Characterization of Low-cycle Fatigue Damage in Inconel 718 by Laser Light Scanning

Abstract

A technique for in situ laser light scanning (LLS) was developed to monitor surface damage on nickel-base superalloy specimens under low-cycle fatigue conditions. This technique characterizes the surface state with a parameter called the defect frequency which minimizes memory requirements and data processing time since it does not involve image processing. As a result, the present technique is capable of scanning speeds that are substantially greater than those achieved with image processing methods. Cylindrical Inconel 718 specimens were tested using an automated servo-hydraulic machine at ambient temperature under fully reversed strain control conditions for constant strain amplitudes ranging from 0.3% to 1%. The fatigue damage was monitored by scanning a laser beam along the gauge section of the specimens during periodic interruptions of the cyclic loading. Acetate replicas of the gauge section surface were also made on some of the specimens to characterize the damage using SEM and image analysis techniques. Comparisons of the results demonstrate the capabilities of the present light-scanning technique for characterizing fatigue damage on the surface of the Inconel 718 specimens. In particular, a rapid rise in the mean defect frequency is shown to correspond to an initial increase in microcrack density that saturates at approximately 20% of the fatigue life. This transient behavior is followed by a plateau in defect frequency which corresponds to crack propagation and interlinkage until failure occurs. The number of cycles to microcrack density saturation as indicated by the defect frequency is found to be linearly related to the number of cycles to failure. Accordingly, the present system provides a characterization of microcrack damage that may be used to predict the low-cycle fatigue life of Inconel 718 specimens long before failure occurs.

Scanning Laser Detection System Used to Measure Propagation of Fatigue Damage of Asphalt Mixes

“…The progressive advancement of laser technology, compact size, and descending cost of laser sources promises to further enhance the development of laser techniques for noncontact characterization. In particular, laser beam scattering schemes have been successfully applied to obtain surface information of various materials (22,23). Laser light scattering intensity has been investigated and correlated to the average surface roughness, defect shape, and defect densities in optical components and semiconductors (22).…”

“…In particular, laser beam scattering schemes have been successfully applied to obtain surface information of various materials (22,23). Laser light scattering intensity has been investigated and correlated to the average surface roughness, defect shape, and defect densities in optical components and semiconductors (22). Localized deformation resulting from cyclic loading generally leads to perturbations in the surface of the component.…”

“…An SLD system, also called laser light scanning, was further developed in the present work to monitor surface damage of asphalt mixtures under fatigue conditions. Scattered light can be related to surface changes leading to crack initiation, and can also be used to determine crack density if multiple cracks are nucleated at the surface (22). The system is capable of detecting cracks with surface lengths as small as 100 µm, with crack opening displacements as low as 1 µm (21).…”

“…Previous studies on fatigue characterization of composite materials using laser scanning system have indicated that an increase in the mean defect frequency (DF) corresponds to an increase in microcrack growth (25,26). This transient behavior is followed by a plateau (peak) in DF that corresponds to crack propagation until failure occurs (21)(22)(23). The number of cycles to failure indicated by DF criterion was found to be linearly correlated to the number of cycles to failure obtained from conventional fatigue approaches (27).…”

Investigation on LCF Behavior of Welded Joint at Different Temperatures for Bainite Steel

“…Actually, a considerable number of structural components' failure can be attributed to the LCF damage, especially for the steam rotors during start/stop stage and power transient period [11][12][13][14][15][16][17]. A series of parameters, including the temperature [18], the strain rate [19], the strain amplitude [20], etc., could influence the LCF behavior of the material. Mishnev et al [21] investigated the microstructure evolution of 10% Cr steel at different temperatures (20-650 °C) under LCF conditions.…”