Tapered Test Specimen for Rapid Damage Precursor Identification

Traditionally, most mechanical testing is conducted on specimens of uniform cross-section and stress magnitude, or only a single position along the specimen length is of interest. Investigations of various stress levels therefore requires separate tests with a unique specimen at each stress level of interest. The impetus of the current work was to develop a method for the design and monitoring of a specimen that simultaneously experiences a continuum of stress magnitudes across various positions. A linearly-tapered specimen was developed and subjected to sinusoidal tension-tension fatigue until specimen failure, with the expectation that a record of damage exists along the length of the specimen due to the varying level of induced stress. Baseline and post-failure scans of x-ray diffraction, electrical resistance via four point probe, nano-indentation, eddy current, and geometric changes were compared. Attempts were made to characterize the pre- and post-test property behaviors as a function of the applied stress, which varied linearly along the specimen, and as a function of fatigue cycles, which were the same along the length of the specimen. The mechanisms of specimen damage due to fatigue cycling were investigated and analyzed to improve durability and damage tolerance understanding.