Influences on the Stiffness of Riveted Joints in Finite-Element-Models
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© ACENTISS 2014 Approved Center of Engineering, T echnology and In Service Support The reproduction and distribution of this document as well as the communication of its contents to others without explicit authorisation by ACENTISS GmbH are prohibited. Offenders will be held liable for the payment of damages Influences on the Stiffness of Riveted Joints in Finite Element Models Dr. C. Katzenschwanz, S. Giehler; Acentiss GmbH; 85521 Ottobrunn; Einsteinstr. 28a EATC Munich 2014
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For the modelling of riveted joints in finite element models it is state of the art to connect the parts by springs (BUSH elements) connecting single nodes on the meshes. The stiffness of the joint is often calculated according to the Huth-formula. The application of this approach for coarser models as used in aerospace industry results in similar joint stiffness compared to analytical models. But it could be observed that with a finer discretisation the stiffness of the joint will be a function of the discretization. Therefore in the model an effect can be seen which is not physical. The classical approach in the finite element method to increase the accuracy of the analysis by finer mesh densities does not hold any more. The predicted load in the model becomes a function of the discretisation! Due to the finer meshes also in the aerospace industry this behaviour becomes more important especially in models with different mesh densities. A modelling technique will be presented which overcomes this drawback and only physical properties are used to describe the joint. Beside this influence there are other estimations for the stiffness of the joints which will be compared to the Huth approach.
Transcript of Influences on the Stiffness of Riveted Joints in Finite-Element-Models
© ACENTISS 2014
Approved Center of Engineering, Technology andIn Service Support
The reproduction and distribution of this document as well as the communication of its contents to others without explicit authorisation by ACENTISS GmbH are prohibited. Offenders will be held liable for the payment of damages
Influences on the Stiffness of Riveted Joints in Finite Element Models
Dr. C. Katzenschwanz, S. Giehler; Acentiss GmbH; 85521 Ottobrunn; Einsteinstr. 28a
EATC Munich 2014
© ACENTISS 2014 2
Table of Content
n Introductionn Modelling of Stiffnessn Finite Element Modelling of Riveted Joints
Influence of the Discretisation on the StiffnessSuggestion to reduce the dependence on the discretisation
n Summary
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Introduction
n Riveted Joints are the most used joints in aerospace industryn Typical Joints are single-, double- and multi-shear
Single shear joint
Double shear joint
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Motivation
n Modelling of riveted joints in finite element models is done by springs(BUSH elements) connecting single nodes on the meshes
n The stiffness of the joint is often calculated according to the Huth-formulan For coarser models as used in aerospace industry similar joint stiffness
compared to analytical models can be foundBut it could be observed that with a finer discretisation the stiffness of the joint will be a function of the discretisationTherefore in the model an effect can be seen which is not physicalThe classical approach in the finite element method to increase the accuracy of the analysis by finer mesh densities does not hold any moreThe predicted load in the model becomes a function of the discretisation!
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Motivation
n Due to the finer meshes also in the aerospace industry this behaviour becomes more important especially in models with different mesh densities
n A modelling technique will be presented which overcomes this drawback and only physical properties are used to describe the joint
n Beside this influence there are other estimations for the stiffness of the joints which will be compared to the Huth approach
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Table of Content
n Introductionn Modelling of Stiffnessn Finite Element Modelling of Riveted Joints
Influence of the Discretisation on the StiffnessSuggestion to reduce the dependence on the discretisation
n Summary
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Analytical Methods of Calculation
n For the calculation of the loads in a riveted joint the components and the rivet are idealized as linear springs (see also HSB 21031-01)
n At multi-row connections there is a statically undetermined system, which load distribution is depending on the stiffness
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Example of a Single-Shear Joint
n In German-speaking countries the Huth-formula is used to calculate the compliance of a riveted joints, also in Airbus methods the formula is used
n But other formulas exist for the stiffness predictionn Typical joints are single-, double- and multi-shear
Component thickness t1 = t2 =2.1 mmKind of Rivet solid rivet with
countersunk headRivet material
Rivet diameter d = 4.8 mmHuth-coefficients a = 0.4; b = 2.2Douglas-coefficients A = 5; B = 0.80
2024-T3E = 72000 MPa
Component material
2024-T4E = 72000 MPa
Example acc. Huth
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Example of a Single-Shear Joint
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Example of a Double-Shear Joint
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Conclusion
n The Huth formula for the modelling of the stiffness of riveted joints is only one out of several methods
n The calculated stiffness of the joint is different for the methods
n Due to the undetermined structures in an aircraft this also changes the load paths!
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Table of Content
n Introductionn Modelling of Stiffnessn Finite Element Modelling of Riveted Joints
Influence of the Discretisation on the StiffnessSuggestion to reduce the dependence on the discretisation
n Summary
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FE-Modelling of Riveted Joints
n Today a riveted joint is often modelled using a spring with 6 degrees of freedom (BUSH-Element)
n The shear-stiffness is calculated with the Huth-formula
n In the past the FE-models are used to determine the load distribution, but today they are directly used for verification, so the mesh density increases
Single-shear joint cut in the symmetry area
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Example for Different Discretisations
n How are the results converging when the mesh density of the components is increased?
u = 0.5mm
u = 0mm
u = 0mm
u = 0.5mm
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coarsefine
Influence of the Discretisation on the Rivet-Load
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Verification on a Cargo Door Model
n To verify the results of the simple model acc. Huth the influence of the discretisation is analysed at a simplified model of a cargo door
n In the marked area of the cargo door the mesh density is increased and the rivet load is analysed
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Influence of the Discretisation on the Rivet Load
Again it can be seen that the Rivet Load decreases when the mesh density is increased
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Table of Content
n Introductionn Modelling of Stiffnessn Finite Element Modelling of Riveted Joints
Influence of the Discretisation on the StiffnessSuggestion to reduce the dependence on the discretisation
n Summary
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Suggestion for an Improved Modelling
n The finite element model shows a non-physical effect when the mesh density is increased
n The reason is the singularity while connecting two components through a single node to the spring element
n An improvement can be reached when the connection is realised through all nodes in a specific area, therefore a RBE3-element is used
NietpositionenNietpositionenRivet-Positions
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Improvement of the Joint Stiffness
A load distribution over all nodes in the area 1.5 to 2.0 times rivet diameter leads to results that are independent
from the discretisation
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Table of Content
n Introductionn Modelling of Stiffnessn Finite Element Modelling of Riveted Joints
Influence of the Discretisation on the StiffnessSuggestion to reduce the dependence on the discretisation
n Summary
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Summary and Outlook
n The increasing mesh densities in finite element models lead to the calculation of a lower joint stiffness, because of a smaller element length
n This fact results in a different load distribution in the components n With the support of RBE3-elements the non-physical effect can be
reduced or even avoided and the results depend on physical and geometrical properties
n There is a local change of the load distribution through the RBE3-elementn So the significance of the stresses at these “disturbed” elements has to be
reviewed for the verification
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Your Point of Contact
Dr. Christoph KatzenschwanzHead of Engineering, Simulation & OptimizationACENTISS GmbH
Einsteinstrasse 28a85521 OttobrunnGermany
Phone +49 89 4111 934 33Fax +49 89 4111 934 95 E-Mail [email protected] www.acentiss.de
Sarah GiehlerEngineering, Simulation & OptimizationACENTISS GmbH
Einsteinstrasse 28a85521 OttobrunnGermany
Phone +49 89 4111 934 34Fax +49 89 4111 934 95 E-Mail [email protected] www.acentiss.de
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