Luís Reis | University of Lisbon (original) (raw)
Papers by Luís Reis
Ultrasonic fatigue testing machine are being used to perform materials testing in the range of 10... more Ultrasonic fatigue testing machine are being used to perform materials testing in the range of 10E9 to 10E10 fatigue cycles. The so-called very high cycle fatigue regime is now under intense studies in which concerns the layout of ultrasonic fatigue testing machines. Nevertheless, the accurate measurement of the parameters that influence fatigue life (stress, strain, displacement …) at ultrasonic frequencies still is a matter of concern and continuous development. The objective of this paper is to describe the design and construction of an ultrasonic fatigue testing machine at 20 kHz working frequency. In order to achieve fully automated tests, a closed loop control system was developed to use monitored temperature and displacement to set the power and the cooling periods of the machine. The monitoring of the displacement, considered here in the bottom face of the specimen, is carried out using a high resolution laser. The specimen´s temperature is monitored online through a pyrometer. To manage and process, a data acquisition device working at 400 kHz from National Instruments is used. The software was developed in house using the LabView package. The present paper describes the advantages and drawbacks of metal fatigue testing at very high frequencies.
International Journal of Fatigue, 2014
ABSTRACT Multiaxial fatigue is a very important physical phenomenon in several mechanical compone... more ABSTRACT 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.
Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications, 2021
A new generation of biodegradable metal alloys with a porous structure has been receiving growing... more 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...
Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications, 2018
Composite sandwich materials are very common in structural uses for a wide range of applications ... more Composite sandwich materials are very common in structural uses for a wide range of applications in the aerospace and automotive industry that require low weight, high bending strength, and high energy absorption. In general, the core of the sandwich structures has a two-dimensional cellular structure, with a regular honeycomb geometry. While with standard manufacturing processes the geometric structures are limited, the emergence of additive manufacturing provides alternatives to conventional designs. The aim of this work is to analyze and evaluate the effect of the core geometry on the flexural properties of the structure. For that purpose, three different cellular configurations were considered, namely regular honeycombs, lotus, and hexagonal honeycombs with Plateau borders. Four relative densities, with average values of 0.1, 0.25, 0.44, and 0.62, for each configuration, were studied. The flexural properties of cellular structures were evaluated with three-point bending tests, both numerically and experimentally. A modeling approach of the tests in the three configurations was performed, for two materials, polylactic acid and pure aluminum, by means of finite element simulations. Fused deposition modeling was used to obtain polylactic acid samples for the aforementioned configurations, which were experimentally tested to evaluate the mechanical response and the failure behavior of the cores. Results differ with the geometry arrangement and showed a strong dependency with the relative density of the structures in the flexural response in what concerns strength, stiffness, and energy absorbed. The arrangements studied present properties, which make them competitive with the traditional core structures for the same density. A promising agreement between experimental and simulation results was obtained.
Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications, 2020
Sandwich structures are frequently used in automotive, aerospace and marine industries, as they p... more 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 flexu...
Procedia Structural Integrity, 2016
During their operation, modern aircraft engine components are subjected to increasingly demanding... more During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Theoretical and Applied Fracture Mechanics, 2014
Many fatigue models proposed in literature are validated by the author's data and usually they ar... more 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.
Theoretical and Applied Fracture Mechanics, 2016
Abstract Nowadays for real world applications, mechanical components in the automotive, aerospace... more Abstract Nowadays for real world applications, mechanical components in the automotive, aerospace, aeronautical and other industries are subjected to multiaxial loading conditions. Although the fatigue behavior of materials like steel alloys, aluminum alloys or even magnesium alloys is fairly well established for uniaxial loading, one should not use this knowledge for biaxial loading. Developing new testing machines and new specimen geometries have been the previous goal of several authors. 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 as their hydraulic counterparts can, and therefore the cruciform specimen needs to be optimized. The authors have previously optimized the cruciform geometry for biaxial fatigue initiation, using a revolved spline to reduce the specimen center thickness. For crack initiation experimental results have proven that the obtained design detail is effective, but there are no studies about the behavior of these specimens for crack propagation. In this paper the authors firstly set out to determine the conditions for crack initiation using traditional criteria like Findley , Brown-Miller , Fatemi-Socie , Smith , Watson and Topper (SWT), Liu I and Chu , as a function of different biaxial loading with phase differences. On a second stage the authors compared the biaxial fatigue crack propagation on the optimal specimens, with the behavior of notched specimens, using the stress intensity factors for mode I and mode II. Several crack and loading parameters were studied, including the starting crack length and angle, and different loading paths. Different biaxial loadings were applied to the model, including 30°, 45°, 60°, 90° and 180° out-of-phase angles. Very similar small crack propagation parameters were obtained for both specimens, although as the crack growths the stress intensity factor for the optimal specimen do not behave as expected. Therefore limiting the use of this specimen for crack propagation.
Ciência & Tecnologia dos Materiais, 2015
Abstract A structural integrity assessment of a damaged crane pedestal/column of a container ship... more Abstract 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.
Procedia Structural Integrity, 2016
During their operation, modern aircraft engine components are subjected to increasingly demanding... more During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Application of Automation Technology in Fatigue and Fracture Testing and Analysis, 2014
Increased safety and reliability in mechanical components became a subject of prime importance ov... more Increased safety and reliability in mechanical components became a subject of prime importance over the past recent years. Therefore, a proper understanding of damage and fracture mechanics in materials and components designed to withstand Very High Cycle Fatigue (VHCF) loadings is extremely important nowadays. However, conventional machines used for fatigue testing are very time-consuming and costly in order to perform VHCF tests. Ultrasonic machines have been introduced as a way to increase the number of cycles in fatigue testing up to 1E 8 to 1E 10 cycles within a considerably reduced amount of time. Nevertheless, the accurate measurement of the parameters that influence fatigue life at ultrasonic frequencies (e.g., stress, displacement, strain-rate, temperature and frequency) is still a matter of concern and ongoing development. Due to the high frequencies involved in VHCF testing, a huge amount of heat is generated over the specimen, which greatly affects the variables determining the fatigue behavior. This paper describes the design and instrumentation of an ultrasonic fatigue testing machine that operates at 20 kHz working frequency. Among other features, it incorporates automated strain and temperature control. In order to run automated tests, a closed loop monitoring and control system was developed based on the measured temperature and displacement amplitudes. Temperature readings are made with a pyrometer and thermography camera and displacement is monitored at the free end of the specimen with a high resolution laser. The machine's power output is continuously adjusted from the displacement readings, so that the stress variations within the specimen are as flat as possible. When temperature increases above a certain set value, a cooling function is triggered and the test is interrupted until the specimen is cooled down. Data is acquired, managed and processed with a data acquisition device working at 400 kHz sampling frequency. The advantages and limitations of metal fatigue testing at very high frequencies are discussed in this paper, with special emphasis on the strain and temperature control issues. Comparison of tests carried out with and without both displacement and temperature control are made on two metallic alloys, copper 99% and carbon steel, with the determination of S-N curves.
Ultrasonic fatigue testing machine are being used to perform materials testing in the range of 10... more Ultrasonic fatigue testing machine are being used to perform materials testing in the range of 10 7 to 10 10 fatigue cycles. The so-called very high cycle fatigue regime is now an established technology in which concerns the layout of ultrasonic fatigue machines, but the accurate measurement of the parameters that influence fatigue life (load, strain, displacement,) at ultrasonic frequencies still is a matter of concern and continuous development. The objective of this paper is to describe the design and construction of a fully instrumented ultrasonic fatigue testing machine at 20 kHz working frequency. In order to achieve fully automated tests, a closed loop control system was developed to use monitored temperature and displacement to set the power and the cooling periods of the machine. The monitoring of the displacement, considered here in the bottom face of the specimen, is carried out using a high resolution laser. The specimen´s temperature is monitored online through a pyrometer. The cooling of the specimen is achieved with cooled dry air. To manage and process the data a data acquisition device working at 400 kHz from National Instruments is used. The software was developed in house using the LabView package.
In order to correctly assess the biaxial fatigue material properties one must experimentally test... more 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 appropriated 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, appropriated 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 is uniform and maximum with two limit phase shift loading conditions. 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.
Theoretical and Applied Fracture Mechanics, 2008
The sintering process of diamond-metal matrix hot-pressed tools, usually used for cutting hard ma... more 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.
International Journal of Fatigue, 2014
The fatigue limit of materials, due to the improvement of fatigue life of structures and mechanic... more The fatigue limit of materials, due to the improvement of fatigue life of structures and mechanical components should be extended from the traditional 10 6-10 7 cycles up to 10 9 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.
International Journal of Fatigue, 2006
This study deals with simulation for cyclic stress/strain evolutions and redistributions, and eva... more 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,
Fatigue & Fracture of Engineering Materials & Structures, 2014
ABSTRACT The shear stress amplitude is one of the most important parameters in the formulation of... more ABSTRACT 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.
Fatigue <html_ent glyph="@amp;" ascii="&"/> Fracture of Engineering Materials and Structures, 2004
... The fatigue life assessment of critical components is based on fatigue tests performed in the... more ... The fatigue life assessment of critical components is based on fatigue tests performed in the laboratory using specimens ... analysis of crack growth and on the determination of the actual part of the cycle that causes ... the correction of the methodology used for the assessment of the ...
Fatigue & Fracture of Engineering Materials & Structures, 2005
ABSTRACT Fatigue crack path prediction and crack arrest are very important for structural safety.... more ABSTRACT Fatigue crack path prediction and crack arrest are very important for structural safety. In real engineering structures, there are many factors influencing the fatigue crack paths, such as the material type (microstructure), structural geometry and loading path, etc. In this paper, both experimental and numerical methods are applied to study the effects of loading path on crack orientations. Experiments were conducted on a biaxial testing machine, using specimens made of two steels: 42CrMo4 and CK45 (equivalent to AISI 1045), with six different biaxial loading paths. Fractographical analyses of the plane of the stage I crack propagation were carried out and the crack orientations were measured using optical microscopy. The multiaxial fatigue models, such as the critical plane models and also the energy-based critical plane models, were applied for predicting the orientation of the critical plane. Comparisons of the predicted orientation of the damage plane with the experimental observations show that the shear-based multiaxial fatigue models provide good predictions for stage I crack growth for the ductile materials studied in this paper.
Fatigue <html_ent glyph="@amp;" ascii="&"/> Fracture of Engineering Materials and Structures, 2006
In real engineering components and structures, many accidental failures are due to unexpected or ... more 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.
Ultrasonic fatigue testing machine are being used to perform materials testing in the range of 10... more Ultrasonic fatigue testing machine are being used to perform materials testing in the range of 10E9 to 10E10 fatigue cycles. The so-called very high cycle fatigue regime is now under intense studies in which concerns the layout of ultrasonic fatigue testing machines. Nevertheless, the accurate measurement of the parameters that influence fatigue life (stress, strain, displacement …) at ultrasonic frequencies still is a matter of concern and continuous development. The objective of this paper is to describe the design and construction of an ultrasonic fatigue testing machine at 20 kHz working frequency. In order to achieve fully automated tests, a closed loop control system was developed to use monitored temperature and displacement to set the power and the cooling periods of the machine. The monitoring of the displacement, considered here in the bottom face of the specimen, is carried out using a high resolution laser. The specimen´s temperature is monitored online through a pyrometer. To manage and process, a data acquisition device working at 400 kHz from National Instruments is used. The software was developed in house using the LabView package. The present paper describes the advantages and drawbacks of metal fatigue testing at very high frequencies.
International Journal of Fatigue, 2014
ABSTRACT Multiaxial fatigue is a very important physical phenomenon in several mechanical compone... more ABSTRACT 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.
Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications, 2021
A new generation of biodegradable metal alloys with a porous structure has been receiving growing... more 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...
Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications, 2018
Composite sandwich materials are very common in structural uses for a wide range of applications ... more Composite sandwich materials are very common in structural uses for a wide range of applications in the aerospace and automotive industry that require low weight, high bending strength, and high energy absorption. In general, the core of the sandwich structures has a two-dimensional cellular structure, with a regular honeycomb geometry. While with standard manufacturing processes the geometric structures are limited, the emergence of additive manufacturing provides alternatives to conventional designs. The aim of this work is to analyze and evaluate the effect of the core geometry on the flexural properties of the structure. For that purpose, three different cellular configurations were considered, namely regular honeycombs, lotus, and hexagonal honeycombs with Plateau borders. Four relative densities, with average values of 0.1, 0.25, 0.44, and 0.62, for each configuration, were studied. The flexural properties of cellular structures were evaluated with three-point bending tests, both numerically and experimentally. A modeling approach of the tests in the three configurations was performed, for two materials, polylactic acid and pure aluminum, by means of finite element simulations. Fused deposition modeling was used to obtain polylactic acid samples for the aforementioned configurations, which were experimentally tested to evaluate the mechanical response and the failure behavior of the cores. Results differ with the geometry arrangement and showed a strong dependency with the relative density of the structures in the flexural response in what concerns strength, stiffness, and energy absorbed. The arrangements studied present properties, which make them competitive with the traditional core structures for the same density. A promising agreement between experimental and simulation results was obtained.
Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications, 2020
Sandwich structures are frequently used in automotive, aerospace and marine industries, as they p... more 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 flexu...
Procedia Structural Integrity, 2016
During their operation, modern aircraft engine components are subjected to increasingly demanding... more During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Theoretical and Applied Fracture Mechanics, 2014
Many fatigue models proposed in literature are validated by the author's data and usually they ar... more 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.
Theoretical and Applied Fracture Mechanics, 2016
Abstract Nowadays for real world applications, mechanical components in the automotive, aerospace... more Abstract Nowadays for real world applications, mechanical components in the automotive, aerospace, aeronautical and other industries are subjected to multiaxial loading conditions. Although the fatigue behavior of materials like steel alloys, aluminum alloys or even magnesium alloys is fairly well established for uniaxial loading, one should not use this knowledge for biaxial loading. Developing new testing machines and new specimen geometries have been the previous goal of several authors. 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 as their hydraulic counterparts can, and therefore the cruciform specimen needs to be optimized. The authors have previously optimized the cruciform geometry for biaxial fatigue initiation, using a revolved spline to reduce the specimen center thickness. For crack initiation experimental results have proven that the obtained design detail is effective, but there are no studies about the behavior of these specimens for crack propagation. In this paper the authors firstly set out to determine the conditions for crack initiation using traditional criteria like Findley , Brown-Miller , Fatemi-Socie , Smith , Watson and Topper (SWT), Liu I and Chu , as a function of different biaxial loading with phase differences. On a second stage the authors compared the biaxial fatigue crack propagation on the optimal specimens, with the behavior of notched specimens, using the stress intensity factors for mode I and mode II. Several crack and loading parameters were studied, including the starting crack length and angle, and different loading paths. Different biaxial loadings were applied to the model, including 30°, 45°, 60°, 90° and 180° out-of-phase angles. Very similar small crack propagation parameters were obtained for both specimens, although as the crack growths the stress intensity factor for the optimal specimen do not behave as expected. Therefore limiting the use of this specimen for crack propagation.
Ciência & Tecnologia dos Materiais, 2015
Abstract A structural integrity assessment of a damaged crane pedestal/column of a container ship... more Abstract 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.
Procedia Structural Integrity, 2016
During their operation, modern aircraft engine components are subjected to increasingly demanding... more During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Application of Automation Technology in Fatigue and Fracture Testing and Analysis, 2014
Increased safety and reliability in mechanical components became a subject of prime importance ov... more Increased safety and reliability in mechanical components became a subject of prime importance over the past recent years. Therefore, a proper understanding of damage and fracture mechanics in materials and components designed to withstand Very High Cycle Fatigue (VHCF) loadings is extremely important nowadays. However, conventional machines used for fatigue testing are very time-consuming and costly in order to perform VHCF tests. Ultrasonic machines have been introduced as a way to increase the number of cycles in fatigue testing up to 1E 8 to 1E 10 cycles within a considerably reduced amount of time. Nevertheless, the accurate measurement of the parameters that influence fatigue life at ultrasonic frequencies (e.g., stress, displacement, strain-rate, temperature and frequency) is still a matter of concern and ongoing development. Due to the high frequencies involved in VHCF testing, a huge amount of heat is generated over the specimen, which greatly affects the variables determining the fatigue behavior. This paper describes the design and instrumentation of an ultrasonic fatigue testing machine that operates at 20 kHz working frequency. Among other features, it incorporates automated strain and temperature control. In order to run automated tests, a closed loop monitoring and control system was developed based on the measured temperature and displacement amplitudes. Temperature readings are made with a pyrometer and thermography camera and displacement is monitored at the free end of the specimen with a high resolution laser. The machine's power output is continuously adjusted from the displacement readings, so that the stress variations within the specimen are as flat as possible. When temperature increases above a certain set value, a cooling function is triggered and the test is interrupted until the specimen is cooled down. Data is acquired, managed and processed with a data acquisition device working at 400 kHz sampling frequency. The advantages and limitations of metal fatigue testing at very high frequencies are discussed in this paper, with special emphasis on the strain and temperature control issues. Comparison of tests carried out with and without both displacement and temperature control are made on two metallic alloys, copper 99% and carbon steel, with the determination of S-N curves.
Ultrasonic fatigue testing machine are being used to perform materials testing in the range of 10... more Ultrasonic fatigue testing machine are being used to perform materials testing in the range of 10 7 to 10 10 fatigue cycles. The so-called very high cycle fatigue regime is now an established technology in which concerns the layout of ultrasonic fatigue machines, but the accurate measurement of the parameters that influence fatigue life (load, strain, displacement,) at ultrasonic frequencies still is a matter of concern and continuous development. The objective of this paper is to describe the design and construction of a fully instrumented ultrasonic fatigue testing machine at 20 kHz working frequency. In order to achieve fully automated tests, a closed loop control system was developed to use monitored temperature and displacement to set the power and the cooling periods of the machine. The monitoring of the displacement, considered here in the bottom face of the specimen, is carried out using a high resolution laser. The specimen´s temperature is monitored online through a pyrometer. The cooling of the specimen is achieved with cooled dry air. To manage and process the data a data acquisition device working at 400 kHz from National Instruments is used. The software was developed in house using the LabView package.
In order to correctly assess the biaxial fatigue material properties one must experimentally test... more 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 appropriated 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, appropriated 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 is uniform and maximum with two limit phase shift loading conditions. 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.
Theoretical and Applied Fracture Mechanics, 2008
The sintering process of diamond-metal matrix hot-pressed tools, usually used for cutting hard ma... more 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.
International Journal of Fatigue, 2014
The fatigue limit of materials, due to the improvement of fatigue life of structures and mechanic... more The fatigue limit of materials, due to the improvement of fatigue life of structures and mechanical components should be extended from the traditional 10 6-10 7 cycles up to 10 9 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.
International Journal of Fatigue, 2006
This study deals with simulation for cyclic stress/strain evolutions and redistributions, and eva... more 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,
Fatigue & Fracture of Engineering Materials & Structures, 2014
ABSTRACT The shear stress amplitude is one of the most important parameters in the formulation of... more ABSTRACT 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.
Fatigue <html_ent glyph="@amp;" ascii="&"/> Fracture of Engineering Materials and Structures, 2004
... The fatigue life assessment of critical components is based on fatigue tests performed in the... more ... The fatigue life assessment of critical components is based on fatigue tests performed in the laboratory using specimens ... analysis of crack growth and on the determination of the actual part of the cycle that causes ... the correction of the methodology used for the assessment of the ...
Fatigue & Fracture of Engineering Materials & Structures, 2005
ABSTRACT Fatigue crack path prediction and crack arrest are very important for structural safety.... more ABSTRACT Fatigue crack path prediction and crack arrest are very important for structural safety. In real engineering structures, there are many factors influencing the fatigue crack paths, such as the material type (microstructure), structural geometry and loading path, etc. In this paper, both experimental and numerical methods are applied to study the effects of loading path on crack orientations. Experiments were conducted on a biaxial testing machine, using specimens made of two steels: 42CrMo4 and CK45 (equivalent to AISI 1045), with six different biaxial loading paths. Fractographical analyses of the plane of the stage I crack propagation were carried out and the crack orientations were measured using optical microscopy. The multiaxial fatigue models, such as the critical plane models and also the energy-based critical plane models, were applied for predicting the orientation of the critical plane. Comparisons of the predicted orientation of the damage plane with the experimental observations show that the shear-based multiaxial fatigue models provide good predictions for stage I crack growth for the ductile materials studied in this paper.
Fatigue <html_ent glyph="@amp;" ascii="&"/> Fracture of Engineering Materials and Structures, 2006
In real engineering components and structures, many accidental failures are due to unexpected or ... more 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.
Frattura ed Integrità Strutturale
With the introduction of new materials and advances in medical science, the endodontic files have... more 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. 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.
Frattura ed Integrità Strutturale
The objective of this work was to characterize the full-field flexural behavior of composite sand... more 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 Citation: Marat-Mendes, R., Martins, R., Garcia, A., Reis, L., Flexural testing and analysis of full-strain-fields in sandwich composites, Frattura ed Integrità Strutturale, 49 (2019) 568-585.