Lockheed Martin just issued a statement on the F-35B test programme:
“During a recent durability ground test, fatigue cracks were discovered in the aft bulkhead of BH-1, an F-35B ground test aircraft. The cracks were discovered during a special inspection when a test engineer discovered an anomaly; the aircraft has logged approximately 1500 hours of durability testing. Precautionary inspections were conducted on all flight test aircraft and the CTOL ground test aircraft. No additional cracks were found and flight testing has not been impacted. The root cause investigation is underway and will determine if the cracks were caused by a test anomaly that is not indicative of flight conditions, an engineering issue or a material failure.Durability testing is conducted early in the development of a new aircraft program to avoid costly sustainment issues later in the life of the aircraft.”
The F-35B is supported by six aluminum bulkheads forged by Alcoa Power and Propulsion. Lockheed has confirmed the fatigue cracks appeared on the 496 bulkhead, which is shown pictured below from an Alcoa marketing brochure. The F-35A and F-35C have a different structural system, sharing a very similar 472 bulkhead but not the F-35Bs 496 structure.