Results of a Pilot Study in Support of a Round-Robin on Creep-Fatigue Crack Growth Testing of a Creep-Brittle Material for ASTM E2760

Abstract

This paper describes pilot test results from a round-robin conducted in support of the standard ASTM E2760, Standard Test Method for Creep-Fatigue Crack Growth Testing, on creep-fatigue crack growth testing. Results from the pilot study are recommended to participants to develop their individual experimental testing. This round-robin is an international effort with 10 participants from five countries from Asia, North America, and Europe. All participants are testing compact type (C(T)) specimens according to the procedure described in the standard. A few participants are evaluating the Kb-bar (surface crack) specimen configuration. The test material is Inconel-718 (IN718) nickel-based alloy, which is a creep-brittle alloy. Pilot creep-fatigue crack growth tests were conducted at 600°C using C(T) specimens under constant load amplitude conditions. The tests were conducted at hold times of 0, 60, and 600 s. Creep-fatigue crack growth rates were analyzed using the methods proposed and included in the test standard. It was found that the procedures followed throughout the pilot test program were sufficient in generating the desired outcomes. As a result, it is proposed that the various participants follow the procedures and test matrix as close as possible and document any deviations. This will allow data from the various participants to be assessed and determine the levels of precision and bias in creep-fatigue crack growth data obtained using the ASTM standard.

Time‐ and cycle‐dependent growth of surface cracks in alloy 718

“…Therefore, the crack growth rate, da/dt, can be predicted by the K level and in creep-fatigue, by the K level during the tensile dwells at the peak load. 19,20 However, under variable amplitude loading, the overloads enhance the crack-tip viscoplastic strain which in turn upon unloading reduces the crack tip stresses. 21 Another important characteristic of fatigue crack growth in IN718 is the acceleration in fatigue crack growth immediately after a tensile dwell.…”

“…IN718 behaves as a creep‐brittle material during sustained loading crack growth with viscoplastic deformation confined to the region ahead of the crack tip. Therefore, the crack growth rate, da/dt, can be predicted by the K level and in creep‐fatigue, by the K level during the tensile dwells at the peak load 19,20 . However, under variable amplitude loading, the overloads enhance the crack‐tip viscoplastic strain which in turn upon unloading reduces the crack tip stresses 21 …”