During the manufacturing process of jet engines, parts are joined using inertia friction welding. This process introduces residual stress into the component, which strongly impacts how long they can safely operate under the high temperature and high stress conditions of an engine.
In this experiment, the focus was on a particular ring-shaped component, approximately 500 mm in diameter. When welding, one of these rings is fixed and pushed against another, rotating, ring. This use scenario leads to extreme localised heating, stresses and distortion. The stresses and strains in the ring vary around its circumference, and it is important for the company to understand these variations.
Predicting how long a part will last before failing is done using a combination of monitoring measurements and finite element modelling. This enables them to make a judgement on when a component needs to be replaced, which is often very conservative. By understanding where on a component the measurement should be taken to define its residual stress state, they can potentially extend its lifetime, reducing maintenance periods and overall environmental impact.
They need neutrons for these experiments because the composition of material and the depth they want to study makes it impossible to see the stresses deep in the component using X-rays. They continue to come to ISIS because of a few factors:
“The Engin-X beamline scientists have a good relationship with industry and make it easy to do experiments. The experience of working with the beamline scientists is why we keep coming back to ISIS for various projects over the last couple of decades," says Andrew Barrow, Senior Specialist – Materials, Rolls-Royce. “The industrial access scheme is good for engaging with industry and makes it very easy to work with the facility to do the measurements."
They are continuing to use both X-ray and neutron techniques, and it is by combining these with hole-drill measurements that they can build a full picture of the stresses inside the component. They are in the middle of a project that aims to standardise the way these measurements are taken, which is in partnership with the ESRF and ILL, as well as other academic and industrial partners. They are also working with John Bouchard from Stress Space with the aim to carry out X-ray and neutron experiments simultaneously.
During this experiment, they measured the welded parts alongside reference samples and will use these to calculate the stress in the assembled components. “Our results will enable us to validate our models so they can predict what happens to the materials during the welding process," says Andrew. This allows them to expedite the development of the manufacturing process without having to do destructive measurements on each part, or save measurements until the end, which comes with more risk. “We are not as reliant on hole drill, surface level measurements," he adds; “now we have a better understanding of the stresses deeper in the sample."
The next stage is to study next-generation nickel-based alloys for the engines of the future. So far, it's not been possible to measure the residual stresses of a whole component after it has been assembled, but a current project has seen PhD students create a jig that means they can measure it in the near future.
