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Forming induced wrinkling of composite laminates: A numerical study on wrinkling mechanisms
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.ORCID iD: 0000-0001-8111-5202
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.ORCID iD: 0000-0002-6616-2964
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
Resource type
Text
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. Vol. 81, p. 41-51
Keywords [en]
A. Prepreg, C. Process simulation, E. Forming
National Category
Applied Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-181434DOI: 10.1016/j.compositesa.2015.10.012ISI: 000369214600005Scopus ID: 2-s2.0-84946781893OAI: oai:DiVA.org:kth-181434DiVA, id: diva2:900771
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
In thesis
1. Towards defect free forming of multi-stacked composite aerospace components using tailored interlayer properties
Open this publication in new window or tab >>Towards defect free forming of multi-stacked composite aerospace components using tailored interlayer properties
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. p. 30
Series
TRITA-AVE, ISSN 1651-7660 ; 2016:19
Keywords
Carbon Fibre, Composites, Prepreg, Forming
National Category
Aerospace Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-185694 (URN)978-91-7595-950-4 (ISBN)
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
2. Improving Forming of Aerospace Composite Components through Process Modelling
Open this publication in new window or tab >>Improving Forming of Aerospace Composite Components through Process Modelling
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:nbn:se:kth:diva-232841 (URN)
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

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Sjölander, JensÅkermo, Malin

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