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Strain hardening ultra high performance fibre reinforced cementitious composites (UH-PFRCC) exhibit increased strength, ductility, and energy absorption capacity when compared to their quasi-brittle, unreinforced counterparts. A mesoscale finite element model can depict the underlying causes for the structural response of UHPFRC and thus help to optimize the fibre content, the fibre dimensions, and the fibre orientation.
Furthermore, it facilitates the investigation of strain rate effects in UHPFRCC under dynamic loading. The mesoscale model can either be used directly or as a representative volume element for a macroscale model.
This work proposes a two-dimensional and a three- dimensional mesoscale finite element model to simulate the structural response of strain hardening UHPFRCC. The mesoscale model employs an implicit gradient enhanced damage model, proposed by Peerlings et al., for the cement matrix and a local bond stress-slip model, proposed by Elige-hausen, Popov, and Bertero, for the bond between the cement matrix and the steel fibres. The steel fibres are modeled discretely as one-dimensional truss elements that are coupled to the cement matrix via bond elements. The implementation of hooked end fibres is realized in the constitutive equations of the bond elements.
The tensile stress-strain response of UHPFRCC is a consequence of local matrix cracking and bond failure. Both phenomena can be depicted when modeling the cement matrix, the steel fibres, and the fibre-to-matrix bond explicitly. In this work, the parameters for the constitutive equations of each constituent are determined through uniaxial Tension tests, bending tests, and fibre pullout tests. Additionally, UHPFRCC specimens are simulated with the same parameters and compared to experimental results.
Luftultraschall wird immer häufiger für die Prüfung von anisotropen Faserverbunden eingesetzt. Meistens wir in Durchschallung geprüft, aber bei der einseitigen Zugänglichkeit muss eine Art Tandemtechnik mit Anregung von geführten Wellen angewendet werden. Bei beiden Techniken ist der Signal-Rausch-Abstand wegen der niedrigen akustischen Impedanz der Luft deutlich kleiner als bei einer typischen Prüfung in Kontakttechnik. Bei der Tandemtechnik wird das Signal zusätzlich durch eine größere Divergenz und Dämpfung geschwächt. Deshalb ist es wichtig, die Anregung, Ausbreitung und Detektion von geführten Wellen in Faserverbunden zu verstehen. In diesem Beitrag wird ein Finite-Elemente-Modell für die Simulation von luftgekoppelter Anregung und Ausbreitung von geführten Wellen in Faserverbunden vorgestellt und mit Messungen verglichen. Insbesondere wurde die Wellenausbreitung und Modenkonversion in der Nähe von Schäden in mehreren Parameterstudien untersucht.
Strain hardening ultra high performance fiber reinforced cementitious composites (UHPFRCC) exhibit increased strength, ductility, and energy absorption capacity when compared to their quasibrittle, unreinforced counterparts. A mesoscale finite element model can depict the underlying causes for the structural response of UHPFRCC and thus help to optimize the fiber content, the fiber dimensions, and the fiber orientation. The mesoscale model can either be used directly or as a representative volume element for a macroscopic model. We present a two-dimensional and a threedimensional mesoscale finite element model to simulate the structural response of strain hardening UHPFRCC. The mesoscale model employs an implicit gradient enhanced damage model for the cement matrix and a local bond stress-slip model for the bond between the cement matrix and the steel fibers. The steel fibers are modeled discretely as one-dimensional truss elements that are coupled to the cement matrix via bond elements. The tensile stress-strain response of UHPFRCC is a consequence of local matrix cracking and bond failure. Both phenomena can be depicted when modeling the cement matrix, the steel fibers, and the fiber-to-matrix bond explicitly. The second part of the talk deals with the efficient modeling of fracture and the prediction of crack initiation, propagation, merging, and branching through the computational domain. Phase-field models and gradient enhanced damage models can solve fracture mechanics problems by integrating a set of partial differential equations for the system and thus avoid the explicit treatment of discontinuities. The main attributes of these approaches are their simplicity and generality. However, they require a fine discretization in the region where the crack evolves. A finite element tearing and interconnecting (FETI) approach for the diffusive crack models is presented to distribute the computational cost among multiple processors and thus speed up the overall computation.
A finite element tearing and interconnecting (FETI) approach for phase-field models and gradient enhanced damage models is presented. These diffusive crack models can solve fracture mechanics problems by integrating a set of partial differential equations and thus avoid the explicit treatment of discontinuities. However, they require a fine discretization in the vicinity of the crack. FETI methods distribute the computational cost among multiple processors and thus speed up the computation.
A finite element tearing and interconnecting (FETI) approach for phase-field models and gradient enhanced damage models is presented. These diffusive crack models can solve fracture mechanics problems by integrating a set of partial differential equations and thus avoid the explicit treatment of discontinuities. However, they require a fine discretization in the vicinity of the crack. FETI methods distribute the computational cost among multiple processors and thus speed up the computation.
The condition for plastic instability is a material characteristic and defines the onset of necking in tensile tests. In large deformation problems of ductile materials it is fundamental to determine the strain at which necking starts as well as the post-necking behaviour in the instability region properly. For verification purposes of material models, usually results of numerical analyses are compared to experimental outcomes. For tensile tests with ductile materials under dynamic loading, it is challenging to obtain comparable experimental and numerical results in terms of the onset of necking and the post-critical deformation behaviour. This paper focuses on the derivation of a theoretical criterion describing the plastic instability in rate-dependent materials based on the time variation of the strain gradient in a tensile specimen under isothermal conditions. We examine the influence of various constitutive equations on the theoretical stability condition predicted by different multiplicative as well as additive approaches. For multiplicative relations, the results indicate that the onset of necking is, in principle, independent of the strain rate, whereas for the considered additive relation, the dynamic necking strain must decrease with increasing strain rate. In conclusion, the theoretical stability condition is related to results from finite element simulations of dynamic tensile tests with various loading rates. It is shown that the simulated and the theoretical predicted onset of plastic instability agree reasonably.
Typical transport packages used in Germany are equipped with encapsulated wooden impact limiting devices. We would like to present the current status regarding the development of a Finite Element (FE) material model for the crush of wood for the FE-code LS-DYNA. The crush of is a phenomenon governed by macroscopic fracture. Here, we would like to reproduce fracture and failure mechanisms over the continuous volume. In a first step we altered an existing LS-DYNA material model for foams, which considers an ellipse shaped yield surface. For the use for longitudinal compression of wood, we modified the existing material model to consider the deviatoric strain for the evolution of the yield surface as well. This is in accordance with the results of crush tests with spruce wood specimens, where the crushing deformation was rather deviatoric for uniaxial stress states and rather volumetric for multiaxial stress states We rate the basic idea of this approach to be reasonable, though other problems exist regarding the shape of the yield surface and the assumption of isotropic material properties. Therefore we developed a new transversal isotropic material model with two main directions, which considers different yield curves according to the multiaxiality of the stress state via a multi-surface yield criterion and a non-associated flow rule. The results show the ability to reproduce the basic strength characteristics of spruce wood. Nevertheless, problems with regularization etc. show that additional investigations are necessary.
During dynamic processes, a certain range of strain rates is often observed along loaded structures and components. For precise numerical simulations, it is necessary to determine rate-dependent properties in dynamic tests and to describe the material behavior correctly within an appropriate domain of strain rates including adiabatic heating effects at higher strain rates, typically higher than 10 1/s. In principle, numerical simulations are compared to experimental results to verify the applied material models. For dynamic tensile tests considering ductile materials and large plastic deformation beyond uniform elongation, it is challenging to obtain comparable results due to plastic instability and necking of the specimen, e.g..
Based on the strain gradient in a general tensile specimen, a theoretical criterion was derived describing the plastic instability in rate-dependent materials under isothermal conditions in. It was applied to different multiplicative and additive constitutive relations and the analytical onset of necking was compared to results from numerical calculations of quasi-static and dynamic tensile tests. The simulations of a sheet-metal specimen with rectangular cross-section were carried out using the Finite Element Method and it was found that the numerical calculated and the theoretical predicted onset of plastic instability agree very good. The analytical criterion for instability holds even for specimens without geometrical or material imperfections and confirms that the onset of plastic instability must be considered a material characteristic.
However, real dynamic problems with higher strain rates are not isothermal, the heat generated by plastic work is not dissipated to the surrounding and the temperature of the material increases significantly. Adiabatic heating and thermal softening must be considered within the constitutive relations of rate-dependent materials and the discussion of plastic instability. In this paper, an enhanced and more generalized approach for the description of the condition for stability is discussed and applied to phenomenological as well as more physical constitutive relations from the literature. This allows an individual assessment of the accuracy and verification of rate-dependent material models with respect to plastic instability.
Modeling the interactions of creep, shrinkage and damage in a multiphysics simulation of concrete
(2018)
The time dependent, mechanical behavior of concrete is affected by multiple phenomena like creep, shrinkage and damage propagation. The interactions of these processes are supposed to have significant influence on the materials response to external loading. For example it can be observed that the compressive strength of concrete rises with lowering the moisture content [Dahms, 1968].
Most of the constitutive models for finite element methods are designed with just a single phenomena in mind. In multiphysics simulations it is quiet common to use a linear superposition, i.e. additive decomposition of the total strain into elastic shrinkage, creep or thermal strains.
In this paper, the interactions of creep, shrinkage and damage models are investigated, in particular for cases where the assumption of linear superposition is questionable. A gradient enhanced damage model proposed by [Peerlings et al., 1996] is employed. Creep is modeled as a Kelvin chain as described in [Jirásek and Bažant, 2001]. Shrinkage is simulated by using two different approaches. The first model simulates shrinkage as an additional moisture dependent strain component. In the second model, shrinkage is simulated as a moisture dependent pore pressure applied to the solid bulk.
The impact of model interactions will be discussed with a focus on simulating the influence of the moisture content on the macroscopic strength. The model is validated by comparison to experimental data.