A double edge notch specimen design for tension–compression fatigue crack growth testing

Abstract

Compact, C(T), specimens are commonly used for fatigue and creep-fatigue crack growth testing under constant-load-amplitude conditions. The use of the standard C(T) specimens [ASTM] is limited to positive load ratios. They are also limited in the amount of crack growth data that can be developed at high stress intensity values due to accumulation of plastic and/or creep strains leading to ratcheting in the specimen. Testing under displacement control can potentially address these shortcomings of the load-controlled tests for which the C(T) geometry is unsuitable. A double edge notch tension–compression, DEN(T–C), specimen to perform displacement controlled creep-fatigue crack growth testing is developed and optimized with the help of finite element and boundary element analyses. The specimen has threaded cylindrical ends for applying the loads. The test section is rectangular with a half width, W, and half height, H, and a constant thickness, B. The height to width ratio, H/W, of the specimen must be kept low to avoid buckling during compressive loading while ensuring that the stress-intensity parameter values are not strongly dependent on the chosen H/W value. Simple optimization analysis shows that a H/W ratio of 1.2 is likely to best meet these requirements. However, only performing actual experiments will confirm whether this choice of H/W value is optimum.In the finite element analyses, the specimen is modeled using an effective height (heff) to width (W) ratio, heff/W, of 1.46 to account for the fillet regions on the two ends of the specimens. Accurate expressions for estimating the fracture mechanics crack tip parameters such as the stress intensity parameter, K, the crack mouth opening displacement (CMOD), and the load-line displacement (LLD) are developed over a wide range of crack sizes for the DEN(T–C) specimen. Expressions are also developed for crack size as a function of specimen compliance.