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Published byCharity O’Neal’ Modified over 9 years ago
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S8-1 SECTION 8 ENFORCED MOTION
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S8-2 ENFORCED MOTION n Used to analyze constrained structures with base input acceleration, velocity, displacement. n Common examples are drop test & shock test and earthquakes (for transient analysis) and swept-sine shaker test simulation (for frequency response analysis).
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S8-3 ENFORCED MOTION n Patran/Marc input for automatic enforced motion defined as u displacement n The analysis could be Transient or Frequency response
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S8-4 n First, a time dependent Load Case is created and is made current. This will ensure that all subsequent LBC forms relate to this time dependent loading. ENFORCED MOTION IN TRANSIENT ANALYSIS
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S8-5 n A time dependent field is set up, in this case a rectangular pulse input ENFORCED MOTION IN TRANSIENT ANALYSIS (Cont.)
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S8-6 n Now define the loading as Displacement n The direction of the displacement is defined in Spatial Dependent. n The Time Dependent is defined in the field. n The grids where the enforced motion are applied are picked in Select Application Region. ENFORCED MOTION IN TRANSIENT ANALYSIS (Cont.) Grid 1 Z direction
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S8-7 USING “LARGE MASS” METHOD IN TRANSIENT RESPONSE n Large Mass Method to apply enforced accelerations. n In this method, apply forces {P(t)} to the structure rather than accelerations. If enforced motion is selected, it is assumed that the user is imposing the motion on a “large mass.” Therefore, the force to move the “large mass” is proportional to acceleration.
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S8-8 USING “LARGE MASS” METHOD IN TRANSIENT RESPONSE (Cont.) n For enforced acceleration n Larger mass should be ~10 6 times the structure mass u If the Large Mass is too small then a coupling may occur between the Large Mass and structural mass. u If the Large Mass is too big, numerical conditioning problems may occur. Large Mass m Rigid Body Structure of Interest (M L = large Mass and Structural Mass, M, is not significant.) M=~10 6 m
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S8-9 USING “LARGE MASS” METHOD IN TRANSIENT RESPONSE (Cont.) n Replace the constraints with connections to the Large Mass using an RBE2 and POINT MASS method. POINT MASS RBE2 Structure Connected to ground n It is vital that the RBE2 connections model the same DOF’s as the original constraints, otherwise the boundary conditions and hence the response, will change.
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S8-10 n Now define the loading as Force n The direction of the force is defined in Spatial Dependent. n The Time Dependent is defined in the field. n The grids where the enforced motion are applied are picked in Select Application Region. ENFORCED MOTION IN TRANSIENT ANALYSIS (Cont.) Grid 1 Z direction
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