General Mechanical

General Mechanical

Acceleration scoped to all bodies vs base acceleration in shock analysis

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    • shanmukhvaranasi
      Subscriber

      I have to perform shock analysis on electronic enclosure with PCBs and other electronic components. Based on shock standards, the input acceleration profile of half sine with (30G, 18ms), (18ms - 40ms) with zero acceleration to capture response after acceleration. When acceleration is given to the base using base excitation option, the acceleration response of components is similar to the input acceleration and gradually acceleration decreased but for input acceleration applied to all bodies with fixed support ( same face for base excitation), the acceleration is very small ( 70mm/s2 compared to 2e5 mm/s2 for base excitation). Acceleration values are compared on the same faces and all other conditions are the same for both simulations. Please explain why the acceleration is so small in the case of acceleration applied to all bodies. The stresses are the same in both the cases.

    • peteroznewman
      Subscriber

      The link below is to a discussion that clearly explains the difference between relative and absolute results.

      https://forum.ansys.com/forums/topic/base-excitation-absolute-result/

    • shanmukhvaranasi
      Subscriber

      Thanks for the response, I have reffered to the question in the link prior to asking this question, but the main part of my question is why is there a difference in results between 'fixed support with acceleration scoped to all bodies' and 'base acceleration with absolute'. Can we say that the 'base  acceleration with relative' case and the 'fixed support with acceleration applied to all bodies'  case are the same? 

    • peteroznewman
      Subscriber

      Yes, base acceleration with relative output is the same as fixed support with acceleration applied to all bodies.

      • shanmukhvaranasi
        Subscriber

         

        Thanks for the response. 

        If base acceleration with relative output and the acceleration scoped to all bodies is same then what is the effect on transmissibiity? Lets asume there is a multi degree of freedom spring mass sytem, when the acceleration is scoped to all bodies, all the masses are supposed to be excited at the same acceleration but in the case of base excitation the acceleration of each mass depends on the connected masses acceleration and their respective transmissibility ratios. Please correct me where I’m wrong. How does transmissibility come into picture when all the bodies are imposed to same acceeration in ” scoped to all bodies”.

         

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