How deep is a deep hole?
8 April 2009
Deep hole drilling is a means of measuring residual stress in complex engineering components.
Residual stresses are introduced into any component that is manufactured. Take a pipe that’s made by welding two sections together. As the hot weld metal cools, it pulls on the two sections of pipe until the weld metal in the centre is under tension. The stress created is what is known as a residual stress, and it can be both detrimental to the life of a product and, perhaps rather surprisingly, beneficial. The rivet holes in an aeroplane wing, for example, are sites of high stress that could cause the wing to fail. In order to avoid this, compressive residual stresses are added to the wing to prevent cracks growing. There are many such industries, from nuclear power plants to the manufacture of submarines, where knowing just how large the residual stresses are in a component is crucial to its safety; but until recently they have been extremely difficult to measure and consequently are poorly understood.
In 1998, Kingston worked for the University for two months over the summer, while trying to find a job after finishing his degree. He was supervised by David Smith, Professor of Engineering Materials in the Department of Mechanical Engineering, “And it wasn’t long before David asked me to do a PhD,” Kingston says. “But I said no! I wanted to work and earn some real money. However, during the summer I really enjoyed it. I particularly enjoyed not being a student anymore, so I carried on and eventually I agreed to do a PhD.” Four years later, so much of Kingston’s research had been commercial work that he wasn’t able to include some of the data in his PhD because they were too confidential. Without realising it, he had started a business.
In 2003, Kingston and Smith put together a business plan and came second in the University’s Enterprise Competition, winning £6,000 in cash, plus £1,000-worth of legal advice from Osborne Clarke solicitors. It was enough to get them started and the following year they formed VEQTER. They haven’t looked back since.
Today VEQTER is the only company in the world carrying out deep hole drilling commercially. The technique involves drilling a small hole, about three millimetres in diameter and up to 750 millimetres in depth, through the component under investigation. They then measure the diameter of the hole using an air gauge which is accurate to within two thousandths of a millimetre. These measurements, taken in many locations, provide the shape of the hole in its stressed state. They then cut around the outside of the hole, taking out a tube of material. In doing so, the residual stresses are released and the hole changes shape as it relaxes.
By measuring the hole again, at exactly the same locations as before, they can compare the stressed shape of the hole to its unstressed shape and calculate what the residual stresses are through the entire thickness of the component. It sounds relatively simple, but the clever bit is in the development of their advanced analysis tools. These, coupled with the company’s extensive expertise in understanding residual stresses, means VEQTER can advise on the management and mitigation of residual stresses in circumstances where they are expected to be detrimental to performance. In doing so, they help to improve the safe working conditions of highly complex industrial components and also reduce the frequency and duration of downtime that can cost some clients more than £1 million a day.