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Publications (10 of 289) 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
Loukil, M. & Varna, J. (2019). Crack face sliding displacement (CSD) as an input in exact GLOB-LOC expressions for in-plane elastic constants of symmetric damaged laminates. International journal of damage mechanics
Open this publication in new window or tab >>Crack face sliding displacement (CSD) as an input in exact GLOB-LOC expressions for in-plane elastic constants of symmetric damaged laminates
2019 (English)In: International journal of damage mechanics, ISSN 1056-7895, E-ISSN 1530-7921Article in journal (Refereed) Epub ahead of print
Abstract [en]

The crack opening and crack sliding displacements of both faces of an intralaminar crack are the main parameters defining the significance of each crack in laminate stiffness degradation, according to the previously published GLOB-LOC approach for symmetric laminates with an arbitrary number of cracks in all plies. In the exact stiffness expressions of this approach, the crack density is always multiplied by crack opening displacement and crack sliding displacement. The dependence of crack opening displacement on geometrical and elastic parameters of adjacent plies was studied previously and described by simple fitting functions. The crack sliding displacement has been analyzed for low-crack densities only and the proposed finite element method-based fitting expressions are oversimplified not including the out-of-plane ply stiffness effects. Based on finite element method analysis, more accurate expressions for so-called non-interactive cracks are suggested in the presented article. For the first time the shear stress perturbations are analyzed and interaction functions are presented with the feature that they always lead to slightly conservative predictions. The presented simple fitting functions, when used in the GLOB-LOC model, give predictions that are in a good agreement with finite element method results and with experimental data for laminates with damaged off-axis plies in cases when crack face sliding is of importance. The significance of including crack sliding displacement in stiffness predictions is demonstrated.

Place, publisher, year, edition, pages
Sage Publications, 2019
Keywords
Composite laminates, crack sliding displacement, damage, finite element analysis, transverse cracking
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-75625 (URN)10.1177/1056789519866000 (DOI)000479429300001 ()
Available from: 2019-08-21 Created: 2019-08-21 Last updated: 2019-08-28
Di Stasio, L., Varna, J. & Ayadi, Z. (2019). Effect of the proximity to the 0°/90° interface on Energy Release Rate of fiber/matrix interface crack growth in the 90°-ply of a cross-ply laminate under tensile loading. Journal of composite materials
Open this publication in new window or tab >>Effect of the proximity to the 0°/90° interface on Energy Release Rate of fiber/matrix interface crack growth in the 90°-ply of a cross-ply laminate under tensile loading
2019 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793XArticle in journal (Refereed) Submitted
Abstract [en]

Models of Representative Volume Elements (RVEs) of cross-ply laminates with different geometric configurations and damage states are studied. Debond growth is characterized by the estimation of the Mode I and Mode II Energy Release Rate (ERR) using the Virtual Crack Closure Technique (VCCT). It is found that the presence of the 0°/90° interface and the thickness of the 0° layer have no effect, apart from laminates with ultra-thin 90° plies where it is however modest. The present analysis support the claim that debond growth is not affected by the plythickness effect.

Keywords
Polymer-matrix Composites (PMCs), Fibre/matrix bond, Debonding, Finite Element Analysis (FEA)
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-76253 (URN)
Available from: 2019-10-05 Created: 2019-10-05 Last updated: 2019-10-05
Loukil, M. S. & Varna, J. (2019). Effective shear modulus of a damaged ply in laminate stiffness analysis: Determination and validation. Journal of composite materials
Open this publication in new window or tab >>Effective shear modulus of a damaged ply in laminate stiffness analysis: Determination and validation
2019 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793XArticle in journal (Refereed) Epub ahead of print
Abstract [en]

The concept of the “effective stiffness” for plies in laminates containing intralaminar cracks is revisited presenting rather accurate fitting expressions for the effective stiffness dependence on crack density in the ply. In this article, the effective stiffness at certain crack density is back-calculated from the stiffness difference between the undamaged and damaged laminate. Earlier finite element method analysis of laminates with cracked 90-plies showed that the effective longitudinal modulus and Poisson’s ratio of the ply do not change during cracking, whereas the transverse modulus reduction can be described by a simple crack density dependent function. In this article, focus is on the remaining effective constant: in-plane shear modulus. Finite element method parametric analysis shows that the dependence on crack density is exponential and the fitting function is almost independent of geometrical and elastic parameters of the surrounding plies. The above independence justifies using the effective ply stiffness in expressions of the classical laminate theory to predict the intralaminar cracking caused stiffness reduction in laminates with off-axis plies. Results are in a very good agreement with (a) finite element method calculations; (b) experimental data, and (c) with the GLOB-LOC model, which gives a very accurate solution in cases where the crack face opening and sliding displacements are accurately described.

Place, publisher, year, edition, pages
Sage Publications, 2019
Keywords
Effective shear modulus, effective stiffness, stiffness prediction, transverse cracking
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-76324 (URN)10.1177/0021998319874369 (DOI)
Available from: 2019-10-09 Created: 2019-10-09 Last updated: 2019-10-09
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). Estimating the average size of fiber/matrix interface cracks in UD and cross-ply laminates. In: A. Turon, P. Maimi, M. Fagerström (Ed.), : . Paper presented at 7th ECCOMAS Conference of the Mechanical Response of Composites (COMPOSITES 2019).
Open this publication in new window or tab >>Estimating the average size of fiber/matrix interface cracks in UD and cross-ply laminates
2019 (English)In: / [ed] A. Turon, P. Maimi, M. Fagerström, 2019Conference paper, Published paper (Refereed)
Abstract [en]

Initiation and propagation of fiber/matrix interface cracks are analyzed in Representative Volume Elements (RVEs) of UD and cross-ply laminates. By studying the distribution of stresses at the fiber/matrix interface in the undamaged case, an estimate of the initial flaw size is derived. By adopting a 2-parameters energy-based criterion for propagation [1], we then proceed to the estimation of the expected debond size in different microstructural arrangements. Finally, the results are compared with microscopic observations available in the literature [2].

Keywords
Fiber Reinforced Polymer (FRP), Debonding, Linear Elastic Fracture Mechanics (LEFM)
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials; Polymeric Composite Materials; Polymeric Composite Materials; Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-76254 (URN)
Conference
7th ECCOMAS Conference of the Mechanical Response of Composites (COMPOSITES 2019)
Available from: 2019-10-05 Created: 2019-10-05 Last updated: 2019-10-05
Di Stasio, L., Varna, J. & Ayadi, Z. (2019). Growth of interface cracks on consecutive fibers: on the same or on the opposite sides?. Paper presented at 12th International Conference on Composite Science and Technology (ICCST 12). Materials Today: Proceedings
Open this publication in new window or tab >>Growth of interface cracks on consecutive fibers: on the same or on the opposite sides?
2019 (English)In: Materials Today: Proceedings, E-ISSN 2214-7853Article in journal (Refereed) Submitted
Abstract [en]

The growth of fiber/matrix interface cracks (debonds) locatedon consecutive fibers along the through-the-thickness (vertical)direction is studied in glass fiber-epoxy UD composites. Debonds couldappear, along the vertical direction, on the same or on opposite sides oftheir respective fibers. Determining which configuration is the mostenergetically favorable to debond growth is the objective of this paper.To this end, two different families of Representative Volume Elements(RVEs) are developed: the first implements the classic condition ofcoupling of the vertical displacements to model a unit cell repeating symmetrically along the vertical direction; the second uses a novel setof boundary conditions, proposed here by the authors, to represent a unitcell repeating anti-symmetrically along the vertical direction. The modelis analyzed in the context of Linear Elastic Fracture Mechanics (LEFM)and the Mode I and Mode II Energy Release Rate are evaluated toinvestigate crack growth. The calculation is performed using the VirtualCrack Closure Technique (VCCT) in the framework of the Finite ElementMethod (FEM). It is found that Mode I dominated propagation is favoredwhen debonds are located on the same sides of their respective fibers;while for larger (Mode II-dominated) debonds, Mode II ERR is higher whenthey lie on the opposite sides. No interaction effect is present when atleast two fully bonded fibers are located between the partially debonded ones.

Keywords
Polymer Matrix Composite (PMC), Fracture mechanics, Debonding, Debond Interaction
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-76255 (URN)
Conference
12th International Conference on Composite Science and Technology (ICCST 12)
Available from: 2019-10-05 Created: 2019-10-05 Last updated: 2019-10-05
Hajlane, A. & Varna, J. (2019). Identification of a Model of Transverse Viscoplastic Deformation for a UD Composite from Curvature Changes of Unsymmetric Cross-Ply Specimens. Mechanics of composite materials, 55(3), 363-384
Open this publication in new window or tab >>Identification of a Model of Transverse Viscoplastic Deformation for a UD Composite from Curvature Changes of Unsymmetric Cross-Ply Specimens
2019 (English)In: Mechanics of composite materials, ISSN 0191-5665, E-ISSN 1573-8922, Vol. 55, no 3, p. 363-384Article in journal (Refereed) Published
Abstract [en]

A novel testing methodology for the characterization of viscoplastic (VP) deformation of unidirectional (UD) composites under transverse tensile loading is described. The law of transverse VP deformation, which, due to specimen failure, is almost impossible to obtain using UD specimens, is identified from curvature changes of an unsymmetrical CF/EP [0/90] specimen. An initially thermally curved specimen is loaded in the axial direction to different high levels of tensile strain, with a specified holding time. After unloading and waiting for recovery of the viscoelastic strains, the curvature of the specimen had changed due to the irreversible transverse VP strain and microdamage had developed in the 90° ply. The damage effect is incorporated into the model by introducing the“effective stiffness” of the damaged layer, which is a function of the crack density and the local delamination length. A model based on the classical laminate theory is used to construct a relationship between the measured curvature after a loading step and the accumulated VP strain, temperature, and the effective stiffness. The back-calculated VP strains are analyzed, and their functional dependence on the stress and time in the viscoplasticity law is obtained. The model and the methodology are validated by comparing calculation results with test data for [902/0] laminates subjected to a similar loading. The methodology suggested could be very accurate on using a laminate with ultrathin plies in the test.

Place, publisher, year, edition, pages
Springer, 2019
Keywords
damage, residual stresses, unsymmetrical laminates, viscoplasticity
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-75643 (URN)10.1007/s11029-019-09818-9 (DOI)000480802300007 ()
Note

Validerad;2019;Nivå 2;2019-08-30 (johcin)

Available from: 2019-08-21 Created: 2019-08-21 Last updated: 2019-08-30Bibliographically 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: 2019-09-06
Xu, J. & Varna, J. (2019). Matrix and interface microcracking in carbon fiber/polymer structural micro-battery. Journal of composite materials, 53(25), 3615-3628
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 1530-793X, Vol. 53, no 25, p. 3615-3628Article in journal (Refereed) Published
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)000484434300009 ()2-s2.0-85064940549 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-09-27 (johcin)

Available from: 2019-06-24 Created: 2019-06-24 Last updated: 2019-09-27Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0001-9649-8621

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