Learning Forum

[Chandra]

Hello everyone,

I want to apply static load of mass 27071.88N or 6086 lbf or 2750 kg to an beam and calculate the dynamic load and then apply them also to that beam find the fatigue failure analysis.

Pls guide me the steps to do.

Thanks.

Regards,

Chandra

[peteroznewman]

Please reply with a sketch of the beam, the location of the mass, and the location where the force is applied. Is the force changing over time?  Describe, perhaps with a graph, how the force changes over time. What is it's minimum and maximum values, does it change sign?  Is the changing force what you mean by a dynamic load?  What is the frequency of oscillation of the force? If the frequency of the force is very low, then the mass just presents a static load, but if the frequency of oscillation of the force is high, then there are dynamic effects due to the mass.  What are the dimensions of the beam and the cross-sectional properties?

[Chandra]

 

Hi Petroznewman,

Picture of the beam is enclosed for your kind perusal and reference. The load is being applied at the center of the beam. The force doesnt change over time. The maximum mass applied to the beam is 6086kg on static load condition. I presume that if I calculate F=mg for static load , I can use those values and calculate the dynamic load which would be +/- of value of static load.  Why I want to calculate the dynamic load is that on road where bumps or speed breakers are there 1.8G is applied on to the beam  where as in static load condition 1G is applied.

I would like to know whether I can calculate F=mg method to calculate or not ? Is there anyother way ? I would appreciate if you could guide me.

Thanks.

Regards,

Chandra

[peteroznewman]

Hi Chandra,

Bumps in the road can create an acceleration load.  So if you mounted an accelerometer, which only measures dynamic acceleration and not gravity forces, to the center of the beam, the maximum value is +/-1.8 G.

Is the mass rigidly bolted to the beam or is it hanging from a cable?

If it is hanging from a cable, when the beam accelerates downward at 1.8 G, the mass that is hanging below can only accelerate downward at 1 G and the cable goes slack. When the beam turns around and accelerates upward at 1.8 G, the cable will snap taut at some point.  This won't happen if the mass is bolted to the beam.

If the mass is hanging from a cable, that would imply that it is sometimes on and sometimes off the beam. Therefore, there are a number of cycles of stress that vary between 0 and 1 G and then a separate cycle count considering bumps in the road.  

You didn't say how the beam is supported. Is the beam hanging from a cable or is it rigidly bolted to the truck frame?  Let's assume the simple case of the beam rigidly bolted to the truck frame at those two tabs on the top.

Let's assume the mass of 6086 kg is bolted to the beam so it never comes off and there is no cable snap to be concerned with, and the only cycles you care about are the bumps in the road. Create a point mass and scope it to the faces that support that point mass on the beam.

The load cycle goes from 2.8 G downward to 0.8 G upward. The range is -0.8 to 2.8 which has a mean of 1.0 and an amplitude of 1.8.

Use a Static Structural model, add the fixed supports, add a Standard Earth Gravity load, add an Equivalent Stress result and add the Fatigue Tool.  In the details of the fatigue tool change it to Ratio and enter these values and pick a Mean Stress theory. Then insert a Life calculation into the fatigue tool to calculate cycles to 50% probability of failure.

 

Regards,
Peter

 

[Chandra]

Hi Peteroznewman,

Thanks for your detailed reply.

I apologize, I should have told you earlier whether this beam is fixed or not.

Yes, this beam is locked to saddle and they have tolerance to move +/-0.5 inches .

How do we calculate a point mass ? Whats the standard earth gravity load ? Whats the reason to choose the loading ratio -0.2857 to be choosen ? Why scaling factor is 2.8 ?

Can I choose the cycle units to days ? After evaluating, Fatigue tool > life> results > minimum ? The minimum value mentioned is the value of its life cycle ?

I would greatly appreciate if you clarify these doubts.

Thanks.

Regards,

Chandra

 

 

[peteroznewman]

To create a point mass in the Model in the Mechanical program, RMB on the Geometry branch of the Outline, Insert > Point Mass.  Now pick the faces that support the mass. Faces that the mass would be bolted to or welded to. These faces will carry the weight of the mass in under the gravity load.

You have a Static Structural analysis so click on that title in the Outline.  Now on the Environment toolbar, you can see an Inertial button. Pull down on that and you will see Standard Earth Gravity.  This creates a 1.0 G downward load on your model. You will also see the Supports button to create Fixed Supports, though you might have something more appropriate for your connection to the saddle, like Remote Displacement that leaves some rotations free.

For the fatigue calculation, you want 2.8 G downward load, hence the scaling factor. If you play around with the ratio, you will find that if you type 0 then the bottom of the sinusoidal load will be 0 G, while if you type in -1 the bottom of the sinusoidal load will be -2.8 G.  You want the bottom of the sinusoidal load to be -0.8 G and if you use a loading ratio of -0.2857, that is what you will get.

One cycle is one bounce of the mass. If you put an accelerometer on the mass and drive 1 km, you can count the number of times the mass bounces in 1 km.  Let's say that the number of bounces in 1 km is 34,000 cycles. You could then calculate life in km by dividing the cycles to failure by 34,000 cycles/km.  If you know that the tractor drives 20 km/day, then you could calculate life in days by dividing the cycles to failure by 680,000 cycles/day.

If you don't have an accelerometer, you can calculate the first natural frequency of the beam with the mass in a Modal analysis.  If the first natural frequency is 32 Hz, and the tractor drives 6 hours/day, then there will be 691,200 cycles/day.

Now not every cycle will have the same amplitude. If you have an accelerometer, it will record the amplitude of each and every cycle. You can change the Fatigue tool to History and give the tool the accelerometer data, then it will accumulate more fatigue damage on the large amplitude cycles and accumulate less fatigue damage on the small amplitude cycles.  If you record an hour of accelerometer data, you will be computing life in hours.

[Chandra]

Thanks again for your detailed response Peteroznewman. Detail explanation on calculating loading ratio was very helpful. Thanks a lot.