1.1.           Purpose

This manual describes the theory and operation processes for selecting instrumented degrees of freedom (DOF) for a modal test and creating a test-analysis model (TAM) using a number of different reduction methods. A test-analysis model (TAM) is a mathematically reduced version of a finite element model (FEM). A FEM will usu­ally contain tens of thousands, hundreds of thousands, or even millions of DOF. In a TAM, only those DOF are retained which correspond to accelerometers in a modal survey, typically one or two hundred. The accuracy of the TAM is a function of the instrumented DOF and the mathematical process used to reduce the FEM to those DOF. A typical FEM and TAM are shown in Figure 1-1.

Figure 1-1. A TAM is a mathematically reduced version of a finite element model.

TAMKIT provides procedures for selecting the instrumented DOF and for reducing the FEM matrices to these DOF. It is implemented as a set of Nastran DMAP alters, with some Matlab functions used to read and interpret the Nastran data. It also includes procedures for comparing two similar models, and for comparing test and analysis modes on completion of the modal test. The overall process covered by TAMKIT along with the corresponding alters and MATLAB functions, is summarized in Table 1-1.

Table 1-1. Summary of Procedures and Alters/Functions Provided in TAMKIT

Step

Alters

MATLAB Functions

Evaluate test article

compare_modes

 

Select instrumented DOF

write_gpke

residual_kinetic_energy

iter_rke

fast_iter_rke

iter_guyan

effind

mass_weighted_effind

read_ke

rpt_ke

pch2aset

Create TAM

dynamic_tam

irs_tam

modal_tam

hybrid_tam

 

Evaluate TAM

ortho

read_tam

rpt_tam

Compare test and analysis

results

ortho

 

read_tam

rpt_tam

Expand test mode shapes

expand_modes

 

Localize errors in the model

expand_modes

ortho