An Ultrasonic Method for Measurement of Size and Opening Behavior of Small Fatigue Cracks

Abstract

A surface acoustic wave (SAW) technique has been observed to be effective in the detection of initiation and measurement of growth behavior of surface microcracks during fatigue cycling. The experimental procedure involving excitation of Rayleigh waves on the surface of a specimen under investigation is described in detail in conjunction with automated data acquisition of the reflected echo from a small surface crack. The effectiveness of a split spectrum processing algorithm to separate specular reflections of isolated cracks from non-specular reflections of microstructural features is also described. A simplified model for predicting the relationship between crack size and the amplitude of the reflected echo from a crack is included along with a description of the technique for measuring opening behavior of individual cracks in-situ during fatigue cycling. Examples of effective utilization of the SAW technique in contemporary investigations of small fatigue crack growth behavior are presented. Finally, the advantages and limitations of the technique are discussed.

In situ measurement of fatigue crack opening stresses by nonlinear vibro-acoustic modulation testing

“…where s 6 is the total stress at the combination frequency v 6 , F 3 is the crack generated force at the combination frequency, and S denotes for the cross-section area of the structure. Finally, using the same derivation process and combining equation (9), equation (11), and equation 12, the displacement amplitudes of the back and forward scattered waves can be presented as follows All symbols in the right of equal-signs in equation 14except k 6 are same with the corresponding symbols in Donskoy et al 16 Under the circumstances of fixed fundamental signal amplitudes A and B, equation 13and equation (14) indicate that, when the crack is subjected to the resultant of a local compressive residual stress s r and a remote tensile stress s ' , the amplitudes of the modulation sidebands increase with increasing remote tension, before s ' .s r . In addition, the modulation intensity of VAM shows the same behavior, if equation 3is taken into account.…”

Real-Time Health Monitoring of Mechanical Structures

“…Although ultrasonic flaw detectors meet the requirements of real-time damage sensing, their implementation is uncommon on structures in service or possibly has not been done to date. Ultrasonic flaw detectors are commonly used in the aerospace and nuclear power industries to detect flaws in structures, and have been used by many researchers for laboratory crack length measurement [9]. While several other ultrasonic NDE techniques are available for off-line inspection, this paper focuses on real time sensing of fatigue crack damage that requires quantification of material degradation using sensor-based automation.…”

Surface Rust Detection Using Ultrasonic Waves in a Cylindrical Geometry by Finite Element Simulation

“…Various nondestructive evaluation/testing (NDE/T) and structural health monitoring (SHM) techniques have been applied to steel structures [1]. Example techniques include visual testing [2], modal analysis [3], eddy current testing [4], thermal infrared testing [5], and ultrasonic testing [6,7]. Among these techniques, piezoelectric transducers (PZTs) and fiber optic sensors (FOSs) are widely used in SHM for corrosion detection.…”