## General Mechanical

#### ANSYS Workbench Fatigue Study (Displacements)

• Student_One
Subscriber

I am interested in conducting a fatigue study, However the "known loading" is a displacement rather than a force. I am familiar with load controlled fatigue but have no idea how to approach this problem.

Details are as follows. I would like to cycle my simple cantilever part through a :

displacement max = 0.3mm

displacement min =0.27mm

Any ideas?

• peteroznewman
Subscriber

I am also interested in fatigue.

A simple cantilever has a length and thickness and you have specified a tip displacement.
Those two parameters will cause the stress from a tip displacement of 0.3 mm to vary widely.
A long, thin cantilever will have stress below the fatigue strength and that will have infinite life.
A short, thick cantilever will have stress above the yield strength and will be bent after one cycle.

Between those extremes are two cantilevers that have fatigue issues that are analyzed differently.
A thinish cantilever will have low stress in the high cycle fatigue range and is appropriate to analyze with stress-life equations.
A thicker cantilever will have high stress in the low cycle fatigue range and is appropriate to analyze with strain-life equations.

ANSYS has a Fatigue Tool that helps to compute cycles to failure, which it calls life, once the nature of the cycle is defined.
Your cycle is from 0.27 mm to 0.3 mm and back. That represents an R=0.9 cycle. Fully reversed stress such as +/- 0.3 mm
would be a Ratio of -1. The R=0.9 ratio implies that there is a mean stress and that can be accounted for in the equations.

Stress-life data is usually acquired using a force-controlled tensile testing machine.
Strain-life data is usually acquired using a displacement-controlled tensile testing machine.

• Student_One
Subscriber

Thank you.

"... Fatigue Tool that helps to compute cycles to failure, which it calls life, once the nature of the cycle is defined. Your cycle is from 0.27 mm to 0.3 mm and back. That represents an R=0.9 cycle.

is the fatigue tool using the loads (max principal in your example) resulting from the applied displacements (0.27 mm to 0.3 mm) for the stress life computations ?

• peteroznewman
Subscriber

The Fatigue Tool uses the stress at 0.3 mm only and uses the ratio to calculate the stress at 0.27 mm.