Computational modelling and experimental characterisation of heterogeneous materials

Heterogeneous materials can exhibit behaviour under load that cannot be described by classical continuum elasticity. Beams in bending can show a relative stiffening as the beam depth tends to zero, a size effect. Size effects are recognised in higher order continuum elastic theories such as micropolar elasticity. The drawback of higher order theories is the requirement of addition constitutive relations and associated properties that are often difficult to establish experimentally. Furthermore the finite element method, of great benefit in classical elasticity, has shown limitations when applied to micropolar elasticity. The determination of additional constitutive properties and the computational modelling of micropolar elasticity will be discussed in the context of a model heterogeneous material loaded in simple 3 point bending. The model material was created by drilling holes in aluminium bar in a regular pattern, with the hole axis normal to the plane of bending. The bending tests show that a size effect is present. These results are compared against modelling the detailed beam geometries in the finite element package ANSYS, which again shows the size effect. These two bending test are used to extract the additional micropolar elastic material properties. A comparison is then made against analytical solutions,numerical solutions using a micropolar beam finite element and a micropolar plane stress control volume method.It will be shown that the need for extensive experimental testing to determine the additional constitutive properties may not be necessary with the appropriate use of numerical methods

Experimental test of magnetic photons

A "magnetic" photon hypothesis associated with magnetic monopoles is tested experimentally. These photons are predicted to easily penetrate metal. Experimentally the optical transmittance T of a metal foil was less than 2 x 10^-17. The hypothesis is not supported since it predicts T = 2 x 10^-12

Mechanical viscoelastic behavior of dental adhesives

Objectives The purpose of the study was to evaluate the mechanical properties of dental adhesive materials at different testing temperatures after dry and wet storage. Methods Specimens (d = 1 mm, l = 18 mm) from six materials were tested: Silorane Adhesive System (SL), Heliobond (HE), One-Step Plus (OS), Optibond Solo Plus (OP), cmf Adhesive System (CF) and Protobond (PR). Static and creep testing was performed by applying a constant torque below the proportional limit of the materials, while dynamic testing consisted of dynamic torsional loading. Experiments were performed after 24 h of dry and wet storage under temperatures from 21 C to 50 C and various viscoelastic parameters were calculated. Results Shear modulus ranged from 0.19 to 1.99 GPa, while flexural modulus from 0.67 to 5.69 GPa. Most of the materials were affected by the presence of water and increase of temperature. OP showed the highest recovery after creep, while SL exhibited the highest permanent deformation. Significance Contact with water after polymerization and increase of temperature resulted in a decline of the mechanical properties, especially for the HEMA-containing adhesives. © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved

Creep and dynamic viscoelastic behavior of endodontic fiber-reinforced composite posts

Purpose: Fiber-reinforced composite (FRC) posts have gained much interest recently and understanding of their viscoelastic properties is important as they can be used in stress-bearing posterior restorations. The aim of this study was to evaluate the creep behavior and the viscoelastic properties of four commercial FRC posts under different temperatures and different storage conditions. Methods: The FRC posts tested were Glassix, C-Post, Carbonite and Snowlight. For the creep measurements a constant load below the proportional limit of the posts was applied and the angular deformation of the specimens was recorded. The viscoelastic parameters were determined by using dynamic torsional loading under four different conditions. Results: All materials were susceptible to creep and exhibited linear viscoelastic behavior. Residual strain was observed in all FRC posts. The viscoelastic properties were affected by the increase of temperature and water storage (p < 0.001) resulting in their decline. Carbon fiber posts exhibited better performance than glass fiber posts. Conclusions: FRC posts exhibit permanent strains under regular masticatory stresses that can be generated in the oral cavity. Their properties are susceptible to changes in temperature, while direct contact with water also affects them deleteriously. © 2009 Japan Prosthodontic Society

The effect of temperature on the viscoelastic properties of nano-hybrid composites

Objectives: The purpose of this study was to examine the viscoelastic properties of nanofilled dental composites under both static and dynamic testing and to determine the influence of temperature, medium of storage and storage time. Methods: Three nanofilled composites, one packable and one ormocer were tested. The specimens were examined dry at 21 °C and wet at 21, 37 and 50 °C after being stored for 24 h and 1 month under both static and dynamic testing. Shear modulus, elastic modulus, loss tangent, Poisson's ratio and other viscoelastic parameters were calculated. Data were analyzed with one-way and two-way analysis of variance (ANOVA) (p = 0.05). Results: All materials tested showed a significant decrease in their moduli with the increase of temperature, while the effect of water storage was different among the composites. Grandio was the composite with the highest Young's modulus followed by Filtek P60. Significance: Most of the materials tested did not have elastic moduli near to that of dentin, making them less satisfactory in posterior restorations. The materials possessing nano-sized filler particles had different elastic properties among them and this implies that filler size is not the only factor that affects the elastic behavior of dental composites. © 2007 Academy of Dental Materials

Fatigue of packable dental composites

Objective: The purpose of the study was to measure the fatigue properties of four dental resin composites using a dynamic mechanical analysis and to relate the results with viscoelastic properties. Methods: Dynamic torsional loading was conducted at resonance at 30-50 Hz. Specimens were thoroughly cured and tested dry at 21 °C. Results: All of the specimens showed a loss of strength following repeated stress, due to material fatigue. The material with the highest shear modulus had the lowest damping and the highest fatigue strength. Significance: Dental composites exhibit a modest loss of strength due to fatigue. Since mastication involves many cycles of stress during the life of a restoration, fatigue properties should be taken into account in restoration design. © 2006 Academy of Dental Materials

The effect of temperature on viscoelastic properties of glass ionomer cements and compomers

The objective of this study was to determine the viscoelastic properties of different types of glass ionomer cements (GICs) and compomers under varying temperature conditions found in the mouth. The materials tested were a conventional GIC (Aqua Ionofil U), a resin modified GIC (Fuji II LC), a highly viscous GIC (Voco Ionofil Molar), and two polyacid modified composite resins/compomers (Glasiosite and Dyract Flow). Six groups of four specimens were prepared from each material. One group was stored dry for 24 h and was subsequently tested dry at 21°C. Each of the remaining five groups was stored for 24 h in distilled water at the temperatures 21, 30.5, 37, 43.5, and 50°C, respectively, and was subsequently tested at that temperature. Shear storage modulus and loss tangent were determined by conducting dynamic torsional loading. Static shear moduli were determined by applying a constant torque (below the proportional limit of the materials) for 10 s and recording the angular deformation of the specimens. Data were analyzed by ANOVA and Duncan's test (α = 0.05). It was found that the viscoelastic properties varied significantly (p < 0.05) across the different materials. The compomer Glasiosite, with the highest filler content, and the highly viscous GIC Voco Ionofil Molar exhibited the highest elastic moduli and lowest loss tangents. Viscoelastic properties varied also significantly (p < 0.05) with temperature levels, but changes in the tested region were not indicative of a glass transition. Dynamic shear storage moduli were highly correlated to the static ones. Storage in water lowered the values of elastic moduli. © 2006 Wiley Periodicals, Inc