Stainless steel shows excellent mechanical properties at elevated temperatures and is, therefore, used very widely in applications involving operation at high temperatures. Martensitic grade stainless steel is used, for example, to manufacture steam turbine blades in power plants. The failure of these turbine blades can result in equipment damage contributing to expensive plant failures and safety concerns. Plastic deformation is one of the primary factors leading to material degradation and the structural failure of key structural components. It is of vital importance to evaluate the level of plastic deformation prior to the initiation of macro defects in order to ensure the viability of these components. The detection of conditions that lead to the development of cracks can reduce overall operational and maintenance costs. Conventional Nondestructive Evaluation techniques such as Ultrasonic Testing (UT) and Eddy Current Testing (ECT) are good candidates for detecting macro defects (cracks, corrosion etc.), but not effective for the evaluation of the material degradation due to fatigue. Fatigue cracks that are too small to be detected using conventional eddy current techniques during an inspection could grow unchecked during the subsequent operation cycle, leading to failure before the next inspection opportunity. A nondestructive technique capable of detecting indications of early fatigue in materials is, therefore, needed. To tackle these challenges, this paper presents a novel nonlinear eddy current technique for garnering information relating to the condition of the material under test. The basic idea is to apply a strong periodic excitation field forcing the material to operate in the nonlinear region of its magnetization characteristic.

  
This paper investigates the feasibility of nonlinear eddy current technique to detect early stage fatigue associated with plastic deformation in ferromagnetic metallic structures. We do so by measuring parameters that are influenced by the underlying microstructural material properties. The test set up comprises a custom sensor assembly consisting of two excitation and two pick-up coils. The two excitation coils are serially connected while the sensor coils are differentially connected to measure the nonlinear response, representing the difference between the high stress region (fatigued) and reference region (unfatigued). The responses induced in the pick-up coil are analyzed in the frequency domain to infer information about material parameters that are influenced by the specimen’s state before conditions conducive to the development and growth of cracks develop. Features such as harmonic peak ratios show correlation with damage caused by fatigue cycles. Experimental results demonstrate that the nonlinear eddy current inspection technique is a promising candidate for characterizing martensitic stainless-steel steam turbine blades and assessing its fatigue life.

  
Acknowledgement:  
This work is supported by Energy Power Research Institute (Task ID: 1-110095-01-04). We wish to thank the project manager Mr. Thiago Seuaciuc-Osorio for his support.


Early Stage Fatigue Prediction for Martensitic Stainless-Steel Samples Using Non-Linear Eddy Current Techniques

No proceedings papers in 2020. Video only.

QNDE2020 - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.