Test specimen is prepared, its geometrical parameters are recorded and it is upset to reduce its thickness. Geometrical parameters are against recorded to define friction factor therefrom. Prior to testing, specimen is shaped to square in plan, square sides and thickness are recorded. After reduction by 15-60%, radius of curvature of the specimen is recorded in the plane orthogonal to depth. Parameter a/R is defined where a is square side, R is radius of curvature in the plane orthogonal to specimen depth, to define friction factor with due allowance there for.