Noninvasive Stress Detection Based on Piezoelectric Micromachined Ultrasonic Transducers for Bolt Loosening Warning

Axial stress monitoring of bolts is significant to various engineering structures and machines because it can assist their reliable and durable operation and prevent major failures and safety accidents. Among various sensing technologies, ultrasound holds prominent advantages in accuracy, noninvasiveness, and nondestructive testing. However, the current ultrasonic technique predominantly relies on conventional bulk-PZT ultrasonic transducers, which suffer large size, high-power consumption, and inferior integration with integrated circuits (ICs), hindering real-time and long-term monitoring and application in small bolts. This article highlights the prospect of piezoelectric micromachined ultrasonic transducers (PMUTs) in aforementioned application situations based on acoustic elastic effects and bolt length variations induced time differences. The corresponding theories for the relationship between the time difference and the stress are validated by finite-element methods. Specific PMUTs with inner circular and outer annular arrays and optimized ultrasound frequencies were proposed to improve sensing performance. As such, the axial stress variation in a steel bolt with a nominal diameter as small as 6 mm is successfully monitored using our fabricated PMUTs chip with a resonant frequency of 850 kHz. Excellent linearity (
$\ge 0.998$
) between the time-of-flight (ToF) difference and applied stress and a high accuracy of less than 3% are demonstrated. Further, the experimental testing of bolt stress under different practical conditions, such as bolt sizes, materials, and connection types validates the performance robustness and widespread applicability of our proposed PMUT-based ultrasonic stress sensing technique.