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In
1920, A. A. Griffith published a paper providing a fracture model based
on the concept of crack propagation in a brittle material. This was postulated
to occur at a critical tensile stress for which there was a balance between
the elastic strain energy decrease in the material and the surface energy
increase due to the creation of the crack. The model related the crack
size, c, the fracture stress, sc,
the crack surface energy, g,
and Young's modulus, E, for the material to give: sc
= (2Eg/pc)0.5.
Experiments on glass fibers confirmed this model but also showed another
interesting effect. If the soda-lime glass fibers were tested immediatly
after drawing from the melt they were found to be stronger than fibers
that were exposed to the atmosphere for some time. The effect of atmospheric
constituents (OH ) reduced the crack surface energy and hence
the fracture stress.
During
the 1914 - 1918 war, Griffith was at the Royal Aircraft Establishment at
Farnborough, U.K. studying the effect of stress risers, such as groves,
on the performance of propeller shafts for aircraft. |
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