WASHINGTON -- Durability testing on the Marine Corps' short-takeoff version of Lockheed Martin's F-35 was halted last month after "multiple" cracks were discovered in the fighter jet, according to a Pentagon's testing office report sent to Congress on Friday.The previously undisclosed halt in high-stress ground testing involves the F-35B, the joint strike fighter's most complicated version, which must withstand short takeoffs and landings on carriers and amphibious warfare vessels, said the report by Defense Department testing chief Michael Gilmore.Flight testing wasn't affected.Development of the F-35, the Pentagon's costliest weapon system, has been marked by delays and cost increases. The Pentagon estimates the total price for development and production of 2,443 F-35s at $395.7 billion, a 70 percent increase since the initial contract with Lockheed was signed in 2001.Durability testing is intended to stress an airframe, assessing its bility to achieve a projected aircraft lifetime of 8,000 "equivalent flight hours."Testing on the Marine version progressed last year until the December halt "after multiple new cracks were found in a bulkhead flange" on the fuselage underside during an inspection after the equivalent of 7,000 hours of testing, the report to Congress said. The cracks were confined to that area.Testing of the F-35B model had been restarted in January 2012 after a 16-month delay caused by the discovery and repair of a crack in the plane's bulkhead.Analysis of the new cracks will continue so as to find the root cause, plan corrective action and determine whether the cracks had been predicted in modeling, the report said."We have implemented the fixes," said Lockheed spokesman Michael Rein, who went to say that static testing could resume as early as late next week."This had no impact on flight testing and this is normal engineering development and test work," Rein added. "This is why we do structural testing in the first place."But Gilmore said in his report that the test results "highlight the risks and costs" of the Pentagon's F-35 concurrent-development strategy, which produces aircraft while they're still in development.The aircraft have two more years of structural testing that may result in more "discoveries," he added.