The principal loading of this hinge was bending about the axis of the screw hole, so the fatigue crack started at the side closest to the hole where the tensile stress was highest. The thin-ness of the multiple-leaf hinge caused additional bending stresses about the orthogonal axis, so the striations on the fracture surface have the shape of chevrons pointing towards the fracture origin.
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|Overall View - 25X
Stack: 0.003 inch increments.
|Fracture Surface at 45X
Stack: 0.001 inch increments.