Deformation and stress theory of surrounding rock of shallow circular tunnel based on complex variable function method

After reviewing many literature foundations, the thesis combines the basic methods of elastic mechanics with mathematical knowledge, sets the bipotential stress potential complex function and analyses the relationship between stress component, strain component and stress potential function, and applies the complex variable function. The expression of the relevant stress component is derived, and the displacement boundary conditions of the surrounding rock of shallow circular tunnel are obtained. Furthermore, the paper applies the basic theory of complex variable function to solve the boundary condition complex variable function for common tunnel sections, and obtains the analytical expression of the surrounding rock stress of shallow circular tunnel. The simulation is carried out by finite element method. The establishment of complex variable function has a good application value in solving the stress of surrounding rock of shallow tunnel.

Constraining tectonic components during a geomechanics-aided successful hydrofracturing campaign of tight gas exploration field

The horizontal stress profile plays an important role, extending from wellbore stability analysis to well completion optimization of tight gas reservoirs. When considering exploration fields with planned wells being drilled to 5500-m TVD, it is imperative to quantify tectonic effects at the well location. In addition, accurately predicting stress profile and fracture initiation values in vertical wells is required to identify sweet zones and barriers. This paper presents the details of a pre-fracture geomechanical model using breakouts and advanced acoustic data for post-fracture analysis. The analysis contains a history match of fracture initiation pressure, which consider the effects of filter cake around permeable sand, variation in tensile strength, and quantification of horizontal stress contrast in the different fields. Overall, three reservoirs have been analyzed, each containing more than eight wells with operations history. Core tests were used to calibrate dynamic-to-static rock elastic and mechanical properties, both of which reduced uncertainty in the model. The poroelastic horizontal strain method was used to build a continuous stress profile. Typically, the rock fabric found in the cores, images, and anisotropy data from the three reservoirs is different and required various dynamic-to static conversions. The Aeolian deposits-based reservoir has a wide variation in horizontal stress, and fracture height is typically governed by the stiffness of the layers. The lower permeability zones have relatively higher tensile strength, compared with higher permeability zones leading to relatively higher fracture initiation values. Overall, the ratio of maximum horizontal stress-to-minimum horizontal stress varies between 1.20 and 1.28 based on post-fracture analysis, which correlates well with regional tectonics and structural data. Depending on lithological variation and structure changes, the horizontal strain component varies at the layer level within regional tectonics. Inversion of fracturing data helped to constrain horizontal strain and stress variations in the field.

A Biaxial Strain Sensor Using a Single MoS2 Grating

In this paper, we report a new type of MoS2-based grating sensor for in-plane biaxial strain gauges with a precision limit of ~ 1‰. The MoS2 grating is numerically simulated with different biaxial strains up to 5%. Our first-principles calculations reveal that the strain sensitivity of the MoS2 reflectance spectrum can be considered an additional strain sensor integrated with the grating structure, enabling the mapping of in-plane biaxial strains. Our experimental studies on a prototype MoS2-grating sensor further confirm that a strain component perpendicular to the grating period can cause intensity peak shifts in the grating’s first-order diffraction patterns. This work opens a new path towards the sensing of in-plane biaxial strain within a single-grating device. Our new approach is applicable for other materials that have predictable reflectance response under biaxial strains and the capacity to form a two-dimensional single-crystal layer.

Distributed Fibre Optic Sensor-Based Continuous Strain Measurement along Semicircular Paths Using Strain Transformation Approach

Distributed fibre optic sensors (DFOS) are popular for structural health monitoring applications in large engineering infrastructure because of their ability to provide spatial strain measurements continuously along their lengths. Curved paths, particularly semicircular paths, are quite common for optical fibre placement in large structures in addition to straight paths. Optical fibre sensors embedded in a curved path configuration typically measure a component of strain, which often cannot be validated using traditional approaches. Thus, for most applications, strain measured along curved paths is ignored as there is no proper validation tool to ensure the accuracy of the measured strains. To overcome this, an analytical strain transformation equation has been developed and is presented here. This equation transforms the horizontal and vertical strain components obtained along a curved semicircular path into a strain component, which acts tangentially as it travels along the curved fibre path. This approach is validated numerically and experimentally for a DFOS installed on a steel specimen with straight and curved paths. Under tensile and flexural loading scenarios, the horizontal and vertical strain components were obtained numerically using finite element analysis and experimentally using strain rosettes and then, substituted into the proposed strain transformation equation for deriving the transformed strain values. Subsequently, the derived strain values obtained from the proposed transformation equation were validated by comparing them with the experimentally measured DFOS strains in the curved region. Additionally, this study has also shown that a localised damage to the DFOS coating will not impact the functionality of the sensor at the remaining locations along its length. In summary, this paper presents a valid strain transformation equation, which can be used for transforming the numerical simulation results into the DFOS measurements along a semicircular path. This would allow for a larger scope of spatial strains measurements, which would otherwise be ignored in practice.

Cyclic Creep Behavior and Modified Life Prediction of Bainite 2.25Cr-1Mo Steel at 455 °C

Uniaxial static and cyclic creep tests were carried out on bainite 2.25Cr-1Mo steel at 455 °C. Effects of the unloading rate from 0.6 to 39 MPa/s and valley stress duration from 0 to 30 min on the cyclic creep deformation behavior were discussed. The results indicated that the fracture behavior under static and cyclic creep conditions showed a consistent ductile mode. The strain accumulation rate under cyclic creep was significantly retarded as compared with static creep due to the presence of anelastic recovery which was apparently influenced by the unloading conditions. For cyclic creep tests, the unrecoverable strain component determined by a systematic classification of the stress–strain curve was the true damage. A modified life prediction method proposed based on the unrecoverable strain component presented a good life prediction for cyclic creep.

Deriving Pavement Deflection Indices from Layered Elastic Theory

Pavement deflection indices such as the surface curvature index or base damage index are widely used to characterize the condition of pavements. Often, the indices are constructed to provide an estimate of a particular strain component somewhere in the pavement structure. Since the pavement damage is a function of the strain, structural indices provide a simple way of estimating the damage rate in a pavement. Despite their widespread use, it has so far proven difficult to derive deflection indices from first principles; instead, new indices are found by evaluating a range of candidates using a trial-and-error approach. In this paper, a systematic method of deriving deflection indices is presented. The method is based on applying the convolution theorem for Hankel transforms to the solution of a layered elastic problem. Besides allowing for easy derivation of new deflection indices, the method can be used to account for the impact of varying layer thicknesses in the pavement.

Mortality among immigrant patients 20–45 years of age with chronic back pain in primary care in Sweden: A 15-year follow-up cohort study

BACKGROUND: Mortality rates among immigrant patients undergoing rehabilitation for musculoskeletal backache are unknown. OBJECTIVE: To study the association between marital status, severe psychosocial strain, receiving long-term time-limited sickness allowance (TLSA) and all-cause mortality (ACM) in a cohort of immigrants aged 20–45 years with long-standing backache in Sweden. METHODS: We studied 318 patients (92% foreign-born, 76% non-European) of known marital status on sick-leave for musculoskeletal backache. They were followed up for ACM until 2015. Socio-demographic data, TLSA and psychosocial strain, including major depression, severe psychosocial stressors and pessimistic thoughts, were analysed using multiple-imputation Cox regression. RESULTS: Over a mean (standard deviation) follow-up time of 15 (5.0) years, 11 (3.5%) participants died. At baseline, 34% were unmarried, 19% were receiving TLSA, and 71% had ⩾ 1 psychosocial strain component (38% depression; 47% severe stressors; 35% pessimistic thoughts). After concomitant risk factors were adjusted for, being unmarried and receiving TLSA were associated with higher mortality by factors of 6.2 (p= 0.005) and 5.8 (p= 0.006), respectively. Psychosocial strain was only significantly associated with higher mortality in the unadjusted analyses. CONCLUSIONS: Being unmarried and receiving TLSA were associated with significantly higher ACM in this highly marginalized group of immigrant patients.

The Potential of Using Dynamic Strains in Earthquake Early Warning Applications

We investigate the potential of using borehole strainmeter data from the Network of the Americas (NOTA) and the U.S. Geological Survey networks to estimate earthquake moment magnitudes for earthquake early warning (EEW) applications. We derive an empirical equation relating peak dynamic strain, earthquake moment magnitude, and hypocentral distance, and investigate the effects of different types of instrument calibration on model misfit. We find that raw (uncalibrated) strains fit the model as accurately as calibrated strains. We test the model by estimating moment magnitudes of the largest two earthquakes in the July 2019 Ridgecrest earthquake sequence—the M 6.4 foreshock and the M 7.1 mainshock—using two strainmeters located within ∼50 km of the rupture. In both the cases, the magnitude based on the dynamic strain component is within ∼0.1–0.4 magnitude units of the catalog moment magnitude. We then compare the temporal evolution of our strain-derived magnitudes for the largest two Ridgecrest events to the real-time performance of the ShakeAlert EEW System (SAS). The final magnitudes from NOTA borehole strainmeters are close to SAS real-time estimates for the M 6.4 foreshock, and significantly more accurate for the M 7.1 mainshock.

Transverse Right Ventricle Strain and Strain Rate Assessed by 2-Dimensional Speckle Tracking Echocardiography in Dogs with Pulmonary Hypertension

Right ventricular (RV) strain analysis using 2-dimensional speckle tracking echocardiography (2D STE) permits practitioners to assess regional and global deformation of the myocardium. Recently, assessment of the longitudinal right ventricle (RV) strain and strain rate using 2D STE has been reported in dogs. Although longitudinal deformation is the dominant component of the RV systole, RV myocardial fibers of the superficial layer are oriented circumferentially and these contribute to the RV pump function (radial deformation). Because this strain component has not been investigated in dogs, we have assessed radial RV strain and strain rate using 2D STE in healthy dogs and dogs with pulmonary hypertension (PH). We have recruited 74 dogs (40 healthy dogs and 34 dogs with PH) in which we have analyzed radial RV free wall strain and strain rate using XstrainTM software. We have used the left apical 4-chamber view optimized for the RV for analysis of the radial strain and strain rate variables (XstrainTM software denoted radial strain as “transverse”). Seven dogs were excluded during the analysis for low quality images. Transverse strain and strain rate obtained in healthy dogs showed no relationship with heart rate, body weight or age (r2 < 0.09 and p > 0.05 for all variables). Moreover, no relationship between transverse strain/strain rate variables and left atrial-to-aortic ratios was observed (r2 < 0.06 and p = 0.2, for both). Transverse strain and strain rate obtained in dogs with PH, showed weak negative relationships with tricuspid regurgitation velocity (r2 < 0.25 and p = 0.006, for both). Transverse RV strain and strain rate using 2D STE is feasible in most dogs and decrease with worsening of the PH, but these advanced echocardiographic indices do not help in identifying dogs with PH.