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Publications (10 of 318) Show all publications
Pupurs, A., Loukil, M. S., Marklund, E., Varna, J. & Mattsson, D. (2024). Transverse Crack Initiation in Thin-Ply Laminates Subjected to Tensile Loading at Low and Cryogenic Temperatures. Mechanics of composite materials
Open this publication in new window or tab >>Transverse Crack Initiation in Thin-Ply Laminates Subjected to Tensile Loading at Low and Cryogenic Temperatures
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2024 (English)In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922Article in journal (Refereed) Epub ahead of print
Place, publisher, year, edition, pages
Springer, 2024
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:ltu:diva-103747 (URN)10.1007/s11029-023-10156-0 (DOI)2-s2.0-85181248582 (Scopus ID)
Note

Funder: To increase the research and innovative capacity of scientific institutions of Latvia (1.1.1.2/VIAA/3/19/408);

Available from: 2024-01-16 Created: 2024-01-16 Last updated: 2024-01-16
Varna, J. & Pupure, L. (2023). Effect of Material State and Temperature on Nonlinear Viscoelastic Response: 3D Constitutive Model and Incremental Formulation for Numerical Analysis. Mechanics of composite materials, 59(2), 193-218
Open this publication in new window or tab >>Effect of Material State and Temperature on Nonlinear Viscoelastic Response: 3D Constitutive Model and Incremental Formulation for Numerical Analysis
2023 (English)In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 59, no 2, p. 193-218Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Springer Nature, 2023
National Category
Applied Mechanics Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-97283 (URN)10.1007/s11029-023-10092-z (DOI)000983881600001 ()2-s2.0-85158096946 (Scopus ID)
Funder
European Regional Development Fund (ERDF), 1.1.1.2/VIAA/4/20/646
Note

Validerad;2023;Nivå 2;2023-08-22 (hanlid)

Available from: 2023-05-24 Created: 2023-05-24 Last updated: 2023-08-22Bibliographically approved
Freire, R. T. .., Nunes, S. G., Amico, S. C., Al-Ramahi, N. J., Joffe, R. & Varna, J. (2023). On Determination of the Linear Viscoelastic Compliance and Relaxation Functions for Polymers in One Tensile Test: [Об определении функций линейной вязкоупругой податливости и релаксации полимеров в одном испытании на растяжение]. Mechanics of composite materials, 58(6), 765-786
Open this publication in new window or tab >>On Determination of the Linear Viscoelastic Compliance and Relaxation Functions for Polymers in One Tensile Test: [Об определении функций линейной вязкоупругой податливости и релаксации полимеров в одном испытании на растяжение]
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2023 (English)In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 58, no 6, p. 765-786Article in journal (Refereed) Published
Abstract [en]

Usually, the viscoelastic (VE) response of polymers for applications in composites is obtained in uniaxial strainor stress-controlled tests. However, analyzing multimaterial structures by the Finite Element Method (FEM) or by other numerical or analytical tools, a material model in terms of a complete set of compliance functions and/or relaxation functions is required. In this paper, a methodology and exact analytical expressions for calculating the whole set of VE functions is presented based on the relaxation modulus E(t)and Poisson’s ratio v (t) determined in strain-controlled tests. The method is based on Laplace transforms, where an exact inversion is possible if a linear VE model with functions in Prony series is used. Results of the analytical model are compared with the FEM simulation, where specific boundary conditions to determine each particular VE function are used. Finally, the applicability of the so-called quasi-elastic method is investigated, where the expressions of elasticity theory are used to calculate a given viscoelastic function at an instant of time tk using the instant values of E(tk) and v(tk). For isotropic materials, the three approaches render almost coinciding results.

Place, publisher, year, edition, pages
Springer Nature, 2023
Keywords
FEM, Laplace transforms, linear viscoelasticity, quasi-elastic method, relaxation
National Category
Applied Mechanics
Research subject
Polymeric Composite Materials; Machine Elements
Identifiers
urn:nbn:se:ltu:diva-95466 (URN)10.1007/s11029-023-10067-0 (DOI)000916623000002 ()2-s2.0-85146393623 (Scopus ID)
Note

Validerad;2023;Nivå 2;2023-02-09 (hanlid);

Funder: Higher Education Improvement Coordination (CAPES/Brazil); ERDF (1.1.1.2/VIAA/4/20/641); Swedish Foundation for International Cooperation in Research and Higher Education (STINT);

Reference to this article in Russian:  Фрейре, Р.Т.С., Нуньес С.Г., Амико, С.К. et al. Об определении функций линейной вязкоупругой податливости и релаксации полимеров в одном испытании на растяжение. Механика композитных материалов 58 (6), 1095-1124 (2022). https://doi.org/10.22364/mkm.58.6.01

Available from: 2023-02-09 Created: 2023-02-09 Last updated: 2023-02-14Bibliographically approved
Varna, J., Loukil, M. S., Pupurs, A. & Joffe, R. (2022). Damage and Failure Analysis for Composites. In: Reference Module in Materials Science and Materials Engineering: . Elsevier
Open this publication in new window or tab >>Damage and Failure Analysis for Composites
2022 (English)In: Reference Module in Materials Science and Materials Engineering, Elsevier, 2022Chapter in book (Refereed)
Abstract [en]

Intralaminar cracking in plies of composite laminates is the first microdamage mode that affects thermo-elastic properties and may trigger local delaminations and final failure of the composite. Intralaminar cracks are like tunnels running along the fiber direction in the ply. The number of cracks increases with the increase of the applied load or with the number of cycles in fatigue loading. In this article, the cracking evolution is analyzed distinguishing two phases in development: initiation and propagation. For laminates with thick plies, the initiation ends with triggering sudden propagation and, therefore, concept of statistical initiation stress distribution in the ply together with Monte Carlo method is used for analysis. For thin-ply laminates, crack initiation does not lead to immediate propagation and the energy release concept is used to analyze crack propagation.

Place, publisher, year, edition, pages
Elsevier, 2022
Keywords
Delaminations, Energy release rate, Fatigue, Intralaminar cracks, Monte Carlo method, Quasi-static loading, Weibull distribution
National Category
Applied Mechanics
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-90827 (URN)10.1016/B978-0-12-822944-6.00020-7 (DOI)978-0-12-803581-8 (ISBN)
Available from: 2022-05-31 Created: 2022-05-31 Last updated: 2022-05-31Bibliographically approved
Pupure, L., Varna, J., Gailitis, R., Al-Maqdasi, Z. & Pakrastins, L. (2022). Development of methodology for experimental parameter identification for inelastic 3D material model. In: Vassilopoulos, Anastasios; Michaud, Véronique, Lausanne (Ed.), ECCM 2022 - Proceedings of the 20th European Conference on Composite Materials: Composites Meet Sustainability. Paper presented at 20th European Conference on Composite Materials (ECCM20), June 26-30, 2022, Lausanne, Switzerland (pp. 282-289). Lausanne: EPFL Lausanne, Composite Construction Laboratory, 6
Open this publication in new window or tab >>Development of methodology for experimental parameter identification for inelastic 3D material model
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2022 (English)In: ECCM 2022 - Proceedings of the 20th European Conference on Composite Materials: Composites Meet Sustainability / [ed] Vassilopoulos, Anastasios; Michaud, Véronique, Lausanne, Lausanne: EPFL Lausanne, Composite Construction Laboratory , 2022, Vol. 6, p. 282-289Conference paper, Published paper (Other academic)
Abstract [en]

Lately due to advancement of materials, structures and modeling capacities demand for 3D nonlinear material models implemented in commercial software’s and codes have increased. In order to obtain such implemented model, firstly the model itself and the methodology for experimental parameter identification have to be established. The overall goal is to create a 3D material model, that could account for viscolpasticity, viscoelasticity and damage. Within this study the viscoplastic behavior in axial and lateral direction will be analyzed, thus obtaining the materials 3D viscoplastic behavior. Experimental data for various materials are analyzed. The results showed that viscoplastic strain has the same time and stress dependency in axial and lateral direction. The only difference is in the ration of axial and lateral strain, which is characterized by a single multiplication factor within viscoplastic material model. 

Place, publisher, year, edition, pages
Lausanne: EPFL Lausanne, Composite Construction Laboratory, 2022
Keywords
Nonlinearity, 3D, viscoplasticity, strain recovery, creep
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-96168 (URN)10.5075/epfl-298799_978-2-9701614-0-0 (DOI)2-s2.0-85149412735 (Scopus ID)978-2-9701614-0-0 (ISBN)
Conference
20th European Conference on Composite Materials (ECCM20), June 26-30, 2022, Lausanne, Switzerland
Funder
European Regional Development Fund (ERDF)
Available from: 2023-03-16 Created: 2023-03-16 Last updated: 2023-05-15Bibliographically approved
Pupurs, A., Loukil, M. & Varna, J. (2022). Digital Image Correlation (DIC) Validation of Engineering Approaches for Bending Stiffness Determination of Damaged Laminates. Applied Composite Materials, 29(5), 1937-1958
Open this publication in new window or tab >>Digital Image Correlation (DIC) Validation of Engineering Approaches for Bending Stiffness Determination of Damaged Laminates
2022 (English)In: Applied Composite Materials, ISSN 0929-189X, E-ISSN 1573-4897, Vol. 29, no 5, p. 1937-1958Article in journal (Refereed) Published
Abstract [en]

During the last decade new models for bending stiffness prediction of damaged composite laminates have been proposed in the literature advancing the earlier developed engineering approaches in accuracy and in complexity. However, experimental data for validation of complex analytical or engineering models are almost non-existent in the literature. In the present work a detailed experimental study was performed to investigate the bending stiffness reduction of composite cross-ply laminates with evolving micro-damage. Intralaminar cracks and local delaminations in the bottom surface 90-degree layer of carbon/epoxy and glass/epoxy cross-ply laminates were introduced in 4-point bending tests. Digital Image correlation (DIC) technique was used to experimentally determine the midplane curvature. The accuracy of beam theory for bending stiffness determination was assessed. The measured bending stiffness reduction with respect to transverse crack density was also compared with FEM predictions. The results show that the beam theory gives slightly underestimated curvature at low deflections, whereas at large deflections the beam theory overestimates the curvature and the moment-curvature relation becomes nonlinear. Nevertheless, the overall agreement between beam theory and DIC-based results is still very good, which leads to conclude that beam theory based data reduction schemes have sufficient accuracy for predicting bending stiffness even for highly damaged laminates.

Place, publisher, year, edition, pages
Springer Nature, 2022
Keywords
Composite laminates, Intralaminar cracks, Bending stiffness, Digital image correlation, Beam theory
National Category
Applied Mechanics
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-92314 (URN)10.1007/s10443-022-10045-0 (DOI)000826833200002 ()2-s2.0-85134507480 (Scopus ID)
Note

Validerad;2022;Nivå 2;2022-11-30 (hanlid)

Available from: 2022-08-03 Created: 2022-08-03 Last updated: 2022-11-30Bibliographically approved
Nunes, S. G., Al-Maqdasi, Z., Fernberg, P., Amico, S. C. & Varna, J. (2022). Does the viscoelastic behavior of fully cured epoxy depend on the thermal history during curing?. Journal of composite materials, 56(22), 3439-3453
Open this publication in new window or tab >>Does the viscoelastic behavior of fully cured epoxy depend on the thermal history during curing?
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2022 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 56, no 22, p. 3439-3453Article in journal (Refereed) Published
Abstract [en]

Residual strains and shape distortions in a polymer-based composite structure may depend on the cure schedule used for manufacture. Aiming to understand the cure history effects, the influence of the curing “path” (time tc and temperature Tc path during curing) on viscoelastic (VE) response of a fully cured (FC) (alpha = 0.992) epoxy was investigated. Five different “families” of the same epoxy were manufactured in constraint-free conditions using different sets of curing parameters. Then, tensile tests were performed at different temperatures (T = 30 to 110 °C), and the time-temperature superposition principle (TTSP) and Schapery’s type of linear viscoelastic (VE) model, accounting for physical aging of specimens tested at high temperature, were used. The results show that the VE properties of the studied epoxy are independent of the curing history provided that at the end all specimens are fully cured. Also, the physical aging rate at high temperatures of all “families” is the same and it can be described by a simple aging-temperature independent equation reported in Nunes et al1 It is expected that curing history of unconstrained and fully cured epoxy has an insignificant effect on final viscoelastic behavior, a knowledge which could assist in developing more time and cost-efficient cure cycles.

Place, publisher, year, edition, pages
Sage Publications, 2022
Keywords
epoxy, curing history, viscoelasticity, physical aging, shift factors, simulations
National Category
Other Materials Engineering
Research subject
Polymeric Composite Materials; Machine Elements
Identifiers
urn:nbn:se:ltu:diva-92323 (URN)10.1177/00219983221115850 (DOI)000826103600001 ()2-s2.0-85134535047 (Scopus ID)
Note

Validerad;2022;Nivå 2;2022-09-26 (hanlid);

Funder: European Regional Development Fund (1.1.1.2/VIAA/4/20/641); Swedish Foundation for International Cooperation in Research and Higher Education, STINT; Higher Education Improvement Coordination CAPES/Brazil

Available from: 2022-07-29 Created: 2022-07-29 Last updated: 2022-09-26Bibliographically approved
Pakkam Gabriel, V. R., Loukil, M. S. & Varna, J. (2022). Intralaminar cracking during cyclic loading in laminates with distributed failure stress in 90-plies. International Journal of Fatigue, 161, Article ID 106909.
Open this publication in new window or tab >>Intralaminar cracking during cyclic loading in laminates with distributed failure stress in 90-plies
2022 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 161, article id 106909Article in journal (Refereed) Published
Abstract [en]

Methodology for cracking evolution simulation in 90-plies during cyclic loading is suggested in this paper. It includes using low crack density (where the neighboring cracks do not interact with each other) data from quasi-static and cyclic testing (at one load) to determine parameters in Weibull transverse failure stress distribution generalized for fatigue. Then, Monte Carlo process is used to assign failure stress to elements in the 90-ply. In the model, the failure stress of the element degrades with number of cycles and a crack appears when the failure stress becomes equal to the stress in the element, calculated with computationally efficient analytical method. The model is successfully used to predict cracking in the whole crack density range. In this model, data from cyclic test at one load is sufficient to predict damage at different load. The results suggest that the dependence on stress is slightly underestimated and the shape parameter in fatigue may be higher than in quasi-static tests. Therefore, fatigue tests at least at two load levels are recommended for reliable predictions.

Place, publisher, year, edition, pages
Elsevier, 2022
Keywords
Intralaminar cracking, Monte Carlo simulations, Fatigue, Damage tolerance, Composite
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-90128 (URN)10.1016/j.ijfatigue.2022.106909 (DOI)000799364700002 ()2-s2.0-85129006085 (Scopus ID)
Note

Validerad;2022;Nivå 2;2022-06-01 (hanlid);

Funder: Swedish Aeronautical Research Program, NFFP7 (2019-02777)

Available from: 2022-04-08 Created: 2022-04-08 Last updated: 2024-02-23Bibliographically approved
Pakkam Gabriel, V. R., Loukil, M. S., Fernberg, P. & Varna, J. (2022). Methodology for transverse cracking simulation in 90 plies of composite laminate under fatigue loading. In: Vassilopoulos, Anastasios P.; Michaud, Véronique (Ed.), ECCM 2022 - Proceedings of the 20th European Conference on Composite Materials: Composites Meet Sustainability. Paper presented at 20th European Conference on Composite Materials (ECCM20), Lausanne, Switzerland, June 26-30, 2022 (pp. 16-22). EPFL Lausanne, Composite Construction Laboratory, 6
Open this publication in new window or tab >>Methodology for transverse cracking simulation in 90 plies of composite laminate under fatigue loading
2022 (English)In: ECCM 2022 - Proceedings of the 20th European Conference on Composite Materials: Composites Meet Sustainability / [ed] Vassilopoulos, Anastasios P.; Michaud, Véronique, EPFL Lausanne, Composite Construction Laboratory , 2022, Vol. 6, p. 16-22Conference paper, Published paper (Other academic)
Abstract [en]

Methodology for crack density evolution simulation in tension-tension fatigue and parameter determination in a model that relies on failure stress distribution (Weibull) in the 90 ply is presented. Cyclic loading is performed at one stress level to obtain detailed crack density dependence on the number of cycles. In addition, one data point (crack density at specified number of cycles) is necessary at a different stress level. Non-interactive crack density region is used to determine Weibull parameters. Then, crack density in a whole crack density range and for arbitrary stress level is predicted using the obtained Weibull parameters, the Monte Carlo method for failure stress distribution and a novel model for stress distribution between cracks. The predictions are in good agreement with test results.

Place, publisher, year, edition, pages
EPFL Lausanne, Composite Construction Laboratory, 2022
Keywords
Intralaminar cracking, Weibull parameter determination, Shape parameter, Scale parameter, Fatigue loading, Failure stress, Monte Carlo simulation
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-90131 (URN)2-s2.0-85149406235 (Scopus ID)
Conference
20th European Conference on Composite Materials (ECCM20), Lausanne, Switzerland, June 26-30, 2022
Projects
Swedish Aeronautical Research Program (NFFP7)
Note

Funder: Swedish Aeronautical Research Program (NFFP7) (2019- 02777);

ISBN för värdpublikation:  978-2-9701614-0-0

Available from: 2022-04-10 Created: 2022-04-10 Last updated: 2023-09-06Bibliographically approved
Nunes, S., Joffe, R., Emami, N., Fernberg, P., Saseendran, S., Esposito, A., . . . Varna, J. (2022). Physical aging effect on viscoelastic behavior of polymers. Composites Part C: Open Access, 7, Article ID 100223.
Open this publication in new window or tab >>Physical aging effect on viscoelastic behavior of polymers
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2022 (English)In: Composites Part C: Open Access, ISSN 2666-6820, Vol. 7, article id 100223Article in journal (Refereed) Published
Abstract [en]

The effect of physical aging on the viscoelastic (VE) behavior of epoxy resin is investigated experimentally performing strain-controlled tests at various temperatures on specimens aged at different temperatures (TA) for different times (tA). The aging effect is analyzed using as a framework Schapery's type of thermo-aging-rheologically simple (T-A-R simple) VE model that contains aging-state and test-temperature dependent shift factor. Experiments show that in first approximation, the shift factor can be presented as the product of aging related shift factor aA and temperature related factor aT. It is found that for short aging times the change rate of the aging shift factor with tA does not depend on TA, whereas for long tA at high TA the rate increases. Shift factors alone are not able to explain differences in relaxation curves for almost “fully” aged specimens aged at different high TA, It is shown that a T-A-R complex VE model with two additional aging-dependent functions can describe the observed discrepancies.

Place, publisher, year, edition, pages
Elsevier, 2022
Keywords
Physical aging, Viscoelasticity, Shift factors, Numerical analysis
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials; Machine Elements
Identifiers
urn:nbn:se:ltu:diva-88621 (URN)10.1016/j.jcomc.2021.100223 (DOI)000819159600009 ()2-s2.0-85121922581 (Scopus ID)
Note

Validerad;2022;Nivå 2;2022-01-01 (johcin)

Available from: 2021-12-30 Created: 2021-12-30 Last updated: 2023-09-06Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-9649-8621

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