Fatigue Ansys.pdf

Fatigue Analysis Using ANSYS

D. Alfred Hancq, Ansys Inc.

Contents

1) Introduction

2) Overview of Capabilities

3) Typical Use Cases

4) Additional Fatigue Resources

1. Introduction

It is estimated that 50-90% of structural failure is due to fatigue, thus there is

a need for quality fatigue design tools. However, at this time a fatigue tool is

not available which provides both flexibility and usefulness comparable to

other types of analysis tools. This is why many designers and analysts use

"in-house" fatigue programs which cost much time and money to develop. It

is hoped that these designers and analysts, given a proper library of fatigue

tools could quickly and accurately conduct a fatigue analysis suited to their

needs.

The focus of fatigue in ANSYS is to provide useful information to the

design engineer when fatigue failure may be a concern. Fatigue results can

have a convergence attached. A stress-life approach has been adopted for

conducting a fatigue analysis. Several options such as accounting for mean

stress and loading conditions are available.

2. Capabilities

A fatigue analysis can be separated into 3 areas: materials, analysis, and

results evaluation. Each area will be discussed in more detail below:

2.1 Materials

A large part of a fatigue analysis is getting an accurate description of the

fatigue material properties. Since fatigue is so empirical, sample fatigue

curves are included only for structural steel and aluminum materials. These

properties are included as a guide only with intent for the user to provide

his/her own fatigue data for more accurate analysis. In the case of

assemblies with different materials, each part will use its own fatigue

material properties just as it uses its own static properties (like modulus of

elasticity).

2.1.1 Stress-life Data Options/Features

•

Fatigue material data stored as tabular alternating stress vs. life points.

•

The ability to define mean stress dependent or multiple r-ratio curves if

the data is available.

•

Options to have log-log, semi-log, or linear interpolation.

•

Ability to graphically view the fatigue material data

•

The fatigue data is saved in XML format along with the other static

material data.

•

Figure 1 is a screen shot showing a user editing fatigue data in ANSYS.

Figure 1: Editing SN curves in ANSYS

2.2 Analysis

Fatigue results can be added before or after a stress solution has been

performed. To create fatigue results, a fatigue tool must first be inserted into

the tree. This can be done through the solution toolbar or through context

menus. The details view of the fatigue tool is used to define the various

aspects of a fatigue analysis such as loading type, handling of mean stress

effects and more. As seen in Figure 2, a graphical representation of the

loading and mean stress effects is displayed when a fatigue tool is selected

by the user. This can be very useful to help a novice understand the fatigue

loading and possible effects of a mean stress.

Figure 2: Fatigue tool information page in ANSYS

2.2.1 Loading

Fatigue, by definition, is caused by changing the load on a component over

time. Thus, unlike the static stress safety tools, which perform calculations

for a single stress, fatigue damage occurs when the stress at a point changes

over time. ANSYS can perform fatigue calculations for either constant

amplitude loading or proportional non-constant amplitude loading. A scale

factor can be applied to the base loading if desired. This option, located

under the “Loading” section in the details view, is useful to see the effects of

different finite element load magnitudes without having to re-run the stress

analysis.

•

Constant amplitude, proportional loading: This is the classic, “back

of the envelope” calculation. Loading is of constant amplitude because

only 1 set of finite element stress results along with a loading ratio is

required to calculate the alternating and mean stress. The loading ratio is

defined as the ratio of the second load to the first load (LR = L 2 /L 1 ).

Loading is proportional since only 1 set of finite element stress results is

needed (principal stress axes do not change over time). No cumulative

damage calculations need to be done. Common types of constant

amplitude loading are fully reversed (apply a load then apply an equal

and opposite load; a load ratio of –1) and zero-based (apply a load then

remove it; a load ratio of 0). Fully reversed, zero-based, or a specified

loading ratio can be defined in the details view under the “Loading”

section.

•

Non-constant amplitude, proportional loading : In this case, again

only 1 set of results are needed, however instead of using a single load

ratio to calculate the alternating and mean stress, the load ratio varies

over time. Think of this as coupling an FEM analysis with strain-gauge

results collected over a given time interval. Cumulative damage

calculations including cycle counting and damage summation need to be

done. A rainflow cycle counting method is used to identify stress

reversals and Miner’s rule is used to perform the damage summation.

The load scaling comes from an external data file provided by the user,

(such as the one in Figure 3) and is simply a list of scale factors.

Figure 3: Chart of loading history

Several sample load histories can be found in the “Load Histories”

directory under the “Engineering Data” folder. Setting the loading type

to “History Data” in the fatigue tool details view specifies non-constant

amplitude loading. Several analysis options are available for non-

constant amplitude loading. Since rainflow counting is used, using a

“quick counting” technique substantially reduces runtime and memory.

In quick counting, alternating and mean stresses are sorted into bins

before partial damage is calculated. Without quick counting, the data is

not sorted into bins until after

partial damages are found. The

accuracy of quick counting is

usually very good if a proper

number of bins is used when

counting. The default setting for

the number of bins can be set in

the Control Panel. Turning off

quick counting is not

recommended and in fact is not

a documented feature. To allow

quick counting to be turned off, set the variable “AllowQuickCounting”

to 1 in the Variable Manager. Another available option when conducting

a variable amplitude fatigue analysis is the ability to set the value used

for infinite life. In constant amplitude loading, if the alternating stress is

lower than the lowest alternating stress on the fatigue curve, ANSYS will

use the life at the last point. This provides for an added level of safety

because many materials do not exhibit an endurance limit. However, in

non-constant amplitude loading, cycles with very small alternating

stresses may be present and may incorrectly predict too much damage if

the number of the small stress cycles is high enough. To help control

this, the user can set the infinite life value that will be used if the

alternating stress is beyond the limit of the SN curve. Setting a higher

value will make small stress cycles less damaging if they occur many

times. The rainflow and damage matrix results can be helpful in

determining the effects of small stress cycles in your loading history.

The rainflow and damage matrices shown in Figure 4 illustrate the

possible effects of infinite life. Both damage matrices came from the

same loading (and thus same rainflow matrix), but the first damage

matrix was calculated with an infinite life if 1e6 cycles and the second

was calculated with an infinite life of 1e9 cycles.

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