A single edge notch specimen for fatigue, creep-fatigue and thermo-mechanical fatigue crack growth testing

The importance of capturing crack growth behavior under creep-fatigue and thermo-mechanical-fatigue is well known. Such service load environments are often simulated by performing tests under load and displacement control conditions where the specimen may be subjected to both tensile and compressive loads. A single edge notch specimen was developed to accommodate tension and compression loading in fatigue, creep-fatigue and thermo-mechanical fatigue crack growth experiments. The stress intensity factor and compliance solutions were developed for the single edge notch specimen using a 3D fracture code (FRANC3D) and 2D boundary element code (FADD2D). Both codes agreed well with each other. It is shown in this work that using simple approximations in a 2D code such as FADD2D, accurate stress-intensity and compliance solutions can be obtained for specimen geometry with varying cross sections. An importance of using correct boundary condition is emphasized by displaying the difference in stress intensity solutions for rectangular edge-cracked plate under uniform-stress, and uniform-displacement cases. Also, it is shown that stress intensity solution for a threaded edge-cracked specimen considered in this paper is different from the solution for rectangular edge-cracked plate under displacement control. As a validation of the developed stress-intensity and compliance equations, fatigue crack growth tests were conducted using the single edge notch, compact type and a surface crack specimen. Tests were conducted under constant amplitude load control of 0.1 stress ratio, R for IN738 nickel base super alloy material. Agreement was found between the different specimen types in the intermediate crack growth rate range.