Monitoring hydrogen embrittlement cracking using acoustic emission technique

Acoustic emission during delayed failure of hydrogen-charged low-alloy high-strength steel has been investigated. Tests were carried out at room temperature using standard ASTM three-point bend specimens. It was found that the cumulative acoustic emission counts rose slowly in discrete steps with increasing time in the initial stage of the embrittlement process, whereas it rose rapidly in the later stage prior to fracture. It was also observed that the initial embrittlement phase consisting of microcrack nucleation is characterized by low-amplitude (35–55 dB) signals only, whereas the rapid crack growth region is marked with high-amplitude (60–100 dB) signals. These observations indicate that such a change in the pattern of cumulative counts together with the level of amplitudes of the generated signals can be used to detect the so called incubation period for hydrogen embrittlement. This kind of early detection of critical cracks may help towards better fracture control.     

Thermal fracture studies in ceramic systems using an acoustic emission technique

An acoustic emission technique for measuring the failure time in thermal shock experiments is described. The technique offers a unique opportunity to measure the heat-transfer coefficient of the test system and hence, to obtain a fully quantitative measure of the peak surface stress generated during the test. Measurements on soda-lime glass have demonstrated that rapid thermal fracture in the material occurs when the surface stress attains a critical value equal to the propagation stress for the most deleterious surface flaw. The effects of slow crack growth on the failure time are also investigated and correlated with recently developed theory.     

Quantitative evaluation of fracture processes in concrete using signal-based acoustic emission techniques

Acoustic emission (AE) techniques can be used for the investigation of local damage in materials. Compared to other observation techniques one advantage is the recording of the damage process during the entire load history without any disturbance to the specimen. This is somehow unique and permits for high-resolution studies of the time-dependent failure of materials including precise determinations of the beginning of fractures and their consecutive progression. There are only a few other experimental techniques in fracture mechanics allowing for similar detailed observations of materials’ time and spatial behaviour. Moreover, these techniques allow a calibration of experiment and simulation comparing for example the output of finite element modelling with AE data analysis. This paper deals with some experimental results obtained during fracture mechanical experiments at concrete specimens demonstrating the capabilities of quantitative AE techniques.     

Electron emission and acoustic emission from the fracture of graphite/epoxy composites

In past studies it has been shown that the fracture of materials leads to the emission of a variety of species, including electrons, ions, neutral molecules, and photons, all encompassed by the term fracto-emission (FE). In this paper we examine electron emission (EE) from the fracture of single graphite fibres and neat epoxy resin. We also combine measurements of EE with the detection of acoustic emission (AE) during the testing of graphite-epoxy composite specimens with various fibre orientation. The characteristics of these signals are related to known failure mechanisms in fibre-reinforced plastics. This study suggests that by comparing data from AE and FE meausrements, one can detect and distinguish the onset of internal and external failure in composites. EE measurements are also shown to be sensitive to the locus of fracture in a composite material.     

Electron and acoustic emission accompanying oxide coating fracture

The behavior of an oxide coating during the tensile deformation of its substrate depends on the physical properties of the oxide and the oxide-substrate interface. These properties strongly influence the rate of appearance and extension of cracks in the oxide layer. We investigate the relationship between the growth of cracks in anodized Al 1350 and electron and acoustic emission. We present evidence that both types of emission are strongly influenced by the energy released during crack propagation.     

Statistical fracture of E-glass fibres using a bundle tensile test and acoustic emission monitoring

Mechanical strength studies have been carried out on fibre bundles used in composite manufacturing. The variability in mechanical properties of glass fibres has been studied using bundles of about 2000 filaments. The fibre strength distributions were analysed using the survival probability-applied strain (S–ε) curve, in relation with various experimental conditions. We also examine the effect of lubricant’s viscosity on the fracture behaviour of E-glass fibre bundles. Acoustic emission (AE) was monitored during the bundle tensile tests in order to verify that individual filament failures are statistically independent. On tensile tests with lubricated bundles of E-glass fibres, it is shown that each individual fibre break can be detected using AE. Hence, AE monitoring of a lubricated bundle of E-glass fibres provides a convenient and relatively quick method to obtain the Weibull parameters of strength distribution.     

On the acoustic emission due to the fracture of brittle inclusions

The far-field characteristics of the emission from a theoretical model for the fracture of brittle inclusions are presented in detail. The model is a circular crack growing at constant speed from zero size until it attains a prescribed size. The far-field radiation pattern is the same as that of a simple combination of force doublets, and some qualitative similarities between force doublets and acoustic dipoles are noted. The initial shape of the far-field pulses due to the growing stage and the stopping is determined, but difficulties arise in accounting for the diffraction of a surface wave on the crack faces generated by the stopping of the crack.     

Studies on indentation fracture toughness on ceramic and ceramic composite using acoustic emission technique

This paper is aimed at investigating the acoustic emission activities during indentation toughness tests on an alumina based wear resistant ceramic and 25 wt% silicon carbide whisker (SiC_w) reinforced alumina composite. It has been shown that the emitted acoustic emission signals characterize the crack growth during loading and unloading cycles in an indentation test. The acoustic emission results indicate that in the case of the composite the amount of crack growth during unloading is higher than that of loading, while the reverse is true in case of the wear resistant ceramics. Acoustic emission activity observed in wear resistant ceramic is less than that in the case of composite. An attempt has been made to correlate the acoustic emission signals with crack growth during indentation test.     

Tool health monitoring using acoustic emission

The concept of CIM and FMS has created a whole new area of what is called ‘tool management system’ (TMS). For efficient operation of TMS, a real-time monitoring of tool condition is desirable. Tool health monitoring is critical in today's automated production and is likely to become crucial as we progress towards the unmanned factory of the future. The paper describes the development of a microcomputer-based acoustic emission (AE) system for monitoring progressive wear of a cutting too!. It also includes results of experiments conducted with the system. The state-of-the-art in AE technique and its comparison with other techniques for tool condition monitoring are also presented.     

A study of the fracture of glasses,pyroceramics and their nanojoints using the method of acoustic emission

The optical systems operating under great ambient temperature drops use materials with special properties, e.g., with invariable geometric characteristics, such as Zerodur glass ceramic with zero coefficient of thermal expansion. Strong precision joints of parts are necessary elements of systems. Fracture processes in loaded materials are studied by the method of acoustic emission, which allows one to record the instant of crack initiation and to study crack development in a brittle material during loading. The evaluation of the strength of brittle optical materials and their nanojoints by the method of acoustic emission has been shown to have good prospects. It has been found that the number of acoustic emission pulses depends on specimen strength. In tests of nanojoints of pyroceramics, two characteristic fracture modes are possible: with crack propagation into the specimen material and without crack propagation beyond the seam limits. The method of acoustic emission assures reliable recording of these fracture modes. __________ Translated from Problemy Prochnosti, No. 1, pp. 94–99, January–February, 2007.     

Analysis of the bending fracture process for piezoelectric composite actuators using dominant frequency bands by acoustic emission

We examined the fracture process of a thin monolithic PZT and a plate-type piezoelectric composite actuator (PCA) subjected to a three-point bending load with the aid of an acoustic emission (AE) monitoring. Fracture surface observations were conducted using a scanning electron microscope (SEM) and optical microscope. AE signals were analyzed during the bending fracture process of a PCA in terms of the dominant frequency band which was processed by a fast Fourier transform (FFT) along with the behavior of AE amplitude. Three dimensional finite element simulations were performed to investigate the stress state in each layer of PCAs. It was concluded that the damage of a PCA under a bending load was initiated in the brittle PZT layer, which induced the interlaminar delamination between PZT layer and adjacent fiber composite layers. As a bending load approached maximum, fiber breakages hindering the main crack propagation in the glass–epoxy bottom layer together with macro-delamination between the PZT and fiber composite layers led to the final failure of PCAs. The fracture process and mechanisms of PCAs were successfully identified by AE characteristics based on the analysis of dominant frequency bands.     

The fracture energy and acoustic emission of a boron-epoxy composite

The fracture surface energy () of a boron fibre-epoxy resin composite has been measured by three different techniques: work of fracture, linear elastic fracture mechanics, and compliance variation. Significant differences were obtained by the different methods. The compliance data were analysed to give at different stages of crack propagation. It was observed that decreased as the crack entered the material and that this variation of could be correlated with the pull-out length of fibres and acoustic emission generated during fracture. The fracture surface energy is explained in terms of a debonding model.     

Monitoring mechanical damage in structural materials using complimentary NDE techniques based on thermography and acoustic emission

This work deals with nondestructive evaluation (NDE) of the fracture behavior of metallic materials by combining thermographic and acoustic emission (AE) characterization. A new procedure, based on lock-in infrared (IR) thermography, was developed to determine the crack growth rate using thermographic mapping of the material undergoing fatigue. The thermography results on crack growth rate were found to be in agreement with measurements obtained by the conventional compliance method. Furthermore, acoustic emission was used to record different cracking events. The rate of incoming signals, as well as qualitative features based on the waveform shape, was correlated with macroscopically measured mechanical parameters, such as load and crack propagation rate. Additionally, since the failure modes have distinct AE signatures, the dominant active fracture mode was identified in real time. The application of combined NDE techniques is discussed for characterizing the damage process which leads to catastrophic failure of the material, thereby enabling life prediction in both monolithic aluminum alloys and aluminum alloy/SiC particle (SiC_p) reinforced composites.     

Mode II wood fracture characterization using the ELS test

This paper describes experimental and numerical studies on the application of the end loaded split test to mode II wood fracture characterization. A new data reduction scheme, based on the specimen compliance and on the equivalent crack concept, is proposed. The method presents three main advantages relatively to the classical methodologies: it does not require crack measurement during propagation; it accounts for the root rotation at the clamping point and includes the effect of the fracture process zone at the crack tip. The new procedure was numerically validated using a two-dimensional finite element analysis including a cohesive damage model, which allows the simulation of crack initiation and growth. The results demonstrated the good performance of the model and the applicability of the end loaded split test for mode II wood fracture characterization.     

Testing E-glass fibre bundles using acoustic emission

In tensile tests on lubricated bundles of a few hundred parallel E-glass fibres it is shown that individual fibre breaks, to the last fibre in the bundle, can be detected using acoustic emission (AE). By this means the single-fibre strength distribution is deduced. Relationships are obtained between some AE signal parameters and the fibre fracture stress which are consistent with theoretical expectations. Studies are made of the distribution of fibre break locations, the occurrences of multiple (stimulated) fibre breaks and the attenuation of the AE signals.     

Detection of fibre fracture and ply fragmentation in thin-ply UD carbon/glass hybrid laminates using acoustic emission

This paper investigates the link between acoustic emission (AE) events and the corresponding damage modes in thin-ply UD carbon/glass hybrid laminates under tensile loading. A novel configuration was investigated which has not previously been studied by AE, where the laminates were fabricated by embedding thin carbon plies between standard thickness translucent glass plies to produce progressive fragmentation of the carbon layer and delamination of the carbon/glass interface. A criterion based on amplitude and energy of the AE event values was established to identify the fragmentation failure mode. Since the glass layer was translucent, it was possible to quantitatively correlate the observed fragmentation during the tests and the AE events with high amplitude and energy values. This new method can be used as a simple and advanced tool to identify fibre fracture as well as estimate the number and sequence of damage events that are not visible e.g. in hybrid laminates with thick or non-transparent layers as well as when the damage is too small to be visually detected.     

Tool wear monitoring using acoustic emission in the existence of chatter

A method using an acoustic emission (AE) technique to monitor automatically a tool wear was examined. A flank wear is calculated using a statistical model which uses RMS value of AE signal and cutting speed as variables. The investigation reveals that the AE signal is influenced by tool vibration, specially during chatter. This contradicts the theory that the AE signal is not susceptible to mechanical vibration. The paper discusses the mechanism that explains the effect of the tool vibration on the AE signals.     

The temperature dependence of the fracture toughness and acoustic emission of polycrystalline alumina

The technique of acoustic emission has been shown to be suitable for the monitoring of fracture-toughness tests over a range of temperatures. Commercial polycrystalline alumina has been tested at temperatures up to 1000° C to determine the effect of microstructure and impurity content on fracture toughness and acoustic emission. For a given alumina there was no significant variation in acoustic response or fracture toughness up to 650° C. The emissions observed prior to fracture in this temperature range were attributed to subcritical crack growth. The number of emissions depended on the amount of subcritical crack growth, the grain size, and the presence and amount of porosity. Above 650° C the fracture behaviour changed due to the flow of a grain-boundary glassy phase. This was associated with a peak in the temperature dependence of the apparentK _IC and was accompanied by a large number of acoustic events of low amplitude and low pulse width. At these elevated temperatures the extent of grain-boundary glassy flow, and hence the acoustic response, increased with decreasing grain size and increasing impurity content.