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Improving Forming of Aerospace Composite Components through Process Modelling
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.ORCID iD: 0000-0001-8111-5202
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In the aerospace industry there is a constant effort to reduce the weight of aircraft. Since weight reduction has a direct impact on fuel consumption. Reducing the fuel consumption leads to botheconomical benefits through less money spent on fuel and environmental benefits through reduced CO2 emissions. One way that weight savings have been achieved in the last couple of decades is by replacing metals with carbon fiber composites in structural components, where a common choice is unidirectional pre-impregnated (UD prepreg) carbon fiber. Traditionally manufacturing is done by hand lay-up where one ply at a time is laid up on a tool. However the need to make large production volumes feasible has led to a need of automated manufacturing processes. One way to rationalize production is to form the whole laminate at once instead of layer by layer. This is done presently with the single and double diaphragm forming techniques. The challenge with forming of stacked laminates is that the individual plies interact with each other as they conform to the geometry increasing the likelihood of defects to develop. This thesis investigates the effect of forming method and process parameters on the development of manufacturing faults and on the geometry of the finished formed part and studies if these faults can be predicted in numerical simulations. First a method for forming stacked laminates using an industrial robot with methods inspired by human forming techniques is presented. Using this system the effect of different forming sequences on the appearance of wrinkles can be investigated. Forming simulations were done to relate the appearance of wrinkles to ply strains detected in the simulated forming process. The method is used to manufacture joggled spars with a length of 1.4 m and a laminate consisting of 20 plies. Thereafter process simulation of hot drape forming (HDF) is used to determine why wrinkling occurs when plies with specific fiber directions are combined with each other in a stack. This study is supported by an experimental study where plies using two different material systems were mixed in the stack to promote or suppress different types of wrinkles. This leads to the discovery that the wrinkles observed could be divided into two main types: global wrinkles were the whole laminate is under compression due to the geometry, and local wrinkling were wrinkling is initiated by compression of one layer due to interaction with surrounding layers. In the fifth paper the impact of forming method on radius thinning is investigated. By comparing hand lay-up and HDF it is shown that a majority of radius thinning of a laminate can occur already in the forming step if HDF is used. In the last study inter-ply shear of prepreg under a variety of different testing parameters is investigated, including different relative fiber directions between the plies. The study shows that the relative fiber direction is an important parameter to take into account when characterizing inter-ply shear as the force required to shear an interface that has a difference of fiber direction of 0° is significantly higher than the force required to shear interfaces with a difference of 45° and 90°. Taking the difference into account also has a significant impact on the results of forming simulations where models that included the difference in inter-ply shear behavior showed a higher tendency for in-plane wrinkling.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. , p. 28
National Category
Aerospace Engineering
Identifiers
URN: urn:nbn:se:kth:diva-232841OAI: oai:DiVA.org:kth-232841DiVA, id: diva2:1236636
Public defence
2018-09-07, F3, Lindstedtsvägen 26, Stockholm, 11:59 (English)
Opponent
Supervisors
Note

QC 20180806

Available from: 2018-08-06 Created: 2018-08-03 Last updated: 2018-08-06Bibliographically approved
List of papers
1. Automated robot forming of stacked UD-prepreg- Experimental and numerical analysis
Open this publication in new window or tab >>Automated robot forming of stacked UD-prepreg- Experimental and numerical analysis
(English)Manuscript (preprint) (Other academic)
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:kth:diva-232837 (URN)
Note

QC 20180810

Available from: 2018-08-03 Created: 2018-08-03 Last updated: 2018-08-10Bibliographically approved
2. 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, p. 41-51Article 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
Keywords
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: 2018-08-03Bibliographically approved
3. 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, p. 234-245Article 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.

Keywords
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: 2018-08-03Bibliographically 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)In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569Article in journal (Refereed) Published
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
Keywords
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)000419819800019 ()2-s2.0-84957836987 (Scopus ID)
Funder
VINNOVA, GF Demo 2012-01031VINNOVA, GF Demo 2013-04667
Note

QC 20180129

Available from: 2016-04-25 Created: 2016-04-25 Last updated: 2018-08-03Bibliographically approved
5. Process Induced Radius Thinning and Shape Distortions during Manufacturing of Composite Aerospace Parts
Open this publication in new window or tab >>Process Induced Radius Thinning and Shape Distortions during Manufacturing of Composite Aerospace Parts
(English)Manuscript (preprint) (Other academic)
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:kth:diva-232838 (URN)
Note

QC 20180814

Available from: 2018-08-03 Created: 2018-08-03 Last updated: 2018-08-14Bibliographically approved
6. Including direction dependent inter-ply friction informing simulation of stacked UD prepreg
Open this publication in new window or tab >>Including direction dependent inter-ply friction informing simulation of stacked UD prepreg
(English)Manuscript (preprint) (Other academic)
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:kth:diva-232839 (URN)
Note

QC 20180810

Available from: 2018-08-03 Created: 2018-08-03 Last updated: 2018-08-10Bibliographically approved

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