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Forming of stacked unidirectional prepreg materials
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

To reduce cost of structural composites, the development of more efficient manufacturing methods is of great interest. An automatic tape layer (ATL) can be used to perform the layup in an efficient way for flat sheets and the second step is by pressure forming the prepreg stack onto a given mould. Sheet forming of thermoset prepreg, also known as hot drape forming, is a promising manufacturing method when combined with automatically stacked laminates. To reach the desired shape, without flaws such as wrinkles and severe fibre angle deviation, knowledge about the forming behaviour of stacked material is important. A simulation of the forming can add to the understanding of mechanisms causing defects and also how to avoid them.

A continuum mechanical finite element approach is used to model the forming of stacked unidirectional prepreg. This with the aim of finding mechanisms causing defects, such as out-of-plane wrinkling and in-plane waviness. Data from experimental characterisation of the materials are required for the constitutive material models. An experimental approach is used to investigate the intraply (within the layer) and interply (between the layers) deformation. The intraply deformation behaviour is investigated by a bias extension test, where resistance to deform and fibre rotation are registered. The interlaminar friction characteristics in the prepreg/prepreg interface are retrieved from tests performed with a specially designed rig. Measurements on the bending stiffness of the prepreg, both transverse to and in the fibre direction, are used in the out-of-plane model.

Different aerospace-grade thermoset prepreg materials are tested and major differences in behaviour are found. Since epoxy is brittle, the resin is toughened by thermoplastics, for some materials in liquid phase and for others by solid particles. These particles seem to influence both the intraply deformation modes and increase the level of friction between layers.

The experimental data from shear, friction and bending tests are used to calibrate the chosen material models. In-plane forming simulations confirm the affect stacking sequence have on the forming behaviour. Full-scale forming simulations on a joggles beam are performed with one of the tested material systems. Two different stacking sequences, with the same amount of fibre in the main directions, are analysed and compared. Both numerical and experimental forming shows that the coupling between layers, due to the interply friction, will affect the forming behaviour and in some cases create wrinkles. Wrinkles can develop both in the plane and out-of-plane, where only the in-plane waviness is present in the performed forming simulations.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , vii, 31 p.
Series
Trita-AVE, ISSN 1651-7660 ; 2012:75
National Category
Composite Science and Engineering
Research subject
SRA - Production
Identifiers
URN: urn:nbn:se:kth:diva-106269ISBN: 978-91-7501-558-3 (print)OAI: oai:DiVA.org:kth-106269DiVA: diva2:573621
Public defence
2012-12-18, E2, Lindstedtsvägen 3, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Projects
KEKS Kostnadseffektiva kompositstrukturerPRICE Producerbarhet i centrumXPRES
Funder
XPRES - Initiative for excellence in production researchTrenOp, Transport Research Environment with Novel Perspectives
Note

QC 20121204

Available from: 2012-12-04 Created: 2012-12-03 Last updated: 2013-04-11Bibliographically approved
List of papers
1. On the in-plane deformability of cross-plied unidirectional prepreg
Open this publication in new window or tab >>On the in-plane deformability of cross-plied unidirectional prepreg
2011 (English)In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 46, no 8, 929-939 p.Article in journal (Refereed) Published
Abstract [en]

The work presented herein aims to characterise the in-plane properties of cross-plied unidirectional prepreg using the bias extension method. The study is focusing on carbon fibre/epoxy material systems and testing is performed at elevated temperatures to enhance formability. Using digital image correlation (DIC), the specimen deformation and fibre rotation is captured during tests. The study shows that the cross-plied unidirectional material deforms at different characteristic length scales as deformation continues: first seemingly continuous and later in form of bands. Further, the different type of prepreg materials investigated behaves differently; for one type the pin-jointed net theory fit well, enabling simple estimation of resulting fibre angle, while for another not. Different loading speed, temperatures and layup methods are investigated in order to pin-point its influence on the deformability.

Place, publisher, year, edition, pages
Sage Publications, 2011
Keyword
prepreg, forming, multi-layer
National Category
Reliability and Maintenance Manufacturing, Surface and Joining Technology Materials Engineering Vehicle Engineering
Research subject
SRA - Production
Identifiers
urn:nbn:se:kth:diva-10677 (URN)10.1177/0021998311412988 (DOI)000302010000006 ()2-s2.0-84859314024 (Scopus ID)
Projects
Kostnadseffektiva kompositstrukturer (KEKS)NFFP Price
Funder
XPRES - Initiative for excellence in production research
Note

QC 20120424

Available from: 2013-01-25 Created: 2009-06-22 Last updated: 2017-12-13Bibliographically approved
2. On the interply friction of different generations of carbon/epoxy prepreg systems
Open this publication in new window or tab >>On the interply friction of different generations of carbon/epoxy prepreg systems
2011 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 42, no 9, 1067-1074 p.Article in journal (Refereed) Published
Abstract [en]

Using pre-stacked material that is formed as a first step in the manufacturing process offers reduced process cycle time for production of complex structural components. The forming is achieved by forcing layers to deform by e.g. intraply deformation and interply slippage, where the latter is the scope of this study. The prepreg-prepreg friction is experimentally determined for four unidirectional carbon/epoxy prepreg systems. The materials differ considering volume fraction of fibres, fibre stiffness and phase of thermoplastic toughener (solved or particles). The study shows large individual differences between the tested materials, where the material systems with particle tougheners seem to obey a boundary lubrication friction, while the other materials show hydrodynamically dominated friction. A large difference between the high and low friction materials, almost a factor of 10, correlates to trends seen in the herein performed surface roughness measurements. Vacuum as well as autoclave consolidated materials are tested.

Place, publisher, year, edition, pages
Elsevier, 2011
Keyword
Prepreg, Carbon fibre, Forming, Surface analysis
National Category
Vehicle Engineering Production Engineering, Human Work Science and Ergonomics Materials Engineering
Research subject
SRA - Production
Identifiers
urn:nbn:se:kth:diva-26091 (URN)10.1016/j.compositesa.2011.04.010 (DOI)000293313900001 ()2-s2.0-79959928978 (Scopus ID)
Projects
NFFP PriceXPRES
Funder
XPRES - Initiative for excellence in production research
Note

QC 20110128.  Uppdaterad från submitted till published 20110816.

Available from: 2010-11-12 Created: 2010-11-12 Last updated: 2017-12-12Bibliographically approved
3. In-plane deformation of multi-layered unidirectional thermoset prepreg - Modelling and experimental verification
Open this publication in new window or tab >>In-plane deformation of multi-layered unidirectional thermoset prepreg - Modelling and experimental verification
2014 (English)In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 56, 203-212 p.Article in journal (Refereed) Published
Abstract [en]

Sheet forming of unidirectional prepregs is gaining increased interest as a cost efficient alternative manufacturing method. Its potential lies within the use of automatically and efficiently stacked flat prepregs, which in a second step can be formed. A successful forming requires understanding of the properties of the uncured material. Here, the in-plane deformation behaviour of two different unidirectional thermoset prepregs is investigated. Experimental measurements are performed, showing the importance of stacking sequence and its effect on the forming behaviour of stacked prepreg. Finite element models are developed, using material models calibrated from bias extension tests and interlaminar friction tests. The method developed can be used to predict the reaction force and fibre reorientation during in-plane forming of thermoset prepreg, for one of the considered material systems. Further, it enables prediction of the effect of stacking sequence, which is promising for future full-scale forming simulations.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Carbon fibres, Prepreg, Finite element analysis (FEA), Forming
National Category
Vehicle Engineering
Research subject
SRA - Production
Identifiers
urn:nbn:se:kth:diva-106510 (URN)10.1016/j.compositesa.2013.10.005 (DOI)000329264900020 ()2-s2.0-84887706645 (Scopus ID)
Funder
XPRES - Initiative for excellence in production research
Note

QC 20140131. Updated from submitted to published.

Available from: 2012-12-04 Created: 2012-12-04 Last updated: 2017-12-07Bibliographically approved
4. Forming simulation of multi-layered unidirectional thermoset prepregs
Open this publication in new window or tab >>Forming simulation of multi-layered unidirectional thermoset prepregs
(English)Article in journal (Other academic) Submitted
National Category
Vehicle Engineering
Research subject
SRA - Production
Identifiers
urn:nbn:se:kth:diva-106511 (URN)
Funder
XPRES - Initiative for excellence in production research
Note

QCR 20160816

Available from: 2012-12-04 Created: 2012-12-04 Last updated: 2016-08-16Bibliographically approved

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