Ricardo de Medeiros

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

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.

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.

Effective properties evaluation for smart composite materials with imperfect fiber–matrix adhesion

A numerical approach is proposed to evaluate the effective properties of piezoelectric fibers embedded in a nonpiezoelectric matrix, considering imperfect contact between fiber and matrix. Firstly, a Representative Volume Element is analyzed via Finite Element Method for different loadings with suitable boundary conditions applied in a unique way. Transversely, isotropic piezoelectric materials with circular and square cross-section fibers are analyzed for square arrangements with different fiber volume fractions as well as with perfect and imperfect contact conditions. The results for circular and square cross-section fibers with perfect contact are compared to the literature data in order to verify the consistency of proposed numerical approach. After that, the results with imperfect contact are discussed, observing the influence of the fiber volume fraction and the level of the imperfection in the fiber–matrix adhesion. Results show that the imperfect contact not only influences the elastic constants, but also the piezoelectric effective values.

Numerical and Experimental Damage Identification in Metal-Composite Bonded Joint

Experimental and numerical analyses were carried out in order to identify damage in metal-composite bonded joints, which were manufactured from carbon-fiber reinforced polymers and titanium plates joined by an epoxy resin. The monitoring was performed by using vibration-based method through changes in frequency response function (FRF). First, free–free vibration tests were performed on four different specimens (with presence or not of damage and with presence or not piezoelectric sensor). Finite element analyses for the conditions without the transducer were also carried out and compared with the experimental data. FRFs were obtained by using the response of the PZT placed over the titanium plate and accelerometers located at other positions of the joint. The damage was reproduced by replacing 50% of the overlap with a layer of Teflon. Lastly, based on damage identification metric, FRFs for the undamaged and damaged structure were compared, evaluating not only the potentialities and limitations of the applied experimental detection technique, but also the computational model. The experimental and numerical results showed that the vibration-based damage identification methods combined to the metrics can be used in structural health monitoring systems.

Experimental and numerical analysis of single lap bonded joints: Epoxy and castor oil PU-glass fibre composites

ABSTRACT Currently, there is a growing concern for the environment. Several studies of new materials to reduce environmental impact have been carried out by different research groups, and many companies have replaced parts made of fossil sources by renewable materials. The use of polyurethane (PU) derived from castor oil as a matrix for composite materials and adhesives is one example. Hence, the present work aims to compare the numerical and experimental analyses of castor oil PU and epoxy resin not only as a matrix of composite materials, but also as an adhesive of bonded joints. The joint coupons were manufactured by using castor oil PU-glass fibre and epoxy-glass fibre as adherents, which were bonded by epoxy or castor oil PU. Thus, four combinations of adherents and adhesives were investigated. Specimens with identical geometry were used in all tests, which were based on guidelines for single lap bonded joints. Computational simulations via Finite Element Method were performed for predictions of the adhesive layer stresses and strength. In addition, a material model is proposed to predict the failure of the adhesive layer. The experimental and numerical results showed that PU derived from castor oil has good mechanical performance, making this material a feasible alternative for bonded joints, mostly nowadays when environment is a major concern.

Structural Health Monitoring of Sandwich Structures Based on Dynamic Analysis

This work aims to contribute to the development of SHM systems based on vibration methods to be applied on sandwich structures. The main objective is focused on experimental damage identification via changes in the Frequency Response Function FRF with the usage of damage metrics. Specimens of sandwich structures made from skins of epoxy resin reinforced by glass fiber and a core of PVC foam are manufactured. First, preliminary nondamped Finite Element FE models are performed, and results obtained are used to define the frequency range of interest for the experimental procedure. After that, vibration experimental analyses are carried out on undamaged specimens. The natural frequencies are compared to the preliminary FE results. Second, experimental analyses are performed on damaged specimens with and without piezoelectric sensors. Then, damage metric values are calculated based on FRFs for damaged and undamaged structures, which were obtained from experimental and FE analyses with damping effects . In addition, a new procedure is proposed to improve the quality of results provided by the damage metric. It is shown that the new procedure is very effective to identify the damage using both amplitude and phase from FRFs. Lastly, it is discussed the potential and limitations of the FE model to predict damage metric values, comparing to experimental data.