Acoustic Emission Measurements Research Articles

Study of Oil Starvation in Journal Bearing Using Acoustic Emission and Vibration Measurement Techniques

Abstract This present article evaluates the state of starvation in a journal bearing using acoustic emission (AE) and vibration measurement techniques. A journal bearing requires a constant supply of oil in an adequate amount to develop a hydrodynamic film, thick enough to separate the surfaces and avoid asperity contacts. On a microscopic level, the surface interaction under starved lubrication results in deformation and fracture of asperities. This causes a proportionate increase in AE and vibration. The AE activities resulting from asperities interaction have significant energy in the frequency range of 100–400 kHz with peak frequencies in the range of 224–283 kHz. Further, the peak frequency shifts from the higher to lower side as the asperity interaction transits from the elastic to plastic contact. This information derived from the spectral analysis of AE signals can be used to develop condition monitoring parameters to proactively control the lubrication and prevent bearing failure.

Effect of CO2/N2 Dilution on Characteristics of Liquid Fuel Combustion in Flameless Combustion Mode

ABSTRACT This paper presents the effect of CO2 and N2 dilution in the oxidizer stream on combustion and emission characteristics in a flameless combustor operating with kerosene and biodiesel as a fuel. The thermal input is varied from 30 kW (7.5 MW/m3) – 53 kW (13.3 MW/m3) and inlet air temperature are varied from 300 K to 800 K. CO2/N2 are added to dilute the oxidizer stream in a proportion of 15% of total air volume to reduce the O2 concentration in the air stream. Reacting and non-reacting flow simulations show that the flow field improves significantly as both the flow velocity and temperature distributions become uniform. Distribution of the reactant dilution ratio becomes more uniform and a maximum value of Rdil = 4.89 is achieved for 30 kW conditions. The reduced O2 concentration in the oxidizer results in enhanced dilution of fresh air-fuel mixture thereby enables uniform temperature and flow field. For the air preheating case, CO2 dilution does not affect the velocity and thermal fields significantly. The measured CO emissions are marginally increased due to CO2 dilution; however, NOx emissions are significantly reduced. N2 dilution results in increased levels of NOx, contrary to the perception of the inert role of N2 addition even though the combustor operating temperatures are lower than 1800 K. The measurements of acoustic pressure emissions show a sharp decrease during flameless combustion mode.

Crack behaviour and mechanical properties of thermally treated kaolin based ceramics: The influence of pore generating agents

In this study, the crack behaviour of porous kaolin based ceramics was experimentally investigated. The samples (ceramic bodies) were fabricated with the inclusion of pore formers in determined proportions and subjected to heat treatment. Next, the apparent porosity of the samples was measured using the water immersion method. The values confirmed the increased porosity (up to 47%) for the samples with embedded pores. We speculated that the open pores on the surface of the samples which were quite evident, especially with styrofoam as pore formers, may also have penetrated through, hence the enhanced porosity values for the samples. Acoustic emission (AE) activity which reveals the formation of microcracks in the ceramics due to the different thermal expansion coefficients in the cooling stage was recorded for the samples. The first signals of AE counts appeared at 800 °C, where the compressive stresses between the different phases (particles with different coefficients of thermal expansion) led to an appearance of microcracks. By introducing porosity to the samples, the AE signals were less pronounced. This was evident for the samples with sawdust as pore formers, and it was inferred that in this sample, microcracking was suppressed. As a validation protocol for AE measurements, mechanical measurements on the produced samples were conducted through the indentation technique to obtain the fracture toughness of the samples. The results conformed to the observation made during AE measurements. The samples embedded with sawdust as porogens produced the highest fracture toughness of 4.77 MPa.m1/2 by reason of the suppression of microcracking after heat treatment.

Mechanical behavior of hydroxyapatite-poly(lactic acid) hybrid porous scaffold

Although autologous bone is currently used for repairing bone defects, this method is highly invasive and poses a heavy burden on patients. Thus, the use of artificial bones has been expected. Among the artificial bone materials, bioactive hydroxyapatite (HA) has been attracting much attention. In this research, in order to overcome the problem such as low initial strength of artificial bone after implantation, poly(lactic acid) (PLA) was coated on HA unidirectional porous body prepared by freeze-casting method. In order to investigate the PLA coating method that most effectively improves the mechanical properties, the coating state of PLA was observed and the compression test was performed. As a result of the compression test of the HA-PLA composite porous body, Young’s modulus and compressive strength were improved by 79% and 184% respectively at maximum. Furthermore, acoustic emission (AE) measurement was conducted to investigate the fracture behavior during the compression test, classifying the main fracture of HA and HA-PLA porous body as shear fracture.

Microscale characterization of damage accumulation in CMCs

The developing roles of damage mechanisms in the failure response of SiC/SiC minicomposites was investigated by the characterization of microscale damage accumulation with respect to microstructure. A multi-modal approach combining spatially resolved acoustic emission (AE) with tensile testing in-SEM (scanning electron microscope) was used to simultaneously examine surface (observed in-SEM) and bulk damage (monitored via AE). Strong agreement was shown between the evolving crack density estimated by AE and in-SEM measurements. The following were observed: (i) in-plane matrix content and distribution impacted crack growth; (ii) spatially-distributed matrix cracks generated varying stress-dependent AE; and (iii) certain individual cracks became more probable failure locations due to unique combinations of damage mechanisms that drove their growth. This approach enabled characterizing potential failure determinants and suggests that early damage behavior is related to certain microstructural features (e.g. surface flaws), while subsequent damage behavior is coupled to interactions of local mechanisms evolving with stress.

Occlusion of the lumbar spine canal during high-rate axial compression

BACKGROUND CONTEXTWhile burst fracture is a well-known cause of spinal canal occlusion with dynamic, axial spinal compression, it is unclear how such loading mechanisms might cause occlusion without fracture. PURPOSETo determine how spinal canal occlusion during dynamic compression of the lumbar spine is differentially caused by fracture or mechanisms without fracture and to examine the influence of spinal level on occlusion. STUDY DESIGNA cadaveric biomechanical study. METHODSTwenty sets of three-vertebrae specimens from all spinal levels between T12 and S1 were subjected to dynamic compression using a hydraulic loading apparatus up to a peak velocity between 0.1 and 0.9 m/s. The presence of canal occlusion was measured optically with a high-speed camera. This was repeated with incremental increases of 4% compressive strain until a vertebral fracture was detected using acoustic emission measurements and computed tomographic imaging. RESULTSFor axial compression without fracture, the peak occlusion (Omax) was 29.9±10.0%, which was deduced to be the result of posterior bulging of the intervertebral disc into the spinal canal. Omax correlated significantly with lumbar spinal level (p<.001), the compressive displacement (p<.001) and the cross-sectional area of the vertebra (p=.031). CONCLUSIONSSpinal canal occlusion observed without vertebral fracture involves intervertebral disc bulging. The lower lumbar spine tended to be more severely occluded than more proximal levels. CLINICAL SIGNIFICANCEClinically, intermittent canal occlusion from disc bulging during dynamic compression may not show any radiographic features. The lower lumbar spine should be a focus of injury prevention intervention in cases of high-rate axial compression.

Influence of Frequency and Distance on Acoustic Emission Velocity Propagating in Various Dielectrics

The results obtained for the measurements of acoustic emission (AE) signal parameters emitted in three dielectric liquids are presented in this paper. In particular, the velocity of AE wave was calculated based on the time of arrival of the AE signal. A frequency modulated signal was generated by a piezoelectric transducer and measured at various distances by a hydrophone. The changes in velocity values at particular distances and for different frequencies were investigated. The analyses include the determination of the dependency of the velocity values from the distance between the communicating devices. A nonlinear regression model was calculated, and the differences between AE velocities propagating in the considered dielectrics were determined. Similarly, the influence of modulation frequency on the AE velocity was determined using nonlinear regression. Based on the calculation data, it can clearly be stated that the velocity of AE wave depends significantly on the frequency and distance at which it is registered. These two factors may have an important influence on the localization of partial discharges (PD) occurring in these types of dielectric liquids.

Algorithm development for acoustic emission measurement in high-frequency ranges and its application on a large two-stroke marine diesel engine

Abstract In this paper, an advance measuring algorithm was designed to measure acoustic emission (AE) signals in a high-frequency range based on a C-Sharp programming language for a large two-stroke marine diesel engine. To verify the ability of this algorithm, an experiment was carried out to investigate the characteristics of the AE signal caused by the combustion process on the large two-stroke low-speed marine diesel engine in the frequency range 100–900 kHz. The results showed that the AE signals emitted from the combustion event at around the top dead center could be well-observed in this frequency range. These outcomes also proved that the AE signal could not propagate to other cylinders due to a big distance between the cylinders. However, the combustion process affected the remaining AE sources. Moreover, this technique could be applied to detect the firing state as well as the combustion time of each cylinder.

Modeling the Local Deformation and Transformation Behavior of Cast X8CrMnNi16-6-6 TRIP Steel and 10% Mg-PSZ Composite Using a Continuum Mechanics-Based Crystal Plasticity Model

A Transformation-Induced Plasticity (TRIP) steel matrix reinforced with magnesium-partially stabilized zirconia (Mg-PSZ) particles depicts a superior energy absorbing capacity during deformation. In this research, the TRIP/TWIP material model already developed in the framework of the Düsseldorf Advanced Material Simulation Kit (DAMASK) is tuned for X8CrMnNi16-6-6 TRIP steel and 10% Mg-PSZ composite. A new method is explained to more accurately tune this material model by comparing the stress/strain, transformation, twinning, and dislocation glide obtained from simulations with respective experimental acoustic emission measurements. The optimized model with slight modification is assigned to the steel matrix in 10% Mg-PSZ composite material. In the simulation model, zirconia particles are assigned elastic properties with a perfect ceramic/matrix interface. Local deformation, transformation, and the twinning behavior of the steel matrix due to quasi-static tensile load were analyzed. The comparison of the simulation results with acoustic emission data shows good correlation and helps correlate acoustic events with physical attributes. The tuned material models are used to run full phase simulations using 2D Electron Backscatter Diffraction (EBSD) data from steel and 10% Mg-PSZ zirconia composites. Form these simulations, dislocation glide, martensitic transformation, stress evolution, and dislocation pinning in different stages of deformation are qualitatively discussed for the steel matrix and ceramic inclusions.

Microscopic Damage Characterization on L-shaped CFRP Laminates for Lower Limb Orthosis with Acoustic Emission Measurement

Microscopic Damage Characterization on L-shaped CFRP Laminates for Lower Limb Orthosis with Acoustic Emission Measurement

Acoustic emission characteristics of a dry sandy soil subjected to drained triaxial compression

Acoustic emission (AE) technique that is capable of diagnosing the failure process of stressed materials has rarely reported its application to sandy soils subjected to triaxial compression. In this paper, drained triaxial compression tests incorporating with a high-performance AE measurement system were conducted for dry sands with different confining stresses and initial relative densities. Generally, an increased confining stress or initial relative density generates more acoustic emissions, while there also exist exceptions due to different failure patterns. A good resemblance between stress–strain and AE hit rate–strain relations was observed, and power functions between the mechanical parameters and AE hit rate were well established regardless of different confining stresses and initial relative densities. Besides, the behavioral state of yield and peak during compression could be also evaluated by AE hit rate, compared with conventional stress–strain determination. Particularly, the peak AE hit rate is found not always synchronous to but fluctuating at around the peak stress depending on different failure patterns, which might provide beneficial insights into the incipient failure of sands. The present good consistencies suggest that AE characteristics could be used as alternative parameters to evaluate and even predict the mechanical behavior of dry sands.

Relation between the shear stress distribution and the resulting acoustic emission variation in concrete beams

Acoustic emission (AE) technology is extensively acknowledged as a promising structural health monitoring (SHM) approach; meanwhile, AE monitoring has long been criticised for a lack of quantitative results. This work revealed a strong correlation between the shear stress distribution and the resulting AE distribution, enabling accurate and real-time quantification of the shear stress states in concrete beams using only AE measurements. This paper started by introducing the AE phenomenon and the theoretical distribution of the shear stresses on a beam cross section, which revealed that the distributions of the stresses and the resulting emissions inherently correlate to each other. Twelve reinforced concrete beams were then tested, deploying devices including an AE system. The results showed that the AE source intensity distribution strongly correlated with that of the shear stresses in a beam segment when the beam was in both the working stage and the failure phase and that the matching degree of these two distribution curves varied from 79.20% to 94.12%. The correlation revealed in the paper will boost a wide range of significant AE-based scientific research and engineering applications, including SHM and non-destructive testing.

Effect of compatibilizing agent on the fiber-matrix adhesion and mechanical properties of lignocellulose fiber reinforced polyolefin

Lignocellulose fiber (LCF)-reinforced high-density polyethylene (HDPE) or polypropylene (PP) composites were prepared with a twin-screw extruder using a maleic anhydride-modified polyolefin as a compatibilizing agent. The effect of a compatibilizing agent on the mechanical properties of the composites was investigated. Acoustic emission (AE) measurements were conducted during tensile testing to evaluate the micro-fracture process of the composites. The mechanical properties of the LCF-polyolefin composites were greatly increased by the addition of a compatibilizing agent. In particular, the tensile strengths for the 20 mass% LCF/HDPE and 30 mass% LCF/PP composites were improved by 30% and 57%, respectively, when a compatibilizing agent was used. Differences in the AE behaviors of composites with and without a compatibilizing agent indicated that the use of a compatibilizing agent changed the fracture mode of the composites from interfacial fracture to CF fracture.

Reinforcement of PP with polymer fibers: Effect of matrix characteristics, fiber type and interfacial adhesion

Two polypropylene polymers with considerably differing properties were modified with poly(ethylene terephthalate) (PET) and poly(vinyl alcohol) (PVA) fibers. Fiber content changed from 0 to 60 vol% and interfacial adhesion was modified with the addition of a maleated PP coupling agent. Mechanical properties were characterized by tensile and impact testing, while structure was evaluated by acoustic emission measurements and scanning electron microscopy. Attention was focused on the balance of stiffness and impact resistance and on the relationship of local deformation processes to macroscopic properties. The results showed that different inherent matrix properties result in a dissimilar combination of properties. The strength of interfacial adhesion together with matrix properties determines the extent of reinforcement and the direction in the change of properties as a function of composition. Adhesion influences local deformation processes very strongly and these processes determine the macroscopic properties of the composites. Shear yielding induced by the debonding of fibers consumes much energy, while fiber fracture is less efficient in increasing impact resistance. Fiber characteristics are important for modulus and strength, but less significant for impact resistance. A good combination of stiffness and impact resistance can be obtained in PP homopolymers modified with PVA fibers, while this type of modification is less efficient and/or advantageous in elastomer modified PP.

Predicting 3D heterogeneous in situ stress field of Gaoshangpu Oilfield northern area, Nanpu Sag, Bohai Bay Basin, China

Research on in situ stress has important theoretical and practical significance for the exploration and development of oil and gas reservoirs. The orientation and magnitude of in situ stress in the Gaoshangpu Oilfield northern area (GO-NA) were analyzed using borehole breakout data and acoustic emission measurements. Mechanical experiments, logging interpretation, and seismic data enabled spatial characterization of rock mechanics parameters. A 3D geological model and 3D heterogeneous rock mechanics field of the GO-NA were constructed. Petrel and ANSYS modeling provided detailed prediction of the 3D stress field in the GO-NA. The results indicate that the maximum horizontal stress orientation in the GO-NA is generally ENE–WSW-trending, with significant changes in in situ stress orientation within and between fault blocks. Along surfaces and profiles, stress magnitudes are discrete and in situ stress is of the Ia-type. Observed inter-strata differences were characterized by five different types of in situ stress profile. Faults are the most important factor in the large distributional differences in the stress field of reservoirs observed within the complex fault blocks, significantly affecting magnitudes and orientations in the stress field. The next most important influence on the stress field is the reservoir’s rock mechanics parameters, which affect in situ stress magnitudes. A strong linear correlation exists between reservoir depth and in situ stress magnitude. This technique provides a theoretical basis for more efficient exploration and development of low-permeability reservoirs. It also serves as a reference for the detailed prediction of inter-well in situ stress in regions with similarly complex fault blocks.

Use of nearest neighbors (k-NN) algorithm in tool condition identification in the case of drilling in melamine faced particleboard

The purpose of this study was to develop an automatic indirect (non-invasive) system to identify the condition of drill bits on the basis of the measurement of feed force, cutting torque, jig vibrations, acoustic emission and noise which were all generated during machining. The k-nearest neighbors algorithm classifier (k-NN) was used. All data analyses were carried out in MATLAB (MathWorks – USA) environment. It was assumed that the most simple (but sufficiently effective in practice) tool condition identification system should be able to recognize (in an automatic way) 3 different states of the tool, which were conventionally defined as “Green” (tool can still be used), “Red” (tool change is necessary) and “Yellow” (intermediate, warning state). The overall accuracy of classification was 76 % what can be considered a satisfactory result at this stage of studies.

Hidden Markov model based rotate vector reducer fault detection using acoustic emissions

This paper proposes a hidden Markov model (HMM) based RV reducer fault detection using acoustic emission (AE) measurements. Compared with the conventional faults from the common rotating machinery (such as bearings and gears), faults from RV reducer are more complicated and undetectable due to its inherent inline and two-stage meshing structure. To this end, this work modifies the HMM model by taking into account not only the current observations and previous states, but the subsequent series of observations within posteriori probability framework. Through this way, the random and unknown disturbance could be suppressed. Besides, HMM is also applied to separate AE signal bulks within one cycle that has 39 subcycles. The proposed method has been evaluated on our collected AE signal dataset from the RV reducer in the industrial robotic platform. The experimental results and analysis validate the effectiveness and accuracy of our RV reducer fault detection model.

Hidden Markov model based rotate vector reducer fault detection using acoustic emissions

This paper proposes a hidden Markov model (HMM) based RV reducer fault detection using acoustic emission (AE) measurements. Compared with the conventional faults from the common rotating machinery (such as bearings and gears), faults from RV reducer are more complicated and undetectable due to its inherent inline and two-stage meshing structure. To this end, this work modifies the HMM model by taking into account not only the current observations and previous states, but the subsequent series of observations within posteriori probability framework. Through this way, the random and unknown disturbance could be suppressed. Besides, HMM is also applied to separate AE signal bulks within one cycle that has 39 subcycles. The proposed method has been evaluated on our collected AE signal dataset from the RV reducer in the industrial robotic platform. The experimental results and analysis validate the effectiveness and accuracy of our RV reducer fault detection model.

Acoustic process monitoring in laser beam welding

Structure-borne acoustic emission (AE) measurement shows major advantages regarding quality assurance and process control in industrial applications. In this paper, laser beam welding of steel and aluminum was carried out under varying process parameters (welding speed, focal position) in order to provide data by means of structure-borne AE and simultaneously high-speed video recordings. The analysis is based on conventionally (e.g. filtering, autocorrelation, spectrograms) as well as machine learning methods (convolutional neural nets) and showed promising results with respect to the use of structure-borne AE for process monitoring using the example of spatter formation.

Avalanches from charged domain wall motion in BaTiO3 during ferroelectric switching

We report two methods for direct observations of avalanches in ferroelectric materials during the motion of domain walls. In the first method, we use optical imaging techniques to derive changes in domain structures under an electric field. All changes occur through small jumps (jerks) that obey avalanche statistics. In the second method, we analyze jerks by their displacement current. Both methods reveal a power law distribution with an energy exponent of 1.6, in agreement with previous acoustic emission measurements, and integrated mean field theory. This new combination of methods allows us to probe both polarization and strain variations during the motion of domain walls and can be used for a much wider class of ferroelectrics, including ceramic samples, than acoustic emission.