Shear Fracture Research Articles

Study on the resistance of UHMWPE laminates against oblique penetration of projectiles

AbstractTo solve the problem of the wall panel of high‐performance shelters protecting against the oblique penetration of projectiles, ballistic experiments were carried out with ultra‐high molecular weight polyethylene (UHMWPE) laminates subjected to normal penetration by high‐velocity projectiles, and the deformation and failure characteristics of UHMWPE laminates were analyzed. The numerical model of the resistance of UHMWPE laminates against penetration of projectiles was established by using the finite element program LS‐DYNA. The validity of the numerical model was verified by the experimental results of damage morphology and the depth of depression of the target plate and the deformation of the warhead. On this basis, the ricochet angle and failure mode of the UHMWPE target plate against projectile penetration were studied numerically, and the impact of the angle of incidence on the residual kinetic energy and normal penetration displacement of the bullet, as well as the damage morphology of the UHMWPE target plate, was revealed by using simulation. The results showed that the ricochet angle of the bullet obliquely penetrating into the UHMWPE target plate was in the range of 45° to 50°; at the initial stage of projectile penetration, the shear fracture failure of the fiber layer mainly occurred at the striking face of the UHMWPE target plate. Under the impact of the warhead deformation, with the reduction of the residual velocity of the bullet, the failure mode gradually changed to the tensile and interlaminar cohesion failure of the fiber layer near the back face; When the angle of incidence was greater than the ricochet angle, the residual kinetic energy of the projectile increased slowly with the increase of the angle of incidence of the bullet and then went up approximately linearly; the normal displacement of the bullet obliquely penetrating into the UHMWPE target plate decreased slowly after reaching the peak, and the peak value of the normal penetration displacement decreased gradually with the increase of the angle of incidence, and the time to peak of the displacement shortened gradually. The damage area of the bulge on the back surface of the UHMWPE target plate after penetration decreased gradually with the increase of the angle of incidence. The research results can provide a theoretical reference for the application of UHMWPE laminates in the design of military bulletproof shelters and armor.

Quantitative Prediction of the Development and Opening Sequence of Fractures in an Ultradeep Carbonate Reservoir: A Case Study of the Middle Ordovician in the Shunnan Area, Tarim Basin, China

Summary Quantitative prediction of reservoir tectonic fracture development characteristics, opening pressures, and opening sequences is critical in the exploration and development of oil- and gas-bearing reservoirs and thus has received widespread attention. Using numerical simulations of the paleostress field during the formation of tectonic fractures and the rock fracture criterion, we predict the development and occurrence of fractures in the Middle Ordovician Yijianfang Formation in the Shunnan region of the Tarim Basin, China. The local paleostress fields reflected by the mechanical properties and occurrence of tectonic fractures obtained from core descriptions, acoustic emission (AE) experiments, paleomagnetic experiments, sound velocity measurements, and borehole breakouts were used to determine the regional paleostress and in-situ stress. We established a geomechanical model by combining the mechanical parameters of the rocks with the finite element method (FEM), optimizing the boundary conditions with a self-adaptive constraint algorithm, and conducting numerical simulations of the in-situ stresses. Fracture occurrence and numerical simulation results of the in-situ stress field were used to determine the opening pressure (Pk) and opening sequence of the fractures. The level of fracture development decreases away from the strike-slip fault in the study area. Fracture development is positively correlated with the Young’s modulus, paleostress difference, and paleostress difference coefficient of the rock. The direction of the maximum horizontal principal stress is from north-northeast (NNE) to northeast (NE). Initially, shear fractures and tensional fractures oriented NNE 30°–35° and NE 40°–45°, respectively, open during the water injection process. Pk is positively correlated with the horizontal stress difference coefficient and the angle between the fracture strike and the maximum horizontal principal stress. At the structural highs (burial depths shallower than 6450 m) and the structural lows (burial depths deeper than 6450 m), the burial depth correlates negatively and positively with Pk, respectively. This investigation of the development, occurrence, Pk, and opening sequence of tectonic fractures and their principal controlling factors will have a positive impact on the future exploration and production opportunities of similar fractured reservoirs.

Computational multiphase micro‐periporomechanics for dynamic shear banding and fracturing of unsaturated porous media

AbstractDynamic shearing banding and fracturing in unsaturated porous media are significant problems in engineering and science. This article proposes a multiphase micro‐periporomechanics (PPM) paradigm for modeling dynamic shear banding and fracturing in unsaturated porous media. Periporomechanics (PPM) is a nonlocal reformulation of classical poromechanics to model continuous and discontinuous deformation/fracture and fluid flow in porous media through a single framework. In PPM, a multiphase porous material is postulated as a collection of a finite number of mixed material points. The length scale in PPM that dictates the nonlocal interaction between material points is a mathematical object that lacks a direct physical meaning. As a novelty, in the coupled PPM, a microstructure‐based material length scale is incorporated by considering micro‐rotations of the solid skeleton following the Cosserat continuum theory for solids. As a new contribution, we reformulate the second‐order work for detecting material instability and the energy‐based crack criterion and J‐integral for modeling fracturing in the PPM paradigm. The stabilized Cosserat PPM correspondence principle that mitigates the multiphase zero‐energy mode instability is augmented to include unsaturated fluid flow. We have numerically implemented the novel PPM paradigm through a dual‐way fractional‐step algorithm in time and a hybrid Lagrangian–Eulerian meshfree method in space. Numerical examples are presented to demonstrate the robustness and efficacy of the proposed PPM paradigm for modeling shear banding and fracturing in unsaturated porous media.

Natural fractures at depth in shale reservoirs: new insights from the southern Sichuan Basin marine shales

Natural fractures are pervasive in southern Sichuan Basin marine shales, China, and provide a useful opportunity to try to understand subsurface fracture networks in shale reservoirs. Based on cores and electrical imaging logs from vertical and horizontal petroleum wells in the southern Sichuan Basin, four types of natural fractures were identified in terms of orientation, size, filling properties and spatial distribution. Uncemented bed-parallel shear fractures develop at or in the vicinity of mechanical interfaces and are inclined to present in shale layers with a dip angle greater than 12°. Cemented bed-parallel fractures are characterized by a crack-seal texture marked by multiple bands of fibrous cement, and their intensity decreases upwards and shows a positive relationship with total organic carbon (TOC) values. Uncemented bed-oblique fractures are a type of fracture that rarely develops, and accommodate limited open space. Cemented bed-oblique/perpendicular fractures are the most developed fracture type and are distributed on a regional scale and are subdivided into two types. The results imply that these shale fractures could be formed sequentially by local and regional tectonic deformation, and by abnormally high pressure. Most natural fractures cannot contribute to reservoir storage or efficiently enhance its permeability, yet can act as planes of weakness and be potentially reactivated during hydraulic fracture treatments.

High-resolution source imaging and moment tensor estimation of acoustic emissions during brittle creep of basalt undergoing carbonation

SUMMARY As the high-frequency analogue to field-scale earthquakes, acoustic emissions (AEs) provide a valuable complement to study rock deformation mechanisms. During the load-stepping creep experiments with CO2-saturated water injection into a basaltic sample from Carbfix site in Iceland, 8791 AE events are detected by at least one of the seven piezoelectric sensors. Here, we apply a cross-correlation-based source imaging method, called geometric-mean reverse-time migration (GmRTM) to locate those AE events. Besides the attractive picking-free feature shared with other waveform-based methods (e.g. time-reversal imaging), GmRTM is advantageous in generating high-resolution source images with reduced imaging artefacts, especially for experiments with relatively sparse receivers. In general, the imaged AE locations are found to be scattered across the sample, suggesting a complicated fracture network rather than a well-defined major shear fracture plane, in agreement with X-ray computed tomography imaging results after retrieval of samples from the deformation apparatus. Clustering the events in space and time using the nearest-neighbour approach revealed a group of ‘repeaters’, which are spatially co-located over an elongated period of time and likely indicate crack, or shear band growth. Furthermore, we select 2196 AE events with high signal-to-noise-ratio (SNR) and conduct moment tensor estimation using the adjoint (backpropagated) strain tensor fields at the locations of AE sources. The resulting AE locations and focal mechanisms support our previously assertion that creep of basalt at the experimental conditions is accommodated dominantly by distributed microcracking.

Explosion Cracking Analysis of Sulfur Resistance Induction Heat Bend Pipe

The explosion cracking reason on induction heat bend pipe was analyzed through macroscopic analysis, chemical composition analysis, mechanical properties testing, metallographic analysis, scanning electron microscopy and XRD. The results showed that the chemical composition, tensile test, Vickers hardness results of the straight section of the bent pipe met the requirements of the technical specifications, and the metallographic structure of the straight section of the bent pipe was pearlite and ferrite. The Charpy impact test in the neutral zone of the bend section had a low shear fracture ratio, and the Vickers hardness test results were on the high side. The metallographic structure was granular bainite. The fracture surface of the failed bent pipe exhibited brittle fracture characteristics on the macro level. The metallographic analysis and hardness test results of the crack source area showed that the fracture surface had typical intergranular propagation characteristics and high hardness values. Energy spectrum analysis of the crack source area revealed the presence of S element. The XRD analysis of the product on the inner wall of the pipe contained sulfides. The comprehensive results indicated that the failed bend pipe undergoes stress corrosion and initiates corrosion cracks on the surface under the combined action of operating pressure, additional bending stress, residual stress, etc. in a sulfide corrosive medium, the cracks continued to propagate along the grain boundaries and ultimately caused the bend pipe to explode.

Macro-meso fatigue fracture and instability of rock-backfill composite structure under increasing-amplitude cyclic loading

The rock-backfill composite structure (RBCS) material in mine stopes is often subjected to stress disturbance. In order to reveal the fracture and instability mechanism of the RBCS exposed to disturbed stresses, a series of multi-stage cyclic loading tests were conducted on RBCS specimens with various cement-tailing ratios (c/t). The results indicate that deformation characteristics, stiffness degradation, damage propagation and failure mode of the RBCS were greatly influenced by the c/t. The volume dilatancy increases as well as the secant modulus (Es) increases and then decreases with decreasing c/t, i.e., from 1:4 to 1:12. For a sample with c/t of 1:4 and 1:8, the damage displays two-phase increasing pattern. However, for a sample with c/t of 1:10 and 1:12, inverted S-shaped damage accumulation pattern is found. As the c/t decreased, failure mode of RBCS specimens transformed from mixed tensile-shear failure to dominant tensile failure. Post-test CT imaging reveals obvious fracture evolution pattern that was characterized by shear fracture within the backfill, a circular tensile fracture along the rock-backfill interface, and tensile-shear fracture within the surrounding rock. It is noteworthy that complicated crack network is prone to forming for RBCS specimens with the c/t of 1:4 and 1:8. It is found that the use of "flexible CTB" with relatively low c/t is beneficial to improve the overall stability of the mine stopes.

Joining high volume fraction SiC particle reinforced aluminum matrix composites using Bi2O3-B2O3-ZnO-BaO-Na2O glass

This paper designs a bismuthate glass 58Bi2O3-25B2O3-13ZnO-2BaO-2Na2O in mol%(BBZ) and uses it to join 65 % volume fraction SiC particle reinforced aluminum matrix composites (65 vol%SiCp/ZL102) in an atmospheric environment. The thermal properties of BBZ glass were studied, and the effect of the brazing process on the mechanical properties of the joint was explored. The results show that the glass transition temperature of the glass is 344.6 °C, and the two crystallization temperatures are 464.7 °C and 544.3 °C. The oxidation of the parent material can improve the mechanical properties of the joint. The increase in welding temperature and the extension of holding time make the mechanical properties of the joint first increase and then decrease. When brazing at 490 °C for 30 min, the pre-oxidized composite material was brazed, the joint was defect-free, and the shear strength of the joint was 19.4 MPa. The analysis of the shear fracture showed that the fracture mainly occurred in the glass brazing material, which is a brittle fracture.

Mechanical behavior, acoustic emission and principal strain field evolution properties of layered cemented paste backfill under unconfined compression

In order to save costs, the layered filling method is adopted by mine enterprise to fill the underground goafs. To understand the mechanical behavior, damage evolution and failure modes the layered cemented paste backfill (LCPB), this study investigated the strength properties, acoustic emission (AE) characteristics and principal strain field evolution of LCPB samples with cement-to-tailings (c/t) ratio of 1:4, 1:6, 1:8, 1:10, and the height (h) of middle layer of 30 mm, 35 mm, 40 mm, 45 mm, 50 mm. The obtained results indicate that: (1) UCS of LCPB decreases with the increases of h and with the decreases of c/t ratio. The influence of c/t ratio on UCS of LCPB is in a good agreement with logarithmic function, while the variation relationship between h and UCS of LCPB satisfies the exponential function. (2) The acoustic emission signals first appeared during the transition from the compression to the elastic stage, reaching a maximum approaching the peak stress, and AE ringing counts also appeared during the post-peak stage. Internal fracture type of LCPB under external loading is a tension-shear composite fracture dominated by shear fracture. (3) The damage of LCPB occurs mainly in the middle layer and then extends to the upper and lower layer with the formation of visible localized zones. The damage of LCPB developed from significant shear damage to tensile-shear damage with the decreased of the c/t ratio, and transitioned from tensile-shear damage to complex damage dominated by shear damage and supplemented by tensile damage as h increased. Ultimately, the findings of this work can provide a reference for the design of LCPB.

Effect of tool rotational speed on friction stir spot welds of AZ31B Mg alloy to AISI 304 stainless steel

Abstract AZ31B Mg alloy were welded to AISI 304 stainless steel (which is a very challenging task because of their huge different chemical and physical features) by friction stir spot welding (FSSW) at different tool rotation speeds (700, 1000, and 1300 rpm) while keeping other conditions the same. Welds were evaluated and compared by examining their cross-sectional geometry, XRD pattern, tensile shear strength, fracture way and area. From cross-sections of welds, it was obtained that joining area expanded when speed was enhanced and the weld made through 1300 rpm had the largest one. Therefore, weld fabricated with the highest speed of 1300 rpm displayed the biggest tensile shear load of 3741 N while that manufactured with the lowest speed of 700 rpm possessed the lowest of 2430 N. All welds showed tensile shear type fracture and fracture occurred from the hooking regions where joining took place. From the fracture joint region, the weld of 700 rpm had the smallest joined area, whereas that of 1300 rpm showed the biggest. Higher speed produced a stronger joint, which agreed with a bigger bonding area and broken joint region. Also, joint became more ductile with increasing speed. Some phases were detected in joints.

Modelling geomechanical stability of a large deep borehole in shale for radioactive waste disposal

Large diameter boreholes drilled to a depth of 1000–2000 m are currently investigated in Australia as a potential solution for waste emplacement and long-term containment for long-lived intermediate-level radioactive waste (ILW). Different potentially suitable types of host rocks are considered, including granite, shale and salt. This study is aimed at assessing the risk of borehole instability for a large diameter deep borehole in shale. The investigation is conducted with numerical simulations using the CSIRO-developed modelling tools FRACOD and FRCAOD3D, and the commercial code Irazu. Previous laboratory experiments on boreholes in deep shale reported in literatures have been used as validation tests of the numerical models. Under hydrostatic stresses, the FRACOD model showed extensive shear-dominated “log-spiral” fractures around a borehole in shale which is similar to the laboratory testings reported in literatures. Subsequently, 2D and 3D numerical models representing a 0.7-m diameter vertical borehole drilled to the depths of 1000 m and 2000 m have been used. “Strong” and “weak” shales are simulated. The strong and weak shales represent the higher and lower ends of the shale strength reported in literature, respectively. Results showed that a “strong” shale exhibited only small borehole breakouts at the depth of 2000 m. The “weak” shale showed large breakouts (>1.5 m) at depths of 1000 m and 2000 m caused by extensive shear fracturing. In the scenario where a pre-existing 3D fracture crosscuts the borehole, the existing fracture may propagate deeper into the rock formation depending on its orientation and on the in situ stress magnitudes. Predictions based on analytical solutions indicate a 0.02-m and 0.04-m thick casing could prevent severe failure of rock mass at 1000 and 2000 m depths, respectively. Insights from model-based investigations of borehole breakouts are critical during the site selection of rocks with suitable geomechanical properties for waste disposal and for optimising the design of large-diameter boreholes for drilling, waste emplacement and long-term safety.

Experiment study on shear behavior and properties of granite fractures under real-time high-temperature conditions

Direct shear tests were conducted on granite fractures with similar joint roughness coefficients under real-time high temperatures (up to 400 °C) and constant normal stiffness conditions. In this study, the shearing process was monitored via acoustic emission, and fracture surface morphology was measured with three-dimensional laser scanning. With increasing temperature, peak and residual shear strengths, peak friction coefficient, and maximum normal dilation became higher. In contrast, peak normal displacement and peak shear displacement decreased. The apparent cohesion and internal friction angle exhibited opposite trends as temperature increased, primarily due to competition between thermal hardening and thermal cracking. The elastic strain energy of stick-slip events increased with temperature. From 100 °C to 400 °C, the degradation of fracture roughness and the volume of sheared-off asperities grew. Real-time temperature influences shear properties and is mainly attributed to physical and geometrical changes. Acoustic emission parameters, including count, energy, and hit, show segmented variation during fracture shearing. Based on parameter analyses of the average frequency and rise angle values, during the shearing process, the proportion of shear cracks at each stage varied with temperature. Shear failure was the predominant failure mode. Results from this study provide insight into the shear behavior of granite fractures under real-time high temperatures.

Shear fracture behavior and damage constitutive model of rock joints considering the effect of pre-peak cyclic loading

The research on mechanical properties and fracture nalysed of rock joints under dynamic loading disturbance is essential for the stability analysis of rock mass engineering. In this work, a series of pre-peak cyclic shear tests considering the number of cycles and cycle amplitude are conducted for rock joints. Firstly, the shear fracture nalysed and damage evolution law of rock joints are investigated in detail. Then, based on the statistical damage theory, the novel damage constitutive model of rock joints under pre-peak cyclic shear loading is established assuming that the microscopic element damage of rock joints material obeys the Weibull distribution. Finally, the real-time response of acoustic emission and the failure mode of joint surface are nalysed. The results demonstrate that the peak and residual shear stress are positively correlated with the cyclic amplitude. The cumulative cyclic displacement grows with the increase in the number of cycles and cycle amplitude. Moreover, the developed damage model considering the effect of pre-peak cyclic loading is verified by comparing the shear test results with the theoretical values, which are in good agreement. The variation of the shear wear zone of joint surface with cyclic parameters is discussed by binarization calculations. The results of this work can provide a valuable theoretical basis for further research on the shear fracture mechanism of rock joints under dynamic loading disturbance and mitigating geological disasters.

Significant inhibition of interfacial Cu6Sn5 IMC and improvement of solder joint strength and reliability by applying (111) nanotwinned Cu substrate and vacuum soldering

In this research, (111) oriented nanotwinned Cu (nt-Cu) substrates, combined with vacuum soldering, were utilized to enhance the strength and reliability of the solder joints. The wettability of the composite solder Sn1.0Ag0.5Cu-Si3N4 (SAC105-Si3N4) was improved on the (111) nt-Cu substrates and the oxidization of the molten solder was avoided by the vacuum environment, ensuring the formation of regular and uniform solder joints. The (111) nt-Cu substrates, characterized by their low surface energy, are demonstrated to be highly efficient in restricting the growth of interfacial IMCs, and this effects becomes particularly pronounced in three-dimensional (3D) solder joints following extended high-temperature aging. In addition, with vacuum soldering, a continuous and uniform interfacial structure was obtained, which is essential for upgrading the reliability of 3D solder joints. Furthermore, the electron back scatter diffraction (EBSD) testing of the solder matrix and interface formed on the nt-Cu substrates revealed that the c-axis of the β-Sn grains was parallel to the (111) nt-Cu substrates, and that the c-axis of the Cu6Sn5 grains was approximated to be perpendicular to the (111) nt-Cu substrates in an air environment. Finally, the shear strength and fracture morphology of the solder joints were evaluated. The macroscopic voids inside the solder joints were suppressed under vacuum, and the solder joints formed on the (111) nt-Cu substrates provided the highest shear strength.

Effect of microstructural inhomogeneity on mechanical anisotropy of Al–Cu–Li alloy plate

In order to reveal the relationship between microstructure inhomogeneity and mechanical anisotropy, the Al–Cu–Li alloy 100 mm thick plate for aerospace was evaluated. The grain structure, crystallographic texture, phase structure and fractography of different thickness positions and directions were observed by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The in-plane and through-thickness anisotropy were characterized by tensile properties, fracture toughness and hardness tests. The results demonstrate a greater in-plane anisotropy in yield strength at the same thickness position of the plate. The Brass texture component increases successively at T, T/4, and T/2 positions, while the Cu texture component exhibits minimal variation. The in-plane anisotropy mainly arises from the Brass texture. Through-thickness anisotropy in the same direction is primarily influenced by the diameter, number density, and volume fraction of the T1 strengthening phase. The average diameter and number density of the primary strengthening phase T1 within grains are highest at the T/2 position and lowest at the T position. The plate contains unrecrystallized, elongated pancake-like grains along the rolling direction. Fracture toughness tests conducted in the L-T and T-L directions predominantly reveal dimple-type transgranular shear fractures, while the S-L direction exhibits intergranular fracture as the primary characteristic. The crack divider delamination toughening mechanism is the main contributor to higher fracture toughness values in the L-T and T-L directions compared to those in the S-L direction. The results reveal the corresponding relationship between the microstructure inhomogeneity and the mechanical anisotropy of Al–Li alloy ultra-thick plates, and provide ideas for the adjustment of process parameters during processing deformation and the regulation of microstructure during heat treatment of large-scale Al–Li alloy thick plates, which has engineering application reference value.

Tailoring the non-coherent interfacial precipitation to overcome the trade-off between strength-ductility and impact energy release in Ti–Zr–Hf–Nb–Ta high entropy alloy

The energetic high-entropy alloys have brought a new idea for the development of new energetic structural materials (ESMs) with high energy density and excellent mechanical properties. However, the energetic high-entropy alloys exhibit a significant trade-off between strength-ductility and impact energy release efficiency. Here, we develop a new strategy to introduce non-coherent interfacial nanoprecipitates into the Ti–Zr-Hf-Nb-Ta BCC energetic high-entropy alloy, which can not only exert the precipitation strengthening mechanism to maintain excellent mechanical properties, but also transform the local shear fracture into multiple cracks along the grain boundary to promote energy release. The nanoprecipitation-strengthened energetic high-entropy alloy achieved a four-times greater fireball area generated by impact energy release than that of the single-phase BCC alloy, while maintaining an excellent mechanical property combination of high dynamic compression strength (1787 MPa) and high fracture strain (40 %). The results provide new insights into the development of energetic high-entropy alloys with superior impact energy release and strength-ductility synergy.

Characteristics and controlling factors of tectonic fractures within the buried hill reservoirs from the Archaean metamorphic basement: a case study in the Bozhong 19–6 condensate gas field, Bohai Bay Basin

The Bozhong 19–6 Condensate gas field is the first giant gas field discovered in the Bohai Bay Basin in recent years. The gas field reservoir has diverse reservoir space and strong heterogeneity.In this paper, we systematically summarize tectonic fractures development patterns and explore the main factors controlling fractures development and clarify the influence of fractures on reservoir quality through core and microscopic thin section observations, physical property data, imaging logging data. The results show that the major types of tectonic fractures in study area are shear fractures, followed by tensile fractures. Study area mainly developed high-angle fractures and completely filled fractures, fractures are commonly filled with carbonate and clay cement. Four groups of tectonic fractures were found in the study area, which corresponds to the four fracture formation period. Tectonic movement controls the fractures development in study area, the multi-stage tectonic movement had caused a complex fractures network system. The faults control the development of associated fractures formation in the fractures zone, different faults control the fractures formation and orientations in the vicinity. Rock type and minrals content is the basis of forming different fractures, such as the high felsic content is the basis of developed fractures in metamorphic rocks, and rock layer thickness, reservoir porosity and permeability are important factors of the fractures development heterogeneity longitudinally.

Volcanic unrest as seen from the magmatic source: Reyðarártindur pluton, Iceland

How the Earth’s crust accommodates magma emplacement influences the signals that can be detected by monitoring volcano seismicity and surface deformation, which are routinely used to forecast volcanic eruptions. However, we lack direct observational links between deformation caused by magma emplacement and monitoring signals. Here we use field mapping and photogrammetry to quantify deformation caused by the emplacement of at least 2.5 km3 of silicic magma in the Reyðarártindur pluton, Southeast Iceland. Our results show that magma emplacement triggered minor and local roof uplift, and that magma reservoir growth was largely aseismic by piecemeal floor subsidence. The occurrence and arrangement of fractures and faults in the reservoir roof can be explained by magmatic overpressure, suggesting that magma influx was not fully accommodated by floor subsidence. The tensile and shear fracturing would have caused detectable seismicity. Overpressure eventually culminated in eruption, as evidenced by exposed conduits that are associated with pronounced local subsidence of the roof rocks, corresponding to the formation of an asymmetric graben at the volcano surface. Hence, the field observations highlight processes that may take place within silicic volcanoes, not accounted for in widely used models to interpret volcanic unrest.

Response characteristics of acoustic emission signal and judgment criteria for different fracture modes

Coal mining is going deeper into the ground, resulting in more intense and frequent mine dynamic disasters. To ensure the early warning of dynamic disasters, it is significant to accurately reveal the differences of acoustic emission (AE) signals generated by rock fractures. This paper focuses on studying how AEs average frequency (AF) and rise angle (RA) change in sandstone during loading, and examines the characteristics of waveform amplitude, energy, and frequency distribution of AE signals from different types of fractures. The findings reveal that the percentage of shear fracture gradually increases with increasing load, and the energy activity of the AE signal is significantly enhanced in the high stress stage. The occurrence of the main fracture was preceded by a large number of low-frequency tensile fracture AE signals, accompanied by a small decrease in RA and a small increase in AF. The spectra of different fracture signals were characterized as “low anomalous F” for shear fractures, “vertical F” for low-frequency tensile fractures, and “high anomalous F” for high-frequency tensile fractures. Their energy distribution in the frequency domain was respectively described as “peak-centered distribution”, “pillar-shaped distribution” and “peak-skewed distribution”. The outcomes of the decomposition and reconstruction from the P- and S-waves reveal that the P wave energy (Ep) dominates in the high-frequency tensile fracture AE signal, while the S wave energy (Es) is greater in the low-frequency tensile fracture and the shear fracture AE signals. The ES/EP threshold value for laboratory discrimination of tensile and shear fracture is 3, as the fracture extends and penetrates, the ES/EP value rises rapidly at the appearance of macroscopic fracture. The findings offer theoretical guidance for monitoring and early warning of coal and rock dynamic hazards as well as revealing the morphology of fracture source.

Creep anisotropy dominated by orientation rotation in Ni-based single crystal superalloys at 750 °C/750 MPa

The creep anisotropy of Ni-based single crystal superalloys was studied at 750 °C/750 MPa. Transmission electron microscopy (TEM) observations showed that stacking faults and micro-twins were the typical dislocation configurations, which all originated from the activity of {111} 〈112〉 slip systems. Then, the orientation rotation caused by the activity of the main {111} 〈112〉 slip system would accelerate the specimen fracturing in shear fracture mode. Furthermore, the orientation rotation would dominate the creep anisotropy. The orientation rotation would occur during the entire creep process, the greater the degree of rotation, the more severe the asynchrony of rotation, which would result in nonuniform deformation and local stress concentration, and exacerbate the failure of the specimen.