Showing papers on "Acoustic emission published in 2011"

Experimental Study of the Brittle Behavior of Clay shale in Rapid Unconfined Compression

Abstract: The mechanical behavior of clay shales is of great interest in many branches of geo-engineering, including nuclear waste disposal, underground excavations, and deep well drilling Observations from test galleries (Mont Terri, Switzerland and Bure, France) in these materials have shown that the rock mass response near the excavation is associated with brittle failure processes combined with bedding parallel shearing To investigate the brittle failure characteristics of the Opalinus Clay recovered from the Mont Terri Underground Research Laboratory, a series of 19 unconfined uniaxial compression tests were performed utilizing servo-controlled testing procedures All specimens were tested at their natural water content with loading approximately normal to the bedding Acoustic emission (AE) measurements were utilized to help quantify stress levels associated with crack initiation and propagation The unconfined compression strength of the tested specimens averaged 69 MPa The crack initiation threshold occurred at approximately 30% of the rupture stress based on analyzing both the acoustic emission measurements and the stress–strain behavior The crack damage threshold showed large variability and occurred at approximately 70% of the rupture stress

Fiber-optic sensor applications in civil and geotechnical engineering

Abstract: Different types of fiber-optic sensors based on glass or polymeric fibers are used to evaluate material behavior or to monitor the integrity and long-term stability of load-bearing structure components. Fiber-optic sensors have been established as a new and innovative measurement technology in very different fields, such as material science, civil engineering, light-weight structures, geotechnical areas as well as chemical and high-voltage substations. Very often, mechanical quantities such as deformation, strain or vibration are requested. However, measurement of chemical quantities in materials and structure components, such as pH value in steel reinforced concrete members also provides information about the integrity of concrete structures. A special fiber-optic chemical sensor for monitoring the alkaline state (pH value) of the cementitious matrix in steel-reinforced concrete structures with the purpose of early detection of corrosion-initiating factors is described. The paper presents the use of several fiber-optic sensor technologies in engineering. One example concerns the use of highly resolving concrete-embeddable fiber Fabry-Perot acoustic emission (AE) sensors for the assessment of the bearing behaviour of large concrete piles in existing foundations or during and after its installation. Another example concerns fiber Bragg grating (FBG) sensors attached to anchor steels (micro piles) to measure the strain distribution in loaded soil anchors. Polymer optical fibers (POF) can be — because of their high elasticity and high ultimate strain — well integrated into textiles to monitor their deformation behaviour. Such “intelligent” textiles are capable of monitoring displacement of soil or slopes, critical mechanical deformation in geotechnical structures (dikes, dams, and embankments) as well as in masonry structures during and after earthquakes.

Probing shear-band initiation in metallic glasses.

Abstract: In situ acoustic emission monitoring is shown to capture the initiation of shear bands in metallic glasses. A model picture is inferred from stick-slip flow in granular media such that the origin of acoustic emission is attributed to a mechanism of structural dilatation. By employing a quantitative approach, the critical volume change associated with shear-band initiation in a metallic glass is estimated to be a few percent only. This result agrees with typical values of excess free volume found in the supercooled liquid regime near the glass transition temperature.

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.

Investigation of pencil-lead breaks as acoustic emission sources

Abstract: Pencil-lead breaks are widely used as a reproducible source for test signals in acoustic emission (AE) applications. Experimental and numerical studies are presented that focus on the differences in surface displacements obtained from pencil-lead breaks under different angles and free lead lengths. Experimentally, a setup for measurement of rupture forces during pencil-lead breaks was designed. The experiments were carried out with different lead diameters, different free lead lengths and under various angles with respect to an aluminum block. The measured forces were used as parameters for validation of finite element modeling of pencil-lead breakage. The model for the pencil-lead break source was developed using multi-scale modeling and dynamic boundary conditions. Various experimental conditions were simulated, and the resulting surface displacement and forces at the contact between lead and aluminum block were evaluated to obtain numerical source functions for pencil-lead breaks. A comparison is made between source functions of finite element simulation and analytical source functions as proposed in literature. The obtained source functions elucidate the difference in signal magnitudes for the various angles and lead lengths and yield insight in the microscopic processes during lead fracture.

Acoustic and Electromagnetic Emissions as Precursor Phenomena in Failure Processes

Abstract: In this work we measured the electromagnetic field, given by the moving charges, during laboratory fracture experiments on specimens made of different heterogeneous materials. We investigated the mechanical behaviour of concrete and rocks samples loaded up to their failure by the analysis of Acoustic Emission (AE) and Electromagnetic Emission (EME). All specimens were tested in compression at a constant displacement rate and monitored by piezoelectric (PZT) transducers for AE data acquisition. Simultaneous investigation of magnetic activity was performed by a measuring device calibrated according to metrological requirements. In all the considered cases, the presence of AE events has been always observed during the damage process, whereas it is very interesting to note that the magnetic signals were generally observed only in correspondence of the final collapse or sharp stress drops.

Damage and rupture dynamics at the brittle-ductile transition: The case of gypsum

Abstract: [1] Triaxial tests on gypsum polycrystal samples are performed at confining pressure (Pc) ranging from 2 to 95 MPa and temperatures up to 70°C. During the tests, stress, strain, elastic wave velocities, and acoustic emissions are recorded. At Pc ≤ 10 MPa, the macroscopic behavior is brittle, and above 20 MPa the macroscopic behavior becomes ductile. Ductile deformation is cataclastic, as shown by the continuous decrease of elastic wave velocities interpreted in terms of microcrack accumulation. Surprisingly, ductile deformation and strain hardening are also accompanied by small stress drops from 0.5 to 6 MPa in amplitude. Microstructural observations of the deformed samples suggest that each stress drop corresponds to the generation of a single shear band, formed by microcracks and kinked grains. At room temperature, the stress drops are not correlated to acoustic emssions (AEs). At 70°C, the stress drops are larger and systematically associated with a low-frequency AE (LFAE). Rupture velocities can be inferred from the LFAE high-frequency content and range from 50 to 200 m s−1. The LFAE amplitude also increases with increasing rupture speed and is not correlated with the amplitude of the macroscopic stress drops. LFAEs are thus attributed to dynamic propagation of shear bands. In Volterra gypsum, the result of the competition between microcracking and plasticity is counterintuitive: Dynamic instalibilities at 70°C may arise from the thermal activation of mineral kinking.

Damage analysis of reinforced concrete buildings by the acoustic emission technique

Abstract: This paper presents a research work in which the stability of a multi-storeyed building in reinforced concrete, with two visible macrocracks periodically subjected to visual inspection, is assessed by the acoustic emission (AE) technique. The observed proportionality between the rates of recorded AE activity from the cracks and the measured crack growth rates confirms significantly the effectiveness of the AE technique for damage evolution assessment in structural elements. The AE activity has been correlated with the size of the source crack advancements by some fitting relationships established using models from Damage Mechanics, Fracture Mechanics and Geophysics. Copyright © 2010 John Wiley & Sons, Ltd.

Experimental analysis of the through-thickness compression properties of z-pinned sandwich composites

Abstract: This paper presents an experimental investigation into the flat-wise compression properties, strengthening mechanisms and failure modes of sandwich composite materials reinforced with orthogonal z-pins. The compression modulus of the sandwich composite increases rapidly with the volume content of z-pins due to their high longitudinal stiffness, however acoustic emission monitoring and X-ray computed tomography reveal that some z-pins are damaged during elastic loading. The compression stress to induce core crushing is increased greatly by z-pinning (up to nearly 700%), although a large percentage of the z-pins fail close to the elastic stress limit by longitudinal splitting and/or kinking. The total absorbed compressive strain energy of the sandwich composite is also improved greatly by z-pinning (more than 600%) due to the z-pins resisting core crushing, even though they are severely damaged. The results and observations presented in this paper have implications on the mechanical modelling of sandwich materials reinforced with brittle z-pins.

Acoustic emission during fatigue crack propagation in a micro-alloyed steel and welds

Abstract: The acoustic emission (AE) behaviors and source mechanisms during fatigue crack propagation in the base metal and weld of Q345 steel were investigated in this study. The fatigue properties and acoustic emission characteristics were analyzed based on the micro-structural and fractographic observations. The source mechanisms of acoustic emission for the three stages during fatigue were proposed, which were crack initiation, plastic activities ahead of the crack tip, and shearing of ligaments between micro-voids and micro-cracks respectively. The effects of the micro-structure and peak load on acoustic emission during fatigue crack propagation were also discussed in this study. The results showed that the acoustic emission was more sensitive to the changes in the fracture mode and could be used to monitor the fatigue damage developed in structures.

Damage development in woven carbon fiber/epoxy composites modified with carbon nanotubes under tension in the bias direction

Abstract: The study investigates the effect of carbon nanotubes (CNTs) on the damage development in a woven carbon fiber/epoxy composite under quasi-static tension in the bias direction. The composite is produced by the resin transfer molding and contains 0.25 wt.% of CNTs in the matrix. The tensile tests are carried out till different strain levels and are accompanied with acoustic emission (AE) registration. The nano-modified composite possesses a higher stiffness and strain-to-failure. It also exhibits a significantly increased AE activity, both in terms of the number of events and the energy level, but reveals a lower crack density. The combined analysis of the AE data and X-ray images indicates that in the nano-modified composite cracks progress through the material in smaller jumps than in the virgin composite. The crack faces in the composite with CNTs also display a fine web of secondary fractures, which is not detected in the virgin composite.