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Pereira, J. C. R., A. M. P. de Jesus, J. Xavier, J. A. F. O. Correia, L. Susmel, and A. A. Fernandes. "Low and ultra-low-cycle fatigue behavior of X52 piping steel based on theory of critical distances." International Journal of Fatigue (2020): 105482. AbstractWebsite

The cyclic failure observed in structural components such as pipelines subjected to extreme loading conditions highlights some limitations concerning the application of existing fatigue damage models. The evaluation and prediction of this type of failure in these steel components under large-scale plastic yielding associated with high levels of stress triaxiality are not sufficiently known nor explored. This fatigue domain is conventionally called ultra-low-cycle fatigue (ULCF) and damage features are representative of both low-cycle fatigue (LCF) and monotonic ductile fracture. Thus, in order to understand the ULCF damage mechanisms both monotonic and LCF tests are required to get representative bounding damage information to model the material damage behaviour under such extreme loading conditions. This paper aims at exploring the Theory of Critical Distances (TCD) in the LCF and ULCF fatigue regimes, including the application of the point, line and area methods. The application of the TCD theories has not been explored so far in the ULCF fatigue regimes, despite its promising results in the LCF and high-cycle fatigue. An experimental program was carried out on several specimens’ geometries made of X52 piping steel. In detail, smooth plane specimens and notched plane specimens were cyclic loaded under tension/compression loading in order to obtain fatigue lives within the range of 101-104 cycles. In addition, cyclic bending tests on notched plane specimens were also incorporated in this study. Finite element simulations of all small-scale tests were conducted allowing to derive elastoplastic stress/strain fields along the potential crack paths. The numerical data were subjected to a post-processing in order to find characteristic lengths that can be treated as a fatigue property according to the TCD. A unified strain-life relation is proposed for the X52 piping steel together with a characteristic material length, consisting of a practical relation for pipeline strain-based design under extreme cyclic loading conditions.

Fernandes, Pedro, José Sena-Cruz, José Xavier, Patrícia Silva, Eduardo Pereira, and José Cruz. "Durability of bond in NSM CFRP-concrete systems under different environmental conditions." Composites Part B: Engineering. 138 (2018): 19-34. AbstractWebsite

Abstract This paper addresses the durability of bond between concrete and carbon fibre reinforced polymer (CFRP) strips installed according to the near-surface mounted (NSM) technique (NSM CFRP-concrete systems) under the effects of two main groups of environmental conditions: (i) laboratory-based ageing conditions; (ii) real outdoor ageing conditions. The bond degradation was evaluated by carrying out direct pullout tests on aged specimens that were previously subjected to distinct environmental conditions for different periods of exposure. Moreover, the degradation of the mechanical properties of the involved materials was investigated. The digital image correlation (DIC) method was used to document the evolution of the deformation fields at the surface over the whole region of interest consisting of concrete and epoxy adhesive at the ligament region. This information supported the discussion about the evolution of the bond resistant mechanism developed in \{NSM\} CFRP-concrete specimens during testing, as well as the assessment of the bond quality of the system. In general, the results obtained from the durability tests conducted have shown that the different exposure environments, which may be considered as quite severe, did not result in significant damage on \{NSM\} CFRP-concrete system. The maximum decrease of about 12% on bond strength was obtained for real outdoor environments. Conversely, a maximum increase of 8% on bond strength was obtained on the specimens exposed to the temperature cycles between -15��C and +60��C. \{DIC\} allowed to document the stress transfer mechanisms established between the \{CFRP\} and the concrete substrate, revealing the crack patterns and the influence widths of the \{CFRP\} reinforcement strips, which were shown to be important for avoiding group effect when using multiple parallel strengthening \{CFRP\} strips.

Pereira, J. C. R., A. M. P. de Jesus, J. Xavier, and A. A. Fernandes. "ULCF assessment of X52 piping steel by means of cyclic bending tests." Journal of Constructional Steel Research. 138 (2017): 663-674. AbstractWebsite

Abstract Pipelines and piping components may experience large cyclic deformations during a reduced number of cycles (Ni = 1�100 cycles), when subjected to extreme cyclic loading events (e.g. hurricanes, support settlements, earthquakes). In accordance with these loading scenarios, a lateral movement can be applied to the pipeline inducing bending stresses that gradually promotes strain localization, due to progressive plastic instabilities (buckling), damage evolution and final failure. This work aims at characterizing ultra-low cycle fatigue (ULCF) behaviour of the \{X52\} piping steel under bending and local buckling state. An experimental program was carried out to derive \{ULCF\} data for smooth, notched and flat-grooved specimen geometries under cyclic bending. Furthermore, the small-scale tests were simulated in ABAQUS� with the objective of computing the parameters governing the fatigue damage models. The classical Coffin-Manson strain-life relation commonly used in the low cycle fatigue (LCF) regime is proposed to model the fatigue lives. In addition, the Xue model, particularly dependent of the monotonic fracture strain was also used for the prediction of the number of cycles until the crack initiation. The numerical data obtained with these models are compared, being achieved similar fatigue lives predictions for notched plane specimens. For the case of flat-grooved specimens which provides plain strain conditions, an overestimation from the Coffin-Manson relation was observed while the Xue model reproduces very good results for both specimens' series.

Silva, A. L. L., A. M. P. de Jesus, J. Xavier, J. A. F. O. Correia, and A. A. Fernandes. "Combined analytical-numerical methodologies for the evaluation of mixed-mode (I+II) fatigue crack growth rates in structural steels." Engineering Fracture Mechanics. 185 (2017): 124-138. AbstractWebsite

Abstract This paper proposes an experimental study aiming to evaluate stress intensity factors (SIFs) for fatigue cracks propagating under pure mode I and mixed-mode I+II for a S235 structural steel. Compact tension (CT) specimens with a side hole were manufactured in order to generate a stress field, ahead of the crack tip, resulting in mixed-mode fatigue crack propagation. Specimens with distinct side hole locations were submitted to fatigue tests under stress controlled conditions for a stress rati

Subramani, P., Sohel Rana, Bahman Ghiassi, Raul Fangueiro, Daniel V. Oliveira, Paulo B. Lourenco, and Jose Xavier. "Development and characterization of novel auxetic structures based on re-entrant hexagon design produced from braided composites." Composites Part B: Engineering. 93 (2016): 132-142. AbstractWebsite

Abstract This paper reports the first attempt of developing macro-scale auxetic structures based on re-entrant hexagon design from braided composite materials for civil engineering applications. Braided composite rods (BCRs) were produced and arranged as longitudinal and horizontal elements to produce three types of auxetic structures: (1) basic re-entrant hexagon structure, (2) basic structure modified by adding straight longitudinal elements and (3): structure-2 modified by changing structural angle. The influence of various material and structural parameters as well as structure type on Poisson's ratio and tensile properties was thoroughly investigated. The auxetic behaviour was found to strongly depend on the structural angle and straight elements, resulting in lower auxeticity with lower angles and in presence of straight elements. Material parameters influenced the auxetic behaviour to a lesser extent and a decrease in auxetic behaviour was noticed with increase in core fibre linear density and using stiffer fibres such as carbon. The reverse effect was observed in case of tensile strength and work of rupture. Among these structures, structure-3 exhibited good auxetic behaviour, balanced tensile properties, and high energy absorption capacity and their auxetic behaviour could be well predicted with the developed analytical model. Therefore, these novel structures present good potential for strengthening of civil structures.

Silva, Patrícia, Pedro Fernandes, José Sena-Cruz, José Xavier, Fernando Castro, Delfim Soares, and Vítor Carneiro. "Effects of different environmental conditions on the mechanical characteristics of a structural epoxy." Composites Part B: Engineering. 88 (2016): 55-63. AbstractWebsite

Abstract With the aim of characterising a commercially available epoxy adhesive used for fibre-reinforced polymers strengthening applications, when submitted to different environmental conditions, mainly thermal (TC), freeze–thaw (FT), and wet–dry (WD) cycles and immersion in pure (PW) and water with chlorides (CW) for periods of exposure that lasted up to 16 months, an experimental program was carried out. Several methodologies were used in its characterization, mainly the scanning electron microscope (SEM), dynamic mechanical analysis (DMA), standard tensile tests (STT) coupled with digital image correlation (DIC). In general the results revealed that the chemical composition was not affected by the environmental conditions. Nevertheless, it was verified through \{DMA\} and \{STT\} that the modulus of elasticity and tensile strength of the epoxy adhesive increased in the TC, while the specimens submitted to \{PW\} and \{CW\} faced a high degradation in terms of its mechanical properties. Eventually, the glass transition temperature (Tg) was not affected by the environmental conditions, apart from the specimens subjected to \{TC\} and FT, presenting a higher and lower Tg, respectively, when compared with the reference specimens.

Xavier, J., A. Majano-Majano, and J. Fernandez-Cabo. "On the identifiability of stiffness components of clear wood from a 3D off-axes prismatic specimen: angle orientation and friction effects." European Journal of Wood and Wood Products. 74 (2016): 285-290. AbstractWebsite

The robustness of the test method based on a single 3D off-axis prismatic specimen for the simultaneous identification of the orthotropic stiffness components of clear wood is addressed. In this method, the specimen is consecutively submitted to uniaxial compression tests along its three orthogonal axes. A data reduction based on anisotropic elasticity is applied to extract active material parameters from 3D full-field deformation measurements provided by stereo-correlation over adjacent faces. Two major limitations of this test method, directly affecting the parameter identification, are analysed and discussed: (1) off-axes angle orientation; (2) friction effects. A numerical study pointed out that radial and tangential rotations of about 29° and 9°, respectively, balances out the strain components in the specimen response. Moreover, friction can be reduced by using mass lubricant or soft material in the contact interface, realising transverse shear deformation.

Furtado, C., A. Arteiro, G. Catalanotti, J. Xavier, and P. P. Camanho. "Selective ply-level hybridisation for improved notched response of composite laminates." Composite Structures. 145 (2016): 1-14. AbstractWebsite

Abstract This work presents an experimental study on the effect of ply-level hybridisation on the tensile unnotched and notched response of composite laminates. In a first assessment, notched tests were performed on laminates with nominal ply thicknesses between 0.03 mm and 0.30 mm. From the understanding of the effect of ply thickness on the damage mechanisms that govern the notched response of laminates, the concept of ply-level hybridisation is introduced, which consists in combining plies of different grades. A uniform combination of thin and conventional plies resulted in a hybrid laminate with intermediate notched response. Selective hybridisation, where thin off-axis plies are combined with thicker 0° plies, resulted in a globally enhanced notched behaviour without compromising the unnotched and fatigue responses. This work clearly shows how ply-level hybridisation, when designed to trigger specific damage mechanisms, can be used to improve the notched response of composite laminates.

Garon-Roca, J., J. M. Sena-Cruz, P. Fernandes, and J. Xavier. "Effect of wet-dry cycles on the bond behaviour of concrete elements strengthened with NSM CFRP laminate strips." Composite Structures. 132 (2015): 331-340. AbstractWebsite

Abstract The near-surface mounted (NSM) strengthening technique is capable of effectively increase the bearing capacity of structural concrete elements. This technique which basically consists of placing \{FRP\} reinforcements inside small grooves cut in the concrete cover, has been widely investigated in terms of structural performance and ability to improve the flexural and shear behaviour of reinforced concrete beams and columns. However, little research has been carried out concerning to the \{NSM\} long-term performance and durability. Motivated by the need of increasing the knowledge on the expected durability of the \{NSM\} technique using \{CFRP\} laminates, this paper presents an experimental program in which direct pull-out tests are carried out for evaluating the bond behaviour of specimens aged through wet-dry cycles. A total of 30 specimens are tested, analysing the effect of the bond length, the groove width, the groove depth and the aging effect on the bond behaviour. Digital image correlation method is also used to identify the bond resistant mechanism developed in an element strengthened using \{NSM\} technique. Finally, using the experimental results, an analytical�numerical strategy is applied to establish the local bond stress�slip relationship.

Xavier, J., J. R. A. Fernandes, J. J. L. Morais, and O. Frazão. "Fracture behaviour of wood bonded joints under modes I and II by digital image correlation and fibre Bragg grating sensors." Ciência & Tecnologia dos Materiais. 27 (2015): 27-35. AbstractWebsite

Abstract Direct identification of cohesive laws in modes I and İI\} of wood bonded joints is addressed by the double cantilever beam (DCB) and end-notched flexure (ENF) tests, respectively. Moreover, the development and extension of fracture process zone (FPZ) ahead of the initial crack tip, is analysed by means of digital image correlation (DIC) and embedded fibre Bragg grating (FBG) sensors. From \{FBG\} spectral response, the spectrum geometric mean is determined and the strain induced by wavelength variation employed to identify the initial and final stages of the FPZ. These stages are used to consistently define the cohesive laws in both modes I and II. Resistance-curves are determined from the compliance-based beam method (CBBM). Besides, the crack tip opening displacements (CTOD) are determined by post-processing displacement field provided by \{DIC\} around the initial crack tip. The strain energy release rate as a function of the \{CTOD\} are then determined for both mode I and mode II. The respective cohesive laws are reconstructed by numerical approximation and differentiation. It is concluded that the proposed data reduction scheme is effective to determine both the \{FPZ\} development phase and the corresponding cohesive laws of wood bonded joints in both mode I and mode II.

Xavier, J., J. R. A. Fernandes, O. Frazão, and J. J. L. Morais. "Measuring mode I cohesive law of wood bonded joints based on digital image correlation and fibre Bragg grating sensors." Composite Structures. 121 (2015): 83-89. Abstract

Abstract This work addresses the experimental identification of mode I cohesive law of wood bonded joints. The approach combines the double cantilever beam (DCB) test with both digital image correlation (DIC) and embedded fibre Bragg grating (FBG) sensors. The spectrum geometric mean of the \{FBG\} reflected spectral response was determined, and the wavelength evolution was used to define the fracture process zone (FPZ) development phase. This evaluation allowed a consistent selection of experimental range of over which the identification procedure of mode I cohesive law is build up. Mode I crack length, Resistance-curve and cohesive law parameters are characterised and discussed. The strain energy release rate (GI) is determined from the P�d curve by the compliance-based beam method (CBBM). The crack tip opening displacement (wI) is determined by post-processing displacements measured by DIC. The cohesive law in mode I (sI�wI) is then obtained by numerical differentiation of the GI�wI relationship.

Fernández-Cabo, J. L., R. Widmann, M. Arce-Blanco, R. Crocetti, J. Xavier, and A. Majano-Majano. "Assessment of wire-frame analysis models of a historical planked timber arch." Proceedings of the Institution of Civil Engineers - Structures and Buildings. 168 (2015): 680-694. AbstractWebsite
Sampayo, L. M. C. M. V., P. M. F. Monteiro, J. A. F. O. Correia, J. M. C. Xavier, De A. M. P. Jesus, A. Fernandez-Canteli, and R. A. B. Calçada. "Probabilistic S-N Field Assessment for a Notched Plate Made of Puddle Iron From the Eiffel Bridge with an Elliptical Hole." Procedia Engineering. 114 (2015): 691-698. AbstractWebsite

Abstract Probabilistic fatigue models are required to account conveniently for several sources of uncertainty arising in the prediction procedures for structural details, such as the scatter in material behaviour. In this paper, a procedure to derive probabilistic S-N fields for structural details [1] is applied to a notched geometry with elliptic central hole made of puddle iron from the Eiffel bridge, based on the local approaches supported by the probabilistic ɛa–N or Smith–Watson–Topper (SWT)–N fields [1,2]. This procedure suggests an extension of the fatigue crack propagation model proposed by Noroozi et al. [3,4] to structural details, in order to cover both the fatigue crack initiation and fatigue crack propagation, based on local strain approaches to fatigue. Both fatigue crack initiation and fatigue crack propagation mechanisms are accounted for in the proposed approach. The numerical results are compared with available experimental S-N fatigue data for the notched plate under consideration.

Subramani, P., S. Rana, D. V. Oliveira, R. Fangueiro, and J. Xavier. "Development of novel auxetic structures based on braided composites." Materials & Design. 61 (2014): 286-295. AbstractWebsite
Pereira, J. C. R., A. M. P. de Jesus, J. Xavier, and A. A. Fernandes. "Ultra low-cycle fatigue behaviour of a structural steel." Engineering Structures. 60 (2014): 214-222. AbstractWebsite
Fernandes, R. M. R. P., J. A. G. Chousal, M. F. S. F. de Moura, and J. Xavier. "Determination of cohesive laws of composite bonded joints under mode II loading." Composites Part B: Engineering. 52 (2013): 269-274. Abstract


Fernandes, A., J. Lousada, J. Morais, J. Xavier, J. Pereira, and P. Melo-Pinto. "Comparison between neural networks and partial least squares for intra-growth ring wood density measurement with hyperspectral imaging." Computers and Electronics in Agriculture. 94 (2013): 71-81. Abstract
Fernandes, A., J. Lousada, J. Morais, J. Xavier, J. Periera, and P. Melo-Pinto. "Measurement of intra-ring wood density by means of imaging VIS/NIR spectroscopy (hyperspectral imaging)." Holzforschung. 67 (2013): 59-65. Abstract
Xavier, J., A. M. R. Sousa, J. J. L. Morais, V. M. J. Filipe, and M. Vaz. "Measuring displacement fields by cross-correlation and a differential technique: experimental validation." Optical Engineering. 51 (2012): 043602. Abstract
Sousa, A. M. R., J. Xavier, M. Vaz, J. J. L. Morais, and V. M. J. Filipe. "Cross-correlation and differential technique combination to determine displacement fields." Strain. 47 (2011): 87-98. Abstract


Sousa, A. M. R., J. Xavier, J. J. L. Morais, V. M. J. Filipe, and M. Vaz. "Processing discontinuous displacement fields by a spatio-temporal derivative technique." Optics and Lasers in Engineering. 49 (2011): 1402-1412. Abstract