Viscoelastic properties of the central region of porcine temporomandibular joint disc in shear stress-relaxation

In this study, shear relaxation properties of the porcine temporomandibular joint (TMJ) disc are investigated. Previous studies have shown that, in fatigue failure and damage of cartilage and fibrocartilage, shear loads could be one of the biggest contributors to the failure. The aim of the present study is to develop an evaluation method to study shear properties of the disc and to do a mathematical characterization of it. For the experiments, twelve porcine discs were used. Each disc was dissected from the TMJ and, then, static strain control tests were carried out to obtain the shear relaxation modulus for the central region of the discs. From the results, it was found that the disc presents a viscoelastic behavior under shear loads. Relaxation modulus decreased with time. Shear relaxation was 10% of the instantaneous stress, which implies that the viscous properties of the disc cannot be neglected. The present results lead to a better understanding of the discs mechanical behavior under realistic TMJ working conditions

Compressive properties of the whole TMJ disc

In this study, the dynamic and static compressive properties of the whole porcine temporomandibular joint (TMJ) disc were investigated. The aim of the study was to develop a new simple method for the evaluation of joint viscoelasticity, enabling examination of the load-bearing capacity and joint flexibility of the entire disc. For the experiments, a novel testing fixture that reproduces the condylar and fossa surfaces of the TMJ was developed to replicate TMJ disc geometry. Ten porcine discs were used in the experiments. Each disc was dissected from the TMJ and sinusoidal compressive strain was applied to obtain the storage and loss moduli. Static strain control tests were carried out to obtain the relaxation modulus. The result of static and dynamic tests indicated that the whole disc presented viscoelastic behavior under compression. Storage and loss moduli increased with frequency and the relaxation modulus decreased over time. The loss tangent showed less frequency dependence, with values ranging from 0.2 to 0.3, suggesting that the viscous properties of the disc cannot be neglected. These results provide a better understanding of whole disc mechanical compression behavior under realistic TMJ working conditions

Optimal discrete-time Prony series fitting method for viscoelastic materials

The most important characteristics of the behaviour of viscoelastic materials are the time and temperature dependence of their properties. Viscoelastic models based on Prony series are usually used due to easy implementation in finite element analysis (FEA) codes. The experimental data are fitted to a Prony series using a user-convenience number of terms represented by two coefficients. The time coefficients τ are previously fixed in the time scale in order to determine the second parameters of the model. Usually, an homogeneous distribution in logarithmic-time scale is used for τ, which produces accurate fittings when a large number of terms in the Prony series are used as well as when the material presents a uniform sigmoidal viscoelastic curve along several decades of time. When short-time curves must be fitted or the relaxation curve shape is not so uniform distributed along time, the homogeneous distribution of time coefficients could be a significant drawback since a large number of coefficients could be needed or even a reasonable fitting with a Prony series model is not possible. In this study, an optimized τi distributed method for fitting master curves of viscoelastic materials based on Prony series model is proposed. The method is based on an optimization algorithm strategy to allocate the time coefficients along the time scale in order to obtain the best fit. The method is validated by using experimental data of temporomandibular joint (TMJ) disc, which is a biological material that presents a short-time and high relaxation rate viscoelastic curve. The results show that the method improves significantly the fitting of the viscoelastic curves when compared with uniform distributed time fittings. Furthermore, the optimized coefficients are also used to obtain the complex moduli of the material using an analytical conversion, which is compared with the experimental complex moduli curves of the material

Evaluation of damping properties of structural glass panes under impact loading

The latest technology and architectural trends have significantly improved the use of a large variety of glass products in construction which, in function of their own characteristocs, allow to design and calculate structural glass elements under safety conditions. This paper presents the evaluation and analysis of the damping properties of rectangular laminated glass plates of 1.938 m x 0.876 m with different thickness depending on the number of PVB interlayers arranged. By means of numerical simulation and experimental verification, using modal analysis, natural frequencies and damping of the glass plates were calculated, both under free boundary conditions and operational conditions for the impact test equipment used in the experimental program, as the European standard UNE-EN 12600:2003 specifies

Caracterización probabilística de vidrio mediante diferentes tipos de ensayos

El vidrio no puede ser tratado como un material estructural convencional desde el punto de vista de la resistencia mecánica. Su naturaleza, como material frágil, junto con la inevitable presencia de microfisuras en su superficie y las consecuencias de accidentes por posibles fallos, exigen métodos rigurosos que garanticen un cálculo seguro de los elementos estructurales de vidrio, cuya resistencia a rotura depende en gran medida del tamaño del elemento y del tipo de carga a la que está sometido. Por lo tanto, su cálculo debe basarse en conceptos probabilísticos y en criterios de mecánica de la fractura, en sustitución de un cálculo convencional de vidrio según tablas deducidas de programas experimentales y posterior aplicación del concepto de tensiones admisibles. Con el fin de analizar y comparar las características mecánicas de vidrios templados, termoendurecidos y recocidos, se realizó un amplio programa experimental de ensayos de flexión a cuatro puntos y de anillos concéntricos de pequeña superficie, seguido de un ajuste de los resultados mediante una función de distribución triparamétrica de Weibull. Glass cannot be handled as a conventional structural material from the point of view of the mechanical strength. Its nature as brittle material, together with the inevitable presence of micro-cracks on its surface and the consequences of eventual failures, demand rigorous methods to achieve a safe design for glass elements, whose stress resistance is very much dependent on the integrity of its surface, element size and loading pattern. Thus, its design must rely on probabilistic concepts and fracture mechanics criteria, substitutive of the conventional glass design based on charts derived from experimental programs and subsequent application of the admissible stress concept. In order to analyze and compare the strength characteristics of tempered, heat-strengthened and annealed glass, a large experimental programme based on four-point bending and coaxial double ring tests was performed and the results were fitted using a three-parameter Weibull cumulative distribution function

Cálculo de elementos de vidrio en edificación: propuesta de norma española

RESUMEN. El presente artículo describe un modelo probabilístico de cálculo de placas de vidrio y el procedimiento que permite la determinación de la carga admisible en determinados tipos de vidrio, como propuesta de una nueva norma española de vidrio estructural. Una vez caracterizado el material, el modelo, implementado en el programa Windesign, permite el cálculo de la probabilidad de fallo para una placa de vidrio monolítico de dimensiones dadas sometida a una determinada carga, o en su caso, la determinación del espesor de placa capaz de resistir una carga conocida para una probabilidad de fallo dada. Asimismo se presenta un estudio comparativo entre los resultados obtenidos para el modelo aquí propuesto y varias normas extranjeras de vidrio, tales como la norma americana ASTM E 1300-02, la italiana UNI 7143-72, la francesa NFP 78-201-1 y la neozelandesa NZS 4223.ABSTRACT. This paper reports on a probabilistic model for design of glass plates and the procedure to determine the allowable load of specified types of glass, as a proposal for a new Spanish code for structural glass. After the experimental material characterization, the design model, implemented in the Windesign software, allows the user to calculate either the probability of failure of a monolithic glass plate for a given dimensions and specific loading, or to determine the plate thickness needed to withstand a known loading for a given probability of failure. Moreover, a comparative analysis among the results obtained by the model proposed here and the current foreign glass design standards, such as the American ASTM E 1300-02, the Italian UNI 7143-72, the French NFP 78-201-1 and the New Zealander NZS 4223 is presented

Study of the Time-Temperature-Dependent behaviour of PVB: application to laminated glass elements

The mechanical behavior of laminated glass elements is governed by material properties of the interlayer, the Polyvinyl Butiral (PVB) being the most used interlayer material in these elements. PVB is a viscoelastic material whose mechanical properties (Young’s modulus, shear modulus, etc.) depend mainly on the load application time and the temperature. Thus an adequate mechanical characterization of the PVB must be performed in order to predict the response of laminated glass elements with a good accuracy In this work, PVB specimens were subjected to static relaxation tests and to dynamic experimental tests (frequency domain) at different temperatures from -〖15〗^o C to 〖50〗^o C using a DMTA equipment. Then the curves at different temperatures were related using the William-Landel-Ferry (WLF) Time-Temperature Superposition (TTS) model to obtain the mastercurve of both the time and frequency domain Young’s moduli of the PVB. Finally, a viscoelastic Prony based model was fitted to the experimental data and used, afterwards, to simulate numerically the static and dynamic behaviour of different laminated glass elements at different temperatures. The numerical simulations were compared with the static and dynamic experimental results achieving a good accuracy in both the static deflections and the natural frequencies. With respect to the damping, the discrepancies are less than 22%

Modelización y contraste experimental del comportamiento mecánico del vidrio laminado estructural = Modelling and experimental contrast of the mechanical behaviour of structural laminated glass

This paper presents a numerical simulation of the mechanical behaviour of laminated glass plates (glass- PVB-glass) and its experimental verification. The viscoelastic characterization of the intermediate layer of PVB has been done by means of stress relaxation tests at various temperatures. The consideration of PVB as a viscoelastic material permits to analyze the real response of the structural element of laminated glass under time variations of temperature, of application of loading, of stress state, etc. Displacements obtained with the numerical analysis have been verified experimentally with laminated glass plates under lateral load using close range photogrammetry and dial gauges indicators. The analysis of results confirms the time dependent behaviour of the glass-PVB-glass laminate and suggests the validity of the proposed model.Este trabajo presenta la simulación numérica del comportamiento mecánico de placas de vidrio laminado (vidrio-PVB-vidrio) y su comprobación experimental. La caracterización viscoelástica de la lámina intermedia de PVB se ha realizado mediante ensayos de relajación de tensiones a diversas temperaturas. La consideración del PVB como material viscoelástico permite analizar la respuesta real del elemento estructural de vidrio laminado ante variaciones en el tiempo de la temperatura, de la velocidad de aplicación de las cargas, del estado tensional, etc. Los desplazamientos obtenidos numéricamente han sido contrastados experimentalmente en placas de vidrio laminado sometidas a carga lateral mediante fotogrametría de objeto cercano y relojes comparadores. Del análisis de los resultados se confirma el comportamiento variable en el tiempo del conjunto vidrio-PVB-vidrio y se deduce la validez de la modelización propuesta

Análisis del amortiguamiento en vidrio estructural bajo cargas de impacto

Los últimos avances tecnológicos y las tendencias arquitectónicas actuales han impulsado notablemente el uso de una gran variedad de productos de vidrio en la construcción que, en función de sus propias características, permiten diseñar y calcular elementos estructurales de este material bajo condiciones de seguridad. Este trabajo presenta la evaluación y el análisis de las propiedades de amortiguamiento de placas rectangulares de vidrio laminado de 1938 mm x 876 mm con distinto espesor, según el número de capas intermedias de PVB dispuestas. Mediante simulación numérica por elementos finitos y su posterior comprobación experimental, utilizando análisis modal, se determinaron las frecuencias naturales y el amortiguamiento de las placas de vidrio, tanto en condiciones de contorno libre, como en condiciones operacionales del equipo de ensayos de impacto utilizado en el programa experimental, siguiendo las especificaciones de la norma europea UNE-EN 12600:2003