Investigating Ultrasound-Induced Acoustic Softening of Aluminum 6061

Blaha and Langenecker are the first to document the phenomena known as “acoustic softening”: a significant reduction of static stress in tensile tests when applying longitudinal ultrasonic waves to various metals. Based on experimental observations, they hypothesized that acoustic heating due to internal friction and energy activation at dislocations was responsible for this temporary weakening of the material. Later research studies investigating the acoustic softening process used different experimental setups leading to alternative theories of the softening process such as superposition of static and dynamic stress. The variation in the design of the experiments leads to significant differences in observations, causing differing interpretations of the results and the formation of competing theories. We reviewed previous experimental studies and found that the optimum setup is similar to Blaha’s and Langenecker’s. Their setup minimizes noise factors such as friction at the oscillator specimen interface, which could contribute to heating and stress reduction. Therefore, we present our experimental setup composed of an MTS tensile test machine and a Branson ultrasonic welder to study the softening of aluminum 6061 and discuss our own results and how they compare with those in the literature. Additionally, we investigate the applicability of competing theories of softening based on our experimental data.