexplicably phenomena of surface degradation, which could be related, in general terms, to an acid attack. But, even considering the combined effect of ultrasonic waves, temperature, humidity and acid attack, the PTFE properties of resistance nominally exclude the risk of the extreme erosion phenomena as observed. The present experimental research aim at investigating this contradiction. A possible explanation could be related to the presence in the cleaning solution of unexpected fluorides, able to produce fluorinating agents and, thus, degrade carbon-fluorine bonds. Considering more the 300 chemical elements a tire compound consists in, it is really complex to preserve the original chemical composition of the cleaning solution. In this research PTFE samples have been treated with different mixtures of acids with the aim at investigating the different aging effects. The thermal analysis has permitted the experimental characterization of PTFE surface properties after acid attack, providing evidence of the degradation phenomena. In particular, the different acid treatments adopted for accelerating the aging of gaskets have highlighted the different behaviour of the PTFE matrix, but also differences between manufacturers

Damage characterization of nano-interleaved CFRP under static and fatigue loading

© 2019 by the authors. The use of high strength-to-weight ratio-laminated fiber-reinforced composites is emerging in engineering sectors such as aerospace, marine and automotive to improve productivity. Nevertheless, delamination between the layers is a limiting factor for the wider application of laminated composites, as it reduces the stiffness and strengths of the structure. Previous studies have proven that ply interface nanofibrous fiber reinforcement has an effective influence on delamination resistance of laminated composite materials. This paper aims to investigate the effect of nanofiber ply interface reinforcement on mode I properties and failure responses when being subjected to static and fatigue loadings. For this purpose, virgin and nanomodified woven laminates were subjected to Double Cantilever Beam (DCB) experiments. Static and fatigue tests were performed in accordance with standards and the Acoustic Emissions (AE) were acquired during these tests. The results showed not only a 130% increase of delamination toughness for nanomodified specimens in the case of static loads, but also a relevant crack growth resistance in the case of fatigue loads. In addition, the AE permitted to relate these improvements to the different failure mechanisms occurring

Buckling analysis of telescopic boom: theoretical and numerical verification of sliding pads

U cilju poboljšanja najviših performansi, materijali u strojarskim konstrukcijama su često vođeni ka sve bližim i bližim kritičnim granicama. Razmotrimo, na primjer, kako progresivno smanjenje debljine može dovesti do nepredviđenih efekata u stabilnosti lima, sve do brzog loma cjelokupne konstrukcije. Ovaj rizik je posebice poznat konstruktorima teleskopskih dizalica, koje se koriste za pokretanje radnih platformi. U ovom radu, numeričkim pristupom i ANSYS kodom, čvrstoća i stabilnost teleskopske dizalice su verificirani. Nakon preliminarne teorijske analize, različite konfiguracije opterećenja i graničnih uvjeta su razmatrane u skladu s uvjetima realne uporabe. Kao opći rezultat, potvrđeno je da su naprezanja bila u okviru elastičnih granica materijala, osim u ograničenom broju kontaktnih površina, gdje su korišteni posebni kontaktni elementi za sprječavanje loma. Osim toga, linearne tehnike izvijanja su pokazale da su kritična opterećenja i odgovarajući moduli izvijanja bili veći od najtežih uvjeta rada; stabilnost je potvrđena. Konačno, FEM simulacije dopuštaju smanjenje brojnih eksperimenata, nudeći time brze metode za poboljšanje konstrukcija.With the aim at improving the highest performances, materials in mechanical structures are constantly pushed closer and closer to their critical limits. Consider, for example, how the progressive reduction in thickness may lead to unforeseen effects in the instability of metal sheets, until the rapid collapse of the whole structure. This risk is specially known by designers of telescopic booms, used for moving aerial platforms. In this paper, by a numerical approach and ANSYS code, structural resistance and stability of a telescopic boom were verified. After a preliminary theoretical analysis, different loads and boundary configurations were considered in accordance with the most common conditions of real utilisation. As general result, it was confirmed that stresses were under the elastic limit of materials, except in a very limited number of contact zones, where specific connecting solutions have to be installed to prevent failures. Furthermore, linear buckling techniques showed that critical loads and corresponding buckling modes were higher than the most extreme working conditions; thus, structural stability was also confirmed. Finally, the large adoption of FEM simulations permitted to reduce the experiments, offering a fast methodology for improvements in design

MEASURING DEFORMATIONS IN THE TELESCOPIC BOOM UNDER STATIC AND DYNAMIC LOAD CONDITIONS

The interest in pushing the mechanical structures closer to their limits of usage makes necessary to combine the traditional design with the implementation of specific tests able to definitely confirm and guarantee their safety. Exploring the case of a large telescopic boom, the present study analyses the response to intense loads prevenient from static and dynamic conditions. The measure of deformations was oriented to validate several design assumptions, but also to investigate the presence of phenomena of local instability, not easily predictable within theoretical formulations