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Ben Kahla, H. (2019). Micro-cracking and delaminations of composite laminates under tensile quasi-static and cyclic loading. (Doctoral dissertation). Luleå: Luleå tekniska universitet
Open this publication in new window or tab >>Micro-cracking and delaminations of composite laminates under tensile quasi-static and cyclic loading
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Mikrosprickor och delamineringar i kompositlaminat under kvasistatisk och cyklisk dragbelastning
Abstract [en]

Aerospace industry is devoted to improving the aircraft performance while reducing its weight and limiting the emissions. Part of this objective can be accomplished with the use of high-performance long fibre reinforced polymer laminated composites. Being the first mode of damage under loading, intralaminar cracks initiate at the free edge of the off-axis plies and propagate along the respective fibre orientation. While these cracks grow as tunnels and increase in number, at some point two close cracks in plies of different off-axis orientation could intersect forming an envelope with the free edge. As loading continues, local delamination is expected within this envelope. The evolution and interactions of the different damage modes and the accumulation of damage under a specific loading are crucial in order to have a good understanding of the mechanisms and hence an accurate prediction of the mechanical properties´ degradation. This thesis is devoted to initiation and evolution of intralaminar cracking in plies and interlayer delamination in composite laminates.

In the first part, quasi-isotropic Carbon Fibre/ Epoxy non-crimp fabric (NCF) laminates were studied under both quasi-static and cyclic loadings. The objective was to develop an efficient testing methodology for statistical damage evolution determination in Fatigue. The sequence of damage occurrences (intralaminar cracks in the different layers, delaminations at the different interfaces) loaded under quasi-static and tension-tension fatigue is first captured. To save characterisation time and costs, a simple model for predicting intralaminar cracking in laminates under cyclic loads was proposed and validated under low stress cyclic loads and low crack density. The model is based on Weibull distribution for the probability of cracking where part of parameters is obtained in quasi-static tests and part in a limited number of cyclic tests. 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 NCF laminates as well as in tape based cross-ply laminates. In position where intralaminar cracks meet the specimen edge, local delaminations initiate due to the high 3D stress state. The delamination is further assisted by cracks in other off-axis plies, usually linking them. The average delamination length dependence on loading parameters is characterized and linked with the extent of the laminate stiffness reduction, showing using a simple ply-discount analysis that delaminations are the main reason for very large axial modulus reduction.

In the second part, local delaminations and their effect on laminate stiffness are analysed using FEM. Expressions for the crack opening displacement (COD) determined using FEM are obtained and a modelling approach based on GLOB-LOC is performed for intralaminar crack case with local delaminations starting from the intralaminar crack. The delamination length is used as a parameter and studies are performed for different materials. Strong effect of delaminations on COD and on the axial modulus of the laminate is found. Finally, the last findings are used to simulate the damaged composite laminate behaviour in 4-point bending test. The bending stiffness of the laminate is significantly reduced by intralaminar cracks with delaminations. An approach, using the concept of the effective stiffness of the damaged ply is used. The so obtained effective stiffness matrix is a function of intralaminar crack density in the ply and the delamination length. The effective stiffness is used to calculate the bending stiffness of the damaged laminate. The laminate curvature calculated in this way is in a very good agreement with the curvature obtained in 3-D FEM simulations of the test with explicitly including cracks and delaminations in the model.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2019
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
Laminate, Fatigue, damage growth, stiffness reduction, NCF
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-73393 (URN)978-91-7790-356-7 (ISBN)978-91-7790-357-4 (ISBN)
Public defence
2019-06-04, E231, Lulea university of technology, 97187 Luleå, Sweden, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2019-04-09 Created: 2019-04-08 Last updated: 2019-05-14Bibliographically 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
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: 2019-04-08Bibliographically approved
Pupurs, A., Varna, J., Loukil, M. S., Kahla, H. B. & Mattsson, D. (2016). Effective stiffness concept in bending modeling of laminates with damage in surface 90-layers (ed.). Composites. Part A, Applied science and manufacturing, 82, 244-252
Open this publication in new window or tab >>Effective stiffness concept in bending modeling of laminates with damage in surface 90-layers
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2016 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 82, p. 244-252Article in journal (Refereed) Published
Abstract [en]

Simple approach based on Classical Laminate Theory (CLT) and effective stiffness of damaged layer is suggested for bending stiffness determination of laminate with intralaminar cracks in surface 90-layers and delaminations initiated from intralaminar cracks. The effective stiffness of a layer with damage is back-calculated comparing the in-plane stiffness of a symmetric reference cross-ply laminate with and without damage. The in-plane stiffness of the damaged reference cross-ply laminate was calculated in two ways: 1) using FEM model of representative volume element (RVE) and 2) using the analytical GLOB-LOC model. The obtained effective stiffness of a layer at varying crack density and delamination length was used to calculate the A, B and D matrices in the unsymmetrically damaged laminate. The applicability of the effective stiffness in CLT to solve bending problems was validated analyzing bending of the damaged laminate in 4-point bending test which was also simulated by 3-D FEM.

Place, publisher, year, edition, pages
Elsevier, 2016
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-3182 (URN)10.1016/j.compositesa.2015.11.012 (DOI)000370997000026 ()2-s2.0-84956666458 (Scopus ID)0f8f7712-3d80-40ce-94ac-1c1535b54d62 (Local ID)0f8f7712-3d80-40ce-94ac-1c1535b54d62 (Archive number)0f8f7712-3d80-40ce-94ac-1c1535b54d62 (OAI)
Note

Validerad; 2016; Nivå 2; 20151117 (andbra)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2019-04-08Bibliographically approved
Kahla, H. B., Varna, J. & Pupurs, A. (2015). Microcracking in layers of composite laminates in cyclic loading with tensile transverse stress component in layers (ed.). In: (Ed.), 20th International Conference on Composite Materials: Copenhagen, 19-24th July 2015. Paper presented at International Conference on Composite Materials : 19/07/2015 - 24/07/2015. ICCM, Article ID 2204-1.
Open this publication in new window or tab >>Microcracking in layers of composite laminates in cyclic loading with tensile transverse stress component in layers
2015 (English)In: 20th International Conference on Composite Materials: Copenhagen, 19-24th July 2015, ICCM , 2015, article id 2204-1Conference paper, Published paper (Refereed)
Abstract [en]

Intralaminar cracking in layers of a quasi-isotropic carbon fiber NCF laminate in tension-tension cyclic loading is studied experimentally. Methodology based on modified Weibull analysis is suggested to combine quasi-static and fatigue testing to identify parameters in the crack density growth model. The validity of the assumptions for the given material is experimentally confirmed. The suggested methodology can lead to significant time and material savings in composites fatigue behaviour characterization.

Place, publisher, year, edition, pages
ICCM, 2015
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-72449 (URN)2-s2.0-85053144209 (Scopus ID)
Conference
International Conference on Composite Materials : 19/07/2015 - 24/07/2015
Available from: 2019-01-04 Created: 2019-01-04 Last updated: 2019-01-04Bibliographically approved
Ben Kahla, H. & Varna, J. Effect of intralaminar cracking induced local delaminations on laminate stiffness.
Open this publication in new window or tab >>Effect of intralaminar cracking induced local delaminations on laminate stiffness
(English)In: Article in journal (Refereed) Submitted
National Category
Engineering and Technology
Research subject
Material Mechanics
Identifiers
urn:nbn:se:ltu:diva-73514 (URN)
Available from: 2019-04-08 Created: 2019-04-08 Last updated: 2019-04-08
Ben Kahla, H., Varna, J. & Ayadi, Z. Intralaminar cracking and speciemen edge interaction induced local delamiantions in quasi-isotropic CF/EP NCF Composites in fatigue.
Open this publication in new window or tab >>Intralaminar cracking and speciemen edge interaction induced local delamiantions in quasi-isotropic CF/EP NCF Composites in fatigue
(English)In: Article in journal (Refereed) Submitted
National Category
Engineering and Technology
Research subject
Material Mechanics
Identifiers
urn:nbn:se:ltu:diva-73513 (URN)
Available from: 2019-04-08 Created: 2019-04-08 Last updated: 2019-04-08
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-3322-8197

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