The failure of a natural-gas pipeline that ruptured three weeks ago, devastating a San Bruno neighborhood, may have started along a weld or in a weakened section of the 54-year-old pipe, said experts who reviewed photographs of the twisted metal sections Thursday.

The National Transportation Safety Board allowed photographers to take pictures of the segments of pipe at its Virginia laboratory. The Mercury News asked three experts to analyze the photographs.

“This pipe basically unpeeled and failed catastrophically,” said Tom Bowman, chairman of the thermosciences division in Stanford University’s mechanical engineering department.

But the exact cause of the failure will have to wait for NTSB metallurgical analysis, the experts said. It’s possible the pipe may have become brittle with age; welds made years ago, before modern manufacturing techniques, could have been weakened by corrosion, and cracks might have grown over time under the pressure of gas running through the line.

The line is like thousands of miles of pipe across the U.S. that was laid half a century or more ago, noted Theofanis G. Theofanous, a chemical and mechanical engineering professor at ﻿UC Santa Barbara.

How the country deals with that aging infrastructure “is a profound question,” he said.

The largest, 27-foot section of the San Bruno pipe unpeeled like a banana, and the force of the explosion sent it flying 100 feet through the air. Eight people died, and 37 homes were destroyed in the Sept. 9 disaster.

The pipeline was buried close to the surface — 3 to 4 feet below ground — where a tremor or a small ground motion could have caused a crack, Theofanous noted. “More importantly, by being so close to the surface, the pipe is exposed to the elements from humidity and water from the rain. That can be an important factor in corrosion,” he said.

“Then that crack propagates” from the pressure inside the pipe, he said.

“It doesn’t fail all at once,” he said. “It begins to fail at one point, and then because of pressure, it creates a crack along the line of the most brittle part of the material — the weld.”

The wide-ranging NTSB investigation includes looking at welds on the pipe and work on a sewer line that crossed beneath the gas pipeline. That work used a technique that can cause ground shaking and soil disruption.

The photographs don’t reveal where the rupture initiated, said Richard Kuprewicz, a Redmond, Wash., pipeline safety expert.

“My first reaction, when I saw this segment, was I don’t see any major evidence of corrosion” in the pipe wall, he said, “but we can’t rule it out. We don’t know if it was the weld or if something else caused the pipe to fail and the weld failed because the force just unzipped it,” he said.

“Another way a weld could fail is it could have been overpressured,” he said.

The NTSB will look for signs of corrosion, fatigued metal or cracks that originated from an outside force, he said.

“Technically, steel pipe doesn’t age like your car, but yes, there are issues associated with the vintage, with when the pipe went in ground. Manufacturing techniques have improved; welding techniques have improved,” he said.

Kuprewicz said he’s seen similar pipeline failures, where the forces of compressed gas in the pipe burst it open, throwing a section out of the ground and leaving two jets of gas coming out of the remaining line at the speed of sound. “Not all of them ignite, but when they do, there’s a big explosion. The heat generated is so hot it can vaporize aluminum and liquefy steel,” he said.

“This is a rather small release compared to some of the other transmission pipeline rupture incidents in the U.S.,” Kuprewicz said.

Said Bowman, “We just have to do things better in the future. It can happen again.” The safety record of natural gas pipelines “is pretty good, but there’s room for improvement,” he said.

Contact Pete Carey at 408-920-5419.