Investigation of fatigue crack closure effect on the evaluation of edge cracks with the fundamental mode of edge waves.

Fatigue cracks often initiate and propagate from edges of structural components. Detection and evaluation of edge cracks could be very challenging, specifically, due to the crack closure phenomenon, which makes fatigue cracks to be partially closed when the applied loading is removed; this usually corresponds to maintenance and NDE inspection conditions. Despite that fatigue crack closure is well investigated, past experimental and theoretical studies related to guided wave-based NDEs largely ignored this phenomenon. In this article, the fundamental symmetric mode of edge waves (ES0) is used to evaluate crack closure effects on the evaluation of fatigue cracks. The experimental studies have demonstrated that the reflected and transmitted signals at different frequencies correlate very well with the length of the open region of fatigue cracks. However, an accurate evaluation of the total crack length can only be conducted under an applied loading, which fully separates the crack faces. Finally, a new FE model has been proposed to simulate the fatigue crack closure and its effects on propagation of ultrasonic bulk and guided waves. The outcomes of FE modelling and experimental study were found in a good agreement.

Fatigue crack detection in edges of thin-walled structures with corners using the fundamental mode of edge waves.

Non-destructive detection and evaluation of fatigue cracks is critical to maintain safety and effective operation of high-value assets working under cyclic loading. However, this can be difficult in the case of the corners of the structural elements, especially at inaccessible locations. In this article, the propagation of the fundamental symmetric mode of edge wave (ES0) along structural features such as sharp and rounded corners are investigated using experimental and numerical methods. The ultimate aim of this study is to demonstrate that the ES0 is a promising for defect detection in geometries with corners. The outcomes of this study show that ES0 wave is able to propagate through sharp and rounded corners and provides a way to inspect difficult-to-reach locations. Further, the numerical simulations indicate that the radius-to-wavelength ratio above 3 has no significant impact on the wave amplitude when the ES0 propagates through the rounded corner. The results also demonstrate that the presence of fatigue crack leads to generation of the second harmonic of the ES0 wave mode, and this phenomenon can be utilised in the development of fatigue crack detection and characterization procedures.

Low-frequency Lamb wave mixing for fatigue damage evaluation using phase-reversal approach.

Fatigue damage is difficult to detect and evaluate non-destructively, specifically at its early stages (before the macro-crack formation). In this study, fatigue damage is evaluated based on the growth rate of the combinational harmonics generated by mixing of two fundamental symmetric mode (S0) of Lamb waves in the low frequency range. The incorporation of the phase reversal approach to the wave mixing method could potentially improve the evaluation of the combinational and second harmonics and avoid the influence of other undesirable harmonics. A series of parametric case studies are carried out using the three-dimensional (3D) finite element (FE) method to investigate the effects of the excitation frequencies and time delay of the incident waves in wave mixing on the transient response of a weakly-nonlinear material. The numerical results and experimental results show that the sum combinational harmonic and second harmonics are sensitive to weak material nonlinearities. Further experiments on damaged samples by cyclic loading demonstrate that the sum combinational harmonic has much better sensitivity to the progressive fatigue damage than the the second harmonics. In general, the outcomes of this study indicate that the damage evaluation of early stage fatigue damage is feasible and effective with the wave mixing method using the S0 waves generated at low frequency, and the phase-reversal approach improves considerably the quality of experimental results in the fatigue damage evaluation.

Rapid Characterization and Modeling of Natural and Undefined Charge-Regulated Surfaces in Aqueous Systems.

The surfaces of most materials in aqueous systems are charged due to the ionization of surface functional groups. When these surfaces interact, the surface charge, electrostatic potential, and pH will vary as a function of separation distance, and this process is termed the charge-regulation effect. Charge regulation is a controlling factor in the adhesion and transport of colloids and microorganisms in aqueous systems, and its modeling requires representation of the pH-charge response of the surfaces, typically provided as the equilibrium constants (K) and site densities (N) of the dominant surface functional groups. Existing methods for obtaining these parameters demonstrate shortcomings when applied to many natural and man-made materials, such as weathered materials, materials with undefined or complex surface structures, and permeable materials, and for materials that do not provide the requisite high surface area in suspension due to small sample sizes. This hinders inclusion of the charge-regulation effect in colloid and microbial transport studies, and most studies of colloidal and microbial surface interactions use simplifying assumptions; a key example is the routine use of the constant potential assumption in DLVO modeling. Here we present a robust method that overcomes these issues and provides a rapid means to characterize charge-regulated surfaces using zeta potential data, without requiring a priori knowledge of the material composition. Applying a combined charge-regulation and Gouy-Chapman model, K and N values are obtained that accurately represent the electrostatic response of a charge-regulated surface. This method is demonstrated using activated carbon, aluminum oxide, iron (hydr)oxide, feldspar, and silica sand. The resulting K and N values are then used to show the variations in surface charge, electrostatic potential, and pH that can occur as these charge-regulated surfaces interact. This method provides a readily applied experimental approach for characterizing charge-regulated surfaces, with the overall goal to promote the inclusion of charge-regulated interactions into adhesion and transport studies with natural and undefined materials.

Genotoxic effect of 2,4,6-trichlorophenol on p53 gene in zebrafish liver.

Trichlorophenol (2,4,6-trichlorophenol [TCP]), a probable human carcinogen, is widely present in the environment. Employing denaturing high-performance liquid chromatography and DNA sequencing analyses, we demonstrated that 10-d exposure of zebrafish to TCP at 5 microg/L resulted in elevated point mutations of p53 gene in the liver genome. The result suggests that TCP might be involved in carcinogenesis by inducing point mutations in the somatic genome.