Project Code: 32898
Develop and investigate the capability, accuracy and limitations of non-destructive ultrasonic techniques for quantification and monitoring of residual stresses in metallic materials.
Develop a procedure for using the ultrasonic method to assess the residual stresses of materials and welds on site.
Generate tables with acoustoelastic constants and calibration blocks for the most common materials to be used in combination with the above procedure.
Use the method to provide validation of weld repair regions, in addition to providing further data for validating the stress relaxation clauses provided in current Structural Integrity Standards.
Stress measurements using ultrasonics rely on the acoustoelasticity theory, which relates the elastic solid stress-state to a macroscopic elastic wave velocity. Basically, the speed of ultrasonic waves travelling through a material are a function of the direction and magnitude of stresses present (in addition to temperature and microstructure/texture). Thus by measuring the time of flight or birefringence of waves through both an unstressed and stressed region of the same material, the magnitude of the residual stresses present in the material can be determined.
In a recent project, TWI’s existing equipment was used to investigate the capabilities of two ultrasonic methods to measure applied tensile stresses in C-Mn Steel. Electromagnetic acoustic transducers (EMAT) with two linearly polarized shear waves and a piezoelectric (PZT) OmniScan with one critically refracted longitudinal wave (LCR method) were used. The first method, EMAT, computed the stress from birefringence measurements and the second method computed the stress from time of flight measurements.
The results were good and demonstrated well how an applied stress in the plate influences the birefringence and time of flight. By performing regression analysis on the data, the acoustoelastic constant of the material, and then the residual stress, could be calculated. Results compared well with the theoretically expected residual stresses for the weld studied.
This project will extend the approach to a wider range of materials, and develop a non-destructive ultrasonic test method to provide both laboratory and on-site residual stress measurement capabilities.
Relevant Industry Sectors
Power Generation; Oil & Gas; Pipelines; Infrastructure; Construction; Transport
Technical and Economic Benefits
Direct measurement of residual stresses can reduce the use of worst-case assumptions in engineering critical assessments (ECA). This will improve the accuracy of ECAs, providing potential benefits in reduced construction and/or operating costs, and improved safety.
None as yet