Inês Pires

Performance of bi-adhesive bonded aluminium lap joints

The stress concentration towards the ends of a bonded lap joint depends to some extent on the relative stiffnesses of the adherend and the adhesive used. For a given adherend, the lower the stiffness of the adhesive used in the bondline, the lower the stress concentration, giving rise to potentially higher joint strength. The paper presents the results of a study of the application of two adhesives with different stiffnesses along the overlap length in single lap joints. A stiff adhesive was applied in the middle portion of the overlap, while a low modulus adhesive was applied towards the edges prone to stress concentrations. The results show measurable increase in strength of the bi-adhesive bonded joints compared with those in which single adhesives were used over the full length of the bondline.

Fabrication of computationally designed scaffolds by low temperature 3D printing

The development of artificial bone substitutes that mimic the properties of bone and simultaneously promote the desired tissue regeneration is a current issue in bone tissue engineering research. An approach to create scaffolds with such characteristics is based on the combination of novel design and additive manufacturing processes. The objective of this work is to characterize the microstructural and the mechanical properties of scaffolds developed by coupling both topology optimization and a low temperature 3D printing process. The scaffold design was obtained using a topology optimization approach to maximize the permeability with constraints on the mechanical properties. This procedure was studied to be suitable for the fabrication of a cage prototype for tibial tuberosity advancement application, which is one of the most recent and promising techniques to treat cruciate ligament rupture in dogs. The microstructural and mechanical properties of the scaffolds manufactured by reacting α/β-tricalcium phosphate with diluted phosphoric acid were then assessed experimentally and the scaffolds strength reliability was determined. The results demonstrate that the low temperature 3D printing process is a reliable option to create synthetic scaffolds with tailored properties, and when coupled with topology optimization design it can be a powerful tool for the fabrication of patient-specific bone implants.

Fume emissions during gas metal arc welding

The control of exposure to welding fumes is of increasing importance in promoting a healthy, safe and productive work environment. This article describes the effects of shielding gas composition on the amount and composition of welding fumes produced during gas metal arc welding (GMAW). The amount of fumes generated during welding was measured for steady current over a range of wire-feed speeds and arc voltages using the standard procedures contained in ANSI/AWS F1.2 [American Welding Society. ANSI/AWS F1.2. Laboratory method for measuring fume generation rates and total fume emission of welding and allied processes. Miami, Florida; 1992]. Results of these measurements show that the fume formation rates (FFRs) increase with CO2 and O2 in the shielding gas mixture. The lowest FFRs were obtained with the mixtures of Ar + 2%CO2 and Ar + 3%CO2 + 1%O2. The highest FFRs were obtained with the mixtures of Ar + 18%CO2 and Ar + 5%CO2 + 4%O2. The welding fumes contains mainly iron, manganese, silicon, titanium and sodium under oxide forms. The fume cluster particles have dimensions between 0.5 and 2 µm. The FFR was found to be governed by the transfer modes of molten metal, i.e. the current intensity and arc voltage, as well as by the shielding gas mixtures composition. Thus these parameters have to be taken into consideration before designing a welding process. Whenever possible, users of GMAW should use the lowest current intensity. However, when this is not possible, due to the constraints of process productivity, welders should use higher currents, but with Ar + 2%CO2 and Ar + 3%CO2 + 1%O2 shielding mixtures, which will lead to smaller fume emissions.

Influence of Pre-treatments on the Surface Condition of 2024-T3 Aluminium Alloy

SUMMARY The influences of various pre-treatments of interest in adhesive bonding on the surface condition of 2024-T3 alloy have been examined. In particular, the morphologies of films formed during alkaline etching (NaOH), electropolishing (perchloric acid/ethanol), acid pickling (Na2Cr2O7/H2SO44 and anodzing (H3PO4), and the consequences of film growth to the composition of the alloy, have been determined using transmission and scanning electron microscopies and Rutherford backscattering spectroscopy. The pretreatments result generally in scalloped surfaces, with deeper cavities associated originally with second phase particles. The topography of the surface is generated primarily in the pre-treatment stages, with subsequent anodizing having a lesser influence. Enhanced attack of the alloy in the vicinity of grain boundaries occurs during acid pickling, which appears to be accentuated during subsequent anodizing. Alkaline etching and electropolishing produce compact films, respectively about 5–8 and 2–3 nm thick, on the main alloy surface. In contrast, acid pickling produces an open-textured, porous film, about 30 nm thick and of fibrous appearance in section, with a thin barrier layer at the alloy/film interface. The porous films produced by anodizing in phosphoric acid are of typical thickness in the range 200–350 nm, with a 10–18 nm thick, barrier layer. The formation of the various films results in significant enrichment of copper in the alloy to the range 4.2 × 1015 to 6.4 × 15 Cu atoms cm−2.

Application of a 3D printed customized implant for canine cruciate ligament treatment by tibial tuberosity advancement.

Fabrication of customized implants based on patient bone defect characteristics is required for successful clinical application of bone tissue engineering. Recently a new surgical procedure, tibial tuberosity advancement (TTA), has been used to treat cranial cruciate ligament (CrCL) deficient stifle joints in dogs, which involves an osteotomy and the use of substitutes to restore the bone. However, limitations in the use of non-biodegradable implants have been reported. To overcome these limitations, this study presents the development of a bioceramic customized cage to treat a large domestic dog assigned for TTA treatment. A cage was designed using a suitable topology optimization methodology in order to maximize its permeability whilst maintaining the structural integrity, and was manufactured using low temperature 3D printing and implanted in a dog. The cage material and structure was adequately characterized prior to implantation and the in vivo response was carefully monitored regarding the biological response and patient limb function. The manufacturing process resulted in a cage composed of brushite, monetite and tricalcium phosphate, and a highly permeable porous morphology. An overall porosity of 59.2% was achieved by the combination of a microporosity of approximately 40% and a designed interconnected macropore network with pore sizes of 845 μm. The mechanical properties were in the range of the trabecular bone although limitations in the cages reliability and capacity to absorb energy were identified. The dogs limb function was completely restored without patient lameness or any adverse complications and also the local biocompatibility and osteoconductivity were improved. Based on these observations it was possible to conclude that the successful design, fabrication and application of a customized cage for a dog CrCL treatment using a modified TTA technique is a promising method for the future fabrication of patient-specific bone implants, although clinical trials are required.

Numerical simulation of mono- and bi-adhesive aluminium lap joints

In single-lap bonded joints the stresses are maximum at the edges, where failure usually begins, while in the centre stresses are the lowest. Stress concentration towards the ends of a bonded lap joint depends, to some extent, on the relative stiffness of the adherend and the adhesive used. For a given adherend, the lower the stiffness of the adhesive used in the bondline, the lower the stress concentration, giving rise to potentially higher joint strength. This paper discusses the failure mechanism of bi-adhesive joints, where a flexible adhesive is used at the joint edges, while a less ductile adhesive is used in the centre of the overlap. The results show an increase in shear strength of the bi-adhesive-bonded joints compared with those in which single adhesives were used over the full length of the bondline. The increase of the apparent lap-shear strength was qualitatively predicted, through finite element modelling.

The role of shell/core saturation level on the accuracy and mechanical characteristics of porous calcium phosphate models produced by 3Dprinting

Purpose – This paper aims to study the influence of the binder saturation level on the accuracy and on the mechanical properties of three-dimensional (3D)-printed scaffolds for bone tissue engineering. Design/methodology/approach – To study the influence of the liquid binder volume on the models accuracy, two quality test plates with different macropore sizes were designed and produced. For the mechanical and physical characterisation, cylindrical specimens were used. The models were printed using a calcium phosphate powder, which was characterised in terms of composition, particle size and morphology, by X-ray diffraction (XRD), laser diffraction and Scanning electron microscopy (SEM) analysis. The sample’s physical characterisation was made using the Archimedes method (porosity), SEM, micro-computer tomography (CT) and digital scan techniques, while the mechanical characterisation was performed by means of uniaxial compressive tests. Strength distribution was analysed using a statistical Weibull approac...

Characterization of sintered hydroxyapatite samples produced by 3D printing

Purpose – The purpose of this study is to characterize sintered hydroxyapatite (HA) samples produced by three-dimensional printing (3DP). This study is part of a project concerned with the fabrication of calcium phosphates implants by 3DP. However, before considering a more complex structure, like scaffolds or implants, a thorough knowledge of the role played by the sintering temperature on physical and mechanical the properties of porous HA is necessary. Design/methodology/approach – The characteristics of sintered HA samples have been analyzed by means of x-ray diffraction, scanning electron microscope (SEM) and uniaxial compression tests. The 3DP parameters used to produce the HA samples were those who led to higher accuracy and mechanical stability. Findings – Sintering temperature and powder morphology are critical factors influencing densification behavior, porosity, phase stability, mechanical strength and tangent modulus of the HA samples produced by 3DP. This study allowed us to conclude about th...

Welding Technologies for Repairing Plastic Injection Moulds

The aim of the present paper is to investigate the metallurgical behaviour of AISI P20 tool steel frequently used in injection moulds, after welding. For this purpose Tig and Laser welding tests were conducted to determine the influence of the welding parameters, materials and preheating. The welding quality was assessed through the microstructural properties and microhardness evolution. The results achieved with the different processes were compared. This work allows to obtain a deeper knowledge concerning tool steels behaviour after welding.

Performance of 2024-T3 aluminium adhesive bonded joints

Abstract Adhesive bonding of aluminium alloys has been investigated due to its growing importance in aeronautic, aerospace, and automotive industries. This paper examines the influences of two pretreatments, alkaline etching and acid pickling, applied singly or in combination with phosphoric acid anodizing on the mechanical behavior of adhesive bonded 2024-T3 aluminium alloy joints, before and after aging by water immersion. The aim of the present study is to investigate the nature of the oxide formed on the aluminium-copper alloy with different surface pretreatments/phosphoric acid anodizing, to analyze the influence of surface features in the durability of bonded joints, the interactions between the oxide and the adhesive, and the influence of water on the mechanical properties of the bulk adhesive, and to understand the phenomena responsible for the degradation of bonded joints.