Validation of Circumferential Notched Tensile (CNT) Test Procedure for KISCC Determination

Abstract

It is well known that a great proportion of failures of industrial structures operating in a corrosive environment occur by stress corrosion cracking (SCC). Life prediction of such components involves the application of linear elastic fracture parameters such as stress intensity factor, K-I that is a measure of material's resistance to crack propagation. K-ISCC is the threshold value of the stress intensity factor below which a present crack stays stationary under the stress corrosion conditions. Therefore, determination of the threshold stress intensity (K-ISCC) is essential during material selection for any mechanical engineering design. K-ISCC has traditionally been determined using specimens of fracture mechanics with geometries named as compact tension (CT) and double cantilever beam (DCB) test specimens. Such specimens are generally expensive to manufacture and test, requiring relatively expensive testing systems. A rapid and cost-effective determination of KISCC is of great interest to design engineers. Recently in literature alternative test piece geometry called Circumferential Notch Tensile (CNT) specimen has been proposed for producing K-ISCC data. Smallness of CNT specimens reduces the cost of the test because of requirement of small loading devices to obtain the desired stress intensities and small amounts of testing material. In this study, for constant loading, a set-up has been designed and manufactured. CNT specimens were produced from AISI 4140 steel after austenizated at 850 degrees C; water quenched and tempered 450 degrees C. The corrosive environment was 33% NaOH (caustic) at 80oC. Specimens were loaded under constant loads until they are broken. Results showed that K-ISCC of AISI 4140 was determined as 55 MPa root m. In literature this value in the same environment was given as 59 MPa root m for compact tension specimen produced from the same steel where crack growth was monitored by the potential drop method. Thereby results validate the CNT testing technique for determination of K-ISCC.