Assembly having first and second members distinguishable by respective first and second compositions each including one or more isotopes of a same element

A gas turbine engine comprising:

• a first member including a first material composition,
• a second member including a second material composition,
• the first and second material compositions each including one or more isotopes of the same element which are arranged to permit distinguishing between the first and second members or parts thereof.

A fan blade assembly comprising:

• a first member including a first material composition,
• a second member including a second material composition,
• the first and second material compositions each including one or more isotopes of the same element which are arranged to permit distinguishing between the first and second members or parts thereof, and the members are fan blades.

A method of testing a rotating fan blade assembly, the assembly including a first member and a second member, the first member including a first material composition, the second member including a second material composition, the first and second material compositions each including one or more isotopes of the same element which are arranged to permit distinguishing between the first and second members or parts thereof, the method including a step in which one of the members is separated from the assembly during rotation, the method including a subsequent step in which the assembly is inspected to identify one or more locations of the first member and of the second member.