Complex sentence: But the material had not been tested extensively enough and showed up to be problematic in service with the Bundesmarine: microscopic cracks in the pressure hull forced the cancellation of 9 of the 12 ordered submarines and the early retirement of the three built boats.
Split into simpler sentences: But the material had been insufficiently tested and proved to be problematic in service with the Bundesmarine. Microscopic cracks in the pressure hull forced the cancellation of 9 of the 12 ordered submarines and the early retirement of the three built boats.