Cumulative Acoustic Emission Counts Research Articles

A Global Method for the Identification of Failure Modes in Fiberglass Using Acoustic Emission

Abstract The various failure mechanisms in bidirectional glass/epoxy laminates loaded in tension are identified using acoustic emission (AE) analysis. AE data recorded during the tensile testing of a single layer specimen are used to identify matrix cracking and fiber failure, while delamination signals are characterized using a two-layer specimen with a pre-induced defect. Parametric studies using AE count rate and cumulative counts allowed damage discrimination at different levels of loading and Fuzzy C-means clustering associated with principal component analysis were used to discriminate between failure mechanisms. The two above methods led to AE waveform selection: On selected waveforms, Fast Fourier Transform (FFT) enabled calculating the frequency content of each damage mechanism. Continuous wavelet transform allowed identifying frequency range and time history for failure modes, whilst noise content associated with the different failure modes was calculated and removed by discrete wavelet transform. Short Time FFT finally highlighted the possible failure mechanism associated with each signal.

The relationship between acoustic emissions and wear particles for repeated dry rubbing

Acoustic emission (AE) is the emission of elastic stress waves resulting from the deformation and fracture of materials. The application of AE measurement techniques permits in-process monitoring of tribological characteristics. For practical applications, it is necessary to determine the relationship between AE signals and tribological phenomena, so we measured AE signals generated by tribological actions for repeated dry rubbings. The experimental results indicate that the features of AE parameters differ depending on the type of wear particles that are produced. The amplitude variation in the AE count rate is proportional to the surface damage at a friction interface. There is a good correlation between the AE cumulative count and the number of wear particles. In addition, there may be a negative correlation between the AE mean value and the elastic energy stored in wear particles between sliding surfaces. The generation and actions of wear particles between sliding surfaces has a marked effect on AE signals detected under repeated dry rubbing.

Fracture characterization of C/C composites under various stress modes by monitoring both mechanical and acoustic responses

C/C composites with different porosities, produced by chemical vapor infiltration have been mechanically tested under quasi-static loading in bending modes using uniform and notched specimens. The acoustic emission (AE) method was used to monitor the damage accumulation profile during loading up to fracture, supported by optical and scanning electron microscope characterization. Three stages in the damage buildup up to fracture were observed: Stage I, with no AE activity, Stage II, gradual growth in AE counts up to an abrupt jump and Stage III, sharp increases in AE counts. Moreover, the similarity in the profile between the cumulative AE counts vs. strain data and the predicted crack density vs. strain by the micro mechanical model suggested for interlaminar cracking, indicates the importance of AE in monitoring the damage evolution in composites in terms of AE counts. Fast Fourier transform analysis of the AE waves revealed three characteristic frequencies in Stage III, which is a sign of three main micro-mechanisms of failure which control the failure progress: fiber fracture, debonding and matrix cracking seem to be the active mechanisms.

Back propagation neural network prediction of failure strength of composite tensile specimens

In this research work, a feed forward back propagation neural network model was generated from acoustic emission cumulative counts data taken during loading of unidirectional carbon/epoxy tensile specimens. Only data collected up to 50% of ultimate load and its corresponding failure strength of 12 specimens were utilised for training the network out of 18 specimens tested, remaining six specimens data were utilised for testing the software. The network structured as 66-15-1 was able to give the prediction results with the worst case error of 4.98%. Fifteen neuron middle layer was mapping the pattern for failure of specimens well before from the AE data. This work indicates that it is possible to proof test the composites more sophisticated at lower loads (may be 50% of ultimate load) then are currently being tested (70% to 80% of ultimate load). Unintentional degradation of composite materials while proof testing could be thus minimised.

Acoustic emissions during fracture toughness tests of steels exhibiting varying ductility

This investigation is primarily aimed at examining steels with varying ductility using characteristics of acoustic emission (AE). Four steels (AISI 1060, AISI 1080, SA333 grade 6 and AISI 304LN) were selected and their structure property relations were characterized using standard metallographic examinations, hardness and tensile properties. Fracture toughness of these steels was determined as per the guidelines of ASTM standard E1820 with simultaneous recording of AE signals. The results of these investigations have been used to demonstrate that: (a) nature of the variation of AE cumulative counts with time is different for linear and non-linear load–displacement plots, (b) synergistic analysis of the rate of change of cumulative energy, cumulative counts and intensity of AE signals provide the point of crack initiation in a material, and (c) fracture toughness of a material estimated using AE parameters is lower compared to that obtained by ASTM standard procedure.

Strength of Quasi-Isotropic Co-Cured Composite Joints Under Quasi-static and Fatigue Loading

Abstract Three composite joint configurations, namely, the straight laminate (SL), the single lap, and the single nested overlap (SNO) joints are investigated to ascertain the improvement of the SNO joint over the single lap joint under quasi-static and cyclic loading. The SNO joint is a co-cured composite joint design proposed previously, while the SL represents a perfect joint and serves as an upper bound for static strength and fatigue life comparisons. Stress-number curves are generated based on constant amplitude tension-tension cyclic loading at a frequency of 5 Hz and stress ratio (R) of 0.1 for quasi-isotropic graphite/epoxy (IM7/8551-7) joints. The maximum fatigue load is varied between an average of 70-90 % UTS for SL, 20–80 % ultimate tensile strength (UTS) for single lap joints and 40–95 % UTS for SNO joints. Fatigue run-out is defined at 1×106 cycles. The fatigue strength improvement of SNO joint is ascertained quantitatively through two indicators. Three quasi-static acoustic emission (AE) count peaks were identified during monotonic loading before eventually reaching a maximum at ultimate failure. The cumulative AE count peaks are used as a collective measurement of the significant damage event incurred under specific loading and subsequently correlated to the fatigue performance of each of these three joint configurations.

Evaluation of the Twinning Behavior of Polycrystalline Magnesium at Room Temperature by Acoustic Emission

In order to study the twinning behavior of polycrystalline magnesium at room temperature, acoustic emission (AE) was measured during the compression process at an intermediate strain rate along the extrusion direction. Microstructure evolution was quantitatively characterized by the twinning area fraction and the strain dependence of twinning size distribution from the observation by optical microscope. Deformation was mainly due to twinning nucleation in the initial stage, and twinning growth and dislocation motions gradually became dominant with the increase of strain. As AE count rate increased greatly in the initial stage of deformation and dropped quickly in the later stage, AE signals in the initial stage were thought to be mainly due to the twinning nucleation. Twinning strain and the fraction of twinning strain rate were calculated by considering the variation of Schmid factor in deformation. A quantitative relation between the twinning strain and the cumulative AE counts was obtained in the initial stage of deformation. [doi:10.2320/matertrans.I-MRA2007849]

Deformation and Anelastic Recovery of Pure Magnesium and AZ31B Alloy Investigated by AE

Extruded pure magnesium and AZ31B alloy were compressed parallel and vertical to extrusion direction. Deformation behavior was investigated in monotonous compression by acoustic emission (AE). Anelastic recovery behavior was observed in cyclic compression-quick unloading-recovery process. AE method was applied in each recovery process to investigate detwinning behavior at different strain levels. It was found that twinned samples vertically compressed easily detwin compared to parallel samples in both pure magnesium and AZ31B alloy. Compared to pure magnesium, twinning in AZ31B alloy is more stable and shows weaker psuedoelasticity. A model of strain dependence on cumulative AE counts from detwinning was proposed and fitting results are in good agreement with experimental data.

AE Characteristics of Bending Fatigue Failure of Super-Carburized Spur Gear Teeth

This paper presents a study on acoustic emission (AE) in the bending fatigue process of super-carburized spur gear teeth. Research was carried out on AE signals, AE cumulative event count, AE event count rate, AE cumulative energy count, frequency spectra of AE signals and crack length in the bending fatigue process of eutectoid-carburized SCM415 steel, super-carburized SCM415 steel and MAC14 steel gear teeth. The relation between the AE parameters and the bending fatigue crack propagation as well as the effects of gear materials on the AE characteristics in the fatigue damaging process of the gear tooth were determined. Wavelet transforms (WT) of AE signals of these gear teeth were carried out, and then WT maps of AE signals were examined, Basic data were obtained for the prediction and diagnosis of bending fatigue failure of the supper-carburized gear tooth by means of AE method.

Detection of failures of adhesively bonded joints using the acoustic emission method

Abstract Wooden glued constructions require touch-free monitoring of destructive processes, especially in adhesive bonds that are most exposed to failure. The objective of the investigations was to describe failure processes in the adhesive bond of wood joints, in particular to determine their initiation, propagation, and destruction. The acoustic emission (AE) method was employed as the carrier of information about changes occurring in glued joints, whereas the numerical method was applied to determine values of distribution of tangential stresses generated in adhesive bonds. The acoustic phenomena examined were described using the AE cumulative counts. The authors analysed acoustic signals generated in loaded wooden and plastic overlap samples glued together using polyethyl methacrylate glue as well as in solid samples. On the basis of the acoustic emissions obtained, it was possible to establish characteristic places and stages of escalating structural defects generated from bonds of adhesive joints. This was utilised later on, in conjunction with results of numerical calculations, to determine correlations occurring between the AE cumulative counts and generated tangential stresses. Dependencies established in this way were used to determine characteristic points during the propagation of destructive phenomena of wood adhesive joints. The results obtained proved that it was possible to predict the development of the destruction of wood adhesive joints on the basis of observations of the increasing AE cumulative counts of acoustic signals in combination with tangential stresses determined using the finite elements method.

Acoustic Emission in Bending Fatigue Process of ADI Spur Gear Teeth

This paper presents a study on acoustic emission (AE) in the bending fatigue process of austempered ductile cast iron (ADI) gear teeth. Research was conducted on AE cumulative event count, AE event count rate, AE amplitude, frequency spectra of AE signals and crack length in the bending fatigue process of ADI and FC 250 spur gear teeth. The relationship between the AE parameters and the fatigue crack propagation was examined, and the AE characteristics in the bending fatigue process of the ADI and FC 250 spur gear teeth were determined. Basic data were obtained for the prediction of bending fatigue crack initiation of the ADI and FC 250 gear teeth by means of the AE method. Furthermore, the effect of gear material on the AE characteristics in the bending fatigue process of spur gear teeth was examined by comparing those of ADI and FC 250 spur gear teeth with those of the normalized and case-hardened spur gear teeth.

Effect of variation of strain rate on thermal and acoustic emission during tensile deformation of nuclear grade AISI type 316 stainless steel

A systematic study has been undertaken to correlate the changes in thermal and acoustic emissions during tensile deformation of AISI type 316 nuclear grade stainless steel (SS) due to variations in the strain rate. Strain rates were varied in the range 3.3 × 10-4s-1 to 1.7 × 10-2s-1. Thermal emissions were monitored using a focal plane array based thermal imaging system. For a given strain rate, the rate of increase in temperature was observed to be gradual and uniform in the work hardening zone, and to increases drastically during necking. With increasing strain rate, the temperature also increased. Based on the experimental results a constitutive equation can be modelled relating the rise in temperature to strain rate. In the case of acoustic emission (AE), the root mean square (RMS) voltage of the AE signal and cumulative counts increase with strain rate due to the increase in source activation. The peak amplitude distribution of AE hits has shown that hits with similar peak amplitude are generated for all strain rates.

Monitoring Acoustic Emissions from Finger-Joints from Tropical African Hardwoods for Predicting Ultimate Tensile Strength

Summary The patterns of acoustic emissions generated during tension test of finger-joints from three tropical African hardwoods, Obeche (Triplochiton scleroxylon), Makore (Tieghemella heckelii) and Moabi (Baillonella toxisperma) were evaluated to assess their potential usefulness for non-destructively predicting ultimate tensile strength. The acoustic emission patterns generated were observed to differ depending on the type of finger profile and the wood species. Regression coefficients from cumulative acoustic emission count versus applied stress squared functions also varied with the profile and species type. When ultimate tensile strength was correlated with these regression coefficients, for stresses applied up to 50% of mean ultimate strength, the logarithmic regression model developed could predict finger-joint strength accurate to ±12%, ±13% and ±18% for Obeche, Makore and Moabi, respectively. The model was also sensitive to the type of finger profile used for all three tropical African hardwoods. The results indicate that this acoustic emission monitoring procedure could be useful for non-destructively predicting ultimate tensile strength of finger-joints from the three tropical African hardwoods.

Mode I fracture and acoustic emission of softwood and hardwood

The Mode I fracture behaviour of two softwoods (spruce and pine) and three hardwoods (alder, oak and ash) was studied in the RL crack propagation system using the splitting test in combination with monitoring acoustic emission (AE) activity. Test parameters measured included notch tensile strength, specific fracture energy, characteristic length and AE cumulative counts, AE amplitudes as well as parameters characterizing the frequency spectra of the emitted acoustic emission events. The notch tensile strength was found to correlate with density. The specific fracture energy and characteristic length showed the different crack propagation process between the softwoods and hardwoods. The softwoods fractured in a more ductile way and the hardwoods showed a more linear elastic behaviour. This finding was supported by the AE measurements showing much less cumulative counts for the hardwoods indicating that less microcracks were formed and that processes like fiber bridging were not so effective. Differences in the frequency domain of the AE signals between softwoods and hardwoods could not be detected.

Non-destructive evaluation of plasma sprayed functionally graded thermal barrier coatings

Acoustic emission (AE) as a non-destructive evaluation technique has recently been used in a number of studies to investigate the performance and failure behavior of plasma sprayed thermal barrier coatings. The mechanism of coating failure is complex, especially when considering the composite nature of the coating. In the present paper, the thermal shock tests with in situ acoustic emission are used to study the cracking behavior of plasma sprayed functionally graded thermal barrier coatings. Each thermal cycle consists of 8 min heating in the furnace at 1000°C and 8 min cooling from 1000°C to the room temperature by a compressed air jet. The AE signals are recorded during the quench stage. Three, four and five layer functionally graded coatings have been tested. The results show that the five layer functionally graded coatings appear to have the best thermal shock resistance in the specimens tested, because of the gradual changes in material properties. Higher AE energy counts and cumulative counts recorded by the tests are associated with the macro-crack initiation and growth. On the other hand, micro cracking and phase transformation only give rise to lower AE signals.

A new method of modelling the rock micro-fracturing process in double-torsion experiments using neural networks

Microfracturing of rock is a complicated damage evolution process. Inaccurate prediction of micro-fracturing behaviours suggests a need for the development of a better modelling method. Analysis of acoustic emission (AE) measurements in double-torsion tests indicates that micro-fracturing behaviours during the loading stage have fractal time structures. This fractal behaviour can be described by C(t) ∝ tD, where D is the correlation exponent, t is the time and C(t) is the correlation integral. Furthermore, by utilizing measured AE data, a new method has been developed to model the AE behaviours of micro-fracturing in rock, in air, and following soaking in water and in a chemical solution of DTAB. The neutral models NN(10,21,2) and NN(10,20,2) were found to describe reasonably well the AE behaviours of micro-fracturing in rock under air and DTAB conditions, and water conditions, respectively. The cumulative AE events and the cumulative AE counts predicted by the neural models agreed well with those measured in experiments. Copyright © 1999 John Wiley & Sons, Ltd.

Acoustic Emission Monitoring of Hybrid FRP-Concrete Columns

Fiber-reinforced plastic tubes can be used as permanent formwork and external reinforcement for concrete columns. Failure of such columns is often accompanied by audible noise from cracking of concrete and shifting and settling of aggregates, snapping of the inner layers of the jacket, and ultimately fiber fracture. Since the state of the concrete core is not readily visible from the outside, nondestructive evaluation tools such as acoustic emission (AE) can help assess its structural integrity. The objective of this research was to investigate the applicability of AE technique to hybrid columns, and to correlate the AE parameters to the state of stress in concrete. The AE signature of hybrid columns was studied on over 40 specimens with different lengths, cross sections, jacket types, and jacket thicknesses. The study showed that (1) rate of change of cumulative AE counts with respect to the applied load correlates well with the degree of damage sustained by the concrete core; (2) longer specimens, thicker jackets, and unbonded tubes display higher AE activities; and (3) the Kaiser effect is not present in hybrid columns.

Acoustic Emission Study of Failure Mechanisms in Ceramic Matrix Composite under Longitudinal Tensile Loading

Acoustic emission (AE) measurements were correlated with macroscopic stress-strain behavior and direct microscopic observations of damage mechanisms in a unidirectional ceramic matrix composite under longitudinal tensile loading. Matrix crack initiation and the onset of AE events correspond to the proportional limit of the material. Both the crack density and cumulative AE counts vary linearly up to the point of crack saturation. Thereafter follows a "strain hardening" region corresponding mainly to fiber-matrix debonding and manifested by an increase of low to moderate amplitude AE activity and leveling off of high amplitude AE signals. At higher strains, beyond the "strain hardening" region, there are two characteristic parts in the AE output, one related to further damage (possibly debonding and fiber fracture) and the other clearly related to interfacial frictional sliding. Upon unloading further AE output related to sliding is observed. AE activity of low to intermediate amplitude continues in the last stages of loading and is believed to be associated with fiber fractures.

切欠き付シートモールディングコンパウンド複合材の破壊挙動

Effect of notch tip radius on fracture behavior of compact tension type specimens of sheet-molding-compound (SMC) composites was studied. Notch tip radii (ρ) were from 0.09mm to 2.1mm. Acoustic emission (AE) measurements and microscopic observations were carried out to examine damage formation procedure around the notch tip. Optical observations by a dye-penetration method exhibited that fracture patterns in the specimens of ρ≤0.56mm were quite different from those in case of ρ=2.1mm. For ρ≤0.56mm, a main-crack initiated at the notch tip and grew in parallel with generation of many sub-cracks surrounding the main-crack tip. For ρ=2.1mm, on the contrary, during main-crack growth, sub-cracks propagated in radial directions. With increasing notch tip radius, there were observed little increase in the maximum load, AE cumulative event counts and standard deviation of AE source locations. Strain energy at the maximum load slightly increased with increasing notch tip radius in a range 0.09mm≤ρ≤0.56mm. States of fiber strands at the notch tip affected parameters on strength more significantly than notch tip radii within ρ=2.1mm.

Acoustic Emission in Bending Fatigue Process of Spur Gear Teeth.

This paper presents a study on acoustic emission (AE) in the bending fatigue process of spur gear teeth. Research was conducted on AE signals, AE cumulative event count, AE event count rate, frequency spectra of AE signals and crack length in the bending fatigue process of case-hardened and normalized spur gear teeth. The relationship between the AE parameters and the fatigue crack propagation, as well as the effects of the gear material and the case depth on the AE characteristics in the fatigue damaging process of the gear tooth, was determined. Basic data were obtained for the prediction of bending fatigue crack initiation of the gear tooth by means of the AE method.