0013-794418310701!1-10$03.w io Per gamon Pr ess L.t d zyxwvutsrqpon CORROSION FATIGUE CRACK PROPAGATION TESTING WITH THE KRAK-GAGE@ IN SALT WATER P. K. LIAW, H. R. HARTMANN+and E. J. HELM Materials Engineering Department, Westinghouse R and D Center, Pittsburgh, PA 15235, U.S.A. Abstract-The application of a relatively new, thin-film bondable transducer, commercially available under the name KRAK-GAGE@t, was evaluated for corrosion fatigue crack propagation tests in a sea water (3S%NaCI) environment at ambient temperature on an HY 80 steel. Fatigue crack growth data generated by this method are shown to be consistent with those obtained by the compliance method and the commonly used optical/visual measurements. Thus, this new instrumentation system appears to be a valuable addition for corrosion fatigue crack growth testing. Furthermore, test data acquisition and analysis of the KRAK- GAGE and the compliance method were computer automated, which results in considerable cost savings for such customarily labor intensive fatigue crack propagation experiments. Additionally, the FRAC- TOMAT/KRAK-GAGE instrumentation system can be utilized for direct test machine control, permitting completely automated fatigue crack growth testing. INTRODUCTION THE SCIENCE of Fracture Mechanics has been successfully used in the life prediction of structural materials and elements. The application of Fracture Mechanics to contemporary engineering design based on the “defect-tolerant” approach is becoming widely accepted. Under certain loading and environmental conditions, a flaw present in a machine component will propagate and eventually cause ultimate failure. The measurement of the length of the flaw or crack has been an important subject in Fracture Mechanics testing. Many methods, such as visual-optical observations, a compliance technique, the direct electrical potential method, acoustic emission or ultrasonic methods have been proposed to measure the length of a propagating crack. However, Fracture Mechanics scientists are constantly searching for more effective, accurate and improved ways to measure the size of a propagating flaw, due to the complexities of the many testing conditions. Recently, Paris and Hayden[l] have reported that a bonded, metal foil-type Gage[2] is a useful new addition to Fracture Mechanics Testing and Instrumentation. It was reported that this system was compatible with ASTM Standard E647-81, the “Tentative Test Method for Constant-Load-Amplitude Fatigue Crack Growth Rates above lo-* m/cycle”, and easily exceeded the accuracy and sensitivity requirements of ASTM E647-81. Fitzgerald and Grifiths[31 and Pearson and Gilbert [4] also showed that the KRAK-GAGE per- formed satisfactorily, and the test data compared favorably with comparative data obtained from other methods in laboratory air and in 100% relative humidity environments, respectively, for aluminum alloys and a steel. The objective of this work was to further evaluate the application of this new in- strumentation system in a corrosive, aqueous test environment. PRINCIPLEOFTHEKRAK-GAGE The basic principle of the KRAK-GAGE employs an indirect d.c. potential measurement technique. A detailed description on the theory of the KRAK-GAGE and associated instrumentation has recently been reported by Hartmann and Churchill[S]. This Gage, or transducer, is a bondable, thin, electrically insulated metal foil of certain dimensions, photo-etched from a constantan alloy. The Gage backing is flexible epoxy-phenolic matrix providing the necessary insulation and bonding surface area, similar to the technology of foil-type strain gages. Conventional, well established foil strain-gage installation techniques [2,6] are applied to the bonding and installation of such a Gage to the test specimen. By an appropriate selection of Gage material, Gage size, backing and adhesive bonding, a crack will simultaneously propagate in both the test specimen and the bonded KRAK-GAGE. A constant current source, in the order of lOOmA, is used to excite the low resistance Gage, Fig. l(a). A propagating crack produces a large change in the resistance of the Gage and yields a high d.c. tHartrun Corporation, Chaska, MN 55318, U.S.A. I21