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# Beginner questions regarding NX Durability (Load ratio and mean stress/maximum stress)

Hi all,

I am using NX10 Durability to simulate fatigue life and to compare the simulation results with experimental findings. I have two questions in regards to the Durability module when performing static analyses:

1. How do I change the load (stress) ratio for the excitation?

The settings for the excitation allows the user to change the load pattern to either a half-unit cyclic function or full-unit cyclic function. Can I assume that half-unit cyclic function refers to load ratio of 0 while the full-unit cyclic function refers to load ratio of -1? In this case, how do I obtain result for load ratio of 0.1?

2. Does the excitation refers to mean stress or maximum stress?

The excitation in Durability grabs the "Subcase - Static Loads 1" (by default) from the structural static analysis to create a cyclic load. Considering a dogbone fatigue test specimen undergoing a simple force-controlled uniaxial fatigue test, does the "Subcase - Static Loads 1" refer to the mean stress or the maximum stress in a constant amplitude cyclical loading?

E.g: if the axial load is 1000N and for load ratio of 0.1, is the maximum and minimum cyclic load 1000N and 100N respectively or is it 1818N and 181N respectively?

Greatly appreciate your help for these questions.

Thanks,

Gregory

3 REPLIES

## Re: Beginner questions regarding NX Durability (Load ratio and mean stress/maximum stress)

1. Define a result path excitation, like this:

Subcase 1, scaled by 1

Subcase 1, scaled by 0.1

Subcase 1, scaled by 1

2. The stresses or strains from the static load case represent the maxima. The means are derived from the pattern or the path.

Philippe Tremblay
phillippe.tremblay.ext@siemens.com
www.mayahtt.com

## Re: Beginner questions regarding NX Durability (Load ratio and mean stress/maximum stress)

Hi Philippe,

Thank you for your response.

I didn't give my attention to the excitation type "Result Path", "Pattern" or "Function" that the user can select previously. I understand how you can set R-ratio of 0.1 now.

If I were to achieve R-ratio of -1, am I right to define a result path excitation with subcase 1, scaled by 1 then subcase 1, scaled by -1 and subcase 1, scaled by 1? In this case, how is it different from setting from the "Load Pattern", and setting scale by 1 and selecting the full unit cycle?

Could you provide some explanations on how the user can make use of "Pattern", "Result Path" and "Function"?

Greatly appreciate your help!

Thank you,

Gregory

Solution
Solution
Accepted by topic author Gregory_555
‎11-23-2016 08:32 PM

## Re: Beginner questions regarding NX Durability (Load ratio and mean stress/maximum stress)

Gregory,

You would do it like this:

Subcase 1: 0

Subcase 1: 1

Subcase 1: 0

Subcase 1: -1

Subcase 1: 0

This should give you exactly the same result as if you were to define the reversed pattern

These 3 excitation types allow you to define load variations, ie varying peak and mean stress or strain histories, from a static load. This is necessary because fatigue cannot occur from the appliction of a purely static load.

• Load Pattern: Half or Full Triangle Wave input
• Result Path: Weighted Linear Concatenation of load cases
• Function: Scaling the load case stress/strain with graph

I hope this is clear,

Phil

Philippe Tremblay
phillippe.tremblay.ext@siemens.com
www.mayahtt.com