Acoustic emission

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

Ultrasonic tomography and acoustic emission in hydraulically fractured Lac du Bonnet Grey granite

Abstract: Ultrasonic tomography and acoustic emission (AE) data were obtained during laboratory hydraulic fracturing tests on two large, unconfined cylinders of Lac du Bonnet grey granite. The cylinders were internally pressurized over four to five cycles prior to final failure. Compressional velocities were measured before and after each pressurization cycle with an array of 16 evenly spaced transducers around the central, cross-sectional plane of each sample. Sixteen channels of whole waveform AE data were recorded during most pressurization cycles and for a period of about 1 hour after final failure in one sample. Compressional velocities were found to be strongly anisotropic, with the in situ vertical direction being the most rapid direction in both samples. The velocity anisotropy is related to the rock's preexisting microcrack fabric. Owing to radial penetration of fluid into the rock, compressional velocities rose over the course of the experiment. A regression analysis showed that the velocity changes can be explained by variations in crack density, inferred from initial velocities, and radial distance from the borehole. Saturation levels consistent with the observed velocity changes were calculated on the basis of the O'Connell and Budiansky theory. Acoustic emissions reoccurred in a few distinct zones over several pressurization cycles. The AE locations allowed two distinct fracture planes to be sharply delineated in one sample. The fracture plane orientations were controlled by the preexisting microcrack fabric in both samples. AE occurred too rapidly during peak pressure failure to permit us to isolate distinct events. Source mechanism analysis of the AE which occurred prior to peak pressure failure in both samples, and during postfailure monitoring in one sample, showed a predominance of double-couple sources. Compressive sources, thought to be related to crushing of asperities during crack closure, and tensile sources, related to mode I crack growth, were also recorded, as well as more complex sources that could not be modeled by the simple source types listed above.

Acoustic characterization of damage and healing of microencapsulation-based self-healing cement matrices

Abstract: Self-healing of cracks in cementitious composites is of great significance to improve the serviceability of concrete structures. In this study, poly urea–formaldehyde (PUF) microcapsules enclosing epoxy resins were synthesized. The damage- and healing process of cement paste incorporating microcapsules was in situ detected by acoustic emission (AE) technique, in which passive AE and active AE were combined to provide complementary information about the damage and crack formation. The two representative AE signals, i.e., matrix cracking and debonding of the interface, were used as calibration for further AE post-processing analysis. The effects of the concentration of microcapsules and the level of pre-damage were investigated. The results revealed the distinguished cracking mechanisms according to the differentiated feature of the signals in terms of the temporal and spectral AE descriptors. The plot of average frequency (AF) versus RA index (rise time/amplitude) confirms that the cracking modes contribute to the characteristic spectrum.

Uniaxial Strength Behavior of Brittle Cellular Materials

Abstract: Two types of brittle reticulated materials were evaluated under uniaxial tensile and compressive loading and analyzed in terms of the Gibson and Ashby model for brittle open-cell solids. The samples consisted of an open-cell alumina-mullite material which was tested as a function of density at a constant cell size and a reticulated vitreous carbon tested at one density and two cell sizes. The samples were mounted such that only the loading direction was varied in the tests. A combination of video photography and acoustic emission was critical to interpreting the results. The model assumes that identical deformation modes, bending failure of the struts, are responsible for failure of the bulk foam in tension and compression. The results of this work indicate a significant difference between the density dependence in tension and compression. Tensile failure in both materials appeared to be characterized by the catastrophic propagation of a single crack. Compressive failure was significantly different between the alumina and glassy carbon foams. The alumina foam failed by a damage accumulation process, whereas the carbon foam failed by the catastrophic collapse of a band of cells perpendicular to the loading direction.

High temperature characterisation of cordierite-mullite refractory by ultrasonic means

Abstract: Refractory materials containing cordierite (2MgO·2Al 2 O 3 ·5SiO 2 ) and mullite (3Al 2 O 3 ·2SiO 2 ) are used as support in furnaces, because of their low thermal expansion properties (coefficient of thermal expansion (CTE) ≈3–4 × 10 −6 K −1 ) which confer them a very good ability to thermal shock resistance. Composed of two phases presenting very different CTE (1.5–3 × 10 −6 for cordierite and 4–6 × 10 −6 K −1 for mullite), these materials can develop damage during thermal cycling due to internal stresses. This paper is devoted to the characterisation of the damage generated by this CTE mismatch, thanks to the application of ultrasonic techniques like long bar mode echography and acoustic emission (AE). The combination of these two techniques allows, during the applied thermal cycle (20–1200 °C), to continuously follow both the evolution of the elastic properties (Young's modulus) and the acoustic emission activity generated within the material. The analysis of these two characteristics, which are closely related to the damage evolution, makes it possible to propose a chronology of the mechanisms (damage, expansion) acting during the heating and the cooling stages.