Impact of contact force on the ultrasonic signal amplitude in carbon steel: A systematic evaluation

Ultrasonic testing is widely used in industrial non-destructive evaluation; however, the influence of transducer contact force on signal stability during conventional contact measurements is still insufficiently documented for metallic specimens. This study investigates the effect of applied contact force on ultrasonic wave velocity and signal amplitude in SAE 1020 carbon steel, using a 5 MHz contact transducer and a controlled loading device. Three cylindrical specimens were tested under increasing, decreasing, and cyclic force protocols within a range of 2 to 20 kgf. The results showed negligible variation in ultrasonic velocity throughout the investigated force range, whereas signal amplitude increased markedly at low forces and reached a plateau near 10 kgf. These findings indicate that contact force primarily affects coupling efficiency and signal amplitude, rather than bulk wave velocity, under the present experimental conditions. The proposed procedure contributes to improved repeatability and supports more reliable contact-based ultrasonic measurements in industrial practice.