An apparatus for simulating stresses experienced by a rotor blade root in a complementary disc fixture, especially in gas turbine engines. The apparatus comprises a mounting member for receiving a blade speciman having a root. The mounting member has an aperture for receiving the root of the blade specimen, and blade contacting regions on either side of said aperture for holding said blade specimen in said aperture. The apparatus further comprises an actuator for applying a tension load to said blade specimen. The blade contacting regions are undercut such that there is a gap between said blade contacting regions and a main body of said mounting member, whereby the blade contacting regions are able to deflect outward into said gap when a tension load is applied to the blade specimen.