Pre-failure Stage Research Articles

Amplitude reconstruction of acoustic emission signals on the base of its mathematical modeling as a stochastic process

The problem of random oscillations generated by inner defect within the neighborhood of the boundary of the elastic massive body under the pre-failure stage is considered. The investigation is based on results of the invariants’ method in acoustic emission (AE) theory consisting in the stability conditions of the mean of statistic distribution parameters of signal characteristics under the same pre-failure stage. The mathematical model of arising wave process of displacement oscillations in the massive body is constructed and the correctness of its using is studied. The problem in question is reduced to the investigation of a boundary integral equation in special stochastic process classes. The task of reconstructing from AE signals and describing the nature of the random process of defect radiation at the free boundary of the body is set. Some numerical are presented.

Simulating the entire rainfall-induced landslide process using the material point method for unsaturated soil with implicit and explicit formulations

Rainfall-induced landslides cause various types of damage, including damage to infrastructure, leading to devastating economic losses and human casualties. Although various numerical methods have been developed to predict landslide occurrence and the extent of sediment flow, three-dimensional analysis of the entire landslide process in a unified manner is still challenging owing to limitations in computational efficiency and the representation of deformation and flow. In this study, we present a numerical method of rainfall-induced landslides using the coupled hydromechanical material point method (MPM) for unsaturated porous media based on implicit and explicit formulations, which enables the efficient analysis of all rainfall-induced landslide processes, including both quasi-static and dynamic processes. The developed method includes an implicit MPM based on a simplified formulation, which is first applied to the quasi-static analysis in the pre-failure stages in which rainwater infiltrates the ground. The explicit MPM is then applied to the dynamic analysis for post-failure stages in which the ground collapses and flows. A constitutive law for soils is improved in the simulation of landslide initiation and sediment flow by incorporating the effect of cohesion in a visco-plastic model for granular materials. The proposed method was applied to a three-dimensional terrain model of Ashikita town, Kumamoto, Japan, where an actual landslide occurred owing to intense rainfall on July 3–4, 2020. Numerical results from the proposed hydromechanical coupling compared with single-phase MPM indicated that pore water plays an important role in understanding all rainfall-induced landslide processes, from landslide initiation to sediment discharge.

MPM-based mechanism and runout analysis of a compound reactivated landslide

Understanding the entire process of hydraulic-related landslide reactivations is crucial for risk assessment, which includes initiation and runout evolves from a small-deformation in the pre-failure stage to large-deformation after failure, with complex interactions between the materials in solid and liquid phases. This paper reproduces the entire process of a reactivated landslide using Material Point Method (MPM). The accuracy of MPM is validated in comparison to Limit Equilibrium Method (LEM) and Finite Element Method (FEM). The effects of antecedent rainfall and pre-existing groundwater on landslide runout and the deposits morphology are discussed. Results show that the antecedent rainwater rises the groundwater level and saturates the front edge of slope where the initial failure occurred. Three computed spatio-temporal distributions of pore water pressure show good agreement and match well with field evidence. The kinematic characteristics show that the landslide has different moving features with different microtopography, which reveals retrogressive failure in front and middle part of slope initially and compound retro- and pro-gressive failures occur at the rear edge. The results of unsaturated two-phase MPM are in better agreement with the measured morphology than full-saturated MPM. The antecedent rainfall and the pre-existing groundwater are the main contributing factors to the landslide runout.

Identification of fracturing behavior in thermally cracked granite using the frequency spectral characteristics of acoustic emission

The frequency characteristics of acoustic emission (AE) during triaxial compression of thermally cracked and unheated (“fresh”) granite samples were investigated with the aim of understanding the influence of pre-existing cracks on precursor information regarding macroscopic failure. The peak frequency during the damage process was the same for thermally cracked and fresh granites. Analysis of AE signals showed that signals with low peak frequency appeared before failure of the sample, implying the initiation of macrocracks with progressive growth of cracks. The peak amplitude of the frequency spectrum recorded in the thermally cracked samples was much lower than that in the fresh samples. This result suggests two reasons for the difference in peak amplitude: reduction in shear modulus and the attenuation filtering phenomenon caused by thermal cracks (i.e., the differences in overall bulk properties). In particular, the maximum value of peak amplitude in the low-frequency band for the thermally cracked samples was smaller than that for fresh samples. This characteristic can be related to the properties of source of AE signals, such as the stress drop and crack size. Assuming that pre-existing thermal cracks grow during the pre-failure stage, the events with low peak frequency and low peak amplitude in the heat-treated samples are interpreted as exhibiting a low stress drop for individual events. Therefore, although AE signals with low frequency can be considered as precursors to rock failure, cracking behavior suggested by events with low frequency depends on the initial damage condition of the rock sample.

Yielding, shear banding, and brittle failure of amorphous materials

Widespread processes in nature and technology are governed by the dynamical transition whereby a material in an initially solid-like state then yields plastically. Major unresolved questions concern whether any material will yield smoothly and gradually (ductile behaviour) or fail abruptly and catastrophically (brittle behaviour); the roles of sample annealing, disorder and shear band formation in the onset of yielding and failure; and, most importantly from a practical viewpoint, whether any impending catastrophic failure can be anticipated before it happens. We address these questions by studying the yielding of slowly sheared athermal amorphous materials, within a minimal mesoscopic lattice elastoplastic model. Our contributions are fourfold. First, we elucidate whether yielding will be ductile or brittle, for any given level of sample annealing. Second, we show that yielding comprises two distinct stages: a pre-failure stage, in which small levels of strain heterogeneity slowly accumulate, followed by a catastrophic brittle failure event, in which a crack quickly propagates across the sample via a cooperating line of plastic events. Third, we provide an expression for the slowly growing level of strain heterogeneity in the pre-failure stage, expressed in terms of the macroscopic stress-strain curve and the sample size, and in excellent agreement with our simulation results. Fourth, we elucidate the basic mechanism via which a crack then nucleates and provide an approximate expression for the probability distribution of shear strains at which failure occurs, as determined by the disorder inherent in the sample, expressed in terms of a single annealing parameter, and the system size.

Experimental Analysis of Steel Circular Hollow Section under Bending Loads: Comprehensive Study of Mechanical Performance

The present study aimed at evaluating the mechanical performance under bending loads of circular hollow sections of steel. Different bending tests have been carried out by applying two-point loads, to determine and examine the effects of the diameter, the thickness of the section, and the span of the beam on the performance of the steel tube. The effects of square opening and variation in the number of openings on the performance of these sections have also been examined. Ten samples of hollow circular beams of varying thickness (2 mm, 3 mm, and 6 mm), varying diameter (76.2 mm, 101.6 mm, and 219 mm), and varying span (1000 mm, 1500 mm, and 2000 mm) were fabricated and tested for pre-failure and post-failure stages. The dimensions of the reference specimen considered were 3 mm in thickness, 101.6 mm in diameter, and 1500 mm in span. The results have shown that on increasing the section thickness by 200%, ductility and bearing strength were enhanced by 58.04% and 81.75%, respectively. Meanwhile, decreasing the section thickness by 67%, ductility and bearing strength were reduced by 64.86% and 38.87%, respectively. Moreover, on increasing the specimen diameter and on decreasing span, a significant increase in bearing strength and stiffness was observed; however, ductility was reduced. Meanwhile, on increasing the span of the specimen, all the parameters observed, i.e., bearing strength, stiffness, and ductility, decreased. On observing the ultimate strength of each specimen with square opening, the ultimate strength was reduced by 17.88%, 19.71%, and 14.23% for one, two-, and three-square openings, respectively. Moreover, the ductility was significantly reduced by 72.40%, 67.71%, and 60.88% for one, two-, and three-square openings/apertures, respectively, and led to the sudden failure of these specimens. The local buckling failure dominated for specimens having a D/t ratio more than 50 and showed very negligible levels of ovalization of the cross-section. Local buckling failure was observed to be prevented after providing the circular rings in the specimen, since bearing strength increased compared with the specimen without rings.

Satellite-based interferometric monitoring of deformation characteristics and their relationship with internal hydrothermal structures of an earthflow in Zhimei, Yushu, Qinghai-Tibet Plateau

Under the influence of climate change, permafrost landforms are sensitive to seasonal heave and contraction, thus exacerbating surface instability and fostering landslides as a consequence. In the pastureland of Zhimei on the Qinghai-Tibet Plateau (QTP), a typical earthflow has drawn significant attention through social media. However, detailed knowledge of the deformation characteristics, internal hydrothermal regime, and structure is still scarce. In this study, we aim to enhance traditional satellite synthetic aperture radar interferometry to divide ground deformation into the seasonal oscillation and slope deformation components and identify the magnitude and spatial distribution of unstable slopes in frozen regions. Then, the use of unmanned aerial vehicles (UAVs) was combined with geophysical monitoring techniques to recognise the deformation dynamics from the pre- to post-failure stages. Sentinel-1 images, covering almost five years, highlighted that obvious creep behaviour dominated at the pre-failure stage, while a seasonal deformation pattern characterised by a piecewise distribution associated with the hydrothermal regime was observed at the post-failure stage. Fast retrogressive erosion on the head scarps at the post-failure stage was clearly identified by multidifferential digital surface models from the UAV observations. To better understand the internal structure, both electrical resistivity tomography and ground-penetrating radar were combined to determine the seasonal frozen thickness, underlying thawing materials, and vertical cracks, which controlled the kinematic evolution from the initial creep to the narrow and long oversaturated flow that represented the terminal portion of the landslide. Finally, by comparing in situ monitoring data with field investigations, the main driving factors controlling the movement mechanism are discussed. Our results highlight the specific kinematic behaviour of an earthflow and can provide a reference for slope destabilisation on the QTP under the influence of climate change.

An Investigation of Rainfall-Induced Landslides From the Pre-Failure Stage to the Post-Failure Stage Using the Material Point Method

The kinematic behavior of rainfall-induced landslides from the pre-failure stage to post-failure stage contains important information for risk assessment and management. Because a complex relationship exists between rainfall conditions, pore water pressure, soil strength, and movement rates, a numerical model is the most efficient way to investigate the behavior of rainfall-induced landslides. In this study, the material point method (MPM) is used to investigate the dynamic behavior of landslides. First, the rainfall boundary conditions are extensively verified by comparing 1-D consolidation tests against other numerical solutions. Then, a numerical model is used to simulate a lab-scale rainfall-induced slope failure. A parametric study shows the influence of rainfall intensity on pore water pressure development, failure triggering time, surface displacement, and velocity. The use of the MPM provides a clear understanding in the failure mechanism and post-failure behavior of a rainfall-induced landslide.

Experimental Study on the Relationship between the Velocity of Surface Movements and Tilting Rate in Pre-Failure Stage of Rainfall-Induced Landslides.

With the development of deformation measuring technology at slope surfaces, prediction methods for rainfall-induced landslides based on the surface movements and tilting of slopes in the pre-failure stage have been recognized as a promising technique for risk reduction of slope failure triggered by rainfall. However, the correlation and possible mechanism for these prediction methods were rarely discussed. In this study, the comparison between the prediction methods of slope failure based on the time history of surface displacements and tilting in the acceleration stage was carried out by conducting a series of laboratory tests and field tests under rainfall, in which the movements and tilting behaviors at the slope surface were measured. The results show that the predicted failure time of tested slopes obtained by different prediction methods is consistent, and the correlation between these landslide prediction methods were also detected. A proportional relationship between the velocity of surface displacements and tilting rate was observed, and a possible mechanism for the sliding behavior was discussed based on this linear relationship as well. In addition, an expression for the linear relationship between the rate of the surface tilting and displacement was also established in this study, and the results could have significance for the understanding of the sliding behavior in the failure process in rainfall-induced landslides.

Modelling the spatio-temporal evolution of a rainfall-induced retrogressive landslide in an unsaturated slope

Numerical modelling, particularly fully-coupled hydro-mechanical large-deformation models, greatly helps in properly simulating the complex failure and post-failure mechanisms of rainfall-induced landslides. The affected soils, in fact, evolve from none or small deformation rates to large deformation rates during the initiation stage and vice-versa during deposition, with relevant interactions between the solid skeleton and interstitial water. The Material Point Method (MPM) has the potential to reproduce entirely those complex processes. However, a comparison with standard tools (e.g. FEM: Finite Element Method, LEM: Limit Equilibrium Method) may guide in the optimal choice (or in the combined use) of the various modelling approaches. A framework is here proposed based on a multi-tool approach consisting in the combination of: a) no-deformation LEM, b) small-deformation FEM, c) large-deformation MPM. The LEM slope stability analyses are performed for a realistic assessment of the major slip surface(s) and to back-analyse uncertain slope parameters. The FEM stress-strain analyses assess the progressive failure, the onset of initial velocity and the later acceleration of the landslide body, until large deformations occur in the slope and numerical convergence of FEM is lost. The MPM analyses are used to reproduce the whole landslide process, from the initiation to propagation and final deposition. Such an integrated framework is tested for an international landslide benchmark (the 1995 Fei Tsui Road landslide in Hong Kong). The results achieved through the different approaches are discussed in relation to the wide scientific literature available for the general topics and the specific case study. The paper highlights that the fully-coupled hydro-mechanical large-deformation model properly reproduces the complex failure and post-failure mechanisms of rainfall-induced landslides. However, no-deformation LEM analyses and small-deformation FEM analyses allow a reasonable understanding of both the pre-failure stage and the failure mechanism. These more traditional tools are confirmed as indispensable tools in the engineering practice and research.

Association Between High-Sensitivity Troponin T on Admission and Organ Dysfunction During Hospitalization in Patients Aged 80 Years and Older with Hip Fracture: A Single-Centered Prospective Cohort Study.

BackgroundPrognostic evaluation of elderly patients with hip fracture is an issue that has been highly concerned by clinicians. Only a few studies have focused on organ dysfunction after hip fracture in the elderly. This study aimed to investigate the association between high-sensitivity troponin T (hs-TnT) at admission and organ dysfunction during hospitalization in elderly patients with hip fracture.MethodsWe enrolled 168 patients with hip fracture who were aged 80 years and older at Geriatric Orthopaedic Center of Sichuan Provincial Orthopedic Hospital between January 2020 and August 2020. Baseline characteristics, perioperative information, and short-term clinical outcomes were analyzed.ResultsOf the 208 patients admitted during the study period, 168 met the inclusion criteria; of these, 91 (54.2%) had higher hs-TnT than the 99th percentile in the normal population. After adjustment for confounders, elevated hs-TnT was independently associated with multiple organ dysfunction syndrome in the elderly (MODSE) (adjusted OR, 5.76; 95% CI, 1.74–19.10; P = 0.004), heart dysfunction (adjusted OR, 7.48; 95% CI, 2.17–25.82; P = 0.001), MODS severity score > 3 (adjusted OR, 5.22; 95% CI, 1.32–20.60; P = 0.018), and length of hospital stay > 14 days (adjusted OR, 2.38; 95% CI, 1.05–5.36; P = 0.037).ConclusionIncreased hs-TnT on admission is an independent risk factor for MODSE after hip fracture in patients aged 80 years and older. Effective measures should be applied to avoid progression of MODSE from pre-failure stage to failure stage.

Centrifuge modelling of rainfall-induced slope failure in variably saturated soil

This paper presents the results of centrifuge tests on rainfall-induced instabilities in variably saturated slopes. The roles of rainfall intensity and initial conditions, such as slope angle, porosity and degree of saturation of the soil, in the failure initiation and postfailure kinematics are considered. The failure patterns, infiltration profile and deformation at prefailure and postfailure stages are characterized. The results indicate that rainfall-induced slope failures usually follow one of the following two failure modes, i.e. slide-to-flow and flowslide failure modes. The former pattern is characterized by soil mass flow after initial failure along a continuous shear surface, while the latter is more relevant to the rapid increase in the saturation at the slope surface, resulting in surface erosion channels followed by the acceleration of the soil mass. The flowslide failure pattern usually gives rise to several superficial shear surfaces and longer run-out distances. The rainfall intensity and profiles of the degree of saturation play the key roles in initiating the slope failure at the prefailure stage and subsequently in mobilizing the soil mass at the postfailure stage. Our test data, together with the data from the literature, are presented in two threshold curves to define the critical condition of slope failure under rainfall infiltration.

An Extension Failure Criterion for Brittle Rock

Under the triaxial compressive state, the compressive strain is supposed to happen in the direction of the maximum principal stress, but tensile strain happens in the direction of the minimum principal stress. Moreover, as the intermediate principal stress is not too high, the corresponding strain can also be tensile. If the brittle rock is assumed as linear elastic in the prefailure stage, a new strength criterion based on the sum of the two tensile strains was presented. The new criterion considers the differences in mechanical parameters (i.e., elastic modulus and Poisson’s ratio) under tension and compression. The parameters of the criterion only include Poisson’s ratio and uniaxial strength. And the effect of the intermediate principal stress σ2 can be reflected. Certain featured failure phenomenon of rock material can be explained well by the proposed criterion. The results of conventional and true triaxial tests can verify the criterion well. Finally, the criterion is compared with the Mohr–Coulomb and Drucker–Prager criteria.

Электрохимическая эмиссия при деформировании и разрушении алюминий-магниевого сплава в водной среде

Spatio-temporal structures of Portevin-Le Chatelier deformation bands at the pre-failure stage, necking and destruction of an aluminum-magnesium alloy, deformable in an aqueous medium, were investigated by a complex of in situ methods, including high-speed video of the surface and an electrochemical emission method. The latter is based on measuring and analyzing jumps in the electrode potential of a deformable metal under the conditions of the manifestation of intermittent deformation. It is established that discrete signals of electrochemical emission in the frequency band of 10 Hz - 10 kHz contain information on the number of deformation bands, the moments of nucleation and stages of their growth, the statistical temporal structure of the bands, etc. A characteristic series of signals at the pre-failure stage is revealed − an electrochemical precursor neck formation and specimen fracture. The connection between the generation of electrochemical emission signals and the localization of plastic deformation and the destruction of an oxide film on an aluminum alloy surface in contact with an aqueous medium is discussed.

Monitoring of displacement evolution during the pre-failure stage of a rock block using ground-based radar interferometry

As a typical abrupt geological hazard, rockfalls are widely distributed and occur frequently. It is often difficult to predict the occurrence, and therefore record and monitor the whole failure process of rockfalls. In this study, with the combined use of terrestrial laser scanning (TLS) and ground-based radar interferometry (GB-InSAR) technologies, a typical small collapse in the Hongshiyan post-earthquake rock slope was monitored. An accurate TLS three-dimensional (3D) rock slope model of the study area was established with high-resolution geometry and morphology information, the discontinuity sets and their orientations and distributions were visually identified and analyzed by means of an automatic discontinuity identification algorithm. A perspective view of the whole dynamic failure process of the small collapse was achieved; the displacement behavior during critical sliding stage was revealed by integrating the high-accuracy (millimetric) GB-InSAR monitoring results into the TLS 3D model. It can be used for a complete analysis of the failure behavior and stability assessment of the rock slope during a phase of emergency. The unstable rock block on the slope exhibited a rapid growth of displacement. The dynamic failure process of the rock block underwent three obvious accelerating periods. There was a correspondence between the spatial expansion of the moving area and the process of cracking propagation of rock bridges. The stability of the perilous rock above the tafoni was controlled by the connectivity of rock bridges. Thanks to monitoring, early warning and preventive measure were taken, which avoided a possible undesirable event. This typical case study can provide a reference for the monitoring of an unstable rock slope and the understanding of the evolution of rock block kinematics before collapse.

Consistent modeling of a catastrophic flowslide at the Shenzhen landfill using a hydro-elasto-plastic model with solid–fluid transition

Flow-type landslides are an important hazard that can cause great destruction due to the rapid flow velocity and large disaster area. This paper presents a catastrophic flowslide that recently occurred at a landfill in Shenzhen, China. This disaster involved an area about 1100 m in length and 630 m in maximum width, and caused the death of 77 people and the destruction of 33 buildings. The precise reason for the landfill’s failure is still unknown, and therefore we try to contribute an increased understanding of the event for future prevention. In this study, the failure mechanism of the studied slope was analyzed and described under partially saturated condition. The solid–fluid transition during the flowslide occurrence was described using a unified constitutive model. The model was used to perform the hydro-elasto-plastic modeling in the pre-failure stage, the viscous modeling in the post-failure stage, and the second-order work criterion was introduced in between to model the solid–fluid transition. The consistent evolution of the flowslide, including initiation, propagation, and deposit stages, was simulated and analyzed using the finite element method with Lagrangian integration points after careful calibration of the viscous parameters. The numerical results were compared with the real case and used to explain the failure mechanism.

Shallow landslide susceptibility assessment in granitic rocks using GIS-based statistical methods: the contribution of the weathering grade map

Shallow landslides (i.e., slide, flow, and complex) are widespread around the world, affecting the soil mantle and upper regolith as a result of the weathering of granitic bedrock, and periodically cause enormous social and economic damages. Shallow landslide hazards are predominantly due to the scarcity of warning signs during the pre-failure stage, high velocities reached in the post-failure phase, and an increase in mobilized volumes caused by the entrapment of material in the downhill path of the phenomena. Owing to the abovementioned aspects, susceptibility assessment of shallow landslides in weathered granitic rocks is a relevant issue for land use planning and design purposes. This study proposes a three-step methodology for the susceptibility assessment of these phenomena. The methodology has been tested and validated at the 1:10,000 scale over a 30.4-km2 area in southern Italy, where weathered granitic rocks are periodically affected by shallow landslides. This methodology is divided into three successive steps: step 1 consists of database creation, with an emphasis on the weathering grade map (including five weathering classes, from class II to class VI, each one characterized by comparable mechanical behavior), and steps 2 and 3 focus respectively on susceptibility map calibration and validation through statistical analyses. The area under the ROC curve (AUC) shows values ranging from 0.95 in step 2 (calibration) to 0.88 in step 3 (validation) and is a testament to the good overall predictive accuracy of the methodology. The obtained results demonstrate both the effectiveness and the consistency of the proposed methodology in performing susceptibility mapping of shallow landslides in weathered granitic rocks, as well as the important role played by the weathering grade map.

Dynamic failure of viscoplastic structures under ASB and micro-voiding

Adiabatic shear bands are known to cause premature structural failure in high strength metals and alloys. The observation of adiabatic shear bands inside partially fractured specimens evidences the presence of micro-voids as the precursor of the ultimate failure. An enriched model containing the effects of adiabatic shear banding and micro-voiding mechanisms was developed and it is here taken through a calibration procedure. The aim of the present work is to evaluate the performances of the enriched model considering an initial-boundary value problem. To that purpose, the model has been implemented as user material in the engineering finite element computation code LS-DYNA. Numerical simulation of the dynamic shearing of hat shaped structures is conducted and the interest of accounting for the pre-failure stage consisting of micro-voiding in the ASB wake is emphasized.

Spectral Characteristics of Acoustic Emission in Loaded Coal Specimens for Failure Prediction

Сhanges in acoustic emission spectra of coal samples from the Kuznetsk and Pechora Basins are experimentally studied in uniaxial and triaxial (Karman-type) compression tests. It is shown that the acoustic emission spectral patterns change considerably at the pre-failure stage in the samples under uniaxial compression: the amplitudes of low-frequency components grow while those of high frequencies reduce. Under the Karman-type compression, the amplitudes of low-frequency signals grow while the high-frequency amplitudes do not change much, because lateral hydrostatic pressure impedes opening of vertical fractures.

Evolution of Macro-Scale Plastic Flow Localization of Tri-Layered Stainless Steel - Low Carbon Steel - Stainless Steel Metal with Digital Image Correlation Method

Evolution of localized plastic deformation in tri-layered metal material casting consistng of the working part (layer) from austenitic stainless steel and bearing part from low-carbon steel was investigated. The pictures of localization of the plastic flow during the process of uniaxial tension were obtained by means of the digital image correlation method (DIC). Using optical microscopy methods, the changes in the fracture surface were investigated. The deformation diagrams were examined for deformed samples of tri-layered metal. These were found to show all the plastic flow stages: the linear, parabolic and pre-failure stages would occur for the respective values of the exponent n from the Lüdwik-Holomon equation. The analysis of the plastic flow stages and localized plastic deformation parameters was performed.