Dynamic Design Analysis Method (DDAM)

Dynamic Design Analysis Method (DDAM)

The purpose of a Dynamic Design Analysis Method (DDAM) in FEA is to find a system’s response to a shock load without the need for a shock curve (acceleration vs. time graph), using empirical correlations instead. The DDAM is used for the analysis of equipment on naval vessels, which have to meet the requirements laid out in the shock standard defined by the US Naval Research Laboratory.

A DDAM is a Response Spectrum Analysis (RSA) (https://www.fea-solutions.co.uk/response-spectrum-analysis/), with modal superposition being used to calculate the systems response (https://www.fea-solutions.co.uk/modal-superposition/).

The loads which are to be applied in a DDAM analysis are calculated based on the frequencies from a modal analysis, with acceleration and velocities being calculated for each mode. The calculation of the loads depends on the type of ship and how the item is mounted to ship (i.e. deck mounted, hull mounted, etc), with the calculations defined in the US naval standard. Many FEA programs perform this calculation automatically.

For the DDAM analysis to be accurate, it is generally required that a total of at least 80% of the modal mass (https://www.fea-solutions.co.uk/modal-mass/) must be captured in all directions. This is important as the modes are used to calculate the loads which are applied in a DDAM analysis, hence missing any mode with a significant modal means that a critical load may be missed.

As a DDAM is a Response Spectrum Analysis (RSA), results cannot be shown at time steps as the analysis is performed in the frequency domain. Instead, a SRSS result (https://www.fea-solutions.co.uk/result-combination-methods/) combines the results for the individual modes.

A separate DDAM analysis is required for each direction. To go from the individual modes to the overall response in a particular direction, the results have to be combined using a common method such as SRSS (see above).

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