- [Instructor] This video looks at the background
to transient analysis.
In transient analysis we calculate the response
to a time-varying input.
Input excitation is always defined in the time domain.
Now the excitation type can be via applied forces or
enforced motions such as
displacement, velocity, or acceleration.
The excitation can be via an external input
like a hit or a strike,
or a base motion input like a shake or a shock.
The excitation can be applied for the whole duration
of the analysis or for just part of the time.
We'll look at an example the plate model where we applied
an impulse load; the shape of the impulse load
and the corresponding table to define it is shown here.
The duration of the impulse is 10 milliseconds.
Putting a lead-in or a zero input for a millisecond or so
like this can be very helpful to many solvers.
I have also extended the tail out to 20 milliseconds;
in other words I've got a flat line.
That ensures that any extrapolation will always happen
along the flat line along the zero axis.
There's always a danger if you close out
other than a flat line extrapolation will take you
into a very large negative or positive gradient
which could basically wreck the loading form.
I also like to put small slopes in
around about .1 millisecond.
What that does is avoid giving the solver two values
at the same time step.
Again dependent on the solver,
it can get into difficulties with that.
So to avoid it, a very slight slope in there.
Now in terms of the transient analysis results,
computed responses are also a function of time.
We can output nodal quantities, such as displacements,
velocities, and accelerations.
Or we can output element quantities,
such as forces, stresses, and strain energies.
We can also output reaction forces as a function of time.
Now transient response output can take several forms.
We can have time history graphs
which have a high capture rate.
Individual element or nodal responses
are presented through time.
We can also generate contour plots
which essentially just need a low capture rate
to give an overall feel for what the structures doing.
Each of these represents a frozen response
at a point in time.
We can animate these through time; however, we don't need
such a high fidelity of frame rate as we do to get
a very accurate plot in a time history plot.
Typically we might want to get time history graph's output
at every calculated point, and we might put a filter of say
every four or five time steps for the animations.
So in this video we've had a brief look at the
forms of input for a transient analysis
and the forms of output for a transient analysis.