R Buckling analysis of fiber reinforced composites ... · Composite Materials and Adaptive ... Buckling analysis of fiber reinforced composites structures Ralph Kussmaul 1 / 14

Department of Mechanical and Process Engineering

Composite Materials and Adaptive Structures Lab

IMES-ST/2015-10-19 – Buckling analysis of fiber reinforced composites structures Ralph Kussmaul 1 / 14

Eidgenössische Technische Hochschule Zürich

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Buckling analysis of fiber reinforced composites structures using NX 8.5 under

the example of a 3-point-bending beam

Ralph Kussmaul

Zurich, 19-October-2015





Contents 1 Introduction .......................................................................................................................... 3

1.1 Task definition ............................................................................................................... 3

2 Pre-Processing .................................................................................................................... 4

2.1 Constraints .................................................................................................................... 4

3 Solving ............................................................................................................................... 11

4 Post-Processing ................................................................................................................. 12





1 Introduction

This manual is based on the 3-point-bending beam introduced in the document “Simulation of fiber reinforced composites using NX 8.5 under the example of a 3-point-bending beam”.

1.1 Task definition In this NX manual it is presented how a linear buckling analysis of a fiber reinforced composite beam subjected to 3-point-bending is carried out. The beam under study is unchanged compared to the beam in the aforementioned document. Therefore, the pre-processing steps creation of geometry, meshing and material definition are not addressed in this manual anymore.





2 Pre-Processing

Starting point of this manual is the .fem model of the meshed beam with assigned material properties. The support and the impactor are not included. This is due to the fact that proper contact modelling is not possible in the linear buckling analysis.

2.1 Constraints As contact modelling is not possible for the buckling analysis, the application of constraints and load is modified. In order to achieve a homogenous load introduction a Rigid Body Element 2 (RBE2) is used. This special element creates an ideally rigid connection between one source node and several target nodes. To create a source node click “Insert” – “Node” – “Create”.





Specify the location of the node as can be seen in the figure below and click “OK”.

Thus, a node is created over the top surface of the beam.





To create the RBE2 click on “1D Connection” and set type to “Node to Node”. Select the created node as “Source”. The selection of the “Target” nodes is depended on the desired load introduction. Assuming a broad, tree-like impactor all nodes over a certain length (here: 10 cm) on the upper surface of the beam are chosen.

As “Element Properties” – “Type” chose “RBE2” and click “OK”. Thus, the RBE2 is created.





In the next step create a new simulation file.

In the appearing window give a suitable name and select “SOL 105 Linear Buckling” as “Solution Type”. Click “OK”.





Now the load and the constraints have to be applied. Select “Load Type” – “Force”. Enter a force “Magnitude” of “1 N”, specify the “–Zc”-direction and assign it to the before created RBE2 source node. Click “OK”.





Next, click “Constraint Type” – “User Defined Constraint”. Select the RBE2 source node and fix its DoF [12456]. Thus a proper load introduction solely along the –Zc-axis is ensured. Click “OK”.

Now the support constraints need to be modelled. Click “Constraint Type” – “User Defined Constraint”. Select the nodes at the lower surface where the support is located and fix their DoF [3]. Click “OK”.





Finally, constrain the DoF [1] of the nodes in the middle of beam. Make sure not to select the nodes that belong to the target nodes of the RBE2, which are already constrained by the RBE2 source node. Click “OK”.

Now the model is sufficiently constrained and can be solved.





3 Solving

To run the prepared FEM model click “Solve” – “OK”.





4 Post-Processing

To access the simulation results click “Open Results”. Now, in the Post Processing Navigator the available results are listed. As can be seen, two results subcases are available for the SOL 105 solution type.

The first subcase is called “Buckling Loads”. There the results of a type SOL 101 Linear Statics solution are available. This linear static analysis is needed in order to calculate the so-called stress stiffness matrix. The second subcase “Buckling Method” contains the results of the linear buckling analysis, i.e. the solution of the eigenvalue problem:

To access the buckling results, click on “Buckling Method”. Selecting “Mode 1” – “Displacement – Nodal” - “Magnitude” displays the eigenvector of the first buckling mode.





As can be seen, the first buckling mode occurs at the upper surface of the beam which is subjected to compressive loads. This result is plausible. The buckling load is obtained by multiplying the eigenvalue with the applied load in the model (here: 1 N). Thus, in this example the first buckling load is found to be 3’229 N. For the eigenmodes 5 and 6 negative eigenvalues are calculated. These buckling modes can only occur if the applied load is reversed (in +Zc-direction). Thus, they can be ignored. Investigating the linear buckling analysis results, it can be seen that failure of the structure due to instability has to be expected prior to the design load. This cannot be accepted. Therefore, countermeasures against buckling like increased wall thicknesses or additional stiffeners are needed. As an example, the wall thickness of the beam is increased from 2 to 3 mm.





As can be seen, this thickness increase results in an increased buckling load of 11’020 N.

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R Buckling analysis of fiber reinforced composites ... · Composite Materials and Adaptive ... Buckling analysis of fiber reinforced composites structures Ralph Kussmaul 1 / 14