| 1. |
- Marengo, Alessandro, et al.
(författare)
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An in-plane phase-field ductile fracture model for orthotropic paperboard material
- 2024
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Ingår i: International Journal of Solids and Structures. - 0020-7683. ; 294
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Tidskriftsartikel (refereegranskat)abstract
- A phase-field ductile fracture formulation for orthotropic paperboard materials is proposed, based on an anisotropic, multi-surface elastoplastic model describing the in-plane behavior of paperboard. A variational statement for the finite-step elastoplastic problem is extended to include the variational description of Griffith-type brittle fracture by a phase-field gradient term. The interaction between plastic and fracture dissipation mechanisms is modeled by introducing a scalar modulation function, assuming plasticity driven damage growth. This function depends on a scalar measure of the plastic strain components in the material orthotropy frame. It modifies the fracture activation criterion in a non-variational fashion, resulting in a direction-dependent material strength against crack propagation. The model performance is assessed by comparing numerical simulations and experimental tests conducted in a climate-controlled laboratory.
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| 2. |
- Motamedian, Hamid Reza, 1981-
(författare)
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Beam-to-Beam Contact and Its Application to Micromechanical Simulation of Fiber Networks
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This doctoral thesis covers the topic of modeling the three-dimensional fiber net- works with the finite element method. It contains the part addressing the numerical aspects of the modeling, namely, the contact formulation and application of the developed methods to the fundamental questions such as the effect of randomness in fiber properties and effect of fines and hygroexpansion.In the approached used in the work, the fibers were meshed with beam elements and the bond between fibers is modeled using point-wise beam-to-beam contact. Contact between beam elements is a specific category of contact problems, which was introduced by Wriggers and Zavarise in 1997 for normal contact [1] and later extended by Zavarise and Wriggers to include tangential and frictional contact [2]. These formulations encompass a large number of derivations and provide the consistent tangent matrix. We showed, however, the resulting numerical implementations based on these consistent formulations are not sufficiently robust in modeling random fiber networks with a large number of contacts. In the first papers, we proposed a simpler non-consistent formulation, which turned out to be superior in terms of convergence stability with respect to the load step size for a wide range of loading cases. Having these advantages, it remained equally accurate as the original formulation. The first paper covered the formulation of normal and tangential contact, and the second paper contains two formulations with both the consistent and non-consistent linearizations for in-plane rotational contact of beams.We use the developed formulations to address fundamental problems within the area of fiber networks, which cannot be solved purely with experimental tools. In the third article, we investigated the effect of fiber and bond strength variations on the tensile stiffness and strength of fiber networks and concluded that in cases of skewed distribution, using mean values for fiber and bond properties instead of the distributions is not always adequate to assess the changes these properties have on the average mechanical characteristics of the entire network.In the fourth paper, the mechanisms behind the improvement of stiffness and strength after PFI refining in the papermaking process is investigated. The PFI refiner is very popular for studying the effect of refining in the lab scale. By using a combination of experimental and numerical tools, we found that density, which is often mentioned as the main reason behind the improvement of mechanical properties after PFI re- fining, cannot solely explain the degree of the change observed experimentally. We concluded the remaining part of the improvement is caused by the fibrillar fines, in particular, by the fines that cannot be detected with modern automated fiber characterization tools due to the limited resolution of such tools.Finally, in the fifth paper, we suggested a multi-scale model to study hygroexpan- sion/shrinkage properties of paper. Due to the anisotropy of the fibers, the stress transfer at the bonded sites has a dominant role in the behavior of paper when exposed to moisture change. While we modeled the bonds between fibers using point-wise contact elements, such stress transfer requires a finite contact area. To solve this limitation and yet preserve the advantages for using beams for modeling fiber networks, we developed a concurrent multi-scale approach. In this approach, the bond model is resolved for every bond in the network, and the exchange between the network and bond model is maintained through the current configuration of the fibers being passed to the bond scale, and the inelastic strains being transferred back to the network scale. We demonstrated the effectiveness of such approach by comparing it with a full-scale continuum model. Using this approach, we were able to complete the existing experimental observation with key insights using the ad- vantage of having unlimited access to the details of the network at each stage of the deformation.
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| 3. |
- Robertsson, Kristofer, et al.
(författare)
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Efficient and accurate simulation of the packaging forming process
- 2018
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Ingår i: Packaging Technology and Science. - : Wiley. - 0894-3214. ; 31:8, s. 557-566
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Tidskriftsartikel (refereegranskat)abstract
- To allow for large-scale forming applications, such as converting paperboard into package containers, efficient and reliable numerical tools are needed. In finite element simulations of thin structures, elements including structural features are required to reduce the computational cost. Solid-shell elements based on reduced integration with hourglass stabilization is an attractive choice. One advantage of this choice is the natural inclusion of the thickness, not present in standard degenerated shells, which is especially important for many problems involving contact. Furthermore, no restrictions are imposed on the constitutive models since the solid-shell element does not require the plane stress condition to be enforced. In this work, a recently proposed efficient solid-shell element is implemented together with a state-of-the-art continuum model for paperboard. This approach is validated by comparing the obtained numerical results with experimental results for paperboard as well as with those found by using 3D continuum elements. To show the potential of this approach, a large-scale forming simulation of paperboard is used as a proof of concept.
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