Evaluation of Total Stress in W-Flange Members Using Ultrasonic Shear Waves

This paper discusses the development of ultrasonic stress measurement (USM) technology as a potential tool for structural heath monitoring and evaluation of buildings and bridges. The proposed approach utilizes acoustic birefringence to assess the total stress in steel members. Acoustic birefringence describes the difference in velocity between orthogonally polarized shear waves as a result of anisotropy in the material. This anisotropy comes from texture produced during the manufacturing process and stresses (i.e., strains) in the plate under loading. The relationship between the birefringence and stresses may be a viable method to evaluate the total stress in a steel member, which includes stresses produced from all applied loads, including both dead loads and live loads. This paper investigates steel wide-flange (W-flange) rolled sections under tension and bending. The results indicate that birefringence shows a strong linear correlation with strain. The variation in the in situ birefringence was found to be significant and varied between different locations on the same cross section. If the acoustic constants (B0 and m1) that relate the birefringence to the strain are known, laboratory tests showed that stress can be approximated to within ±1.1 ksi (±7.6 MPa) (2% of the material yield strength, Fy). If average values of constants across all laboratory testing were used, the error increased to ±14.79 ksi (±102.0 MPa) (28% Fy). These results indicate that ultrasonic stress measurement based on acoustic birefringence measurements may be a viable method for the evaluation of total stress in steel members but needs additional research to identify and mitigate the sources of variation in critical acoustic constants that affect the accuracy of the approach.