# Confused on a few things for a beam and cycles to failure

1. Apr 20, 2017

I have a problem which is a beam with 2 machines on the beam and one of them has an operating frequency between 200 and 600 rad/s that transmits a force to the beam. I have to determine the number of cycles to failure of the beam. It's stated in the problem statement to account for the mean stress that arises from the weight of the machines and the beam.

I've found out the bending moments for the transmitted force and the weights of each machine and for the beam's weight. And then used that to determine the stresses. I've calculated that the stresses from just the weights add up to roughly 72 MPa. And the stress from just the transmitted force is 113 MPa.

Now I'm told to use the soderberg failure criterion.
σsoderberg=-(σfatigueyieldmeanfatigue
σalternating≤σsoderberg

I'm given the yield stress and I need to find the fatigue stress to calculate for cycles to failure. So I need to calculate the mean and alternating stresses. From what I've learned is that the mean stress is (σmaxmin)/2 and alternating stress is (σmaxmin)/2. I'm confused on my problem on what the min and max is. Well obviously the min would be 72 for the weight of the beam when the machine isn't transmitting a force. Would the max be 113 or would it be the total of the stress from weight plus the stress from the transmitted force for 185 MPa.

The other part I'm confused on is that I need to also find the cycles to failure from factors of safety for 1, 1.25, 1.5, 1.75, 2. Now I know to take that into account in a basic problem. You would take the yield stress over the factor of safety to find the working stress. But I'm not sure how to apply it for the soderberg equation?

If anyone could help me out that would be so greatly appreciated. Thanks.

2. Apr 26, 2017

### PF_Help_Bot

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