Calculating stress in top flange of a beam 1

[bsmet95]

I need to perform a pull test on a proprietary beam (patented monorail track), as a point load, beam simply supported. Strain gages will be used on the top flange. How do I calculate stress from the measured strain?

 

[ToadJones]

Hooke's Law

 

[rb1957]

and remember poission ...

 

[haynewp]

Unless you already have the actual yield stress of the material provided, you need to get it by plotting when a tensile test specimen goes from increasing tension and increasing strain to when it is increasing strain with no increase in tension. There are standard test setups for this in combination with using Hooke's Law.

Assuming the beam stays all within the elastic (less than yield strength) range, calculate the inertia of the section then get the section modulus (first moment of the area), divide the moment from the load by the section modulus to get the max stress, all stress points in between the max are linear varying from zero at the neutral axis to the max at the extreme most fiber from the neutral axis. This should verify the stress you calculate from the test beam's strain you measure that has to be less than the yield strain. But you don't have to get the area moments if you have the beam strains and with Hooke's Law.

If the beam strain exceeds the yield strain, the beam stress is the yield stress until the point in the beam where the strain is less than the yield strain, it is linear from this point to zero at the new calculated neutral axis. You can also calculate this stress profile using the modified section modulus you recalculate (using the past yield strain producing moment) since you know the yield stress of the material.

 

[bsmet95]

Thanks. The yield stress is known for the material and the applied load will be kept low enough so that the calculated stress will be no greater than 0.60Fy.

 

[JStephen]

Note that the strength of the rail in bending may be governed by buckling rather than yielding, so you can't necessarily deduce the strength of the assembly from the test.

 

[haynewp]

If the test beam buckles there should be strain data from the gauges just prior to buckling.

 

[msquared48]

Is this extruded Aluminum or Steel?

If it's Aluminum the .6 Fy will not apply - you will have to use the Aluminum code stress limitations that are very sensitive to the shape.

Mike McCann
MMC Engineering

 

[JStephen]

My point was that if you load it to 0.6Fy and it doesn't buckle, you also don't know how far you are from failure, either.

Of course, the post didn't say WHY this was being done, so it may not matter.

 

[haynewp]

I don't know why the limit to 0.6fy during the test. We tested to failure.

 

[bsmet95]

The beam is a special steel section, certified Fy>36,000. It will be tested at various spans, and the load applied for each will be calculated as per Fb=12e6/LdAf.

 

[bsmet95]

My error: should be Fb=12e6/L(d/Af).