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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
Al-Ramahi, N., Joffe, R. & Varna, J. (2018). Investigation of end and edge effects on results of numerical simulation of single lap adhesive joint with non-linear materials. International Journal of Adhesion and Adhesives, 87, 191-204
Open this publication in new window or tab >>Investigation of end and edge effects on results of numerical simulation of single lap adhesive joint with non-linear materials
2018 (English)In: International Journal of Adhesion and Adhesives, ISSN 0143-7496, E-ISSN 1879-0127, Vol. 87, p. 191-204Article in journal (Refereed) Published
Abstract [en]

This paper presents systematic numerical study of stresses in the adhesive of a single-lap joint with the objective to improve understanding of the main material and geometrical parameters determining performance of adhesive joints. For this purpose a 3D model as well as 2D model, optimized with respect to the computational efficiency by use of novel displacement coupling conditions able to correctly represent monoclinic materials (off-axis layers of composite laminates), are employed. The model accounts for non-linearity of materials (adherend and adhesive) as well as geometrical non-linearity. The parameters of geometry of the joint are normalized with respect to the dimensions of adhesive (e.g. thickness) thus making analysis of results more general and applicable to wide range of different joints. Optimal geometry of the single-lap joint allowing to separate edge effect from end effects is selected based on results of the parametric analysis by using peel and shear stress distributions in the adhesive layer as a criterion. Three different types of single lap joint with similar and dissimilar (hybrid) materials are considered in this study: a) metal-metal; b) composite-composite; c) composite-metal. In case of composite laminates, four lay-ups are evaluated: uni-directional ([08]T and [908]T) and quasi-isotropic laminates ([0/45/90/-45]S and [90/45/0/-45]S). The influence of the abovementioned parameters on peel and shear stress distributions in the adhesive layer is examined carefully and mechanical parameters governing the stress concentrations in the joint have been identified, this dependence can be described by simple but accurate fitting function. The effect of the used material model (linear vs non-linear) on results is also demonstrated.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Composites, Single-lap adhesive joint, Hybrid joints, Finite element method, Stress distribution, Stress analysis
National Category
Applied Mechanics Manufacturing, Surface and Joining Technology Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-71462 (URN)10.1016/j.ijadhadh.2018.10.007 (DOI)000449894000022 ()2-s2.0-85055248042 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-12-07 (johcin)

Available from: 2018-11-19 Created: 2018-11-19 Last updated: 2018-12-10Bibliographically approved
Pupure, L., Varna, J., Joffe, R., Berthold, F. & Miettinen, A. (2018). Mechanical properties of natural fiber composites produced using dynamic sheet former. Wood Material Science & Engineering
Open this publication in new window or tab >>Mechanical properties of natural fiber composites produced using dynamic sheet former
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2018 (English)In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280Article in journal (Refereed) Epub ahead of print
Abstract [en]

Composites formed from wood fibers and man-made cellulosic fibers in PLA (polylactic acid) matrix, manufactured using sheet forming technique and hot pressing, are studied. The composites have very low density (due to high porosity) and rather good elastic modulus and tensile strength. As expected, these properties for the four types of wood fiber composites studied here improve with increasing weight fraction of fibers, even if porosity is also increasing. On the contrary, for man-made cellulosic fiber composites with circular fiber cross-section, the increasing fiber weight fraction (accompanied by increasing void content) has detrimental effect on stiffness and strength. The differences in behavior are discussed attributing them to fiber/ fiber interaction in wood fiber composites which does not happen in man-made fiber composites, and by rather weak fiber/matrix interface for man-made fibers leading to macro-crack formation in large porosity regions.

Place, publisher, year, edition, pages
Taylor & Francis, 2018
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials; Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-69464 (URN)10.1080/17480272.2018.1482368 (DOI)
Available from: 2018-06-13 Created: 2018-06-13 Last updated: 2018-06-13
Pupure, L., Varna, J. & Joffe, R. (2018). Methodology for macro-modeling of bio-based composites with inelastic constituents. Composites Science And Technology, 163, 41-48
Open this publication in new window or tab >>Methodology for macro-modeling of bio-based composites with inelastic constituents
2018 (English)In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 163, p. 41-48Article in journal (Refereed) Published
Abstract [en]

Methodology for development of a macro-scale model (with strain as an input) for Regenerated Cellulose fiber (RCF) composites with highly non-linear (viscoelastic (VE) and viscoplastic (VP)) constituents is presented and demonstrated. The VE is described by Schapery's models and Zapas' model is used for VP. For a purely VE constituent the model can be identified from stress relaxation in constant strain tests. In the presence of VP the constant strain test does not render VE stress relaxation functions, because part of the applied strain is VP and the VE strain is changing. As an alternative creep and strain recovery tests are suggested to find the plasticity law and also the nonlinear creep compliances to identify the VE model where stress is an input. The incremental form of this model is then inverted and used to simulate the VE relaxation tests and the simulated relaxation functions are used to identify the VE model with VE strain as an input.

Models for constituents are used in micromechanics simulations of the composite behavior in arbitrary ramps including the composite VE relaxation test. Using the latter, a macro-model is developed and its validity and accuracy are demonstrated.

Place, publisher, year, edition, pages
Elsevier, 2018
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-68687 (URN)10.1016/j.compscitech.2018.05.015 (DOI)000438323000006 ()2-s2.0-85046756325 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-05-15 (rokbeg)

Available from: 2018-05-09 Created: 2018-05-09 Last updated: 2018-08-07Bibliographically approved
Kahla, H. B. & Varna, J. (2018). Similarities and differences in micro-damage mechanisms during tensile quasi-static and cyclic loading of NCF composites. In: : . Paper presented at 18th European Conference on Composite Materials - ECCM 18, Athens, 25-28 June, 2018. (pp. 8).
Open this publication in new window or tab >>Similarities and differences in micro-damage mechanisms during tensile quasi-static and cyclic loading of NCF composites
2018 (English)Conference paper, Published paper (Refereed)
Abstract [en]

A comparative study of damage mechanisms governing the mechanical behavior of a [-45/90/45/0]s HTS40/RTM6 NCF laminate under quasi-static and tension-tension cyclic loading was performed. Intralaminar cracking in 90˚-layers is described assuming the Weibull distribution of transverse cracking initiation strength. Based on the assumption that the non-uniform fiber distribution is the reason for strength variation, the fatigue model parameters are determined using the crack density in a quasi-static tensile test and in a cyclic test with just one stress level. A triggering mechanism was observed when the cracking which starts in the 90˚-layer leads to immediate cracking in the neighboring off-axis layers even if the average stress there is low. In the quasi-static test, delaminations are small even at high strain levels. At low fatigue strain level, a similar behavior was observed, however in high strain cyclic tests, the delaminations are growing faster at the edges and inside the composite. The delamination length is the largest at the edge and it decreases towards the middle of the laminate. The rate of the decrease depends on the interface and the fatigue strain applied. Delaminations which have propagated inside the specimen increase the opening and the sliding displacements of the intralaminar cracks thus causing much larger stiffness reduction than cracks without delaminations.

Keywords
NCF, Fatigue, Quasi-static, intralaminar cracks, delamination growth
National Category
Engineering and Technology Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-71651 (URN)
Conference
18th European Conference on Composite Materials - ECCM 18, Athens, 25-28 June, 2018.
Available from: 2018-11-19 Created: 2018-11-19 Last updated: 2019-01-21Bibliographically approved
Kahla, H. B., Ayadi, Z., Edgren, F., Pupurs, A. & Varna, J. (2018). Statistical model for initiation governed intralaminar cracking in composite laminates during tensile quasi-static and cyclic tests. International Journal of Fatigue, 116, 1-12
Open this publication in new window or tab >>Statistical model for initiation governed intralaminar cracking in composite laminates during tensile quasi-static and cyclic tests
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2018 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 116, p. 1-12Article in journal (Refereed) Published
Abstract [en]

A simple model for predicting intralaminar cracking in laminates under cyclic loads is proposed and validated. The model is limited to low stresses and low crack density and is based on the assumption that the non-uniformity of the fiber distribution is the main reason for the observed large variation of cracking resistance along the transverse direction of the layer. Hence, the resistance variation in quasi-static and in cyclic loading can be described by the same parameter. At low crack density the failure resistance variation is more significant than the variation of the stress state in the specimen, the latter becoming dominant at high crack density. At low crack density the Weibull distribution for probability of intralaminar cracking is used for crack density growth simulation during cyclic loading. Assuming the non-uniformity of the fiber distribution as the cause for variation of cracking resistance, the Weibull shape parameter in cyclic loading is the same as in quasi-static loading case while the scale parameter is assumed to degrade with the applied number of cycles and this dependence is described by a power function. Thus, the determination of parameters is partially done using quasi-static tests and partially using cyclic tests, significantly reducing the necessary testing time. The predictions of dependency of the cracking on the stress and number of cycles are validated against experimental observations of cracking in the 90-plies of quasi-isotropic non-crimp fabric (NCF) laminates as well as in tape based cross-ply laminates.

Place, publisher, year, edition, pages
Elsevier, 2018
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-69097 (URN)10.1016/j.ijfatigue.2018.05.030 (DOI)000450377100001 ()2-s2.0-85048258679 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-06-21 (svasva)

Available from: 2018-06-04 Created: 2018-06-04 Last updated: 2018-12-05Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0001-9649-8621

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