The natural variation of strength properties within brittle materials leads to the well-known phenomenon of size effect whereby large members tend to display lower strengths than smaller members when subjected to the same environmental and loading conditions. An important implication of size effect comes about when using multi-axial failure criteria to determine load carrying capacity of a structure in a constitutive model. The uniaxial strengths (needed as input for the strength theory) must be representative of the volume of material to which the strength theory is being applied - normally the element size in the finite element model. This means that one must first adjust the strength based on smaller laboratory specimens to that based on the finite element size.
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