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Testing rig - modal analysis

Testing rig - modal analysis

Request: Customer needs to validate testing rig dynamic properties.

Solution:

  • Obtain a 3D CAD model
  • Natural frequencies evaluation
  • Proposed new design
  • Validation of new design

Figure 1 shows the structure model supplied by the customer. The frame serves for testing of the car hoses and hoses are tested at 60Hz. Therefore the first natural frequencies of the frame must be higher than 60Hz to avoid affecting the results. Thus, analysis is performed of the current frame to obtain natural frequencies. The frame is made of AA 6061.

Fig. 1 – Customer CAD model

The meshing procedure is applied to the CAD model, i.e. a regular mesh of elements is created. In this case, quadratic tetrahedron elements with 10 nodes are used. The tetrahedron has 4 vertices, ie. that it has 4 nodes as a linear element. These 4 nodes then form one tetrahedron element. The quadratic tetrahedron also contains nodes between vertices and thus the total number of nodes is equal to 10.

Fig. 2 –  Mesh model

Fig. 3 – Tetra element (left – linear, TETRA4, right – quadratic, TETRA10)

Boundary conditions: Modal analysis does not require any boundary conditions, but the first 6 eigenfrequencies should be excluded in the evaluation. They represent Rigid body motion, ie. movement and rotation in space.

After defining the material properties and determining the contacts between the profiles, modal analysis is performed. The first natural frequency is 105 Hz. This is more than the 60 Hz test frequency for hose testing.

Fig. 4 – The first natural frequency of the frame supplied by the customer
However, to guarantee the correct results when testing the hoses, we have designed an additional frame reinforcement to achieve a first resonant frequency of at least 120 Hz.
Fig. 4 – The first natural frequency of the frame supplied by the customer

Conclusion: Increasing the rigidity of the frame ensures that there is no impact on the measurement results.

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