Active sensing of fatigue damage using embedded ultrasonics

Abstract

Embedded ultrasonics has demonstrated considerable utility in structural health monitoring of aeronautical vehicle. This active sensing approach has been widely used to detect and monitor cracks, delaminations, and disbonds in a broad spectrum of metallic and composite structures. However, application of the embedded ultrasonics for active sensing of incipient damage before fracture has received limited attention. The aim of this study was to investigate the suitability of embedded ultrasonics and nonlinear acoustic signatures for monitoring pre-crack fatigue damage in aerospace structural material. A harmonic load was applied to structural specimens in order to induce fatigue damage accumulation and growth. Specimens of simple geometry were considered and piezoelectric active sensors were employed for generation and reception of elastic waves. The elastic wave signatures were analyzed in the frequency domain using nonlinear impedance and nonlinear resonance methods. A relationship between fatigue severity and linear as well as nonlinear acoustic signatures was investigated and considered in the damage classification procedure. Practical aspects of the active sensing of the fatigue damage before fracture were discussed and prospective avenues for future research were suggested.

Influences of mechanical contact on damage evaluation with electromechanical impedance technique

“…We know that the resonant frequency, as an intrinsic parameter of the structural dynamics, is widely accepted as a critical damage evaluation index. Although the value might vary with the shape, dimension, material property, and testing frequency band, the results of our and others’ experiments have all demonstrated that the Δf value corresponding to the early-stage damage of a dog-bone sample was about 10 −1 kHz (Li et al, 2012; Palomino et al, 2011; Zagrai et al, 2009). For the ASS sample used in previous work, the sub-millimeter crack damage and a plastic deformation zone around the pre-notch resulted a 0.2–0.3 kHz peak frequency decrease (Luo et al, 2013).…”

“…The material mechanical damage behavior evaluated by EMI technique has been reported by several researchers (Giurgiutiu et al, 1999;Soh, 2010, 2011;Palomino, 2011;Park et al, 2010;Sevostianov et al, 2010;Zagrai et al, 2009). Aluminum alloys were widely used because of its high availability and the low price.…”

“…The initiation and propagation of the fatigue crack have also been monitored. The two damage indexes, Δf and RMSD, were successfully incorporated to qualitatively distinguish the fatigue stage and quantitatively evaluate the damage extent (Giurgiutiu et al, 1999; Li et al, 2012; Zagrai et al, 2009). Other methods, such as linear elastic fracture mechanics (LEFM) (Lim and Soh, 2010) and statistical model (Palomino et al, 2011), were also introduced to estimate the fatigue life or the stress state.…”

“…It is well known that contact mechanical tests are commonly performed in laboratories to study the damage behavior of materials. Parts contacting with the fixtures would inevitably produce some superficial indentations, scratches, and so on (Giurgiutiu et al, 1999; Li et al, 2012; Lim and Soh, 2010, 2011, 2012; Luo et al, 2013; Palomino et al, 2011; Park et al, 2010; Zagrai et al, 2009). The damage level is the same or even more serious against that of the early-stage material we concerned about.…”

Active sensing of fatigue damage using embedded ultrasonics

“…Two methods were used to interpret the data. The linear frequency shift metric used by Zagrai et al (2009) and was applied to the peak responses of the low and high frequency HP4192A measurements, and the fundamental frequency RITEC-Shunt measurements. An average amplitude metric was applied to all measurements, including the RITEC-Shunt second harmonic measurements, to determine the effect of fatigue on measurement amplitude.…”