writes "NASA engineers have finally discovered the root cause of the cracks that have been found on the space shuttle Discovery's main external tank. The main tank, one of the "Super Lightweight Tank" models developed by Lockheed-Martin employs an aluminum-lithium alloy developed by Lockheed-Martin specifically for this application. The new alloy is used in various structural stringers throughout the SLWT design. Unfortunately, the batch of this alloy used in the tank that is currently mated with the Discovery shuttle, appears to be of low quality. The alloy used in the stringers has a "mottled" appearance, compared to the nominal appearance typically used in the main tank stringers (see picture in article). This appearance is indicative of a fracture threshold that is significantly lower than typical. NASA has determined, through testing, that this low grade alloy has only 65% of the fracture strength of the nominal alloy typically used.
NASA engineers have devised a potential fix to the problem that they are currently testing to ensure that the repair will cause no unintended consequences. NASA plans to have the Discovery shuttle ready to launch again by February 24th, 2011."Link to Original Source