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Fatigue bearing failure of CFRP composite in biaxially loaded bolted joints at elevated temperature
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering. Saab AB, SE-58188 Linkoping, Sweden.
Saab AB, SE-58188 Linkoping, Sweden.
Swedish Def Research Agency, Sweden.
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
2015 (English)In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 127, 298-307 p.Article in journal (Refereed) Published
Abstract [en]

Hybrid composite-aluminium structures develop internal loads when exposed to elevated temperatures, due to the different thermal expansion properties of the constituent materials. In aircraft structures with long rows of bolted joints, the mechanical and the thermally induced bolt loads are oriented in different directions, creating a biaxial bearing load state. In this study, the bearing fatigue failure process and the influence of the biaxial load state on the failure are investigated. An experimental set-up was designed, where both the mechanical and the thermally induced bolt loads were applied by means of mechanical load actuators. Two-bolt, double-lap joints with quasi-isotropic carbon-epoxy composite specimens were subjected to uniaxial and biaxial cyclic loading at 90 degrees C. A microscopy study of the bearing plane revealed that the main fatigue driving mechanisms were matrix cracking and fibre-matrix debonding. Motivated by these findings, a fatigue prediction model based on the kinetic theory of fracture for polymer matrices was run in a finite element code and the results showed a satisfactory correlation to the experimental results. The biaxial loading resulted in a longer fatigue life than the uniaxial loading, for the same peak resultant force, which was explained by the smaller effective stress range in the biaxial case.

Place, publisher, year, edition, pages
Elsevier , 2015. Vol. 127, 298-307 p.
Keyword [en]
Hybrid bolted joint; Carbon-epoxy; Thermally induced load; Fatigue bearing failure
National Category
Composite Science and Engineering
Identifiers
URN: urn:nbn:se:liu:diva-118835DOI: 10.1016/j.compstruct.2015.03.031ISI: 000354139800028OAI: oai:DiVA.org:liu-118835DiVA: diva2:818204
Note

Funding Agencies|Swedish Armed Forces, Swedish Defence Materiel Administration; Swedish Governmental Agency for Innovation Systems

Available from: 2015-06-08 Created: 2015-06-04 Last updated: 2015-11-02
In thesis
1. Static and Fatigue Failure of Bolted Joints in Hybrid Composite-Aluminium Aircraft Structures
Open this publication in new window or tab >>Static and Fatigue Failure of Bolted Joints in Hybrid Composite-Aluminium Aircraft Structures
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The use of fibre composites in the design of load carrying aircraft structures has been increasing over the last few decades. At the same time, aluminium alloys are still present in many structural parts, which has led to an increase of the number of hybrid composite-aluminium structures. Often, these materials are joined at their interface by bolted connections. Due to their different response to thermal, mechanical and environmental impact, the composite and the aluminium alloy parts are subject to different design and certification practices and are therefore considered separately.The current methodologies used in the aircraft industry lack well-developed methods to account for the effects of the mismatch of material properties at the interface.One such effect is the thermally induced load which arises at elevated temperature due to the different thermal expansion properties of the constituent materials. With a growing number of hybrid structures, these matters need to be addressed. 

The rapid growth of computational power and development of simulation tools in recent years have made it possible to evaluate the material and structural response of hybrid structures without having to entirely rely on complex and expensive testing procedures.However, as the failure process of composite materials is not entirely understood, further research efforts are needed in order to develop reliable material models for the existing simulation tools.

The work presented in this dissertation involves modelling and testing of bolted joints in hybrid composite-aluminium structures.The main focus is directed towards understanding the failure behaviour of the composite material under static and fatigue loading, and how to include this behaviour in large scale models of a typical bolted airframe structure in an efficient way. In addition to that, the influence of thermally induced loads on the strength and fatigue life is evaluated in order to establish a design strategy that can be used in the industrial context.

The dissertation is divided into two parts. In the first one, the background and the theory are presented while the second one consists of five scientific papers.

Place, publisher, year, edition, pages
Linköping University Electronic Press, 2015. 53 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1706
Keyword
Hybrid structures; Composites; Bearing failure; Bolted joints
National Category
Aerospace Engineering
Identifiers
urn:nbn:se:liu:diva-122349 (URN)10.3384/diss.diva-122349 (DOI)978-91-7685-942-1 (print) (ISBN)
Public defence
2015-12-03, C3, Hus C, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2015-11-02 Created: 2015-10-29 Last updated: 2015-11-02Bibliographically approved

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