1,721,129 research outputs found
Concrete wall subjected to fragment impacts - Numerical analyses of perforation and scabbing
No full textConcrete subjected to fragment impact is mainly related to protective structures such as civil defence shelters where the main threat arise from explosions caused by military weapons. For analysing impacts on concrete, material models are used in numerical analyses to simulate the reality. Such a model is the RHT model which is used to describe the material behaviour of concrete in the software Autodyn. The model has been evaluated for constructions subjected to compression and the result were satisfying. However, the function of the model is not satisfying for constructions subjected to tensile stresses. Therefore there exists a need for deepening the knowledge about penetration and scabbing of concrete subjected to fragment impact, about the parameters involved, and how the impact should be modelled in a accurate way. The intentions are to give recommendations on how scabbing and perforation by fragment impact will be modelled in Autodyn. The numerical calculations, based on the RHT model, were verified by results of already performed full-scale tests. The model was calibrated from information gained from a parameter study of the most important parameters such as tensile strength, softening energies, strain rate and shear strength. It was found out that by a calibration of the parameters conduction strain rate and shear resistance, in the material model, a result was obtained that was in accordance with the full-scale tests at the backside of the specimen. On the front side the result was not that convincing but showed tendencies that with further evaluation of the material model it could reach satisfaction
Modelling of Concrete Subjected to Cyclic Loading [Elektronisk resurs]
No full textThe infrastructure of today depends heavily on concrete structures. Most of these structuresare subjected to repeated loads, known as fatigue or cyclic loads: the loads weakenthe structure. As this phenomenon is of high cost to society, a deeper understanding ofthe deterioration process of cyclic loading would be beneficial. The aim of this thesis isto add to the knowledge of the deterioration phenomenon and to develop models that candescribe the response of concrete subjected to cyclic loading.In analyses of structures of today, finite element modelling is gaining ground and is beingused more frequently in research and structural design. The investigations here use onlythe finite element method.Many structures incorporate details that are subjected to complicated loading, which resultsin complex crack patterns. A suitable tool for describing these crack patterns isanisotropic damage material models. However, anisotropic models are difficult to implementand are often computationally inefficient. One of the investigations in this thesisaims to find out what makes the anisotropic formulation suitable for complex crack patterns.This is done by implementing a model which can control the amount of couplingbetween volumetric- and deviatoric strains. It was found that this coupling is essentialfor describing complex crack patterns.To deepen the understanding of concrete subjected to cyclic loading, the phenomenonwas investigated on the meso-scale level. An interface model was developed and appliedto a three phase representation of concrete that incorporates: aggregates, cement paste,and interfacial zones around the aggregate. The model in itself does not yield cyclicbehaviour, i.e. no hysteresis loops were generated at the constitutive level. Instead, thecyclic response was generated by the meso-structure. It was found that the interfacialtransition zones are crucial in amount and strength.Concrete subjected to cyclic loading was also investigated on the macro-level, with theambition to describe the response of concrete structures subjected to cyclic loading. Twoinvestigations were made: one aims to describe cyclic response in tension and the otheraims to cover tension and the transition to reasonable high states of compression. Theinvestigations are based on the theory of plasticity and damage mechanics, which arecombined in both a serial and a parallel fashion. In the serial configuration the nominalstress is computed by adding the damage to the effective stress; for the parallel configuration,the damage stress and the effective stress are evaluated separately for the samestrain and then added to yield the nominal stress. Furthermore, both models use twoyield surfaces to describe the hysteresis loops. The result of the analyses show an overallagreement with experimental observations
Implementation of constitutive model for concrete subjected to tensile and compressive loading
No full textDeveloping a model consist not only of finding constitutive relations and adequate equations that represent an identified problem. The equations are also implemented in for example a FE environment to be tested and verified. The implementation should be in a readable way, so that others can use it or/and extend it for other purposes, but also so that the writer later can find what was done and how it was solved. This report serves as a help to understand and read the model described in Rempling (2006).To increase the understanding of concrete fatigue and improve the methods of anal-ysis, the constitutive model was developed to describe concrete subjected to tensilecyclic loading. The model uses the concept of bounding surface to describe the hys-teresis loops observed in cyclic loading. An energy balance law is used to describe theenergy dissipation by hysteresis loops. An experiment with a concrete bar subjected todirect cyclic tensional loading was simulated. The general response was described withgood agreement
Modelling of Concrete Subjected to Cyclic Loading
Full text linkThe infrastructure of today depends heavily on concrete structures. Most of these structuresare subjected to repeated loads, known as fatigue or cyclic loads: the loads weakenthe structure. As this phenomenon is of high cost to society, a deeper understanding ofthe deterioration process of cyclic loading would be beneficial. The aim of this thesis isto add to the knowledge of the deterioration phenomenon and to develop models that candescribe the response of concrete subjected to cyclic loading.In analyses of structures of today, finite element modelling is gaining ground and is beingused more frequently in research and structural design. The investigations here use onlythe finite element method.Many structures incorporate details that are subjected to complicated loading, which resultsin complex crack patterns. A suitable tool for describing these crack patterns isanisotropic damage material models. However, anisotropic models are difficult to implementand are often computationally inefficient. One of the investigations in this thesisaims to find out what makes the anisotropic formulation suitable for complex crack patterns.This is done by implementing a model which can control the amount of couplingbetween volumetric- and deviatoric strains. It was found that this coupling is essentialfor describing complex crack patterns.To deepen the understanding of concrete subjected to cyclic loading, the phenomenonwas investigated on the meso-scale level. An interface model was developed and appliedto a three phase representation of concrete that incorporates: aggregates, cement paste,and interfacial zones around the aggregate. The model in itself does not yield cyclicbehaviour, i.e. no hysteresis loops were generated at the constitutive level. Instead, thecyclic response was generated by the meso-structure. It was found that the interfacialtransition zones are crucial in amount and strength.Concrete subjected to cyclic loading was also investigated on the macro-level, with theambition to describe the response of concrete structures subjected to cyclic loading. Twoinvestigations were made: one aims to describe cyclic response in tension and the otheraims to cover tension and the transition to reasonable high states of compression. Theinvestigations are based on the theory of plasticity and damage mechanics, which arecombined in both a serial and a parallel fashion. In the serial configuration the nominalstress is computed by adding the damage to the effective stress; for the parallel configuration,the damage stress and the effective stress are evaluated separately for the samestrain and then added to yield the nominal stress. Furthermore, both models use twoyield surfaces to describe the hysteresis loops. The result of the analyses show an overallagreement with experimental observations
Betongvägg utsatt för splitterbelastning - Numeriska analyser av genomträngning och utstötning
No full textConcrete subjected to fragment impact is mainly related to protective structures such as civil defence shelters where the main threat arise from explosions caused by military weapons.
For analysing impacts on concrete, material models are used in numerical analyses to simulate the reality. Such a model is the RHT model which is used to describe the material behaviour of concrete in the software Autodyn. The model has been evaluated for constructions subjected to compression and the result were satisfying. However, the function of the model is not satisfying for constructions subjected to tensile stresses. Therefore there exists a need for deepening the knowledge about penetration and scabbing of concrete subjected to fragment impact, about the parameters involved, and how the impact should be modelled in a accurate way.
The intentions are to give recommendations on how scabbing and perforation by fragment impact will be modelled in Autodyn. The numerical calculations, based on the RHT model, were verified by results of already performed full-scale tests. The model was calibrated from information gained from a parameter study of the most important parameters such as tensile strength, softening energies, strain rate and shear strength.
It was found out that by a calibration of the parameters conduction strain rate and shear resistance, in the material model, a result was obtained that was in accordance with the full-scale tests at the backside of the specimen. On the front side the result was not that convincing but showed tendencies that with further evaluation of the material model it could reach satisfaction
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Implementation of constitutive model for concrete subjected to tensile and compressive loading
No full textDeveloping a model consist not only of finding constitutive relations and adequate equations that represent an identified problem. The equations are also implemented in for example a FE environment to be tested and verified. The implementation should be in a readable way, so that others can use it or/and extend it for other purposes, but also so that the writer later can find what was done and how it was solved. This report serves as a help to understand and read the model described in Rempling (2006).To increase the understanding of concrete fatigue and improve the methods of anal-ysis, the constitutive model was developed to describe concrete subjected to tensilecyclic loading. The model uses the concept of bounding surface to describe the hys-teresis loops observed in cyclic loading. An energy balance law is used to describe theenergy dissipation by hysteresis loops. An experiment with a concrete bar subjected todirect cyclic tensional loading was simulated. The general response was described withgood agreement
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