Using Work and Energy to Characterize Mechanical Shock

Timothy Edwards, Sandia National Laboratories

Abstract:
By far the most widely used tool in shock data analysis is the shock response spectrum (SRS). The SRS has gained popularity because of several primary considerations. It has physical significance, it is simple to understand and it is believed to indicate shock severity. Despite its popularity, the SRS has limitations. Foremost among them is the underlying assumption that shock severity is proportional to a time derivative of position, which does not agree with accepted material failure models. Additionally, the SRS cannot distinguish between naturally occurring, complex shocks and the chirps sometimes used to achieve the underlying SRS using electrodynamic shakers with inadequate force capabilities. A relatively new concept has been introduced whereby similar spectral analysis is made on the work done on SDOF structures by the excitation force. Since work is equal to the change in the energy of a system, this quantity is closely related to failure models based on strain energy such as the Von Mesis criterion. This paper explores a method for characterizing shock motion based on the input energy. The input energy spectrum has attractive properties which include intuitive physical significance, insensitivity to system parameters such as damping, the ability to distinguish between realistic shocks and chirps, and a close relation to accepted material failure models.

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