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Publications (10 of 283) Show all publications
Varna, J. & Pupure, L. (2019). Characterization of viscoelasticity, viscoplasticity, and damage in composites (2ed.). In: Rui Miranda Guedes (Ed.), Creep and Fatigue in Polymer Matrix Composites: (pp. 497-530). Elsevier
Open this publication in new window or tab >>Characterization of viscoelasticity, viscoplasticity, and damage in composites
2019 (English)In: Creep and Fatigue in Polymer Matrix Composites / [ed] Rui Miranda Guedes, Elsevier, 2019, 2, p. 497-530Chapter in book (Refereed)
Abstract [en]

Empirical inelastic constitutive material models for composites and testing methodology for parameter determination in these models are analyzed. Short fiber as well as long unidirectional fiber reinforced composites are analyzed in situations when the main sources of inelastic behavior are a combination of (a) nonlinear viscoelasticity; (b) nonlinear viscoplasticity; and (c) microdamage-induced reduction of thermoelastic properties, all three evolving with time and stress. These phenomena are included in a common material model. The necessary tests for model identification are tensile quasistatic loading-unloading tests and creep tests at different stress levels with recorded strain recovery after load removal. The methodology is demonstrated presenting models for (a) shear in layers of [45/−45]s laminates; (b) response of short fiber composites (SMC with glass fiber bundles; composites with natural or man-made cellulosic fibers in bio-based resins).

Place, publisher, year, edition, pages
Elsevier, 2019 Edition: 2
Series
Woodhead Publishing Series in Composites Science and Engineering
Keywords
Fiber composites, Stiffness reduction, Viscoelasticity, Viscoplasticity, Creep
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-73438 (URN)10.1016/B978-0-08-102601-4.00016-3 (DOI)978-0-08-102601-4 (ISBN)
Available from: 2019-04-05 Created: 2019-04-05 Last updated: 2019-04-05Bibliographically approved
Di Stasio, L., Varna, J. & Ayadi, Z. (2019). Energy release rate of the fiber/matrix interface crack in UD composites under transverse loading: Effect of the fiber volume fraction and of the distance to the free surface and to non-adjacent debonds. Theoretical and applied fracture mechanics (Print), 103, Article ID 102251.
Open this publication in new window or tab >>Energy release rate of the fiber/matrix interface crack in UD composites under transverse loading: Effect of the fiber volume fraction and of the distance to the free surface and to non-adjacent debonds
2019 (English)In: Theoretical and applied fracture mechanics (Print), ISSN 0167-8442, E-ISSN 1872-7638, Vol. 103, article id 102251Article in journal (Refereed) Published
Abstract [en]

The effects of crack shielding, finite thickness of the composite and fiber content on fiber/matrix debond growth in thin unidirectional composites are investigated analyzing Representative Volume Elements (RVEs) of different ordered microstructures. Debond growth is characterized by estimation of the Energy Release Rates (ERRs) in Mode I and Mode II using the Virtual Crack Closure Technique (VCCT) and the J-integral. It is found that increasing fiber content, a larger distance between debonds in the loading direction and the presence of a free surface close to the debond have all a strong enhancing effect on the ERR. The presence of fully bonded fibers in the composite thickness direction has instead a constraining effect, and it is shown to be very localized. An explanation of these observations is proposed based on mechanical considerations.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Polymer-matrix Composites (PMCs), Thin-ply, Energy Release Rate, Debonding, Finite Element Analysis (FEA)
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-75072 (URN)10.1016/j.tafmec.2019.102251 (DOI)2-s2.0-85066800172 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-06-27 (johcin)

Available from: 2019-06-27 Created: 2019-06-27 Last updated: 2019-06-28Bibliographically approved
Di Stasio, L., Varna, J. & Ayadi, Z. (2019). Investigation of Scaling Laws of the Fiber/Matrix Interface Crack in Polymer Composites through Finite Element-based Micromechanical Modeling. In: Book of abstracts of the 10th EEIGM International Conference on Advanced Materials Research: . Paper presented at 10th EEIGM International Conference on Advanced Materials Research - April 25-26, 2019 - Moscow, Russia. Moscow, Russia
Open this publication in new window or tab >>Investigation of Scaling Laws of the Fiber/Matrix Interface Crack in Polymer Composites through Finite Element-based Micromechanical Modeling
2019 (English)In: Book of abstracts of the 10th EEIGM International Conference on Advanced Materials Research, Moscow, Russia, 2019Conference paper, Oral presentation with published abstract (Refereed)
Place, publisher, year, edition, pages
Moscow, Russia: , 2019
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-73928 (URN)
Conference
10th EEIGM International Conference on Advanced Materials Research - April 25-26, 2019 - Moscow, Russia
Available from: 2019-05-13 Created: 2019-05-13 Last updated: 2019-05-13
Xu, J. & Varna, J. (2019). Matrix and interface microcracking in carbon fiber/polymer structural micro-battery. Journal of composite materials
Open this publication in new window or tab >>Matrix and interface microcracking in carbon fiber/polymer structural micro-battery
2019 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793XArticle in journal (Refereed) Epub ahead of print
Abstract [en]

In this paper, the propagation of radial matrix cracks and debond cracks at the coating/matrix interface in unidirectional carbon fiber structural micro-battery composite are studied numerically. The micro battery consists of a solid electrolyte-coated carbon fiber embedded in an electrochemically active polymer matrix. Stress analysis shows that high hoop stress in the matrix during charging may initiate radial matrix cracks at the coating/matrix interface. Several 2-D finite element models of the transverse plane with different arrangements of fibers and other matrix cracks were used to analyze the radial matrix crack growth from the coating/matrix interface of the central fiber in a composite with a square packing of fibers. Energy release rates of radial cracks along two potential propagation paths are calculated under pure electrochemical loading. The presence of a radial matrix crack imposes changes in the stress distribution along the coating/matrix interface, making debonding relevant for consideration. Results for energy release rates show that the debond crack growth is governed by mode II.

Place, publisher, year, edition, pages
Sage Publications, 2019
Keywords
Carbon fiber composite, intercalation, micro-battery, microcracking, modeling
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-74910 (URN)10.1177/0021998319843616 (DOI)2-s2.0-85064940549 (Scopus ID)
Available from: 2019-06-24 Created: 2019-06-24 Last updated: 2019-06-24
Saseendran, S., Berglund, D. & Varna, J. (2019). Stress relaxation and strain recovery phenomena during curing and thermomechanical loading: Thermorheologically simple viscoelastic analysis. Journal of composite materials
Open this publication in new window or tab >>Stress relaxation and strain recovery phenomena during curing and thermomechanical loading: Thermorheologically simple viscoelastic analysis
2019 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793XArticle in journal (Refereed) Epub ahead of print
Abstract [en]

Stress relaxation and strain recovery phenomena during curing and changed thermal conditions are analyzed using a viscoelastic model developed for thermorheologically complex materials (VisCoR). By making several simplifying assumptions regarding the material behavior, the incremental form of the VisCoR model is reformulated to a version describing thermorheologically simple material and presented in one-dimension for simplicity. The model (called VisCoR-simple) is used to analyze material behavior under various conditions, including stress relaxation behavior at varying temperatures and time scales; tensile loading and unloading tests at high temperatures; stress build up and “frozen-in” strains during curing and following cool-down and strain recovery during the next step of heating. Furthermore, the differences between the so-called “path-dependent” model, which is a linear elastic model with different elastic properties in glassy and rubbery regions, and the presented viscoelastic model are studied. The path-dependent model is an extreme case of the viscoelastic model presented. The importance of considering viscoelasticity when considering temperature and curing effects on polymers and the shortcomings of the path-dependent model are revealed and discussed.

Place, publisher, year, edition, pages
Sage Publications, 2019
Keywords
Viscoelasticity, process simulation, stress relaxation, strain recovery, temperature and degree of cure
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-74974 (URN)10.1177/0021998319848818 (DOI)2-s2.0-85065747166 (Scopus ID)
Available from: 2019-06-25 Created: 2019-06-25 Last updated: 2019-06-25
Xu, J., Lindbergh, G. & Varna, J. (2018). Carbon fiber composites with battery function: Stresses and dimensional changes due to Li-ion diffusion. Journal of composite materials, 52(20), 2729-2742
Open this publication in new window or tab >>Carbon fiber composites with battery function: Stresses and dimensional changes due to Li-ion diffusion
2018 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 52, no 20, p. 2729-2742Article in journal (Refereed) Published
Abstract [en]

Structural composite materials that simultaneously carry mechanical loads, while storing electrical energy offers the potential of significantly reduced total component weight owing to the multifunctionality. In the suggested micro-battery, the carbon fiber is employed as a negative electrode of the battery and also as a composite reinforcement material. It is coated with a solid polymer electrolyte working as an ion conductor and separator while transferring mechanical loads. The coated fiber is surrounded by a conductive positive electrode material matrix. This paper demonstrates a computational methodology for addressing mechanical stresses arising in a conceptualized micro-battery and dimensional changes of the cell during electrochemical cycling, caused by time-dependent gradients in lithium ion concentration distribution.

Place, publisher, year, edition, pages
Sage Publications, 2018
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-67699 (URN)10.1177/0021998317752825 (DOI)000441034800003 ()2-s2.0-85051282917 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-08-08 (rokbeg)

Available from: 2018-02-20 Created: 2018-02-20 Last updated: 2018-08-23Bibliographically approved
Kahla, H. B. & Varna, J. (2018). Characterization and modelling of multiple intralaminar cracking initiation under tensile quasi-static and fatigue loading. In: : . Paper presented at 17th International Conference on Fracture and Damage Mechanics, FDM 2018; Bangkok; Thailand; 4-6 September 2018; Code 217369 (pp. 467-472). Trans Tech Publications, 774
Open this publication in new window or tab >>Characterization and modelling of multiple intralaminar cracking initiation under tensile quasi-static and fatigue loading
2018 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The first failure mode in tensile quasi-static and in tension-tension fatigue (cyclic) loading of composite laminates is intralaminar cracking in layers with off-axis fiber orientation. These tunnel-building cracks are result of combined action of in-plane transverse and shear stresses. We assume that due to non-uniform fiber distribution (clustering) which leads to local stress concentrations, different positions in the layer have different resistance to crack initiation (initiation strength). If so, the weakest position in quasi-static loading is also the weakest in fatigue and some of the distribution parameters for fatigue behavior can be obtained in quasi-static tests, thus significantly reducing the number of required fatigue tests. Methodology is suggested and validated for cases when the cracking is initiation governed- initiated crack almost instantly propagates along fibers. Distribution parameters are identified using data in low crack density region where stress perturbations from cracks do not interact. Monte- Carlo simulations are performed for cracking in layers under quasi-static and cyclic loading using novel approach for computationally efficient stress state calculation between existing cracks. 

Place, publisher, year, edition, pages
Trans Tech Publications, 2018
Series
Key Engineering Materials, ISSN 1013-9826, E-ISSN 1662-9795 ; 774
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-70942 (URN)10.4028/www.scientific.net/KEM.774.467 (DOI)9783035713503 (ISBN)
Conference
17th International Conference on Fracture and Damage Mechanics, FDM 2018; Bangkok; Thailand; 4-6 September 2018; Code 217369
Available from: 2018-09-24 Created: 2018-09-24 Last updated: 2018-09-24Bibliographically approved
Di Stasio, L., Varna, J. & Ayadi, Z. (2018). Effect of boundary conditions on microdamage initiation in thin ply composite laminates. In: : . Paper presented at ECCM18 - 18 th European Conference on Composite Materials, Athens, Greece, 24-28 th June 2018.
Open this publication in new window or tab >>Effect of boundary conditions on microdamage initiation in thin ply composite laminates
2018 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The Energy Release Rate (ERR) and the contact zone size for a fiber/matrix interface debond are studied for a thin-ply glass fiber/epoxy laminate. The main objective is to analyze the effect on the debonding process of the presence of a traction-free specimen surface or an adjacent material, in the form of a stiffer UD ply or by considering it as part of a thick 90° layer, at different levels of fiber content. To this end, a model of Representative Volume Element (RVE) subjected to different combinations of boundary conditions is proposed. It is found that the constraining effect of the adjacent ply favors at high fiber volume fractions the opening of small debonds (10 − 40°) for the same level of strain. The results agree well and provide a mechanical explanation to previous microscopic observations available in the literature [4].

Keywords
Polymer-matrix Composites (PMCs), Thin-ply, Transverse Failure, Debonding, Finite Element Analysis (FEA)
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-71656 (URN)
Conference
ECCM18 - 18 th European Conference on Composite Materials, Athens, Greece, 24-28 th June 2018
Available from: 2018-11-19 Created: 2018-11-19 Last updated: 2019-01-21Bibliographically approved
Zhuang, L., Pupurs, A., Varna, J., Talreja, R. & Ayadi, Z. (2018). Effects of Inter-Fiber Spacing on Fiber-matrix Debond Crack Growth in Unidirectional Composites under Transverse Loading. Composites. Part A, Applied science and manufacturing, 109, 463-471
Open this publication in new window or tab >>Effects of Inter-Fiber Spacing on Fiber-matrix Debond Crack Growth in Unidirectional Composites under Transverse Loading
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2018 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 109, p. 463-471Article in journal (Refereed) Published
Abstract [en]

The energy release rate (ERR) of a fiber-matrix debond crack in a unidirectional composite subjected to transverse tension is studied numerically. The focus of the study is the effect of the proximity of the neighboring fibers on the ERR. For this, a hexagonal pattern of fibers in the composite cross-section is considered. Assuming one fiber to be debonded at certain initial debond arc-length, the effect of the closeness of the surrounding six fibers on the ERR of the crack is studied with the inter-fiber distance as a parameter. Using an embedded cell consisting of discrete fibers in a matrix surrounded by the homogenized composite, a finite element model and the virtual crack closure technique are used to calculate the ERR. Results show that the presence of the local fiber cluster accelerates the crack growth up to a certain initial crack angle, beyond which the opposite effect occurs. It is also found that the residual stress due to thermal cooldown reduces the ERR. However, the thermal cooldown is found to enhance the debond growth in plies within a cross-ply laminate.

Place, publisher, year, edition, pages
Elsevier, 2018
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-68101 (URN)10.1016/j.compositesa.2018.03.031 (DOI)000432508500044 ()2-s2.0-85044595517 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-04-03 (rokbeg)

Available from: 2018-03-28 Created: 2018-03-28 Last updated: 2018-06-08Bibliographically approved
Al-Ramahi, N., Joffe, R. & Varna, J. (2018). Fem analysis of stresses in adhesive single-lap joints with non-linear materials under thermo-mechanical loading. In: ECCM18: . Paper presented at 18th European Conference on Composite Materials Athens, Greece, 24-28th June 2018.
Open this publication in new window or tab >>Fem analysis of stresses in adhesive single-lap joints with non-linear materials under thermo-mechanical loading
2018 (English)In: ECCM18, 2018Conference paper, Published paper (Refereed)
Abstract [en]

This study presents comprehensive numerical stress analysis in the adhesive layer of a single-lap joint subjected to various loading scenarios (mechanical and thermal loading). For this purpose numerical model (finite element method) with novel displacement coupling conditions able to correctly represent monoclinic materials (off-axis layers of composite laminates) has been developed. This model includes nonlinear material model and geometrical nonlinearity is also accounted for. The effect of thermal residual stresses (in adhesive) is analysed for various methods of manufacturing of single lap joint. The sequences of application of thermal and mechanical loads for the analysis of the thermal residual stresses in joints are proposed. It is shown that the most common approach used in many studies of linear superposition of thermal and mechanical stresses works well only for linear materials and produces wrong results if material is non-linear. The present study demonstrates suitable method to apply combined thermal and mechanical loads to get accurate stress distributions. Based on the analysis of these stress distributions the conclusions concerning the effect of the thermal residual stresses on peel and shear stress concentrations are made. The comparison between effect of thermal stresses in case of the one-step and two-step joint manufacturing techniques is made.

National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-70262 (URN)
Conference
18th European Conference on Composite Materials Athens, Greece, 24-28th June 2018
Available from: 2018-08-07 Created: 2018-08-07 Last updated: 2018-08-13Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-9649-8621

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