Acoustic emission assessment of through-thickness fatigue crack growth in steel members

The understanding of crack behavior under fatigue remains a critical issue in addressing the performance of steel bridges. Single edge notches in general provide a very well defined load and fatigue crack size and shape environment for estimation of the stress intensity factor K, which is not found in welded elements. ASTM-E647 SE(T) specimens do not appear to provide ideal boundary conditions for proper recording of acoustic wave propagation and crack growth behavior observed in steel bridges, but do provide standard fatigue crack growth rate data. Acoustic emission (AE) has been increasingly used for assessment and prediction of fatigue cracks in steel bridge members. In steel bridge members, AE transducers are commonly attached to the surface of the plate from which the crack is initiated, hence producing a through-thickness crack growth monitored by the transducer. A modified version of the SE(T) specimen was developed in order to maintain similitude with the field crack propagation orientation and to provide a small-scale specimen with improved AE characteristics while maintaining accuracy of fatigue crack growth rate (da/dN) versus stress intensity factor range (ΔK). The specimen simulates fatigue cracks in flanges or early stage of crack growth in webs of steel bridge members. Effects of load ratio (R) and material on AE data recorded during the crack growth was addressed. Applicability of AE to capture, locate and predict the behavior of the growing crack was positively verified. R ratio showed to have a significant effect on evolution of AE data captured during the test.