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In order to obtain a valid plane-strain fracture toughness value, a test specimen must meet the E 1304 criteria for a minimum specimen size. This minimum size is a function of the plastic zone size at the crack tip and hence a function of the ratio KIv/Sys where:
Sys is the tensile yield strength of the material. For a valid KIv result, the specimen must meet the following criteria:
![]() where B is the diameter of a short rod test specimen or the width of a short bar test specimen. In selecting a specimen size to be tested, one should know the tensile yield strength of the material and then make an estimate of the expected KIv value. With this information, a minimum specimen diameter can be determined. It is then desirable to select a standard specimen diameter larger than this minimum diameter. Standard Specimen Sizes Although, any specimen size can be tested, it is desirable to use standard sizes that allow use of existing loading grips and displacement transducers. Standard specimen sizes are as follows:
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