Neutron scanner tests for welding stress

Engineering giant TWI's new technique for pipeline joints is put through its paces. James Mitchell Crow reports.

Pipes need testing as localised heating from welding can introduce internal strain in the material as it expands and then contracts again. – Monty Rakusen/Getty Images

When the oil and gas industry builds a new pipeline wide enough that you could crawl through it, it needs to know it won’t fail. So when UK engineering research group TWI developed an efficient new welding method for joining sections of pipe, it shipped a sample half way around the world to ANSTO’s Bragg Institute in Sydney for safety testing.

ANSTO is home to the newest and one of the most powerful “strain scanner” instruments in the world. Known as KOWARI, it uses a beam of neutrons to non-destructively peer inside a material and measure internal stress.

Localised heating from welding can introduce internal strain in the material as it expands and then contracts again. “These stresses are frozen in the material and can cause a failure,” says Anna Paradowska, a scientist at the Bragg Institute. Sometimes even small amounts of additional pressure can be enough to fracture the pipe. “These high internal stresses can cause a fracture just because it is a very cold morning.”

To assess whether the new technique would unduly stress the pipe, TWI analysed a sample using KOWARI. The instrument maps the variation in the distance between a material’s constituent atoms – a key measure of internal stress. But measuring residual stress in such a large pipe presents challenges. Analysing a 25 kg component on KOWARI would normally be considered large, says Paradowska. The section of pipe was a “600 kilo beast”.

The instrument’s flexible design was the reason TWI brought their pipe to ANSTO for testing. All of KOWARI’s key parts are movable, which helps when scanning such an unwieldy object. Before the pipe arrived, the ANSTO team had carefully planned the procedure on their “virtual instrument”, a 3D computer model of the scanner and pipe. Any accidental collision of the pipe with the scanner’s optics will knock the carefully calibrated instrument out of alignment.

The scans showed residual stress around the weld was low.

Paradowska recently became the industrial liaison manager for all instruments at the Bragg Institute. But assessing welds was the subject of her PhD research and remains close her heart. “Welding is important to me, it is a part of any engineering project these days,” she says. “By optimising welding we are making structures like cars, trains, planes, pipelines and bridges more economical, safer and more reliable.”

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