J. T. Staley, Bodycote Materials Testing, Skokie, IL; T. Bartel, Elgiloy Specialty Metals, Elgin, IL

Summary:

Corrosion resistant, high temperature foil (as thin as 0.025mm) is used in many critical aerospace applications under demanding environmental and loading conditions. One essential property of this thin foil is its resistance to stress rupture or failure at high temperature under a constant load. Microstructure and surface finish are only a couple of material characteristics necessary to produce foil that will meet the rigorous user and Aerospace Materials Specification (AMS) stress rupture requirements. However, providing evidence of meeting these requirements by stress rupture testing of foil is difficult. Adherence to American Society for Testing and Materials (ASTM) specification E139 (a standard test method used for conducting stress rupture tests) can prove challenging. And although E139 provides useful guidelines for testing various product shapes, details for testing foil are not provided. Some of the challenges in testing foil include machining the specimens (including special edge preparation techniques), gripping the specimens in the furnace, attaching thermocouples to the specimen gage length and ensuring axiality during loading and testing.

This paper describes in detail the steps taken to ensure that valid stress rupture tests are conducted up to 1200°C per ASTM E139 on thin foil material. The qualification and use of abrasive waterjet cutting to produce precisely shaped specimens with superior edge finish is discussed. In addition, specialized grip designs that work inside the furnace are explained as well as an innovative way to attach thermocouples to a specimen without deflection or detachment. Design and fabrication of custom stress rupture frames is documented including the unique load train assembly that ensures axiality and proper preloading. All other necessary steps and equipment are revealed and the text is complimented with detailed photographs of various procedures followed to develop the test system.