Ultrasonic Measurement of Stress in Gusset Plates
The objective of this research is to develop non destructive methods for measuring the stress in steel gusset plates in highway bridges.
A significant challenge in determining the adequacy of a gusset plate is determining the actual forces carried in the plate resulting from both dead loads and live loads. Commercially available sensors like strain gauges can measure the effects of live loads, but the dead load effects can only be estimated from design plans. As a result, there can be significant uncertainty in the estimated forces in the plate and the calculated compressive and tensile stresses required to assess the adequacy of the plate. Ultrasonic methods of stress measurement, in contrast to conventional strain gage measurements, evaluate the total stress in a metal as compared to the natural, unstressed state. As a result, these methods are able to assess the true stresses in a metal, including those resulting from dead load forces, live load forces, and/or residual stresses caused by thermal treatment or welding.
To evaluate the true stresses in a gusset plate, this research is utilizing the acoustoelastic effect which causes a variation in ultrasonic wave velocities due to the presence of stress in a metal. To assess the true stresses in a gusset plate, where both tensile and compressive stresses may exist in different portions of the same plate, acoustic birefringence measurement are being used. In an unstressed isotropic medium, an ultrasonic shear wave will propagate at a constant velocity (≈ 3200 m/s in steel) regardless of wave polarization. The presence of stress creates anisotropy in the material that makes the velocity dependent on the wave polarization direction. A shear wave propagating in the thru-thickness direction of a gusset plate, and polarized parallel to the applied stress axis will have a different velocity then a wave polarized orthogonally to the stress axis. The normalized difference in these velocities is known as the acoustic birefringence. The measured birefringence is proportional to the stress in the plate, and thus the magnitude and orientation of the applied stress can be deduced from the ultrasonic velocity measurements. This research is performing proof-of-concept laboratory testing to evaluate the potential for this technology to provide a unique tool for the condition assessment of in-service highway bridges and other structures that utilize gusset plate connections. Such a tool could be used to screen gusset plates to identify plates that may be at or near stress limits, reduce uncertainty in safety assessments, and ensure the safety of bridges and other structures.
Support for this research has been provided by Collins Engineers, Inc., Chicago, IL, USA