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Isaksson, Per
Publications (10 of 51) Show all publications
van Dijk, N. P., Wu, D., Persson, C. & Isaksson, P. (2019). A global digital volume correlation algorithm based on higher-order finite elements: Implementation and evaluation. International Journal of Solids and Structures, 168, 211-227
Open this publication in new window or tab >>A global digital volume correlation algorithm based on higher-order finite elements: Implementation and evaluation
2019 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 168, p. 211-227Article in journal (Refereed) Published
Abstract [en]

We propose a DVC technique that is based on higher-order finite-element discretization of the displacement field and a global optimization procedure. We use curvature penalization to suppress non-physical fluctuations of the displacement field and resulting erroneous strain concentrations. The performance of the proposed method is compared to the commercial code Avizo using trabecular bone images and found to perform slightly better in most cases. In addition, we stress that the performance of a DVC method needs to be evaluated using double scans (zero strain), virtual deformation (imposed deformation) and real deformation. Double scans give insight into the presence of noise and artifacts whereas virtual deformation benchmarks allows evaluation of the performance without noise and artifacts. Investigation of the performance for actual deformed heterogeneous materials is needed for evaluation with noise, artifacts and non-zero strains. We show that both decreasing the resolution of the displacement field (increasing subvolume size) as well as (increasing) curvature penalization (regularization) have a similar effect on the performance of evaluated DVC methods: Decreasing the detrimental effect of noise, artifacts and interpolation errors, but also decreasing the sensitivity of a DVC method to displacement peaks, discontinuities and strain concentrations. The needed amount of regularization is a trade-off between accuracy and precision of the estimated strain fields and their resolution. The obtainable accuracy and precision of the estimated displacement fields are influenced by interpolation errors in the DVC procedure and the relative amount of detail, noise and artifacts in the images. Errors in the displacement field are typically magnified during the strain calculation. Based on the tests and subvolume sizes (16-50 voxels) in this study, the expected order of magnitude of the accuracy and precision is 0.1 micro-voxels and 1 milli-voxels for the displacements and 0.1 and 1 milli-strains of the strain fields. 

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2019
Keywords
Digital volume correlation, Accuracy, Precision, Benchmarks, Evaluation, Trabecular bone
National Category
Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-387714 (URN)10.1016/j.ijsolstr.2019.03.024 (DOI)000469906300017 ()
Funder
Swedish Research Council, 2016-04608Göran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of Technology
Available from: 2019-06-26 Created: 2019-06-26 Last updated: 2019-08-12Bibliographically approved
Chen, S. & Isaksson, P. (2019). A note on the defect sensitivity of brittle solid foams. Engineering Fracture Mechanics, 206, 541-550
Open this publication in new window or tab >>A note on the defect sensitivity of brittle solid foams
2019 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 206, p. 541-550Article in journal (Refereed) Published
Abstract [en]

The fracture behavior of brittle solid foams of different densities and regularities is numerically analyzed in finite element models. The findings provide insight into the complex fracture phenomenon in cellular materials and reveal a size influence from a dominant microstructure on the global fracture mechanism. It is observed that a crack of length of about three times the average cell size in the foam is needed to obtain localization of nucleated fractures to the vicinity of the initial defect At cracks smaller than this critical size, the fractures nucleate at randomly positioned high-stressed regions in the foam far away from the initial crack, i.e. the structure is seemingly insensitive to the initial defect Further, it is found that irregular (i.e. randomly positioned cells) foams are more insensitive to defects than perfectly ordered foams if all other parameters are similar and thus indicate that classical fracture theories for solid foams have to be slightly modified.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2019
Keywords
Microstructure model, Defect sensitivity, Material characteristic length
National Category
Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-373305 (URN)10.1016/j.engfracmech.2018.11.012 (DOI)000454511700034 ()
Funder
EU, Horizon 2020, 734485
Available from: 2019-01-16 Created: 2019-01-16 Last updated: 2019-01-16Bibliographically approved
Espadas-Escalante, J. J., van Dijk, N. P. & Isaksson, P. (2019). A phase-field model for strength and fracture analyses of fiber-reinforced composites. Composites Science And Technology, 174, 58-67
Open this publication in new window or tab >>A phase-field model for strength and fracture analyses of fiber-reinforced composites
2019 (English)In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 174, p. 58-67Article in journal (Refereed) Published
Abstract [en]

A phase-field model for brittle fracture is proposed and evaluated for strength and fracture analyses of composites. In addition to the elastic properties, this approach makes use of only the fracture toughness and the strength of the material. The capability of the method is shown in analyses of composites at two scales. In laminates, strengths of notched laminates are estimated, including hole size effects. In a lamina, cracks developed in both transverse tension and compression are analyzed and compared to other numerical methods in the literature. The effects of a spectral and a hydrostatic-deviatoric decomposition of the strain energy density, two variants often used in phase-field formulations, are studied. It is shown that the choice of the decomposition affects the fracture development. Results are compared to experiments and simulations in the literature showing the capabilities of the phase-field approach.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Computational mechanics, Strength, Fracture, Finite element analysis (FEA)
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:uu:diva-368254 (URN)10.1016/j.compscitech.2018.10.031 (DOI)000469407500008 ()
Available from: 2018-12-03 Created: 2018-12-03 Last updated: 2019-07-05Bibliographically approved
Espadas-Escalante, J. J. & Isaksson, P. (2019). A study of induced delamination and failure in woven composite laminates subject to short-beam shear testing. Engineering Fracture Mechanics, 205, 359-369
Open this publication in new window or tab >>A study of induced delamination and failure in woven composite laminates subject to short-beam shear testing
2019 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 205, p. 359-369Article in journal (Refereed) Published
Abstract [en]

Failure in woven composite laminates subject to global shear load is studied. Laminates are manufactured, tested and analyzed using X-ray computed tomography, scanning electron microscopy and finite element models. It is found that the stress distribution along the thickness direction is dependent on the layer shifting that alters different yarn interactions, which in turn, affects delamination and failure onset A suggested failure mechanism is in agreement with experimental observations.

National Category
Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-368250 (URN)10.1016/j.engfracmech.2018.10.015 (DOI)000453766000026 ()
Available from: 2018-12-03 Created: 2018-12-03 Last updated: 2019-07-05Bibliographically approved
Carlsson, J. & Isaksson, P. (2019). Crack dynamics and crack tip shielding in a material containing pores analysed by a phase field method. Engineering Fracture Mechanics, 206, 526-540
Open this publication in new window or tab >>Crack dynamics and crack tip shielding in a material containing pores analysed by a phase field method
2019 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 206, p. 526-540Article in journal (Refereed) Published
Abstract [en]

Many naturally occurring materials, such as wood and bone, have intricate porous micro-structures and high stiffness and toughness to density ratios. Here, the influence of pores in a material on crack dynamics in brittle fracture is investigated. A dynamic phase field finite element model is used to study the effects of pores with respect to crack path, crack propagation velocity and energy release rate in a strip specimen geometry with circular pores. Four different ordered pore distributions are considered, as well as randomly distributed pores. The results show that the crack is attracted by the pores; this attraction is stronger when there is more energy available for crack growth. Crack propagation through pores also enables higher crack propagation velocities than are normally seen in strip specimens without pores (i.e. homogeneous material), without a corresponding increase in energy release rate. It is further noticed that as the porosity of an initially solid material increases, the crack tip is increasingly likely to become shielded or arrested, which may be a key to the high relative strength often exhibited by naturally occurring porous materials. We also find that when a pore is of the same size as the characteristic internal length then the pore does not localise damage. Since the characteristic internal length only regularises the damage field and not the strain end kinetic energy distributions, crack dynamics are still affected by small pores.

Keywords
Dynamic fracture, Phase field method, Porous material, Crack tip shielding
National Category
Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-372271 (URN)10.1016/j.engfracmech.2018.11.013 (DOI)000454511700033 ()
Funder
Swedish Energy Agency, 37206-2Swedish Research Council, 2016-04608EU, Horizon 2020, 734485
Available from: 2019-01-07 Created: 2019-01-07 Last updated: 2019-01-15Bibliographically approved
Jonsson, H., Öhman-Mägi, C., Alderborn, G., Isaksson, P. & Frenning, G. (2019). Crack nucleation and propagation in microcrystalline-cellulose based granules subject to uniaxial and triaxial load. International Journal of Pharmaceutics, 559, 130-137
Open this publication in new window or tab >>Crack nucleation and propagation in microcrystalline-cellulose based granules subject to uniaxial and triaxial load
Show others...
2019 (English)In: International Journal of Pharmaceutics, ISSN 0378-5173, E-ISSN 1873-3476, Vol. 559, p. 130-137Article in journal (Refereed) Published
Abstract [en]

Cracking patterns in four kinds of granules, based on the common pharmaceutical excipient microcrystalline cellulose (MCC) and subject to compressive load, were examined. The initial pore structure and the location of initial failure under uniaxial compression were assessed using X-ray micro-computed tomography, whereas contact force development and onset of cracking under more complex compressive load were examined using a triaxial testing apparatus. Smoothed particle hydrodynamics (SPH) simulations were employed for numerical analysis of the stress distributions prior to cracking. For granules subject to uniaxial compression, initial cracking always occurred along the meridian and the precise location of the crack depended on the pore structure. Likewise, for granules subject to triaxial compression, the fracture plane of the primary crack was generally parallel to the dominant loading direction. The occurrence of cracking was highly dependent on the triaxiality ratio, i.e. the ratio between the punch displacements in the secondary and dominant loading directions. Compressive stresses in the lateral directions, induced by triaxial compression, prevented crack opening and fragmentation of the granule, something that could be verified by simulations. These results provide corroboration as well as further insights into previously observed differences between confined and unconfined compression of granular media.

Keywords
Granule, Uniaxial, Triaxial, Cracking, Fragmentation
National Category
Pharmaceutical Sciences
Research subject
Pharmaceutics
Identifiers
urn:nbn:se:uu:diva-347457 (URN)10.1016/j.ijpharm.2018.12.064 (DOI)000459871500012 ()30599228 (PubMedID)
Funder
Vinnova, 2017-02690
Note

Title in thesis list of papers: Crack nucleation and propagation in microcrystalline cellulose-based granules subject to uniaxial and triaxial load

Available from: 2018-04-03 Created: 2018-04-03 Last updated: 2019-03-21Bibliographically approved
Espadas-Escalante, J. J. & Isaksson, P. (2019). Mesoscale analysis of the transverse cracking kinetics in woven composite laminates using a phase-field fracture theory. Engineering Fracture Mechanics, 216, Article ID 106523.
Open this publication in new window or tab >>Mesoscale analysis of the transverse cracking kinetics in woven composite laminates using a phase-field fracture theory
2019 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 216, article id 106523Article in journal (Refereed) Published
Abstract [en]

A phase-field approach to fracture is used to simulate transverse cracking kinetics in composite laminates. First, a typical unidirectional tape laminate is modeled and the transverse cracking evolution with the consequent reduction in the in-plane modulus of elasticity is estimated. Then, a four-layered plain weave composite is modeled using different layer shifting configurations. Predictions in the transverse cracking evolution become improved as the shifting configuration of the laminate model become closer to experimental observations. Simulations predict that some cracks do not form perpendicularly to the loading direction, as it has been observed experimentally in similar locations. Only the fracture toughness and the in situ transverse strength of the ply are required without prior knowledge of the position of the cracks or an ad hoc criterion for crack evolution. All the simulations are compared qualitatively and quantitatively to experiments published elsewhere.

Keywords
Composites, Fibre reinforced materials, Fracture mechanics, Damage mechanics, Crack growth
National Category
Applied Mechanics
Research subject
Engineering science with specialization in Applied Mechanics
Identifiers
urn:nbn:se:uu:diva-388533 (URN)10.1016/j.engfracmech.2019.106523 (DOI)000477573000028 ()
Available from: 2019-07-01 Created: 2019-07-01 Last updated: 2019-09-20Bibliographically approved
Espadas-Escalante, J. J., Bednarcyk, B. A., Pineda, E. J. & Isaksson, P. (2019). Modeling the influence of layer shifting on the properties and nonlinear response of woven composites subject to continuum damage. Composite structures, 220, 539-549
Open this publication in new window or tab >>Modeling the influence of layer shifting on the properties and nonlinear response of woven composites subject to continuum damage
2019 (English)In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 220, p. 539-549Article in journal (Refereed) Published
Abstract [en]

The influence of relative layer shifting on the elastic and damage response of plain weave composite laminates is analyzed using a continuum damage mechanics approach in combination with the finite element method. First, the homogenized properties of the woven composite as a function of the number of layers and of layer shifting are presented. Next, the damage development in various shifting configurations is studied using different damage constitutive models for the matrix and the fiber bundles. It is shown that the impact of layer shifting on both the elastic response and the nonlinear damage response is significant. Most notably, the model captures changes in the damage mechanisms within the woven composite that occur due to layer shifting, resulting in stiffer, more brittle behavior, which has been shown experimentally in the literature. Model results in the linear and nonlinear regimes are shown to be consistent with both an independent analytical model and reported experiments.

Place, publisher, year, edition, pages
ELSEVIER SCI LTD, 2019
Keywords
Textile composites, Layered structures, Damage mechanics, Multiscale modeling
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:uu:diva-385955 (URN)10.1016/j.compstruct.2019.04.006 (DOI)000465495700048 ()
Available from: 2019-06-19 Created: 2019-06-19 Last updated: 2019-06-19Bibliographically approved
Heldin, M., Carlsson, J., Isaksson, P. & Wiklund, U. (2019). On tool engagement in groundwood pulping - in-situ observations and numerical modelling at the microscale. In: The 11th Fundamental Mechanical Pulp Research Seminar, Norrköping, Sweden, April 2-4: . Paper presented at The 11th Fundamental Mechanical Pulp Research Seminar, Norrköping, Sweden, April 2-4, 2019..
Open this publication in new window or tab >>On tool engagement in groundwood pulping - in-situ observations and numerical modelling at the microscale
2019 (English)In: The 11th Fundamental Mechanical Pulp Research Seminar, Norrköping, Sweden, April 2-4, 2019Conference paper, Oral presentation only (Other academic)
National Category
Paper, Pulp and Fiber Technology Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Identifiers
urn:nbn:se:uu:diva-389856 (URN)
Conference
The 11th Fundamental Mechanical Pulp Research Seminar, Norrköping, Sweden, April 2-4, 2019.
Funder
Swedish Energy Agency, 37206-2
Available from: 2019-07-30 Created: 2019-07-30 Last updated: 2019-07-30
Chen, S. & Isaksson, P. (2018). An experimental analysis of the defect sensitivity of solid foams. Theoretical and applied fracture mechanics (Print), 96, 768-774
Open this publication in new window or tab >>An experimental analysis of the defect sensitivity of solid foams
2018 (English)In: Theoretical and applied fracture mechanics (Print), ISSN 0167-8442, E-ISSN 1872-7638, Vol. 96, p. 768-774Article in journal (Refereed) Published
Abstract [en]

Single edge notched bending (SENB) and single edge notched tensile (SENT) fracture experiments were conducted to study the influence of the defect size on the global fracture behavior of three different brittle low density PVC solid foams. It was found that for sufficiently small defects (initial cracks), the continued fracture process was through breakage of cell edges located at random positions far away from the defect while the global fracture load was fairly constant and hence unaffected by the initial defects. At defects larger than about four cells, however, the continued cell edge fractures were localized to the near vicinity of the defect, resulting in a decreasing global fracture load in accordance with classical linear elastic fracture mechanics theory. Hence a size of about four cells is considered to be a critical microstructure transition length, meaning that the foam is unaffected for defects smaller than this transition length from a fracture point of view.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2018
Keywords
Defect sensitivity, Closed cell foam, Critical transition crack length
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:uu:diva-362843 (URN)10.1016/j.tafmec.2017.11.004 (DOI)000441485500063 ()
Available from: 2018-10-15 Created: 2018-10-15 Last updated: 2018-10-15Bibliographically approved
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