Titanium activation foils for neutron studies were exposed inside the JET tokamak vessel during the entire 15 months long experimental campaign with deuterium fuelling. Afterwards, it was found that the structure of some foils was only slightly affected, while others were totally or partly disintegrated into dust: flakes and powder. The analyses indicated that defects produced in materials by cutting during the sample preparation eventually lead to the degradation of the titanium crystal structure upon the exposure in JET to neutral particles including hydrogen. In some cases, a passive oxide layer on the foil surface effectively prevented the access of hydrogen, and by this protected the metal against partial or complete transformation into the titanium hydride powder.