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Towards defect free forming of multi-stacked composite aerospace components using tailored interlayer properties
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.ORCID iD: 0000-0003-0613-2680
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Use of lightweight materials is an important part of reduction of fuel consumption by commercial aircraft. A considerable number of structural aircraft parts are therefore built of thin layers of epoxy pre-impregnated carbon fibres stacked to laminates. Manufacturing these by hand is costly and different methods of automation have therefore been developed. One cost-effective way of manufacturing is Automated Tape Lay-up of flat stacks followed by a Hot Drape Forming operation. A well-known problem in the industry within forming is fibre wrinkling, which can cause a serious strength knock down. The focus of this thesis has therefore been on understanding how and why wrinkles develop during forming of multi-layer stacks and, based on this, investigate different methods for process and material improvements.

The work presented initially investigates the dependency between stacking sequence and wrinkle development. It is shown that wrinkle free forming can be obtained by changing the fibre stacking order. In the following investigation it is shown that the wrinkles cannot be entirely eliminated by local stiffening of the critical layers. In a, related study it is shown that different kinds of wrinkles develops during forming; wrinkles may be either due to global buckling of the entire lay-up or local compression of single layers. Global buckling is due to excessive material. Local compression occurs as the material shear during forming.

The work presented leads to an understanding of the importance of making the beneficial neighbouring fibre layers interact during forming. One way to connect neighbouring layers is to tailor the interlayer properties. A study is presented that shows how local manipulation of interlayer properties may steer the multi-layered material into a different deformation mechanisms. The manipulation in this thesis is performed using Multi Wall Carbon Nano Tubes, thermoplastic veils or consolidation of thermoplastic toughener particle interlayers.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 30 p.
Series
TRITA-AVE, ISSN 1651-7660 ; 2016:19
Keyword [en]
Carbon Fibre, Composites, Prepreg, Forming
National Category
Aerospace Engineering
Research subject
Aerospace Engineering
Identifiers
URN: urn:nbn:se:kth:diva-185694ISBN: 978-91-7595-950-4 (print)OAI: oai:DiVA.org:kth-185694DiVA: diva2:922883
Public defence
2016-06-01, Sal F3, Lindstedtsvägen 26, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
VINNOVA, NFFP5 2010-01257VINNOVA, NFFP6 2013-01220VINNOVA, GF Demo 2012-01031VINNOVA, GF Demo 2013-04667
Note

QC 20160425

Available from: 2016-04-25 Created: 2016-04-25 Last updated: 2016-05-16Bibliographically approved
List of papers
1. An experimental study of mechanisms behind wrinkle development during forming of composite laminates
Open this publication in new window or tab >>An experimental study of mechanisms behind wrinkle development during forming of composite laminates
Show others...
2013 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 50, 54-64 p.Article in journal (Refereed) Published
Abstract [en]

A general problem when forming quasi-isotropic, multilayer unidirectional (UD) prepreg over a double curved geometry is out-of-plane wrinkling. The presented study aims to show the existence of compression in the laminate in a recess area that is globally under tension during forming. Further it aims to investigate the influence of compression on the out-of-plane wrinkle development.

An experimental study is performed with pre-stacked UD prepreg on a forming tool with varying cross section. Investigated parameters include lay-up sequence, prepreg ply thickness, inter-ply friction and prepreg ply impregnation. Experimental evaluation is performed on the out-of-plane defect height, type, location and number. Further, measurements are performed on the outer-ply deformation in the z-direction and inside the formed component using Micro CT. The study show that compression is to some degree always developed during forming of a recess area but that the lay-up sequence has a dominant effect on the wrinkling development.

Place, publisher, year, edition, pages
Elsevier, 2013
Keyword
A. Carbon fiber, A. Laminates, E. Forming, E. Prepreg
National Category
Vehicle Engineering
Research subject
SRA - Production
Identifiers
urn:nbn:se:kth:diva-116488 (URN)10.1016/j.compositesa.2013.03.013 (DOI)000320149000007 ()2-s2.0-84876567357 (Scopus ID)
Funder
XPRES - Initiative for excellence in production research
Note

QC 20130719

Available from: 2013-01-21 Created: 2013-01-21 Last updated: 2017-12-06Bibliographically approved
2. Forming induced wrinkling of composite laminates with mixed ply material properties; an experimental study
Open this publication in new window or tab >>Forming induced wrinkling of composite laminates with mixed ply material properties; an experimental study
2015 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 78, 234-245 p.Article in journal (Refereed) Published
Abstract [en]

One disadvantage of multi-layer forming of unidirectional (UD) prepreg tape is the risk of out-of-plane wrinkling. This study aims to show how mixed ply material properties affect global wrinkling behaviour. An experimental study was performed using pre-stacked UD prepreg on a forming tool with varying cross sections. Parameters studied include local interply friction, effects of co-stacking and fibre stresses in critical fibre directions. Experimental evaluation was performed on out-of-plane defect height, type and location. The study shows that fibre stresses in some fibre directions were crucial for the shearing required to avoid wrinkling. The same fibre stresses may cause wrinkling if the lamina is stacked in a non-beneficial order. Changing the friction locally, or reducing the number of difficult combinations of fibre angles, improves the forming outcome slightly. However, in order to make a significant improvement, co-stacking or different fibre stacking is required.

Keyword
Carbon fibre, Prepreg, Forming
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-176329 (URN)10.1016/j.compositesa.2015.08.025 (DOI)000362605100026 ()2-s2.0-84940757122 (Scopus ID)
Funder
XPRES - Initiative for excellence in production research
Note

QC 20151117

Available from: 2015-11-17 Created: 2015-11-03 Last updated: 2017-12-01Bibliographically approved
3. Forming induced wrinkling of composite laminates: A numerical study on wrinkling mechanisms
Open this publication in new window or tab >>Forming induced wrinkling of composite laminates: A numerical study on wrinkling mechanisms
2016 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 81, 41-51 p.Article in journal (Refereed) Published
Abstract [en]

When manufacturing composite aircraft components consisting of uni-directional prepreg laminates, Hot Drape Forming (HDF) is sometimes used. One issue with HDF is that, in contrast to hand lay-up where normally only one ply is laid up at a time, multiple plies are formed together. This limits the in-plane deformability of the stack, thus increasing the risk of out-of-plane wrinkling during forming. In this paper mechanisms responsible for creating different types of wrinkles are explained. It is shown through simulations how the wrinkles are created as a result of interaction between two layers with specific fibre directions or due to compression of the entire stack. The simulations are compared to experimental results with good agreement.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
A. Prepreg, C. Process simulation, E. Forming
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-181434 (URN)10.1016/j.compositesa.2015.10.012 (DOI)000369214600005 ()2-s2.0-84946781893 (Scopus ID)
Funder
XPRES - Initiative for excellence in production research
Note

QC 20160205

Available from: 2016-02-05 Created: 2016-02-02 Last updated: 2017-11-30Bibliographically approved
4. Forming of composite spars including interlayers of aligned, multiwall, carbon nanotubes: an experimental study
Open this publication in new window or tab >>Forming of composite spars including interlayers of aligned, multiwall, carbon nanotubes: an experimental study
2016 (English)Article in journal (Refereed) Epub ahead of print
Abstract [en]

Carbon nanotubes offer the potential for improved or changed matrix properties, thereby enabling the creation of novel, multifunctional composite materials. By using highly-aligned, multiwall, carbon nanotubes (MWCNT) with thermoset resins, good dispersion and distribution of nanotubes is obtained. To date, research has mainly focused on improving the growth process of the aligned MWCNTs, however little has been done on the processing of composites that include MWCNTs as interlayers in the stack.  The aim of this work is to study how the aligned MWCNTs are affected within composite part forming. The study shows that MWCNTs are influenced by the shearing that occurs during forming, but still maintain their integrity. To some extent, the shear pattern observed in the MWCNTs after deformation provides an indication of deformation modes. However, the presence of MWCNTs also significantly influences the forming characteristics of the prepreg stack.

Place, publisher, year, edition, pages
John Wiley & Sons, 2016
Keyword
Carbon Fibre, Prepreg, Forming, Multi wall carbon nanotubes
National Category
Aerospace Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-185696 (URN)10.1002/pc.23918 (DOI)
Funder
VINNOVA, GF Demo 2012-01031VINNOVA, GF Demo 2013-04667
Note

QCR 20160428

Available from: 2016-04-25 Created: 2016-04-25 Last updated: 2016-05-31Bibliographically approved
5. Interface manipulation towards wrinkle-free forming of stacked UD prepreg layers
Open this publication in new window or tab >>Interface manipulation towards wrinkle-free forming of stacked UD prepreg layers
(English)Article in journal (Other academic) Submitted
Abstract [en]

The aim of this paper is to experimentally study how forming behaviour can be changed by local manipulation of prepreg interfacial characteristics. Different methods for surface modification are examined, however all were aimed at enabling significantly increased interply friction. The paper shows that by using increased friction in between layers, these neighbouring pairs will act together during forming, thereby either improving or exacerbating the forming outcome. For the geometry utilized, wrinkle free forming was obtained when the cross-plied layers were paired to deform mainly through intraply shear during forming. The method is supported by the appended numerical analysis and interaction between forming mechanisms and radius thinning is instigated as part of the experimental outcome.

Keyword
Carbon Fibre, Composites, Prepreg, Forming
National Category
Aerospace Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-185716 (URN)
Funder
VINNOVA, NFFP6 2013-01220
Note

QC 20160428

Available from: 2016-04-25 Created: 2016-04-25 Last updated: 2016-04-28Bibliographically approved

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