Calculating Fatigue Limit | STRESS vs. Stress Amplitude | Homework Help

[1question]

Homework Statement

First of all, fatigue limit is defined in terms of STRESS, NOT stress amplitude, right?

Calculate the fatigue limit based on the given data (see my attempt for data).

Homework Equations

N/A

The Attempt at a Solution

Here is a link to the data + my graph (image too large for attachment): https://docs.google.com/file/d/0BzPjRy-fIhozc19YaUkyUzdKU0E/edit?usp=sharing I found the area through A = pi(d/2)^2 and each stress through stress = F/A.

Ok, I've got data going from log(N) cycles vs. 994,000 Pa to ~2mil pascals. The 994,000 Pa is at the right end that is almost perfectly horizontal and is the last of 3 points to make that horizontal line. Therefore, I pick it as my fatigue limit value. Unfortunately, the quiz I am doing only offers 280 MPa, 330 MPa, 400 MPa and no fatigue limit as answers. The curve clearly has a fatigue limit and all 3 values on the horizontal line are very close (994K Pa, 101K Pa, 102K Pa), so the particular number I picked is not the issue. In fact, not one of my stress values is close to any of the options given (see link's graph or table).

I have no idea how I am doing this wrong. The other questions' magnitudes are also far from the ones I get. Can anyone shed some light on this? Thanks.

 

[rock.freak667]

Based on your experiment, I assume you subjected the cantilever to bending. If that is the case, then your stress is bending stress and not a tensile one so using Force/Area won't give you bending stress.

\sigma_{bending} = \frac{My}{I}

where M = bending moment

y = distance from neutral axis

I = second moment of area about the bending axis.