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I need info on HF equivalent stiffness testing

Currently, I can test the low frequency equivalent stiffness of brackets welded on light truck frames. Some people call this "point mobility testing" Now I need to test stiffer brackets that have higher natural resonant frequencies. I need to develop a high frequency equivalent stiffness test, but almost no literature can be found on this subject.
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I do not have any information concerning high frequency equivalent stiffness testing, but if you would like to describe your application in a bit more detail, I would be happy to point you in the right direction in regards to the NI products that may be a good fit for what you are trying to accomplish.

Spencer S.
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I'm looking for techniques. By attaching an accelerometer to the underside of a steel bracket that is welded onto a structure, you can determine the stiffness of that bracket using standard modal test equipment. An input force is applied with a modal hammer to the top of the bracket. The FRF (Frequency Response Function) from the accel is converted from A/F to V/F to D/F (deflection/force). This is called "equivalent stiffness testing" The technique works on relatively thin sheet metal brackets with resonance peaks under 200Hz. How do I do it for stiffer brackets over 4mm thick where the resonance peaks are over 200Hz? An expert in Modal Analysis may know the answer.
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