THE RESISTANCE OF MATERIALS TO FRACTURE PROPAGATION AND GUNFIRE DAMAGE
NAVAL RESEARCH LAB WASHINGTON DC
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The report briefly describes the theory and procedures used in the Ballistics Branch, Naval Research Laboratory for determining the toughness of materials. It is necessary to separate the total work for crack propagation into two terms a the work per unit crack area used in producing permanent set or that at best slowly recoverable, and b the work per unit crack area not associated with permanent set. The permanent set work is often highly dependent on the dimensions of the test piece or structure, as well as on the material and temperature. The non-permanent set work per unit area designated G sub c in this report is relatively insensitive to specimen geometry and dimensions and it is therefore more nearly a materials property. It is not completely so, however. It can be shown that G sub c in a given material depends on the roughness of the fracture and this in turn can be temperature dependent. In general it is less temperature dependent than the permanent set work. By the judicious choice of a probable value of G sub c for a given material it is possible to predict with fair accuracy the strength of a structural member containing a crack of known or assumed dimensions. Calculations of the strength of a damaged piece have been in good agreement with experiments. Gunfire damage experiments were performed not to provide design data or to duplicate battle conditions but rather to reveal some of the important factors which must be kept in mind in planning and interpreting such tests.