The modal testing experiment presented here is performed by one university laboratory using ModalVIEW software and National Instruments DSA hardware as the modal testing and analysis tool. The analysis result provides instructive guidance of their research on FEM model.

Experiment scenario

The testing object is a hydrogen tank of composite material:

To identify closed spaced and conjugate modes of this symmetrical structure, accurate modal analysis method of poly-reference, multiple DOF, global fitting is required. ModalVIEW modal analysis software provides advanced modal analysis method based on mode stabilization diagram to extract modes from poly-reference FRF measurements, which meet testing requirements. National Instruments 24-bit portable module NI -9233 is used to guarantee the accurate measurement of weak vibration signal of measured points far away from excitation point. The portable NI DSA hardware is suitable for field modal testing experiment.  

System configuration

In this modal testing experiment, the following software and hardware are used：

1．ModalVIEW1.0 Software.

2．NI DSA System (Module carrier NI-9162, 4 channel DSA module NI-9233)

3．Hammer (Lance LC1302) and Charge amplifier (Lance LC0106).

4．Tri-axial accelerometer.

 Analysis results

Modal analysis for the structure is finished with ModalVIEW’s advanced mode estimation method based on mode stabilization diagram。The following figure shows the clear mode stabilization diagram from 70*3 measured FRF data.

The closely spaced modes can be selected from stabilization diagram. The real modes are kept with mathematical or faked modes being discarded. Part of the picked meaningful modes are listed in following table :

The following figure displays the plot of synthesized FRFs from estimated modes (red line) and measured original FRFs. The synthesized FRFs should match original FRFs well, which indicates that estimated modes are accurate enough.  

One of the mode shapes is displayed as following :

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