ABSTRACT

Accounting for all crack growth mechanisms is critical in designing tests to predict the performance of materials in critical applications and establish appropriate inspection intervals or schedule-based maintenance. The objective of this work was to quantify the effects of testing frequency on the fatigue crack growth rates (FCGRs) of AISI 4340 in the 180 ksi (1241 MPa) condition to determine an appropriate knockdown factor for lifing and integrity of a critical offshore component. A frequency scan at a relatively high cyclic crack driving force (ΔK = 36 ksi√in. (39.6 MPa√m) and R = 0.2) was performed on the material in fully aerated fresh water test solution. With decreasing testing frequency, the crack growth rates accelerated, and at the lowest test frequency of 0.0001 Hz, the crack growth rate was approximately 1000 times greater compared to the highest test frequency (1 Hz) in fully aerated fresh water. Upon examination of the fracture surfaces, the fracture morphology transitioned from transgranular to intergranular with decreasing frequency, and stress corrosion cracking was identified to be the prominent crack growth mechanism during lower frequency loading segments. Further testing was designed and executed to quantify these effects on a FCGR test over a cyclic crack driving force ranging from 15 to 80 ksi√in. (16.5 to 87.9 MPa√m) and to determine the static cracking arrest threshold (KISCC). Additional frequency scan testing was also executed such that the maximum crack driving force (Kmax) was below the KISCC of the material. At the lowest testing frequency, the crack growth rates were only accelerated 10 times as compared to 1000 times with Kmax greater than KISCC.

INTRODUCTION

Crack growth rate properties are used to assess life and integrity to determine appropriate operating conditions and service life; permissible flaw sizes; and defining minimum detectable flaw size requirements for non-destructive evaluations (NDE). In corrosion-fatigue environments, it is important to characterize the corrosion-fatigue crack growth behavior at a frequency that allows maximum interaction between the material and the environment to ensure that test conditions are suitably representative of service conditions and appropriately conservative.

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