Creep of CarbFix basalt: influence of rock–fluid interaction

Geological carbon sequestration provides permanent CO2 storage to mitigate the current high concentration of CO2 in the atmosphere. CO2 mineralization in basalts has been proven to be one of the most secure storage options. For successful implementation and future improvements of this technology, the time-dependent deformation behavior of reservoir rocks in the presence of reactive fluids needs to be studied in detail. We conducted load-stepping creep experiments on basalts from the CarbFix site (Iceland) under several pore fluid conditions (dry, H2O saturated and H2O + CO2 saturated) at temperature, T≈80 ∘C and effective pressure, Peff=50 MPa, during which we collected mechanical, acoustic and pore fluid chemistry data. We observed transient creep at stresses as low as 11 % of the failure strength. Acoustic emissions (AEs) correlated strongly with strain accumulation, indicating that the creep deformation was a brittle process in agreement with microstructural observations. The rate and magnitude of AEs were higher in fluid-saturated experiments than in dry conditions. We infer that the predominant mechanism governing creep deformation is time- and stress-dependent subcritical dilatant cracking. Our results suggest that the presence of aqueous fluids exerts first-order control on creep deformation of basaltic rocks, while the composition of the fluids plays only a secondary role under the studied conditions.

Acoustic emissions in directed energy deposition processes

Acoustic emissions in directed energy deposition processes such as wire arc additive manufacturing and directed energy deposition with laser beam/metal are investigated within this work, as many insights about the process can be gained from this. In both processes, experienced operators can hear whether a process is running stable or not. Therefore, different experiments for stable and unstable processes with common process anomalies were carried out, and the acoustic emissions as well as process camera images were captured. Thereby, it was found that stable processes show a consistent mean intensity in the acoustic emissions for both processes. For wire arc additive manufacturing, it was found that by the Mel spectrum, a specific spectrum adapted to human hearing, the occurrence of different process anomalies can be detected. The main acoustic source in wire arc additive manufacturing is the plasma expansion of the arc. The acoustic emissions and the occurring process anomalies are mainly correlating with the size of the arc because that is essentially the ionized volume leading to the air pressure which causes the acoustic emissions. For directed energy deposition with laser beam/metal, it was found that by the Mel spectrum, the occurrence of an unstable process can also be detected. The main acoustic emissions are created by the interaction between the powder and the laser beam because the powder particles create an air pressure through the expansion of the particles from the solid state to the liquid state when these particles are melted. These findings can be used to achieve an in situ quality assurance by an in-process analysis of the acoustic emissions.

Study on Mechanical Characteristics and Failure Modes of Coal–Mudstone Combined Body with Prefabricated Crack

There are normally pre-existing cracks that can be observed in the coal seam and immediate roof that influences the stability of the rib spalling and the movement law of overlying strata. In this study, comprehensive research methods (e.g., theory analysis, experimental tests and numerical simulations) were adopted to reveal the mechanical characteristics, acoustic emission behaviors and failure modes of a coal–mudstone combined body with a single prefabricated non-penetrating crack. The results show that the influence of the crack angle on the elastic modulus of the coal–mudstone combined body samples was limited. With the increase in the crack angle, the unconfined compressive strength of samples decreased first and then increased in a V-shaped trend. In addition, the minimum unconfined compressive strength could be observed at a crack angle of 45°. Moreover, the number of acoustic emissions significantly increased with the process of continuous loading. In addition, the stress reduction zone could be observed in both ends of the prefabricated cracks at the initial stage of loading. The high- and low-stress zones were transformed with the process of continuous loading. Under an unconfined compression test, the failure models of the coal body part in the samples were mainly caused by shear failure, and only a few cracks occurred in the upper tip of the prefabricated cracks of the mudstone part. Therefore, airfoil cracks could be observed in the samples due to the strength difference of the coal mass and mudstone.

In situ X-ray and acoustic observations of deep seismic faulting upon phase transitions in mantle olivine

Activity of deep earthquakes, which increases with depth from ~400 km to a peak at ~600 km and abruptly decreases to zero at 680 km, is enigmatic, because brittle failure is unlikely to occur under the corresponding pressures of 13−24 GPa. It has been suggested that pressure-induced phase transitions of olivine in subducted slabs are responsible for occurrence of the deep earthquakes, based on deformation experiments under pressure. However, most experiments were made using analogue materials of mantle olivine and at pressures below ~5 GPa, which are not applicable directly to the actual slabs. Here we report the results of deformation experiments combined with in situ X-ray observations and acoustic emission measurements on (Mg,Fe)2SiO4 olivine at 11−17 GPa and 860−1250 K, equivalent to the conditions of colder regions of the slabs subducted into the mantle transition region. We find that faulting occurs only at very limited temperatures of 1100−1160 K, accompanied by intense acoustic emissions from both inside and outside of the sample, immediately before the rupture. The formation of lenticular packets filled with nanocrystalline olivine and wadsleyite is confirmed in the recovered sample without faulting, indicating that the faulting is caused by adiabatic shear heating along the weak layer of the connected lenticular packets, where nanocrystalline olivine plays important roles. Our study suggests that the transformational faulting occurs on the isothermal surface of the metastable olivine wedge in subducted slabs, leading to deep earthquakes in limited regions and depth range.

RESEARCH OF ACOUSTIC EMISSIONS FROM THE SYSTEM OF COMPLANARY CRACKS

The dynamic problem of the displacement field in an elastic half-space caused by the time-steady displacement of the surfaces of the system of disc-shaped coplanar cracks is solved. The solutions are obtained by the method of boundary integral equations. The dependences of elastic displacements on the surface of the half-space on the wave number, the number of defects and the depths of their occurrence are constructed.

Application of Acoustic Emissions Technique in Assessment of Cracking Performance of Asphalt Pavement Materials

This chapter focuses on various applications of acoustic emissions (AE) technique in evaluation of cracking in asphalt pavements including (1) assessment of low-temperature cracking of asphalt binders and mixtures and (2) quantitative characterization of rejuvenators’ efficiency in restoring aged asphalt pavements to their crack-resistant state. The AE-based embrittlement temperature results of 24 different asphalt materials consisting of eight different binders, each at three oxidative aging levels are presented. Results show that embrittlement temperatures correlated well with corresponding bending beam rheometer (BBR-based) critical cracking temperatures with R2 = 0.85. This chapter also presents application of AE for evaluation of rejuvenators’ efficiency on asphalt materials at various oxidative aging levels. The Geiger’s iterative source location method was employed to accurately determine embrittlement temperatures throughout the thickness of rejuvenator-treated asphalt samples. Results showed that the low temperature cracking properties of oxidative aged materials after 2 weeks of dwell time of rejuvenator have been recuperated. Moreover, it was observed that cracking characteristics of aged asphalt 6–8 weeks after applying rejuvenator far exceeded that of the virgin materials. The promising results suggest that the AE technique can be considered as a viable approach for the assessment of low temperature behavior of asphalt pavements.

Automatic Crack Classification by Exploiting Statistical Event Descriptors for Deep Learning

In modern building infrastructures, the chance to devise adaptive and unsupervised data-driven structural health monitoring (SHM) systems is gaining in popularity. This is due to the large availability of big data from low-cost sensors with communication capabilities and advanced modeling tools such as deep learning. A promising method suitable for smart SHM is the analysis of acoustic emissions (AEs), i.e., ultrasonic waves generated by internal ruptures of the concrete when it is stressed. The advantage in respect to traditional ultrasonic measurement methods is the absence of the emitter and the suitability to implement continuous monitoring. The main purpose of this paper is to combine deep neural networks with bidirectional long short term memory and advanced statistical analysis involving instantaneous frequency and spectral kurtosis to develop an accurate classification tool for tensile, shear and mixed modes originated from AE events (cracks). We investigated effective event descriptors to capture the unique characteristics from the different types of modes. Tests on experimental results confirm that this method achieves promising classification among different crack events and can impact on the design of the future of SHM technologies. This approach is effective to classify incipient damages with 92% of accuracy, which is advantageous to plan maintenance.