Nonlinear Acoustic Modeling and Measurements during the Fatigue Process in Metals (original) (raw)
Related papers
2014
A dedicated modeling technique for comprehending nonlinear characteristics of ultrasonic waves traversing in a fatigued medium was developed, based on a retrofitted constitutive relation of the medium by considering the nonlinearities originated from material, fatigue damage, as well as the ''breathing'' motion of fatigue cracks. Piezoelectric wafers, for exciting and acquiring ultrasonic waves, were integrated in the model. The extracted nonlinearities were calibrated by virtue of an acoustic nonlinearity parameter. The modeling technique was validated experimentally, and the results showed satisfactory consistency in between, both revealing: the developed modeling approach is able to faithfully simulate fatigue crack-incurred nonlinearities manifested in ultrasonic waves; a cumulative growth of the acoustic nonlinearity parameter with increasing wave propagation distance exists; such a parameter acquired via a sensing path is nonlinearly related to the offset distance from the fatigue crack to that sensing path; and neither the incidence angle of the probing wave nor the length of the sensing path impacts on the parameter significantly. This study has yielded a quantitative characterization strategy for fatigue cracks using embeddable piezoelectric sensor networks, facilitating deployment of structural health monitoring which is capable of identifying small-scale damage at an embryo stage and surveilling its growth continuously.
Experimental analysis of the nonlinear behaviour of fatigued metallic samples
2003
The nonlinear behaviour of intact and fatigued metallic bars flexurally and/or extensionally vibrating has been studied by means of a new experimental procedure for ultrasonic fatigue damage initiation and detection. The procedure is based on the measurement of the vibration velocity of stepped bar samples vibrating at their first resonant mode for different excitation levels. The reverberation of the vibration velocity signal, picked-up by a laser vibrometer, is automatically acquired and analysed by classical FFT methods. The tests have been done with samples of titanium and aluminium alloys. The fatigue of the material is produced by using the driving signal at high excitation levels in the same experimental setup developed for detection. Two types of experimental studies have been carried out: determination of the fatigue stress at high frequency and comparison between the nonlinear behaviour of intact and fatigued samples. The results show a notable increase of the nonlinear characteristics of the fatigued samples with respect to the original intact samples. In particular, the variation of the third harmonic becomes very relevant.
Quantification of fatigue damage accumulation using non-linear ultrasound measurements
International Journal of Fatigue, 2007
This article attempts to relate ultrasonic second harmonic generation to dislocation structures in fatigued materials. By focusing on the fact that asymmetric dislocation motion is required to generate the second harmonic, a theoretical derivation of the non-linearity parameter (ratio of the amplitude of second harmonic to square of the amplitude of the fundamental) from dislocation pile-ups is presented. In order to verify the theory, harmonic generation was measured along a failed fatigue sample of DA718, a nickel base superalloy that exhibits planar slip, and consequently dislocation pile-ups. TEM studies along the length of the failed fatigue sample revealed a banded dislocation structure, composed of pile-ups near the fracture zone and a sparse dislocation network closer to the grip region. The non-linearity parameter was found to increase by between 90% and 140% from the grip region to the fracture zone and this correlated well with the calculations based on theoretical expressions derived here.
Baseline-free estimation of residual fatigue life using a third order acoustic nonlinear parameter
The Journal of the Acoustical Society of America, 2011
Prediction of crack growth and fatigue life estimation of metals using linear/nonlinear acousto-ultrasound methods is an ongoing issue. It is known that by measuring nonlinear parameters, the relative accumulated fatigue damage can be evaluated. However, there is still a need to measure two crack propagation states to assess the absolute residual fatigue life. A procedure based on the measurement of a third-order acoustic nonlinear parameter is presented to assess the residual fatigue life of a metallic component without the need of a baseline. The analytical evaluation of how the cubic nonlinear-parameter evolves during crack propagation is presented by combining the Paris law to the Nazarov-Sutin crack equation. Unlike other developed models, the proposed model assumes a crack surface topology with variable geometrical parameters. Measurements of the cubic nonlinearity parameter on AA2024-T351 specimens demonstrated high sensitivity to crack propagation and excellent agreement wit...
Experimental investigation of nonlinear acoustic effect at crack
NDT & E International, 2009
The nonlinear ultrasonic technique using the feature of higher-harmonic generation that is one of nonlinear acoustic behavior at interfaces has been considered as a positive method for the detection of micro-cracks. However, most previous studies were limited to model development and its verification by testing two blocks contacted together. This paper investigates experimentally the nonlinear acoustic effect at a real crack. For this, we constructed a measurement system using contact PZT transducers, which was able to keep all equipments and their setup conditions consistent and thus enabled a reliable measurement of nonlinear parameter. A CT specimen of Al6061 with a crack initiated by a fatigue test was tested, and the nonlinear parameter was measured by scanning along the crack depth direction. We could see that the nonlinear parameter had good correlation with the crack, which was identical to the theoretical prediction. From these results, we could show the applicability of the proposed methodology to micro-crack detection.
Linear Versus Nonlinear Acoustic Probing of Plasticity in Metals: A Quantitative Assessment
2018
The relative dislocation density of aluminum and copper samples is quantitatively measured using linear Resonant Ultrasound Spectroscopy (RUS). For each metallic group, four samples were prepared with different thermomechanical treatments in order to induce changes in their dislocation densities. The RUS results are compared with Nonlinear Resonant Ultrasound Spectroscopy (NRUS) as well as Second Harmonic Generation (SHG) measurements. NRUS has a higher sensitivity by a factor of two to six and SHG by 14% to 62%. The latter technique is, however, faster and simpler. As main a result we obtain a quantitative relation between the changes in the nonlinear parameters and the dislocation density variations, which in a first approximation is a linear relation between these differences. We also present a simple theoretical expression that explains the better sensitivity to dislocation content of the nonlinear parameters with respect to the linear ones. X-Ray diffraction measurements, altho...
MRS Proceedings, 1999
ABSTRACTVariation in acoustic nonlinearity has been monitored in real time during fatigue, on four dogbone specimens of Ti-6A1-4V, under low cycle fatigue conditions, from the virgin state all the way to fracture. The results of these experiments show that the acoustic nonlinearity undergoes large changes during the fatigue and follows a similar trend for the material under given fatigue test conditions. Transmission electron microscopic (TEM) examination of the samples with similar composition fatigued to different stages indicates a gradual change in the microstructure and dislocation density, which correlates with the changes in acoustic nonlinearity.
Nonlinear Acoustics for Damage Detection and Evaluation in 2017A Aluminium Alloy
2016
The present study focuses on the experimental ultrasonic evaluation of cracked metal structures in other to asses fatigue damage. In this framework, two different techniques for quantifying damage were tested, a linear approach based on the classic track of stiffness and a nonlinear ultrasonic approach by monitoring the acoustic nonlinear parameter. Fatigue tests were performed on compact tension (CT) specimens made of 2017A aluminum alloy. In this context the analysis tool of fatigue cracks relies on a thorough examination of the temporal and spectral acoustic signatures of ultrasonic transmitted waves in cyclically loaded specimens. Indeed, unlike an uncharged medium, a cracked medium exhibits a high acoustic nonlinearity which is manifested by the appearance of harmonics in the frequency spectrum of the received signal. This approach for detection, sizing and prediction of fatigue cracks could be a general strategy for in situ monitoring of structural element damage. The analysis...
Ultrasonics, 2018
In this paper, a third harmonic was used to investigate microstructural changes in Al6061-T6 due to different fatigue cycles and a relationship between fatigue cycle and third order nonlinearity has been observed. Piezoelectric measurement harmonic generation technique was applied for the specimens with 0%, 55%, 75% and 85% fatigue cycles, respectively. The results shows that the third order harmonics gradually increased up to 55% and rapidly decreased after wards, it was attributed to the behavior of dislocation, dislocation-precipitation interaction and voids with increasing fatigue cycle. Further, it was verified with scanning electron microscope (SEM). We also observed that third order nonlinearity is more sensitive to small change in area of fraction of voids than second order nonlinearity after 55% fatigue life and could be a good candidate to investigate Al6061-T6 specimen with voids.
The signatures of acoustic emission waveforms from fatigue crack advancing in thin metallic plates
Smart Materials and Structures
The acoustic emission (AE) waveforms from a fatigue crack advancing in a thin metallic plate possess diverse and complex spectral signatures. In this article, we analyze these waveform signatures in coordination with the load level during cyclic fatigue. The advancing fatigue crack may generate numerous AE hits while it grows under fatigue loading. We found that these AE hits can be sorted into various groups based on their AE waveform signatures. Each waveform group has a particular time-domain signal pattern and a specific frequency spectrum. This indicates that each group represents a certain AE event related to the fatigue crack growth behavior. In situ AE-fatigue experiments were conducted to monitor the fatigue crack growth with simultaneous measurement of AE signals, fatigue loading, and optical crack growth measurement. An in situ microscope was installed in the load-frame of the mechanical testing system (MTS) to optically monitor the fatigue crack growth and relate the AE signals with the crack growth measurement. We found the AE signal groups at higher load levels (75%-85% of maximum load) were different from the AE signal groups that happened at lower load levels (below 60% of load level). These AE waveform groups are highly related to the fatigue crackrelated AE events. These AE signals mostly contain the higher frequency peaks (100 kHz, 230 kHz, 450 kHz, 550 kHz). Some AE signal groups happened as a clustered form that relates a sequence of small AE events within the fatigue crack. They happened at relatively lower load level (50%-60% of the maximum load). These AE signal groups may be related to crack friction and micro-fracture during the friction process. These AE signals mostly contain the lower frequency peaks (60 kHz, 100 kHz, 200 kHz). The AE waveform based analysis may give us comprehensive information of the metal fatigue.