| 1. |
- Grytsan, Andrii, 1986-
(författare)
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Abdominal aortic aneurysm inception and evolution - A computational model
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Abdominal aortic aneurysm (AAA) is characterized by a bulge in the abdominal aorta. AAA development is mostly asymptomatic, but such a bulge may suddenly rupture, which is associated with a high mortality rate. Unfortunately, there is no medication that can prevent AAA from expanding or rupturing. Therefore, patients with detected AAA are monitored until treatment indication, such as maximum AAA diameter of 55 mm or expansion rate of 1 cm/year. Models of AAA development may help to understand the disease progression and to inform decision-making on a patient-specific basis. AAA growth and remodeling (G&R) models are rather complex, and before the challenge is undertaken, sound clinical validation is required.In Paper A, an existing thick-walled model of growth and remodeling of one layer of an AAA slice has been extended to a two-layered model, which better reflects the layered structure of the vessel wall. A parameter study was performed to investigate the influence of mechanical properties and G&R parameters of such a model on the aneurysm growth.In Paper B, the model from Paper A was extended to an organ level model of AAA growth. Furthermore, the model was incorporated into a Fluid-Solid-Growth (FSG) framework. A patient-specific geometry of the abdominal aorta is used to illustrate the model capabilities.In Paper C, the evolution of the patient-specific biomechanical characteristics of the AAA was investigated. Four patients with five to eight Computed Tomography-Angiography (CT-A) scans at different time points were analyzed. Several non-trivial statistical correlations were found between the analyzed parameters.In Paper D, the effect of different growth kinematics on AAA growth was investigated. The transverse isotropic in-thickness growth was the most suitable AAA growth assumption, while fully isotropic growth and transverse isotropic in-plane growth produced unrealistic results. In addition, modeling of the tissue volume change improved the wall thickness prediction, but still overestimated thinning of the wall during aneurysm expansion.
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| 2. |
- Islam, Md Shafiqul, 1984-
(författare)
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Fracture and Delamination in Packaging Materials : A Study of Experimental Methods and Simulation Techniques
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Packages are the means of preservation, distribution and convenience of use for food, medicine and other consumer products. The introduction of a new package-opening technique for a better opening experience requires additional prototype development and physical testing. In order for the design process to be more rapid and robust, finite element (FE) simulations are widely used in packaging industries to compliment and reduce the amount of physical testing.The goal of this work is to develop some building blocks for complete package-opening FE-simulation. To begin with, the study focuses on mechanical testing of packaging materials’ fracture and delamination; especially shear fracture. Use of tools like digital image correlation (DIC) and scanning electron microscope (SEM) greatly aided to the strain measuring technique and observation of fractured and delaminated surfaces respectively.A modified shear test specimen for polymer sheet testing was developed and its geometry was optimized by FE-simulation. A geometry correction factor of shear fracture toughness for the proposed specimen was derived based on linear elastic fracture mechanics (LEFM). It was found that the specimen ligament length should vary between twice the thickness and half the ligament width of the modified shear specimen to measure the essential work of fracture.Thin-flexible laminate of low-density polyethylene (LDPE) and aluminium (Al) is another key packaging material addressed in this study. The continuum and fracture testing of individual layers provided the base information and input for FE-modelling. The FE-simulation material parameters were calibrated from the physical test response through inverse analysis. Identification process of the laminate interface fracture energy (Gc) from peel tests was studied experimentally and theoretically. A successful FE-simulation optimization framework using artificial neural network and genetic algorithm was developed for the calibration of Gc. To address the challenge in quantifying shear Gc of laminate with very thin substrates, a convenient test technique was proposed. In a separate case, the tearing response of LDPE/PET (polyethylene terephthalate) laminate was studied to examine crack propagation, crack path deviation and delamination of the laminate in mode III fracture. Several tear EWF evaluation theories were proposed along with a cyclic tear test method.
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| 3. |
- Elmukashfi, Elsiddig
(författare)
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Modeling of fracture and damage in rubber under dynamic and quasi-static conditions
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Elastomers are important engineering materials that have contributed to the different technical developments and applications since the 19th century. The study of crack growth mechanics for elastomers is of great importance to produce reliable products and therefore costly failures can be prevented. On the other hand, it is fundamental in some applications such as adhesion technology, elastomers wear, etc. In this thesis work, crack propagation in rubber under quasi-static and dynamic conditions is investigated.In Paper A, theoretical and computational frameworks for dynamic crack propagation in rubber have been developed. The fracture separation process is presumed to be described by a cohesive zone model and the bulk behavior is assumed to be determined by viscoelasticity theory. The numerical model is able to predict the dynamic crack growth. Further, the viscous dissipation in the continuum is found to be negligible and the strength and the surface energy vary with the crack speed. Hence, the viscous contribution in the innermost of the crack tip has been investigated in Paper B. This contribution is incorporated using a rate-dependent cohesive model. The results suggest that the viscosity varies with the crack speed. Moreover, the estimation of the total work of fracture shows that the fracture-related processes contribute to the total work of fracture in a contradictory manner.A multiscale continuum model of strain-induced cavitation damage and crystallization in rubber-like materials is proposed in Paper C. The model adopts the network decomposition concept and assumes the interaction between the filler particles and long-chain molecules results in two networks between cross-links and between the filler aggregates. The network between the crosslinks is assumed to be semi-crystalline, and the network between the filler aggregates is assumed to be amorphous with the possibility of debonding. Moreover, the material is assumed to be initially non-cavitated and the cavitation may take place as a result from the debonding process. The cavities are assumed to exhibit growth phase that may lead to complete damage. The comparison with the experimental data from the literature shows that the model is capable to predict accurately the experimental data.Papers D and E are dedicated to experimental studies of the crack propagation in rubber. A new method for determining the critical tearing energy in rubber-like materials is proposed in Paper D. The method attempts to provide an accurate prediction of the tearing energy by accounting for the dissipated energy due to different inelastic processes. The experimental results show that classical method overestimates the critical tearing energy by approximately 15%. In Paper E, the fracture behavior of carbon-black natural rubber material is experimentally studied over a range of loading rates varying from quasi-static to dynamic, different temperatures, and fracture modes. The tearing behavior shows a stick-slip pattern in low velocities with a size dependent on the loading rate, temperature and the fracture mode. Smooth propagation results at high velocities. The critical tearing depends strongly on the loading rate as well as the temperature.
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| 4. |
- Fallqvist, Björn, 1985-
(författare)
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On the mechanics of actin and intermediate filament networks and their contribution to cellular mechanics
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The mechanical behaviour of cells is essential in ensuring continued physiological function, and deficiencies therein can result in a variety of diseases. Also, altered mechanical response of cells can in certain cases be an indicator of a diseased state, and even actively promoting progression of pathology. In this thesis, methods to model cell and cytoskeletal mechanics are developed and analysed.In Paper A, a constitutive model for the response of transiently cross-linked actin networks is developed using a continuum framework. A strain energy function is proposed and modified in terms of chemically activated cross-links.In Paper B, a finite element framework was used to assess the influence of numerous geometrical and material parameters on the response of cross-linked actin networks, quantifying the influence of microstructural properties and cross-link compliance. Also, a micromechanically motivated constitutive model for cross-linked networks in a continuum framework was proposed.In Paper C, the discrete model is extended to include the stochastic nature of cross-links. The strain rate dependence observed in experiments is suggested to depend partly on this.In Paper D, the continuum model for cross-linked networks is extended to encompass more composite networks. Favourable comparisons to experiments indicate the interplay between phenomenological evolution laws to predict effects in biopolymer networks.In Paper E, experimental and computational techniques are used to assess influence of the actin cytoskeleton on the mechanical response of fibroblast cells. The influence of cell shape is assessed, and experimental and computational aspects of cell mechanics are discussed.In Paper F, the filament-based cytoskeletal model is extended with an active response to predict active force generation. Importantly, experimentally observed stiffening of cells with applied stress is predicted.
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| 5. |
- Sharifimajd, Babak
(författare)
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On the continuum muscle modeling
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Modeling muscle behavior using techniques developed in continuum mechanics is a growing eld of research. The developed models allow for a more generalized way of computing stresses and deformations, specially, when it comes to using nite element techniques. Current continuum muscle models mostly focus on the kinetics of the muscle contraction, while other fundamental physiological processes such as, the membrane excitation and the activation process are disregarded. These processes are essential to initiate the contraction, and to determine the amount of generated force, respectively. In this thesis, muscle modeling is carried out in a thermodynamically consistent framework where the physiological processes governing muscle contraction are included. The behavior of the muscle is described by dissipative constitutive equations derived from applying the principles of thermodynamics. The muscle model is then validated through comparing the model response to available experimental data.
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| 6. |
- Abelen, Aisha, et al.
(författare)
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Modelling of rate-dependent inelasticity and damage in semi-crystalline polymers using an Eulerian framework
- 2023
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Ingår i: International Journal of Engineering Science. - : Elsevier. - 0020-7225 .- 1879-2197. ; 193
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Tidskriftsartikel (refereegranskat)abstract
- The present paper concerns the mechanical response of semi-crystalline polymers during cyclic loading, and it includes both modelling and experimental testing. The model is Eulerian in the sense that it is independent of measures of total deformation and plastic/inelastic deformations. It is able to account for such essential phenomena as strain-rate dependence, work hardening, and damage. The model was applied to uniaxial tension tests performed on high-density polyethylene (HDPE), which is a semi-crystalline polymer widely used in the industry. Two types of tests were conducted: monotonic tests, and loading-unloading tests. The model was able to reproduce the experimental results very well. The proposed model was also implemented as a UMAT in Abaqus, including an analytic tangent. The UMAT was used for simulating two 3D geometries. The implementation seems to be robust, and no convergence problems were observed.
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| 7. |
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| 8. |
- Kovacikova, Janka, et al.
(författare)
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Mechanical properties of fiberboard composite bonded with polymer matrixcomputed by mean-field homogenization methods
- 2021
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Ingår i: 25th International Congress of Theoretical and Applied Mechanics, August 22-27, 2021. - Milano, Italy. - 9788365550316 ; , s. 1984-1985
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Nowadays, the design of composite materials considering sustainability and the environmental impact of the production is conspicuous. Therefore, in this work, we focus on investigating the mechanical behaviour and structure of a new green wood-based fibrous composite material bonded with a novel polymer matrix. The constitutive prediction models employing the material and structure design approaches simultaneously are proposed here to describe the material's microstructure. The goal is speeding up trials and reducing experiments expenses by replacing tests with computer simulations. Additionally, the relationship between material behaviour and structure is established and will be later used to generate Representative Volume Elements (RVEs) for finite element analysis (FEA).
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| 9. |
- Kroon, Martin, Professor, 1974-, et al.
(författare)
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A simple scalar directional hardening model for the Bauschinger effect compared with a tensorial model
- 2020
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Ingår i: Journal of Mechanics of Materials and Structures. - : Mathematical Sciences Publishers. - 1559-3959 .- 2157-5428. ; 15:4, s. 511-537
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Tidskriftsartikel (refereegranskat)abstract
- Modeling the Bauschinger effect is usually accomplished by introducing a second-order back-stress or directional hardening tensor. The objective of this paper is to propose a simpler scalar model of the Bauschinger effect based on a scalar directional hardening parameter that is determined by integration of an evolution equation. The behavior of this scalar model is compared to a tensorial model for a number of load cases. Strongly objective numerical algorithms are developed for integrating the evolution equations for both the tensorial and scalar models. Also, a consistent tangent is developed for both models. Obviously, the numerical implementation of the scalar model is significantly less complicated than for the tensorial model. Examples show that the tensorial and scalar models predict the same results for cyclic proportional triaxial extension and triaxial compression loadings. In contrast, the tensorial model predicts a Bauschinger effect for cyclic proportional pure torsion loading which is not predicted by the scalar model. More complicated examples with nonproportional loading paths and inhomogeneous deformations indicate that, relative to the tensorial model, the scalar model accounts for directional hardening fairly well and the simplicity of the model makes it an attractive option to add to isotropic hardening models.
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| 10. |
- Kroon, Martin, Professor, 1974-, et al.
(författare)
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A strongly objective, robust integration algorithm for Eulerian evolution equations modeling general anisotropic elastic-inelastic material response
- 2020
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Ingår i: Finite elements in analysis and design (Print). - : Elsevier. - 0168-874X .- 1872-6925. ; 177, s. 1-15
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Tidskriftsartikel (refereegranskat)abstract
- A background to the constitutive modeling of elastic-inelastic material response is provided to highlight the uniqueness of the Eulerian formulation of general nonlinear fully anisotropic thermoelastic-inelastic materials proposed in Rubin (1994) [1]. This model introduced Eulerian evolution equations for a triad of microstructural vectors that characterize elastic deformations and anisotropic orientations. Components of tensors which transform like the Cauchy stress referred to these vectors are insensitive to superposed rigid body motions so they can be used to formulate general elastically and inelastically anisotropic constitutive equations. This paper develops a strongly objective, robust numerical algorithm for integrating the evolution equations for the microstructural vectors. This algorithm can easily be implemented into computer codes to simplify the use of general anisotropic constitutive equations for thermoelastic-inelastic material response.
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| 11. |
- Kroon, Martin, Professor, 1974-, et al.
(författare)
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An Eulerian constitutive model for the inelastic finite strain behaviour of isotropic semi-crystalline polymers
- 2023
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Ingår i: European journal of mechanics. A, Solids. - : Elsevier. - 0997-7538 .- 1873-7285. ; 100
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Tidskriftsartikel (refereegranskat)abstract
- A constitutive model for isotropic, semi-crystalline polymers is proposed. The model is Eulerian in the sense that it is independent of measures of total deformation and plastic/inelastic deformations. It is able to account for such essential phenomena as strain-rate dependence, work hardening, stress relaxation, volumetric inelastic deformations, and damage. The model was applied to uniaxial tension tests performed on polyoxymethylene (POM), which is a semi-crystalline polymer widely used in the industry. Three types of tests were conducted: monotonic tests at different strain rates, stress relaxation tests, and loading-unloading tests. The model was able to reproduce the experimental results well. The proposed model was also implemented as a VUMAT in Abaqus, and the deformation of a 3D geometry was simulated.
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| 12. |
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| 13. |
- Kroon, Martin, Professor, 1974-, et al.
(författare)
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An Eulerian model for orthotropic elasticity and inelasticity applied to injection-moulded low-density polyethylene
- 2022
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Ingår i: Mechanics of materials. - : Elsevier. - 0167-6636 .- 1872-7743. ; 167
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Tidskriftsartikel (refereegranskat)abstract
- Anisotropic elasticity and inelasticity is of relevance in many practical applications. The Eulerian formulation for anisotropic elastic and inelastic response based on microstructural vectors is used here to model an injection-moulded low-density polyethylene. In contrast with Lagrangian models of inelasticity, the Eulerian formulation is insensitive to arbitrariness of the reference and intermediate configurations as well as to measures of total and inelastic deformations. A specific strain-space-type anisotropic yield function is proposed that depends on anisotropic measures of elastic deformation and anisotropic hardening variables. Use is also made of a rate-independent model with a smooth elastic-inelastic transition. The material parameters were calibrated to reproduce uniaxial test data for loading in three directions in the moulding plane. In addition, a strongly objective numerical implementation is presented and used to simulate stretching of a plate with a circular hole. In contrast with metals, this polyethylene experiences elastic deformations of about 10%. Although the inelastic spin rate could not be determined by the available test data, simulations of loadings in different material directions yield observable influences of inelastic spin rate.
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| 14. |
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| 15. |
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| 16. |
- Kroon, Martin, Professor, 1974-, et al.
(författare)
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An Eulerian thermomechanical elastic-viscoplastic model with isotropic and directional hardening applied to computational welding mechanics
- 2021
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Ingår i: Acta Mechanica. - : Springer. - 0001-5970 .- 1619-6937. ; 232, s. 189-218
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Tidskriftsartikel (refereegranskat)abstract
- An Eulerian thermomechanical elastic-viscoplastic model with isotropic and directional hardening is used to analyse the residual mechanical state resulting from the arc welding of a multi-pass weld. Details of the weld test plate, weld filler material, and numerical implementation of the model are provided, including integration algorithms and consistent tangent modulus. For the computational welding mechanics analyses, the austenitic ASME stainless steel grade 316L was considered so that no phase transformations of solid states needed to be considered. The maximum residual stresses were found to be about 500-600 MPa, which is of the order of the yield stress of the base material. Variations in the heat input and the resulting weld cooling time had a significant influence both on the residual stress state and on the resulting geometry of the weld. The predicted stress levels were compared to the experimental results. Overall, the proposed Eulerian framework seems to be a promising tool for analysing melting/solidification processes and residual mechanical states.
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| 17. |
- Kroon, Martin, Professor, 1974-, et al.
(författare)
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Experimental and theoretical study of stress relaxation in high-density polyethylene
- 2024
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Ingår i: Acta Mechanica. - : Springer. - 0001-5970 .- 1619-6937. ; 235:4, s. 2455-2477
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Tidskriftsartikel (refereegranskat)abstract
- Stress relaxation of high-density polyethylene is addressed both experimentally and theoretically. Two types of stress relaxation testing are carried out: uniaxial tensile testing at constant test specimen length and compression testing of a 3D structure producing inhomogeneous deformation fields and relaxation. A constitutive model for isotropic, semi-crystalline polymers is also proposed. The model has the ability to model stress relaxation at different time scales. The developed model was implemented as a user subroutine in Abaqus (UMAT). The implicit integration scheme including an algorithmic tangent modulus is described in detail. The material model is calibrated by use of the uniaxial tensile tests, and the model is then validated by simulating the compression tests of the 3D structure. The model is able to describe the uniaxial tension tests well, and the comparison between the simulations and experimental testing of the 3D structure shows very good agreement. © 2024, The Author(s).
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| 18. |
- Kroon, Martin, Professor, 1974-, et al.
(författare)
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Modelling Large Inelastic Dilatational and Distortional Deformations in Semi-crystalline Polymers Using an Eulerian Framework
- 2023
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Ingår i: USNCCM17. - : USACM.
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Konferensbidrag (refereegranskat)abstract
- A constitutive model for isotropic, semi-crystalline polymers is proposed. The model is Eulerian in the sense that it is independent of measures of total deformation, plastic/inelastic deformations, and all state variables are defined in the current state of the material. The deformation state of the material is represented by a unimodular tensor, characterizing elastic distortional deformation, and an elastic dilatation. The model is able to account for such essential phenomena as strain-rate dependence, work hardening, stress relaxation, volumetric inelastic deformations, and damage. Uniaxial tension tests were performed on polyoxymethylene (POM), which is a semi-crystalline polymer widely used in the industry. Three types of tests were conducted: monotonic loading tests at different strain rates, stress relaxation tests, and loading-unloading tests. These tests produced large elastic and inelastic deformations which were reproduced well by the model. The model was also implemented as a VUMAT in Abaqus, and the deformation of a 3D geometry was simulated. Specifically, a simple structure of the POM material was deformed and then unloaded. Thenresulting elastic spring-back and residual stress state was investigated.
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| 19. |
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| 20. |
- Kroon, Martin, Professor, 1974-, et al.
(författare)
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Modelling of rate-dependent and volumetric inelasticity of semi-crystalline polymers using an Eulerian framework
- 2023
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Ingår i: 17<sup>th</sup> International Conference on Computational Plasticity (COMPLAS 2023), Barcelona, Spain, September 2023. - : International Center for Numerical Methods in Engineering (CIMNE).
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Konferensbidrag (refereegranskat)abstract
- A constitutive model for isotropic, semi-crystalline polymers is proposed. The model is Eulerian in the sense that it is independent of measures of total deformation and plastic/inelastic deformations. Hence, all state variables are defined in the current state of the material. It is able to account for such essential phenomena as strain-rate dependence, work hardening, stress relaxation, volumetric inelastic deformations, and damage. The model includes 15 model parameters and was applied to uniaxial tension tests performed on polyoxymethylene (POM), which is a semi-crystalline polymer widely used in the industry. Three types of tests were conducted: monotonic tests at different strain rates, stress relaxation tests, and loading-unloading tests. The model was able to reproduce the experimental results well. The proposed model was also implemented as a VUMAT in Abaqus. The deformation of a 3D geometry was simulated. The 3D geometry consisted of a “tooth” that was deformed by a rigid block, causing permanent deformation and spring-back of the tooth. The elastic spring-back and relaxation of the tooth is demonstrated and also the residual state in the tooth after the inelastic deformation.
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| 21. |
- Kroon, Martin, Professor, 1974-, et al.
(författare)
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Numerical and experimental analysis of inelastic and rate-dependent buckling of thin injection-moulded high-density polyethylene structure
- 2024
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Ingår i: International Journal of Solids and Structures. - : Elsevier. - 0020-7683 .- 1879-2146. ; 290
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Tidskriftsartikel (refereegranskat)abstract
- Semi-crystalline polymers is an important group of materials that is used in a vast array of products. In this study, the rate-dependent properties of high-density polyethylene (HDPE) are investigated, both experimentally and theoretically. Experimental compression testing of a three-dimensional HDPE structure is performed and analysed numerically by use of the finite element method. In addition, an Eulerian constitutive material model for isotropic, semi-crystalline polymers is proposed. The model is able to account for such essential phenomena as strain-rate dependence, work hardening, pressure-dependence of inelastic deformations, and damage. The proposed material model was implemented in Abaqus as a VUMAT, which is an explicit implementation. The material model was calibrated by use of uniaxial tensile tests performed on HDPE dog-bone shaped samples, and the model was further explored by applying the VUMAT implementation to the compression tests of the HDPE structure. The simulation model was able to reproduce the experimental results well, both the uniaxial tests and the compression tests. In particular, the friction present in the compression tests seems to play an important role in determining the buckling mode of the structure.
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| 22. |
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| 23. |
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| 24. |
- Kroon, Martin, Professor, 1974-, et al.
(författare)
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Prediction of welding residual stresses using an Eulerian plasticity model
- 2022
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Ingår i: Proceedings M2D2022 - 9th International Conference on Mechanics and Materials in Design, Funchal/Portugal 26-30 June 2022. - 9789895475636 ; , s. 167-168
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Konferensbidrag (refereegranskat)abstract
- Solidification processes play an important role in several industrial processes, such as weldingand additive manufacturing, and the resulting residual stress state is often critical for thestructural integrity of the material. A thermomechanical model was developed for the analysisand prediction of such solidification processes. The approach is based on a model proposed byRubin (Int. J. Eng. Sci. 25, 1175-1191, 1987). This plasticity model was formulated for finitestrains using an Eulerian evolution equation for a unimodular elastic distortional deformationtensor. This evolution equation automatically ensures thermodynamic consistency andpreservation of isochoric inelastic deformations. A 2D problem in the form of a butt weld jointwas analyzed. The main outcome from the analyses were the residual stress distributions. Theresults are compared to experimental data from the literature.
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| 25. |
- Kroon, Martin, Professor, 1974-
(författare)
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Rate-Dependent Crack Propagation in Polyethylene
- 2019
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Ingår i: Proceedings of the First International Conference on Theoretical, Applied and Experimental Mechanics. - Cham : Springer. - 9783319919898 - 9783319919881 ; , s. 194-198
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Konferensbidrag (refereegranskat)abstract
- Crack growth in semi-crystalline polymers, represented by polyethylene, is considered. The material considered comes in plates that had been created through an injection-molding process. Hence, the material was taken to be orthotropic. Material directions were identified as MD: molding direction, CD: transverse direction, TD: thickness direction. Uniaxial tensile testing was performed in order to establish the direction-specific elastic-plastic behaviour of the polymer. In addition, the fracture mechanics properties of the material was determined by performing fracture mechanics testing on plates with side cracks of different lengths. The fracture mechanics tests were filmed using a video camera. Based on this information, the force vs. load-line displacement could be established for the fracture mechanics tests, in which also the current length of the crack was indicated, since crack growth took place. Crack growth was modelled using a rate-dependent cohesive zone. The problem was analyzed using Abaqus, and the crack growth experiments were simulated. The experiments could be well reproduced. Furthermore, the direction-specific work of fracture had been established from the experiments and these energies could be compared to the values of the J-integral from the simulations for the different crack lengths.
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| 26. |
- Kroon, Martin, Professor, 1974-, et al.
(författare)
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Regularization of damage and failure using a non-local hardening variable in an Eulerian formulation of inelasticity
- 2023
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Ingår i: 15th International Conference on Fracture (ICF15)June 11-16, 2023 Atlanta, GA USA.
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Konferensbidrag (refereegranskat)abstract
- It is known that damage or inelastic softening can cause an ill-posed problem leading to localization and mesh-dependence in finite element simulations. Here, a nonlocal hardening variable is introduced in a finite deformation Eulerian formulation of inelasticity. This nonlocal variable is defined over an Eulerian region of nonlocality, which is a sphere with radius equal to a characteristic length, defined in the current deformed geometry of the material. The influence of the nonlocal hardening variable is studied using an example of a plate that is loaded by a prescribed boundary displacement causing formation of a shear band. Predictions of the applied load vs. displacement curves and contour plots of the total distortional deformation of the plate and the hardening variable are studied. It is shown that the characteristic material length controls the structure of the shear band developed in the plate.
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| 27. |
- Kroon, Martin, Professor, 1974-, et al.
(författare)
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Regularization of localization due to material softening using a nonlocal hardening variable in an Eulerian formulation of inelasticity
- 2022
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Ingår i: International Journal of Engineering Science. - : Elsevier. - 0020-7225 .- 1879-2197. ; 176
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Tidskriftsartikel (refereegranskat)abstract
- It is known that damage or inelastic softening can cause an ill-posed problem leading to localization and mesh-dependence in finite element simulations. In this paper, a nonlocal hardening variable, ̄? , is introduced in a finite deformation Eulerian formulation of inelasticity with a rate-independent smooth elastic–inelastic transition. This nonlocal variable is defined over an Eulerian region of nonlocality, which is a sphere with radius equal to the characteristic length, ?c, defined in the current deformed geometry of the material. Two models of this nonlocal hardening variable are explored. One model where ̄ ? is the minimum value of the local hardening ? within the region of nonlocality, and another model where ̄? is the average of ? in the same region. The influence of the nonlocal hardening variable is studied using an example of a plate that is loaded by a prescribed boundary displacement causing formation of a shear band. Predictions of the applied load vs. displacement curves and contour plots of the total distortional deformation of the plate and the hardening variable ? are studied. The model based on the minimum value of ? in the nonlocal region predicts mesh-independent post-peak response of the load vs. displacement curve. Also, it is shown that the characteristic material length, ?c, controls the structure of the shear band developed in the plate.
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| 28. |
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| 29. |
- Stoltz, Björn, et al.
(författare)
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A modified neo-Hookean model for semi-crystalline thermoplastics assessed by relaxation and zero-stress creep tests of recycled polypropylene and polyoxymethylene
- 2024
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Ingår i: Mechanics of time-dependant materials. - : Springer. - 1385-2000 .- 1573-2738. ; 28, s. 43-63
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Tidskriftsartikel (refereegranskat)abstract
- The mechanical behavior of thermoplastics is strongly rate-dependent, and oftentimes it is difficult to find constitutive models that can accurately describe their behavior in the small to moderate strain regime. In this paper, a hyperelastic network model (modified neo-Hookean) and a set of experiments are presented. The testing consists of monotonic tensile loading as well as stress relaxation and zero stress creep. Two materials were tested, polyoxymethylene (POM) and recycled polypropylene (rPP), representing one more rigid and brittle and one softer and more ductile semi-crystalline polymer. The model contains two main novelties. The first novelty is that the stiffness is allowed to vary with the elastic deformation (in contrast to a standard neo-Hookean model). The second novelty is that the exponent governing viscous relaxation is allowed to vary with the viscous deformation. The basic features of the new model are illustrated, and the model was fitted to the experimental data. The model proved to be able to describe the experimental results well.
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