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How to Calculate the Modulus of Rupture of Wood Beams

In materials science, you will frequently need to characterize materials, including brittle ones, in terms of their mechanical properties. The modulus of rupture, for instance, is the maximum stress to which a material can be subjected before it breaks. You can calculate the modulus of rupture of a beam of wood by experimentally determining the maximum stress that a wood beam can take and plugging that into a formula that includes the depth and width of the beam.

Things You'll Need

Wooden beam
Load testing machine

Instructions

- 1
  Place your wooden beam in a load testing machine, following the manufacturer's instructions. Generally, you will have to clamp each end of the wood beam to the machine.
- 2
  Use the machine to constantly apply a load to the wood specimen until it breaks. The machine will record this point of rupture in pounds.
- 3
  Plug the rupture stress into the formula for the modulus of rupture, which states that:
  
  modulus of rupture = (rupture stress)/(b*d^2),
  
  in which b is the width of the beam and d is the depth. For instance, if you have a beam of width 4 inches and depth 5 inches and a rupture stress of 20 pounds, your modulus of rupture will be:
  
  (20 pounds)/(4 inches*(5 inches)^2),
  
  or 1 pound/5 inches^3.

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