Fernando Luiz Bastian

Oxidation behavior of HVOF sprayed nanocrystalline NiCrAlY powder

Abstract This paper describes recent progress on the research into improving the oxidation behavior of the bond coat using a HVOF nanostructured NiCrAlY coating. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed onto Ni-based alloy to form a nanocrystalline bond coat. The powder and coating structure were characterized by XRD, SEM, and TEM. Oxidation experiments were performed on the coating to form the thermally grown oxide layer (TGO). After heat treatment at 1000 °C for 24 and 95 h, a homogeneous α-Al2O3 layer was formed on top of the bond coat. The oxide layer was analyzed and compared to the coating sprayed using the as-received powder. As shown in the results, the nanostructured characteristic of the coating and the presence of Al2O3 within the cryomilled powders (oxidation occurred during cryomilling process) seem to affect the nucleation of the alumina layer on the top of the coating. The formation of a continuous TGO layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions, such as those presented in the coating sprayed using the as-received powder.

The effects of water absorption on an ester vinyl resin system

Derakene is a vinyl ester resin largely employed as matrix for polymeric based composite systems. In this work, the performance of such polymeric system was evaluated considering the effect of hygrothermal aging. The mechanical and thermal properties were examined for the processed material before and after ageing in water at 60 oC for a maximum period of 64 days. Both analyses indicated the occurrence of post cure of the system after 16 days due to exposition at 60 oC in water, which was also confirmed by analyses in samples with post cure treatment. Moreover, it was observed plasticizing of resin after 36 and 64 days of ageing. Micro structural and fracture surface analyses were carried out in order to characterize the samples. The appearance of surface voids was also observed.

Critical fracture toughness, JC and δ5C, of unidirectional fibre–metal laminates

Abstract Fibre–metal laminates (FMLs) are structural composites designed aiming to produce a damage-tolerant and high strength material. Their main characteristic is their very low fatigue crack propagation rates when compared to traditional aeronautical Al alloys. Their application in aeronautical structures demands a deep knowledge of a wide set of mechanical properties and technological values, including both fracture toughness and residual strength. The objectives of the present work were to present critical toughness values (JC and δ5C) of unidirectional FMLs obtained following a recently proposed methodology and to use them for critical crack length and residual strength predictions. Residual strengths of middle centre-cracked panels of Arall® 2 and 3 were predicted and compared to experimental values from the literature. The results showed that all FMLs evaluated presented higher fracture toughness and crack tolerance than their constituent alloys and that the measured fracture toughness was useful for an accurate prediction of residual strength in centre-cracked plates of Arall.

Fracture Micromechanisms of Fibre-Metal Laminates: In-Situ SEM Observations

The monotonic fracture micromechanisms of an aramid-aluminum laminate were studied using very small single edge bend specimens, SE(B) tested in a small-instrumented testing machine inside a scanning electron microscope. Thefracture process was followed by simultaneous observation of the sample and recording the loading variables. The instability fracture toughness was strongly dependent on the fibre-reinforced epoxy layer and the crack-growth in the external aluminum layers, prior to fracture instability, was influenced by the notch acuity. The load versus loadline displacement behaviourand load instabilities were similar to the ones obtained by conventional testing procedures. The technique implemented proved to be a powerful tool to study the fracture micromechanisms of aramid-aluminum laminates or other fibre-metal laminates and to correlate them with the recorded macro events.

Fatigue crack growth resistance and crack closure behavior in two aluminum alloys for aeronautical applications

Aluminum-lithium alloys are candidate materials for many aerospace applications because of their high specific strength and elastic modulus. These alloys have several unique characteristics such as excellent fatigue crack growth resistance when compared with that of the conventional 2000 and 7000 series alloys. In this study, fatigue crack propagation behavior has been examined in a commercial thin plate of Al-Li-Cu-Mg alloy (8090), with specific emphasis at the fatigue threshold. The results are compared with those of the traditional Al-Cu-Mg alloy (2024). Fatigue crack closure is used to explain the different behavior of the compared alloys.

Fracture toughness evaluation of unidirectional fibre metal laminates using traditional CTOD (δ) and Schwalbe (δ5) methodologies

Abstract Fibre metal laminates (FMLs) were developed for the aeronautical industry, which requires thin sheets with high resistance to fatigue crack growth, high damage tolerance and high specific strength. Considering all these requirements, FMLs are an advantageous choice when compared to metal alloys currently used. In order to employ FMLs in aircraft structures, designers must have a deep knowledge of a wide set of properties including fracture toughness. The aim of this work was to evaluate the available methodologies for critical CTOD measurement of unidirectional FMLs. To achieve this, tests were performed to obtain traditional (BSI/ASTM) and Schwalbe’s CTODs by using experimental procedures especially adapted to these laminates. Results achieved point out that there are differences between both CTOD parameters, that Schwalbe method proved more appropriate, and also that the standard plastic-hinge model does not work properly in FMLs.

Development and characterization of composite materials for production of composite risers by filament winding

Industry has been challenged to provide riser systems which are more cost effective and which can fill the technology gaps with respect to water depth, riser diameter and high temperatures left open by flexibles, steel catenary risers (SCRs) and hybrid risers. Composite materials present advantages over conventional steel risers because composite materials are lighter, more fatigue and corrosion resistant, better thermal insulators and can be designed for improving the structural and mechanical response. This paper contains a study of the toughening mechanism of an epoxy resin under rubber addition by means of fractographic analysis and its relation with the fracture process and increase of strength of a composite riser employing this polymeric matrix. Initially, an epoxy resin system was toughened by rubber CTBN addition (10 wt. (%)) as a way of improving the flexibility of future risers. Mechanical and thermal analyses were carried out for characterizing the polymeric systems. Later, composite tubes were prepared and mechanically characterized. The influence of matrix toughening on the mechanical behavior of the tubes was also studied. Split-disk tests were used to determine the hoop tensile strength of these specimens. The results indicate that the matrix plays an important role in composite fracture processes. The adding rubber to the polymeric matrix promoted a simultaneous increase of stress and elongation at fracture of the tubes manufactured herein, which is not often reported. These results, probably, is function of better adhesion between fibers and polymeric matrix observed in the CTBN-modified composite rings, which was evidenced in the fractografic analysis by SEM after the split-disk tests.

Determination of CTOD C in Fibre Metal Laminates by ASTM and Schwalbe Methods

Fibre Metal Laminates (FMLs) have arisen as a demand of the aeronautical industry to use thin sheets with high resistance to fatigue crack growth, high damage tolerance, corrosion resistance and high specific strength. Considering these requirements, FMLs are an advantageous choice when compared to metal alloys currently used. In order to employ FMLs in aircraft structures, designers must hold a deep knowledge of a wide set of their properties including fracture toughness. The aim of this work was to evaluate the available methodologies to measure fracture toughness at instability (CTODC) in unidirectional fibre metal laminates reinforced with aramid fibres (ARALL®). To achieve this, tests were performed to obtain traditional and Schwalbe CTODs by using experimental ASTM based techniques, especially adapted to these laminates. Results achieved point out that Schwalbe method is more appropriate and also that there are differences between both CTOD parameters.

Characterization of optical, electronic and topographic images in fatigue research

The characterization of images containing fatigue manifestations often implies the detection and quantification of thin and elongated objects such as fatigue striations, slip bands and micro cracks. In this paper, we propose a classification of these images in three categories: (1) fractured surface imaging, (2) damaged surface imaging, and (3) deformed surface imaging. Each category has a preferential mode of acquisition (electronic, optical microscope or laser scanner), and a particular processing (conventional matrix processing, directional filter via Fourier transform, Haralicks segmentation process, shape classification). The accuracy and reliability of the algorithms presenting a novelty will be further discussed. The objectives are to measure the orientation and spacing between striations on fractured surfaces, to evaluate the relative surface of slip bands on damaged surfaces, and to assess the shape and size of a crack tip plastic zone qualitatively via deformed surface imaging.

A new method for calculating the elastic component of the crack opening displacement (COD)

Abstract A criticism is made in the method of calculating the elastic component of the Crack Opening Displacement (COD) following the British standard. The problem with this standard is the criterion for selecting the value of force, from a force-displacement curve, to be used for calculating the elastic component of COD. A theoretical discussion is made showing the impropriety of the criterion with the standard; experimental evidence also points to this fact. A new method for calculating the elastic component of COD is proposed. It is also shown that for the materials and conditions of the present work the values of the elastic component do not depend upon the geometry and orientation of the COD specimens.