Uncorrelated blocked force determination on plate and shell structures

Inverse force determination is commonly used to determine input forces when they are not directly measurable. If transfer functions are measured with the source component attached, the inversely determined forces are, by definition, blocked forces. The primary advantage of using blocked forces is that the receiving structure may be modified and blocked forces, in theory, are unchanged. In this research, blocked forces are determined for a plastic engine cover connected to a base plate and a compressor attached to two different structures. At lower frequencies, blocked forces are determined using routine approaches where phase is included in both transfer function and operational response measurements. At high frequencies, it is demonstrated that predictions are improved if phase is ignored and blocked forces are assumed to be uncorrelated with respect to each other. It is also shown that the uncorrelated blocked forces are still valid even when changes are made to the receiving structure.