On the mechanical characteristics of a self-setting calcium phosphate cement

Objective: To perform a mechanical characterization of a self-setting calcium phosphate cement in function of the immersion time in Ringer solution. Materials and methods: Specimens of self-setting calcium phosphate cement were prepared from pure alpha-TCP powder. The residual strains developed during hardening stage were monitored using an embedded fiber Bragg grating sensor. Additionally, the evolution of the elastic modulus was obtained for the same time period by conducting low-load indentation tests. Micro-computed tomography as well as microscope-assisted inspections were employed to evaluate the porosity in the specimens. Moreover, diametral compression tests were conducted in wet and dried specimens to characterize the material strength. Results: The volume of the estimated porosity and absorbed fluid mass, during the first few minutes of the material's exposure in a wet environment, coincide. The immersion in Ringer solution lead to a noticeable increase in the moduli values. The critical value of stresses obtained from the diametral compression tests were combined with the data from uniaxial compression tests, to suggest a Mohr-Coulomb failure criterion. Conclusions: This study presents different techniques to characterize a self-setting calcium phosphate cement and provides experimental data on porosity, mechanical properties and failure. The investigated material possessed an open porosity at its dried state with negligible residual strains and its Young's modulus, obtained from micro-indentation tests, increased with hardening time. The failure loci may be described by a Mohr-Coulomb criterion, characteristic of soil and rock materials.

An experimental-numerical investigation of hydrothermal response in adhesively bonded composite structures

John Botsis, Luis Pablo Canal Casado, Hans Georg Limberger, Véronique Michaud, Roohollah Sarfaraz Khabbaz, Georgios Violakis

Water absorption and thermal response of adhesive composite joints were investigated by measurements and numerical simulations. Water diffusivity, saturation, swelling, and thermal expansion of the constituent materials and the joint were obtained from gravimetric experiments and strain measurements using embedded fiber Bragg grating (FBG) sensors. The mechanical response of these materials at different temperatures and water content was characterized by dynamic mechanical analysis. Thermal loading and water absorption in joint specimens were detected by monitoring the FBG wavelength shift caused by thermal expansion or water swelling. The measured parameters were used in finite element models to simulate the response of the embedded sensor. The good correlation of experimental data and simulations confirmed that the change in FBG wavelength could be accurately related to the thermal load or water absorption process. The suitability of the embedded FBG sensors for monitoring of water uptake in adhesive composite joints was demonstrated. (C) 2015 Elsevier Ltd. All rights reserved.

Elsevier2015

Stochastic analysis of clear timber as a structural material

Alireza Farajzadeh Moshtaghin

Wood/timber has been widely used for house and bridge construction. It is a widely available natural material that necessitates low energy for the production, following simple processes. The environmentally friendliness, together with the low cost of raw material makes it an efficient building material. Moreover, timber possesses attractive mechanical properties such as high specific strength and stiffness. In contrast, timber constructions have, to a large extent, been based on experience and craftsmanship, which prevents taking full advantage of this material. There are several reasons for this. Timber has a complex mechanical behavior being a natural highly anisotropic fiber composite, with properties that are also affected by moisture content. For specific species, geographical location, local growth conditions and moisture content, the material properties depend, among others, on the age, the structural imperfections, the location of timber within the tree, and load history. Consequently, the mechanical properties of timber are, inherently, highly variable. Variability of timber properties includes statistical and spatial variabilities, referred to as random spatial variability (RSV). This entails adopting a probabilistic/stochastic approach to analysis of timer structures. The aim of this research is to understand and model the effect of the RSV on the clear timber mechanical properties, as well as the experimental characterization of RSV for clear timber, and also to develop a stochastic finite element framework for random response assessment of clear timber components. A size effect model was developed which takes into account the RSV in the strength field. The theory of random fields was used for this purpose. Using the spectral representation scheme, realizations of strength field in each specimen were generated. The stochastic response was obtained via the Monte Carlo method. The model results was compared to the existing experimental data in the literature. Also, an analytical expression was provided to facilitate the application of the model. Clear timber specimens of different lengths were fabricated for longitudinal tensile tests. Local deformations along the lengths of the specimens were recorded during the tests in order to characterize the RSV in longitudinal properties. A connection between the mesostructure of the clear wood and its local elastic modulus was observed. Statistics concerning the elastic modulus, strength and strain to failure and the effect of length change on these properties were extracted. The correlations between the strength, the elasticity and the density were obtained. Transverse properties were also investigated which are of particular importance in some applications such as mechanical and adhesively-bonded timber joints. Regularly positioned and randomly positioned specimens were cut from different timber boards. Statistics and size effects concerning the elastic modulus, strength and strain to failure as well as the correlation between the properties were studied. The spatial variability in the transverse elastic modulus, the tensile strength and the failure strain was also experimentally studied. Mesostructural patterns of clear timber were shown to have a direct effect on the local elastic modulus. Finally, a stochastic finite element framework was established by combining the spectral representation scheme for RSV modelling and the finite element software ABAQUS in a non-intrusive manner. This framework can be used for the stochastic structural response assessment of timber structural components made of clear timber. To show the applicability of the model in real applications, the failure of adhesively bonded double-lap timber joints were simulated under tensile loading. The effect of size on the strength was also taken into account. The results were in a fairly well agreement with the available experimental data in the literature.

EPFL2016

A granular model of solidification as applied to hot tearing

Stéphane Vernède

Hot cracking is a spontaneous failure of an alloy during solidification. It is a severe problem for casting industry as it reduces the productivity of cast houses and limits the range of alloys compositions that can be industrially produced. Hot tears occurs at the end of solidification, when solid grains are separated by thin liquid films. At this stage of solidification, fluid flow between the grains is difficult and the solid network is not continuous enough to transmit stresses. The material is extremely brittle. Moreover, the deformation induced by the thermal shrinkage of the material tends to localize in hot spots, i.e., in zones which solidify at last. These zones become in tension and the concomitant effect of the tensile stress with the intrinsic brittleness of the material result in a hot crack. Even though the general mechanisms of hot tearing are understood, the importance of each physical parameter remains ill defined. The present work is mainly focused on the derivation of a granular model of mushy zone (the zone where solid and liquid coexist) for aluminium alloys, i.e., a model which explicitly considers the behaviour of each grain while being sufficiently simple to allow the computation of large mushy zones. First, a solidification model based on the Voronoi diagram of a random set of nuclei is derived. This model computes the solidification in each polyhedron considering back-diffusion and coalescence. Second, a pressure drop calculation is performed in the network of connected liquid channels assuming the centre of solid grains to be fixed. This model considers a Poiseuille flow in each channel, Kirchhoff's conservation of flow at nodal points and flow Losses compensating solidification (KPL model). Finally, the displacement of the solid grain centres is considered and the mechanical behaviour of the mushy zone is computed assuming perfectly rigid solid grains. This model shows the progressive formation of grain clusters and the localization of fluid flow at high solid fraction. Therefore, it allows to define transitions in the behaviour of the mushy zone. These transitions are essential for hot cracking models, but should be introduced as parameters in continuum approaches. Finally, the mechanical model shows a strong localization of deformation in liquid channels normal to the stress direction. Such channels create ideal sites for the nucleation and the propagation of hot cracks.