Aluminum Beam Analysis

In the winter of 2025, I was able to take a course in Aerospace Structural Dynamic Testing. A crucial element of this class is being able to utilize concepts learned about modal analysis and experimental testing in order to adequately characterize the dynamics of a system. Two labs of the class involved accurately characterizing the natural frequencies, damping ratios, and mode shapes of a metal beam. Additionally, a finite element model was created and a MAC matrix analysis was performed. 

Modal testing is necessary in various industries to verify that finite element models (FEM) of test specimens are valid and applicable. When data from a modal test is obtained, FEM may be altered to more accurately reflect the physical structure of the test specimen. The objective of this lab is to successfully perform a modal impact test on a beam test specimen and acquire relevant data that could be used to evaluate the validity of a finite element model. 

The test specimen was an aluminum 6061-T6 beam of dimensions 24 x 2 x 3/16 in. with free-free boundary conditions. In lieu of a finite element model, an analytical approach was used to determine the natural frequencies and damping ratios of the first 6 bending modes of the beam. Experimentally acquired natural frequencies and damping ratios were determined from the modal test data using an Algorithm of Modal Identification (AMI) software system. Analytical and experimental data were compared to identify the maximum level of agreement, and various experiment parameters were modified to analyze the effects on the damping ratios of each mode. 

The first 6 bending modes and first 2 torsion modes. The analytical mode shapes are compared with measured mode shapes as calculated with AMI processing. 

Afinite element model of the beam was created using the ‘modeling_demo_SteelBeam.m’ file in the osfern package from the class materials. The dimensions and material properties calculated in the previous lab for the beam were modified to match the beam specimen used in the lab. Next, the number of nodes was adjusted in both the length (x) and height (y) directions until each of the six bending and two torsion modes were able to be identified from the finite element model (FEM). This resulted in six nodes to be used for the mesh in the y-direction, and 121 nodes to be used in the x-direction. A few of the extracted mode shape plots are seen below:

The full reports including all analyses of the beam may be viewed below.