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Journal Article
Pereira, F. A. M., M. F. S. F. de Moura, N. Dourado, J. J. L. Morais, J. Xavier, and M. I. R. Dias. "Direct and inverse methods applied to the determination of mode I cohesive law of bovine cortical bone using the DCB test." International Journal of Solids and Structures. 128 (2017): 210-220. AbstractWebsite

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.

Xavier, J., M. Oliveira, P. Monteiro, J. J. L. Morais, and M. F. S. F. Moura. "Direct Evaluation of Cohesive Law in Mode I of Pinus pinaster by Digital Image Correlation." Experimental Mechanics. 54 (2014): 829-840. AbstractWebsite
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Pinto, V. C., T. Ramos, A. S. F. Alves, J. Xavier, P. J. Tavares, P. M. G. P. Moreira, and R. M. Guedes. "Dispersion and failure analysis of PLA, PLA/GNP and PLA/CNT-COOH biodegradable nanocomposites by SEM and DIC inspection." Engineering Failure Analysis. 71 (2017): 63-71. AbstractWebsite

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.

Morais, J. J. L., M. F. S. F. de Moura, F. A. M. Pereira, J. Xavier, N. Dourado, M. I. R. Dias, and J. M. T. Azevedo. "The double cantilever beam test applied to mode I fracture characterization of cortical bone tissue." Journal of the Mechanical Behavior of Biomedical Materials. 3 (2010): 446-453. Abstract

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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.

Arteiro, A., G. Catalanotti, J. Xavier, P. Linde, and P. P. Camanho. "Effect of tow thickness on the structural response of aerospace-grade spread-tow fabrics." Composite Structures. 179 (2017): 208-223. AbstractWebsite

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.

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.

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.

Ghiassi, B., J. Xavier, D. V. Oliveira, A. Kwiecien, P. B. Lourenço, and B. Zajac. "Evaluation of the bond performance in FRP-brick components re-bonded after initial delamination." Composite Structures. 123 (2015): 271-281. Abstract

Abstract The bond behavior between Fiber Reinforced Polymers (FRPs) and masonry substrates has been the subject of many studies during the last years. Recent accelerated aging tests have shown that bond degradation and \{FRP\} delamination are likely to occur in FRP-strengthened masonry components under hygrothermal conditions. While an investigation on the possible methods to improve the durability of these systems is necessary, the applicability of different bond repair methods should also be studied. This paper aims at investigating the debonding mechanisms after repairing delaminated FRP-strengthened masonry components. FRP-strengthened brick specimens, after being delaminated, are repaired with two different adhesives: a conventional epoxy resin and a highly flexible polymer. The latter is used as an innovative adhesive in structural applications. The bond behavior in the repaired specimens is investigated by performing single-lap shear bond tests. Digital image correlation (DIC) is used for deeper investigation of the surface deformation and strains development. The effectiveness of the repair methods is discussed and compared with the strengthened specimens.

Majano-Majano, Almudena, Antonio José Lara-Bocanegra, José Xavier, and José Morais. "Experimental Evaluation of Mode II fracture Properties of Eucalyptus globulus L." Materials. 13 (2020). AbstractWebsite

Eucalyptus globulus Labill is a hardwood species of broad growth in temperate climates, which is receiving increasing interest for structural applications due to its high mechanical properties. Knowing the fracture behaviour is crucial to predict, through finite element models, the load carrying capacity of engineering designs with possibility of brittle failures such as elements with holes, notches, or certain types of joints. This behaviour can be adequately modelled on a macroscopic scale by the constitutive cohesive law. A direct identification of the cohesive law of Eucalyptus globulus L. in Mode II was performed by combining end-notched flexure (ENF) tests with digital image correlation (DIC) for radial-longitudinal crack propagation system. The critical strain energy release for this fracture mode, which represents the material toughness to crack-growth, was determined by applying the Compliance Based Beam Method (CBBM) as data reduction scheme and resulted in a mean value of 1.54 N/mm.

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.

Silva, TEF, S. Gain, D. Pinto, A. M. P. de Jesus, J. Xavier, A. Reis, and P. A. R. Rosa. "Fracture characterization of a cast aluminum alloy aiming machining simulation." Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications. 233 (2019): 402-412. AbstractWebsite

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.

Silva, F., M. de Moura, N. Dourado, J. Xavier, F. Pereira, J. Morais, M. Dias, P. Lourenço, and F. Judas. "Fracture Characterization of Human Cortical Bone Under Mode I Loading." Journal of Biomechanical Engineering. 137 (2015): 121004. AbstractWebsite

A miniaturized version of the double cantilever beam (DCB) test is used to determine the fracture energy in human cortical bone under pure mode I loading. An equivalent crack length based data-reduction scheme is used with remarkable advantages relative to classical methods. Digital image correlation (DIC) technique is employed to determine crack opening displacement at the crack tip being correlated with the evolution of fracture energy. A method is presented to obtain the cohesive law (trapezoidal bilinear softening) mimicking the mechanical behavior observed in bone. Cohesive zone modeling (CZM) (finite-element method) was performed to validate the procedure showing excellent agreement.

Silva, F. G. A., M. F. S. F. de Moura, N. Dourado, J. Xavier, F. A. M. Pereira, J. J. L. Morais, M. I. R. Dias, P. J. Lourenço, and F. M. Judas. "Fracture characterization of human cortical bone under mode II loading using the end-notched flexure test." Medical {&} Biological Engineering {&} Computing. 55 (2017): 1249-1260. AbstractWebsite

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.

Kuhn, P., G. Catalanotti, J. Xavier, P. P. Camanho, and H. Koerber. "Fracture toughness and crack resistance curves for fiber compressive failure mode in polymer composites under high rate loading." Composite Structures. 182 (2017): 164-175. AbstractWebsite

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.

Koerber, H., J. Xavier, P. P. Camanho, Y. E. Essa, and Martín F. de la Escalera. "High strain rate behaviour of 5-harness-satin weave fabric carbon-epoxy composite under compression and combined compression-shear loading." International Journal of Solids and Structures. 54 (2015): 172-182. AbstractWebsite

Abstract The strain rate dependent mechanical behaviour was studied for the common out-of-autoclave aerospace textile composite 5-harness-satin carbon�epoxy. End-loaded 15 � , 30 � and 45 � off-axis and 90 � compression tests were carried out at three different strain rate levels ( 4 � 10 - 4 s - 1 , 200 s - 1 and 1000 s - 1 ) to determine the effect of strain rate for transverse compression and combined transverse compression/in-plane shear loading. The dynamic tests were carried out on a split-Hopkinson pressure bar, where high speed photography and digital image correlation allowed a detailed study of the specimen deformation and failure process. Quasi-static reference tests were carried out on an electro-mechanical test machine using the same specimen type and a static \{DIC\} system. Pronounced strain rate effects on the axial stress�strain response were observed for all specimen types. Failure envelopes for the combined s 22 c - t 12 stress state were derived from the experimental data and compared with the maximum stress criterion, which appears well suited to approximate the experimental failure envelope at all strain rate levels. It was observed that the failure envelope was simply scaled up with increasing strain rate, while the overall shape was found to be strain rate independent.

Koerber, H., J. Xavier, P. P. Camanho, Y. E. Essa, and Martín F. de la Escalera. "High strain rate behaviour of 5-harness-satin weave fabric carbon–epoxy composite under compression and combined compression–shear loading." International Journal of Solids and Structures. 54 (2015): 172-182. AbstractWebsite

Abstract The strain rate dependent mechanical behaviour was studied for the common out-of-autoclave aerospace textile composite 5-harness-satin carbon–epoxy. End-loaded 15 ° , 30 ° and 45 ° off-axis and 90 ° compression tests were carried out at three different strain rate levels ( 4 × 10 - 4 s - 1 , 200 s - 1 and 1000 s - 1 ) to determine the effect of strain rate for transverse compression and combined transverse compression/in-plane shear loading. The dynamic tests were carried out on a split-Hopkinson pressure bar, where high speed photography and digital image correlation allowed a detailed study of the specimen deformation and failure process. Quasi-static reference tests were carried out on an electro-mechanical test machine using the same specimen type and a static \{DIC\} system. Pronounced strain rate effects on the axial stress–strain response were observed for all specimen types. Failure envelopes for the combined σ 22 c - τ 12 stress state were derived from the experimental data and compared with the maximum stress criterion, which appears well suited to approximate the experimental failure envelope at all strain rate levels. It was observed that the failure envelope was simply scaled up with increasing strain rate, while the overall shape was found to be strain rate independent.

Catalanotti, G., P. Kuhn, J. Xavier, and H. Koerber. "High strain rate characterisation of intralaminar fracture toughness of GFRPs for longitudinal tension and compression failure." Composite Structures. 240 (2020): 112068. AbstractWebsite

The elastic parameters, strengths, and intralaminar fracture toughness are determined for an E-Glass polymer composite material system, statically and at high strain rate, adapting methodologies previously developed by the authors for different carbon composites. Dynamic experiments are conducted using tension and compression Split-Hopkinson Bars (SHBs). A unique set of experimental parameters is obtained, and reported together with the experimental set-up, in order to ensure reproducibility. While in-plane elastic and strength properties were obtained by testing one specimen geometry, intralaminar fracture properties required the testing of different sized notched specimens with scaled geometries. This allowed the use of the size-effect method for the determination of the dynamic R-curve. When comparing these results with those previously obtained for a carbon/epoxy material system, it is observed that the dynamic fracture toughness exhibits a much more significant increase in both tension and compression. The obtained results permit the identification of the softening law at different strain rates, allowing its use in any analytical or numerical strength predictive method.

Gomes, F., J. Xavier, and H. Koerber. "High strain rate compressive behaviour of wood on the transverse plane." Procedia Structural Integrity. 17 (2019): 900-905. AbstractWebsite

The high strain rate compressive behaviour of Pinus pinaster Ait. wood along the radial and tangential material axes was addressed in this work. Both quasi-static and dynamic tests were considered for comparation purposes. The quasi-static compression tests were performed on rectangular prismatic specimens along the radial and tangential directions coupled with digital image correlation. The high strain rate tests were carried out using a classical split-Hopkinson pressure bar coupled with a high-speed imaging system allowing independent kinematic measurements through digital image correlation. From these tests and material symmetry orientations, the constitutive curves were determined from which the Young modulus, Poisson’s ratio and yield stress were evaluated and compared over the two different regimes over the strain rate spectrum. The mechanical properties observed for this species under quasi-static compression loading agree with reference values. A qualitative comparison between quasi-static and high strain rate regimes reveals a significant increase of some mechanical properties by increasing the strain rate. Quantitatively, by comparing mean values at the two strain rates, it was found that, in the radial direction, the modulus of elasticity increased by 6.3%, the yield stress showed an increase of 130.3% and the Poisson’s ratio is slightly higher by 3.0%. Furthermore, in the tangential direction, it was found that the modulus of elasticity increased by 21.9% while the value of the yield stress showed an increase of 111.8%, and finally the Poisson‘s ratio presented a reduction of 24.3%.

Pereira, B., J. Xavier, F. Pereira, and J. Morais. "Identification of transverse elastic properties of the diaphysis of cortical bone." Journal of Mechanical Engineering and Biomechanics. 2 (2018): 50-55. AbstractWebsite
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Kuhn, P., H. Koerber, G. Catalanotti, and J. Xavier. "Intralaminar fracture toughness of UD glass fiber composite under high rate fiber tension and fiber compression loading." EPJ Web Conferences. 183 (2018): 02018. AbstractWebsite

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Arteiro, A., G. Catalanotti, J. Xavier, and P. P. Camanho. "Large damage capability of non-crimp fabric thin-ply laminates." Composites Part A: Applied Science and Manufacturing. 63 (2014): 110-122. AbstractWebsite
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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, 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
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