WASHINGTON — On-the-ground stress testing for the Marine Corps version of Lockheed Martin Corp.’s F-35 fighter, which is assembled in Fort Worth, may be halted for as long as a year after cracks were found in the bulkheads, Pentagon officials said.Durability testing was stopped in late September after inspections turned up cracks in three of six bulkheads on a plane used for ground testing, said Joe DellaVedova, spokesman for the F-35 program.The previously undisclosed suspension of the stress testing may increase scrutiny of the Marine Corps’ F-35B, the most complex of the three versions of the plane, during congressional hearings on the Defense Department’s fiscal 2015 budget. The department plans to request funds for 34 F-35s, eight fewer than the 42 originally planned, according to officials. Six of those planes would be for the Marines.“We consider this significant but by no means catastrophic,” Frank Kendall, the Pentagon’s undersecretary for acquisition, said in an emailed statement. While the program office is still performing an assessment, “based on preliminary analysis, a redesign” of some F-35B structures will be required, said Kendall, who has a master’s degree in aerospace engineering.Michael Gilmore, chief of the Defense Department’s weapons testing office, said in his annual report last month that during ground testing in late September “the cracks continued to grow” until a “bulkhead severed and transferred loads, which caused cracking in the adjacent” bulkhead.Redesigning the bulkheads could cause the Marines’ F-35 to regain some of the weight saved by using aluminum bulkheads instead of the titanium ones in the Air Force and Navy models. That move was part of an effort in 2004 and 2005 to lighten the increasingly heavy Marine Corps version.The test office said in its annual report that the Marine Corps model gained 37 pounds last year. Changes to the bulkhead risk adding more weight to a plane that’s now within 202 pounds of the 32,577-pound maximum specified in the contract for it.“Managing weight growth with such small margins will continue to be a significant program challenge,” Gilmore wrote in his report.‘Not surprising’The cracking “is significant enough to warrant changes to the design” of the bulkhead, Jennifer Elzea, spokeswoman for the Pentagon test office, said in an email. “This is a new defect that must now be addressed through a production change and a retrofit plan.”“The crack was not predicted to occur by prior analyses or modeling,” she said. “We can’t know all the changes that must be made to the structures until the testing is complete, and it is not surprising when discoveries like this occur.”The purpose of “durability testing is to intentionally stress the aircraft to its structural limits so we can identify any issues and corrective actions needed to fix them,” the Pentagon’s DellaVedova said in an emailed statement. “These discoveries are expected and planned for in a developmental program.”The F-35 program office and Lockheed are making repairs with a goal of restarting testing by Sept. 30, DellaVedova said. The Bethesda, Md., company concurs with his statement, spokeswoman Laura Siebert said.Redesigning the bulkhead to make it more durable “would take some time,” George A. Lesieutre, a professor of aerospace engineering at Penn State University, said in an e mailed statement.Ground testing stresses an airframe to simulate flight conditions and determine whether a plane can reach its projected lifetime, which in the case of the Marines’ F-35B is 8,000 flying hours.To provide an extra margin of assurance, the Marine, Air Force and Navy versions of the F-35 are all required to undergo tests for the equivalent of 16,000 flight hours. The Marine version was supposed to complete its second 8,000 hours of testing by the end of this year.The ground testing aircraft had accumulated 9,480 hours “when testing was stopped to conduct root-cause analysis on discovered bulkhead cracks,” DellaVedova said.“Because of the high hours accumulated,” this “discovery does not affect current F-35B flying operations,” he said, adding that the suspension of ground testing won’t affect the Marine Corps’ goal of declaring its first squadron operational no later than December 2015.‘Nowhere else to go’Richard Aboulafia, a defense aerospace analyst with the Fairfax, Va.-based Teal Group, said the testing halt is a “setback for the program, but this is the Marines’ version, and they have absolutely nowhere else to go” because they need the aircraft in order to operate from amphibious vessels.The Pentagon projects that an eventual fleet of 2,443 F-35s will cost $391.2 billion, a 68 percent increase from a 2001 estimate for 409 more planes, measured in constant dollars. The testing office has repeatedly questioned the plane’s progress, finding last month that it wasn’t reliable enough in training flights last year.The Marine Corps plans to buy 340 of the F-35B, which can take off like a conventional fighter and land like a helicopter. While the ground testing is suspended, pilots can continue development and training flights on the 38 fighters already delivered, according to DellaVedova.The Defense Department plans to request funds for nine Marine Corps planes in fiscal 2016 and 20 in 2019, according to internal budget figures. The short takeoff and vertical landing model is also being bought by the British and Italian militaries.