jmc-xavier

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.

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.

Abstract Ancient structures are part of the inheritance our elders left us. These historical inheritance needs to be preserved, so the historic structures need to be rehabilitated and restored, protecting the cultural patrimony and attending to the comfort and habitability required nowadays. In order to accomplish a good and economic rehabilitation is essential to study the behaviour of traditional structures elements (masonry and wood) in order to develop adequate assessment measures and techniques. In this context it was carried out an experimental campaign to characterize the tensile mechanical behavior of the woods from the �sequeiro� wood structure, integral part of the �Quinta de Lobeira de Cima� farm. This building from the 20th century is located in Minho, Portugal. Tensile Tests were carried out for two different species of wood, chestnut and oak. The tensile tests were performed to obtain the tensile strength parallel to the fibers, using the digital image correlation (DIC) for the extension measurement.

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

Abstract This work addresses the determination of the cohesive law under mode I loading of bovine cortical bone tissue using the Double Cantilever Beam (DCB) test. Direct and inverse methods were proposed to assess the cohesive laws representative of bone fracture under mode I loading. The direct method combines the evolution of the strain energy release rate under mode I loading with the crack tip opening displacement that is monitored by digital image correlation technique. According to this method, the cohesive law is obtained by differentiation of such relation with respect to the crack opening. The inverse procedure is performed through a finite element analysis including cohesive zone modelling, conjointly with a developed optimization algorithm. This identification strategy does not require a pre-established shape of the cohesive law as with the conventional inverse based procedures, which is viewed as a novelty of this work. It was concluded that both methods provide consistent results, being appellative tools concerning systematic and methodical studies dedicated to bone fracture characterization.

Biodegradable polymers such as PLA have been studied for medical applications, human ligament repair is one of such cases. However, these materials can be applied in other sectors as aerospace, aeronautics, automotive, food packaging. PLA presents a relatively brittle on the mode I fracture behavior, being often blend with other biodegradable or non-degradable polymers to improve its fracture energy. For some existing applications, PLA components exhibit accumulated permanent deformation resulting from dynamic mechanical inputs, resulting on failure by laxity of parts. Aiming the improvement of PLA mechanical properties, the inclusion of carbon nanofillers into PLA matrix, in particular, CNT-COOH and GNP have been developed, due to their strong sp2 carbon-carbon bondings and their geometric arrangement that enhance mechanical properties of the polymer matrix. PLA and nanocomposites were produced by melt blending followed by compression moulding in a hot press, with small weight percentages of nanofillers added to the matrix. Nanocomposites dispersion was evaluated by SEM. Quasi static tensile tests were performed on a mechanical testing machine (Instron� ElectroPuls E1000) along with strain field measurements of specimens with centred crack with digital image correlation, revealing strain distribution along specimens.

The effect of ply thickness on the onset of intralaminar and interlaminar damage is extremely important for the structural response of laminated composite structures. This subject has gained particular interest in recent years due to the introduction in the market of spread-tow, ultra-thin carbon-fibre reinforcements with different configurations. In the present paper, an experimental test campaign was carried out to study the structural response of aerospace-grade plain weave spread-tow fabrics (STFs) of different areal weights. The results showed that, in spite of an apparent superior longitudinal tensile strength of the thick STF, the multidirectional thin-STF laminate exhibited an improved tensile unnotched strength over the thick-STF laminate, attributed to its damage suppression capability. However, damage suppression was also responsible for similar tensile notched strengths. In compression, the thin-STF laminate performed substantially better than the thick-STF laminate in both unnotched and notched configurations. Finally, a similar bearing response was obtained in both STF laminates, in spite of a slightly higher resistance of the thin-STF laminate to the propagation of subcritical damage mechanisms.

Fracture characterization of human cortical bone under mode II loading was analyzed using a miniaturized version of the end-notched flexure test. A data reduction scheme based on crack equivalent concept was employed to overcome uncertainties on crack length monitoring during the test. The crack tip shear displacement was experimentally measured using digital image correlation technique to determine the cohesive law that mimics bone fracture behavior under mode II loading. The developed procedure was validated by finite element analysis using cohesive zone modeling considering a trapezoidal with bilinear softening relationship. Experimental load-displacement curves, resistance curves and crack tip shear displacement versus applied displacement were used to validate the numerical procedure. The excellent agreement observed between the numerical and experimental results reveals the appropriateness of the proposed test and procedure to characterize human cortical bone fracture under mode II loading. The proposed methodology can be viewed as a novel valuable tool to be used in parametric and methodical clinical studies regarding features (e.g., age, diseases, drugs) influencing bone shear fracture under mode II loading.

Abstract This work presents an experimental method to measure the compressive crack resistance curve of fiber-reinforced polymer composites when subjected to dynamic loading. The data reduction couples the concepts of energy release rate, size effect law and R-curve. Double-edge notched specimens of four different sizes are used. Both split-Hopkinson pressure bar and quasi-static reference tests are performed. The full crack resistance curves at both investigated strain rate regimes are obtained on the basis of quasi-static fracture analysis theory. The results show that the steady state fracture toughness of the fiber compressive failure mode of the unidirectional carbon-epoxy composite material IM7-8552 is 165.6kJ/m2 and 101.6kJ/m2 under dynamic and quasi-static loading, respectively. Therefore the intralaminar fracture toughness in compression is found to increase with increasing strain rate.

AbstractThe behaviour of standard boltedsteel connections submitted to monotonic loads, through the use of numerical models, is presented in the current paper.The bolted connections allow speed up constructive processes in an increasingly competitive and globalized world in which the costs are a decisive factor in the development of a project. The use of computational tools in the analysis of bolted connections becomes determinant, mainly for new solutions or solutions less explored in terms of design codes.Throughout the years, bolted connections have been suffering transformations resulting from research activities performed by many authors. Rivets have been replaced by bolts, the main achievement being the pre-stressed bolts. Methodologies based on finite element analyses were proposed for double shear bolted connection. The non-linear behaviour of these connections is investigated and their performances are compared. In the numerical modelling of the bolted connection, linear elastic and elastoplastic analyzes reveal that there are two slip levels associated with local non-linearities caused by the contact pairs, which vary with clamping stresses.

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.

This study presents two alternative methods to determine the cohesive law of bovine cortical bone under mode II loading, employing the End Notched Flexure (ENF) test. The direct method results from the combination of the progress of the mode II strain energy release rate with the crack tip shear displacement, obtained by digital image correlation. The resulting cohesive law is determined by differentiation of this relation relatively to the crack shear displacement. The inverse method employs finite element analyses with cohesive zone modelling, in association with an optimization procedure. The resulting strategy enables determining the cohesive law without establishing a pre-defined shape. The significant conclusion that comes out of this work is that both methods offer consistent results regarding the estimation of the cohesive law in bone. Given that the inverse method dispenses the use of sophisticated equipment to obtain the cohesive law in bone, it can be used as a more convenient procedure to accomplish efficient studies in the context of bone fracture characterization under mode II loading.

This paper presents the determination of the crack resistance curve of the unidirectional carbon-epoxy composite material IM7-8552 for intralaminar fiber tensile failure under dynamic loading. The methodology, proposed by Catalanotti et al. (2014) for quasi-static loading conditions, was enhanced to high rate loading in the order of about 60 ?s-1. Dynamic tests were performed using a split-Hopkinson tension bar, while quasi-static reference tests were conducted on a standard electromechanical testing machine. Double-edge notched tension specimens of different sizes were tested to obtain the size effect law, which in combination with the concepts of the energy release rate is used to measure the entire crack resistance curve for the fiber tensile failure mode. Digital image correlation is applied to further verify the validity of the experiments performed at both static and dynamic loading. The data reduction methodology applied in this paper is suitable for intralaminar fiber failure modes without significant delamination. Sufficient proof is given that quasi-static fracture mechanics theory can also be used for the data reduction of the dynamic tests. It is shown, that the intralaminar fracture toughness for fiber tensile failure of UD IM7-8552 increases with increasing rate of loading.

The prediction of the fracture behaviour through reliable and practical criteria in the design of structural timber elements and connections has become of great importance and demands a proper fracture characterization of the material. Eucalyptus globulus Labill is envisioned as a hardwood species with great potential for high performance structural purposes because of its major mechanical and durability properties, being so far mainly used in paper industry. Experimental research on the identification of the resistance curves to derive the critical strain energy release rate in Eucalyptus globulus L. under pure mode I and RL crack propagation system is performed by means of Double Cantilever Beam tests. Three different data reduction schemes are compared: the Modified Experimental Compliance Method; and two approaches of the Compliance Based Beam Method. These methods take into account the non negligible damage mechanisms at the fracture process zone and have the advantage of being based exclusively on the specimen compliance following an equivalent crack concept, for which crack length monitoring during testing is not required. The Compliance Based Beam Method turns out to be the most appropriate data reduction scheme to obtain the critical energy release rate in eucalyptus because of its simplicity. Concerning this, a high average value of 720�J/m2 was obtained confirming Eucalyptus globulus L. as a promising hardwood species for timber structural design.

The non-linear shear behaviour of bovine cortical bone is measured by coupling the Arcan test with digital image correlation. The experimental study is carried out on specimens taken from bovine femur and oriented along the longitudinal-tangential material plane. An ad hoc Arcan fixture is built to transfer the cross-head displacement of the testing machine into a shear loading at the centre of the V-notched section. A validation of predominant shear behaviour at the gauge section is shown from full-field deformation measurements. Moreover, direct evaluation of the shear modulus is obtained by integrating the shear strain component along the V-notches, avoiding numerical correction factors required in the classical data reduction scheme. The shear modulus of bovine cortical bone is found in good agreement with references from literature. Moreover, the shear stress at maximum load is understood as a suitable estimation of the shear strength. Furthermore, the Ramberg�Osgood model is found to provide an accurate description of the non-linear shear behaviour of bone tissue.

This work addresses the reconstruction of strain gradient fields at the wood growth ring scale from full-field deformation measurements provided by digital image correlation. Moreover, the spatial distribution of the earlywood and latewood radial modulus of elasticity is assessed. Meso-scale tensile tests are carried out on Pinus pinaster Ait. wooden specimens oriented in the radial–tangential plane under quasi-static loading conditions. A parametric analysis of the two-dimensional digital image correlation extrinsic and intrinsic setting parameters is performed, in a balance between spatial resolution and resolution. It is shown that the parametric module is an effective way to quantitatively support the choice of digital image correlation parameters in the presence of the high deformation gradient fields generated by the structure–property relationships at the scale of observation. Under the assumption of a uniaxial tensile stress state, the spatial distribution of the radial elastic modulus across the growth rings is obtained. It is observed that the ratio of the radial modulus of elasticity between latewood and earlywood tissues can vary significantly as a function of the digital image correlation parameters. It is pointed out, however, that a convergence value can be systematically established. Effectively, earlywood and latewood stress–strain curves are obtained and elastic properties are determined assuming the converged digital image correlation setting parameters.

Abstract The authors have recently proposed an innovative connector system that consists on a Glass Fibre Reinforced Polymer (GFRP) perforated plate that is embedded into Steel Fibre Reinforced Self-Compacting Concrete (SFRSCC) layers. The connection is strongly based in the mechanical interlock assured by the dowels originated from the \{SFRSCC\} passing through the holes opened on the \{GFRP\} plates. In this study, an analytical framework to evaluate the load capacity of the connections when loaded transversally was developed based on experimental pull-out tests presented in the companion paper (Part I). For a better understanding of the mechanical behaviour of the connections and to allow to make estimations of the load capacity of connection when it is conditioned by the rupture of the connector itself, pull-out pin-bearing tests with single-hole plates were executed to assess the effect of the type of \{GFRP\} on the strain distribution in the vicinity of the holes until the failure, as well as the estimated failure modes and load capacities of the connections.

The enhanced mechanical performance of thin-ply laminates results from their ability to delay the onset of damage typically observed in composite materials. However, in notched structures, subcritical damage growth causes beneficial stress redistributions in the vicinity of the notch, blunting the stress concentration. Precluding these damage mechanisms, as in thin-ply laminates, may potentially lead to inferior notched responses. To obviate this limitation of thin-ply laminates, a strategy based on the combination of standard grade 0� plies and thin transverse and off-axis plies is analysed in this paper. A detailed study of the effect of 0� ply blocking is carried out, with particular emphasis on the blunting mechanisms and notched response. Tests on scaled notched panels loaded in tension, with notch sizes between 6?mm and 30?mm, show that the combination of standard grade 0� ply blocks with thin transverse and off-axis plies promotes localised fibre-matrix splitting, which acts as an important notch blunting mechanism, while preventing matrix cracking and delamination. This results in an improved notched response and superior large damage capability. It is also shown that thicker 0� ply blocks provide higher stability in composite bolted joints, while the thin transverse and off-axis plies contribute for matrix-dominated damage suppression, resulting in an improved bolt-bearing response. The improvements of the large damage capability and bolt-bearing performance are obtained without compromising the superior unnotched tensile and compressive strengths intrinsic to thin-ply laminates.

Purpose The antique structures are part of the inheritance that our elders left, being important to preserve their memories. It is important to preserve, rehabilitate and restore the historic buildings protecting the cultural patrimony, attending to the actual comfort and habitability requirements. It is necessary to study the behaviour of the various elements that compose antique structures (masonry and wood) in order to develop assessment measures according to the characteristics of the original materials. The paper aims to discuss these issues. Design/methodology/approach An experimental campaign to characterize the mechanical behaviour of the wood of the roof of the �sequeiro� of �Quinta Lobeira de Cima�, a building from the twentieth century located in Minho, was carried out. The tested wood specimens are from two different species: chestnut and oak. Compression, tension and static flexion tests according to parallel to the grain direction were performed. Other parameters, such as density, moisture content and longitudinal modulus of elasticity in compression and in tension, were also obtained. The measurement of displacements was made with Digital Image Correlation (DIC). Findings The results of this study show the similarity between experimental and empirical values for the studied woods species. Originality/value This original study aimed at characterizing the mechanical properties using DIC of wood of the roof of the �sequeiro� of �Quinta Lobeira de Cima�, a building from the twentieth century located in Minho (Portugal). This study is part of master thesis of Jo�o Martins, an original research work.

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.

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Orthotropic stiffness components of Pinus pinaster Ait. wood are simultaneously determined by means of a heterogeneous plate bending test. The proposed inverse identification approach couples full-field slope measurements provided by deflectometry with the virtual fields methods. Wooden plates oriented in the longitudinal–radial and longitudinal–tangential material planes were manufactured. A procedure was implemented to allow suitable specular reflective coating of the wooden plates, required in the deflectometry technique. Reconstructed curvature fields, applied load and plate dimensions were input in virtual fields methods for material parameter identification, assuming Kirchhoff–Love classical plate theory. Several virtual fields and load cases were analysed to address the identifiability of the method. The values of the orthotropic elastic constants obtained from the proposed approach were found in good agreement with regard to reference ones for the same species and determined from classical tensile, compression and shear mechanical tests.

An optical experimental procedure for evaluating the J-Integral from full-field displacement fields under dynamic loading is proposed in this work. The methodology is applied to measure the J-integral in the dynamic compressive loading of fiber-reinforced composites and to calculate the dynamic fracture toughness associated with the propagation of a kink-band. A modified J-Integral that considers inertia effects is calculated over the full-field measurements obtained by digital image correlation, for double edge-notched specimen of IM7-8552 laminates dynamically loaded in a split-Hopkinson pressure bar (SHPB). A sensibility study is conducted to address the influence of the speckle parameters. The results show good agreement with experimental observations obtained by using a different data reduction method, suggesting the existence of a rising R-curve for the studied material under dynamic loading. Furthermore, it was noticed that the inertia effect can be negligible, indicating a state of dynamic equilibrium in which quasi-static approaches may comfortably be used.

Despite extensive research regarding metal cutting simulation, the current industrial practice very often relies on empirical data when it comes to tool design. In order accurately simulate the cutting process it is not only important to have robust numerical models that closely portray the phenomenon, but also to properly characterize the material taking into account the cutting conditions. The goal of this investigation focuses on the mechanical characterization of the cast aluminum alloy AlSi9Cu3 by conducting both compression and fracture tests. Due to its very good castability, machinability, and attractive mechanical properties, this alloy is widely used in casting industry for the manufacture of automotive components, among others. Besides the experimental characterization, a numerical methodology is proposed for the modeling of the cast alloy, making use of the Johnson–Cook constitutive material model, in Abaqus/CAE. The material model is calibrated based on compression tests at multiple conditions (quasi-static, incremental dynamic and high temperatures). The identified model is then validated by simulation of the ductile fracture tests of notched specimens. The obtained numerical results were consistent with the experimentally obtained, contributing to the validity of the presented characterization technique.