Detecting and monitoring of stress on beams using lamb waves

Abstract

A stress magnitude higher than the ultimate stress is harmful to the structural reliability of many critical components in the aerospace, civil and mechanical (ACM) industries. However, very often the presence of excessive stress (even if it is nominal) tends to be undetectable as they may not initially crack the ACM structures. The use of smart material based structural health monitoring (SHM) is of great significance to estimate even the minimal rise of stress levels, and to improve the reliability of the structure. The non-destructive testing methods like the visual inspection, magnetic particle and ultrasonic are effective only for crack detection but not stress detection. The persistence of such stresses can lead to structural failure at later stages unless routine maintenance or inspections are being carried out. This paper thus investigates a lamb wave propagation based SHM method using peizoelectric transducers (PZTs) to investigate the minimal increments of the stress on a beam. Experiments were conducted with the introduction of stresses at mid points by attaching weights on the beam. Lamb waves are then generated on the specimen using one of the attached PZT transducers as an actuator, and the resulting waves are received on the other end by the another PZT, which acts as a sensor. These PZT transducers were reusable with excellent repeatability. This paper explores the suitability and sensitivity of lamb wave based wave propagation method for transverse load monitoring with nominal stress increments.