ABSTRACT
The aim of this study is to investigate means of efficiently assessing the effects of structural modifications on the dynamic properties of a complex structure such as attachment of new weapon systems to a helicopter airframe. The dynamic properties of the modified structure can be determined by experimental testing or numerical simulation, both of which are complex, expensive and time consuming. Assuming that the original dynamic characteristics are already established and that the modification is a relatively simple attachment, the modified dynamic properties may be determined numerically without solving directly the equations of motion of the full modified structure. The frequency response functions (FRFs) of the modified structure can be computed through a distributed mass system approach with additional degrees of freedom (DOFs) by using the original FRFs and a delta dynamic stiffness matrix [ΔB] for the modification introduced. The validity of this approach is investigated by first applying it to a cantilever beam to which a smaller beam is attached as modification. The original FRFs were obtained experimentally as well as numerically. The matrix [ΔB] was determined numerically by modeling the attachment and part of the original structure including the attachment points. The FRFs of the modified beam were then computed using the mathematical approach outlined above. Reasonably good agreement is obtained by comparing the results to the FRFs of the modified beam determined experimentally as well as by numerical modeling of the complete modified structure.
