Structural Health Monitoring (SHM) of fatigue crack growth in welded joints

• Main Author: Tan, Yong Quan.
• Other Authors: Pang Hock Lye, John
• Format: Final Year Project
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Civil engineering::Structures and design
• Online Access: http://hdl.handle.net/10356/54099
• Institution: Nanyang Technological University

In day-to-day operations, any structures such as engine component, bridge or aircraft will be subjected to fatigue and will require periodic assessment on its structural health integrity. Latest industry statistics placed 60% of damages encountered to be due to signs of fatigue cracks caused by continual fluctuating stresses on localized but progressive damage on structures. The problem is even more severe if a welded joint is subjected to such fatigue behavior where damage often initiates at the weld toes of joint connections. This research aims to employ a comparative work of techniques comprising of Electromechancial Impedance (EMI) based on frequency response, Strain gauge measurement and Lamb Wave propagation based on pulse-echo and pitch-catch, to study and monitor the welded joint behavior during cyclic loading for fatigue cracks. Numerical simulations using finite element analysis also served to provide valuable insights of the occurrence of possible crack locations and resonance frequency mode shape behaviors. Experiments were conducted on two steel welded cruciform joint specimens with piezoceramic transducers surface bonded at several locations near the weld in determining crack initiation, location and propagation. In both experiments, EMI was found to be efficient in identifying the initial crack occurrence and were used as indicators in estimating the structural health integrity of the welded joint. For lamb wave propagation, compared to pitch catch, pulse echo responded more responsively during crack propagation where voltage drop was more significantly in the later cyclic stages with the possibility of lifespan being predicted. Strain gauge measurement served the purpose of determining the exact location of crack as any permanent damage in structural stiffness will cause a shift in strain reading. However, Strain gauges were only feasible if the specimen had passed its elastic region for result to be appreciable In addition, the trends of statistical root mean square deviation (RMSD) index ...... (RESTRICTED)