Catastropic propagation of ductile cracks in metallic foils Public Deposited


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  • The sudden failure of metal structures under load, which has been a design problem for many years, is characterized by a crack propagation rate which approaches the speed of sound in the material. These failures, which are often known as catastrophic fractures, may be divided into two types which depend on the particular material in question. Fractures which cause a very small amount of plastic deformation associated with low energy absorption are known as brittle fractures, whereas high energy absorption during fracture, due to large amounts of plastic deformation, is associated with ductile fracture. The Griffith theory for spontaneous fracture in brittle materials states that the strain energy released during fracture must be greater than the energy needed to form the new crack surface area. This theory, which has been verified experimentally for brittle materials, is not applicable to ductile materials since the energy of plastic deformation during crack propagation must be taken into account. The revised and simplified theory for ductile materials states that the strain energy must exceed the energy needed to cause plastic deformation ahead of the crack. The steps in the fracture of ductile materials are initiation, slow crack growth, and rapid or catastrophic crack propagation, and the variable plastic deformation involved in these steps causes the process to be complex in behavior and analysis. However, an understanding of the causes of slow and rapid crack growth can be obtained by studying the effects of plastic deformation at the tip of a propagating crack. The objective of this thesis was to investigate the factors which affect the tendency for a ductile crack to propagate catastrophically. This was accomplished through the use of two types of investigations. In the first investigation an attempt was made to obtain catastrophic failure in 18 x 32 inch uniaxially stressed sheets of commercial household aluminum foil by introducing a crack in the center of the sheet in such a manner that the crack was elongated outwards in each direction. The catastrophic failure which was observed, however, was due to the addition of extra energy to the system boundaries which was not accounted for in the theory. Slow crack propagation was also observed and led to the second investigation. The plastic deformation at the crack tip was studied by visual of the was done observation using a metallograph and by actual measurement amount of deformation occurring during crack growth. This with a Tukon microhardness tester in which indentations were placed on the foil specimens, which were later given a small crack at the center, and which were mounted in a drill press vise on the tester. The crack was then propagated by turning the screw on the vise and measurements of the deformation between the indentations were made. The photographs and measurements of the plastic deformation at the crack tip showed that deformation existed throughout the entire specimen and was not localized in a given area at the crack tip as has been assured in some theories. The existance of elastic strain in the material was shown by the occurrence of elastic recovery in the strain relieved areas of the cracked material although no differentiation could be made between the area of elastic and plastic deformation. The existance of a stable configuration of iso-strain contours surrounding the tip of the crack was also shown. Calculations of the strain energy needed to satisfy the plastic deformation energy requirements showed that a crack length of about 49 inches would be needed to cause catastrophic failure. This is much larger than the specimen size used and would explain the lack of rapid crack growth in the tests run under near theoretical conditions.
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