Understanding the development and distribution of residuals stresses (RS) during manufacturing is a key factor in the development of methods to minimise distortions in components during machining.
Residual stress monitoring is also crucial for the overall control and maintenance of high performance components operating in extreme environments due to its influence on lifecycle. Therefore understanding the evolution and interaction of residual stresses during the whole manufacturing chain is of utmost importance; this is why one of the cores areas at the AMRC is the modelling and monitoring of bulk and near surface residual stress. The Residual Stress Measurement Group at the AMRC has developed competence in residual stress measurements (RS) using different technologies, including X-ray Diffraction, Ultrasonic, Contour Technique for RS measurements, as we as, Digital Image Correlation (DIC) Technique for strain measurements.
The Residual Stress Measurement Group has the aim to provide measurement capability on range of materials encountered at AMRC as a ‘wholesale’ RS service, to support ongoing machining projects on-site, and to validate process models in-house.
The group also has taken part in ‘Round Robin’ tests for X-Ray Diffraction (XRD) for RS measurements, involving research centres in Singapore, Germany and UK, designed to demonstrate the robust and stable nature of each centre’s measuring capabilities. The group also has collaboration with Research Centres for residual stress measurements using Neutron diffraction.
The Residual Stress Measurement Group at the AMRC has interest in investigating the factors that influence the generation and distribution of residual stresses in manufacturing processes, including i) development of an understanding of the factors that influence the generation and distribution of RS during heat treatment; ii) effect of process variability on RS; iii) machining-induced stresses; iv) influence of RS on machining distortions; v) interaction of near and bulk RS; vi) validation of process models in-house for RS predictions.
Portable XRD System
X-ray diffraction can be used to measure residual stress using the distance between crystallographic planes, i.e., d-spacing, as a strain gage. When the material is in tension, the d-spacing increases and, when under compression the d-spacing decreases. Stresses can be determined from the measured d-spacings. X-rays diffract from crystalline materials at known angles 2θ according to Bragg’s Law.
Features of X-ray diffraction testing:
- Determination of surface and subsurface residual stresses in metal alloy components.
- Measurements can be made in a wide variety of alloys, including carbon steels, ferritic and austenitic stainless steels, nickel base alloys, aluminium, and titanium based alloys.
- Quantification of residual stresses due to processes, such as turning, grinding, milling, welding, shot peening.
Ultrasonic Stress Measurement System
The portable, semi-automatic device for Ultrasonic Measurements of Applied and Residual Stress is designed for measurement of bulk and surface residual and applied stresses in samples, parts, welded elements and structures non-destructively.
Ultrasonic measurement of mechanical stresses is based on the acoustic-elastic effect. According to this phenomenon, the velocity of propagation of ultrasonic waves in solids is dependent on mechanical stresses. When material properties are known, the stress measurement could be done by determination of the velocities of propagation of longitudinal and shear polarized (in orthogonal direction) ultrasonic waves.
Features of ultrasonic testing:
- Magnitude and sign of uni- and biaxial applied and residual stresses in samples and real structural elements.
- Uniaxial stresses and forces in pins and bolts.
- Parameters of the acoustic-elastic characteristics of materials.
- Residual stress change as a result of post-welding treatment and service loading.
- The thickness of parts and structural elements.
- Young Modulus and Poisson Ratio
Contour measurements are used to measure bulk residual stresses. Contour measurement is performed as following:
1) EDM machine; 2) CMM machine and 3) FE Modelling
Once the average has been calculated to remove the errors caused by shear stresses or sensitivity of the probe head measured using CMM, this data is modelled in a FE model. A force is then applied on the model to “push” back the model back to its original form. The stress is then calculated and this gives the stress profile of the component before the WEDM cut.
Digital Correlation Technique
Digital Image Correlation (DIC) is an optical method that employs tracking & image registration techniques for accurate measurements of changes in images. This technique is growing in popularity as non-contact method due to its relative ease of implementation and use. The DIC can be implemented in heat treatment, machining, shot peening processes in order to monitor the strain/displacement field and infer level of induced residual stress left in the process. The combination of this optical method and the numerical technique could have the potential use to assess the stresses.
Technical Lead: Dr. Sabino Ayvar-Soberanis
Team members: Ignacio Blanco, Ravi Bilkhu, Krunal Rana, Dr. Vaibhav Phadnis, Dr. Shaoming Yao.