A gas turbine engine, comprising: a component having a gas path wall with a gas path facing surface and an opposing air cooled surface, wherein the air cooled surface receives a supply of cooling air; a chamber which holds the cooling air for delivery to the air cooled surface; a test sample mounted within the chamber so as to be located in the supply of cooling air which is delivered to the air cooled surface, wherein the test sample is constructed of the same material as the gas path wall of the component.