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This paper is focused on the formability of wire-arc additively manufactured AISI 316L stainless steel sheets with the purpose of analysing the feasibility of including this material in hybrid additive manufacturing chains involving sheet metal forming operations. Conventional tensile tests performed in specimens obtained from different orientations to the building direction were carried out to characterise the mechanical properties, the strain loading paths and the limiting strains at fracture. Microstructure observations combined with fractography analysis add insight to the results by establishing a link between the forming limits by fracture and the crack opening mechanisms. Results obtained for wrought commercial AISI 316L stainless steel sheets are included for comparison purposes and reveal that the additively manufactured sheets have a much stronger anisotropic behaviour and poorer formability due to their dendritic-based microstructure. Still, the forming limits obtained from the experiments allow concluding that the additively manufactured sheets can withstand large plastic deformations and, therefore, can be used in hybrid additive manufacturing routes.

Additive manufacturing technologies are becoming increasingly popular due to their advantages over traditional subtracting manufacturing technologies. Despite advances in this field, fixed and maintenance costs for additive manufacturing with metals remain high. The introduction of low-cost metal machines in the additive manufacturing market considerably reduces the cost of acquiring and maintaining this type of equipment. This work aims to establish the process requirements for a low-cost selective powder deposition process, and validate it through the production of specimens in the laboratory and evaluate their mechanical properties. Tin bronze specimens were produced under different manufacturing conditions, namely powder dimensions, type of crucible and coke, firing segments and casting strategy. The morphology and chemical composition of the specimens were carried out combining the scanning electron microscopy and energy dispersive X-Ray spectroscopy techniques, respectively. It was observed that crucibles and coke with impurities that react with the metal powders and infill in a reducing atmosphere have influence in the final quality of parts. Tested samples displayed high variability of results which can be correlated with different manufacturing conditions. The selection of the appropriate print parameters led to the manufacture of tin bronze specimens with mechanical properties comparable to those reported in the literature. Overall, low-cost selective powder deposition is a promising technology, if identified manufacturing issues are addressed.

A new generation of biodegradable metal alloys with a porous structure has been receiving growing attention as temporary bone scaffolds for tissue regeneration. The mechanical response of the scaffolds depends upon several factors including the properties of the metal itself, the amount of porosity, the geometrical topology and the immersion conditions. The purpose of this study is to evaluate the degradation behaviour and the mechanical properties of porous iron samples with porosities in the range of 20–30%. Besides the amount of porosity, the effect of topology was evaluated with the study of different arrangement of pores, as well as pore shapes. The specimens were subjected to chemical degradation by immersion of the iron samples in body fluid simulation conditions. The mechanical properties of the samples prior and after the degradation process were assessed by three-point bending tests. Numerical simulations were carried out and the results were compared with the experimental results. The degradation operated by body fluids tends to reduce the mechanical properties. In comparison with the compact structures, porous structures exhibit lower mechanical strength, but still with reasonable values for the use in temporary implants, which also allows reducing the stress shielding effect, keeping the biodegradable advantages. The present work also confirms that the topological design has a strong influence on the mechanical properties of the specimens and on the biodegradation behaviour.

Sandwich structures are frequently used in automotive, aerospace and marine industries, as they provide adequate functional properties. The two-dimensional regular hexagonal cell shape, i.e. honeycomb is the most used core structure in sandwich panels. Recently, a new type of cellular structures composed of lattice struts has been proposed, as they combine high stiffness, strength and energy absorption with low weight. The main purpose of this research is to investigate the effect of the lattice topology on the flexural behaviour of sandwich panels. Five lattice geometries inspired in crystalline structures were designed, namely, body-centred parallelepiped, body-centred parallelepiped with struts in z-axis, body- and face-centred parallelepiped with struts in z-axis, face-centred parallelepiped with struts in z-axis and parallelepiped simple. The relative density of all the lattices was kept constant as 0.3. Both numerical and experimental approaches were used to evaluate the flexural properties and failure behaviour of the sandwich structures under three-point bending tests. The numerical analysis was undertaken with the finite element software NX Nastran. Taking advantage of additive manufacturing technologies, material extrusion was used to produce polylactic acid samples with the configurations aforementioned. The sandwich panels are composed by a single layer formed by the lattice core and two thin plates, at the bottom and top. The three parts of the panel were manufactured all together. The simulation results indicate that, among the lattices studied, topologies body-centred parallelepiped with struts in z-axis and body- and face-centred parallelepiped with struts in z-axis exhibit higher strength, while body- and face-centred parallelepiped with struts in z-axis shows higher stiffness and higher energy absorption, attaining values that do not differ much from the ones obtained with a two-dimensional hexagonal cellular structure, with the same relative density. As a consequence, some of the geometries studied may have the potential to be considered as alternatives to conventional structures in the design of sandwich structures.

Fatigue is one of the main causes for in service failure of mechanical components and structures. The need for the weight reduction of components and a growing need for greater lifespans of equipment, forced the understanding of the fatigue behaviour of materials under multiaxial loading cycles and increased number of loading cycles. It is generally recognized that multiaxial stresses occur in many full-scale structures, being rare the occurrence of pure uniaxial stress states. Nevertheless, the high cost of fatigue materials testing under multiaxial loading and for very high number of cycles, was a serious drawback for the research under these testing conditions. This study is a description of the present status of fatigue testing machines working at a frequency of 20 kHz, and the research that can be carried out in order to perform multiaxial fatigue tests under bi-axial stress states and very high cycle fatigue. Special emphasis will be given to the performance of multiaxial fatigue tests under axial/torsion and in plane bi-axial testing using low cost piezo-electric transducers either in classical cylindrical or in cruciform specimens and associated instrumentation. Together with the description of the testing machines, experimental results of fatigue tests will be presented in order to compare the behaviour of a metallic material from a Railway wheel under multiaxial tension/torsion at very high cycle fatigue.

This work aims to provide research guidance for the development of multiple extrusion manufacturing, with the objective of reducing large parts manufacturing time. The manufacture of parts with multiple collaborative extrusion heads emerge as a potential solution to overcome the limitations of speed, cost, and scale of the conventional extrusion processes. The existence of multiple independent extrusion modules imposes the existence of intersections areas in parts. These intersections are treated as seams. Four different types of specimens made of PLA, differing in seam dimensions, were produced with two independent extrusion modules. Samples were subjected to a tensile test and results suggest that in material extrusion, the seams act in a part by creating a localized fragility preventing it from entering a plastic domain and diminishing its tensile properties. Seam dimensions have little influence on the tensile strength of specimens, with the average values varying between 49.16 MPa for samples with seam without overlap areas and 46.69 MPa in samples with a 10 mm overlap. Samples with 20 mm overlap have an average tensile strength of 48.99 MPa. This work concludes that the existence of seams could be considerate in the design stage of a product and that the development of multiple extrusion manufacturing, with the goal of reducing large parts manufacturing time without compromising geometrical accuracy is worthy of studying.

Magnesium alloys have been attractive to use in structural components due to their high strength to weight ratio, low density and high damping capacity. However, magnesium alloys show peculiar plastic deformation mechanisms under cyclic loads (twinning and de-twinning) that causes the asymmetric material behaviour and limits their use in structural components. Recent researches indicate that this type of plastic deformation mechanism cannot be fully characterized using the typical tools used in steels. Therefore, the phenomenological Hypo-strain (HYPS) model has been developed to capture the asymmetric behaviour of magnesium alloys under uniaxial and multiaxial loadings. This study aims to evaluate the phenomenological Hypo-strain approach for AZ31B-F magnesium alloy and to implement the HYPS model on an external subroutine (UMAT) to run on Abaqus. The goal is to reach a numerical tool that can be used to accurately describe the cyclic elastic-plastic behaviour of magnesium alloys in synergy with finite element packages. In order to characterize the cyclic behaviour of AZ31B-F magnesium alloy, experimental tests were performed considering proportional and non-proportional loadings. To evaluate the implemented model in UMAT, these results were correlated with the experiments and with the analytical HYPS approach. Moreover, the estimates were also correlated with the Armstrong-Frederick model available on Abaqus/Standard 6.14 library. The results have shown that the HYPS model was successful implemented on the UMAT subroutine with a good correlation between experimental tests and the HYPS model. Some remarks between the HYPS and Armstrong-Frederick models are drawn.

Mechanical and material testing and analysis are of indisputable importance. As the variety of materials and/or the complexity of products grows, so does the need for new material characterisation methods. In this study, experimental results from a new method for ultrasonic fatigue testing of equibiaxial and pure shear cruciform specimens is presented and validated. The method uses specially designed cruciform specimens’ geometries for ultrasonic fatigue testing machines. Three different cruciform geometries were tested. The responses from the specimens were first studied by an adapted modal calculation method FDD. The new method allows determining fatigue strength of cruciform specimens in the VHCF regime with very satisfactory results.

In this work, the mechanical behavior of the AZ31B-F magnesium alloy under cyclic loading is analyzed with the goal of contributing to the advancement of its use in the design of AZ31B-F components and structures. To achieve this goal, an experimental program was implemented to evaluate the cyclic response of the AZ31B-F under specific proportional loads with different stress amplitude ratios. Afterwards, regression methods were applied to extend the experimental data to a wide range of proportional loads. As a result, the AZ31B-F damage map, a material property that stablishes the damage scale between normal and shear stresses for finite life loading regimes, was obtained. In addition, a safety factor was developed for the AZ31B-F material when subjected to proportional loading. The achieved results have a direct application in mechanical design of components/structures made of AZ31B-F contributing to its reliability.

Additive Manufacturing (AM) has carved a significant leap in mechanical design history, offering an almost infinite geometric freedom. It enables complex geometries with lower weight and enhanced behaviour, like topology optimisation. AM material mechanical properties is under continuous research due to the ever-increasing available materials, methods and an overall higher number of influential variables, such as printing parameters and powder shape and size (associated with metal AM). A rough and low quality-built surface is obtained from most AM methods. It has been proven to have a far from neglectable influence on the final mechanical properties and fatigue strength. Different post-treatment methods have shown a positive impact on correcting the rough surface impact on fatigue strength. In this work, the Ti6Al4V titanium alloy additively manufactured by Selective Laser Melting (SLM) is placed under Ultrasonic Fatigue Testing (UFT) within the Very High Cycle Fatigue (VHCF) regime. Ti6Al4V is widely applied in the aerospace industry due to its high strength, low-density relation. This work aims to evaluate the influence of surface roughness, defect size and location on fatigue strength. Several non-heat treated Ti6Al4V specimens were manufactured in one printing session. The present defect type, size and density, were quantified, and their tension, ductility and fatigue life properties evaluated. Tensile UFT were conducted with R =-1 to as-built and polished mirror-like specimens. The resulting fatigue fracture showed surface crack initiation for as-built specimens and sub-surface in polished specimens. A significant fatigue strength increase accompanied the crack initiation change. Sub-surface crack initiation occurred on the most common AM present defects, gas pores and smooth facets. Their location in respect to the surface appeared to influence fatigue resulting life directly.

Sandwich panels are widely used in many engineering applications where saving weight while maintaining high strength and stiffness is required. The most common core structure in sandwich panels is the two-dimensional regular hexagonal cell shape, denoted as Honeycomb. In recent times, bioinspired materials and structures have become increasingly attractive to researchers, as they provide adequate functional properties. The goal of the present work is to study two new bioinspired structures aimed at improving the performance of sandwich panel cores. Among all the large amount of structures that nature provides, two novel cores inspired in the structures of enamel and of bamboo were chosen. The compressive and flexural properties of these two innovative cellular structures were assessed and compared with the classic honeycomb. All the arrangements were numerically simulated for different relative densities. The fused deposition modelling technique enables to print selected samples in polylactic acid that were experimentally tested in compression and in bending. Results show that the mechanical properties depend strongly on the core geometry, on the relative density and on the cell thickness distribution. A satisfactory agreement was found between finite element results and experimental data. For the same relative density, the bioinspired natural structures proposed in the present study are potential competitors with the traditional core structures in what concerns strength, stiffness and energy absorption.

With the development of new materials, it is now known that there is no such thing as a fatigue endurance limit, i.e., materials do not have infinite life when the stress level is such that there is no fracture up to 10 million (1E7) cycles. The problem of testing materials above this number of cycles is that most testing equipment operates well below 150 Hz, making testing up to 1 billion (1E9) cycles or above an impracticality. The recent developments of ultrasonic testing machines where frequencies can go as high as 20 kHz or above enabled tests to be extended to these ranges in just a few days. This is now known as Very High Cycle Fatigue (VHCF). On the other hand, critical components used in Engineering applications are usually subjected to multi-axial loads, as is the case of the fuselage and wings of aircrafts which are subjected to biaxial states of stress. In this paper, VHCF cruciform test specimens purposely designed to develop orthogonal biaxial stresses with different biaxiality ratios will be analysed. The specimens are composed from Aluminium 6082-T651, a medium strength alloy used in many highly stressed engineering applications, including trusses, cranes, bridges and transportation. The specimens work as tuning forks with determined mode shapes at 20±0.5 kHz, where maximum principal stresses are developed at the centre of the specimen. Finite Element Analysis (FEA) is used to assess the dynamic behaviour of the specimens. The framework on how to design and manufacture cruciform specimens with different biaxiality ratios will be explained in a clear way so it can be used by other engineers in the field.

In this work the failure analysis of a coupled shaft from a shredder is presented. The failure analysis was performed using both experimental methods (visual and microscopic inspection, tensile and hardness tests) and theoretical calculations (stress analysis) aiming to determine the root causes of failure of the coupled shaft. The failure of the shaft occurred due to fatigue, on a perpendicular plane to the rotation axis, in the region of the coupling transverse hole. The shaft fracture surface presents characteristics of fatigue due to torsion combined with bending high loads, with an associated strong plastic deformation. The contribution of bending solicitations related with radial misalignment between the shaft and the shredder module was evaluated. The radial misalignment plays an important role on the failure of the shaft. For the reported life service of the shaft under maximum torsional moments, and considered working conditions, the design guidelines of radial misalignments compromise the structural integrity of the shaft in the shredder system. Recommendation regarding the design and assembly of the shaft are also presented.

A case study of a catastrophic failure of a diesel motor crankshaft is presented. The aim of this failure analysis is to investigate the root cause of this important mechanical component. This crankshaft belonged to a particular vehicle (140 cv, at 4000 rpm, 1968 cm3 displacement) that fractured after 180,000 km and 8 years in service. The motor was disassembled, and the crankpin No. 3 and the main bearing cap No. 4 were broken. Defects of material and machining defects were not found at the crack initiation sites by optical and SEM microscope. Results shown that the crankpin and the main bearing cap clearly failed by fatigue, and the root cause seems to be related with deficient tightening of the main bearing cap No. 4, which fractured due to a crack developed on the its outer side. The main journal No. 4 did run out of support, and the crankthrow No. 3 increased the alternating stress amplitude, whereby an inevitable catastrophic crankshaft failure happened.

Fused deposition modelling (FDM) shows a great potential for the production of complex shapes in the case of several materials including nylon, which is a type of polyamide. In such materials, water absorption is an issue that requires attention and there is interest in studying the application of protective products, with the goal of improving the sealing properties of the parts. The objective of this work is to analyze and evaluate the water absorption, mechanical properties and consequent failure surfaces of nylon parts obtained by FDM, with two raster angles, after being coated with a protective material. Two polymeric products were used to coat the parts, polyurethane elastomer and silicone. The performance of the coated parts was evaluated with water absorption tests, as well as compression and tensile tests. The treatment was found to reduce the open porosity and to decrease the mechanical properties of the specimens in different raster angles. Failure occurs at surfaces making angles of zero or forty five degrees from the applied force, depending on the raster angle, both in treated and untreated samples.

The aim of this work is to evaluate the mechanical properties and failure analysis of cellular core structures with different geometries that were obtained by additive manufacturing. Sandwich panels are widely used in the aerospace and automotive industry. In general, the core of the panels is made of a two dimensional cellular with a honeycomb geometry. With the development of additive manufacturing methods it is possible to produce samples with complex geometries which may compete with conventional designs. Thus an investigation was conducted to evaluate the mechanical behavior of three core geometries, specifically, regular honeycombs, lotus and hexagonal honeycombs with Plateau borders. Samples were produced in PLA (polylactic acid) by fused deposition modelling (FDM). Experimental compressive loading in three different directions, and finite element simulations of the samples permit to evaluate their deformation and failure mechanisms. Load direction angles were found to have a strong influence in the failure mode. Among the three structures, and for the same relative density, the lotus geometry exhibited the highest stiffness and strength. However, the absorbed energy was found to be higher for honeycomb, at two loading directions. Some of the structures studied may be alternative to conventional designs pursuing the strategy of design with low weight and high stiffness.

This paper reports a failure mode analysis of a 1900 cm3 turbo diesel engine of a well-known commercial brand. The engine is a supercharged diesel engine with turbocharger, producing a maximum power of 81 kW; it was produced in 1999 and collapsed at 120,000 km without warning. A fracture occurred at the crankpin n°1 of the crankshaft in the region of web-fillet. Crankshafts are mechanical power transmission components with complex geometries and experience multiaxial stress states in main journals and crankpins. The objective of this work is to determine the root cause that led to the crankshaft collapse. A fractographic, metallographic, and numeric analysis were performed to understand the crankshaft failure mode and its mechanical mechanisms. Results show that the crankshaft failure resulted from a fatigue process governed by normal stresses raised by two possible processes, namely, a notch in the crack initiation spot, or the crankshaft misalignment.

This work aims to study the influence of protective coatings on the water absorption and mechanical properties of 3D printed polyꖖlactic acid (PLA) parts. The PLA parts were fabricated with different levels of the 3D printing process parameters, aiming to define samples with distinct strength and ductility/toughness characteristics. Water absorption tests following the standard ASTM D570�98 were performed on uncoated and coated PLA specimens. The effectiveness of two protective coatings based on acrylic and polyurethane varnish on reducing water absorption was evaluated. Both protective coatings have shown being effective on preventing water absorption by the PLA, with polyurethane presenting the best performance reducing water absorption by 38%. Tensile tests were carried out to determine the ultimate tensile strength, elastic modulus, yield tensile strength, fracture strain and toughness of specimens, before and after the application of protective coatings. The polyurethane protective coating also benefits the tensile properties of PLA parts, increasing the strength and ductility/toughness characteristics of specimens up to 24%.

This work intends to evaluate the use of epoxy composite materials reinforced with basalt fibers as replacement to metallic mechanical parts of a highly efficient electrical prototype. The analysis of the behavior of the original metallic bracket was made and an optimization process was carried out in order to achieve the most suitable geometry and stacking sequence if produced in composite material. Finite element analysis using Siemens NX12 and experimental tests to the produced composite part were performed in order to access it. It was verified that the total weight of the composite part shows a 45% reduction. The composite part shows a higher deformation than the metallic one due to basalt fiber’s higher flexibility. However, the advantages added by the new component largely compensate for the disadvantages that may have been added without compromising its performance. Obtained results show that the use of basalt fiber reinforced composites as the material of mechanical parts of a highly efficient electrical prototype that is a good alternative.

Fatigue crack growth is usually studied using C(T) or M(T) specimens with through-thickness cracks. The objective of the present study is to propose a cylindrical specimen with central crack, produced by additive manufacturing. This geometry allows to have pure plane strain state along the whole crack front, avoiding the complexities associated with corner points, crack shape, and variation of crack closure along crack front. Additionally, this geometry may be used to develop studies in vacuum, avoiding expensive vacuum equipment, since the air is not in contact with the crack front. Cylindrical specimens of Ti6Al4V titanium alloy were produced by Selective Laser Melting and tested at a stress ratio R = 0. Marking with overloads was the solution adopted to measure the length of the internal cracks. The fracture surfaces presented circular crack fronts and the da/dN-ΔK curves showed a great influence of atmosphere on fatigue crack growth. An average difference of 50% was found between the results in air and vacuum. Therefore, this geometry with internal crack is an interesting alternative to through-thickness geometries.

International audience; The aerospace, automotive, and marine industries are heavily reliant on sandwich panels with cellular material cores. Although honeycombs with hexagonal cells are the most commonly used geometries as cores, recently there have been new alternatives in the design of lightweight structures. The present work aims to evaluate the mechanical properties of metallic and polymeric honeycomb structures, with configurations recently proposed and different in-plane orientations, produced by additive and subtractive manufacturing processes. Structures with configurations such as regular hexagonal honeycomb (Hr), lotus (Lt), and hexagonal honeycomb with Plateau borders (Pt), with 0°, 45°, and 90° orientations were analyzed. To evaluate its properties, three-point bending tests were performed, both experimentally and by numerical modeling, by means of the finite element method. Honeycombs of two aluminum alloys and polylactic acid were fabricated. The structures produced in aluminum were obtained either by selective laser melting technology or by machining, while polylactic acid structures were obtained by material extrusion using fused filament fabrication. From the stress distribution analysis and the load–displacement curves, it was possible to evaluate the strength, stiffness, and absorbed energy of the structures. Failure modes were also analyzed for polylactic acid honeycombs. In general, a strong correlation was observed between numerical and experimental results. The results show that the stiffness and absorbed energy increase in the order, Hr, Pt, Lt, and with the orientation through the sequence, 45°, 90°, 0°. Thus, Lt structures with 0° orientation seem to be good alternatives to the traditional honeycombs used in sandwich composite panels for those industrial applications where low weight, high stiffness, and large energy-absorbing capacity are required.

Fatigue behavior for any material or composite is an ever-evolving complex subject of big engineering importance. Within the very high 106 to 109 cycle regime the use of ultrasonic fatigue has been a big research topic in recent years, ranging from all metals and composite studies, new experimental methods and induced stress states, to the frequency effect and associated fracture mechanics. Following the associated cruciform ultrasonic fatigue testing research in development, new and improved cruciform specimens were acquired and subjected to experimental analysis. The conducted fatigue cruciform testing method induces an axial-axial in phase stress state through the use of a piezoelectric ultrasonic testing machine built at Instituto Superior Tecnico laboratories. A modal response analysis was performed within the frequency working range of the used piezoelectric transducer using a Polytec laser vibrometer. A modal calculation method termed Frequency Domain Decomposition (FDD) was adapted to the measured cruciform vibration displacements. The points of measure were in correlation to a previous published numerical research. The results were then compared to previous cruciform specimens’ behavior and numerical results. Strain gauges were also used for the strain and stress study at the specimen’s center. With both conducted experiments showing promising results, the new and improved specimens were led to fatigue failure and a preliminary fracture analysis made.

The structural integrity of several structures could be determined by their joints strength. Over the years, adhesively bonded joints have been often chosen to achieve a compromise between mass reduction and higher mechanical strength. Among others, the reduction in stress concentrations, the ability of producing smooth surfaces with no discontinuities and the reduced weight penalties are some of the factors that make this type of joints so attractive. Normally, to increase the bond strength, the materials to be bonded must be subjected to a kind of surface treatment. For metals, and more specifically, for aluminium alloys, phosphoric acid anodizing and chromic acid anodizing have been the most used treatments worldwide. However, recent investigations show that these kinds of anodizing are detrimental to health due to the release of carcinogenic substances. With this in mind, it is of the utmost importance to find alternative surface treatments that can ensure an effective bond. In this paper, a vast experimental study was performed based in the single lap joint ASTM D 1002 standard method, with the objective of determining the best alternative surface treatment (Sulfuric Acid Anodizing and Boric-Sulfuric Acid Anodizing), for aluminium-to-aluminium joints, using two types of adhesives, namely the AF 163 and the EA 9658 AERO. Results show that the optimum surface treatment is different for each type of adhesive and this fact has a huge influence on mechanical behavior of this type of aeronautical adhesive joints. info:eu-repo/semantics/publishedVersion

In this paper, a railway wheel material is under evaluation using multiaxial fatigue testing. The experiments were conducted using a servo-hydraulic machine with standardized specimens. All samples were machined from a single worn-out railway wheel. The damage scale between normal and shear stresses was evaluated in the normal stress space for proportional and non-promotional loadings. Moreover, the uniaxial SN curves were obtained. A critical plane analysis was performed using theoretical criteria and experimental results. Results show a strong influence of heat treatments on the material fatigue behavior.

It has been experimentally proven that the shear stress level needed to cause fatigue failure is lower than the axial one. This fact has led to consider a Stress Scale Factor (SSF) between shear and axial stress to reduce different applied stresses to the same shear stress space or principal stress space, consequently facilitating the yielding analysis or fatigue damage evaluations. Most of multiaxial fatigue models use an SSF, and materials can be classified as shear sensitive (low SSF values) or tensile sensitive (large SSF values), depending on the main fatigue microcrack initiation process under multiaxial loadings. The use of SSF is quite common in many multiaxial fatigue criteria based on the critical plane approach. Such criteria adopt a SSF value assumed constant for a given material, sometimes varying with the fatigue life (in cycles) but not with the SAR (Stress Amplitude Ratio), the stress amplitude level, or the loading path shape. In this work, in-phase proportional tension-torsion tests related to 42CrMo4 steel specimens for several values of SAR are presented. The SSF approach is then compared with critical-plane models, based on their predicted fatigue lives and the observed ones for the studied tension-torsion histories.

In this work, two special aluminium cruciform specimens are designed and tested in an ultrasonic fatigue machine. They were designed based on Single-Input-Multiple-Output (SIMO) modal analysis to induce in-plane biaxial stress combinations (in-phase T-T and out-of-phase C-T) when at resonance at 20 kHz. The geometries were subjected to both numerical analysis and experimental testing to understand if they can indeed create the intended biaxial state of stresses. Both numerical and experimental results showed an impact of nearby resonant modes of non-interest on the correct functioning of the specimens, especially regarding the T-T specimen where a large deviation from the mode of interest was measured. This means that future work includes re-designing T-T specimens taking into account these mode shapes. Only out-of-phase specimens demonstrated to work properly and tests until failure were conducted. The first failure results showed to be consistent with literature when out-of-phase biaxial stress is applied cyclically.

Fatigue studies of materials in simple or complex loading systems for any given lifetime is object of continuous research. This is due to the advancements on mechanical and structural components, as well as for new and innovative materials, which implies the knowledge of a materials response to all dynamic loads. The fatigue failure regime beyond what was once considered to be the fatigue limit (infinite life) is characterized between 107 and 109, known as Very High Cycle Fatigue regime. Due to the time consuming and wide energy consumption of conventional fatigue testing for such regime, fatigue tests under ultrasonic actuators are being used, capable of applying the dynamic loads at around 20 kHz. Nowadays, several variants of ultrasonic fatigue tests were already proposed and tested but it is still a somewhat limited fatigue test if compared to the conventional servo-hydraulic fatigue testing machines of general use. In this study, biaxial in plane stresses are induced in specially designed cruciform specimens with ultrasonic fatigue testing resonant principals. Two geometries were numerically analysed, manufactured and experimentally tested, the in-phase tension-tension (T-T) specimen and the out-of-phase compression-tension (C-T) specimen. All specific designed geometries go under a thorough numerical and several experiments analysis for their validation. The specimens showing a correct and as intended behaviour are led to failure.

The objective of this work was to characterize the full-field flexural behavior of composite sandwich beams. Finite element analysis was used to estimate the behavior of sandwich beams under three- and four-point-bending tests and were compared with experimental results obtained via digital image correlation and strain-gauges. Two different polyurethane core thicknesses and two different sandwich lengths were used to simulate short- and long-beam. Two distinct sandwich beams were used by means of two different faces: aluminum and basalt fiber reinforced polymer composite. Full-strain-fields and flexural displacements results were obtained showing that BFRP sandwiches exhibited higher flexibility and higher capacity of absorption energy than the aluminum specimens however with a higher prospect of core shear failure. For both face materials short-beams present higher strains than the long-beams and 4PB test specimens showed negative strain distribution in the upper side of the specimen and positive in the underside contrary to 3PB that presents positive strain distribution along the fulfilled. Moreover, results show that the strain distribution in not symmetric. Obtained experimental results are in good agreement with estimations; the digital image technique attested to be a complementary method to strain gages measurements in experimental full-strain-fields analysis.

With the introduction of new materials and advances in medical science, the endodontic files have changed since the early days of root canal treatments. In the late days, we have seen an increasing use of Nickel-Titanium (NiTi) alloys, to the detriment of more conventional alloys. At body temperature, NiTi alloys present a superelastic behaviour, which allows to be more effective in the removal of the tooth pulp tissue, and in the protection of the tooth structure. Anyhow, these NiTi instruments will eventually fracture, usually without any visual signal of degradation. Thus, there is a need of studying these alloys, as they present a high hysteresis cycle and non-linearities in the Elastic domain. Currently, there is no international standard to test NiTi endodontic files, so various authors have attempted to design systems that can test them under fatigue loads, usually based on empirical setups. Following a systematic approach, this work presents the results of rotary fatigue tests for two Alfa Aesar® Nitinol wires with different diameters (0.58mm and 0.25mm).The formulation is presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure, as well numerical FEM simulation to verify the analytical model predictions.

This paper is focused on the utilisation of double edge notched tension, staggered and shear tests to determine fracture toughness and the formability limits by fracture in principal strain space. The experiments were performed in test specimens with different geometries and ligament angles, and the influence of strain hardening was taken into consideration by selecting two materials (aluminium AA1050-H111 and pure copper), with very different strain hardening exponents. Results are plotted in principal strain space, and the discussion is focused on the link between formability limits, fracture toughness and macroscopic fractography characteristics of the specimens that fail by mode I, mode II or mixed-mode.

Railway mechanical components are subject to thousands of fatigue cycles. Fatigue damage and life assessment is still an open issue. Under service multiaxial loading conditions several challenges can arise. In this study an evaluation of a railway wheel material is performed, i.e. the material properties and the working conditions are taking into account and evaluated. Different mechanical tests are carried out, namely fatigue tests under uniaxial (LCF+HCF) and biaxial (HCF) conditions, applied to several specimens made from the railway wheel. Multiaxial fatigue models were considered regarding the fatigue life estimation. Moreover, fatigue crack plane measurements were compared with estimations from several critical plane models. The applied models provided very satisfactory results, regarding the fatigue life estimation and the initial crack initiation plane under the multiaxial loadings conditions.

Dental implants are widely used in replacing missing teeth. However, some problems may occur such as screw loosening or screw fracture which lead to implant failure. In this regard, it is of utmost importance to analyse some of the factors that affect the primary stability and consequently the osseointegration of dental systems. In the present study, particular attention is given to the bone density and the bone-implant contact in order to evaluate the implant performance. Given the difficulty in working with bone in vivo, in the present study two implant systems were inserted in polymeric samples, of different densities, known as Sawbones, which simulate the structure of trabecular bone. The implant systems chosen were external hexagon and Morse taper. The performance of the implants was evaluated through experimental fatigue tests that simulate the chewing or mastication cycles. The qualitative analysis of the damage in the cellular structure of the samples was performed using scanning electron microscopy (SEM). The finite element (FE) method was used to model the penetration of the implants and enable the determination of the stress at the implant and deformations at the Sawbones-implant vicinity. Prior to penetration simulations, a well-known analytical model of indentation was compared with indentation simulations, in order to validate the numerical model. The major difference between numerical and analytical model results, in the case of the displacements, was found to be around 9%. The failure of the Sawbones occurred with the propagation of the collapsed cells according to SEM observations. The fatigue results showed that the Morse taper implant exhibited lower displacements than the external hexagon. Higher density Sawbones also exhibits lower deformations, while keeping the other parameters constant. It means that Morse taper and high dense materials show a better fatigue performance than the other systems and materials. The FE results for penetration showed that a threaded geometry implant when compared to smooth geometry provided a reduction of the stresses in the implant and reduces the deformations in the Sawbones. The results of the penetration simulations follow the same trend as the indentation simulations and analytical model. Simulation results show that the displacements are reduced with the increase of the density of the polymer foam. This is in agreement with the fatigue results. Comparing bones with foams, it could be concluded that, the increase of the bone density will induce a higher stability of the implants.

Damping properties for the AZ31B-F magnesium alloy were evaluated for pure axial and pure shear loading conditions at room temperature. Hysteretic damping results were measured through stress–strain controlled tests. Moreover, the magnesium alloy viscous damping was measured with frequency response functions and free vibration decay, both results were obtained by experiments. The axial and shear damping ratio (ASDR) has been identified and described, specifically for free vibration conditions.

In the research herein presented, mode III and mode I fatigue loading tests were carried out on standard compact tension (CT) specimens using a bi-axial servo-hydraulic Instron 8874 machine. Thin sheet CT specimens were made of two types of austenitic stainless steels, namely an AISI 316L stainless steel and a high-strength Cr-Mn stainless steel. Specimens were pre-cracked in mode I, at room temperature, and crack growth paths were experimentally determined. Fracture surfaces of rectangular sections were observed using optical devices and crack growth revealed to be higher at the external surfaces than at the mid plane of the specimens tested under mode III fatigue loading; in addition, smooth slant crack growth occurred, as well as crack branching. Fracture surfaces obtained under mode I loading revealed that crack grew flat, coplanar and normal to the tensile axis, as expected. Additionally, stress intensity factors present at tip of each branch crack loaded under mode III were calculated through finite element analyses (FEA). Hence, although an out-of-plane torsional loading was remotely applied, a mixture of modes was foreseen to be applied locally at crack tip during crack growth, namely mode I, mode III and mode II loading, in that order of relevance, and equivalent stress intensity factor values at the crack tip were calculated using Pook, Richard or Schollmann et al. criterion.

The very high cycle region, up to 10E10 cycles of the S-N fatigue curve has been the subject of intensive research on the last decades, with special focus on axial, bending, torsional and fretting fatigue tests. This can be achieved using ultrasonic exciters which allow for frequency testing of up to 30 kHz. Still, the multiaxial fatigue analysis is not yet developed for this type of fatigue analyses, mainly due to conceptual limitations of these testing devices. In this paper, a special designed cylindrical specimen is tested at 20 kHz on a bi-axial loading condition, with axial and torsional components on the specimen throat, using a new biaxial ultrasonic fatigue testing machine. The characterization of the specimen loading is measured by rotational and axial laser transducers at the bottom of the specimen and additionally by strain gauges applied at its throat. Strain gage data from very high frequency excitation is compared to strain gage data obtained from equivalent specimen geometry at 0.5 Hz in a biaxial servohidraulic machine. Results indicate good correlation between strain measurements for both fatigue tests at low and ultrasonic frequency tests. This data will help to extend very high cycle fatigue curves for biaxial loading conditions and serve as groundwork for future research on this field.

In this work, the performance of the SSF criterion is evaluated under variable amplitude loading conditions. The main objective was to inspect the validity of the hypothesis in which the SSF damage map remains valid for any high strength steel. In order to achieve that, fatigue life correlation of the 1050QT steel and 304L stainless steel was analyzed under multiaxial loading conditions. The loading block considered in the study comprises 360 proportional loading cycles with different stress amplitude ratios and stress levels. Despite being made of proportional branches, this loading block is a non-proportional loading due to its principal directions variation. This feature allows the evaluation of combined loading effects under variable amplitude loading conditions, which makes this loading block suitable to mimic the loading effects usually found in the field. Results show very good agreements, which reinforces the aforementioned hypothesis.

Endodontic rotary file instruments used to treat root canals in dentistry suffered breakthrough transformations in recent years when stainless steel was replaced by Nickel-Titanium (NiTi). NiTi alloys used in Endodontics possess superelastic properties at body temperature (37C) that bring many advantages on the overall performance of the root-canal treatment. They can follow curved root canals more easily than stainless steel instruments and have been reported to be more effective in the removal of the inflamed pulp tissue and protection of the tooth structure. However, these instruments eventually fracture under cyclic bending loading due to fatigue, without any visible signals of degradation to the practitioner. This problem brought new challenges on how new instruments should be tested, as NiTi alloys are highly non-linear and present a large hysteresis cycle in the Elastic domain. Current existing standards are only available for Stainless Steel testing. Thus, many authors have attempted to design systems that can test NiTi endodontic files under fatigue loads. However, no approach has been universally adopted by the community yet, as in most cases they are based on empirical set ups. Following a more systematic approach, this work presents the results of rotary fatigue tests for several NiTi wires from different manufacturers (Memry™ and Euroflex™). The formulation is presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure, as well numerical FEM simulation to verify the analytical model predictions.

With the increasing interest in sustainable solutions in material design in the last decade, research on natural materials (animal, vegetal or mineral) has increased at a rapid pace. Of these materials, Basalt Fibers for composite construction provide an interesting set of mechanical properties, equal or above to those of Glass Fibers, with advantages in terms of cost effectiveness and production to vegetable based Natural Fibers. Basalt fibers offer some advantages versus current materials, it is fireproof, requires no material addition, has better mechanical properties than most types of E-Glass, and it is cheaper than Carbon Fiber. This paper studies the mechanical properties of a Basalt Fiber composite in an Unsaturated Polyester matrix produced by Resin Transfer Molding (RTM), with the composites subjected to tensile, compression, shear and flexural tests. The results aligned with the predicted values by using the mixing rule, albeit with a high coefficient of variation, which microscopic analysis confirmed to arise from production issues with RTM.

The main purpose of the research presented herein was to study the fatigue crack propagation under loading modes I, II or III, either for plane strain or plain stress state. Therefore, several finite element analyses (FEA) were carried out and some experimental tests were performed in a bi-axial servo-hydraulic machine. J-integral values were determined through numerical simulations and stress intensity factors, K I , K II and K III were inferred at the crack tip of both thick and thin C(T) specimens, assuming several crack’s lengths, and either for plane strain or plane stress state. Load opening-mode I shown to be the predominant mode of crack propagation under plane-strain state, leading to the highest J-integral values calculated, followed by Mode III. In addition, shearing load (mode II) induced the lowest stress intensity factor values at the crack tip both for plane stress state and for plane strain state. Finally, some fatigue crack growth rates (FCG) were determined under loading modes I or III, for thin specimens, at room temperature. C(T) specimens used in the experimental tests were made of two austenitic stainless steels, namely the AISI 316L and the Cr-Mn stainless steel, which are commonly used in several specific engineering applications. Results were compared and some conclusions could be drawn.

Dental implants are in general made of titanium, since this material promotes a stable and functional connection between the bone and the surface of the implant. Efforts produced during the chewing cycles may interfere with this union, affecting the process of osseointegration and eventually compromising the stability of the implant. Given the difficulty in working with bone in vivo, in the present study two implant systems were inserted in polymer samples, known as Sawbones, which simulate the structure of trabecular bone. The performance of the implants was evaluated through experimental fatigue tests. The qualitative analysis of the damage in the structure of the samples was performed using scanning electron microscope images. Determination and comparison of stress fields and deformations at the Sawbone-implant interface using an analytical model of indentation and the finite element method (FEM) to model indentation and penetration were undertaken. The experimental results showed that the performance of the Morse taper implant was greater than the external hexagonal implant when both were tested cyclically in samples of different densities. It was shown that the diameter, length, density and type of implant-abutment interface affected the behaviour of the implants. The numerical results of indentation model were very similar to those obtained by the analytical model. The results of the FEM penetration model had the same tendency as the experimental values and the indentation analysis with increasing the density of the polymer foam. It could be concluded that, as in foams, the increase of the bone density will induce an increased stability to the implants.

The very high cycle region of the S-N fatigue curve has been the subject of intensive research on the last years, with special focus on axial, bending, torsional and fretting fatigue tests. This can be achieved using ultrasonic exciters which allow for frequency testing of up to 30 kHz. Still, the multiaxial fatigue analysis is not yet developed for this type of fatigue analyses, mainly due to conceptual limitations of these testing devices. In this paper, a specimen is tested at 20 kHz on a bi-axial loading condition, with axial and torsional components on the specimen throat. The behavior of the specimen is measured by rotational and axial laser transducers at the bottom of the specimen and by strain gauges applied at its throat. Strain gauge data from very high frequency excitation is compared to strain gauge data obtained from equivalent specimen geometry at 0.5 Hz. Results indicate good correlation between both low and very high frequency tests. This data will serve as groundwork for future research on this field.

A critical parameter in a hybrid CFRP/steel composites is the shear stress between CFRP plies and steel foils.This paper presents an experimental and numerical research on the single lap shear stress in hybrid CFRP/Steel specimens in accordance with DIN EN ISO 1465. Four different types of specimens were tested: Vacuum basting surface treatment and pickling treatment. For each type of surface treatment two types of specimens were tested: Dry specimens and specimens immersed in water for 1000 h. The results show that vacuum blasting surface treatment is the one that withstands a higher shear stress and is simpler and less hazardous than pickling surface treatment. The results also show that this type of composite has low sensitivity to water absorption. A significant difference between the displacement measured by digital image correlation and the displacement measured by a LVDT was observed. Finite element models are able to predict the maximum shear stress of hybrid composites provided that the constitutive parameters of cohesive elements are validated by experimental results.

The presence of a biomimetic HAP coating on titanium surface, which reduces the structural stiffness, is essential to improve implants biocompatibility and osteointegration. In this study, new citrate-HAP (cHAP) coatings were produced by a simple hydrothermal method on pure titanium (Ti) surface, without requiring any additional pretreatment on this metal surface. The formed cHAP coatings consisting of nanorod-like hydroxyapatite particles, conferred nanoroughness and wettability able to endow improved biological responses. Indeed, the presence of citrate species in the precipitate medium seems to be responsible for controlling the morphology of the new coatings. The presence of citrate groups on the surface of cHAP coatings, identified by chemical composition analysis, due to their implication in bone metabolism can additionally bring an add-value for bone implant applications. From a mechanical point of view, the Finite Element algorithm showing that cHAP coatings tend to decrease the mechanical stress at pure Ti, further favors these new coatings applicability. Overall, the simple and expedite strategy used to developed new biomimetic coatings of citrate-HAP resulted in improved physicochemical, morphological and mechanical properties of Ti, which can endeavor improved implantable materials in bone healing surgical procedures.

The goal of this research is to investigate the detrimental effect of non-metallic inclusions on the fatigue strength of the AISI 4140 steel under multiaxial loading conditions. In order to do so, two multiaxial fatigue models based on the critical plane approach are coupled with Murakami’s √area parameter. The proposed adapted multiaxial criteria are proved simple to calibrate. Experiments under combined push-pull and torsional loading in and out-of-phase are conducted and compared with the estimates provided by the proposed adapted models. The adaptation is conservative with errors around 20%. An analysis of the fracture surfaces is carried out to compare the estimated crack initiation planes with the observed ones.

Ultrasonic fatigue testing is a relative recent fatigue study methodology that applies resonant principles for the induction of stress cycles in a specific designed material specimen. Such experimental method can apply fatigue damage at high frequencies, most commonly at 20 kHz. It was created with the main purpose of studying material fatigue life in the Very High Cycle Fatigue regime between 10E07 and 10E09 cycles with a higher performance of time and energy wise in comparison to conventional servo-hydraulic machines. In this study an ultrasonic fatigue testing machine was used in order carry out multiaxial tension/torsion fatigue tests at a frequency of 20 kHz. The main objective was to reach a reliable multiaxial fatigue testing method, by modifying only the specimen, with the ability to choose the shear-axial stress ratio. The improved design of the testing method was focused in innovative design specimen by conducting both numerical and experimental analysis. Thermographic imaging, laser displacement measurements, and the application of rosette strain gauges to the main stress region of the specimen were carried out in order to validate the new improved design concepts. These strain and displacement results were compared with the ones obtained by finite element and a good agreement was achieved. A first series of fatigue tests were carried out intension/torsion multiaxial fatigue and the results are discussed in order to understand the feasibility of multiaxial fatigue tests at ultrasonic frequencies.