Acoustic Emission Assessment of Impending Fracture in a Cyclically Loading Structural Steel
2. Materials and Methods
- The crack nucleates within the first 15 ± 10 loading cycles at stress concentrators. The average number of cycles to failure was 20,900 at 296 K and 26,900 at 233 K, i.e., as desired, the tests correspond to low-cycle fatigue conditions whereby by the fatigue life is controlled by material’s tolerance to fatigue crack propagation . The increase in fatigue life (by a factor of 1.3) at low temperature agrees with results reported in [12,13] and supports our suggestion that fatigue fracture is ductile in all cases investigated and is mediated by plastic deformation, which is impeded to some extent by low temperature.
- AE does not reveal any periodicity in appearance of transient signals associated with brittle fracture due to fatigue crack advance. The latter occurs typically at the maximum cyclic load [1,2]. The AE stream is formed by continuous-like signals, thus making it impossible to use any traditional hit detection approach based on an amplitude threshold. For example, Figure 3 shows what would be recorded if the amplitude threshold was set as indicated by a red line. Obviously, the result of such a detection would not represent the real picture of damage development in terms of either the number of hits detected or their amplitude distribution. Therefore, the high ambiguity of the AE output obtained with the threshold-based detection prevents conclusive decisions regarding the fracture process and the damage state of the material.
- The AE behavior changes synchronously with the loading cycle. It was systematically observed in all tests that the shape of the AE peak changes with the number of cycles as the critical state approaches. Specifically, a wide AE peak, which is commonly observed at on the early stages of cyclic deformation cycle, splits up into two sharper peaks with fatigue crack advance. This pattern becomes increasingly clearly visible as final rupture approaches, differentiating AE signals corresponding to early and late stages of fatigue crack propagation. Compared with signal classification methods based on single features detected via threshold-based acquisition, pattern recognition techniques are computationally intense. However, they are significantly more adequate for processes investigated and, fortunately, can now be implemented in a variety of efficient and elegant mathematical ways .
- The most important finding, which was observed in all tests at both temperatures, is that, as final rupture approached, the AE energy per cycle increased, while the spectral density function progressively shifted to the low-frequency domain, as reflected in the median frequency reduction. The similar behavior in the AE power spectral density has been reported earlier in [15,16] for cyclically and monotonically tested metals when strain localization occurred prior to fracture. Thus, the drop in AE median frequency, which reflects the increasing memory of the past and increasing correlation in the ensemble of emitting AE sources, appears to be a stable indicator and a reliable harbinger of the approaching critical state and failure of a material under load.
4. AE Data Analysis
- II class (C) corresponds to an active AE source. This is a source that should be monitored continuously and inspected by alternative NDT methods.
- III class (D) corresponds to a critical AE source. This kind of source is placed under continuous surveillance with the use of an arsenal of NDT methods available.
- IV class (E) corresponds to a catastrophically active AE source. An operator is directed to stop operating the equipment immediately and exercise safety measures after detecting this kind of source.
4.1. Criterion A: Frequency-Energy
- (kE = 1, kf = 1). This combination indicates that no changes occur in the behavior of E and fm, i.e., both parameters remain at their initial levels corresponding to the incubation fatigue stage preceding crack initiation. Thus, the hazard class I is not assigned, and the source class is labeled as “0”;
- (kE = 1, kf > 1). This combination indicates the increase in the median frequency while the energy remains unchanged, which is observed at the stage of crack nucleation. This behavior is associated with the active AE source II;
- (kE = 1, kf < 1). This combination represents the steady energy behavior, but the decreasing median frequency, which is commonly observed during the ductile fatigue crack growth stage featured by low values of energy increments. It can be associated with the critical active AE source IV;
- (kE > 1, kf = 1). This combination arises when the AE energy increases while the median frequency remains constant. This pattern was also observed during the crack growth stage; therefore, it was labeled as the critically active AE source III;
- (kE > 1, kf > 1). This combination represents the case when both the energy and median frequency increase. It is observed during the initial crack growth stage; therefore, the AE source belongs the class of critically active sources III;
- (kE > 1, kf < 1).This combination reflects the increasing AE energy and concurrently falling median frequency. It is observed before rupture; therefore, it is classified as the catastrophically active AE source IV;
- The combinations (kE < 1, kf < 1), (kE < 1, kf > 1), and (kE < 1, kf = 1) have not been observed experimentally during fatigue crack growth in the tested specimens. Therefore, they represent a hypothetical passive source labeled by “0”.
4.2. Criterion B: Cluster-Energy
- the AE changes synchronously with the load during each cycle with good reproducibility;
- AE energy features systematically change with fatigue crack advance, i.e., the AE energy curve per cycle shows a single wide peak in the beginning of loading, which then splits into two sharper peaks as the crack grows up and the critical stage approaches;
- the AE energy increments progressively when the crack propagates on the late stage of fatigue before failure.
4.3. Verification of the Criteria
Conflicts of Interest
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|Relative Frequency Parameter||Relative Energy Parameter|
|kE < 1||kE = 1||kE > 1|
|kf = 1||0||0||III|
|kf > 1||0||II||III|
|kf < 1||0||IV||IV|
|Type of AE Energy Curve||Energy Indicator|
|Δe < z||z < Δe < 1||Δe > 1|
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Rastegaev, I.; Danyuk, A.; Afanas’yev, M.; Merson, D.; Berto, F.; Vinogradov, A. Acoustic Emission Assessment of Impending Fracture in a Cyclically Loading Structural Steel. Metals 2016, 6, 266. https://doi.org/10.3390/met6110266
Rastegaev I, Danyuk A, Afanas’yev M, Merson D, Berto F, Vinogradov A. Acoustic Emission Assessment of Impending Fracture in a Cyclically Loading Structural Steel. Metals. 2016; 6(11):266. https://doi.org/10.3390/met6110266Chicago/Turabian Style
Rastegaev, Igor, Alexey Danyuk, Maksim Afanas’yev, Dmitry Merson, Filippo Berto, and Alexei Vinogradov. 2016. "Acoustic Emission Assessment of Impending Fracture in a Cyclically Loading Structural Steel" Metals 6, no. 11: 266. https://doi.org/10.3390/met6110266