How to Determine the Modulus of Rupture

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The modulus of rupture is the ultimate strength determined in a flexure or torsion test. The flexure test is based on the maximum fiber stress at failure, and the torsion test is based on the maximum shear stress in the extreme fiber of a circular member at failure. Usually, the modulus of rupture refers to a 3-point flexure test on brittle material such as ceramic or concrete. Knowing how to determine and calculate the modulus of rupture for a specific material is important, because it provides insight on the maximum force a substance can withstand before breaking.

Modulus of Rupture/Flexural Test

    Dry surface of each beam to attach angle brackets and label. Use epoxy to glue angle brackets to the top center of bottom face, approximately 10 1/2 inches from the ends. The top face is the finished, rough surface. Mark this location before gluing.

    Mark the beams 1 1/2 inches from the ends and 3 inches from the bottom/top on the rough and smooth bottom/top surfaces. These markings will be used for the deflection frame.

    Mark the beams 1 1/2 inches, 7 1/2 inches, and 13 1/2 inches from the end.

    Load the sample into the 20-kip testing frame. Use a loading head with two rollers spaced 6 inches apart. Attach the pin and rollers from the bottom of the base plate with bolts.

    Attach the deflection frame by screwing it into the points marked on the beam with the LVDT holder just under the angle bracket.

    Set up the load controller. Connect the controller to the load cell and LVDT. Test the load.

Calculating Modulus of Rupture

    Record data from tests including load at breaking, distance between edges on which the sample is supported, average sample breadth, and average specimen depth. Convert the breaking load to pounds and all the other measures to inches.

    Multiply the breaking load by three and by the distance between edges on which the sample is supported.

    Multiply two by the average sample breadth and the square of the average sample depth.

    Divide the first number by the second number. The result is the value of the modulus of rupture in pounds per square inch.


    • Make sure all screws and nuts are tightened on the frame against the beam. If any of the components are loose, the beam may spontaneously break. Always make sure "Load Protect" is on when setting up the beam or equipment. Make sure all the equipment works properly before following steps. Read the manuals for all the controllers before continuing. Do not attempt this unless you already know how to operate all machinery used. There is a chance that beams will break, which may cause physical harm.


About the Author

Lela Bast is a professional writer whose articles have appeared on websites such as Essortment. She is currently pursuing her Bachelor of Science in engineering at the University of Pennsylvania, majoring in bioengineering and minoring in engineering entrepreneurship and computer science.

Photo Credits

  • Hemera Technologies/ Images

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