GEBT - results for dynamic analysis

[Theja]

Hello Sir,
             We are doing a non-linear dynamic analysis of an anisotropic bar using GEBT and the eigen values we get are of very small magnitude. 

             Hence we have tried to validate the results by performing static analysis of a cantilever beam with a tip transverse load and the results from GEBT matches with the theoretical results. 

            Now we have done a free vibration analysis and got the eigen values and frequencies using GEBT of very small magnitude which are not matching with the theoretical values mentioned in the word document.  We want to confirm that the analysis that is being carried out is correct. We need your help in confirming the values we obtained are reliable.

The attached word document contains the input file used in GEBT and the inputs and results obtained.

Thank You Sir,

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With regards,

M. Thejasvi,

Aerospace Structures,

IIT Bombay,

India.

[Qi Wang]

Hi Theja,

There are some mistakes in your input file. For example, the flexibility matrix is not correct. This matrix is calculated as the inverse of the stiffness matrix, where you can put EA, GJ, EI_2,EI_3 as the diagonal terms of a 4 by 4 matrix and then calculate the inverse matrix. Then put them in the corresponding positions in a 6 by 6 matrix, which is required by GEBT. Also there are mistakes in the mass matrix. For your case, the Mass(4,4) should be the summation of Mass(5,5) and Mass(6,6). And in the file you provided it is not. Please read the manual carefully and check the input file.

Qi

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[Wenbin Yu]

I noticed that although you are doing an analysis of anisotropic bar. You used simple analytical formula to get the stiffness, which is far from correct. You can request the computer code VABS to carry out a cross-sectional analysis to get accurate beam inertial and stiffness properties.

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[Theja]

Hello Sir,

              The formulae I have used are for isotropic material, this is for validation purpose. If it is valid for simple isotropic problems then I can use it for my research work. I wont use the same formulae for the anisotropic bar. 

              As I am including a delamination in my research work, I have to give the modified stiffness and hence cant use the code VABS directly for obtaining the stiffness properties. I can get the inertial properties from VABS directly, I'll look into it and come back to you.

Thank You Sir,

Aerospace Structures.