Volnei Tita

Theoretical and experimental dynamic analysis of fiber reinforced composite beams

The composite materials are well known by their excellent combination of high structural stiffness and lowweight. Their inherent anisotropy allows the designer to tailor the material in order to achieve the desired performance requirements. Thus, it is of fundamental importance to develop tools that allow the designer to obtain optimized designs considering th e structural requirements, functional characteristics and restrictions imposed by the production process. Within these requirements, this work considers the dynamic behavior of components manufactured from fiber reinforced composite materials. some beams were made using the hand -lay up process followed by a molding under pressure and heating. Experimental dynamic tests were carried out using specimens with different fiber orientations and stacking sequences. From the results, the influence of the fibers orientations as well as the stacking sequences on the natural frequencies and modal damping were investigated. Also, these experiments were used to validate the theoretical model and the results obtained from the finite element analysis.

Effective properties evaluation for smart composite materials

The purpose of this article is to present a method which consists in the development of unit cell numerical models for smart composite materials with piezoelectric fibers made of PZT embedded in a non-piezoelectric matrix (epoxy resin). This method evaluates a globally homogeneous medium equivalent to the original composite, using a representative volume element (RVE). The suitable boundary conditions allow the simulation of all modes of the overall deformation arising from any arbitrary combination of mechanical and electrical loading. In the first instance, the unit cell is applied to predict the effective material coefficients of the transversely isotropic piezoelectric composite with circular cross section fibers. The numerical results are compared to other methods reported in the literature and also to results previously published, in order to evaluate the method proposal. In the second step, the method is applied to calculate the equivalent properties for smart composite materials with square cross section fibers. Results of comparison between different combinations of circular and square fiber geometries, observing the influence of the boundary conditions and arrangements are presented. Keywords: smart composite materials, piezoelectric fiber composite, active fiber composite, finite element analyses, effective properties

Study of an anisotropic polymeric cellular material under compression loading

This paper emphasizes the influence of micro mechanisms of failure of a cellular material on its phenomenological response. Most of the applications of cellular materials comprise a compression loading. Thus, the study focuses on the influence of the anisotropy in the mechanical behavior of cellular material under cyclic compression loadings. For this study, a Digital Image Correlation (DIC) technique (named Correli) was applied, as well as SEM (Scanning Electron Microscopy) images were analyzed. The experimental results are discussed in detail for a closed-cell rigid poly (vinyl chloride) (PVC) foam, showing stress-strain curves in different directions and why the material can be assumed as transversely isotropic. Besides, the present paper shows elastic and plastic Poissons ratios measured in different planes, explaining why the plastic Poissons ratios approach to zero. Yield fronts created by the compression loadings in different directions and the influence of spring-back phenomenon on hardening curves are commented, also.

A procedure to estimate the dynamic damped behavior of fiber reinforced composite beams submitted to flexural vibrations

This work proposes a procedure to estimate the dynamic damped behavior of fiber reinforced composite beams in flexural vibrations. A set of experimental dynamic tests were carried out in order to investigate the natural frequencies and modal shapes. These results are used to evaluate the damping factors by the program FREQ. These damping factors are then used as input to a damped dynamic analysis by the Finite Element Method, using Rayleigh Model. A good agreement between theoretical and experimental results was obtained. Thus, it became possible to validate the proposed procedure to evaluate dynamic damped behavior of composite beams.

Numerical simulation of anisotropic polymeric foams

This paper shows in detail the modelling of anisotropic polymeric foam under compression and tension loadings, including discussions on isotropic material models and the entire procedure to calibrate the parameters involved. First, specimens of poly(vinyl chloride) (PVC) foam were investigated through experimental analyses in order to understand the mechanical behavior of this anisotropic material. Then, isotropic material models available in the commercial software AbaqusTM were investigated in order to verify their ability to model anisotropic foams and how the parameters involved can influence the results. Due to anisotropy, it is possible to obtain different values for the same parameter in the calibration process. The obtained set of parameters are used to calibrate the model according to the application of the structure. The models investigated showed minor and major limitations to simulate the mechanical behavior of anisotropic PVC foams under compression, tension and multi-axial loadings. Results show that the calibration process and the choice of the material model applied to the polymeric foam can provide good quantitative results and save project time. Results also indicate what kind and order of error one will get if certain choices are made throughout the modelling process. Finally, even though the developed calibration procedure is applied to specific PVC foam, it still outlines a very broad drill to analyze other anisotropic cellular materials.

Considerations for the design of polymeric biodegradable products

Abstract Several biodegradable polymers are used in many products with short life cycles. The performance of a product is mostly conditioned by the materials selection and dimensioning. Strength, maximum strain and toughness will decrease along its degradation, and it should be enough for the predicted use. Biodegradable plastics can present short-term performances similar to conventional plastics. However, the mechanical behavior of biodegradable materials, along the degradation time, is still an unexplored subject. The maximum strength failure criteria, as a function of degradation time, have traditionally been modeled according to first order kinetics. In this work, hyperelastic constitutive models are discussed. An example of these is shown for a blend composed of poly(L-lactide) acid (PLLA) and polycaprolactone (PCL). A numerical approach using ABAQUS is presented, which can be extended to other 3D geometries. Thus, the material properties of the model proposed are automatically updated in correspondence to the degradation time, by means of a user material subroutine. The parameterization was achieved by fitting the theoretical curves with the experimental data of tensile tests made on a PLLA-PCL blend (90:10) for different degradation times. The results obtained by numerical simulations are compared to experimental data, showing a good correlation between both results.

Numerical and analytical analyses for active fiber composite piezoelectric composite materials

This study consists of the calculation of the effective properties for active fiber composites made of either circular or square cross-section fibers not only by using finite element analysis and representative volume elements, but also based on the asymptotic homogenization method. Thus, there is an investigation about different approaches, which have specific mathematical formulations and unique characteristics. The comparison between numerical and analytical approaches shows that the numerical results are in good agreement with investigations performed by both analytical and semi-analytical methods, mainly the predictions for loading applied in fiber direction. For active fiber composites made of circular cross-section fibers, the maximum difference between asymptotic homogenization method and finite element analysis is from 1.29% to 5.49% for mechanical and piezoelectric effective properties, respectively, considering representative volume element in square arrangement. However, for active fiber composites made of square cross-section fibers, the maximum difference between semi-analytical method and finite element analysis is from 2.15% to 17.09% for mechanical and piezoelectric effective properties, respectively, considering representative volume element in square arrangement.

Damage-induced hydrolyses modelling of biodegradable polymers for tendons and ligaments repair.

The use of biodegradable synthetic grafts to repair injured ligaments may overcome the disadvantages of other solutions. Apart from biological compatibility, these devices shall also be functionally compatible and temporarily displayed, during the healing process, adequate mechanical support. Laxity of these devices is an important concern. This can cause failure since it may result in joint instability. Laxity results from a progressive accumulation of plastic strain during the cyclic loading. The functional compatibility of a biodegradable synthetic graft and, therefore, the global mechanical properties of the scaffold during degradation, can be optimised using computer-aiding and numerical tools. Therefore, in this work, the ability of numerical tools to predict the mechanical behaviour of the device during its degradation is discussed. Computational approaches based on elastoplastic and viscoplastic constitutive models are also presented. These models enable to simulate the plastic strain accumulation. These computational approaches, where the material model parameters depend on the hydrolytic degradation damage, are calibrated using experimental data measured from biodegradable suture fibres at different degradation steps. Due to durability requirements the selected materials are polydioxone (PDO) and polylactic acid and poly-caprolactone blend (PLA-PCL). Computational approaches investigated are able to predict well the experimental results for both materials, in full strain range until rupture and for different degradation steps. These approaches can be further used in more complex fibrous structures, to predict its global mechanical behaviour during degradation process.

Experimental analyses of metal-composite bonded joints: damage identification

The advent of composite co-cured and co-bonded integrated construction in aircraft structures has lead to the replacement of fastened joints with bonded joints between the skins and the stiffeners. Skin-stiffener debondings could occur due to impact or other operational reasons and it is usually internal failure. Damage identification of bonded components, which are often vital elements in many structures, is crucial for the prevention of failure of the entire structure. Thus, different researchers have investigated vibration-based methods as an alternative technique to be used in the structural health monitoring (SHM) systems. Hence, this work consists of investigating experimentally through the vibration-based method, the dynamic behavior changes in a bonded metal-composite structure by using piezoelectric transducer and accelerometers in order to monitory the damage. The damage is an artificial debonding in the joint, which was simulated by inserting Teflon™ tapes within the joint. In-situ inspection as ensured by accelerometer and piezoelectric transducers (PZT) bonded to the structure. Indeed, with a simple comparison of the frequency response functions is difficult to conclude if there is damage in the structure, unless a large damage is presented. However, by using damage metrics, it is possible to identify the damage with more accuracy. Thus, the experimental results obtained by the accelerometers were compared to the data provided by the smart composite sensors (PZT). Finally, it was discussed the advantages and limitations of the experimental analyses and the identification technique proposal.

Computational methodology of damage detection on composite cylinders: structural health monitoring for automotive components

A numerical investigation of the damage effects on the structural response of the composite cylinders damaged by impact loading was performed. A computational methodology, which consists of carrying out four-step finite element (FE) analyses in progressive sequence, was used. Firstly, modal analyses were carried out for the intact structure to determine the natural frequencies and modal shapes. Then, vibration analyses were performed for intact structure to obtain the frequency response function (FRF). After that, impact analyses were performed by using a material model, which is accessed to predict the damage. Based on damaged FE model, vibration analyses, again, were carried out to determine the new FRF. Thus, the results of the damaged structure were combined to intact model results by using a specific metric in order to indicate the damage or not in the composite cylinders. Finally, it was discussed about the advantages and limitations of SHM systems, which use vibration-based methods and piezoelectric sensors.