Carlos Alberto Cimini

Abfraction and anisotropy-effects of prism orientation on stress distribution

This work discusses the effect of enamel anisotropy in the stress concentration at the cement-enamel junction (CEJ), a probable cause of fracture in enamel leading to abfraction. Usual simplifications when developing computer models in dentistry are to consider enamel isotropic, or that the direction of the prisms is orthogonal to either the dentine-enamel interface or the tooth outer surface. In this paper, a more refined model for the material behavior is described, based on laboratory observation and on the work of Fernandes and Chevitarese [1] . The material description is used in a two-dimensional (2D) finite element model of the first upper premolar, and the analysis is performed for two different situations: vertical loads, typical of normal mastication and horizontal loads, dominant in bruxism. The analyses were performed using a unit load, which under the hypothesis of linear response of the tooth, allows the combinations described in the text to simulate different functional and parafunctional loads. The results indicate that a realistic enamel description in terms of mechanical properties and spatial distribution of its prisms alters significantly the resulting stress distribution. For all cases included in this study, the detailed description of prism orientation and resulting anisotropy led to improved response in terms of stress distribution, even when loading was horizontal.

Fracture mechanics via sensitivity analysis and numerical evaluation of J integral in cracked sheets

Abstract In this paper the formulation of the J integral is derived from the shape sensitivity analysis for plane cracked domains. Numerical evaluation of the J integral is performed using the results obtained from a finite element calculation. Finally, three examples of cracked sheets are analyzed and the results compared to available values in the literature.

Characterization of moisture effect on static and fatigue performance of epoxy resin using thin-film specimen on dynamic mechanical analyzer

A novel method of characterizing moisture effect on mechanical performance of epoxy resin is presented in this paper. A 50-µm-thick layer of cured epoxy resin was fabricated and cut into strips of 4 mm wide and 30 mm long as specimens to be tested on a dynamic mechanical analyzer equipped with thin-film tension clamp. Static tension and force-controlled tension–tension fatigue tests were first carried out using thin-film specimens made from Momentive 135/137 and BASF 5400/5440 epoxy resin systems without applying moisture, and results were compared with those obtained using conventional dog-bone specimens to validate the proposed testing method. Another batch of thin-film specimens were then immersed into deionized water, and the weight gain was recorded regularly until full saturation to obtain the absorption curve. Static and fatigue tests were performed using thin-film specimens made from BASF 5400/5440 with 55% and 100% saturation of moisture respectively, to evaluate moisture-induced material degradation. The aging effect on BASF 5400/5440 caused by cyclic water immersion and drying process was also assessed by performing static and fatigue tests using fully dried thin-film specimens after aging. It was concluded that the combination of thin-film specimen and dynamic mechanical analyzer would yield as good measurements of tensile strength and fatigue life as conventional dog-bone specimen does, and the small thickness of thin-film specimen would greatly reduce the time to reach a certain level of moisture content, facilitating further studies on effect of moisture ingression on polymeric matrix composites using multi-scale approaches.

Thermal Fatigue Analysis of a PWR NPP Steam Generator Injection Nozzle Model Subjected to the Thermal Stratification Phenomenon

This work is related to an experimental thermal stratification study aiming to quantify thermal fatigue damages in the pipe material. Thermal fatigue damages appear as a consequence of non-linear longitudinal and circumferential loads and thermal stripping present in pipes whit thermal stratified flows. In this work an experimental section, simulating the injection nozzle of a NPP steam generator, was subjected to the effects of thermal fatigue due to thermal stratification. The experimental section was made of stainless steel pipe type AISI 304L and its geometric characteristics allowed the same range of Froude numbers of a Pressurized Water Reactor (PWR) NPP. Temperatures were measured externally and internally in three positions and deformations just externally in seven positions. Up-and-down fatigue tests were done to assess the amount of damage induced in the material experimental section. Preliminary numerical simulations were done using a coupled analysis in the ANSYS code with temperatures and pressure inputs taken from thermo hydraulic experimental results. The objectives in this work are quantify the thermal fatigue intensity imposed to the pipe material by thermal stratification experiments, verify the agreement between numerical and experimental thermal stratification results and obtain the material fatigue limit testing specimens made of pipe experimental section and from the virgin pipe. In this work is possible to conclude that stratified flows could be developed in the experimental section, thermal stratification induces considerable thermal stresses and strains in the experimental section pipe material, thermal stratification reduces the material fatigue limit, numerical and experimental results agreed appropriately in some pipe region and disagreed in others.Copyright

Finite element evaluation of the effects of curvature in Lamb waves for composites structural health monitoring

This work presents the effect of the high curvature to thickness ratio on the characteristics of Lamb Waves propagating over the skins of composite structures. It is accessed how the curvature on composite skins affects the group velocity of the symmetrical (S0) and asymmetrical (A0) wave modes. It is also accessed the gradient of curvature effect, when the wave propagates from a flat to a curved part on the skin. The results are intended to be used for the improvement of structural health monitoring of wings and wind turbine blades. This is accomplished through dynamic explicit linear finite element method simulations of plates with flat and curved parts made of Eglass-epoxy bidirectional laminate. As the skin structures are often designed to withstand torsional loads, the fibers are aligned with 45 degrees from the leading edge (curved region) throughout the analysis. Several skins with different curvature to thickness ratios are generated and simulated. Results and trends are presented and can be used to improve the algorithms for damage detection on those structures. It is shown that the group velocity of the incoming waves change considerably with the presence of curvature, for both main modes of vibration.

Strength analysis of composite cables

Carbon Fiber Reinforced Polymer (CFRP) cables, due to their outstanding performance in terms of specific stiffness and strength, are usually found in civil construction applications and, more recently, in the Oil & Gas sector. However, experimental data and theoretical solutions for these cables are very limited. On the contrary, several theoretical and numerical approaches are available for isotropic cables (metallic wire ropes), some of them with severe simplifications, nonetheless showing good agreement with experimental data. In this study, experimental tensile results for 1×7 CRFP cables were compared to a simplified analytical model (assumed transversally isotropic) and to a 3D finite element model incorporating the experimental uncertainty in important input parameters: longitudinal elastic modulus, Poisson’s ratio, static friction coefficient and ultimate tensile strain. The average experimental breaking load of the cable was 190.25 kN (coefficient of variation of 1.74%) and the agreement with the numerical model predictions were good, with an average-value deviation of –1.15%, which is lower than the experimental variations. The simplified analytical model yielded a discrepancy above 10%, indicating that it needs further refinement although much less time consuming than the numerical model. These conclusions were corroborated by statistical analyses (i.e. Kruskal–Wallis and Mann-Whitney).

Theory and validation of the master ply concept for invariant-based stiffness of composites:

The objective of this paper is to unveil the background theory behind the universal master ply assumption, based on the invariant approach, to describe ply elastic properties. It was demonstrated that using ply-based constitutive relations, trace-normalized stiffness properties can be derived for different materials. Theoretical predictions for trace-normalized parameters were plotted as functions of the unidirectional ply longitudinal modulus (Ex), which defines the particular material system. Ply stiffness extensive empirical data were obtained from literature for four types of material systems (high modulus carbon/epoxy, standard modulus carbon/epoxy, aramid/epoxy, and glass/epoxy) and correlated quite well to theoretical predictions. Theoretical curves presented a nonlinear region for low Ex which gradually evolves to a plateau as Ex increases. It was verified that the master ply concept averaging the trace-normalized ply stiffness matrix elements can be applied for high modulus carbon/epoxy, standard modulus carbon/epoxy, and aramid/epoxy material systems. However, glass/epoxy systems can not be represented by this concept. The exposed theoretical background supports trace-based approach and enhances its effectiveness as a design tool, encompassing all the consequent advantages.

A Simple and Robust Subsonic Boundary Layer Method to Be Used with the Panel Method for Wing Calculations Using a Wing Strip Approach

This paper will present the development of a simple subsonic boundary layer method suitable to be used coupled with panel methods in order to estimate the aerodynamic characteristics, including viscous drag and maximum lift coefficient, of 3D wings. The proposed method does not require viscous-inviscid iterations and is based on classical integral bi-dimensional boundary layer theory using Thwaites and Head ́s models with bi-dimensional empirical corrections applied to each wing strip being therefor robust and efficient to be used in the early conceptual stage of aircraft design. Presented results are compared to the Modified CS Method in an IBL scheme and experimental data and are shown to provide good results.

Residual Stresses and Cracking in Dental Restorations due to Resin Contraction Considering In-Depth Young's Modulus Variation

Composite resins have been increasingly used as restorative material, both in anterior and posterior teeth, where they replace metal restorations. Its aesthetic characteristics, coupled with improved physical properties have made their use extend from just anterior teeth to also include posterior teeth. The use of such material in oral regions subjected to higher loading makes it important to account for the effect of residual stresses arising during polymerization, induced by resin contraction (Ausiello, Apicilla and Davidson 2002). Different reports can be found in the literature focused in this aspect and using different approaches, such as X-ray micro-computed tomography (Sun, Eidelman and Gibson 2009), 3D evaluation of the marginal adaptation (Kakaboura et al 2007) and 3 D deformation analysis from MCT images (Chiang et al 2010). These authors agree in the critical role played by resin contraction in restoration success. The Finite Element Method can therefore be a useful tool to investigate the cracking of interfaces, stress concentrations in the internal angles and effects of variations in the mechanical properties in the overall behavior of the resin-based dental restorations.

Evaluation of pitch up of two-axle solid waste collection compactor trucks in the static condition

The study of pitch up limit for solid waste collection compactor trucks in tilted public roads is of great relevance both for the planning of waste collection, mainly in cities with very uneven street gradient, or for use as a design parameter in projects involving public roads. Considering the typical construction and use of rear loader waste compactor equipment, the centre of gravity moves towards the rear of the vehicle as it is loaded, resulting in overload in the rear axle at the end of the waste collection period. In the city of Belo Horizonte (Brazil), several cases of pitch up have been reported for this type of vehicle, in streets with different inclinations and with loading situations in which the load box was not completely full. The present study investigated the variation of the imminence pitch up angles of the two-axle, rear-loading, solid waste collection compactor truck in a static condition, determined by the variation of its centre of gravity coordinates, which were obtained experimentally by means of a testing programme for different loading situations. The critical inclination angle was 0.347 rad (19.89°), which corresponds to a ramp inclination of 36.17%, for the condition of total weight of 157.06 kN (16 027 kgf) with payload of 63.42 kN (6472 kgf).