Static Stress Loading Research Articles

The 2022–2023 seismic sequence onshore South Evia, central Greece: evidence for activation of a left-lateral strike-slip fault and regional triggering of seismicity

On 29 November 2022, an earthquake of ML 5.0 (Mw 4.8) occurred onshore South Evia Island (central Greece) preceded by a ML 4.7 (Mw 4.6) event. The pattern of relocated aftershocks indicates the activation of a single, near-vertical fault segment, oriented NW-SE at shallow crustal depths (6–11 km). We suggest that both events ruptured a blind, left-lateral strike-slip fault, about 5 km southeast of village Almyropotamos. We observed that a clear foreshock activity (N=55 events) existed before the two moderate events. The impact of the static stress loading on neighboring fault planes diminishes after a distance of 7 km from the November 2022 epicenters, where the static stress falls below +0.1 bar. We further explore triggering relationships between the 29 November events and the late December 2022 moderate events (ML 4.9) that occurred about 60 km toward NW in the Psachna and Vlahia regions of central Evia. We present evidence of possible delayed dynamic triggering of the late December 2022 central Evia sequence, based on marked changes in seismicity rates and on measured peak ground velocities (PGVs) and peak dynamic strains, both exhibiting local maxima in their map distributions. The causes of the delayed triggering may be related to the well-known geothermal field in central/north Evia and the NW-SE strike of the seismic fault.

Roadway fracture mechanism investigation subjected to static and blast loading under different lateral pressure coefficients

Blasting disturbance generated by the drilling and blasting is a significant factor in inducing damage to the rock mass surrounding a nearby roadway. Understanding the roadway fracture mechanism triggered by blasting disturbance is crucial for improving support efficiency. This study conducted the Brazilian disk splitting and direct shear tests on cement mortar to determine the classification for distinguishing between tensile and shear cracks based on acoustic emission (AE) parameters, i.e., rising time/amplitude and the average frequency (RA-AF). In the study context, various roadway failure experiments were conducted under the combined effects of static stress loading and repeated blasting disturbances. The temporal-spatial evolution characteristics of AE activities and identifying crack types were investigated to reveal roadway failure mechanisms in response to blasting disturbances under different lateral pressure coefficients. The results show that as the increases of blast disturbance numbers and stress level, the damage within the surrounding rock around the roadway increases and peaks at the moment of roadway destruction. Roadway failure involves a transition from tensile to shear fractures. When the stress level within the rock is low, blasting disturbance leads to compressive deformation and mainly causes tensile fractures, while it promotes dislocation formation and primarily results in shear cracks at a high-stress level. Different proportions of tensile and shear cracks were counted quantitatively around the roadway, and tensile cracks are more likely to cluster on the sides of facing blasting as the lateral pressure coefficient rises.

Investigation on overbreak and underbreak of pre-stressed tunnels under the impact of decoupled charge blasting

Decoupled charge blasting has shown its superior performance in achieving smooth tunnel profiles in underground excavation. In this study, through establishing an underground tunnel blasting similarity modeling (UTBSM) test system that incorporates a static stress loading device (SSL), a highspeed digital image correlation device (DIC) and a 3D laser scanning device (3DLS), field blast tests with concrete models involving set-aside circular and rectangular openings are first conducted to examine the influence of filling medium on the depth and area of overbreak and underbreak. Subsequently, a 3D finite element model is developed using LS-DYNA and validated against one of the test results, and the influences of decoupled coefficient, decoupled charge mode, filling medium, and in-situ stress condition are comprehensively investigated. Finally, an energy analysis encompassing internal energy and kinetic energy is performed. The experimental and numerical results show that water-decoupled blasting results in the largest overbreak and smallest underbreak due to the highest internal energy and kinetic energy, and the expanded polystyrene foam (EPS) effectively reduces the blasting energy exerted on concrete so that causes the minimum overbreak depth and volume. Both the overbreak and damage in burden decrease with increasing decoupled coefficient. Radial decoupled charge mode produces a higher energy transfer efficiency than axial charge mode, whereas eccentric charge mode outperforms in controlling overbreak and improving the excavation efficiency. Additionally, the increase of in-situ stress leads to a reduction in overbreak depth and damage volume in the burden, indicating that in-situ stress plays a favorable role in overbreak control during contour blasting. However, the restraining effect of in-situ stress on rock fracture warrants sufficient attention as it may result in blasting failure.

Experimental and theoretical investigation of roadway fracture induced by static-blast loading using acoustic emission

Underground metal mine is characterized by a large mining range involving complex spatial roadways layout and high mining intensity along with strong blast disturbances. Roadway destruction is easily triggered by blast-induced disturbances from different directions. In this study, the relationships between the fracture pattern of the roadway and static-blast loading were investigated by the means of theoretical analysis and experiment. The results showed that the macroscopic cracks around the roadway could be attributed to the combined action of the static loading and blast-induced disturbances. Under static loading, the stresses of the spandrel and wall corners were concentrated, as well as the AE events were clustered significantly. Blast-induced disturbances aggravated the damage to both the blast-facing side and high-stress areas, resulting in generations of large-scale AE events and deformation inside the rock. The differences in the spatial distribution of AE parameters indicated that the damage caused by static stress loading was stronger than that caused by blast-induced disturbances. To control the progressive fracture of rock surrounding the roadway caused by dynamic disturbances, it is essential to strengthen the support for the high-stress area and the blast-facing side respectively.

Fatigue and cyclic plasticity of 304L stainless steel under axial-torsional loading at room temperature

This work investigates the axial-torsional fatigue and cyclic deformation behaviour of 304L stainless steel at room temperature. The testing programme included four fully reversed strain-controlled loading paths (axial, torsional, proportional axial-torsional, and 90° out-of-phase axial-torsional) and a fully reversed shear strain-controlled with static axial stress. The experiments were carried out at equivalent strain amplitudes ranging from 0.20% to 1.00%, resulting in fatigue lives from 102 to 106 cycles. For axial, torsional, and torsional with axial static stress loading, secondary hardening related to martensitic transformation was observed, while it did not occur for nonproportional loading. For proportional loading, secondary hardening occurred only for an equivalent strain amplitude equal to or greater than 0.80%. Neither the accumulated plastic strain nor the equivalent plastic strain amplitude could predict whether secondary hardening would occur for the investigated loading conditions. Almost no variation in the cyclic Young’s and shear moduli with the number of loading cycles was observed even for the tests that exhibited considerable secondary hardening. Fatigue life estimates calculated using the stress-strain hysteresis loops at maximum softening and at half-life were similar for the Smith–Watson–Topper, Fatemi–Socie, and Jiang models. It was observed that macroscopic surface crack orientation depends on the equivalent strain amplitude for torsional, proportional, nonproportional and torsional with static stress loading. The capability of the models to estimate the observed crack orientations is discussed.

Two‐dimensional elastoplastic analysis of cylindrical cavity problems in Tresca materials

SummaryThis paper presents analytical elastic‐plastic solutions for static stress loading analysis and quasi‐static expansion analysis of a cylindrical cavity in Tresca materials, considering biaxial far‐field stresses and shear stresses along the inner cavity wall. The two‐dimensional static stress solution is obtained by assuming that the plastic zone is statically determinate and using the complex variable theory in the elastic analysis. A rigorous conformal mapping function is constructed, which predicts that the elastic‐plastic boundary is in an elliptic shape under biaxial in situ stresses, and the range of the plastic zone extends with increasing internal shear stresses. The major axis of the elliptical elastic‐plastic boundary coincides with the direction of the maximum far‐field compression stress. Furthermore, considering the internal shear stresses, an analytical large‐strain displacement solution is derived for continuous cavity expansion analysis in a hydrostatic initial stress filed. Based on the derived analytical stress and displacement solutions, the influence of the internal shear stresses on the quasi‐static cavity expansion process is studied. It is shown that additional shear stresses could reduce the required normal expansion pressure to a certain degree, which partly explains the great reduction of the axial soil resistance due to rotations in rotating cone penetration tests. In addition, through additionally considering the potential influences of biaxial in situ stresses and shear stresses generated around the borehole during drillings, an improved cavity expansion approach for estimating the maximum allowable mud pressure of horizontal directional drillings (HDDs) in undrained clays is proposed and validated.

Space‐time model for repeating earthquakes and analysis of recurrence intervals on the San Andreas Fault near Parkfield, California

AbstractWe propose a stochastic model for characteristically repeating earthquake sequences to estimate the spatiotemporal change in static stress loading rate. These earthquakes recur by a cyclic mechanism where stress at a hypocenter is accumulated by tectonic forces until an earthquake occurs that releases the accumulated stress to a basal level. Renewal processes are frequently used to describe this repeating earthquake mechanism. Variations in the rate of tectonic loading due to large earthquakes and aseismic slip transients, however, introduce nonstationary effects into the repeating mechanism that result in nonstationary trends in interevent times, particularly for smaller‐magnitude repeating events which have shorter interevent times. These trends are also similar among repeating earthquake sites having similar hypocenters. Therefore, we incorporate space‐time structure represented by cubic B‐spline functions into the renewal model and estimate their coefficient parameters by maximizing the integrated likelihood in a Bayesian framework. We apply our model to 31 repeating earthquake sequences including 824 events on the Parkfield segment of the San Andreas Fault and estimate the spatiotemporal transition of the loading rate on this segment. The result gives us details of the change in tectonic loading caused by an aseismic slip transient in 1993, the 2004 Parkfield M6 earthquake, and other nearby or remote seismic activities. The degree of periodicity of repeating event recurrence intervals also shows spatial trends that are preserved in time even after the 2004 Parkfield earthquake when time scales are normalized with respect to the estimated loading rate.

Three-dimensional finite element analysis of lumbar vertebra loaded by static stress and its biomechanical significance

To explore the mechanical behavior of lumbar spine loaded by stress and provide the mechanical basis for clinical analysis and judgement of lumbar spine fracture classification, mechanical distribution and static stress. By means of computer simulation method, the constructed lumbar spine three-dimensional model was introduced into three-dimensional finite element analysis by software Ansys 7.0. The lumbar spine mechanical behavior in different parts of the stress loading were calculated. Impact load is 0-8000 N. The peak value was 8000 N. The loading time is 0-40 minutes. The values of the main stress, stress distribution and the lumbar spine unit displacement in the direction of main stress were analyzed. The lumbar spine model was divided into a total of 121 239 nodes, 112 491 units. It could objectively reflect the true anatomy of lumbar spine and its biomechanical behavior and obtain the end-plate images under different stress. The stress distribution on the lumbar intervertebral disc (L(3)-L(4)) under the axial, lateral flexion and extension stress, and the displacement trace of the corresponding processus articularis were analyzed. It is helpful to analyze the stress distribution of lumbar spine and units displacement in static stress loading in the clinical research of lumbar spine injury and the distribution of internal stress.

Lessons on the calculation of static stress loading from the 2003 Bingol, Turkey earthquake

The 2003 Bingol earthquake ( M w = 6.4) occurred very close to a region along the east Anatolian fault zone which was identified in 2002 as posing a particularly high seismic risk. This damaging earthquake occurred on a conjugate right-lateral blind fault that was inconsistent with the stress-change field calculated for preceding large earthquakes in the region. In this paper, four reasons which might be responsible for this stress discrepancy are identified and investigated individually. Firstly, co-seismic stress changes are considered. The time frame of the previous stress calculations is extended to include the large earthquakes in the 1780s which were not included in the earlier study. A sensitivity analysis is then conducted on the more recent events since 1822 to examine the effect of errors in their location and sizes. The possibility of the occurrence of a small magnitude event close to the 2003 epicentre is considered. We argue that, barring the occurrence of a low-probability, unmodelled local event, the Bingol earthquake was unlikely to have been triggered by co-seismic stress transfer from any known sequence of previous earthquakes. Finally we examine and modify the secular loading model used in the 2002 study and show that loading which is properly constrained by regional GPS data produces a positive stress change on the 2003 rupture. As a result of our examination of the stressing history of the Bingol hypocentre we argue that it is through a combination of historical seismology, guided and constrained by structural geology, directed paleoseismology in which the locations and extent of historical events are confirmed, and stress modelling which has been informed by detailed GPS data, that an integrated seismic hazard program might have the best chance of success.

Earthquake Time Cluster in North-East India During February to April 1988

During the time interval of 3 February to 20 April 1988 four independent main shocks occurred in North-East (NE) India: 03/02/1988, M = 5.5; 06/20/1988, M = 5.8; 15/04/1988, M = 6.2 and 20/04/1988, M = 5.8. The seismicity rate within this 78 day interval is increased by a factor of 15 and 6 with respect to the mean, long term seismicity for M ≥ 5.5 and M ≥ 6.1, respectively. In terms of probability, it has been found out that the probability of observing by chance four events of M ≥ 5.5 or one events of M ≥ 6.1 in NE-India is equal to only 0.4243 and 0.3680, respectively. These results imply that the observed seismicity has a non-random time clustering. Similar earthquake time clusters were identified to have occurred in NE-India in 1930 and 1951. A triggering mechanism has been proposed to interpret the earthquake clustering: the first event of the earthquake sequence produces transient stress changes that cause an acceleration to the static stress loading, and then to seismic failure, to remote highly pre stressed regions.

Mechanical Stress Induced Cellular Orientation and Phenotypic Modulation of 3-D Cultured Smooth Muscle Cells

The effect of periodic stretch on cellular orientation and intracellular ultrastructure of three-dimensionally (3-D) cultured arterial smooth muscle cells (SMCs) was investigated. Ring shaped hybrid tissues were prepared by thermal gelation of a mixed solution of Type I collagen and SMCs derived from bovine aorta. The gels were subjected to three modes of stress loading: floated (control), stretched isometrically (static stress), and periodically stretched and recoiled with 10% amplitude at 60 rpm frequency (dynamic stress). After 4 weeks of stress loading, the gels were morphologically investigated with a light microscope and a transmission electron microscope (TEM). Irrespective of static or dynamic stress loading, SMCs in stress loaded gels exhibited an elongated bipolar spindle shape and were oriented parallel to the direction of stretch, whereas those in control gels were polygonal shaped and randomly oriented. TEM observation showed that SMCs in control and static stress loaded gels were intracellularly filled with organelles, such as rough endoplasmic reticulum, free ribosomes, Golgi complexes, and mitochondria, indicating that the cells were of the synthetic phenotype. On the other hand, SMCs in dynamic stress loaded gels tended to have increased contractile apparatus, such as myofilaments, dense bodies, and basement membranes, suggesting that periodic stretch plays an important role in phenotypic modulation of SMCs from the synthetic to the contractile state, as well as cellular orientation.