Bending stress in laminate beam

[PEinOHIO]

i have a simply supported laminate beam (composite with steel on top and bottom) with a point load in the center. beam is about 4 ft long, point load about 400 lbs. i found a method for finding the max bending stress for each material in the beam in the Mech Eng Reference Manual for PE Exam.

what i am unsure of is how to determine when the beam will fail. can i just compare the maximum bending stress of each material with the yield stress of that material? should i use the distortion energy theory or some other theory? do i need to include the shear stress? thanks in advance

 

[sbozy25]

Lowest fatigue life wins...

Cabbages, knickers, It hasn't got A BEAK!

 

[PEinOHIO]

does it matter what theory is used? guess i'm not sure if i can just compare the bending stress to the yield stress, or if i need to use a method such as distortion energy theory which would include the shear stress too.

 

[ESPcomposites]

The distortion energy theory works pretty well for ductile materials, so you could probably use that.

You can include the shear, but for most beams the shear is not significant:

- For Mc/I, the location of c has zero shear (free surface). For a rectangular cross section, the max shear is at the center (parabolic distribution), at the same point where the bending stress is zero. I sections, etc. are different though.

- Even if you use the average shear in the cross section, it won't matter much unless the beam is relatively short. For long beams, the shear component will be in the noise.

How about you calculate it both ways and see if there is much of a difference.

Brian

http://www.espcomposites.com

 

[msquared48]

More than likely, the point of the exercise here is to find when the interstitial bond will fail, eliminating the composite action, and causing the non-composite sections to fail.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[PEinOHIO]

you're right, shear doesn't change it much. i think i'll go with distortion energy theory, its giving some reasonable numbers,. thanks