Acoustic Emission Source Mechanisms Research Articles

A multi-scale experimental investigation for fatigue limit and fatigue crack initiation behavior of powder bed fusion-laser beam 316L stainless steel

In this study, the fatigue limit of powder bed fusion-laser beam (PBF-LB) 316L prepared by various printing power intensities was studied. After finding the PBF-LB printing parameters giving the best fatigue limit, the fatigue crack initiation (FCI) behavior of dense PBF-LB 316L manufactured by the parameters corresponding to the best fatigue limit was investigated. To this end, the fatigue limit was evaluated by self-heating experiments, and a novel method based on the acoustic emission (AE) technique was suggested to assess the FCI stage during the whole fatigue process. In addition, the AE source mechanism was studied to validate the proposed method, and the fatigue behavior and the fatigue cracking mechanism were deeply clarified through detailed characterizations of microstructures and metallurgical defects. The results indicate that the fatigue limit of dense PBF-LB 316L is affected by a combined effect of the subgrain size and gas pores. However, for the defective PBF-LB 316L, the LOF (lack of fusion) defects induced by low printing power play a crucial role in deteriorating the fatigue limit mainly by promoting FCI. Besides, the printing parameter corresponding to the best fatigue limit is found to be 60 J/mm3 in terms of the volume energy density (VED). Furthermore, as for the FCI behavior, it is shown that the percentage of the FCI stage of dense PBF-LB 316L ranges from 12.4% to 45.3% during the whole fatigue process, and the fatigue crack scale after FCI is on a micro level (from 3.8 to 46.6 μm). Moreover, areas around the fine grains (FGs) and coarse grains (CGs) regions and the junctions between ferrite and austenite are the sites where fatigue cracks are prone to initiate.

In-situ fatigue damage monitoring of rolled Al-Zn-Mg alloy using an advanced acoustic emission technique

In the present study, damage evolution in rolled Al-Zn-Mg alloy and its welds is evaluated using the acoustic emission (AE) method and crack initiation is detected using digital imaging during fatigue tests. The AE characteristics and source mechanisms are analysed based on microstructural and fractographic observations. The experimental results show that AE energies are effective indicators for detecting fatigue crack initiation in Al-Zn-Mg alloys. The results obtained were verified through digital images of the notch tip region of the Al-Zn-Mg alloy samples. For small percentages of the applied load range close to the peak load, the AE count rates show a reasonable correlation with the crack propagation rates. These correlations can be applied to predict the remaining service life of fatigue-damaged structures. The analyses performed demonstrate that the AE technique is sensitive to variations in the fracture mode and could be applied to monitor fatigue damage evolution in welded structures.

Microscopic Failure Mechanism Analysis of Rock Under Dynamic Brazilian Test Based on Acoustic Emission and Moment Tensor Simulation

The dynamic tensile failure of rock is a main failure mode in deep underground engineering projects. The microscopic failure mechanism analysis of this failure mode plays a key role in dynamic disaster warning. Moment tensor inversion is a very well-known method used to analyze failure mechanisms. However, an acoustic emission (AE) event cannot be accurately distinguished in rock dynamic experiments at the laboratory scale, because there are hundreds of AE events generated within a few hundred microseconds in one dynamic test. Therefore, moment tensor analysis is rarely applied in rock dynamic tests with laboratory scale. In this paper, AE and moment tensor simulations with the discrete element method (DEM) are introduced to analyze the microscopic failure mechanism of rock under a dynamic Brazilian test. Comparing the simulation results of AE and moment tensor analysis with the simulation results of micro-crack with DEM, the moment tensor discriminant method can obtain the mechanical mechanism and energy level of micro-cracks. Furthermore, R, which is the ratio of isotropic and deviatoric components of the moment tensor, is used to analyze the AE source mechanism. The implosion, shear, and tensile of the AE source mechanism can better explain the evolution process of main axial crack and the shear failure zones of the Brazilian disc specimen under dynamic tensile simulation. These findings contribute to a better understanding of the microscopic failure mechanism of rock under a dynamic tensile test than the statistical types of micro-cracks based on break bonds with DEM.

Quantitative Acoustic Emissions Source Mechanisms Analysis of Soft and Competent Rocks through Micromechanics-Seismicity Coupled Modeling

This research studied seismic source mechanisms of acoustic emission (AE) events generated during the failure of intact and jointed rock samples using a microscale mechanical-seismicity coupled microscale model. During rock failure, forces and displacements around microcracks were measured in the model to determine seismic moment tensor. Interpretation of AE data was carried out by decomposing the moment tensor into a double couple (DC), a compensated linear vector dipole (CLVD), and isotropic component (ISO). In this study, microscale numerical models of Lac du Bonnet (LDB) granite (intact and jointed) and a Berea sandstone sample were built to represent the mechanical behavior of hard and soft rocks. Subsequently, several numerical triaxial tests under different confining pressures were conducted to analyze source mechanisms of AE events during rock failure. Numerical analysis shows the significant contribution of positive non-DC component during the LDB granite rock failure and significant DC component during the Berea rock failure. The source mechanisms of AE move to the negative non-DC component at higher confining for both granite and sandstone samples. Joint failure causes non-DC component toward crack opening at low confining pressure and a non-DC component toward crack closure events under higher confining pressure. Simulation results, presented in this work, show how micromechanical properties of rocks and joints as well as stress conditions give rise to different types of AE source mechanisms during rock failure.

A new method for determining the crack classification criterion in acoustic emission parameter analysis

At a microscopic scale, the failure of brittle materials results from crack initiation, propagation and coalescence. Acoustic emission (AE) technique, especially parameter analysis, has been widely applied to investigate cracking process and mechanism in civil engineering. However, crack classification in AE parameter analysis mostly derives from the empirical relation between the RA value and the average frequency, and the crack classification criterion, i.e., the optimal transition line between shear and tensile cracks in the parameter analysis has not been determined yet. Based on statistical analysis of dominant frequency characteristics of AE signals, a new method is proposed for determining the optimal transition line for crack classification in AE parameter analysis. Spectrum analyses of AE waveform data in the representative specimens are carried out to acquire the dominant frequency of AE waveforms. Proportions of waveforms distributed in low and high dominant frequency bands (L-type and H-type waveforms) are determined. The ratios of tensile and shear cracks, viewed as measurements, are determined by the statistical analysis of dominant frequency characteristics of AE waveforms. For a series of different transition line, the predicted ratios of tensile and shear cracks in AE parameter analysis are determined. The optimal transition line is determined to be the one corresponding to the least square difference between predicted data and measurements. The determined optimal transition line can be directly applied for crack classification in AE parameter analysis in the subsequent experiments of this brittle material. The reliability of the proposed method were validated by laboratory tests of rock subjected to compression. It can be found that the optimal transition line in the parameter analysis is approximately from 1:100 to 1:500 for brittle rock under compression. The findings in this study contributes to the enhancement of the accuracy and efficiency of AE source mechanism and damage process analysis.

Impact of the Test Device on Acoustic Emission Signals From Nuclear Safety Experiments: Contribution of Wave Propagation Modeling to Signal Processing

A strongly energetic spectral component has been observed in the operating noise of a research reactor before the beginning of reactivity-initiated accidents (RIAs). We identify the source of this component by means of analytical derivations of torsional wave propagation in the test device. Numerical simulations, performed with a spectral-element method, confirm that this component is a resonant frequency of the device, and allow to evaluate the impact of wave travelpath in the test device on the acoustic emission (AE) signals recorded during RIA experiments. The transfer function of the test device is strongly dependent on both the source and the receiver locations, which precludes signal processing by deconvolution when the location of the AE source mechanism is unknown. Moreover, the geometry and the configuration of the test device may not influence the signals in the low-frequency range and hence the signals generated by source mechanisms located on the fuel clad such as clad failures. Finally, replacing the heat transfer fluid (pressurized water) by sodium leads to an increase in the recorded signal amplitude and a small shift of the frequency content toward the high frequencies. The results obtained in this paper may be useful to improve the design of future RIA experiments. They are also useful to understand wave propagation in the core of the reactor and to choose appropriate signal processing tools for recovering the source of AE signals generated during RIA experiments.

Analysis of Acoustic Emission Source Mechanisms for Tensile and Shear Cracks of Rock Fractures

Analysis of Acoustic Emission Source Mechanisms for Tensile and Shear Cracks of Rock Fractures

The Effect of Grain Size on Fatigue Crack Propagation in Commercial Pure Titanium Investigated by Acoustic Emission

The effect of grain size on fatigue crack propagation and the corresponding acoustic emission (AE) characteristics of commercial pure titanium (CP-Ti) were investigated at room temperature. After a four-point bending fatigue testing, the fatigue features and AE source mechanisms were discussed, combined with microstructural and fractographic observations. The results showed that the increased grain size had little effect on the stable propagation rate of fatigue crack; however, a significant increase in the AE counts rate was observed. During crack stable propagation, the relationship between the AE counts rate and the fatigue stress intensity factor range was generally in accordance with the Pairs law, with the exception of some local fluctuations due to regional twin paling. While lenticular twins appeared dispersively along the crack, twin palings were observed occasionally at the edge of the crack. Twin paling occurrence was more frequent in the specimens with larger grains than in those with smaller grains. This suggests that twin discontinuously played a role in the fatigue process in this CP-Ti, and that the AE technique is sensitive to crack propagation and twinning events during fatigue.

Frequency as a key parameter in discriminating the failure types of thermal barrier coatings: Cluster analysis of acoustic emission signals

A key parameter in discriminating the failure types of thermal barrier coatings (TBCs) was found out by using the k-means cluster analysis of acoustic emission (AE) signals. It is shown that there are five classes of mechanisms, including surface vertical cracks, opening interface cracks, sliding interface cracks, substrate deformation and macroscopic cleavage or spallation. Except for the last one, the other four classes can be clearly distinguished from their peak frequency distributions in the ranges of 170–250, 400–500, 260–350 and 40–150kHz, respectively. However, AE signals overlap with each other in other parameter spaces, e.g., amplitude, energy, rise time, and duration time. The results indicate that the frequency can be applied to identify the AE source mechanisms in TBCs.

Determination of the damage threshold in woven-ply thermoplastic laminates at T > Tg: Acoustic emission and microscopic damage analysis

An original in situ measurement of acoustic emission (AE) was applied to monitor damage progress in discrete steps during gradual load/unload tensile tests on [±45°]7 C/PPS laminates at temperatures T>Tg, when matrix ductility is enhanced. In order to understand the specific damage behavior of such materials under severe environmental conditions, AE analysis was accompanied by microscopic observations to detect the damage initiation threshold as well as the damage mechanisms within the composite material. Once the AE source mechanisms have been separated into classes thanks to the pattern recognition software Noesis, they have been identified to match physical phenomena. Earliest cracks events occur at the crimps where the rotation of warp/weft fibres takes place, followed by the intra-bundles splitting on free surface. It is observed that the onset of intralaminar cracking and debonding is affected by the presence of matrix-rich regions between the plies, because of an extensive plasticization of the PPS matrix. The study of the specific acoustic activity of neat PPS resin specimens confirms that the local plastic deformation in matrix-rich areas contributes to delay the initiation of damage, and subsequent AE signals. Finally, AE proved to be a relevant technique to investigate damage mechanisms and to determine accurately the damage threshold in TP-based composites to be used in aeronautical applications at T>Tg.

Brittle failure and fracture reactivation in sandstone by fluid injection

We performed laboratory experiments on sandstone specimens to study brittle failure and the reactivation of an experimentally produced failure plane induced by pore-pressure perturbations using constant force control in high compressive stress states. Here, we focus on the shear failure of a dry sample and the later on induced fracture plane reactivation due to water injection. Acoustic Emission (AE) monitoring has been used during both experiments. We also used ultrasonic wave velocities to monitor pore fluid migration through the initially dry specimen. To characterise AE source mechanisms, we analysed first motion polarities and performed full moment tensor inversion at all stages of the experiments. For the case of water injection on the dry specimen that previously failed in shear, AE activity during formation of new fractures is dominated by tensile and shear sources as opposed to the fracture plane reactivation, when compressive and shear sources are most frequent. Furthermore, during the reactivation of the latter, compressive sources involve higher compressive components compared to the shear failure case. The polarity method and the moment tensor inversion reveal similar source mechanisms but the latter provides more information on the source components.

Effects of micro-structure on fatigue crack propagation and acoustic emission behaviors in a micro-alloyed steel

The effects of the change in micro-structures due to heat treatment and weld on fatigue and corresponding acoustic emission (AE) behaviors of Q345 steels were investigated. Four point bending fatigue tests were carried out, and AE monitoring was performed simultaneously. Based on the micro-structural characterization, mechanical properties studies, fractographic observations, and AE investigations, the AE source mechanisms and effects of micro-structure on AE during fatigue crack propagation were proposed. The results showed that the micro-structures, including the phase conditions, inclusions and precipitates, could significantly influence the fatigue crack propagation behaviors, thus influence AE source mechanisms and characteristics such as AE counts, counts rate, and AE transition phenomenon. By relating the AE signals to the micro-plastic and micro-fracture events, these effects of micro-structures were also reasonably explained in this study. These results suggest that the effects of micro-structures must be considered to obtain a more accurate estimation of fatigue damage through AE method.

Acoustic emission source mechanism analysis and crack length prediction during fatigue crack propagation in 16Mn steel and welds

The acoustic emission (AE) signals during four-point bending fatigue crack propagation in the base metal and weld of 16Mn steel were monitored and analyzed. The fatigue properties and acoustic emission characteristics were analyzed based on the micro-structural and fractographic observations. The results show that: AE signal was divided into three stages, correspond with the three stages of fatigue damage process, which were crack initiation, stable crack extension and unstable crack extension. The corresponding dominated AE source were micro-crack initiation in the first stage, stacking and slipping of dislocations ahead of the crack tip in the intermediate stage, shearing of ligaments and connectivity between dimples in the final stage. Fatigue crack growth rate and AE count rate is greater in weld specimen than in the base material, mainly due to the coarse ferritic grain in widmanstatten structure and a large number of oxide inclusions.The fatigue crack length was predicted by BP neural network method and AE parameter fitting method respectively. The relative error of each specimen was less than 15%. The results showed that acoustic emission technology could be used for fatigue life prediction and safety assessment in 16Mn steel and welds.

Acoustic emission during fatigue crack propagation in a micro-alloyed steel and welds

► Different AE source mechanisms during fatigue in Q345 steel were proposed. ► AE behaviors of the weld were studied and compared with those of the base metal. ► The AE transition was compared with the linear elastic fracture mechanics. ► Effects of the micro-structure and load range on AE behaviors were discussed. The acoustic emission (AE) behaviors and source mechanisms during fatigue crack propagation in the base metal and weld of Q345 steel were investigated in this study. The fatigue properties and acoustic emission characteristics were analyzed based on the micro-structural and fractographic observations. The source mechanisms of acoustic emission for the three stages during fatigue were proposed, which were crack initiation, plastic activities ahead of the crack tip, and shearing of ligaments between micro-voids and micro-cracks respectively. The effects of the micro-structure and peak load on acoustic emission during fatigue crack propagation were also discussed in this study. The results showed that the acoustic emission was more sensitive to the changes in the fracture mode and could be used to monitor the fatigue damage developed in structures.

Effects of strain rate and notch on acoustic emission during the tensile deformation of a discontinuous yielding material

The acoustic emission (AE) generated during tensile tests performed on Q345 steel specimens was investigated in this study. Two typical types of AE signals were obtained during the test: Type A signals with burst type waveforms in the micro-yielding and work-hardening stages and Type B signals with continuous-type waveforms during the discontinuous yielding (Luders band propagation) stage. The effects of the strain rate and notch on these two types of signals were studied. The AE energy of Type B signals increased with increasing strain rate and was much higher in notched specimens compared to unnotched ones at high strain rates. However, for Type A signals, less AE energy was generated in notched specimens than in unnotched specimens, and it showed little sensitivity to changes in the strain rate. These results were fully analyzed by relating the AE behavior to the deformation properties and transmission electron microscope (TEM) observations of the materials. Thus, the AE source mechanisms and the effects of the strain rate and notch during plastic deformation were determined for discontinuous yielding materials.

Evidence of dilatant and non-dilatant damage processes in oolitic iron ore:P-wave velocity and acoustic emission analyses

Uniaxial and triaxial compression experiments were performed on oolitic iron ores to investigate damage processes. Most of these experiments included four indirect measurements of damage evolution, that is, P-wave velocity and maximum amplitude received during pulse transmission experiments, elastic properties (apparent Young´s modulus and apparent Poisson´s ratio) and acoustic emission (AE) monitoring. The mechanical behaviour deduced from strain measurements is dilatant for some samples and non-dilatant for the other samples. However, variations in elastic properties indicate damage processes for all samples. AE source mechanism analysis shows two different microscopic damage processes: (1) for dilatant rock, the development of axial extensive microcracks as well as their interaction and coalescence lead to the formation of shear macroscopic discontinuities; (2) for non-dilatant oolitic iron ore, both compressive and shear micromechanisms take place and interact with macroscopic fractures. A particular consistency between the four types of measurements employed was observed.

A study of the tribological behaviour of piston ring/cylinder liner interaction in diesel engines using acoustic emission

The novel use of non-intrusive acoustic emission (AE) measurements to provide information pertaining to the interaction between piston rings and cylinder liners in a range of diesel engines is investigated in this paper. In doing so, this technique is shown to offer a new method of investigation into this important interface in engine operation. AE generated during normal engine operation is known to consist of contributions from a number of different sources such as injector and valve activity. A recent finding has been the identification of AE signals associated with the ring/liner interface which presents the opportunity for in-service monitoring. This work discusses the possible AE source mechanisms, such as asperity contact, lubricant flow and/or blowby, through reference to a number of tests on motored and in-service small HSDI diesel engines and large, 2-stroke, marine diesel engines. The influence of various factors such as engine speed, load and lubrication is considered.

Kaiser effects in acoustic emission from composites during thermal cyclic-loading

Kaiser effects in acoustic emission (AE) behavior of composite laminates under repetitive thermal cyclic-loads are quantitatively analyzed to identify AE source mechanisms. The repetitive thermal loads brought about a large reduction, i.e. an exponential decrease, in AE total ring-down counts and AE amplitudes. It was thought that generation of most thermo-AE events during the first thermal cycle was not caused by crack propagation, but by secondary micro-fracturing due to abrasive contact between crack surfaces. For subsequent thermal cycles, on the other hand, a small number of weak thermo-AE events were generated due to frictional sliding contact. Such behavior of thermo-AE showed different characteristics according to specimen types and the maximum temperature in the thermal load cycles.

The acoustic emission source mechanism for fatigue crack propagation in 7075 aluminum: Journal of Acoustic Emission, Vol. 7, No. 4 pp. 225–230 (Oct.–Dec. 1988)

Le mecanisme d'emission acoustique resulte d'une plasticite localisee a l'extremite de fissures, et non de la rupture des inclusions comme montre auparavant. Une telle plasticite conduit l'extremite de la macrofissure a une inclusion rompue

A study of acoustic emission generated during orthogonal metal cutting—1: Energy analysis

The application of acoustic emission (AE) sensing in metal cutting process monitoring requires a knowledge of the signal dependence on the variables encountered in the process and an understanding of the source mechanisms responsible for AE generation. In this paper, we study the dependence of the AE signal energy on orthogonal machining variables such as cutting velocity, uncut chip thickness and the chip-tool contact length. Controlled contact length tools were used in orthogonal machining of tubular 6061-T6 aluminum, at varying cutting velocities and feed rates (the feed rate in this case is equal to the uncut chip thickness). The root mean square (RMS) value of the AE signal was found to be linearly proportional to the cutting velocity. Based on this observation, the damping of dislocation motions is proposed as a possible AE source mechanism at the high strain rates encountered in metal cutting. The validity of the dislocation damping based model for AE generation is supported by experimental results and observations.