Understanding excitation loads for static events

The excitation load transforms a static state of stress or strain into a cyclic stress or strain time history, from which durability results are computed. The following types of excitation loads are available:

• Load pattern

• Result path

• Excitation function

Load patterns and excitation functions are defined by the following:

• Stress or strain results from the static solution subcase

• A function defining one cycle

• Load scaling factor

• Load offset

The loading history, F(t), is calculated as follows:

F(t) = S ∙ (a∙f(t) + Lm)

where:

• S is the static stress or strain.

• a is the scale factor.

• f(t) is a unit cyclic function for load patterns or a general function for excitation function.

• Lm is the load offset.

Scale factor

The scaling factor scales the stress results linearly. Using the scaling factor helps you to avoid performing repeated solves for different loading values.

Load offset

Applying a mean stress offset to the excitation load modifies the stress free state of cyclic functions. When the structure is in the unloaded state instead of being in the stress free state, it has the mean stress applied to it.

Cyclic function

The cyclic function describes the shape of the stress or strain history when it is loaded and unloaded. The following cyclic pattern types are supported by the load pattern excitation:

• Half-unit cycle

• Full-unit cycle

Half unit cycle

In a half-unit cyclic function, the structure is initially at rest or in a stress free state. The structure is loaded to maximum stress and then unloaded back to the stress free state.

time, t00.51f(t)010	time, t	0	0.5	1	f(t)	0	1	0
time, t	0	0.5	1
f(t)	0	1	0

Full unit cycle

In the full-unit cyclic function, the structure is initially at rest or in a stress-free position. It is then loaded to maximum stress, unloaded to the stress free state, reloaded to maximum negative stress, and finally unloaded again to the stress free state.

time,t00.250.50.751f(t)010-10	time,t	0	0.25	0.5	0.75	1	f(t)	0	1	0	-1	0
time,t	0	0.25	0.5	0.75	1
f(t)	0	1	0	-1	0

General function

When you create an excitation function type of excitation load, you define a general AFU table function that represent the shape of the stress or strain history for one cycle.

To create the general AFU table function, you use the XY Function Manager and XY Function Editor commands.

When you create the AFU table function, you must set:

• The Purpose and Function Type to General.

• The abscissa data type to time that is evenly spaced.

• The ordinate data type to one of the following: Unknown, Unitless Scalar, Unitless Real, or Unitless Integer.

Cyclic function created from result path

For result path excitation, you define a cyclic stress or strain function from existing solution steps. You construct an excitation load by repeating the static loading subcases with different scaling factors.

Example:

Assume that you have a static solution with the following two subcases:

• Subcase 1 has a stress loading of 10 000 Pa.

• Subcase 2 has a stress loading of 12 000 Pa.

To generate the result path excitation shown, you need to add the subcases and scale factors in the following order to the Result Path dialog box:

1. Subcase 1 with scale factor equal to 1.

2. Subcase 2 with scale factor equal to -2.

3. Subcase 1 with scale factor equal to 3.

4. Subcase 2 with scale factor equal to 3.

5. Subcase 1 with scale factor equal to -2.

6. Subcase 2 with scale factor equal to 1.

How do I

Create a static durability event

Add a load pattern to a static event

Create a result path excitation

Create an excitation function

Learn more

Static events

Excitation loads

Durability objects

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Understanding excitation loads for static events, Simcenter 3D 2021.1 Series

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