Stress Intensities and Crack Growth in Photoelastic Motor Grain Models
VIRGINIA POLYTECHNIC INST AND STATE UNIV BLACKSBURG DEPT OF ENGINEERING SCIENCE AND MECHANICS
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Two dimensional thermal shrinkage tests on photoelastic models and tensile tests on single fin sections of motor grain have suggested that, for a specific fin geometry consisting of a small edge radius coalescing with a large central fin tip radius, the critical locus for stress in a homogeneous model lies at the points of confluence for the two radii on the fin surface. On the other hand, some motor grain manufacturers have reported cracks emanating from the fin tip along its own axis of symmetry as a result of defects collected there during the casting process. One aspect of the problem just beginning to be studied is how cracks grow from these two critical points. Cotterell classified cracks that extended in a particular direction as Class I cracks, i.e., cracks located on the axis of symmetry of a fin were cracks that were always under Pure Mode I loading due to symmetry in both load and geometry. Other cracks, such as those emanating from the points of coalescence of the two fin tip radii off-axis cracks were initially called Class II cracks, the growth direction being initially unknown due to mixed mode states along the crack border, but after turning and growing in a new direction become Class I cracks. A series of experiments on photoelastic motor grain models under internal pressure were conducted on models containing such cracks in which the frozen stress method was used together with a two-parameter algorithm to extract the Mode I and Mode II stress intensity factors SIFs at certain points along the crack borders. The experiments showed that both SIF values and crack geometry during growth were quite variable due to shear modes during the class II stage for the off-axis cracks. On the other hand, the symmetric cracks on the fin axis were quite predictable in their behavior, as were the off-axis cracks after eliminating the shear modes. 1 table, 5 figures, 4 refs.
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