Study of Use of Ultrasonic Waves for the Detection of Residual Stresses
TWI Industrial Member Report 1188-2024 [pdf / 1,841KB]
By Kyriakos Flentzeris
Industrial Need
Residual stresses (RSs) are critical to the strength, fatigue life and corrosion resistance of structural components. They are often inherent to a fabrication process i.e. welding, forging and heat treatments and occur due to differences in thermal expansion and contraction of neighbouring material.
Understanding the stress distribution within a manufactured component, due to residual stress, as well as their magnitude, is of utmost importance to ensure the delivery of safe and reliable structures. Currently, several residual stress measurements exist, including those classified as destructive (i.e. contour mapping, slitting and boring), semi-destructive (i.e. hole-drilling) and non-destructive (i.e. neutron, synchrotron and X-ray diffraction). These techniques are well validated with some providing high-resolution residual stress distributions. They are, however, often costly and time-consuming to perform with most requiring a laboratory with dedicated facilities. This means that there is a requirement for a portable method for characterising the residual stresses within component that can be deployed non-destructively.
This project sought to investigate the application of ultrasonic critically refracted longitudinal waves (LCR) for the measurement of residual stresses within a component. Using ultrasonic techniques, such as those investigated here, would be advantageous for implementation on operational plants because it offers a portable and non-destructive means of testing.
Key Findings
- It was concluded, during this work, that there is a correlation between the stress applied to a material and the time of flight (ToF) measured for longitudinal surface wave required to travel from the transmitter to the receiving transducer. As the stress or load increases, the ToF increases.
- The LCR technique could easily distinguish between the weld and heat affected zones (HAZ) within the samples and capture the difference in ToF values between the weld metal and base metal.
- The effectiveness of the LCR technique is likely to be affected by the peak residual stress with high gradient. This is because the ultrasonic wave calculates stress in a length of material and the results are averaged across the scanned length.
- The LCR technique may be useful in monitoring the stress distribution in welded samples rather than obtaining precise stress values. Laboratory trials proved that it is not possible to identify areas of high stress without destruction of the component under test.
- A new experimental setup was developed for scanning larger areas without the need for mechanical movement of the transducer. This setup was tested in comparison to the conventional method and demonstrated equivalent performance.
Impact
The main objective of this project was to develop process parameters and manufacture a defect-free and transparent diffusion bonded micro-channelled thermoplastic demonstrator.
- The results on leak testing and optical microscopy analysis on the bonded demonstrators suggest that the micron/sub-micron scale channels can be produced through laser machining and can be diffusion bonded without any defects or deformation.
- With the recent advancement in technology and industries looking for clean and sustainable processes, diffusion bonding with laser channelling can prove to be one of the potential candidates for manufacturing the next generation of microfluidic devices.