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Effects of Non-uniform Fiber Distribution on Fiber/matrix Interface Crack Propagation in Polymeric Composites
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. (Polymeric Composites)
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Fiber/matrix interface cracking plays an important role in determining the final failureof unidirectional (UD) composites. When subjected to longitudinally tensile loading,fiber/matrix interface debonds originate from fiber breaks or initial defects propagatealong loading direction. Depending on the quality of fiber/matrix interface, debondscould keep growing longitudinally which leads to the degradation of compositestiffness or kink out of interface and connect with neighboring debonds or fiberbreaks that forms a so called critical fracture plane which leads to the final failure ofUD composite. For UD composite subjected to transversely tensile loading, theinitiation, growth and coalesce of arc-shape fiber/matrix interface debonds result inthe formation of macro-size transverse cracks, the propagation and multiplication ofthese transverse cracks, although would not directly lead to the final failure ofcomposite, could cause significant stiffness degradation of composite structures.In the presence thesis, the growth of a fiber/matrix interface debond of a UDcomposite with hexagonal fiber packing under longitudinal and transverse tensileloading was investigated numerically, with the special focus on the influence ofneighboring fibers. In the current study, energy release rate (ERR) is considered as thedriving force for the debond growth and was calculated based on J Integral andVirtual Crack Closure Technique (VCCT) using finite element software ANSSY.Papers A – C in the present thesis deal with the influence of neighboring fibers on theERR of a debond emanating from a fiber break under longitudinal loading condition.In longitudinal loading case, debond growth is mode II dominated. In paper A, anaxisymmetric model consisting 5 concentric cylinders that represent broken fiber withdebond, surrounding matrix, neighboring fibers, surrounding matrix and effectivecomposite was generated. It’s found that there are two stages of debond growth, thefirst stage is when debond length is short, the ERR decreases with increasing debondlength, and the presence of neighboring fibers significantly increase the ERR ofdebond. For relatively long debond, the debond growth is steady when ERR is almostconstant regardless of debond length. In steady state of debond growth, the presenceof neighboring fibers have little effect on the ERR. In papers B and C, a 3-D modelwas generated with broken fiber and its 6 nearest fibers in a hexagonal packed UDcomposite were modelled explicitly, surrounded by the homogenized composite.

Based on the obtained results, it’s shown that ERR is varying along debond front, andhas its maximum at the circumferential location where the distance between two fibercenter is the smallest. This indicates that the debond front is not a circle. For steadystate debond, the presence of neighboring fibers have little effect on averaged ERR(averages of ERR along debond front). For short debond, the presences ofneighboring fibers increases the averaged ERR, and that increase is more significantwhen inter-fiber distance is the smallest. Paper D investigates the growth of afiber/matrix debond along fiber circumference under transverse loading. It’s foundthat debond growth in this case is mixed-mode, and both mode I and mode II ERRcomponents increase with increasing debond angle and then decreases. Debondgrowth is mode I dominated for small debond angle and then switch to mode IIdominated. The presence of neighboring fibers have an enhancement effect on debondgrowth up to certain small debond angle and then changes to a protective effect. InPaper E, the interaction between two arc-size debond under transverse loading isinvestigated. It’s found that when two debonds are close to each other, the interactionbetween two debond becomes much stronger, and that interaction leads to the increaseof ERR of each debond significantly, which facilitates further growth for bothdebond.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2017.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
URN: urn:nbn:se:ltu:diva-62974ISBN: 978-91-7583-862-5 (print)ISBN: 978-91-7583-863-2 (electronic)OAI: oai:DiVA.org:ltu-62974DiVA: diva2:1087889
Public defence
2017-05-24, E246, Luleå tekniska universitet, Luleå, 09:00
Opponent
Available from: 2017-04-10 Created: 2017-04-10 Last updated: 2017-05-08Bibliographically approved
List of papers
1. Effect of neighboring fibers on energy release rate during fiber/matrix debond growth
Open this publication in new window or tab >>Effect of neighboring fibers on energy release rate during fiber/matrix debond growth
2014 (English)In: 16th European Conference on Composite Materials, ECCM 2014, European Conference on Composite Materials, ECCM , 2014Conference paper, Published paper (Refereed)
Abstract [en]

In this paper fiber/matrix interface debond growth in unidirectional composites subjected to mechanical tensile loading is analyzed using fracture mechanics principles of energy release rate (ERR). The objective of the present study is to analyze the effect of neighboring fibers on the ERR. 5-cylinder axisymmetric FEM models with adjustable inter-fiber distance were used for ERR calculations. The results show that the ERR slightly increases with the inter-fiber distance in the case of long debonds. For short debonds, however, because the stress-state is more complex, it was found that the debond propagates in a mixed Mode I and Mode II and contribution of each mode to the ERR depends on the actual debond length. It was found that for very small debond lengths ERR significantly increases with the inter-fiber distance.

Place, publisher, year, edition, pages
European Conference on Composite Materials, ECCM, 2014
Keyword
Debonding, Energy release rate, FEM modeling, Fiber breaks
National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-26843 (URN)84915745571 (Scopus ID)019ed06f-336e-4ad7-97fe-b0433966b881 (Local ID)9780000000002 (ISBN)019ed06f-336e-4ad7-97fe-b0433966b881 (Archive number)019ed06f-336e-4ad7-97fe-b0433966b881 (OAI)
Conference
European Conference on Composite Materials : 22/06/2014 - 26/06/2014
Note
Godkänd; 2014; 20141217 (andbra)Available from: 2016-09-30 Created: 2016-09-30 Last updated: 2017-05-08Bibliographically approved
2. Effect of fiber clustering on debond growth energy release rate in UD composites with hexagonal packing
Open this publication in new window or tab >>Effect of fiber clustering on debond growth energy release rate in UD composites with hexagonal packing
2016 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 161, 76-88 p.Article in journal (Refereed) Published
Abstract [en]

Steady-state energy release rate (ERR) for fiber/matrix interface debond growth originated from fiber break in unidirectional composite is calculated using 3-D FEM models with hexagonal fiber arrangement. In the model the debonded fiber is central in the hexagonal unit which is surrounded by effective composite. The effect of neighboring fibers focusing on local fiber clustering on the ERR is analyzed by varying the distance between fibers in the unit. The steady-state ERR is calculated from potential energy difference between a unit in the bonded region far away from the debond front and a unit in the debonded region far behind the debond front. The ERR for different modes of crack propagation is obtained from a FEM model containing a long debond by analyzing the stress at the debond front.Results show that in mechanical axial tensile loading fracture Mode II is dominating, it has strong angular dependence (effect of closest fibers) but the average ERR is not sensitive to the local fiber clustering. In thermal loading the Mode III is dominating and the average ERR is highly dependent on the distance to neighboring fibers. However, for realistic loads the thermal ERR is much smaller than the mechanical.

National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-9352 (URN)10.1016/j.engfracmech.2016.04.037 (DOI)7f3b1433-1d2e-447f-a4a5-32cbaf110f8b (Local ID)7f3b1433-1d2e-447f-a4a5-32cbaf110f8b (Archive number)7f3b1433-1d2e-447f-a4a5-32cbaf110f8b (OAI)
Note
Validerad; 2016; Nivå 2; 20160502 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-06-29Bibliographically approved
3. Fiber/matrix debond growth from fiber break in unidirectional composite with local hexagonal fiber clustering
Open this publication in new window or tab >>Fiber/matrix debond growth from fiber break in unidirectional composite with local hexagonal fiber clustering
2016 (English)In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 101, 124-131 p.Article in journal (Refereed) Published
Abstract [en]

Energy release rate (ERR) for fiber/matrix debonding in composite with local fiber clustering, subjected to axial tension, has been investigated numerically by a 3-D finite element (FE) model. In the model, broken fiber is central in a hexagonal unit which is embedded in an effective composite. Fiber/matrix debond with circular front is assumed to be originated from the fiber break. The effect of the local fiber clustering on ERR is studied by varying distance between the broken fiber and the neighboring fibers. For very short debonds as well as for long debonds (almost steady-state growth) the ERR was calculated by both the J integral and the Virtual crack closure technique (VCCT). Results show that the debond growth is Mode II dominated and that the ERR strongly depends on the angular coordinate. The local fiber clustering has larger effect on the angular variation for shorter debonds and the effect increases with larger local fiber volume fraction. The results obtained from the 3-D hexagonal model are compared with those obtained previously using 5-cylinder axisymmetric model developed by the same authors. The ERR values from 5-cylinder axisymmetric model could be considered as upper bound for the 3-D hexagonal model.

National Category
Composite Science and Engineering
Research subject
Polymeric Composite Materials
Identifiers
urn:nbn:se:ltu:diva-11675 (URN)10.1016/j.compositesb.2016.07.005 (DOI)000381652000013 ()ab1b33ad-2fa0-4c00-9238-6769005a61d8 (Local ID)ab1b33ad-2fa0-4c00-9238-6769005a61d8 (Archive number)ab1b33ad-2fa0-4c00-9238-6769005a61d8 (OAI)
Note

Validerad; 2016; Nivå 2; 20160707 (andbra)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-06-29Bibliographically approved

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