Acoustic emission

About: Acoustic emission is a research topic. Over the lifetime, 16293 publications have been published within this topic receiving 211456 citations.

The role of inclusions in the fracture of ceramic materials

Abstract: The stress concentrations that occur at inclusions due to thermal expansion and elastic modulus mismatch are discussed and the stress intensity factors at interface cracks that result from these stresses are calculated. It is shown that conservative failure prediction based on an equivalence between inclusion size and crack size is usually acceptable if the shear modulusμ or thermal expansion coefficientα for the inclusion is larger than the matrix values. If, however,μ andα are smaller for the inclusion than the matrix, extensive cracking can develop at the inclusions which may lead to premature failure. For this case the only effective methods for failure prediction are techniques which give directly the maximum stress intensity factor, i.e., proof testing and/or acoustic emission.

Identification of factors that dominate size effect in micro-machining

Abstract: In micro-machining the so called “size effect” is identified as critical in defining process performance. Size effects refers to the phenomenon whereby the reduction of the undeformed chip thickness to levels below the cutting edge radius, or gain size of the workpiece material begins to influence workpiece material deformation mechanisms, chip formation and flow. However, there is no clear agreement on factors that drive this size effect phenomenon. To explore the significance of cutting variables on the size effect, micro-milling tests were conducted on Inconel 718 nickel alloy using 500 μm diameter carbide end mill. The experimental design was based on an L9 Taguchi orthogonal array. Fast Fourier transform (FFT) and wavelet transform (WT) were applied to acoustic emission (AE) signals to identify frequency/energy bands and hence size effect specific process mechanism. The dominant cutting parameters for size effect characteristics were determined by analysis of variance (ANOVA). These findings show that despite most literature focussing on chip thickness as the dominant parameter on size effect, the cutting velocity is also a dominant factor. This suggests that manipulating the cutting speed is also an effective strategy in reducing burr thickness, optimising surface finish and in breaking the lower limit of micro-machining.

Evidence for universal intermittent crystal plasticity from acoustic emission and high-resolution extensometry experiments

Abstract: Plasticity, a key property in the mechanical behavior and processing of crystalline solids, has been traditionally viewed as a smooth and homogeneous flow. However, using two experimental methods, acoustic emission and high-resolution extensometry, to probe the collective dislocation dynamics in various single crystals, we show that its intermittent critical-like character appears as a rule rather than an exception. Such intermittent, apparently scale-free plastic activity is observed in single-slip as well as multislip conditions and is not significantly influenced by forest hardening. Strain bursts resulting from dislocation avalanches are limited in size by a nontrivial finite size effect resulting from the lamellar character of avalanches. This cutoff explains why strain curves of macroscopic samples are smooth, whereas fluctuations of plastic activity are outstanding in submillimetric structures.

Influence of damage in the acoustic emission parameters

Abstract: Acoustic emission (AE) is a Non Destructive Inspection Technique, widely used for monitoring of structural condition of different materials like concrete, masonry and rock. It utilizes the transient elastic waves after each fracture occurrence, which are captured by sensors on the surface. Several parameters of the AE behavior enlighten the damage stage within the material. These may be the cumulative AE activity, which is connected to the density of cracks and the emission energy which is connected to the cracks’ intensity. Additionally, AE waveform parameters like duration and frequency content depend on the motion of the crack tip and therefore, carry information about the mode of the crack. Study of the AE indices enlightens the fracture process, enabling predictions on the remaining life. However, the experimental conditions crucially affect the waveforms captured by the sensors. Specimen size, as well as sensor type and sensors separation distance exercise strong influence in the acoustic emission parameters. Since AE features like amplitude and energy are used for characterization purposes in the framework of an energy density approach and frequency is used in cracking mode classification schemes, the influence of the above mentioned experimental parameters should be certainly taken into account in order to lead to more accurate results and increase reliability. This would help to expand the use of AE in situ which so far is hindered by geometric and other technical reasons that allow only a case-specific approach. In the present paper fracture experiments in different specimen sizes of cementitious and rock materials are described while the sensor location relatively to the cracking zone is altered. The aim of this study is to validate the use of cracking characterization in laboratory and check the extension for similar schemes in real size structures with a multiscale methodology.