S8-1

SECTION 8

ENFORCED MOTION

S8-2

ENFORCED MOTION Used to analyze constrained structures with base input

acceleration, velocity, displacement.

Common examples are drop test & shock test and

earthquakes (for transient analysis) and swept-sine shaker

test simulation (for frequency response analysis).

S8-3

ENFORCED MOTION

Patran/Marc input for automatic enforced motion defined as displacement

The analysis could be Transient or Frequency response

S8-4

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

S8-5

A time dependent field is set up, in this case a rectangular pulse input

ENFORCED MOTION IN TRANSIENT ANALYSIS (Cont.)

S8-6

Now define the loading as Displacement

The direction of thedisplacement is defined in Spatial Dependent.

The Time Dependent is defined in the field.

The grids where the enforced motion are applied are picked in Select Application Region.

ENFORCED MOTION IN TRANSIENT ANALYSIS (Cont.)

Grid 1

Z direction

S8-7

USING “LARGE MASS” METHOD INTRANSIENT RESPONSE

Large Mass Method to apply enforced accelerations.

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.

m

Fu

or

umF

S8-8

USING “LARGE MASS” METHOD INTRANSIENT RESPONSE (Cont.)

For enforced acceleration

Larger mass should be ~106 times the structure mass If the Large Mass is too small then a coupling may occur between the Large Mass and

structural mass. If the Large Mass is too big, numerical conditioning problems may occur.

Large Mass

m

Rigid Body

Structure of

Interest

PM

UL

b

1 (ML = large Mass and Structural Mass, M, is not significant.)

bU

M=~106m

S8-9

USING “LARGE MASS” METHOD INTRANSIENT RESPONSE (Cont.)

Replace the constraints with connections to the Large Mass using an RBE2 and POINT MASS method.

POINT MASS

RBE2

StructureStructure

Connected to ground

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.

S8-10

Now define the loading as Force

The direction of theforce is defined in Spatial Dependent.

The Time Dependent is defined in the field.

The grids where the enforced motion are applied are picked in Select Application Region.

ENFORCED MOTION IN TRANSIENT ANALYSIS (Cont.)

Grid 1

Z direction