This paper presents a new method of signal processing for vibro-acoustic modulation (VAM) methods in order to detect damage accumulation in steel samples. Damage in the tested samples was produced by cycle loading, which, with a small amplitude, was used as a pump wave to modulate an ultrasonic probe wave. Multiple sideband peaks were observed, which were used to characterize the modulation effect. We propose the effectiveness sideband peak number (SPN) method as an indicator of any damage accumulation when the load cycle is applied. Moreover, after comparing the SPN with the previously used modulation index (MI), we concluded that, for some of the samples, the SPN provided better damage indication than the MI. The presented results can be explained by a simple model of bilinear crack nonlinearity. This model demonstrates that the amplitude dependences of the sideband components on the pump and the probe wave amplitudes are very different from the quadratic crack model that is usually used for MI test explanation.

/pdf/sideband-peak-count-in-a-vibro-acoustic-modulation-method-3fadzz6w.pdf

Sideband Peak Count in a Vibro-Acoustic Modulation Method for Crack Detection

Nonlinear ultrasonic techniques have the benefit of high sensitivity to micro or early-stage defects. Among the various nonlinear techniques, the newly proposed sideband peak count (SPC) technique investigates defect-induced nonlinearity by counting the spectral sidebands from a broadband ultrasonic response in the frequency domain. In this study, SPC analysis is transformed into the time–frequency plane through synchrosqueezed wavelet transform (SWT) for transient nonstationary ultrasonic signals. The proposed new SPC technique was then adopted for in situ porosity monitoring in directed energy deposition (DED)—a typical metal additive manufacturing process. Porosity is one of the most critical defects in DED and has detrimental effects on the mechanical properties and fatigue performance of products. For in situ porosity monitoring, a fully noncontact ultrasonic measurement was achieved with a laser ultrasonic system, and its detectability was improved by laser polishing. Time and frequency windows were properly selected to suppress the effects of wave characteristic variations on the SPC analysis in the SWT domain. The performance of the proposed technique was verified by monitoring porosity in stainless steel 316L samples manufactured in the DED process. The test results demonstrated that the proposed nonlinear technique is much more sensitive to porosity than conventional linear techniques, and hence, is more suitable for in situ porosity monitoring.

Application of nonlinear ultrasonic analysis for in situ monitoring of metal additive manufacturing

A modified sideband peak count based nonlinear ultrasonic technique for material characterization

Modeling and simulation of zero-group velocity combined harmonic generated by guided waves mixing.

Abstract Ultrasonic wave based techniques are widely used for damage detection and for quantitative and qualitative characterization of materials. In this study, ultrasonic waves are used for probing the response of additively manufactured 316L stainless steel samples as their porosity changes. The additively manufactured stainless steel specimens were fabricated using a laser powder bed fusion (LPBF) metal 3D printer. Four different levels of porosity were obtained by suitably controlling the LPBF process parameters. For generating ultrasonic waves, lead zirconate titanate (PZT) transducers were used. The signals were generated and propagated through the specimens in a transmission mode setup. Both linear and nonlinear analyses were used during the signal processing of the recorded signals for damage characterization. Linear ultrasonic parameters such as the time-of-flight (related to wave velocity) and signal amplitude (related to wave attenuation) were recorded. The nonlinear ultrasonic parameter, Sideband Peak Count—Index (SPC-I), was obtained by a newly developed nonlinear analysis technique. The experimental results obtained for the specimens were analyzed and compared for both linear and nonlinear ultrasonic analyses. Finally, the effectiveness of the SPC-I technique in monitoring porosity levels in additively manufactured specimens is discussed. 

S. Park

Papers

An improved nonlinear ultrasonic technique for detecting and monitoring impact induced damage in composite plates.