M. Freitas

Identification of material properties of composite plate specimens

Abstract An indirect identification technique to predict the mechanical properties of composite plate specimens is presented. This technique makes use of experimental eigenfrequencies, the corresponding numerical eigenvalue evaluation, sensitivity analysis and optimization. The laminate analysis is formulated in terms of non-dimensional material parameters and the discrete model is based on the linear shear deformation theory of Mindlin. The constrained minimization of an error functional expressing the difference between measured higher frequencies of a plate specimen and the corresponding numerical ones is then carried out to find the desired optimum parameters. The required sensitivities with respect to changes in the non-dimensional material parameters have the option of being evaluated analytically, semi-analytically or alternatively by finite difference. Results which show the validation of the sensitivities and the limitations of the model to predict the required quantities and its range of application and accuracy are demonstrated through test cases.

Characterization of material parameters of composite plate specimens using optimization and experimental vibrational data

A numerical/experimental method for identification of material parameters of composite materials is proposed in this paper. This method combines experimental techniques for the determination of vibration eigenfrequencies of plates made of composite materials along with a finite element numerical method for the determination of the corresponding numerical eigenfrequencies. The identification process makes use of optimization techniques, through the minimization of an error measurement that estimates the deviation between numerical and experimental eigenvalues, for a given set of material parameters. The accuracy of the proposed technique is discussed through test cases.

Combined numerical-experimental model for the identification of mechanical properties of laminated structures

Abstract A combined numerical–experimental method for the identification of six elastic material modulus of generally thick composite plates is proposed in this paper. This technique can be used in composite plates made of different materials and with general stacking sequences. It makes use of experimental plate response data, corresponding numerical predictions and optimisation techniques. The plate response is a set of natural frequencies of flexural vibration. The numerical model is based on the finite element method using a higher-order displacement field. The model is applied to the identification of the elastic modulus of the plate specimen through optimisation techniques, using analytical sensitivities. The validity, efficiency and potentiality of the proposed technique is discussed through test cases.

Stress Intensity Factors for semi-elliptical surface cracks in round bars under bending and torsion

Abstract Stress Intensity Factor calculations for semi-elliptical surface cracks in round bars subjected to bending and torsion using a three-dimensional finite element model are presented. The configuration of the semi-ellipse follows the equation b=(2s)/π, experimentally obtained, where b is the crack depth and 2s is the arc crack length. First, Stress Intensity Factors are obtained for bending loading (Mode I) and compared with available literature results in order to validate the proposed model. Second, Stress Intensity Factors under torsion (Mode III) are computed by employing a finite element model of the whole round bar, since such loading is non-symmetrical. Results showed that in torsion case the Mode III loading also induces a significant KII at the points where the crack front intercepts the free surface of the round bar, whereas at the deepest point only KIII exits. The aims of this study have in view the determination of the crack growth rate in rotating load-transmission devices with this type of crack.

A Procedure for Fast Evaluation of High-Cycle Fatigue Under Multiaxial Random Loading

This paper presents a fast evaluation procedure for high-cycle fatigue (HCF) under multiaxial random loading. The recent multiaxial cycle counting method of Wang and Brown is used to identify the loading reversals. For each identified reversal, the effective shear stress amplitude is directly calculated from the component stress ranges by an equation derived from the MCE approach, which is a newly developed method to account for nonproportional loading effect. This shear stress amplitude and the maximum hydrostatic stress during the time period of an identified reversal are used to evaluate the fatigue damage for that reversal by Crosslands criterion. The fatigue damage of the loading block is then calculated by summing the damages of all the identified reversals by Miners rule. Comparisons with other multiaxial HCF approaches show that the procedure is a computationally efficient and conservative engineering approach.

Insertion reactions of heterocumulenes into the niobium-hydride bond of isocyanide and carbonyl niobocene complexes

Abstract The reaction of the hydride niobocene complexes, Nb( η 5 -C 5 H 4 SiMe 3 ) 2 (H)(CNR) (R=2,6-dimethylphenyl (Xyiyl) orcyclohexyl (Cy)) and Nb( η 5 -C 5 H 4 SiMe 3 ) 2 ( H )( CO ) with the heterocumulenes PhN=C=X (X=0 or S) affords the new formamido and thioformamido complexes Nb(( η 5 -C 5 H 4 SiMe 3 ) 2 ) 2 (CNR)(η 1 -XC(H)NPh) (R=Xylyl, X=0 (1); R=Xylyl, X=S (2); R=Cy, X =O (3); R-Cy, X=S (4)) and Nb(( η 5 -C 5 H 4 SiMe 3 ) 2 ) 2(CO) (η1-XC(H) NPh) (X=0 (5); X=S (6) ). The protonation of these derivatives using HCF3COO yields the trifluoroacetato complexes Nb(( η 5 - C 5 H 4 SiMe 3 ) 2 2(CNR) ( η 1 -OOCCF 3 ) (R=Xylyl (7); R =Cy (8)) and Nb(( η 5 -C 5 H 4 SiMe 3 ) 2 )2(CO) ( η 1 -000CF 3 ) ( 9 ) and the imine derivatives HOC(H)=NPh and HSC(H)=NPh. Thermal treatment of 5 gives the η 2 -formamido, complex Nb( η 5 -C 5 H 4 SiMe 3 ) 2 ( η 2 -OC(H)NPh) (10). All these complexes have been characterised by spectroscopic methods.

Eco-composite: the effects of the jute fiber treatments on the mechanical and environmental performance of the composite materials

In this study, untreated and treated jute fiber composites were investigated as candidates to replace glass fibers as reinforcement to produce structural composites with better environmental performance. The surface of the jute fibers was modified by drying and bleaching/drying treatments to improve the wetting behavior of the apolar polyester, improving the mechanical properties of the composites. The mechanical characterization of the composites was obtained according to the ASTM standards (D-3039/D-790) and dynamic mechanical analysis. The environmental characterization was obtained by life-cycle assessment method. The treatment characterization was obtained by horizontal attenuated total reflectance infrared spectroscopy and thermogravimetry. Finally, jute composites were compared with glass composites and results show that the jute fiber treatments imply a significant increase of the mechanical properties of the composites without damaging their environmental performances.

A New Criterion for Evaluating Multiaxial Fatigue Damage under Multiaxial Random Loading Conditions

Generally, mechanical components or structures are subjected to random and a three-dimensional stress state; there are very few field loading paths which can be experimentally fully simulated in laboratory. Loading path parameters such as load sequence, stress level or proportionality/non-proportionality presences are unknown variables with unknown levels under random loading conditions which are impossible to modulate in laboratory because the load spectra is unknown. The load spectrum depends on numerous factors such as environmental, mechanical or user behavior. At design stages the fatigue life estimation is based on typical loading paths or typical loading spectra, however that assumption may be very different from the usage regime. From here it can be concluded that the random multiaxial fatigue issue is of utmost importance to monitoring the in-field damage accumulation. This work presents a proposal to estimate the accumulated damage resulted from multiaxial random loadings based on the SSF equivalent stress and SSF virtual cycle counting concept.

Automation in Strain and Temperature Control on VHCF with an Ultrasonic Testing Facility

Increased safety and reliability in mechanical components has become a subject of prime importance in recent years. Therefore, a proper understanding of damage and fracture mechanics in materials and components designed to withstand very high cycle fatigue (VHCF) loadings is extremely important nowadays. However, the use of conventional machines for fatigue testing is very time consuming and costly for VHCF tests. Ultrasonic machines have been introduced as a way to increase the number of cycles in fatigue testing up to IE8 to IE10 cycles within a considerably reduced amount of time. Nevertheless, the accurate measurement of the parameters that influence fatigue life at ultrasonic frequencies (e.g., stress, displacement, strain rate, temperature, and frequency) is still a matter of concern and ongoing development. Because of the high frequencies involved in VHCF testing, a huge amount of heat is generated over the specimen, which greatly affects the variables determining the fatigue behavior. This paper describes the design and instrumentation of an ultrasonic fatigue testing machine that operates at a working frequency of 20 kHz. Among other features, it incorporates automated strain and temperature control. In order to run automated tests, a closed-loop monitoring and control system was developed based on the measured temperature and displacement amplitudes. Temperature readings are made with a pyrometer and thermography camera, and displacement is monitored at the free end of the specimen with a high-resolution laser. The machines power output is continuously adjusted from the displacement readings, so that the stress variations within the specimen are as flat as possible. When the temperature increases above a certain set value, a cooling function is triggered and the test is interrupted until the specimen is cooled down. Data are acquired, managed, and processed with a data acquisition device working at a 400 kHz sampling frequency. The advantages and limitations of metal fatigue testing at very high frequencies are discussed in this paper, with special emphasis on strain and temperature-control issues. Comparisons are made of tests carried out with and without both displacement and temperature control on two metallic alloys, copper 99 % and carbon steel, with the determination of strength-life (S-N) curves.

A study on the influence of Ni-Ti M-Wire in the flexural fatigue life of endodontic rotary files by using Finite Element Analysis

The aim of this paper is to analyze the cyclic performance of two different Ni-Ti endodontic rotary files made from different alloys under bending using Finite Element Analysis (FEA). When experimentation is not available, this is not a trivial task and most papers on the subject rely on static analysis only. Two Ni-Ti rotary instruments are selected, ProFile GT and a GT Series X (GTX). The latter file is made from M-Wire, which has been thermo-mechanically processed to have larger flexibility, according to its manufacturer. The mechanical response was studied by considering different scenarios in the FEA package, in which the material properties were introduced according to existing literature. The method and results are presented and discussed so that this paper can be used as a guideline for future works. Although not fully reflective of the instruments behavior in a dynamic rotation intra-canal system, the models used constitute a good approximation when a comparison between two instruments is at stake. It is shown that the GTX file has a lower risk of fatigue fracture during its clinical use when compared to the GT file, especially when the root canal makes the file deform into an extreme geometry. However, if the root canal does not make the file deform more than a certain amount, the GT file is equally good from the point of view of mechanical endurance.