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ABSTRACT: In this paper we present the methods of determination and the stress obtained at the periphery of a cold expanded hole in
a 2024-T3 Alclad aluminum alloy sheet. The measurements in the aluminum clad were performed by the sin2Ψ method, taking experimental precautions to deal with the texture effects. In the core aluminum a special method had to be
implemented to determine the stress values in a direction not accessible to the X-ray diffraction. The strains were measured
in sample orientations selected according to the texture characteristics and stress factorsF
ij
were used to calculate the stress tensor. TheF
ij
values were determined assuming a quasi-isotropic material behavior, after concluding that the stress results were not significantly
affected by factors calculated for textured material. The residual stress profile, both in the clad and in the sheet, shows
a nearly axisymmetric stress state. Compressive stresses were observed near the periphery of the hole, with values that are
higher on the exit than on the entrance face. Residual stresses were also higher in the hoop direction than in the radial
direction. They decreased with the radial distance to the hole and affected the previous stress state over a distance of 6
mm. The plastic deformation induced by the cold expansion is well evidenced by the FWHM values, which in the affected zone
decrease with increasing distance from the hole edge.
Experimental Mechanics 04/2012; 45(1):83-88. · 1.52 Impact Factor
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ABSTRACT: Fibre Bragg grating (FBG) sensors are finding increased usage in experimental mechanics for monitoring service conditions in structures and other equipment and are currently being tested for process monitoring. In FBG sensors, strain and temperature cause a shift in the Bragg wavelength reflected by the grating contained in these fibres. In situ monitoring of strain and temperature during welding processes increases knowledge of the welded material and the welding process itself. In the present work, two welding processes are monitored using FBG sensors and the complete measurement approach including sensor selection, calibration, instrumentation, welding monitoring and result interpretation is presented. Calibration for strain measurements at constant temperature was performed using a four-point bending test, and temperature calibration was carried out using an oven. Results for a sensor length of 5 mm are presented. Both transient and residual strains were recorded during experiments on metal inert gas and friction stir welding and the possible impact of this monitoring technology is discussed in the light of process optimization and subsequent structural health monitoring.
Measurement Science and Technology 06/2010; 21(8):085105. · 1.49 Impact Factor
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ABSTRACT: The present paper reports research work carried out to characterise the fatigue behaviour of the Portuguese Pinhão riveted road bridge, built in 1903 over the Douro river. The present traffic conditions are completely different from those foreseen by the bridge designer, rising new concerns, with respect to the bridge integrity, namely its fatigue behaviour. An experimental programme was performed using original material removed from the bridge. The chemical composition and microstructures of the removed materials were characterised. Also, the notch toughness, at room temperature, was evaluated using both notch impact and Crack Opening Displacement (COD) tests. Fatigue crack growth tests were also used to evaluate the fatigue crack growth behaviour. Finally, fatigue tests of riveted joints were conducted in order to define an appropriate S-N curve. The experimental results were used to evaluate the residual fatigue strength of the bridge, adopting both S-N and Fracture Mechanics approaches. The analysis revealed a good tolerance to fatigue cracking, even in the presence of small fatigue cracks, detected in the joints.
Strain 04/2009; 47(s1):e402 - e415. · 1.10 Impact Factor
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ABSTRACT: The increasing use of aluminium alloys in transportation industry, not only in aeronautics but also in automotive industry,
creates the need for research on more efficient and reliable welding processes to be used. In order to allow the industry
to use novel manufacturing techniques as Laser Beam Welding and Friction Stir Welding, which promise high efficiency, research
work on S-N and crack growth fatigue data from the weld zone is required to provide tools to assess the damage tolerance.
This chapter is a contribution to this effort, contrasting the fatigue behaviour of joints made using a traditional process,
Metal Inert Gas welding, with those made with Friction Stir and Laser Beam Welding.
01/2008: pages 85-111;
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ABSTRACT: The residual stress effect due to cold-working is studied in relation to fatigue striation spacing. Cold-working introduces a compressive stress field around the hole reducing the tendency for fatigue cracks to initiate and grow under cyclic mechanical loading. It is known that fatigue lifetime assessment requires a detailed knowledge of the residual stress profile. X-ray diffraction and 3D finite element analysis (FEA) can be used to determine the resid-ual stress profile. Scanning electron microscopy (SEM) measurements were performed for measuring the striation spacing.
10/2004;
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ABSTRACT: Cold-working of riveted holes reduces the stress intensity factor associated with cracks that may develop at the hole boundary, by creating a compressive residual stress field. The residual stress field is determined using the finite-element method and the reduction of the stress intensity factor for different values of the interference is evaluated with the weight function method, in the case of an infinite plate made from an elastic–perfectly plastic material, and having a hole with two symmetrical cracks. Once the weight function of the structure is known, further calculation of the stress intensity factors for different loadings such as a remote uniform stress, or a point load that simulates the action of the rivet can be performed without difficulty.
Fatigue & Fracture of Engineering Materials & Structures 08/2004; 27(9):879 - 886. · 0.85 Impact Factor
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ABSTRACT: ABSTRACT The paper presents a new model for three-dimensional progressive failure analysis of adhesive joints. The method uses interface elements and includes a damage model to simulate progressive debonding. The interface finite elements are placed between the adherents and the adhesive. The damage model is based on the indirect use of fracture mechanics and allows the simulation of the initiation and growth of damage at the interfaces without considering the presence of initial flaws. The application of the model to single lap joints is presented. Experimental tests were performed in aluminium/epoxy adhesive joints. Linear elastic and elastoplastic analyses were performed and the predicted failure load for the elastoplastic case agrees with experimental results.
Fatigue & Fracture of Engineering Materials & Structures 04/2003; 26(5):479 - 486. · 0.85 Impact Factor
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ABSTRACT: The multiple-site damage (MSD) phenomenon is discussed, and exemplified by the behaviour of riveted lap-joint specimens of aluminium alloy 2024-T3 alclad. The tests performed, on which the paper is based, are part of the contribution of IDMEC to a project on the fatigue behaviour of ageing aeronautical structures—the BRITE-EURAM project 'SMAAC', partially funded by the European Union. The study involves fatigue testing under constant amplitude loading of 1.6-mm-thick riveted lap-joints, and includes examination of the specimens during and subsequent to testing (post-mortem analysis of the fracture surface in a scanning electron microscope) in order to determine the time of occurrence, location and extent of fatigue damage. Crack growth rates are determined from periodic crack length measurements with a travelling microscope. Stress measurements are made using extensometry and the SPATE infrared technique to determine loading distribution of the lap-joints and redistribution due to cracking of fastener holes. Data on the initiation and growth of cracks and on residual static strength are used to assess the predictive model based on the finite element method.
International Journal of Fatigue 01/2000; 22:319-338. · 1.55 Impact Factor
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ABSTRACT: A methodology for in-situ stress intensity factor (SIF) determinations that can be used for the analysis of cracked structures is shown. The proposed method is demonstrated in the laboratory for the case of a central cracked plate, subjected to uniaxial tension. Three steps are used for the SIF determination. First the strain field around the crack tip is acquired using a digital image correlation based optical technique. Secondly, the stresses are calculated based on the equation from the theory of elasticity. In the third step an over determined system of equations is solved containing the stress field around the crack tip and the stress intensity facto among others. A comparison of the obtained results was performed with results obtained using the Dual Boundary Element Method (DBEM) together with the J-Integral method for SIF determination. A good agreement can be noticed for both the stress distribution around the crack tip and for the SIF calculated based on these stresses, proving thus the ability to measure the SIF in-situ. KEYWORDS: Fracture mechanics, in-situ stress intensity factor determination.
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ABSTRACT: Fatigue is one of the main problems in the provision of service life and safety of aircraft structures. The menace of fatigue cracking is accentuated in areas of stress concentration, e.g., joints of structural components. An example is the fuselage where riveting is used. One of the techniques for improving the fatigue life of these connections is the cold expansion of the rivet hole. As part of a larger project on the fatigue behaviour of aeronautical structures, an experimental study of open-hole specimens in Al-alloy 2024-T3, with and without hole expansion, is presented. The residual stress field created by the cold expansion was experimentally assessed by using the X-ray technique and predicted by FEA. Fatigue tests were supplemented by SEM measurements of fatigue striation spacing along longitudinal and transverse directions in the crack surface of each specimen. Empirical models and fractographic techniques developed by Nedbal et al. are used for the analysis of the experimental data, and results of quantitative microfractography are presented. Crack tunnelling was quantified based on the reconstituted crack history and on the surface crack growth measurements.
Engineering Fracture Mechanics.
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ABSTRACT: Friction stir welding (FSW) is a solid-state joining process which emerged as an alternative technology to be used in high strength alloys that were difficult to join with conventional techniques. Notwithstanding the widespread interest in the possibilities offered by FSW, data concerning the fatigue behaviour of joints obtained using this process still is scarce. In this work, a comparative study between fatigue crack growth behaviour of friction stir welds of 6082-T6 and 6061-T6 aluminium alloys is carried out. Fatigue crack growth curves were determined for cracks growing in different locations of the weldments, including the base material, the heat affected zone and the welded material. Generally, friction stir material exhibited lower strength and ductility properties than the base material. However, an enhanced crack propagation resistance is observed in the welded material. The 6082-T6 and 6061-T6 base materials exhibit very similar crack propagation behaviours. On the other hand the friction stir 6061-T6 material shows lower crack propagation rates than corresponding 6082-T6 friction stir material. Particular features of the distinct microstructures of the welded and surrounding material are illustrated using scanning electron microscopy.
Theoretical and Applied Fracture Mechanics.
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ABSTRACT: The cold working process is a well-known technique used to improve the fatigue life in aircraft structures. It has been shown that the only way to accurately simulate the cold working process using finite elements is modelling the plate, mandrel and sleeve. Usually, two-dimensional, two-dimensional axisymmetrical and three-dimensional models are used to predict the resulting residual stress field. This type of problem requires complex calculations and is time consuming since it involves the use of contact between the elements assembled and non-linear geometric and material models to simulate the behaviour of the plate. In engineering problems, simpler analyses are preferable as long as the errors caused by the simplifications can be quantified. This paper presents and compares the results obtained using models with different levels of complexity. The consequences of using some of the normal approximations concerning to the geometry, boundary conditions and type of model used, are evaluated.
Finite Elements in Analysis and Design.
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ABSTRACT: Three-dimensional finite element analyses are used in this paper to calibrate the stress intensity factor in a cracked stiffened plate subjected to remote uniform traction. An accurate numerical determination of the stress field and stress intensity variation through the thickness of a central cracked plate was first carried out in order to evaluate three-dimensional effects. A stiffened cracked plate was then analysed, taking into account the results and the conclusions obtained in the previous study. Such a structure was chosen due to the growing interest for large integral metallic structures for aircraft applications, following the continuous need for low cost and the emergence of new technologies. The J-Integral technique was used to calculate the values of the stress intensity factor along the plate thickness. The plane strain behaviour near the crack front and the variation of the opening stress are discussed.
Engineering Fracture Mechanics.
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ABSTRACT: Closed-form and semi-analytical solutions are obtained for the residual stress distributions in a plate caused by pressure acting on a central circular hole, representing the cold-work process. The material is elastic–perfectly plastic. Both Tresca and von Mises yield criteria are used and the corresponding residual stress distributions are compared. The relation between the dimension of the plastic zone and the value of internal pressure is presented. The relation between the magnitude of the residual stresses and the remote uniform tensile stress required to open symmetrical radial cracks is also presented. The reduction of the stress intensity factors of cracked open and riveted holes as a function of the internal pressure applied (or mandrel radial displacement) is investigated using numerical models for both an elastic–perfectly plastic material and for an Al 2024-T3 Alclad aluminum alloy.
Theoretical and Applied Fracture Mechanics.
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ABSTRACT: The multiple-site damage (MSD) phenomenon is discussed, and exemplified by the behaviour of riveted lap-joint specimens of aluminium alloy 2024-T3 alclad. The tests performed, on which the paper is based, are part of the contribution of IDMEC to a project on the fatigue behaviour of ageing aeronautical structures—the BRITE-EURAM project ‘SMAAC’, partially funded by the European Union. The study involves fatigue testing under constant amplitude loading of 1.6-mm-thick riveted lap-joints, and includes examination of the specimens during and subsequent to testing (post-mortem analysis of the fracture surface in a scanning electron microscope) in order to determine the time of occurrence, location and extent of fatigue damage. Crack growth rates are determined from periodic crack length measurements with a travelling microscope. Stress measurements are made using extensometry and the SPATE infrared technique to determine loading distribution of the lap-joints and redistribution due to cracking of fastener holes. Data on the initiation and growth of cracks and on residual static strength are used to assess the predictive model based on the finite element method.
International Journal of Fatigue.
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ABSTRACT: Notwithstanding the widespread interest in the possibilities offered by friction stir welding (FSW), data concerning its mechanical behaviour is still needed. Due to design constraints, friction stir welded components may include notches in the weldment (friction stirred material), and there is a lack of information on the fatigue behaviour of the weldment in those conditions. In this work the influence of FSW on the fatigue life of specimens of aluminium alloy 6063-T6, containing notches in the thermo-mechanically affected zone was studied. For this purpose, welded and unwelded notched specimens were fatigue tested under load control at different stress levels. Compared to base material specimens, FSW specimens were found to have longer fatigue lives but generally of the same order of magnitude. The work included hardness, tensile and fatigue tests, and the finite element method analysis of the stress and strain distribution. Microstructure examination and microhardness measurements were performed, and in order to identify welding defects and fracture surface features a scanning electron microscopy analysis was carried out. The notch-strain analysis was used for approximate modelling of the fatigue behaviour observed.
Journal of Materials Processing Technology.
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ABSTRACT: The linear elastic problem is solved by means of Trefftz functions which automatically satisfy the elasticity equations in a 2D domain. Using Kolosov–Muskhelishvili’s complex variable representation, complex potentials are expanded in power series. Trial elementary elastic fields are derived from each expansion term. The Galerkin weighted residuals formulation is used to derive the system of equations in which the unknowns are the retained expansion coefficients. For crack problems, special expansions that satisfy the zero traction condition along crack edges are used to obtain the approximating elastic field, which allow the direct determination of the stress intensity factors. Several numerical results, obtained for typical crack problems using Trefftz Boundary Element Method, are presented and compared with those published by other authors. A simple example of multiple site damage with two offset parallel cracks is also analyzed.
Engineering Fracture Mechanics.
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ABSTRACT: A mechanical and metallurgical characterization of friction stir welded butt joints of aluminium alloy 6061-T6 with 6082-T6 was carried out. For comparison, similar material joints made from each one of the two alloys were used. The work included microstructure examination, microhardness, tensile and bending tests of all joints. An approximate finite element model of the joint, taking into account the spatial dependence of the tensile strength properties, was made, modelling a bending test of the weldments. This study shows that the friction stir welded dissimilar joint present intermediate mechanical properties when compared with each base material. In tensile tests the dissimilar joint displayed intermediate properties. For instance in the hardness profile the lowest values were obtained in the AA6082-T6 alloy plate side where rupture occurred, and in the nugget all type of joints present similar values.
Materials & Design.
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ABSTRACT: One of the main problems of aeronautical structures is the onset and growth of defects due to fatigue loading. This phenomenon is accentuated in areas of stress concentration, for example in the connection of components, such as the riveted joints of fuselage panels. Several geometric configurations can be used in riveted joints of fuselage panels. This work is focused on one geometry, a single-lap splice with three rivets rows and one rivet column. A three-dimensional stress analysis using the finite element method was carried out in order to analyze the load transfer as a function of crack geometry and length, and to determine the stress intensity factors for one or two cracks emanating from the edge of the hole located at the critical cross section.
Materials & Design.
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ABSTRACT: Over the last years, many efforts have been dedicated to the understanding of fatigue damage of aircraft structures. In order to achieve this goal, attention must be focused on the fatigue phenomena of simpler structural details, conducting research at different scales in terms of specimen size and structural complexity. This paper discusses the fatigue behaviour of different types of specimens with increasing level of geometrical and mechanical complexity. Problems such as the effects of residual stresses due to the cold-working process of rivet holes, load transfer and stress intensity factor calibration of riveted lap-joints, and finally the problem of multiple site damage, are addressed. The residual stress field created by cold expansion was experimentally assessed by using the X-ray technique. Finite element analysis of the different structural details was performed in order to model the residual stresses, stress intensity factor, load transfer and finally fatigue crack propagation. Fatigue tests were supplemented by post-mortem analyses of fracture surfaces in order to measure the location and extent of fatigue damage and the spacing between fatigue striations. The work presented is part of the contribution of IDMEC, Porto to two European Union research projects on the fatigue behaviour of aeronautical structures: ‘SMAAC’ and ‘ADMIRE’.
Materials Science and Engineering: A.
