Experimental investigation of high frequency pulse loading on fatigue crack growth in 5052-H32 series aluminum

Abstract

In the marine industry, designers utilize 5052-H32 marine grade series aluminum for its strength to weight ratio, and the ability of this series to resist marine corrosive environments. During the operating lifetime of a marine vessel, it experiences constant and variable amplitude loading conditions from the sea environment, plus overloads/underloads in the form of high frequency pulses. These high frequency pulses can influence the fatigue crack growth behavior of the aluminum structure. Foundational models like Wheeler and Willenborg have previously been used to predict effects of underloads/overloads on fatigue crack growth, but have proven to be inadequate for all high frequency pulse loading conditions. In this paper, we coupled observations of the crack growth path obtained using an optical microscope with detailed measurements of surface strain fields using Digital Image Correlation (DIC) to elucidate the effects resulting from these high frequency pulses. An experimental test matrix was developed for applying pulses of varying amplitude and frequency to gain fundamental understanding of the characteristics of the high frequency pulses on crack growth behavior. Experimental results indicated there is a novel crack kinking behavior which results in retardation of the fatigue crack growth. Using DIC, we identified changes in the plastic zone ahead of the crack tip that mimic a hard inclusion, affecting not only the crack path, but also the crack opening displacements and subsequent crack growth rates. This work provides a foundation for understanding effects of high frequency pulses on crack growth to enable future development of a model incorporating the effects of the plastic zone on the growth mechanisms.