Elaine Thomas, 67, of Auburn, Washington, pleaded guilty on Monday to falsifying test results that measured the strength and toughness of steel used in navy subs, the Justice Department said.

Thomas, former director of metallurgy at Bradken Inc., a steel foundry in Washington state, admitted to carrying out the fraud for more than 30 years, assigning passing grades to steel that had failed tests.

Bradken is the leading supplier of cast high-yield steel used by prime contractors such as General Dynamics and Huntington Ingalls Industries to build submarine hulls.

The steel castings must meet "rigorous" navy specifications for strength and toughness and the Justice Department said about half of the Bradken castings had failed laboratory tests.

"(Thomas) falsified test results to hide the fact that the steel had failed the tests," the department said in a statement.

"Thomas falsified results for over 240 productions of steel, which represent a substantial percentage of the castings Bradken produced for the Navy," it said.

The fraud was discovered in May 2017 by a lab employee who found that test cards had been altered by Thomas and alerted Bradken management.

Thomas worked at the Tacoma foundry from 1977 to May 2017 and was named director of metallurgy in 2009. She pleaded guilty to falsifying tests from about 1985 until her retirement in 2017.

"Thomas's false statements and misrepresentations caused the prime contractor to install substandard components on naval submarines, and caused the Navy to accept those submarines and place them into service, thereby potentially placing naval personnel and naval operations at risk," the government complaint against Thomas said.

The US Navy had no immediate comment on Thomas's conviction but the Justice Department said the navy "has taken extensive steps to ensure the safe operation of the affected submarines."

"Those measures will result in increased costs and maintenance as the substandard parts are monitored," it said.

The Pentagon took delivery of dozens of submarines between 1985 and 2017, about 40 of which are still in service.

According to the complaint, Thomas told investigators that she had used her "engineering judgment" while changing the results of certain tests.

Thomas criticized a particular test that was conducted at -100 degrees Fahrenheit (-73 degrees Celsius) on the grounds that it was a "stupid requirement" and a "stupid number" to test because nothing operated at -100F in the water.

Thomas will be sentenced on February 14, 2022 by US District Court Judge Benjamin Settle. She faces up to 10 years in prison and a $1 million fine.