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Multi-fastener single-lap joints in composite structures
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This thesis deals with composite joints. Designing such joints is more difficult than metallic joints due to the mechanical properties of composite materials. Composites are anisotropic and have a limited ability of yielding. The low degree of yielding means that stress concentrations are not relieved by plastic deformation, which is important in multi-fastener single-lap joints. The distribution of load between the fasteners may be more uneven than in metallic joints due to that the stress concentrations around the holes are not relieved. Single-lap joints have an eccentric load path which generates a nonuniform bolt-hole contact pressure through the plate thickness. This generates out-of-plane deflection of the joint, termed secondary bending.

Such nonuniform contact stress severely limits the strength of the joint. The nonuniform contact stress distribution is affected by several factors, e.g. bolthole clearance and secondary bending. The first part of the work is devoted to investigating secondary bending, and its effect on stresses in the joint. A novel technique to study secondary bending has been developed and used in a parametric study. It is based on the calculation of specimen curvature from out-of-plane deflections measured with an optical technique. It is shown that the specimen curvature is correlated to the conventional definition of secondary bending, which involves strain measurements on both sides of the plate. The two most important parameters affecting specimen curvature was found to be the overlap length and the thickness of the plates. The finite element method was used to study the influence of secondary bending on joint strength. Secondary bending was changed in magnitude by altering the length of the overlap region in a two-fastener specimen. It was found that secondary bending affects the local stress field around the fasteners and that it may change the strength and the mode of failure.

The second part is concerned with the load distribution and prediction of joint strength. A detailed finite element model was developed to calculate the load distribution while accounting for bolt-hole clearances, bolt clamp-up, secondary bending and friction. An experimental programme was conducted in order to validate the finite element model by means of instrumented fasteners. Good agreement between simulations and experiments was achieved and it was found that bolt-hole clearance is the most important factor in terms of load distribution between the fasteners. Sensitivity to this parameter was found to be large, implying that temperature changes could affect the load distribution if member plates with different thermal expansion properties are used.

Calculating the load distribution in structures with a large number of fasteners is in general not feasible with detailed finite element models based on continuum elements. A simplified, computationally effective model of a multi-fastener, singlelap joint has been developed by means of structural finite elements. The model accounts for bolt-hole clearances, bolt clamp-up, secondary bending and friction. Comparisons with the detailed finite element model and experiments validated the accuracy of the simplified model. A parametric study was conducted where it was found that an increased stiffness mismatch between the plates generates a more uneven load distribution, while reducing the length of the overlap region has the opposite effect. Increasing the stiffness of a fastener shifts some of the load from the nearest fasteners to that particular fastener. An idealized optimization study was conducted in order to minimize bearing stresses in the joint with restrictions on the increase of joint weight and net-section stresses. Maximum bearing stress was reduced from 220 MPa to 120 MPa while both weight and net-section stresses decreased.

A procedure to predict bearing strength based on the results from the simplified model was developed. It was established by an experimental programme that fiber micro-buckling is the initial failure mode. The stress state in the laminate was determined through force and moment equilibrium, based on output from the finite element model. An existing criterion was used to predict the fiber microbuckling, and thus the initial failure. Predictions were compared with experiments which validated the method. The small computational cost required by the procedure suggests that the method is applicable on large scale structures and suitable to use in conjunction with iterative schemes such as optimization and statistical investigations.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , p. ix, 28
Series
Trita-AVE, ISSN 1651-7660 ; 2006:22
National Category
Vehicle Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4006ISBN: 91-7178-396-2 (print)OAI: oai:DiVA.org:kth-4006DiVA, id: diva2:10358
Public defence
2006-06-09, Sal D1, Lindstedtsvägen 17, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20110121Available from: 2006-05-30 Created: 2006-05-30 Last updated: 2011-01-21Bibliographically approved
List of papers
1. Secondary Bending in Multi Fastener, Composite-to-Aluminium Single Shear Lap Joints
Open this publication in new window or tab >>Secondary Bending in Multi Fastener, Composite-to-Aluminium Single Shear Lap Joints
2005 (English)In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 36, p. 195-208Article in journal (Refereed) Published
Abstract [en]

A novel technique to study secondary bending due to the eccentric load path in single shear, composite-to-aluminium lap joints has been developed and used in a parametric study. The method is based on the specimen curvature, which is derived from lateral displacements of the surface. The lateral displacements are measured with an optical measurement system or calculated with the finite element method. It was found that the specimen curvature is equivalent to the secondary bending which is usually measured with strain gauges in the vicinity of the bolt holes. The two most important parameters affecting specimen curvature is the overlap length and the thickness of the aluminium plate in the joints.

Keywords
A. Laminates, C. Finite element analysis (FEA), Bolted joint
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-5840 (URN)10.1016/j.compositesb.2004.09.001 (DOI)000229298300003 ()2-s2.0-12444333610 (Scopus ID)
Note
QC 20110121Available from: 2006-05-30 Created: 2006-05-30 Last updated: 2017-12-14Bibliographically approved
2. Effect of Secondary Bending on Strength Prediction of Composite, Single Shear Lap Joints
Open this publication in new window or tab >>Effect of Secondary Bending on Strength Prediction of Composite, Single Shear Lap Joints
2005 (English)In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 65, p. 953-965Article in journal (Refereed) Published
Abstract [en]

The influence of secondary bending on strength prediction of composite joints has been studied by means of detailed finite element analyses. Secondary bending was changed in magnitude by altering the length of the overlap region, in a two-fastener specimen, while leaving all other joint parameters unchanged. It was found that secondary bending influences the various macroscopic failure modes, and thus has the potential to change the mode of failure and affect the ultimate failure load. Under tensile loading, secondary bending increased the contact area between the fasteners and the hole edges which reduced the bearing stresses and increased the bearing strength. Secondary bending also increased the bending of the plates and thus generated more severe net-tensile stresses which resulted in reduced net-tensile strength. Under compressive loading, secondary bending reduced the net-compressive strength due to the increased plate bending. The effect of secondary bending on bearing strength is difficult to predict and must be evaluated on an individual basis. It was concluded that secondary bending needs to be accounted for in accurate strength predictions.

Keywords
bolted joint, finite element analysis, strength
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-5841 (URN)10.1016/j.compscitech.2004.10.020 (DOI)000227939700013 ()2-s2.0-13844253958 (Scopus ID)
Note
QC 20110121Available from: 2006-05-30 Created: 2006-05-30 Last updated: 2017-12-14Bibliographically approved
3. Load Transfer in Multirow, Single Shear, Composite-to-Aluminium Lap Joints
Open this publication in new window or tab >>Load Transfer in Multirow, Single Shear, Composite-to-Aluminium Lap Joints
2006 (English)In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 66, p. 875-885Article in journal (Refereed) Published
Abstract [en]

A three-dimensional finite element model has been developed in order to determine the load transfer in multifastener single shear joints. The model is based on continuum elements and accounts for all important mechanisms involved in load transfer, such as bolt-hole clearances, bolt clamp-up and friction. In particular, member plates can be of different thickness and stiffness and with different coefficient of thermal expansion. An experimental programme was conducted in order to validate the finite element model through measurements of fastener loads, by means of instrumented fasteners. Good agreement between simulations and experiments was achieved and it was found that bolt-hole clearance is the most important factor in terms of load distribution between the fasteners. Any variation in clearance between the different holes implies that load is shifted to the fastener where the smallest clearance occurs. Sensitivity to this variation in clearance was found to be large, so that temperature changes could significantly affect the load distribution if member plates with different thermal expansion properties are used. It was found that good accuracy in load transfer predictions requires that all aforementioned factors are taken into consideration and that nonlinear kinematics is accounted for in the solution process.

Keywords
bolted joint, load transfer, finite element method, instrumented fastener
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-5842 (URN)10.1016/j.compscitech.2005.08.015 (DOI)000236567300002 ()2-s2.0-33644878878 (Scopus ID)
Note
QC 20110121Available from: 2006-05-30 Created: 2006-05-30 Last updated: 2017-12-14Bibliographically approved
4. Finite Element Modeling and optimization of Load Transfer in Multi-Fastener Joints using Structural Elements
Open this publication in new window or tab >>Finite Element Modeling and optimization of Load Transfer in Multi-Fastener Joints using Structural Elements
2008 (English)In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 82, no 2, p. 245-256Article in journal (Refereed) Published
Abstract [en]

A computationally effective model of a multi-fastener, single-lap, composite-to-aluminium joint has been developed by means of structural finite elements. The model is geared towards accurate predictions of load distribution between the fasteners and accounts for bolt-bole clearances, bolt clamp-up and member plate friction. Comparisons with previously conducted experiments and detailed finite element analyses validated the accuracy of the model. A parametric study was conducted where it was found that an increased stiffness mismatch between the plates generates a more uneven load distribution, while reducing the length of the overlap region has the opposite effect. Increasing the stiffness of a fastener shifts some of the load from the nearest fasteners to that particular fastener. An idealized optimization study was conducted in order to minimize bearing stresses in the joint with restrictions on the increase of joint weight and net-section stresses. Maximum bearing stress was reduced from 220 to 120 MPa and both weight and net-section stresses were decreased.

Keywords
composite joint, load transfer, FEA, optimization
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-5843 (URN)10.1016/j.compstruct.2007.01.005 (DOI)000250399700010 ()2-s2.0-34548679135 (Scopus ID)
Note
QC 20110121Available from: 2006-05-30 Created: 2006-05-30 Last updated: 2017-12-14Bibliographically approved
5. Prediction of Bearing Failure in Composite Single-lap Joints
Open this publication in new window or tab >>Prediction of Bearing Failure in Composite Single-lap Joints
(English)Manuscript (preprint) (Other academic)
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-5844 (URN)
Note
QS 2006Available from: 2006-05-30 Created: 2006-05-30 Last updated: 2012-01-27Bibliographically approved

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