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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%.

Monopiles for offshore wind are the most used foundations by farm operators due to their low production costs, when compared to other bottom-fixed or floating foundations. In this research, a monopile foundation for offshore wind power was evaluated for its soil interaction and grout behavior, and an appropriate numerical model for the structural analysis of the foundation and tower was developed. FAST 8, an aero-hydro-servo-elastic numerical code developed by NREL, was used to obtain the loads applied on the supporting structures. These loads were pre-processed before they were inputted on the finite element model, developed using the finite element software ANSYS. The considered conical grout connection, which connects the monopile to the transition piece through friction, was modeled under a changing-status nonlinearity condition. To model the soil&ndash, pile interaction, a p-y model was applied using the ANSYS APDL commands. Static, modal, and transient structural analyses were produced to study the structure suitability for its use on offshore environments. Different soil interactions were modeled, and their results were then compared within the transient and modal analysis, indicating that the angle of the grout connection strongly affects the loading conditions on the grout. Moreover, scouring affects the dynamic behavior of the overall supporting structures, thus protection against this phenomenon is suggested.

A failure mode analysis of a diesel motor (110 kW) crankshaft from an automobile vehicle is presented. After 120,000 km in service, an abnormal vibration was detected which was increasing with the time. The diesel motor was first disassembled for determining the root cause, however without success. No defect was detected, but since a suspicion of damage was present, and being this failure recurrent in this type of diesel motor series, the crankshaft was disassembled again. Then the crankshaft was subjected to a simple vibration analysis and a preliminary indication of possible existence of a crack was concluded. The crankshaft was then replaced by a new one, and the old was subjected to a failure analysis for determining the root cause. A crack was found at the crankpin web-fillet and after a complete opening of the crack, the failure analysis showed that fatigue was the dominant failure mechanism. Observations were carried out by optical and Scanning Electronic Microscope. Material defects at the crack initiation zone were not found. The root cause of damage seems to be a misalignment of the main journals and a weakness of design close to the gear at the region where the crack was initiated. Therefore, probably a poor design and a deficient assembling of the crankshaft helical gear coupled to the main journal end was the first cause of the failure.

Fatigue damage has special relevance on the life span of mechanical components and structures, as it takes responsibility for the majority of the registered structural failures. Although its mechanisms have been the subject of continuous research, the growing need for greater lifespans forced the understanding of the behavior of materials under the very high cycle regime, also known as Very High Cycle Fatigue. This field of research, which studies the mechanical behavior of materials for fatigue lives beyond 1E7 cycles, only recently gained notoriety, largely due to the appearance of ultrasonic fatigue testing machines, working at 20 kHz and due to the acquisition and control equipment capable of handling signals at very high frequencies. On the other hand the behavior of materials under multiaxial fatigue has been the subject of research and development, but not in the region of very high cycles, due to the inexistence of appropriate testing machine to perform these tests. The authors of this work have already a large experience on the performance of multiaxial fatigue tests under axial/torsion loading under servo-hydraulic fatigue testing machines and on very high cycle fatigue tests. In this paper, and using a new developed multiaxial fatigue testing machine operating at 20 kHz frequency, experimental results, using strain gauges and axial and shear stresses determined by finite element modelling on a new specimen design tested with biaxial loadings, tension/torsion at 20 kHz, are discussed and analyzed. Results show that it is possible to carry out multiaxial (axial/torsion loading) fatigue tests at very high frequencies which will allow to perform reliable very high cycle multiaxial fatigue tests.

Critical plane criteria seek the plane in which their damage parameter has its maximum value. On this plane, the fatigue damage of both normal and shear stresses is updated to the same damage scale by a constant, typically based on material normal/shear fatigue limits. This paper focuses on the damage scale concept under a stress-based critical plane standpoint, and the main objective is to evaluate its performance. To perform this study, data from the high strength steel 42CrMo4 were considered. Five proportional loading paths with different stress amplitude ratios and stress levels were analysed using 2 different multiaxial fatigue criteria, namely, the Findley critical plane model and the stress scale factor equivalent shear stress. Results show that the damage scale between normal and shear stress components varies accordingly to the loading plane direction considered. info:eu-repo/semantics/publishedVersion

The railway system has an important role in developed countries: it is possible to see, nowadays, passenger trains crossing the Old Continent and achieving impressive speeds in the East; at the same time, cargo wagons are hitting load-by-axle records in North America. Railway wheels are a critical component to this system, as any failure can lead to derailment, potentially causing financial loss and/or fatal accidents. The present work aims to analyze the mechanical properties of forged wheels manufactured according to the American standard AAR class C, usually applied in freight cars due its chemical composition (around the eutectoid point) which achieves high mechanical resistance combined with moderated toughness. The mechanical tests to evaluate the mechanical strength, ductility, toughness and hardness were performed in accordance with the European standard BS EN 13262, as follows: tensile tests at different temperatures, impact tests, toughness tests and hardness Brinell tests. The obtained results are in accordance with the microstructure and chemical composition of class C material, and will be employed in future investigations for the numerical validation of the mechanical behavior for multiaxial fatigue conditions and development of new materials (micro alloyed steel).

Additive Manufacturing (AM) is a manufacturing process through which a 3D component is produced by consecutively adding material. One of the most promising AM processes is SLM. In SLM a laser completely melts metallic powder particles together forming a 3D component. SLM is known for its freedom of manufacturing constraints allowing complex geometries and high material efficiency. Topology Optimisation (TO) is an optimisation type that calculates the optimal material distribution for a given problem. The combination of SLM with TO is being developed to create lightweight components. In this work, the whole development process, from optimisation to design, production and testing is addressed. Initially, an aircraft bracket topology was optimised to be produced by means of SLM. The TO solution was interpreted and designed for AM. During the interpretation and design process, a design methodology was defined in order to facilitate and make more accurate the TO solution design and make it ready for AM. After the optimised component was produced, metrological and mechanical tests were performed in order to validate the final design and the computer analysis. The optimised component showed considerable weight reduction with an increase of the factor of safety. The experimental tests revealed a good relation to the computer analysis evidencing, however, room for improvement, both in the computer model and the experimental tests.

The railway system has an important role in developed countries, it is possible to see, nowadays, passenger trains crossing the Old Continent and achieving impressive speeds in the East; at the same time, cargo wagons are hitting load-by-axle records in North America. Railway wheels are a critical component to this system, as any failure can lead to derailment, potentially causing financial loss and/or fatal accidents. The present work aims to analyze the mechanical properties of forged wheels manufactured according to the American standard AAR Class B (produced at the MWL Brasil facility), usually applied in passenger cars due its chemical composition (around the eutectoid point) which achieves high mechanical resistance combined with moderated toughness. The mechanical tests to evaluate the mechanical strength, ductility, fracture toughness and hardness were performed in accordance with the European standard BS EN 13262 (location of sample and method test), as follows: tensile tests, impact tests, toughness tests and hardness Brinell tests (hardness survey/hardness map). The results are in accordance with the microstructure and chemical composition, and will be employed in future investigations for the numerical validation of the mechanical behavior for multiaxial fatigue conditions and for failure analysis reports.

A new generation of smaller and more efficient biaxial fatigue testing machines has arrived on the market. Using electrical motors these machines are not able to achieve the higher loads their hydraulic counterparts can, and therefore the cruciform specimen needs to be optimized. Following the authors previous work, several different optimal specimens’ configurations were produced, using the base material sheet thickness as the main design variable. Every design variable was optimized in order to produce the highest stress level on the specimen center, while the stress distribution is still uniform on a 1 mm radius of the specimen center. Also it was guaranteed that the stress level on the specimen arms was always considerably lower, in order to achieve failure at the specimen center. In this paper traditional criteria like Findley, Brown-Miller, Fatemi-Socie, Smith, Watson e Topper (SWT), Liu I and Chu were considered to determine crack initiation direction for several loads in this biaxial in-plane specimens. In order to understand the fatigue propagation behavior, the stress intensity factors for mode I and mode II was determined for different cracks introduced on the geometry. Several crack and loading parameters were studied, including the starting crack length and angle, and different loading paths. Several biaxial loads were applied to the model, including 30°, 45°, 60°, 90° and 180° out-of-phase angles.

A failure analysis of two damaged crankshafts are presented: one obtained from a diesel engine of a mini backhoe, and another one from an automobile vehicle. The diesel motor suffered a serious mechanical damage after 3 years and 5000 hours in service: the connecting rod n° 3 broke and, in consequence, the crankcase and motor block suffered damage. The motor was repaired by a non-authorized workshop, but maintaining the same crankshaft without being properly inspected. After 1100 hours working the crankshaft failed on the 3rd crankpin. The second crankshaft failed after 105 000 km in service. In both crankshafts a crack grew from the crankpin-web fillet, and their symmetric semi-elliptical crack front profiles confirms the effect of a pure mode I (reversed bending). Fractographic analyses show the semi-elliptical beach marks and results indicate that fatigue fracture was the dominant failure mechanism of these two crankshafts.

Barriers (guardrails and balustrades) prevents people from falling, for example, from balcony, open windows and stairs. Barriers also retain, stop or guide person in buildings. To increase the transparency of these components, traditional materials such as bricks, wood and metal are being replaced by glass or an organic material, which has mechanical behavior different from traditional materials. Regulation usually specify some action to take into account in the design of barriers, but do not define the required resistance. There are no international standards (ISO or EN) to assess the fitness for use of barriers, only national standards, with different testing loading conditions and mechanical resistance requirements. In this paper is presented a comparison of requirements and experimental testing conditions specified in standards from Portugal, Spain, France, UK, USA and Brazil. The goal of this research is to find some equivalence between standards, regarding the mechanical resistance behavior of different materials (brittle/ductile materials) and set a worst case scenario as the basis for the guardrails mechanical resistance profile. Some relations between the service limits state (plasticity) of metal guardrails and maximum deflection are proposed.

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.

Axles and shafts are of prime importance concerning safety in energy equipment, transportation industry and general machinery. Case studies relating failure of axles and shafts are widely reported in the literature. Design rules for axles and shafts are mainly based on endurance curves for the material used according to long time established standards and procedures. Safe life design, based on endurance curves is being complemented by damage tolerance, where crack growth is taken into account. Recently, the knowledge of fatigue crack growth under typical loading conditions of axles and shafts with rotating bending and steady torsion has been object of research in order to apply damage tolerance concepts, mainly for maintenance and failure analysis purposes. The effect of a steady torsion on fatigue crack growth under rotating bending is focused in this paper. While axles and shafts in the transportation industry are traditionally designed on steels, the need for weight reduction due to fuel economy and eco-design constraints, lightweight materials must be considered for these applications. In this study, fatigue crack growth on rotating bending axles and shafts with or without an applied steady torsion is presented. Fracture mechanics approaches are used to analyze the results based on stress intensity factors developed for bending and torsion in shafts. Results are compared for the same stress ratio and relevant differences on fatigue crack growth are commented based on crack closure concepts. Discussion of the effect of crack growth retardation due to steady torsion is also discussed. Fractographic observations are presented and helped to explain fatigue crack growth under this mixed mode loading when steady torsion is applied to rotating bending.

In order to correctly assess the biaxial fatigue material properties one must experimentally test different load conditions and stress levels. With the rise of new in-plane biaxial fatigue testing machines, using smaller and more efficient electrical motors, instead of the conventional hydraulic machines, it is necessary to reduce the specimen size and to ensure that the specimen geometry is appropriate for the load capacity installed. At the present time there are no standard specimen’s geometries and the indications on literature how to design an efficient test specimen are insufficient. The main goal of this paper is to present the methodology on how to obtain an optimal cruciform specimen geometry, with thickness reduction in the gauge area, appropriate for fatigue crack initiation, as a function of the base material sheet thickness used to build the specimen. The geometry is optimized for maximum stress using several parameters, ensuring that in the gauge area the stress distributions on the loading directions are uniform and maximum with two limit phase shift loading conditions ( δ = 0° and δ = 180°). Therefore the fatigue damage will always initiate on the center of the specimen, avoiding failure outside this region. Using the Renard Series of preferred numbers for the base material sheet thickness as a reference, the reaming geometry parameters are optimized using a derivative-free methodology, called direct multi search (DMS) method. The final optimal geometry as a function of the base material sheet thickness is proposed, as a guide line for cruciform specimens design, and as a possible contribution for a future standard on in-plane biaxial fatigue tests.

This paper reports an investigation carried out on two damaged crankshafts of single cylinder diesel engines used in agricultural services for several purposes. Recurrent damages of these crankshafts type have happened after approximately 100 h in service. The root cause never was imputed to the manufacturer. The fatigue design and an accurate prediction of fatigue life are of primordial importance to insure the safety of these components and its reliability. This study firstly presents a short review on fatigue power shafts for supporting the failure mode analysis, which can lead to determine the root cause of failure. The material of these damaged crankshafts has the same chemical composition to others found where the same type of fracture occurred at least ten years ago. A finite element analysis was also carried out in order to find the critical zones where high stress concentrations are present. Results showed a clear failure by fatigue under low stress and high cyclic fatigue on crankpins.

The working principle of the Wells turbine, used in the ocean energy extraction, can sometimes lead to stall in the turbine blades and stators’ vanes, which is an unintended action considering that creates potentially damaging conditions to the mechanical components, plus reducing the efficiency of the take-off system. This work focused on the cracking damage caused to the earth-side guide vanes of the Pico Island Wave Power Plant due to the loading from turbulent air flow and consequent vibrations, as a result of turbine stall. Comprehensive analysis of the design and manufacturing phases, as well as of the constitutive metal properties (AISI 316L stainless steel), including the consequences of welding, was carried out. Strain gages were used to evaluate the loading profile in real-sea working conditions and different suggestions were made in order to postpone and prevent crack nucleation and propagation. The appearance of cracks seems to have been caused by mistakes made at both design and manufacturing phases, eliminating from the guide vanes the ability to endure the aggressive loads which they were subjected to. The lack of an active control strategy on the power plant resulted in consistent turbine stall events, thus contributing to the excessive loadings imposed to the vanes.

The fatigue strength and its correct assessment play an important role in design and maintenance of marine crankshafts to obtain operational safety and reliability. Crankshafts are under alternating bending on crankpins and rotating bending combined with torsion on main journals, which mostly are responsible for fatigue failure. The commercial management success substantially depends on the main engine in service and of its design crankshaft, in particular. The crankshaft design strictly follows the rules of classification societies. The present study provides an overview on the assessment of fatigue life of marine engine crankshafts and its maintenance taking into account the design improving in the last decades, considering that accurate estimation of fatigue life is very important to ensure safety of components and its reliability. An example of a semi-built crankshaft failure is also presented and the probable root case of damage, and at the end some final remarks are presented.

One of the big challenges in fatigue characterization is to evaluate accumulated damage under asynchronous loading conditions. This multidisciplinary subject requires joining all multiaxial issues that are usually studied separately. Multiaxial fatigue models used in accumulated damage evaluation under random conditions are generally validated under loading blocks, which is a particular case of random loadings. In this work is evaluated the accumulated damage of asynchronous loading blocks using the stress scale factor (SSF) damage parameter, the virtual cycle counting, and a Palmgren–Miner type rule. Results show good agreement between the experimental data and the estimations.

A structural integrity assessment of a damaged crane pedestal/column of a container ship is presented. Crane cabins and pedestal/columns are subjected to fatigue coupled with sudden overloads during cargo operations and corrosion effects due to sea environment. Significant number of surface cracks was detected around the pedestal/column, at inner and outside of the crane foundation, close to the main weld seam and, in consequence, the ship-owner ordered a survey. The main weld seam and the sites where cracks were found are evaluated in the present study. For the purpose, the sample material (hot rolled steel plate and upper ring) of the pedestal was provided by the shipyard and macro and micrographics were observed. Results did not show any cracks, although they have been found by the NDT technical services of the shipyard which have decided to remove them by the grinding process. The weld seam did not also show relevant defects, whereby the replacing of the pedestal/column by a new one would not had been necessary. Regardless of some occasional overloads, the surface cracks found on the pedestal/column could be a consequence of the normal operation conditions of the crane during the last 5 years and also due to poor maintenance.

Nowadays the development of new testing machines and the optimization of new specimen geometries are two very demanding activities. In order to study complex material stress and strain distributions, as in-plane biaxial loading, one must develop new technical solutions. A new type of testing machine has been developed by the present authors, for the fatigue testing of cruciform specimens, but the low capacity of the testing machine requires the optimization of the specimen in order to achieve higher but uniform stress and strain distributions on the specimen center. In this paper, the authors describe the procedure to optimize one possible geometry for cruciform specimens, able to determine the fatigue initiation life of material subjected to out of phase in plane biaxial fatigue loadings. The high number of design variables were optimized using the direct multi-search method, considering two objective functions, the stress level on the specimen center and the uniformity of the strain distribution on a 1.0mm radius of the specimen center. Several Pareto Fronts were obtained for different material thickness, considering the commercially available sheet metal thickness. With the optimal solution, the influence of every design variable was studied in order to provide others with a powerful tool that allows selecting the optimal geometry for the desired application. The results are presented in the form of design equations considering that the main design variable, the material thickness, was chosen from a Renard series of preferred numbers. The end user is then able to configure the optimal specimen for the required fatigue test., This work was supported by FCT, Fundação para a Ciência e Tecnologia - Portugal, through IDMEC - LAETA, project UID/EMS/50022/2013.

Multiaxial fatigue is a very important physical phenomenon in several mechanical components. Fatigue life study under cyclic stresses is of utmost importance to avoid unexpected failure of equipments, vehicles or structures. Among several fatigue characterization tools, a correct definition of a loading cycle under multiaxial fatigue loading conditions shows to be crucial to estimate multiaxial fatigue life. The aim of this work is to achieve a correct definition for a multiaxial fatigue loading cycle and accomplish a multiaxial fatigue model to estimate block’s fatigue life under multiaxial loading conditions. To reach this goal, several loading paths were carried out using the 42CrMo4 low alloy steel under different loading conditions. Sequential, proportional, non-proportional and asynchronous loading effects were modulated through eleven loading blocks. Furthermore, two models were proposed: a cycle counting method and a fatigue life evaluation criterion. The results from the proposed models were correlated with the fatigue data and compared with two well known cycle counting models: the Bannantine and Socie and the Wang and Brown criteria. The proposed models were successfully validated by experimental data. Results show that the new proposals lead to an improved multiaxial fatigue characterization under complex loading conditions.

Railway sector plays an important role in the means of transportation and the safety of wheels and rails is a greater concern for the manufacturers and operators of the railway network. In the wheel–rail contact, damage may occur due to wear or fatigue and in the latter case Rolling Contact Fatigue (RCF) is a growing problem due to the operation increase of high speed railway trains, where increased number of loading cycles due to the passage of the wheels over the rails, will be present in the wheel–rail contact. The present study focuses on the RCF failure analysis, analytical, numerical and experimental approaches are applied to investigate the problem. It is widely known that Hertz contact theory was the first approach to explain the stress state in the contact area through the calculation of the curvature of ellipse of contact and the stresses governing rolling contact between wheel and rail. The contact zone was studied for semi-elliptical shape, governed with multiaxial, out-of-phase state of stress with constant change in directions of principal stresses occurring due to rolling. Following, Finite Element analysis of the wheel–rail contact was implemented with the realistic loads, boundary conditions and material properties. Since a multiaxial stress state is present in the contact zone with normal and shear stresses, multiaxial fatigue tests in tension torsion were carried out on specimens taken from wheels and rails. Results from the multiaxial fatigue tests were analysed through multiaxial fatigue models, either stress invariant or critical plane models, such as modified Sines, Findley, Wang–Brown, Fatemi–Socie and Smith–Watson and Topper. Orientation of fatigue crack paths were also predicted by applying the above models and comparisons with measured crack orientation were carried out. The difficulties of the experimental measurement and analysis of the crack initiation are discussed and analysed within this complex stress state where alternate shear stresses are present in a strong compression stress and in a confined zone environment.

Many fatigue models proposed in literature are validated by the author’s data and usually they are not corroborated with other author’s lab work. In this paper, two multiaxial fatigue criteria, the Minimum Circumscribed Ellipse (MCE) and the Stress Scale Factor (SSF), are used to assess fatigue life under multiaxial loading conditions. Two different sets of fatigue data obtained from two different steels and from two different research groups were used in order to evaluate the accuracy of the models MCE and SSF regarding multiaxial fatigue life assessment. The fatigue data used here have two major features: stress concentrations, always present in mechanical components, and the loading path effects, with the inherent changing on the principal stress directions and/or the variation of the stress amplitude ratio (tau/sigma). Furthermore, different failure criteria were adopted for each material and different testing setup. For the Ck45 material, the failure criterion was the crack initiation, for the C40 material was the total rupture. Another factor to point out is that the Ck45 fatigue results were obtained under bending/torsion conditions and the C40 fatigue results were achieved in axial/torsion loading conditions. Results show good correlation between experiments and fatigue life estimations. Some remarks and conclusions concerning the performance of the MCE and SSF models are presented.

This paper presents a detailed analysis of two in service failures concerning trunnions of landing gears of military aircrafts. Several tools were used in order to find out the primary cause of failure, namely, visual examination, chemical analysis and macroscopic, microscopic and microstructural examinations. The progress of the fatigue cracks were determined based on the observation of beach marks caused by the loading history and by striation marks caused by cyclic plastic deformation mechanisms related with events occurring in the loading history of the component. Additionally, the topology of the fracture surfaces provided an indication of the stress system that produced failure, which is closely dependent on the mechanical behaviour of the material. In both examples the failures occurred at the transition of two sections. The visual observation of the fractured surface immediately indicates that these failures occurred by a fatigue process. A main area of the fracture surfaces present high stress failure facies. A more detailed analysis of the fractured surfaces concludes that the surfaces that failed by fatigue is clearly smaller than the one that failed by an overload; this measuring capacity allows the investigator to consider that failures occurred by fatigue but in the presence of high stress levels at the crack initiation zone. In the first case multiple crack initiation was detected while in the second case the cracking occurred at the corner of the lateral surface of the trunnion structure. Any type of initial defect was not observed in the fracture surfaces. Therefore it can be concluded that failure probably occurred by fatigue but in the presence of higher loads in the components than those that were expected.

The fatigue limit of materials, due to the improvement of fatigue life of structures and mechanical components should be extended from the traditional 106–107 cycles up to 109 and more, but with traditional testing hardware this is a difficult technical task due to the length of time needed for the completion of tests. Ultrasonic fatigue testing machines seem to be adequate for very high cycle fatigue (VHCF) tests. We propose here to evaluate the behavior of the hysteretic damping in an attempt to associate that with damage parameter. The approach here presented is based on the fact that the fatigue issue can be understood in terms of the energy available for irreversible process triggering. This nonconservative energy will be involved in micro-structural changes in the material before being dissipated as thermal energy. In fact, the balance between the energy supplied to and returned by the material is positive and the hysteretic damping factor represents the inelastic fraction of energy in each cycle. Aiming at building a model to correlate the hysteretic cycle parameters and the fatigue process, both energy loss and material response of the specimens are measured during the fatigue test. The fatigue tests are carried out with an ultrasonic machine test, operated at 20 kHz with amplitude or temperature control, under HCF and VHCF for copper specimens. The results show the behavior of hysteretic damping parameter during fatigue life, the equivalent dissipated energy per cycle and a good correlation between the hysteretic damping factor parameter and the fatigue process S–N curve, suggesting that factor as a promising fatigue life parameter useful for some cases of fatigue life prediction.

The shear stress amplitude is one of the most important parameters in the formulation of many multiaxial fatigue models proposed in literature. The shear stress amplitude is usually evaluated by the longest projection or the minimum circumscribed circle approach in a shear stress space. Further, the von Mises and Tresca stress spaces are the most used ones under multiaxial loading conditions, where the damage ratio between axial and shear damages is a constant value: 0.577 for von Mises and 0.5 for Tresca. However, the damage scale in each stress space's axes may vary significantly depending on materials' type. Therefore, evaluating proportional and non-proportional damages using conventional stress spaces (von Mises or Tresca) may lead to erroneous interpretations. In this work, systematic fatigue experiments under proportional and non-proportional loading conditions are presented for three structural steels: Ck45, AISI303 and the 42CrMo4. To quantify the relative damage between proportional and non-proportional loading paths, a material's non-proportional sensitivity parameter (Y parameter), determined based on materials experimental tests, is proposed. From this research, it can be drawn that the appropriate axial/shear damage relation should be used to estimate multiaxial fatigue life.

Generally, mechanical components or structures are subjected to random and a three-dimensional stress state; there are very few field loading paths which can be experimentally fully simulated in laboratory. Loading path parameters such as load sequence, stress level or proportionality/non-proportionality presences are unknown variables with unknown levels under random loading conditions which are impossible to modulate in laboratory because the load spectra is unknown. The load spectrum depends on numerous factors such as environmental, mechanical or user behavior. At design stages the fatigue life estimation is based on typical loading paths or typical loading spectra, however that assumption may be very different from the usage regime. From here it can be concluded that the random multiaxial fatigue issue is of utmost importance to monitoring the in-field damage accumulation. This work presents a proposal to estimate the accumulated damage resulted from multiaxial random loadings based on the SSF equivalent stress and SSF virtual cycle counting concept.

In this paper the multiaxial loading path effect on the fatigue crack initiation, fatigue life and fracture surface topology are evaluated for two different crystallographic microstructures (bcc and hc): high strength low-alloy 42CrMo4 steel and the extruded Mg alloy AZ31B-F, respectively. A series of multiaxial loading paths were carried out in load control, smooth specimens were used. Experimental fatigue life and fractographic results were analyzed to depict the mechanical behavior regarding the different microstructures. A theoretical analysis was performed with various critical plane models such as the Fatemi–Socie, SWT and Liu in order to correlate the theoretical estimations with the experimental data. A new approach based on maximum stress concentration factors is proposed to estimate the crack initiation plane, estimations from this new approach were compared with the measured ones with acceptable results. To implement this new approach a virtual micro-notch was considered using FEM. Moreover, the multiaxial loading path effect on stress concentration factors is also studied. The obtained results clearly show the effect of the applied load conditions on local microstructures response.

To model damping effects in structural dynamics, the literature provides a wide range of alternatives. The different models claim different advantages and fields of use where they are useful but, in the end, the compliance of the model with the experimental results, within a given tolerance, is the ultimate criterion for assessing its quality. In the present paper, the behaviour of a simple steel specimen is studied, taking as a focus the time response of the material. Since the harmonic response is well established, the authors propose to submit the specimen to a trapezoidal periodic load and study the behaviour of the response to the load changes. A simple setup was used, using a steel specimen loaded in the linear regime. A carefull test procedure was carried out in order to characterize the dissipation in a quasi-static periodic load. The experimental results show that the response delay to a ramp load depends on the durations of both the previous flat and the ramp itself.

A case study of a crankshaft catastrophic failure of a motor vehicle and its failure analysis is presented. The crankshaft suffered a mechanical seizure on the crankpin no. 2 after 3 years in service. It was repaired and after 30,000 km the vehicle had a damage again, with a catastrophic failure on the same crankpin. A transversal macrograph of the crankpin revealed that the crankpin was rectified and filled with a metal alloy for the same nominal diameter. Two fatigue cracks growing to the center of the crankpin where the final fracture occurred. The symmetric semi-elliptical crack front profile confirms the effect of a pure mode I under alternating bending. The catastrophic failure was a consequence of the inadequate repairing by a non-authorized manufacturer.

A failure analysis of two helical gear wheels of a ducted azimuth thruster is presented. The research work consisted of a fracture examination of the material in order to determine the damage root causes. The samples for the failure analysis were obtained from two broken teeth of two helical gear wheels. An analysis through the scanning electron microscope (SEM) was carried out close to the crack initiation. It was found that the damage in the bevel gears were by the fatigue fracture mode. The SEM analysis showed that the gear teeth were under severe contact stress during the operation aggravated by an inappropriate lubricating. A possible misalignment between the pinions and the gear wheels teeth could also contribute for the premature failure.

With the current challenges that the industrial world faces regarding the unavoidable environmental impact of manufacturing goods, companies have been turning to sustainable design in order to reduce this impact and to minimize the damage to the environment while at the same time reaping the marketing bonus that is the claim of a greener product. This reduction of environmental impact is being done at multiple levels and especially at the design stage and one of the ways taken by companies to reduce this impact is to replace fuel-based materials such as polymers with natural materials. But in order for this replacement to take place, engineers and designers need to know the behavior of these materials. With that idea a set of mechanical tests and studies, namely bending and compression tests, have been performed on cork composites. Those composites were chosen for its importance to the Portuguese economy and its peculiar growth cycle and harvesting techniques, in order to ascertain the mechanical properties of cork composites and how it stands against polymers.

Nowadays, the world faces unprecedented challenges in social, environmental and economical dimensions, in which the industrial design has showed an important contribution with solutions that provide positive answers regarding these problems. In particular, due to its relevance, the automotive industry confronts a moment of crises, and based on the ecodesign of products it has been transforming the challenges in opportunities. In this context, the use of natural fiber composites, produced in developing countries, have presented several social, environmental and economical advantages to design “green” automotive components. Thus, this work through LCA method demonstrates the possibility to use natural fibers through a case study design which investigates the environmental improvements related to the replacement of glass fibers for natural jute fibers, to produce a structural frontal bonnet of an off-road vehicle (Buggy). Results pointed out the advantages of applying jute fiber composites in Buggy enclosures.

A computational strategy is developed to model the effects and interactions of non-metallic inclusions with metal bases. As an industrial case study, the present paper characterises the local elasto-plastic deformation and residual stress distribution induced through the sintering process of the diamond–metal matrix hot-pressed tool, which is a special metal matrix composite (MMC) reinforced by diamond particles, with average diameters ranging from 10 μm to 0.5 mm, to increase the capability for cutting hard materials such as stones. The residual stress fields in the nearby region of a diamond particle via plastic deformation due to sintering process are examined to study the effects of the sintering temperatures and the compression pressure on the upper surface of the metal matrix during the sintering process. In the simulations, the metal matrix is modelled as an elastic–plastic material with combined isotropic–kinematic hardening. The diamond particle is modelled as isotropic and linear elastic. For simplicity, two geometries of the diamond particles are considered to study their effects on local plastic deformation and residual stress distributions. All simulations were performed using the ABAQUS finite element code. For the diamond particles embedded inside the metal matrix, 8-node axisymmetric quadrilateral, reduced integration ABAQUS solid element type CAX8 is used. For the diamond particles on the surface of the matrix, 4-node reduced integration ABAQUS solid element type C3D4 is used in the simulations. Simulating the sintering process and the cutting forces in service, it is demonstrated that the diamond retention capacity induced by the metal matrix (important for extending the life of a diamond tool) is mainly dependent on the sintering process. Optimum design of the diamond cutting tools can be achieved by selecting the appropriate sintering temperatures, the stress–strain behaviour of the metal matrix, the diamond particle shape and the compression pressure on the upper surface of the metal matrix during the sintering process.

The objective of this work is to develop analysis methods based on 3D-FEM simulations for optimum design of the diamond cutting tools under various loading conditions, considering the sintering process of diamond–metal matrix to originate residual stresses. The work concerns the use of finite element simulation for modelling of thermal residual stresses generated during the sintering process of metal matrix diamond tools normally employed by the industry. Stress distribution fields were determined for the diamond shape using a 4-node, reduced integration ABAQUS solid element type C3D4. The residual stress fields in the nearby region of a diamond particle are examined to study the effects of the sintering temperatures, the stress–strain behaviour of the metal matrix and the compression pressure on the upper surface of the metal matrix. Through the simulations of the cutting forces on the diamond, it is demonstrated that the diamond retention capacity induced by the metal matrix (important for extending the life of a diamond tool) is principally dependent on the sintering process. Optimum design of the diamond cutting tools can be achieved by selecting the appropriate sintering temperatures, the stress–strain behaviour of the metal matrix and the compression pressure on the upper surface of the metal matrix during the sintering process.

The increasing use of biofuels, namely blends of gasoline with ethanol, requires the identification of its impact on the fuel admission components of internal combustion engines that have not been specially design to work with high levels of ethanol. This paper provides a discussion on performance testing of corkrubber gaskets from a commercial company (ACC – Amorim Cork Composites) to seal these engines using gasoline and gasoline-ethanol blends. In order to evaluate the chemical compatibility of four corkrubber composites, several samples were immersed in different blends of gasoline with ethanol to determine the changes of hardness, volume, thickness and weight. The results of these tests differed between materials, but all of them showed less damage tolerance to blends with low and medium percentages of ethanol (e.g., E5, E10, E25, E50) when compared to gasoline or blends with higher percentages of ethanol (E85).An aggressiveness factor is proposed to compile results from the immersion tests. Validation tests were carried out in conditions as close as possible to the service and accordingly to similar test standards. This study allows concluding on which of these corkrubber composites have adequate sealing properties for gasoline as well as blends of gasoline with ethanol.

The environment is a prominent issue today. Designing environmentally sustainable products is an attempt to address this question. In many cases, natural materials are environmentally friendly for product design manufacturing. The goal of this work is to study the mechanical behaviour of NL10 and NL30 cork agglomerates. Compression, shear and bending tests in sandwich specimens made of glass or jute fiber in facings and cork agglomerates as core were carried out. The sandwich specimens were manufactured by Resin Transfer Moulding (RTM) process. Results show that NL30 has a higher compression strength and shear resistance than NL10 agglomerate due to its manufacturing process, which originates superior density, but the NL30 agglomerate superior density is undesirable. Sandwich test specimens that presented failure by rupture of the core in both types of tests, core shear tests and three point bending tests, showed that the failure is mainly adhesive occurring between the adhesively joined cork grains. Since grains are unaffected and remain intact, it is possible to improve these materials by using better agglutinants and new bonding techniques with the intent of getting cork agglomerates with higher shear and flexural strength.

Fatigue damage prediction under a general multiaxial service loading consists of three main steps: multiaxial cycle counting, damage evaluation for an identified cycle (or reversal), and damage accumulations. The accuracy of fatigue life predictions depends on all the above steps. This paper reviews the evolutions of various multiaxial fatigue damage models, a comparative study is conducted about the physical basis, the computational efficiency, and the application range of the approaches. Based on the comparative studies, a new procedure is proposed to evaluate fatigue damage under general multiaxial random loading, which uses the Wang and Brown´s multiaxial cycle counting method for identifying cycles (or reversals), the modified procedure of the minimum circumscribed ellipse (MCE) approach for fatigue damage evaluation for an identified cycle (or reversal), and the Miner´s linear damage law for fatigue damage accumulations. By comparisons of the predicted life results with experimental results and with other approaches, it is shown that the proposed procedure is very efficient and suitable for computer aided structural optimization against fatigue.

In real engineering components and structures many accidental failures occur due to unexpected or additional loadings, such as additional bending or torsion. There are many factors influencing the fatigue crack paths, such as the material type (microstructure), structural geometry and loading path. It is widely believed that fatigue crack nucleation and early crack growth are caused by cyclic plasticity. This paper studies the effects of multiaxial loading paths on the cyclic deformation behaviour, crack initiation and crack path. Three types of structural steels are studied: Ck45, medium carbon steel, 42CrMo4, low alloy steel and the AISI 303 stainless steel. Four biaxial loading paths were applied in the tests to observe the effects of multiaxial loading paths on the additional hardening, fatigue crack initiation and crack propagation orientation. Fractographic analyses of the plane orientations of crack initiation and propagation were carried out by optical microscope and SEM approaches. It is shown that these materials have different crack orientations under the same loading path, due to their different cyclic plasticity behaviour and different sensitivity to non-proportional loading. Theoretical predictions of the damage plane were conducted using the critical plane approaches, either based on stress analysis or strain analysis (Findley, Smith–Watson–Topper, Fatemi–Socie, Wang–Brown–Miller, etc). Comparisons of the predicted crack orientation based on the critical plane approaches with the experimental observations for the wide range of loading paths and the three structural materials are shown and discussed. Results show the applicability of the critical plane approaches to predict the fatigue life and crack initial orientation in structural steels.

Cork is a material of great value to the Portuguese economy. Unfortunately, its use is still restricted to traditional areas, with the agglomerate form in particular not being used to its full potential. The objective of this article is to analyze the viability of using cork-based material as core materials in sandwich structures in aeronautical and aerospace applications. The use of cork-based material is proposed because of its isolation properties (both thermal and acoustic) and there is no significant performance loss, when compared with the currently used materials. It presents other advantages, as well as, less wastage of energy in manufacturing and a better environmental integration, both in the transformation stage and in the end of life recycling stage. The objective of this work is to study the mechanical behavior of different sandwich specimens, with carbon/epoxy faces, and cores of different cork agglomerates and their comparison with the results obtained with similar specimens using current material cores. Experimental shear tests and three-point bending tests were carried out and the evolutions of the load— displacement curves of the different cork agglomerates/sandwiches were analyzed and discussed. The obtained results show that significant room for improvement still exists in use of cork-based core materials.

In spite of the high levels of reliability of modern aeroengine components resulting from rigid standards and practices, failures of compressor and turbine blades during normal operational environments are common situations which compromise the flight safety. The investigation of real failures affecting these components allows gaining a deeper knowledge concerning the mechanisms of crack initiation and propagation which, in turn, can be used in order to prevent future incidents or accidents. This paper presents the analysis of two in service failures involving blades breakage belonging to different compressor stages. Crack growth mechanisms were evaluated based on the visual inspection of the affected components and with both macroscopic and microscopic observations of the fracture surfaces. The origins of crack initiation were evaluated through the examination of crack path and beach marks on the fracture surface. Mechanical analyses were carried out to identify the possible causes of the failures by examining anomalies in the mechanical behaviour of the materials such as hardness tests, chemical composition and surface coating analysis through SEM observations. The analysis of the different fracture surfaces shows that crack propagation is mainly related with fatigue mechanisms whilst crack initiation can be attributed to distinct causes, either the presence of defects in the surface of the blade due to impact of debris or intrinsic material defects or some degrading mechanisms affecting the internal microstructure of the material. In some cases the blades were in service after having being object of an overhaul procedure, which can justify some crack initiation cases that caused in service failures.

In order to study the use of a local approach to predict crack-initiation life on notches in mechanical components under multiaxial fatigue conditions, the study of the local cyclic elasto-plastic behaviour and the selection of an appropriate multiaxial fatigue model are essential steps in fatigue-life prediction. The evolution of stress-strain fields from the initial state to the stabilized state depends on the material type, loading amplitude and loading paths. A series of biaxial tension-compression tests with static or cyclic torsion were carried out on a biaxial servo-hydraulic testing machine. Specimens were made of an alloy steel 42CrMo4 quenched and tempered. The shear stress relaxations of the cyclic tension-compression with a steady torsion angle were observed for various loading levels. Finite element analyses were used to simulate the cyclic behaviour and good agreement was found. Based on the local stabilized cyclic elastic-plastic stress-strain responses, the strain-based multiaxial fatigue damage parameters were applied and correlated with the experimentally obtained lives. As a comparison, a stress-invariant-based approach with the minimum circumscribed ellipse (MCE) approach for evaluating the effective shear stress amplitude was also applied for fatigue life prediction. The comparison showed that both the equivalent strain range and the stress-invariant parameter with non-proportional factors correlated well with the experimental results obtained in this study.

In real engineering components and structures, many accidental failures are due to unexpected or additional loadings, such as additional bending or torsion, etc. Therefore, it has attracted more research attentions to study the mechanical behavior of materials under complex loading conditions. Two typical structural materials are studied and compared in this paper: AISI 303 stainless steel and 6060-T5 Aluminum alloy. The objective is to study the effects of multiaxial loading paths on the crack initiation and orientation of the two materials studied. Fatigue tests were conducted in a biaxial testing machine. Fractographic analyses of the fracture surface were carried out by optical microscope and SEM approaches. In addition to the experimental studies, theoretical predictions of the damage plane were made using critical plane approaches. Comparisons of the predicted orientation of the damage plane with the experimental observations are shown. The applicability of the multiaxial fatigue criteria for the two materials is discussed. It was shown that the two materials studied have different crack orientations under the same loading path. This observation appears to show that the applicability of the fatigue models is dependent on the material type and multiaxial microstructure characteristics.

The main objective of the present work is to present and discuss the effects of the residual stresses generated in the matrix. Both, elastic and elastic-plastic models using explicit finite element code Abaqus were developed to investigate the thermal residual stresses generated around a diamond particle embedded in a metal matrix (a binder) during the sintering process for obtaining a diamond tool. The first part of the work consists in examining the thermal residual stress field generated by using three sintering temperatures from which the bodies are cooled from (T1=800°C, T2=700°C and T3=600°C) and two diamond shapes modelled with 2D axisymmetric elements, one from a circle the other from an octagon, respectively. In this case only one type of binder showing typical elastic behaviour is considered. The second part of this work consists in comparing the finite element solutions using binders with different tensile behaviour (elastic vs. elastic-plastic behaviour). This last study has used a particle shape generated from the octagon, since this shape allows, in a great number of real situations, the simulation of a particle with a larger number of facets, in line with what it is observed when looking at a conventional diamond tool.

The sintering process of diamond–metal matrix hot-pressed tools, usually used for cutting hard materials (e.g., stone cutting) originates residual stresses, which should be taken into account in the performance of the tool. The work concerns the use of finite element simulation for modelling of thermal residual stresses generated during the sintering process of metal matrix diamond tools normally employed by the industry. Stress distribution fields were determined for two different diamond shapes (modelled with 2D axisymmetric elements, with the sphere shape generated from the revolution of one circle and the octahedron shape generated from the revolution of one octagon, respectively) using an 8-node biquadratic axisymmetric quadrilateral, reduced integration element type CAX8. The thermal residual stress field in the nearby region of a diamond particle with the shape generated from the octagon is examined by using three different matrix materials, each one sintered at different temperatures. The analyses have demonstrated how much the residual stresses are sensitive to the stress–strain behaviour of the metal matrices.

This study deals with simulation for cyclic stress/strain evolutions and redistributions, and evaluation of fatigue parameters suitable for estimating fatigue lives under multiaxial loadings. The local cyclic elastic–plastic stress–strain responses were analyzed using the incremental plasticity procedures of ABAQUS finite element code for both smooth and notched specimens made of three materials: a medium carbon steel in the normalized condition, an alloy steel quenched and tempered and a stainless steel, respectively. Emphasis is on the studying of ‘intelligent’ material behaviors to resist fracture, such as stress redistribution and relaxation through plastic deformations, etc. For experimental verifications, a series of tests of biaxial low cycle fatigue composed of tension/compression with static and cyclic torsion were carried out on a biaxial servohydraulic testing machine (Instron 8800). Different multiaxial loading paths were used to verify their effects on the additional cyclic hardening. The comparisons between numerical simulations and experimental observations show that the FEM simulations allow better understanding on the evolutions of the local cyclic stress–strain and it is shown that strong interactions exist between the most stressed material element and its neighboring material elements in the plastic deformations and stress redistributions. Based on the local cyclic elastic–plastic stress–strain responses, the energy-based multiaxial fatigue damage parameters are applied to correlating the experimentally obtained lives. Improved correlations between the predicted and the experimental results are shown. It is concluded that the improvement of fatigue life prediction depends not only on the fatigue damage models, but also on the accurate evaluations of the cyclic elasto-plastic stress/strain responses. q 2005 Elsevier Ltd. All rights reserved.

In real engineering components and structures, many accidental failures are due to unexpected or additional loadings, such as additional bending or torsion, etc. Fractographical analyses of the failure surface and the crack orientation are helpful for identifying the effects of the non-proportional multi-axial loading. There are many factors that influence fatigue crack paths. This paper studies the effects of multi-axial loading path on the crack path. Two kinds of materials were studied and compared in this paper: AISI 303 stainless steel and 42CrMo4 steel. Experiments were conducted in a biaxial testing machine INSTRON 8800. Six different biaxial loading paths were selected and applied in the tests to observe the effects of multi-axial loading paths on the additional hardening, fatigue life and the crack propagation orientation. Fractographic analyses of the plane orientations of crack initiation and propagation were carried out by optical microscope and SEM approaches. It was shown that the two materials studied had different crack orientations under the same loading path, due to their different cyclic plasticity behaviour and different sensitivity to non-proportional loading. Theoretical predictions of the damage plane were made using the critical plane approaches such as the Brown-Miller, the Findley, the Wang-Brown, the Fatemi-Socie, the Smith-Watson-Topper and the Liu's criteria. Comparisons of the predicted orientation of the damage plane with the experimental observations show that the critical plane models give satisfactory predictions for the orientations of early crack growth of the 42CrMo4 steel, but less accurate predictions were obtained for the AISI 303 stainless steel. This observation appears to show that the applicability of the fatigue models is dependent on the material type and multi-axial microstructure characteristics.

This work describes the present methodology used on Portuguese Air Force (PoAF) aircrafts for fatigue life assessment to maintain its structural integrity. This methodology uses experimental flight data collected by airborne instrumentation systems. The recorded data are analysed for the prediction of the crack growth of defects at critical components of the aircraft and this information is then used for planning non-destructive inspections, maintenance, logistical support and operational usage of the various fleets. This methodology is exemplified by one application on DASSAULT/DORNIER ALPHA-JET aircraft.

Biaxial low-cycle fatigue composed of tension/compression with a static torsion was carried out on specimens of alloy steel used for shafts in mechanical structures. The behaviour of both the uniaxial and torsional stress during low cycle fatigue is analyzed for different levels of elasticity and plasticity. Results show a relaxation of the torsion loading during cyclic fatigue that depends on the plasticity level. Elastic-plastic finite element analysis was carried out on notched geometries, where local plasticity is attained at the notch, for cyclic bending and static torsion till a stabilized cyclic behaviour was reached, using the cyclic stress strain curve, kinematic hardening model with von Mises yield criterion and associative flow rule. Results of the local stress and strain analysis on the notches with cycling where imposed plasticity is localized also show a similar relaxation of the torsional stress as experimentally observed.

Chitosan biocompatibility and biodegradability properties make this biopolymer promising for the development of advanced internal fixation devices for orthopedic applications. This work presents a detailed study on the production and characterization of three dimensional (3D) dense, non-porous, chitosan-based structures, with the ability to be processed in different shapes, and also with high strength and stiffness. Such features are crucial for the application of such 3D structures as bioabsorbable implantable devices. The influence of chitosan’s molecular weight and the addition of one plasticizer (glycerol) on 3D dense chitosan-based products’ biomechanical properties were explored. Several specimens were produced and in vitro studies were performed in order to assess the cytotoxicity of these specimens and their physical behavior throughout the enzymatic degradation experiments. The results point out that glycerol does not impact on cytotoxicity and has a high impact in improving mechanical properties, both elasticity and compressive strength. In addition, human mesenchymal stem/stromal cells (MSC) were used as an ex-vivo model to study cell adhesion and proliferation on these structures, showing promising results with fold increase values in total cell number similar to the ones obtained in standard cell culture flasks.