Fatigue and Fracture Assessment of Cracks in Steel Elements Using Acoustic Emission

Single edge notches 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 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. A modified versions of the SE(T) specimen has been examined to provide small scale specimens with improved acoustic emission(AE) characteristics while still maintaining accuracy of fatigue crack growth rate (da/dN) versus stress intensity factor (ΔK). The specimens intend to represent a steel beam flange subjected to pure tension, with a surface crack growing transverse to a uniform stress field. Fatigue test is conducted at low R ratio. Analytical and numerical studies of stress intensity factor are developed for single edge notch test specimens consistent with the experimental program. ABAQUS finite element software is utilized for stress analysis of crack tips. Analytical, experimental and numerical analysis were compared to assess the abilities of AE to capture a growing crack

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 (Î???K). 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

Effects of Test Parameters on Fracture and Fatigue Characteristics of a SE(T) Steel Specimen

The understanding of crack behavior under fatigue remains a critical issue in addressing the performance of bridge structures. In this study, effects of test parameters on the stress intensity factor (K) of a modified single edge notched tension [SE(T)] steel specimen with a through width notch was experimentally, analytically and numerically investigated. Equations were developed to include the effects of fix boundary condition on K and crack mouth opening displacement (CMOD) of the crack. Effects of axial and angular misalignments on K along the crack length were examined and a progressively decreasing influence on K was observed when the crack size increased. Two dimensional (2D) and three dimensional (3D) Finite Element Method (FEM) was used to evaluate the effect of the two end bounding conditions on K value of non-uniform crack fronts. Influence of ungripped length of the specimen on K was evaluated and showed to be more substantial for short specimens. Finally, a framework was proposed to map the crack front evolution through high cycle fatigue tests for a modified SE(T) specimen with initial crack inclination

Nondestructive evaluation techniques and acoustic emission for damage assessment of concrete bridge in marine environment

Non-destructive evaluation techniques were used to assess the condition of a 40-year old concrete bridge operating in an aggressive marine environment. The bridge’s superstructure includes both reinforced and prestressed concrete one-way slabs, and experienced widening, repairs, and recently strengthening by means of externally bonded carbon fiber reinforced polymer (CFRP) laminates. Phase I of the investigation focused on evaluating deterioration of concrete and steel reinforcement by means of in-situ and laboratory testing. A 24 in. by 24 in. [610 by 610 mm] grid was marked on the bottom surface of the supporting slabs to map indicators of physical damage. Measurement of carbonation, pH, chloride content, corrosion potential, and visual inspection were implemented and rendered as layered maps to identify damaged areas. Phase II includes acoustic emission (AE) monitoring under service loads. AE amplitude, duration, energy and hits were analyzed to identify structural activity associated with damage phenomena, such as concrete cracking, slip between corroded reinforcement and surrounding concrete, and debonding of CFRP laminates. The database acquired from Phase I and Phase II was used for damage assessment. Combined results from the different techniques show promise in determining areas of concern with reduced uncertainty than when using a single measurement technique