Relationship between acoustic emission and CTOD testing for a structural steel

Abstract

Acoustic emission testing may be employed as a nondestructive technique to detect flaws in materials and equipment. For pressure vessel construction, the test requires loading by internal pressure in order to open the flaw tips and to generate acoustic emission signals. The present work tries to investigate whether the required pressure level causes flaw propagation, risking vessel integrity. The material employed was a fine grained normalized C-Mn structural steel (ASTM A 516 gr. 60). Fracture mechanics crack tip opening displacement (CTOD) tests were conducted and the acoustic emission probes were placed on the testpieces, which enabled a correlation to be established at any moment between the crack acoustic behaviour and its CTOD value. A relevant result was that intense acoustic activity was detected before the first stable crack extension occurs. On a quantitative basis, it was found that the CTOD value corresponding to the first detectable acoustic emission from the loaded crack tip is lower than the CTOD for the initiation of stable crack propagation. As a final conclusion from this work, it can be said that for this specific steel, acoustic emission testing may be reliably performed at sufficiently low CTOD values, where the risk of fracture may be discarded.