ANSYS Day2 Transient Analysis

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Module 3

TRANSIENT DYNAMIC ANALYSIS

A. Define transient dynamic analysis and its purpose.B. Learn basic terminology and concepts underlying

transient analysis.

C. Learn how to do a transient analysis in ANSYS.D. Work on a transient analysis exercise.

TRANSIENT ANALYSIS

A. DEFINITION & PURPOSE

What is transient dynamic analysis?

• A technique to determine the response of a structure to arbitrary

time-varying loads such as an explosion.

• Input

• Loads as a function of time.

• Output

•Time-varying displacements and other derived quantitiessuch as stresses and strains.

TRANSIENT ANALYSIS

… DEFINITION & PURPOSE

Transient dynamic analysis is used in the design of:

• Structures subjected to shock loads, such as automobile doors

and bumpers, building frames, and suspension systems.

• Structures subjected to time-varying loads, such as bridges,earth moving equipment, and other machine components.

• Household and office equipment subjected to “bumps and

bruises,” such as cellular phones, laptop computers, and

vacuum cleaners.

TRANSIENT ANALYSIS

B. TERMINOLOGY & CONCEPTS

Topics covered:

• Equation of motion

• Solution methods

• Integration time step

TRANSIENT ANALYSIS - TERMINOLOGY & CONCEPTS

EQUATION OF MOTION

t F u K uC u M

• Equation of motion for a transient dynamic analysis is the

same as the general equation of motion.

• This is the most general form of dynamic analysis. Loading

may be any arbitrary function of time.

• Depending on the method of solution, ANSYS allows all types

of nonlinearities to be included in a transient dynamic analysis- large deformation, contact, plasticity, etc.

TRANSIENT ANALYSIS - TERMINOLOGY AND CONCEPTS

… INTEGRATION TIME STEP

Response frequency• Different types of loads excite different

natural frequencies of the structure.

• Response frequency is the weightedaverage of all frequencies excited by a

given load.• The ITS should be small enough to

capture the response frequency .

• Twenty time points per cycle should besufficient, i.e,

Dt = 1/20f

where f is the response frequency.

Response period

TRANSIENT ANALYSIS

C. PROCEDURE

• We will discuss the Full method only in this section.

• Five main steps:

• Build the model

• Choose analysis type and options

• Specify BC’s and initial conditions

• Apply time-history loads and solve

• Review results

TRANSIENT ANALYSIS PROCEDURE

BUILD THE MODEL

• All nonlinearities are allowed.

• Remember density!

Build the model

Choose analysis type and options:

• Enter Solution and choose

transient analysis.• Choose Full transient

• Solution options - discussed

• Damping - discussed next.

CHOOSE ANALYSIS TYPE & OPTIONS

… CHOOSE ANALYSIS TYPE & OPTIONS

Solution options

• Choose large displacement transient or small

displacement transient .

• When in doubt, choose large displacement

transient

Specify output controls

(discussed next)

Specify time at end of load step.

Automatic time stepping

(discussed next)

Specify initial, min and max

values of t for this load step.

Automatic time stepping

• An algorithm that automatically calculates appropriate ITS sizesduring the transient.

Recommendation is to activate it and also specify minimum andmaximum values of ITS.

• If nonlinearities are present, use the “Program Chosen” option.

Output controls

• Used to determine what is written to the results file.

• Use the OUTRES command or choose Solution > Sol’n Control.. > Basic in the

• Typical choice is to write all items at every substep to the results file.

• Allows smooth plots of results vs. time.

• Might cause results file to be large.

… CHOOSE ANALYSIS TYPE & OPTIONSTurn transient effects on/off

• useful for setting up initial conditions

Ramp or Step apply load

• Specify damping)

• Use default values for time integration

parameters

Damping

• Both alpha damping and beta damping are available.

• In many cases, alpha damping (viscous damping) is ignored and

only beta damping (damping due to hysteresis) is specified:

b = 2/w

where is the damping ratio and w is the dominant response

frequency (rad/sec).

• Material damping (e.g. rubber) and element damping (e.g. shock

absorber) are also available.

• Choose solver

• By default ANSYS chooses Sparse solver

• For large problems (>100000 dofs) use PCG solver

… SPECIFY BC’S & INITIAL CONDITIONS

Load step 2:• Transient effects ON.

• Release the object, i.e, delete

DOF constraints on the object.

• Specify ending time and continue

with the transient.

t 0.0005 0.001Load step 1

Application of Temporal Acceleration

• Load step 2:

• Transient effects ON.

• Delete the imposed displacement.

• Specify ending time and continue with the transient.

Example - A stationary plate subjected to an impulse load

• In this case u0 = v0 = a0 = 0.

• These are the default initial conditions in ANSYS, so there is no

need to specify them!

• Simply apply the boundary conditions and the impulse load,

then solve.

APPLY TIME-HISTORY LOADS & SOLVE

Build the model

Choose analysis type and options

Specify BC’s and initial conditions

Apply time-history loads and solve

• Time-history loads are loads that

vary with time.

• Three ways to apply them:

• Function tool

• Tabular input

• Multiple load steps

… APPLY TIME-HISTORY LOADS & SOLVE

Function Tool

• Allows you to apply complicated boundary conditions. To access thefunction editor, choose Solution > Define Loads > Apply > Functions >Define/Edit

• Recommendation: do not use the Function Tool if the boundary conditionscan be expressed directly with tabular input

For more informationrefer to “Applying LoadsUsing Function Boundary

Conditions” in the BasicAnalysis Guide.

Method 1

A l i f d 2

Acceleration response of node 2

Method 2

Acceleration response of node 2

Method 3

Tabular input

• Allows you to define a table of load vs. time (using array parameters) and apply the table

as a load.

• Very convenient, especially if there are several different loads, each with its own time

history.

• For example, to apply the force-vs-time curve shown:

1. Choose Solution > Define Loads > Apply > Structural > Force/Moment > On Nodes,

then pick desired nodes.

1.0 1.5

2. Choose the force direction and “New

table”, then OK.

3. Enter table name and no. of rows (no. of

time points), then OK.

4. Fill in time and load values, then File >

Apply/Quit.

5. Specify ending time and integration time step.

• Solution > Load Step Opts > Time/Frequenc >

Time - Time Step

• There is no need to specify the stepped or ramped

condition. It is implied by the load curve.

6. Activate automatic time stepping, specify output controls,

and solve (discussed later.)

Multiple load step method

• Allows you to apply each segment of the load-vs-time curve in

a separate load step.

• No need to use array parameters. Simply apply each segment

and either solve the load step or write it to a load step file

(LSWRITE).

… APPLY TIME-HISTORY LOADS & SOLVE• For example, to apply the same force-vs-time curve as before:

1. Plan the approach. We will need three load steps in this

case: one for the up-ramp load, one for the down-ramp

load, and one for the step removal of the load.Force

0.5 1.0 1.52. Define load step 1:

Apply force = 22.5 units at the desired nodes.

Specify the ending time (0.5), integration time step, and ramped

loads.

Activate automatic time stepping, specify output controls*, and

either solve or write the load step to a load step file.

*Discussed later

3. Define load step 2:

• Change force values to 10.0.

• Specify the ending time (1.0). No need to respecify the

integration time step or ramped condition.• Solve or write the load step to a load step file.

4. Define load step 3:• Delete the forces or set their values to zero.

• Specify the ending time (1.5) and stepped loads.

• Solve or write the load step to a load step file.

Solution

• Use SOLVE command (or LSSOLVE if

load step files were written).

• At each time step, ANSYS calculates

load values based on the load-vs-timecurve.

REVIEW RESULTS

Build the model

Review Results

• Consists of three steps:

• Plot results vs. time at specific points in the

structure.

• Identify critical time points.

• Review results over entire structure at those time

points.

… REVIEW RESULTS - POST26

Identify critical time points

• Use the List Extremes menu.

• Note down the time points at which the minimum and maximum

values occur.

Build the model

Review Results

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