High strain rate identification of Pinus pinaster Ait. on the transverse plane by the image-based inertial impact test

• Autor(a) principal: Cunha, Filipa Alexandra Grifo
• Data de Publicação: 2019
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10362/89176
• topic: Image-based inertial impact test; Grid method; High strain rate testing; Virtual Fields Method; Wood; Dynamic behaviour; Domínio/Área Científica::Engenharia e Tecnologia::Engenharia Mecânica

Computer-aided engineering systems typically rely on constitutive models and material parameters to describe the mechanical behaviour of material over a spectrum of high strain rate regimes. The constitutive parameters are to be determined experimentally by suitable test methods. At high strain rate regimes, a few test methods have been proposed, with advantages and drawback. More recently, an image-based inertial impact (IBII) test was proposed to overcome limitation of quasi-static stress equilibrium (neglecting inertia effects) and 1D wave propagation theory (neglecting dispersion effects) of the classical split-Hopkinson pressure bar (SHPB) test. This dynamic test rely on full-field deformation measurements provided by an optical technique and ultra high speed imaging to resolve both spatial and temporal resolutions. Wood and wood-based products are gaining momentum due to policies of sustainability and green economy. The extension of using such materials under a higher strain rate regimes would be therefore of interest in engineering applications. This work aims the identification of linear elastic constitutive parameters of Pinus pinaster Ait. (maritime pine) wood subjected to high strain rates, using the image-based inertial impact test. In this dynamic test, images of specimen deformation are recorded by means of an ultra high speed camera. The recorded images are processed by the grid method yielding displacement fields over the whole external surface of the specimen. Through the displacements field, the strain and acceleration fields are reconstructed, respectively, by means of spatial and temporal derivations. By using the virtual fields method (VFM), it is possible to identify the constitutive parameters of wood. This characterisation is performed without measuring external forces applied to the specimen under study by selecting VFM admissible virtual fields. This is performed using acceleration fields as a load cell, thereby taking advantage of the non-negligible inertial forces introduced during the dynamic test. In this work, two experimental analyses are carried out on Pinus pinaster Ait. species by using specimens oriented on the RT (Radial-Tangential) and TR (Tangential-Radial) planes. Data from the experimental tests are further processed by the VFM to identify properties such as Young’s modulus (E) and Poisson’s coefficient (n) of the material under study. In this analysis an approximation was performed, having been considered an isotropic constitutive model in the VFM, since the RT and TR planes have a low anisotropy ratio. In this study, the stiffness components (Qxx and Qxy) of the material were determined, with average values of 2.23 GPa and 0.80 GPa, respectively, for the RT specimens. Similarly, the average values for the stiffness components (Qxx and Qxy) are 0.98 GPa and 0.80 GPa for the TR specimens. These dynamic elastic parameters are of the same order of magnitude of quasi-static references values. Therefore, it may be concluded that high strain rate loading has a non significant influence on the elastic transverse properties of for the Pinus pinaster Ait. species. Taking into account the advent of digital technology, the image-based inertial impact test may become a conventional test method to study the materials properties of structural engineering when subjected to high strain rate loads.