R. Rihan

Circumferential Notch Tensile Testing Role of Imposed Electrochemical Potentials in Susceptibility of Steel to Caustic Cracking

A fracture mechanics based novel approach, i.e., circumferential notch tensile (CNT) testing has been employed for determination of threshold stress intensity factor for susceptibility of engineering materials to stress corrosion cracking (K ISCC ). Using CNT technique, K ISCC of a mild steel at an open circuit potential (E corr ) in 12.5 M NaOH at 150°C was determined to be 29 MPa m 1/2 . CNT tests have also been performed under imposed electrochemical potentials to understand the mechanistic aspects of caustic cracking as well as to develop guidelines for mitigation of caustic cracking. An imposed potential in the active-passive potential regime (E a-p ) caused much more rapid failure (than was observed at E corr ). The fractography of the CNT specimens tested at E corr and E a-p showed evidence of stress corrosion cracking. At an imposed potential in the passive region (E p ), the specimen did not fail even after an order of magnitude longer testing, indicating a considerable improvement in resistance to caustic cracking as a result of this application of E p . The resulting fractograph confirmed the absence of caustic cracking. The study has established the use of CNT testing as a simple, relatively fast and economical approach for generating the K ISCC data, and showed that the data are consistent with an electrochemical mechanism for caustic cracking.

Circumventing Practical Difficulties in Determination of Threshold Stress Intensity for Stress Corrosion Cracking of Narrow Regions of Welded Structures

Determination of the threshold stress intensity for stress corrosion cracking (KIscc) of narrow areas such as weld and heat-affected zone (HAZ) of a weldment is a nontrivial task because of the requirements of large specimens in testing by the traditional techniques and the difficulty of restricting crack propagation to narrow regions in such specimens. This article describes a successful application of the circumferential notch tensile (CNT) technique to determine the KIscc of narrow regions of the weld and HAZ. Also, the microstructure of the HAZ of the manual metal arc-welded steel was simulated over a relatively small length of specimens and its KIscc in a hot caustic solution was determined successfully. Intergranular stress corrosion cracking was confirmed with a scanning electron microscope.

ISCCKCircumferential notch tensile (CNT) tests for determination of , using small fracture mechanics specimens

Publisher Summary This chapter discusses a technique for rapid and cost-effective determination of stress intensity factor (KI) and fracture toughness (KIC) using small circumferential notch tensile (CNT) specimens. Ahead of the notch of the CNT specimen, a uniform precrack is developed by subjecting the specimen to a controlled fatigue in a bending-rotating set-up. The precracked CNT specimen is then subjected to a constant load, until the specimen fails. KIC is determined by correlating the stress intensity factor KI with one of the crack propagation parameters such as crack-growth velocity or time-to-failure. One of the critical issues in determination of KIC by CNT method is the possible effect of the eccentric fatigue crack and the ligaments produced by the rotating bending fatigue crack machine of specimens. CNT is the smallest possible specimen that can produce valid plane strain crack loading conditions. KIC values determined using CNT specimens are claimed to be within –3% and +3% of the data generated using the ASTM compact tension (CT) specimen. In tests conducted earlier at Monash University, acceptable results have been achieved using 9.5 and 15 mm diameter CNT specimens, whereas for the same material, KIC determination, using standard CT specimens, requires widths up to 80 mm.