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Static and Fatigue Failure of Bolted Joints in Hybrid Composite-Aluminium Aircraft Structures
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
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 [en]
Hybrid structures; Composites; Bearing failure; Bolted joints
National Category
Aerospace Engineering
Identifiers
URN: urn:nbn:se:liu:diva-122349DOI: 10.3384/diss.diva-122349ISBN: 978-91-7685-942-1 (print)OAI: oai:DiVA.org:liu-122349DiVA: diva2:865707
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
List of papers
1. Conceptual studies of a composite-aluminum hybrid wing box demonstrator
Open this publication in new window or tab >>Conceptual studies of a composite-aluminum hybrid wing box demonstrator
2014 (English)In: Aerospace Science and Technology, ISSN 1270-9638, E-ISSN 1626-3219, Vol. 32, no 1, 42-50 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents a study of two different hybrid composite-aluminum concepts applied to a winglike structure which is exposed to mechanical  and thermal load. The aim of the study is to determine the most suitable  hybrid concept to later on be used in structural fatigue and static testing. In both concepts, the mass is optimized with respect to two different sets of requirements, one of which is currently in use in the fighter aircraft industry and one which is a modified version of the current requirement set. The issues considered in the study are mass, thermal behavior, buckling, bolted joints, failure criteria and fatigue damage, and they are examined in the frame of both requirement sets. The results clearly indicate the order of criticality between the different criteria in the different parts of each concept. Also, the comparison of two requirement sets gives an idea of the degree of influence of the modified criteria on the hybrid concepts and their mass. Based on the mass and the structural behavior in a thermal-mechanical loading one of the hybrid concepts is chosen for further studies and testing.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Hybrid structure, Wing structure, Composite-aluminum, Thermal load, Conceptual study
National Category
Aerospace Engineering
Identifiers
urn:nbn:se:liu:diva-91892 (URN)10.1016/j.ast.2013.11.002 (DOI)000331921900006 ()
Available from: 2013-05-03 Created: 2013-05-03 Last updated: 2017-12-06Bibliographically approved
2. Finite element modeling of mechanically fastened composite-aluminum joints in aircraft structures
Open this publication in new window or tab >>Finite element modeling of mechanically fastened composite-aluminum joints in aircraft structures
2014 (English)In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 109, 198-210 p.Article in journal (Refereed) Published
Abstract [en]

A three-dimensional, solid finite element model of a composite-aluminum single-lap bolted joint with a countersunk titanium fastener is developed. The model includes progressive damage behavior of the composite and a plasticity model for the metals. The response to static loading is compared to experimental results from the literature. It is shown that the model predicts the initiation and the development of the damage well, up to failure load. The model is used to evaluate the local force-displacement responses of a number of single-lap joints installed in a hybrid composite-aluminum wing-like structure. A structural model is made where the fasteners are represented by two-node connector elements which are assigned the force-displacement characteristics determined by local models. The behavior of the wing box is simulated for bending and twisting loads applied together with an increased temperature and the distribution of fastener forces and the progressive fastener failure is studied. It is shown that the fastener forces caused by the temperature difference are of significant magnitude and should be taken into account in the design of hybrid aircraft structures. It is concluded that, the account of the non-linear response of the joints results in a less conservative load distribution at ultimate failure load.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Bolted joints, Composite-aluminum, Finite element modeling, Hybrid wing structures
National Category
Aerospace Engineering
Identifiers
urn:nbn:se:liu:diva-91893 (URN)10.1016/j.compstruct.2013.10.056 (DOI)000331671700020 ()
Available from: 2013-05-03 Created: 2013-05-03 Last updated: 2017-12-06Bibliographically approved
3. Quasi-static bearing failure of CFRP composite in biaxially loaded bolted joints
Open this publication in new window or tab >>Quasi-static bearing failure of CFRP composite in biaxially loaded bolted joints
2015 (English)In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 125, 60-71 p.Article in journal (Refereed) Published
Abstract [en]

Hybrid composite-aluminium bolted joints develop internal loads at elevated temperatures, due to the difference in thermal expansion properties of their constituent materials. In aircraft joints, the thermally induced bolt loads are commonly directed perpendicular to the mechanical loads, inducing a biaxial bearing load state. In this work, carbon-epoxy laminate specimens were tested in uniaxial and biaxial quasi-static bearing failure experiments in a specially designed test rig, at elevated temperature. A microscopy study of a failed specimen revealed that the failure process was mainly driven by fibre kinking, although extensive matrix cracking and delaminations were also found. The experiments were simulated by three-dimensional, explicit, finite element analyses, which included intralaminar damage and delamination. The experimental and simulated bearing failure loads differed by 1.7% in the uniaxial case and 2.1% in the biaxial case. It was suggested that the load-displacement response is influenced by the interaction of all damage mechanisms. Delamination modelling was, however, not essential for the prediction of the maximal bearing strength. The same effective bearing strengths were obtained for the biaxially loaded specimens as for the uniaxially loaded ones, but the damage accumulation process and the resulting damage distributions were different. (C) 2015 Elsevier Ltd. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2015
Keyword
Hybrid joint; Carbon-epoxy; Thermally induced load; Bearing failure; Finite element analysis
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:liu:diva-118234 (URN)10.1016/j.compstruct.2015.01.038 (DOI)000353177600008 ()
Note

Funding Agencies|Swedish Armed Forces; Swedish Defence Materiel Administration; Swedish Governmental Agency

Available from: 2015-05-22 Created: 2015-05-22 Last updated: 2017-12-04
4. Fatigue bearing failure of CFRP composite in biaxially loaded bolted joints at elevated temperature
Open this publication in new window or tab >>Fatigue bearing failure of CFRP composite in biaxially loaded bolted joints at elevated temperature
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
Keyword
Hybrid bolted joint; Carbon-epoxy; Thermally induced load; Fatigue bearing failure
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:liu:diva-118835 (URN)10.1016/j.compstruct.2015.03.031 (DOI)000354139800028 ()
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: 2017-12-04
5. Fatigue bearing failure of CFRP composite in bolted joints exposed to biaxial variable amplitude loading at elevated temperature
Open this publication in new window or tab >>Fatigue bearing failure of CFRP composite in bolted joints exposed to biaxial variable amplitude loading at elevated temperature
2016 (English)In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 142, 71-77 p.Article in journal (Refereed) Published
Abstract [en]

Hybrid structures than contain composite-aluminium interfaces tend to develop internal loads at elevated temperatures. In long bolted joints, the thermally induced bolt loads are superimposed onto the mechanically applied load and can induce a biaxial bearing load state. This paper presents an experimental and numerical study of the bearing fatigue failure of carbon-epoxy laminate specimens, exposed to uniaxial and biaxial variable amplitude loading at 90C. A specifically designed experimental rig was used, where both the mechanical and the thermally induced bolt loads were applied by means of mechanical load actuators. A fatigue model based on the kinetic theory of fracture for polymers, which was previously implemented for constant amplitude loading, is expanded to account for the variable amplitude load history. The results suggest that the biaxial loading gives a longer fatigue life than the uniaxial loading for the same maximum peak resultant force. This result can be utilized as a conservative dimensioning strategy by designing biaxially loaded joints in terms of maximum peak resultant bearing load using uniaxial fatigue data.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Carbon-epoxy, Thermally induced load, Fatigue bearing failure, Variable amplitude loading
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:liu:diva-122420 (URN)10.1016/j.compstruct.2016.01.064 (DOI)000372691300008 ()
Note

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Funding agencies:  Swedish Armed Forces; Swedish Defence Materiel Administration; Swedish Governmental Agency for Innovation Systems

Available from: 2015-11-02 Created: 2015-11-02 Last updated: 2017-12-01Bibliographically approved

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