Abstract: The aim of this work was to design a kirigami-based metamaterial with optical properties. This idea came from the necessity of a study that can improve common camouflage techniques to yield a product that is cheap, light, and easy to manufacture and assemble. The author investigated the possibility of exploiting a rotation to achieve transparency and color changing. One of the most important examples of a kirigami structure is a geometry based on rotating squares, which is a one-degree-of-freedom mechanism. In this study, light polarization and birefringence were exploited to obtain transparency and color-changing properties using two polarizers and common cellophane tape. These elements were assembled with a rotating-square structure that allowed the rotation of a polarizer placed on the structure with respect to a fixed polarizer equipped with cellophane layers.

Keywords: camouflage | color changing | kirigami | rotating squares | transparency

Abstract: Flexible metamaterials have been increasingly harnessed to create functionality through their tunable and unconventional response. Herein, chiral unit cells based on Archimedean spirals are employed to transform a linear displacement into twisting. First, the effect of geometry on such extension-twisting coupling is investigated. This unravels a wide range of highly nonlinear behaviors that can be programmed. Additionally, it is demonstrated that by combining the spirals with polarizing films one can create mechanical pixels capable of modulating the transmission of light through deformation. Guided by experiments and numerical analyses, pixels are arranged in 2D arrays to realize black and white and color displays, which reveal distinct images at different states of deformation. As such, the study puts forward a methodology for the design of an emerging class of flexible devices that can convert nonlinear elastic deformation to tunable optical transmittance.

Keywords: deployability | mechanical metamaterials | optics

Abstract: In this work, the authors aim at developing a reliable and fast methodology to evaluate the wear evolution in tire starting from a complete optical 3D scanning. Starting from a data cloud, a semi-automatic methodology was implemented in MATLAB to extract mean tread radial profiles in correspondence of the desired angular position of the tire. These profiles could be numerically evaluated to establish the presence of irregular wear and the characteristic parameter of the groove depth. The reliability and the robustness of this methodology was firstly tested by applying it to several synthetic case studies modeled in CATIA V5®, where ovalization and presence of defects were also simulated. The groove depth was determined with an error lower than 1% for the ideal model, while the introduction of ovalization and defects leaded to an error of 2.6% in the worst condition. In a second time, the methodology has been successfully applied to experimental measurements carried out in two different wear life of the tire, allowing the tracking of the wear phenomena through the evaluation of the progressive lowering of tread radial profiles.

Keywords: 3D scanning | discrete models | tire wear evaluation | tread profile

Abstract: In this work, the fatigue damage of CFRP uniaxial composite specimens were studied using thermal methods to determine the fatigue behavior. The aim was to evaluate the fatigue damage as a function of the number of cycles. Consequently, the damage process was studied in terms of a global indicator, considering the stiffness decay, and in terms of local parameters, considering the evolution of temperature maps acquired during the fatigue tests. A direct correlation between the damage index, corresponding to 90% of the fatigue life, and the temperature variation of the most stressed area was found. Another parameter taken into consideration was the heating rate during the application of the first thousands cycles. This parameter was proportional to the stress amplitude, making it a useful parameter since it refers to the initial part of the specimen fatigue life.

Keywords: composites | damage | fatigue | thermal methods

Abstract: Residual stress relaxation induced by the application of mechanical loads is determined by the nature of residual stress, the elasto-plastic material properties, and the type of applied load. Despite the importance of the first load cycle, analytical models available in the literature generally assumed residual stress relaxation as a continuous process. Residual stress induced by machining on Inconel 718Plus superalloy cylindrical specimens was measured before and after the application of load cycles under strain control. Low-cycle fatigue tests were carried out at room temperature for different strain amplitudes, and X-ray diffraction measurements were performed before and after 10 and 100 cycles. A comprehensive analytical model was derived to describe the relaxation process associated with the initial cycles and that associated with the continuous application of load cycle, which is based on the plastic strain energy per cycle W and requires the evaluation of parameters that are only dependent on the material and not on the strain amplitude.

Keywords: low-cycle fatigue | residual stress | residual stress relaxation | superalloy

Abstract: Since the presence of a notch in a mechanical component causes a reduction in the fatigue strength, it is important to know the kf value for a given notch geometry and material. This parameter is fundamental in the fatigue design of aeronautical components that are mainly made of composites. kf is available in the literature for numerous types of notch but only for traditional materials such as metals. This paper presents a new practice, based on thermographic data, for the determination of the fatigue notch coefficient kf in composite notched specimens. The innovative aspect of this study is therefore to propose the application on composite materials of a new thermographic procedure to determine kf for several notch geometries: circular, U and V soft and severe notches. It was calculated, for each type of notch, as the ratio between the fatigue limits obtained on the cold and hot zone corresponding to the smooth and notched specimen, respectively. Consequently, this research activity provides, for the first time, a little database of kf for two particular typologies of composite materials showing a fast way to collect further values for different laminates and notch geometries.

Keywords: Composites | Fatigue | Geometric effect | Notch sensitivity | Thermography

Abstract: An innovative active thermography technique is proposed for the inspection of typical wind blade material. The proposed technique is based on the use of a multifunctional material obtained adding a grid of Shape Memory Alloy wires, which would serve also as a protection against lightning, to a traditional glass fibre composite panel. This technique, called SMArt thermography, which exploits the SMA wires as internal heat sources, has been compared to a traditional pulsed thermography in the case of a representative panel of unidirectional glass fibre and epoxy matrix with embedded SMA wires and artificial defects. The experimental results of the two techniques are reported and compared to the result of a numerical FEM transient model, in order to establish the reliability and the detectability limit of the proposed technique. The FEM model has been proven to be a useful tool for the definition of the multifunctional material at a design stage.

Keywords: Defect detection | Multifunctional materials | Shape memory alloy | SMArt thermography | Wind blades

Abstract: The aim of this paper is to model and to compare the results of the mechanical characterization, carried out on numerical models and real specimens, of uniform P-scaffolds with different porosity values. The analysis includes the morphological characterization of 3D printed specimens and the implementation of a FEM shell model to reproduce a compressive test suitable for mechanical properties evaluation of PLA scaffolds. Young modulus and yield strength were also obtained, in order to verify the numerical model accuracy, by experimental tests on 3D printed PLA scaffolds. Numerical results showed that the shell model was able to reproduce, more efficiently compared to a solid model proposed in a previous work, both elastic and plastic behavior of the scaffolds, providing elastic modulus values very close to the experimental ones. On the other hand, the not very high quality of the 3D printing, detected by MicroCT analysis, caused a significant dispersion in the yield strength numerical values respect to the real data. Anyway, an inverse correlation between mechanical properties and porosity was found as expected. The elastic modulus values were similar to the typical values of the trabecular bone for whose regeneration this kind of scaffolds is usually employed.

Keywords: Experimental tests | FEM analysis | MicroCT | TPMS scaffold

Abstract: This work investigates the possibility to apply the improved hybrid capacitor discharge welding (CDW) process, based on projection welding principles, to aluminium alloy Al 5754. The CDW process is an electrical resistance welding technology, realised with high-intensity current pulses discharged by large capacitors. The innovative aspect is the effective possibility to weld aluminium alloys with CDW process and improve the mechanical weld characteristics and the presence of defects as a function of the technological parameters; intrinsic CDW process characteristics need to be investigated on the basis of interaction between the technological and geometrical aspects and the related mechanical properties, in order to improve welding shape and reduce defect size. In order to optimise the process, visual and ultrasonic inspections of the most significant welded joints were performed, and residual stress values were checked; in addition, high-cycle fatigue tests after room temperature tensile tests were executed to optimise the weldments.

Keywords: Al 5754 | Capacitor discharge welding | Fatigue tests | Residual stress | Ultrasonic test | Welded joints

Abstract: SMArt termography is an innovative and promising technique that could be useful for the detection of damages of large components subjected to in-service loads, like wind blade. This technique requires building traditional carbon or glass fiber reinforced composite laminates adding a regular net of Shape Memory Alloy (SMA) wires in the matrix. These wires confer to the composite material additional features. In particular, the electro-thermal properties of SMA could be used as an internal heat source to be used for the control of the component using the traditional numerical technique used to elaborate the raw thermal data. Despite of other thermography techniques, SMArt thermography is characterized by a quite reduced amount of heating power, which produces a limited increasing of the temperature of the component subjected to control. On the other hand, the numerical elaboration of thermal data acquired from IR camera is more sensitive and require a deeper comprehension of the phenomena. In this work, a GFRP composite panel containing several artificial defects has been studied both from a numerical and experimental point of view, in order to determine the sensitivity of the technique, the limit of applicability and practical indications about the reliability of the technique.

Keywords: Multifunctional materials | Shape Memory Alloy | SMArt thermography

Abstract: The development of techniques able to check the structural health of a wind blade is very important. An innovative and promising technique applicable at this aim is the SMArt thermography. It exploits the electro-thermal properties of SMArt composites, in order to detect the structural flaws using an embedded source. Such a system enables a built-in, fast, cost-effective and in-depth assessment of the structural damage as it overcomes the limitations of standard thermography. With the aim for developing a reliable diagnostic method based on SMArt thermography, a preliminary numerical model was implemented in order to simulate the heating and the subsequent cooling of a GFRP composite laminate with embedded SMA wires. The heat source was represented by the Joule effect originated in the SMA wires and supplied as power density. The analysis of the resulting thermal maps at different values of power density provided the optimal levels of current amplitude and period to be applied in the subsequent experimental applications.

Keywords: FEM analysis | Shape Memory Alloy | SMArt thermography

Abstract: In this article, a methodology to analyze the shear behavior of aluminum foam with closed cells is proposed. A biaxial load device was expressly designed and the elaboration with digital image correlation technique of the data acquired during the test with a charge coupled device (CCD) camera allowed determining the displacement and strain fields. This procedure made it possible to evaluate the procedure as suitable or not for conducting a shear characterization for metallic foam. The τ-γ curves obtained showed an initial elastic outline followed by the yield plateau, a peak load, and a rapid load drop. © 2014 Copyright Taylor & Francis Group, LLC.

Keywords: Aluminum foam | digital image correlation | shear characterization

Abstract: Purpose - The aim of this work is the development of a procedure able to model the highly irregular cellular structure of metallic foams on the basis of information obtained by X-ray tomographic analysis. Design/methodology/approach - The geometric modelling is based on the feature "pore" characterized by an ellipsoidal shape. The data for the geometric parameters of the instances are obtained with a methodology which is driven by the pore volume distribution curve. This curve shows how much the cells, whose diameter belongs to a given dimensional range, contribute to the reduction of the total volume. Findings - The presented methodology has been implemented into a CAD tool consisting of a Matlab routine identifying the instances of the feature "pore" and a CATIA's macro modelling the closed cells foam. Originality/value - The presented methodology allows to obtain in an automatic way the CAD model of the complex structure of closed cell aluminium foam approximating by considerable accuracy both the density and the volume distribution of the real foams. Copyright © 2013 Emerald Group Publishing Limited. All rights reserved.

Keywords: Aluminium foam | CAD modelling | Computer aided design | Foams | Microstructural model | OBB overlapping | X-ray tomography

Abstract: The residual stresses introduced into a component as a result of the mechanical processes necessary for its manufacture significantly influence the fatigue behaviour. The application of loads themselves can, however, alter the initial residual stress distribution, so it is feasible to believe that the residual stress field of a fatigue-stressed structure develops over its entire lifespan. The aim of this article is to analyse the development that the residual stresses initially present in a welded butt joint undergo following the application of an external cyclic load. Comparisons between two residual stress measurements, conducted on the same joint before and after the application of fatigue cycles, have made it possible to obtain interesting information regarding the development of residual stresses in components subjected to fatigue. In particular, it has been found that unlike what is commonly reported, there are particular conditions where the application of a cyclic load results in an increase in the residual stresses initially present, rather than their relaxation. This phenomenon should be taken into consideration in order to avoid unexpected failure in components subjected to fatigue. © 2012 Copyright Taylor and Francis Group, LLC.

Keywords: butt joints | fatigue life | fatigue loading | fatigue strength | fatigue tests | MIG welding | research and development | residual stresses | stress distribution | structural steels | welded joints

Abstract: In this paper an associative-parametric approach is proposed in order to model the mesh of an aeronautical concept starting from a set of high-level structural primitives. This approach allows the designer to carry out the geometric modelling and the automatic mesh generation within one software environment in a fast and interactive way. The structural optimisation process is then simplified, with a relevant man-hours saving. A lower number of data transfers between different software is, moreover, involved with less problems related to the data corruption. To assure orders of continuity higher than C0 between adjacent instances, a suitable mathematical description of the structural primitives has been proposed. This description assures the maintenance of the required continuity constraints when the mesh is modified. Appropriate schemes of dependences are identified to guarantee the automatic propagation of the modifications complying with the continuity constraints.

Abstract: In this work, a scheme of representation for aircraft structural concepts is identified. Based on this scheme, a parametric-associative geometrical modelling of the aeronautic structure, consisting in a quad-mapped mesh, is proposed. The mesh generation is based on a hierarchical scheme ensuring the one-to-one correspondence between mesh elements belonging to adjacent primitives. The automatic propagation of modifications is efficiently implemented according to well-defined schemes of dependence thanks to which the modifications involve only the concerned instances. This scheme is implemented in an original software, called MeshFEM and developed using C++, Matlab and the VTK library for 3D graphic visualisation. © 2012 Civil-Comp Ltd and Elsevier Ltd. All rights reserved.

Keywords: Aeronautic structures | CAD/CAE tool | Design conceptualisation | Parametric-associative modelling | Quad-mapped mesh | Wireframe primitives

Abstract: Residual stresses, introduced into a component by manufacturing processes, significantly affect the fatigue behaviour of the component. External load application produces an alteration in the initial residual stress distribution, so it is reasonable to suppose that residual stress field into a component subject to a cyclic load presents an evolution during the total life. In this work, the authors analysed the evolution that the residual stress field, pre-existing in a butt-welded joint, suffers following the application of cyclic load. The comparison between two residual stress measurements, carried out on the same joint before and after the cyclic load application, allowed to obtain interesting information about the residual stress evolution. It was found that in particular condition, unlike the general opinion, a cyclic load application produces an increasing in the residual stress level rather then a relaxation. This phenomenon is to take well in account in order to avoid unexpected failure in components subjected to a fatigue load. © 2011 Nova Science Publishers, Inc.

Keywords: Fatigue | Four-point bending load | Mechanical relaxation | Residual stress

Abstract: An experimental analysis is presented to assess the residual stress evolution in cold-rolled steels. The study aims to establish the influence of a yield strength gradient on the increase of residual stress in a hardened material when it is subject to a bending fatigue load. Rectangular specimens, obtained from cold-rolled plates and having different hardening levels, are analysed and the residual stress evolution, due to three-point bending load application, is evaluated with respect to the initial value. The comparison between tests on different specimens shows that the yield strength gradient plays an important role in the residual stress field evolution. © 2010 Elsevier Inc. All rights reserved.

Keywords: Fatigue | Hardening | Residual stress

Abstract: In this work, a scheme of representation of structural concept for the aeronautical field is identified. It is based on an original set of 2D and 3D wireframe primitives, representing the main structural components of the aeronautical concept. Starting from them, an associative parametric geometrical modelling of the aeronautic structure, consisting in a quad-mapped mesh, is obtained. The automatic propagation of modifications is implemented so that several structural concepts can be efficiently modelled and modified during the very early phases of the design process. The propagation process aims at the automatic regeneration of the whole mesh thanks to a well-defined hierarchy (or relationship of dependences) among the parametrically defined primitives. Based on the above-mentioned considerations, a CAD/CAE tool, called MeshFEM, has been developed using C++ and Matlab languages and the VTK library for the 3D graphic visualization. © 2010 Civil-Comp Press.

Keywords: Aeronautic structures | CAD/CAE tool | Design conceptualisation | Quad-mapped mesh | Wireframe primitives

Abstract: Many factors are involved in determining the fatigue strength of welded joints. It is, however, very difficult to consider their relative importance. The aim of this paper is to isolate the effect of residual stress from other factors, establishing a relation between the amount of residual stress and fatigue life. A geometrical notch due to the weld bead is removed by milling the upper surface of the welded plates. Moreover, specimens are subjected to four-point bend loading. Before conducting the fatigue test, the magnitude of residual stress for each specimen is experimentally evaluated, and then linked to the number of cycles to failure. This relation is analyzed for three different plate thicknesses and for different stress amplitude levels in the high cycle regime. The results clearly show the significant influence of residual stress on fatigue behaviours when the load level is near the fatigue limit. © KSME & Springer 2010.

Keywords: Fatigue | Milling | Residual stress | Welded joints

Abstract: In this paper the possibility to employ the Digital Image Correlation (DIC) technique for the mechanical behaviour analysis of metallic foam was investigated. Image Correlation allowed to measure displacement and strain fields on closed and open cells aluminium foam specimens under two different loading conditions (compression and shear) and to characterise, with opportune calculation, information on the mechanical behaviour of foams. The adopted technique is suitable to conduct a deep analysis and also to appreciate the local heterogeneities that appear in a specimen during test. The parameters obtained with DIC analysis are confirmed by the global data obtained by testing machine proving the adopted methodology represents a valid tool for the study of these new materials.

Abstract: The aim of this work is the development of microstructural numerical models of metallic foams. In particular, attention is focused on closed cell foam made of aluminium alloy. By means of a finite elements code, the material cellular structure was shaped in different ways: firstly the Kelvin cell, with both plane and curved walls; finally an ellipsoidal cell defined by random dimensions, position and orientation has been adopted as base unit. In order to validate the foam numerical models, static tests were performed to obtain the typical stress-strain curves and then compared with the numerical analysis results. © 2009 Elsevier Ltd. All rights reserved.

Keywords: Aluminium foams | FEM | Mechanical characterisation | Microstructural model

Abstract: In this study, we evaluated the residual stress field which arose because of a new welding process named 'friction stir welding'. We analysed aluminium alloy butt-welded joints. Both similar and dissimilar joints were considered in 2024-T3 and 6082-T6 aluminium alloys of 0.8 and 3mm thick. For each joint, the longitudinal and transversal residual stress distributions were obtained in a direction perpendicular to the weld cord. In the thicker dissimilar joints, the longitudinal residual stress distribution present is very similar to the distribution present in traditional welded joints. It presents, in fact, a tensile region near the weld cord, which is balanced by compressive regions away from weld line. On the contrary, other joints present a low compressive stress at the weld toe and a tensile stress state elsewhere. Moreover, the effect of the shoulder geometry on the residual stress field was evaluated on 1.5-mm-thick joints. © 2008 The Authors. Journal compilation © 2008 Blackwell Publishing Ltd.

Keywords: Aluminium alloys | Friction stir welding | Hole-drilling method | Residual stress | Thin sheet

Abstract: The tool geometry is a crucial characteristic of the friction stir welding (FSW) process; its design is the key to the successful FSW application for a wide range of materials and thicknesses improving the weld strength and fatigue life. The present study investigates the influence of three shoulder geometries on the FSW joint performance, in terms of residual stresses state, microhardness profile and mechanical properties of 1.5 mm thick AA 6082-T6 FSW joints in the butt-joint configuration. The three tool geometries are characterized by three different shoulders: a shoulder with scroll, a shoulder with a shallow cavity, and a flat shoulder. Transverse and longitudinal tensile tests at room temperature were performed in order to evaluate the mechanical properties, respectively, of the joints and of the stirred zone, while the fatigue tests were performed transversally to the joint line. © KSME & Springer 2009.

Keywords: AA6082t6 | Fatigue | Friction stir welding | Residual stresses | Shoulder geometry

Abstract: Residual stresses, introduced into a component by manufacturing processes, significantly affect the fatigue behaviour of the component. External load application produces an alteration in the initial residual stress distribution, so it is reasonable to suppose that residual stress field into a component subject to a cyclic load presents an evolution during the total life. In this article, the authors analysed the evolution that the residual stress field, pre-existing in a butt-welded joint, suffers following the application of cyclic load. The comparison between two residual stress measurements, carried out on the same joint before and after the cyclic load application, allowed to obtain interesting information about the residual stress evolution. In particular, it was found that in particular conditions, unlike the general opinion, a cyclic load application produces an increasing in the residual stress level rather then a relaxation. This phenomenon is to take well in account in order to avoid unexpected failure in components subjected to a fatigue load.

Keywords: Butt joints | Fatigue life | Fatigue loading | Fatigue strength | Fatigue tests | MIG welding | Research and development | Residual stresses | Stress distribution | Structural steels | Welded joints

Abstract: In the last ten years, aerospace and automotive industries have addressed large attention to a new welding technology based on a solid-state process, called friction stir welding (FSW). FSW process shows several advantages, in particular the possibility to weld dissimilar aluminium alloys. In this paper, thin aluminium alloy 2024-T3 and 6082-T6 sheets, 0.8 mm thick, have been welded in the rolling direction by μFSW (FSW for ultra-thin sheets). Both similar and dissimilar joints have been successfully produced and analysed. Mechanical characterization has been executed through static and uniaxial fatigue tests with a constant load amplitude. Finally, microhardness and residual stress measurements have been executed on welded sheets for each joint typology. © 2007 Elsevier Ltd. All rights reserved.

Keywords: Fatigue | Micro friction stir welding | Residual stress

Abstract: Residual stresses develop during most manufacturing processes involving material deformation, heat treatment, machining or processing operations that transform the shape or change the properties of a material. They have a not negligible effect on the material strength, especially on fatigue. For this reason, it is important that some knowledge of the internal stress state can be deduced either from measurements or from modelling predictions. The object of this paper is forecasting the modification and the evolution that a residual stress field, originated by welding, suffers after chip-forming machining, such as milling and cutting. Numerical results have been critically compared to experimental measurements and show the potentiality but also the limitations of numerical techniques. © 2006 Elsevier Ltd. All rights reserved.

Keywords: Cutting | Finite element analysis | Milling | Residual stress | Welding

Abstract: The aim of this work is to evaluate by numerical simulation the real stress state of a welded component subjected to a sinusoidal external load. In particular, the relaxation process of the residual stress, due to the application of a cyclic external load, has been studied. Longitudinal welded joints, laser welded and 3 mm thick, are analysed in this work following the residual stress evolution during a fatigue test. A numerical procedure, implemented by the ABAQUS code, carries out the residual stress field originated by a welding process and it is taken as a pre-stress condition in the present ABAQUS simulations. In order to evaluate the influence of the amplitude level on the residual stress relaxation, eight different sinusoidal loads at the load ratio R = 0.1 are applied to the model. The results show a significant reduction in the initial residual stress level, even after the first load cycle. They offer a very precise explanation of the fatigue behaviour of this kind of welded joints, confirming the behaviour experimentally observed.

Keywords: Cyclic loading | Fatigue strength | Finite element analysis | Residual stress relaxation | Welded joints