Rotary Bend Fatigue of Nitinol to One Billion Cycles.

Nitinol implants, especially those used in cardiovascular applications, are typically expected to remain durable beyond 108 cycles, yet literature on ultra-high cycle fatigue of nitinol remains relatively scarce and its mechanisms not well understood. To investigate nitinol fatigue behavior in this domain, we conducted a multifaceted evaluation of nitinol wire subjected to rotary bend fatigue that included detailed material characterization and finite element analysis as well as post hoc analyses of the resulting fatigue life data. Below approximately 105 cycles, cyclic phase transformation, as predicted by computational simulations, was associated with fatigue failure. Between 105 and 108 cycles, fractures were relatively infrequent. Beyond 108 cycles, fatigue fractures were relatively common depending on the load level and other factors including the size of non-metallic inclusions present and the number of loading cycles. Given observations of both low cycle and ultra-high cycle fatigue fractures, a two-failure model may be more appropriate than the standard Coffin-Manson equation for characterizing nitinol fatigue life beyond 108 cycles. This work provides the first documented fatigue study of medical grade nitinol to 109 cycles, and the observations and insights described will be of value as design engineers seek to improve durability for future nitinol implants.

A practical model for sunlight disinfection of a subtropical maturation pond.

Maturation ponds are a type of waste stabilisation pond (WSP) designed to reduce carbon, nutrients and pathogens in the final stages of a WSP wastewater treatment system. In this study, a one-dimensional plug-flow pond model is proposed to predict temperature and E. coli concentration distributions and overall pond disinfection performance. The model accounts for the effects of vertical mixing and ultraviolet light-dependent die-off rate kinetics. Measurements of radiation, wind-speed, humidity and air temperature are recorded for model inputs and good agreement with measured vertical temperature distributions and outlet E. coli concentrations is found in an operational, subtropical maturation pond. Measurements and the model both show a diurnal pattern of stratification during daylight hours and natural convective mixing at night on days corresponding to low wind speeds, strong heat input from solar radiation and clear night skies. In the evenings, the thermal stratification is shown to collapse due to surface energy loss via longwave radiation which triggers top-down natural convective mixing. The disinfection model is found to be sensitive to the choice of die-off kinetics. The diurnal mixing pattern is found to play a vital role in the disinfection process by ensuring that pathogens are regularly transported to the near-surface layer where ultraviolet light penetration is effective. The model proposed in this paper offers clear advantages to pond designers by including geographical specific, time-varying boundary conditions and accounting for the important physical aspects of vertical mixing and sunlight inactivation processes, yet is computationally straightforward.