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Papers by Armando Ramalho

ABSTRACT. Crack closure influences fatigue crack growth rate and must be included in the design o... more ABSTRACT. Crack closure influences fatigue crack growth rate and must be included in the design of components. Plasticity induced crack closure is intimately linked with the crack tip plastic deformation, which becomes residual as the crack propagates. The objective here is to study numerically the effect of crack propagation on crack tip fields. The transient effect observed at the beginning of crack propagation is linked to the hardening behavior of material. The effect of mesh refinement is studied, and a singular behavior is evident, which is explained by the sharp crack associated with mesh topology, composed of a regular pattern of square elements. The plastic zone size measured perpendicularly to crack flank in the residual plastic wake is quantified and compared with literature models. Finally, the removal of material at the first node behind crack tip with load cycling was observed for plane strain state and some hardening models in plane stress state.

Frattura ed Integrità Strutturale, 2015

Crack closure concept has been widely used to explain different issues of fatigue crack propagati... more Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, different authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Accordingly, 3D-FE numerical models with and without contact were developed for a wide range of loading scenarios and the crack tip parameters usually linked to fatigue crack growth, namely range of cyclic plastic strain, crack tip opening displacement, size of reversed plastic zone and total plastic dissipation per cycle, were investigated. It was demonstrated that: i) LEFM concepts are applicable to the problem under study; ii) the crack closure phenomenon has a great influence on crack tip parameters decreasing their values; iii) the ?Keff concept is able to explain the variations of crack tip parameters produced by...

Materials Today: Proceedings

Crack closure concept has been widely used to explain different issues of fatigue crack propagati... more Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, different authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Accordingly, 3D-FE numerical models with and without contact were developed for a wide range of loading scenarios and the crack tip parameters usually linked to fatigue crack growth, namely range of cyclic plastic strain, crack tip opening displacement, size of reversed plastic zone and total plastic dissipation per cycle, were investigated. It was demonstrated that: i) LEFM concepts are applicable to the problem under study; ii) the crack closure phenomenon has a great influence on crack tip parameters decreasing their values; iii) the Keff concept is able to explain the variations of crack tip parameters produced by the contact of crack flanks; iv) the analysis of remote compliance is the best numerical parameter to quantify the crack opening level; v) without contact there is no effect of stress ratio on crack tip parameters. Therefore it is proved that the crack closure concept is valid.

International Journal of Fatigue, 2013

Numerical models have been successfully developed to predict plasticity induced crack closure (PI... more Numerical models have been successfully developed to predict plasticity induced crack closure (PICC). However, despite the large research effort a full understanding of the links between physical parameters, residual plastic wake and PICC has not been achieved yet. The plastic extension of material behind crack tip, Dyp, obtained by the integration of vertical plastic deformation perpendicularly to crack flank, is proposed here to quantify the residual plastic field. The values of Dyp and PICC were obtained numerically in a M(T) specimen using the finite element method. An excellent correlation was found between PICC and Dyp which indicates that this parameter controls the phenomenon, and can be used to quantify the effect of physical parameters. An empirical model was developed to predict PICC assuming that the residual plastic field is a set of vertical plastic wedges, that the linear superposition principle applies and that the influence of a particular wedge exponentially decreases with distance to crack tip. The model was applied successfully to predict PICC for different residual plastic fields which provided an additional validation of Dyp as the parameter controlling PICC.

This experimental study focuses on the comparison between two different sensors for vibration sig... more This experimental study focuses on the comparison between two different sensors for vibration signals: a magnetoresistive sensor and an accelerometer as a calibrated reference. The vibrations are collected from a variable speed inductor motor setup, coupled to a ball bearing load with adjustable misalignments. To evaluate the performance of the magnetoresistive sensor against the accelerometer, several vibration measurements are performed in three different axes: axial, horizontal and vertical. Vibration velocity measurements from both sensors were collected and analyzed based on spectral decomposition of the signals. The high cross-correlation coefficient between spectrum vibration signatures in all experimental measurements shows good agreement between the proposed magnetoresistive sensor and the reference accelerometer performances. The results demonstrate the potential of this type of innovative and non-contact approach to vibration data collection and a prospective use of magne...

Plasticity induced crack closure (PICC) is closely linked to the monotonic and reversed plastic d... more Plasticity induced crack closure (PICC) is closely linked to the monotonic and reversed plastic deformation occurring at the crack tip. The objective of the paper is to identify the different physical and numerical parameters affecting PICC, and develop a sensitivity analysis to quantify their relative importance. The main parameters affecting PICC are the load parameters, the yield stress, the size of finite elements and the numerical parameter considered to quantify PICC. The numerical predictions should be independent of numerical parameters, therefore further work is required to optimize the numerical models.

The presence of residual stresses in a structure influences his behaviour. Fatigue strength of we... more The presence of residual stresses in a structure influences his behaviour. Fatigue strength of welded joints is significantly influenced by the presence of residual stresses, of flaws and notch sharpness at the weld toe. Plasma dressing at the weld toe is frequently used to remove flaws and to lessen the notch sharpness at the weld toe. With this re-melt, the stress field in the weld toe is also profoundly modified. The Finite Element Method (FEM) proves to be effective to simulate the stress fields generated by welding. The simplified two-dimensional models (2D) are efficient in this estimation. This paper presents a 2D finite element model to predict the residual stresses generated by Plasma dressing at the weld toe of a T-joint. The welded T-joints are made in St 52-3 steel, are obtained by covered electrode process and improved with plasma dressing. The analysis was developed with the MSC.Marc finite element code. The estimated stress field is validated with experimental stress ...

Procedia Structural Integrity, 2016

Abstract Crack closure concept has been widely used to explain different issues of fatigue crack ... more Abstract Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, some authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Numerical models with and without contact of crack flanks were built for a wide range of loading parameters. The non-linear crack tip parameters studied were the range of cyclic plastic strain, the crack tip opening displacement (CTOD) and the size of reversed plastic zone. Well defined relations between non linear parameters and K were obtained without contact of crack flanks which reinforces the validity of linear elastic fracture mechanics. When the CTOD with contact is plotted versus ΔK eff the values are coincident with those obtained without contact, which shows that the crack closure concept is valid and able to explain the influence of mean stress. The analysis of overloads and high-low load blocks demonstrated the validity of crack closure concept for variable amplitude loading.

The presence of residual stresses in a structure influences his behaviour. Fatigue strength of we... more The presence of residual stresses in a structure influences his behaviour. Fatigue strength of welded joints is significantly influenced by the presence of residual stresses, of flaws and notch sharpness at the weld toe. Plasma dressing at the weld toe is frequently used to remove flaws and to lessen the notch sharpness at the weld toe. With this re-melt, the stress field in the weld toe is also profoundly modified. The Finite Element Method (FEM) proves to be effective to simulate the stress fields generated by welding. The simplified two-dimensional models (2D) are efficient in this estimation. This paper presents a 2D finite element model to predict the residual stresses generated by Plasma dressing at the weld toe of a T-joint. The welded T-joints are made in St 52-3 steel, are obtained by covered electrode process and improved with plasma dressing. The analysis was developed with the MSC.Marc finite element code. The estimated stress field is validated with experimental stress ...

Frattura ed Integrità Strutturale

Crack closure influences fatigue crack growth rate and must be included in the design of componen... more Crack closure influences fatigue crack growth rate and must be included in the design of components. Plasticity induced crack closure is intimately linked with the crack tip plastic deformation, which becomes residual as the crack propagates. The objective here is to study numerically the effect of crack propagation on crack tip fields. The transient effect observed at the beginning of crack propagation is linked to the hardening behavior of material. The effect of mesh refinement is studied, and a singular behavior is evident, which is explained by the sharp crack associated with mesh topology, composed of a regular pattern of square elements. The plastic zone size measured perpendicularly to crack flank in the residual plastic wake is quantified and compared with literature models. Finally, the removal of material at the first node behind crack tip with load cycling was observed for plane strain state and some hardening models in plane stress state.

ABSTRACT. Crack closure influences fatigue crack growth rate and must be included in the design o... more ABSTRACT. Crack closure influences fatigue crack growth rate and must be included in the design of components. Plasticity induced crack closure is intimately linked with the crack tip plastic deformation, which becomes residual as the crack propagates. The objective here is to study numerically the effect of crack propagation on crack tip fields. The transient effect observed at the beginning of crack propagation is linked to the hardening behavior of material. The effect of mesh refinement is studied, and a singular behavior is evident, which is explained by the sharp crack associated with mesh topology, composed of a regular pattern of square elements. The plastic zone size measured perpendicularly to crack flank in the residual plastic wake is quantified and compared with literature models. Finally, the removal of material at the first node behind crack tip with load cycling was observed for plane strain state and some hardening models in plane stress state.

Frattura ed Integrità Strutturale, 2015

Crack closure concept has been widely used to explain different issues of fatigue crack propagati... more Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, different authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Accordingly, 3D-FE numerical models with and without contact were developed for a wide range of loading scenarios and the crack tip parameters usually linked to fatigue crack growth, namely range of cyclic plastic strain, crack tip opening displacement, size of reversed plastic zone and total plastic dissipation per cycle, were investigated. It was demonstrated that: i) LEFM concepts are applicable to the problem under study; ii) the crack closure phenomenon has a great influence on crack tip parameters decreasing their values; iii) the ?Keff concept is able to explain the variations of crack tip parameters produced by...

Materials Today: Proceedings

Crack closure concept has been widely used to explain different issues of fatigue crack propagati... more Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, different authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Accordingly, 3D-FE numerical models with and without contact were developed for a wide range of loading scenarios and the crack tip parameters usually linked to fatigue crack growth, namely range of cyclic plastic strain, crack tip opening displacement, size of reversed plastic zone and total plastic dissipation per cycle, were investigated. It was demonstrated that: i) LEFM concepts are applicable to the problem under study; ii) the crack closure phenomenon has a great influence on crack tip parameters decreasing their values; iii) the Keff concept is able to explain the variations of crack tip parameters produced by the contact of crack flanks; iv) the analysis of remote compliance is the best numerical parameter to quantify the crack opening level; v) without contact there is no effect of stress ratio on crack tip parameters. Therefore it is proved that the crack closure concept is valid.

International Journal of Fatigue, 2013

Numerical models have been successfully developed to predict plasticity induced crack closure (PI... more Numerical models have been successfully developed to predict plasticity induced crack closure (PICC). However, despite the large research effort a full understanding of the links between physical parameters, residual plastic wake and PICC has not been achieved yet. The plastic extension of material behind crack tip, Dyp, obtained by the integration of vertical plastic deformation perpendicularly to crack flank, is proposed here to quantify the residual plastic field. The values of Dyp and PICC were obtained numerically in a M(T) specimen using the finite element method. An excellent correlation was found between PICC and Dyp which indicates that this parameter controls the phenomenon, and can be used to quantify the effect of physical parameters. An empirical model was developed to predict PICC assuming that the residual plastic field is a set of vertical plastic wedges, that the linear superposition principle applies and that the influence of a particular wedge exponentially decreases with distance to crack tip. The model was applied successfully to predict PICC for different residual plastic fields which provided an additional validation of Dyp as the parameter controlling PICC.

This experimental study focuses on the comparison between two different sensors for vibration sig... more This experimental study focuses on the comparison between two different sensors for vibration signals: a magnetoresistive sensor and an accelerometer as a calibrated reference. The vibrations are collected from a variable speed inductor motor setup, coupled to a ball bearing load with adjustable misalignments. To evaluate the performance of the magnetoresistive sensor against the accelerometer, several vibration measurements are performed in three different axes: axial, horizontal and vertical. Vibration velocity measurements from both sensors were collected and analyzed based on spectral decomposition of the signals. The high cross-correlation coefficient between spectrum vibration signatures in all experimental measurements shows good agreement between the proposed magnetoresistive sensor and the reference accelerometer performances. The results demonstrate the potential of this type of innovative and non-contact approach to vibration data collection and a prospective use of magne...

Plasticity induced crack closure (PICC) is closely linked to the monotonic and reversed plastic d... more Plasticity induced crack closure (PICC) is closely linked to the monotonic and reversed plastic deformation occurring at the crack tip. The objective of the paper is to identify the different physical and numerical parameters affecting PICC, and develop a sensitivity analysis to quantify their relative importance. The main parameters affecting PICC are the load parameters, the yield stress, the size of finite elements and the numerical parameter considered to quantify PICC. The numerical predictions should be independent of numerical parameters, therefore further work is required to optimize the numerical models.

The presence of residual stresses in a structure influences his behaviour. Fatigue strength of we... more The presence of residual stresses in a structure influences his behaviour. Fatigue strength of welded joints is significantly influenced by the presence of residual stresses, of flaws and notch sharpness at the weld toe. Plasma dressing at the weld toe is frequently used to remove flaws and to lessen the notch sharpness at the weld toe. With this re-melt, the stress field in the weld toe is also profoundly modified. The Finite Element Method (FEM) proves to be effective to simulate the stress fields generated by welding. The simplified two-dimensional models (2D) are efficient in this estimation. This paper presents a 2D finite element model to predict the residual stresses generated by Plasma dressing at the weld toe of a T-joint. The welded T-joints are made in St 52-3 steel, are obtained by covered electrode process and improved with plasma dressing. The analysis was developed with the MSC.Marc finite element code. The estimated stress field is validated with experimental stress ...

Procedia Structural Integrity, 2016

Abstract Crack closure concept has been widely used to explain different issues of fatigue crack ... more Abstract Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, some authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Numerical models with and without contact of crack flanks were built for a wide range of loading parameters. The non-linear crack tip parameters studied were the range of cyclic plastic strain, the crack tip opening displacement (CTOD) and the size of reversed plastic zone. Well defined relations between non linear parameters and K were obtained without contact of crack flanks which reinforces the validity of linear elastic fracture mechanics. When the CTOD with contact is plotted versus ΔK eff the values are coincident with those obtained without contact, which shows that the crack closure concept is valid and able to explain the influence of mean stress. The analysis of overloads and high-low load blocks demonstrated the validity of crack closure concept for variable amplitude loading.

The presence of residual stresses in a structure influences his behaviour. Fatigue strength of we... more The presence of residual stresses in a structure influences his behaviour. Fatigue strength of welded joints is significantly influenced by the presence of residual stresses, of flaws and notch sharpness at the weld toe. Plasma dressing at the weld toe is frequently used to remove flaws and to lessen the notch sharpness at the weld toe. With this re-melt, the stress field in the weld toe is also profoundly modified. The Finite Element Method (FEM) proves to be effective to simulate the stress fields generated by welding. The simplified two-dimensional models (2D) are efficient in this estimation. This paper presents a 2D finite element model to predict the residual stresses generated by Plasma dressing at the weld toe of a T-joint. The welded T-joints are made in St 52-3 steel, are obtained by covered electrode process and improved with plasma dressing. The analysis was developed with the MSC.Marc finite element code. The estimated stress field is validated with experimental stress ...

Frattura ed Integrità Strutturale

Crack closure influences fatigue crack growth rate and must be included in the design of componen... more Crack closure influences fatigue crack growth rate and must be included in the design of components. Plasticity induced crack closure is intimately linked with the crack tip plastic deformation, which becomes residual as the crack propagates. The objective here is to study numerically the effect of crack propagation on crack tip fields. The transient effect observed at the beginning of crack propagation is linked to the hardening behavior of material. The effect of mesh refinement is studied, and a singular behavior is evident, which is explained by the sharp crack associated with mesh topology, composed of a regular pattern of square elements. The plastic zone size measured perpendicularly to crack flank in the residual plastic wake is quantified and compared with literature models. Finally, the removal of material at the first node behind crack tip with load cycling was observed for plane strain state and some hardening models in plane stress state.