The test in which a simple penny shaped disk is loaded along a diameter until it splits is a simple one to perform on brittle materials. However, the interpretation of the results obtained relative to uniaxial tensile strength is far from simple because of the biaxial state of stress at the center of the disk where cracks initiate. Results on relatively brittle tungsten carbide disks and uniaxial tensile specimens are brought into good agreement by adopting a maximum tensile strain criterion for strength, a hydrostatic stress effect on fracture strain, and a small size effect. Weibull extreme value statistics are employed to handle the dispersion of results common to all brittle fracture studies. It is clearly demonstrated that tensile strength is not a material properly for a brittle material but depends upon the state of stress at the critical point of fracture.

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