Experimental evaluation of mode II fracture properties of Eucalyptus globulus L.

research projects BIA 2015-64491-P. UIDB/00667/2020 (UNIDEMI).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.publishersversionpublishe

Evaluation of R-curves and cohesive law in mode I of European beech

This work addresses the determination of the cohesive laws in Mode I and tangential–longitudinal (TL) crack propagation system of Fagus Sylvatica L. This species is one of the ever-growing and most widely used hardwood species in Europe for engineered timber products. Double Cantilever Beam (DCB) tests are performed. The strain energy release rate ( G I) is derived from the R-curves by applying the Compliance-Based Beam Method (CBBM), which has the advantage of not requiring the measurement of the crack length during propagation, but only the global load–displacement curves. The cohesive law of the material is determined from the relationship between G I, and the crack tip opening displacement (CTOD) monitored for each specimen using Digital Image Correlation (DIC). Numerical finite element models are developed by implementing the average cohesive law through Cohesive Zone Models (CZM). An average G I value of 0.46 kJ / m 2 is obtained for this species. The numerical load–displacement curves are consistent with the experimental results, which demonstrates the suitability of the method for the identification of the cohesive laws in beech. The fracture properties obtained are essential in the development of advanced and reliable numerical models in timber engineering design using this species.publishersversionpublishe

Shear traction‐separation laws of European beech under mode II loading by 3D digital image correlation

Funding Information: Part of the work was undertaken during a short-term scientific stay by the first author at the Faculty of Engineering (University of Porto) in 2021, with the financial support provided by Programa Propio de I+D+i 2021 de la Universidad Politécnica de Madrid. The work is part of the R&D&I Project PID2020-112954RA-I00 funded by MCIN/AEI/10.13039/501100011033. The authors gratefully acknowledge also Fundação para a Ciência e a Tecnologia (FCT-MCTES) for the financial support of the Laboratório Associado de Energia, Transportes e Aeronáutica (LAETA) by the project MCIN/AEI/10.13039/501100011033 and the Research and Development Unit for Mechanical and Industrial Engineering (UNIDEMI) by the project UIDB/00667/2020. Publisher Copyright: © 2022, The Author(s).An experimental and numerical study on mode II fracture behaviour of European beech (Fagus sylvatica L.) in the RL and TL crack propagation systems is performed. It is a hardwood species that has attracted increasing interest for structural use in Europe in recent years. Three-point end notched flexure tests are performed. The R-curves of both crack propagation systems are obtained, from which the critical strain energy release rate (GIIc) is derived by applying the compliance-based beam method. This data reduction scheme avoids crack length monitoring during its propagation, which is an advantage in wood. Using a direct method, the shear traction‐separation laws in mode II loading are determined. Full field displacements around the crack tip are monitored by 3D digital image correlation technique, and the crack tip shear displacements are analysed. The proposed method is numerically validated by finite element analysis. Cohesive zone models are developed implementing a shear traction–separation law with exponential damage evolution zone and the average value of the experimental elastic and fracture properties. The numerical results for the different properties including upper and lower limits represent well the experimental data.publishersversionepub_ahead_of_prin

Measuring the cohesive law in mode I loading of Eucalyptus globulus

This work was undertaken during a short-term scientific stay by the first and second authors at the University of Tras-os-Montes e Alto Douro, within the framework of the research project BIA 2015-64491-P Analysis of the stress relaxation in curved members and new joints solutions for timber Gridshells made out of Eucalyptus globulus, co-financed by the Ministry of Economy and Competitiveness of the Spanish Government and ERDF funds. The short-term stay was supported by a STSM Grant from COST Action FP1402 Basis of Structural Timber Design-from research to standards, ref. COST-STSM-ECOST-STSM-FP1402-091116-080058. This work is also supported by: European Investment Funds by FEDER/COMPETE/POCI-Operacional Competitiveness and Internacionalization Programme, under Project POCI-01-0145-FEDER-006958 and National Funds by FCT-Portuguese Foundation for Science and Technology, under the project UID/AGR/04033/2013.Assessing wood fracture behavior is essential in the design of structural timber elements and connections. This is particularly the case for connections with the possibility of brittle splitting failure. The numerical cohesive zone models that are used to simulate the fracture behavior of wood make it necessary to assume a cohesive law of the material that relates cohesive tractions and crack opening displacements ahead of the crack tip. This work addresses the determination of the fracture cohesive laws of Eucalyptus globulus, a hardwood species with great potential in timber engineering. This study centres on Mode I fracture loading for RL and TL crack propagation systems using Double Cantilever Beam tests. The Compliance-Based Beam Method is applied as the data reduction scheme in order to obtain the strain energy release rate from the load-displacement curves. The cohesive laws are determined by differentiating the relationship between strain energy release rate and crack tip opening displacement. The latter is measured by the digital image correlation technique. High strain energy release rates were obtained for this species, with no big differences between crack propagation systems. The difference between the crack systems is somewhat more pronounced in terms of maximum stress that determines the respective cohesive laws.publishersversionpublishe

Determining mode I cohesive law of Pinus pinaster by coupling double cantilever beam test with digital image correlation

The direct identification of the cohesive law in pure mode I of Pinus pinaster is addressed. The approach couples the double cantilever beam (DCB) test with digital image correlation (DIC). Wooden beam specimens loaded in the radial-longitudinal (RL) fracture propagation system are used. The strain energy release rate in mode I (GI ) is uniquely determined from the load-displacement ( P ?? ) curve by means of the compliance-based beam method (CBBM). This method relies on the concept of equivalent elastic crack length ( eq a ) and therefore does not require the monitoring of crack propagation during test. The crack tip opening displacement in mode I ? ? I w is determined from the displacement field at the initial crack tip. The cohesive law in mode I I I (? ? w ) is then identified by numerical differentiation of the I I G ? w relationship. Moreover, the proposed procedure is validated by finite element analyses including cohesive zone modelling. It is concluded that the proposed data reduction scheme is adequate for assessing the cohesive law in pure mode I of P. pinaster

Direct and inverse cohesive law identification of hardwood bonded joints with 1C-PUR adhesive using DCB test

Funding Information: The work is part of the R&D&I Project PID2020-112954RA-I00 funded by MCIN/AEI/10.13039/501100011033 . The authors acknowledge the Portuguese Foundation for Science and Technology (FCT) for the financial support associated to “Laboratório Associado de Energia, Transportes e Aeronáutica” (LAETA) by the project UID/EEA/04436/2019, and UNIDEMI by the project UIDB/00667/2020. Publisher Copyright: © 2023 The AuthorsFracture characterisation under mode I loading of Eucalyptus globulus Labill. bonded joints with one-component polyurethane adhesive (1C-PUR) is addressed in this work. The objective is to estimate the cohesive law representative of the fracture behaviour of these joints. A direct and two inverse procedures were employed to determine the softening laws. The direct method is based on local measurement of crack tip displacements using digital image correlation in the course of Double Cantilever Beam (DCB) tests, while the inverse methods rely on load–displacement data and on load-crack tip opening displacement relations by finite element modelling. It was verified that consistent results can be obtained from the three methodologies leading to the conclusion that the classical inverse procedure is the most appealing one owing to its simplicity.publishersversionpublishe

Crack Propagation Analysis of Synthetic vs. Steel vs. Hybrid Fibre-Reinforced Concrete Beams Using Digital Image Correlation Technique

Improvement in fracture behaviour of fibre-reinforced concrete (FRC) due to the inclusion of various types and combinations of fibres is widely reported. The fracture behaviour of FRC needs to be fully understood for the optimum use of these fibres in structural elements. Fracture behaviours of synthetic fibre-reinforced concrete (SynFRC), hybrid fibre-reinforced concrete (HFRC) and steel fibre-reinforced concrete (SFRC) are investigated in this study using digital image correlation (DIC) technique. This work focuses on improvement in the structural performance of FRC through a comprehensive study of the change in the crack length, crack opening and fracture process zone (FPZ) due to different fibres addition and their combinations. Three distinct fibre dosages of 0.50%, 0.75%, and 1.00%, of macro-polyolefin fibres, hooked end steel fibres and their hybrid combination are regarded as research parameters. Test outcomes indicate that HFRC offers higher post-cracking resistance when compared to SynFRC. SFRC showcases superior fracture performance than that of HFRC and SynFRC. Full-field strain measurements from DIC are used to measure the crack openings at different load levels during the fracture tests. Results of DIC analysis show good agreement with experimental measurements. Continuous monitoring of strain contours using DIC reveals the effective engagement of fibres along the depth at higher dosages for HFRC when compared to that of SynFRC. Also, HFRC had longer cracks than SFRC at a particular load

Fracture Characterisation of bonded joints between cortical bone tissue and bone cement

The process of evolution has led to the creation of fantastic materials. Bone is one of these materials and has a complex, anisotropic, hierarchical and heterogeneous microstructure, characterised by an excellent mechanical performance. However, as it is an almost fragile material, it often ends up fracturing. These fractures decrease the patient's quality of life and entail high costs for the health system. In order to deal with these fractures, the BoFraPla project arose, which aims to develop a fibrous system for fixing bone fractures. Therefore, in the scope of the activities of this project, the present dissertation emerges, proposing to mechanically characterise the bond between the bone and bone cement. From a thorough literature review, it was found that although the bone cement has been used for more than fifty years, there are few references to its mechanical characterisation and none were found that have made a clear characterisation of the fracture process between the bone and the cement. Hence, in this report, a pure mode I characterisation (tension) through a Double Cantilever Beam (DCB) test and another pure mode II characterisation (shear) through an End-Notched Flexure (ENF) test are presented. In order to overcome the difficulties in monitoring the crack length during the laboratory tests, a method based on the equivalent crack length was used. With this method, the resistance curve was drawn for each specimen and, in most of them, the existence of thresholds was remarkable, which allowed a correct measurement of the fracture energy for each test. In order to validate the results obtained, numerical simulations were performed with cohesive models. Through the results, it was possible to determine that the chosen tests, the defined dimensions and the adopted procedure, can be used to characterise the fracture of these two materials in the studied loading modes.O processo de evolução natural levou à existência de materiais fantásticos. O osso é um destes materiais e apresenta uma microestrutura complexa, anisotrópica, hierárquica e heterogénea, caracterizada por um excelente desempenho mecânico. Contudo, como se trata de um material quase frágil, inúmeras vezes acaba por fraturar. Estas fraturas diminuem a qualidade de vida do paciente e acarretam elevados custos para o sistema de saúde. Com vista ao tratamento destas fraturas, surgiu o projeto BoFraPla, que se propõe a desenvolver um sistema fibroso de fixação de fraturas ósseas. Assim, no âmbito das atividades deste projeto, surge a presente dissertação que se propõe a caracterizar mecanicamente a ligação entre o osso e um cimento ósseo. A partir de uma profunda revisão bibliográfica apurou-se que, apesar do cimento ósseo ser utilizado há mais de cinquenta anos, existem poucas referências à sua caracterização mecânica, não sendo encontrada nenhuma que fizesse uma clara caracterização do processo de fratura entre o osso e o cimento. Assim, neste relatório é apresentada uma caracterização em puro modo I (tensão), através de um ensaio Double Cantilever Beam (DCB) e outra caracterização em puro modo II (cisalhamento), através de um ensaio End-Notched Flexure (ENF). Para colmatar as dificuldades de monitorização do comprimento de fenda durante os ensaios laboratoriais, recorreu-se a um método baseado no comprimento de fenda equivalente. Com este método foi traçada a curva de resistência para cada provete, sendo notória a existência de patamares na sua maioria, o que permite uma correta medição da energia de fratura para cada ensaio. Com vista a validar os resultados obtidos, foram realizadas simulações numéricas com modelos coesivos. Através dos resultados, foi possível apurar que os ensaios escolhidos, as dimensões definidas e o procedimento adotado, podem ser utilizados para caracterizar à fratura entre estes dois materiais nos modos de carregamento estudados