E. M. Castrodeza

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.

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.

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.

Processing and characterization of dual phase steel foam

Porous materials featuring cellular structures are known to have many interesting combinations of physical and mechanical properties. Some of them have been extensively used in the transportation field (i.e. balsa wood). Steel foams presented promising theoretical properties for both functional and structural applications in transportation, but processing of such a kind of foams is complex due to their high melting point. Recently a technique for processing Cu-based alloys open-cell foams through the molten metal infiltration of a leachable bed of amorphous SiO2 particles was proposed. A variation of the proposed technique that uses SiC particles as space holder is now presented and was recently successfully applied for dual phase steel foam processing. Results from a processing of dual phase DP500 steel foams, including some morphological, micro-structural and mechanical characterization, are here presented.

Characterization and Comparative Study of Pseudo-Elastic Cu-Zn-Al Foams Synthesized by Two Different Methods

Low cost Cu-Zn-Al foams present, in specific composition ranges, good pseudoelastic properties associated with interesting damping capacities, which establish these materials as attractive from the point of view of structural applications. These foams are manufactured by infiltration of small SiO2 spheres into the molten alloys. After alloy solidification the SiO2 spheres are removed by immersing the material in a solution of aqueous hydrofluoric acid. In this work, we have investigated the effect of two different foam preparation techniques: inductive and resistive melting. The paper details both the physico-chemical and mechanical characterization of the foams synthesized with the two different methods. The purpose of this study is to determine the quality of each synthesis method in order to identify the best alternative.

Production of 17CrMoV5-11 steel sponges utilising powder metallurgical replication technique with SiC as space holder

Porous materials, characterised by their cellular structure, have interesting combinations of physical and mechanical properties. In such a context, steel sponges could be employed as an alternative to light alloys in several applications needing a good compromise between weight and mechanical properties. In this work, a new approach is presented that uses a powder metallurgical replication technique to produce steel foams while using SiC spheres as a space holder. The processed sponges were characterised from a morphological, microstructural and mechanical point of view. The results of the characterisation demonstrated the feasibility and the repeatability of the proposed production route.