Average Stress State Research Articles

Segment Cracking and Damage Mechanism of Curved Shield Tunnels During Construction by Field Investigation and Modeling

This study was based on the field statistical results of segment cracking and damage of a curved shield tunnel engineering. Combined with the collected data of 11 related tunneling parameters of 120 segments in the curve section, the one-way analysis of the variance method was used to reveal the main tunneling parameters that affect the cracking and damage state of the segment. Then, the finite element analysis model was established to analyze the mechanism of jack thrust and yawing angle affecting segment cracking and damage. The main results are as follows: (1) The phenomenon of segment cracking and damage gradually becomes serious from the crown to the invert. (2) The jack thrust and shield machine yawing angle are the main factors affecting the cracking and damage state of the segment. For larger values of yawing angle and jack thrust difference, the cracking and damage state of the segment is more serious. (3) The first three segments near the jack are significantly affected by the jack offset angle and uneven jack thrust. During the construction of a curved shield tunnel, under the action of the jack offset angle and uneven jack thrust, the segment spring line is more likely to incur tensile cracking. (4) To ensure the normal stress state of the segment during construction, it is suggested to control the jack offset angle at not more than 0.5° and the jack thrust non-uniform coefficient [Formula: see text] of the left- and right-hand sides within 0.75.

Determining the optimal sampling interval for 3D morphology measurements of different-sized natural rock joints

Efficiently studying the scale effect of surface roughness and peak shear strength of natural rock joints, especially unfilled and well-matched ones, requires an optimal sampling interval to realize cost-effective three-dimensional morphology measurements for different-sized natural rock joints with advanced non-contact optical devices. However, rationally determining such an optimal sampling interval remains controversial. To this end, inspired by digital signal processing, the optimal sampling interval is creatively determined by Shannon's sampling theorem using the Fourier series' optimum order. An empirical formula relating the joint size to the optimum order is correspondingly developed. The proposed empirical formula was found to work well under conditions of high normal stress, where the relative errors for the surface roughness and peak shear strength of series-sized natural rock joints were mostly less than 10% and 5%, respectively, but should be used cautiously under conditions of low normal stress. Furthermore, different from previous perceptions, a higher joint roughness level was discovered to not necessarily imply the need for a smaller optimal sampling interval, and conversely. The proposed empirical formula can facilitate the determination of the optimal sampling interval for measuring different-sized natural rock joints' surface morphologies in engineering practice.

Analysis and prediction of contact characteristics of rock fracture surfaces under normal loading

The real contact area of rock fractures is smaller than the nominal area, and actual stresses on contacts are significantly higher. Evaluating contact characteristics and predicting potential contact areas are crucial for understanding mechanical and hydraulic properties of geological fractures under compression or shear. We measured the real contact area of mated tensile fractures using pressure-sensitive films and studied their contact characteristics and evolution under different normal stresses through point cloud analysis. It is revealed that the initial aperture of mated tensile fractures determines the contact state. Under conditions of low normal stress, regions exhibiting higher mean curvature are more susceptible to contact formation, particularly when aperture sizes are comparable. This propensity is further influenced by the directional orientation of the normal vector components within these regions. Increasing normal stress leads to faster development of new contacts in regions with smaller initial apertures. Additionally, we predicted contact characteristics and real contact area of tensile fractures using back propagation artificial neural network. Comparison between predictions and laboratory measurements demonstrated accurate prediction of contact distribution and area values by the neural network. Our findings enable the prediction of real stress states, potential failure areas, and hydraulic conductivity of rock fractures in slope and underground rock engineering.

Study of the influence of ZrB2 content and thermal shock on the elastic modulus of spark plasma sintered ZrB2-B4C composites using a non-destructive ultrasonic technique

Non-destructive ultrasound phase spectroscopy (UPS) was used to study the influence of thermal shock on the longitudinal elastic constant (C11) of ZrB2-B4C composites. C11 of the composites was measured in the as-fabricated condition and after different numbers of thermal shock cycles. The relative change in C11 was used as the thermal shock resistance parameter. Composites with 70 and 50 vol% ZrB2 were subjected to three thermal shock temperature differences (ΔT) of 600 °C, 800 °C, and 1000 °C, respectively. The rate of drop in C11 increased significantly with an increase in ΔT and B4C content. The mechanism of thermal shock damage has been discussed considering the influence of processing-induced thermal residual stresses and the average stress state in both phases during thermal shock. It was shown that the powder particle arrangement plays a major role in the damage mechanism and B4C-B4C particle contacts provide easy paths for crack propagation.

Experimental Investigations on load‐bearing Capacity and characteristic Preload of Lockstud Systems

AbstractThe joining technology Lockstud System combines the advantages of a bolt thread for making a tapped thread joint and the advantages of a lockbolt. On the tapped thread joining side, the bolt has a metric ISO thread. On the assembly side, the bolt has a grooved geometry like lockbolts of type A, B and C. This article publishes results on the mechanical‐technological properties, the assembly process with the initial preload, the preload losses due to embedding under long period of time and the load‐bearing capacity subjected to static and fatigue normal stress for Lockstud systems of nominal diameters M12, M16 and M20 (strength grade 10.9) compared to conventional bolt assemblies and lockbolt systems.

Softening/Hardening Damage Model and Numerical Implementation of Seabed Silt-Steel Interface in Yellow River Underwater Delta

The interaction between soil and structure is a research hotspot in ocean engineering, and the shear performance of interfaces is an essential factor affecting the bearing capacity of offshore structures. Taking the Yellow River Underwater Delta as the research area, the Softening/Hardening damage model of the silt–steel interface and the determination method of model parameters are proposed based on the statistical damage theory. Through the interface monotonic shear test under the conditions of different normal stress, roughness and water content, the shear mechanical properties and volumetric deformation laws on the silt–steel interface are analyzed, and the damage model parameters are obtained. Finally, a FRIC subroutine for the damage model was developed based on ABAQUS. The research results indicate the following: (1) The interface between silt and steel exhibits two characteristics, softening/hardening and shear shrinkage/expansion, under different conditions. Roughness significantly impacts interfacial cohesion, while water content mainly affects the internal friction angle. (2) The softening model based on the classic rock damage model can better simulate the stress–strain relationship of the silt–steel interface under high normal stress and low water content. In contrast, the hardening model based on the classic hyperbola model can better simulate the stress–strain relationship under low normal stress and high water content. The calculated results of the softening/hardening model agree with the experimental results, and the model has 7 parameters. (3) The developed FRIC subroutine can effectively simulate the nonlinear mechanical behavior of the interface between silt and steel. The research results provide a reference for exploring the stability analysis of offshore structures considering interface weakening effects.

Determination of stress in thin films using micro-machined buckled membranes

In this work, optical profilometry and finite-element simulations are applied on buckled micromachined membranes for the stress analysis of ion-beam-sputtered Ta2O5 and SiO2 thin films. Layers with different thicknesses are grown on silicon substrates, and then several membranes with different geometries are manufactured with standard microsystem technologies; due to a high level of films’ compressive stress, buckled membranes are obtained. Thermally grown silica membranes are also produced for comparison. The residual stress values are determined by comparing the measured and simulated deflections of the membranes. The average stress state of Ta2O5 thin films is found to be −209 MPa. The SiO2 thin films are in a higher compressive stress state whose average value is −576 MPa. For comparison, the average stress in thermal SiO2 thin layers grown at 1130°C is found equal to −321 MPa, in good agreement with the literature.

Research on the Aging Effects of Adhesive and Composites on the Degradation Mechanism of Composite Adhesive Joints in Thermal-Humidity-Cycles for Automobiles

ABSTRACT The existing studies on the degradation mechanism of composite adhesive joints mostly focus on the aging of adhesives, but the aging effect of composites is often ignored. In this paper, the degradation mechanism of adhesive, CFRP, as well as shear joints (SJ) and butt joints (BJ) in thermal-humidity-cycles (T-H-C) for automobiles were investigated by chemical detection and mechanical testing. The aging effects of adhesives and composites are compared and analyzed by the variation in chemical and mechanical properties to reveal the degradation mechanism of composite adhesive joints. Experimental testing show that the adhesive and CFRP underwent hydrolysis and oxidative decomposition respectively, and there was a reduction in Tg (glass transition temperature) and thermal stability. The joint strength of composite adhesive joints fell by around 40% after 30d (days), and interfacial failure appeared. The variation in the composition of adhesive and CFRP, interfacial failure and thermal stress contributed to the degradation of composite adhesive joints. Degradation mechanism analysis shows that SJ under shear stress state were less affected by CFRP aging, while BJ under normal stress state were more affected by CFRP aging and the effect became more obvious with time growth.

In situ stress variation and coal reservoir permeability response of the Jurassic Yan'an formation in the southwestern Ordos basin, China: Its impact on coalbed methane development

The in situ stress (ISS) and permeability determine the coalbed methane (CBM) development method and extraction efficiency to a certain extent. Based on the data of the CBM well test and microseismic monitoring in the Jurassic Yan'an Formation, a prediction model of the ISS is established, and its influence on permeability and CBM development in the southwestern Ordos Basin, China, is discussed. The results show that the orientations of maximum principal stresses (Shmax) in the study area's eastern, central, and western parts are NE–SW, SEE–NWW, and NNW–SSE, respectively. With the increased burial depth, the main ISS parameters show an S-shaped variation. The study area's ISS state can be divided into Shmax > vertical stress (Sv) > horizontal minimum principal stress (Shmin) (<420 m), Sv > Shmax > Shmin (420 m–800 m), and Shmax ≈ Sv > Shmin (>800 m), and the transformation depth of the ISS field is 550 m and 1000 m. The lateral pressure coefficients (λ) ranged from 0.30 to 1.19, with an average of 0.69. The λ, Shmax/Shmin and Shmin/Sυ of the stress field are small, which indicates that the horizontal tectonic stress is weak. The coal reservoir's permeability is distributed between 0.0084 and 6.84 mD (av. 1.13 mD). Affected by the variation of the ISS state in the vertical direction, the coal reservoir permeability shows obvious zoning. The tensile stress field is beneficial to high-permeability coal reservoir formation, but it is constrained by the coupling constraints of a coal reservoir's confining pressure and effective stress. The coal reservoirs are in a normal fault stress state, which is beneficial for improving their hydraulic fracturing effect. Since the Shmin in the study area is always smaller than the Sυ, the height of the fracturing fracture should be controlled in the fracturing of the coal reservoir to avoid the overlying aquifer being connected by fracturing fractures. The above new insights have important guiding significance for the hyperpermeability CBM reservoir prediction, well-bore structure and hydraulic fracturing design in the Ordos Basin.

Efficient characterization on the interlayer shear strengths of 3D printing polymers

Shear strength measurement becomes increasingly important because three-dimensional (3D) printing materials are used in load-bearing structures subjected to a combined normal and shear stress state. Hence, this paper presents a combined experimental and numerical investigation of the interlayer shear strength measurement approach, and its application to four kinds of 3D printing polymers including acrylonitrile butadiene styrene (ABS), polycarbonate (PC), and polylactic acid (PLA) made with fused deposition modeling (FDM), and polyamide 12 (PA12) made with selective laser sintering (SLS). First, a traditional Iosipescu shear specimen was employed to validate the uniform shear strain field using digital image correlation. Second, a new necking-shaped shear specimen was proposed to measure the shear strength efficiently with the aid of an effective alignment assistant beam. Third, 3D finite element analysis was conducted to highlight the stress comparisons of the Iosipescu shear specimen and the necking-shaped shear specimen. In order to quantify the anisotropic strengths, three kinds of shear specimens with different build directions were designed and tested. The results demonstrated that the minimum shear strength of a 3D printing polymer specimen was the interlayer shear strength when the shear force was acting along the printing surface. Our measurement was consistent with the shear strength measurements of the same type of 3D printing PA reported by other researchers. Moreover, our measurement was more conservative and accurate due to the pure shear stress state of all specimens.

Digital image-based performance evaluation of GCL-sand interfaces under repeated shearing

Inadequate shear strength mobilization at the interfaces results in translational failures in Geosynthetic Clay Liners (GCL). Periodic addition of solid waste into the landfill causes additional normal and shear stresses in GCLs. The mechanical response of GCLs and fibre strength are highly time-dependent. Hence the interface shear resistance reduces under the application of repeated shear cycles. To simulate these conditions, a repeated interface shearing test was conceptualized in this study. A natural river sand and a manufactured sand of identical gradation were used in experiments. Each GCL-sand interface was subjected to eight cycles of shearing in dry and hydrated conditions under three different static normal stresses. Results showed that the variation of the peak interface shear stress has different phases, governed by different mechanisms. Digital image analysis of tested GCL surfaces after each shearing cycle provided important clues for this response. Entrapment of sand particles into GCL surfaces is beneficial initially because of increased friction at the interface and this benefit is more pronounced in the case of manufactured sand, due to the irregular shape of particles. Quantification of sand particle entrapment and surface changes to GCL helped in understanding these micro-level interaction mechanisms.

Characteristics and application of present in situ stress field of a strike-slip fault: a 3D finite-element simulation study

AbstractPrevious hydrocarbon explorations in the middle of the Tarim Basin indicate that strike-slip faults play an important role in the development of Ordovician carbonate reservoirs and hydrocarbon accumulation. The SB5 fault in the Tarim Basin was the target of this investigation. An evaluation of the stress in situ was carried out and provided boundary conditions to build a 3D geomechanical model. The distribution and application of present in situ stress in the strike-slip fault were studied. The results show good agreement between the absolute measured stress in situ and the modelled stresses, revealing a different stress regime along the strike-slip fault. The uplift segment belongs to a strike-slip stress state, and other areas belong to a normal fault stress state. The strike-slip fault has a significant influence on the present in situ stress distribution. The direction of the maximum horizontal stress deflects near the fault and tends to be parallel to the fault strike. This work introduces a comprehensive evaluation of the present in situ stress of the fractured carbonate reservoirs controlled by the strike-slip fault system. The present in situ stress direction can clarify the propagation direction of hydraulic fracturing and serve to evaluate the effectiveness of natural fractures.

Mechanical Characteristics and Acoustic Emission Characteristics of Mortar-Rock Binary Medium

The stability of the interface between mortar and rock is very important in engineering construction. In this paper, the all-digital acoustic emission (AE) system is used to detect the direct shear test of the mortar-rock binary medium interface with different sawtooth angles under different normal stress states. The stress-displacement information and AE signal during the whole shearing process are extracted. The coupling relationship between stress and AE characteristic parameters is discussed. The quantitative relationship between sawtooth angle and shear strength of binary medium is established, and three AE characteristic parameters that can be used to predict structural instability are proposed. The research shows that: With the increase of the normal stress and the sawtooth angle, the shear strength of the mortar-rock binary medium increases. The relationship of that is obtained by least squares fitting. The shear stress-displacement curve is divided into five stages according to the change of deformation law. Through the analysis of AE characteristic parameters, it is found that increasing the sawtooth angle makes the AE count and AE cumulative count increase. Based on the analysis of the characteristic parameters of RA-AF, the changes of shear cracks and tensile cracks within the whole shearing process were obtained, respectively. In the process of binary medium shearing, the AE peak frequency is in the range of 120–340 kHz. Three acoustic emission parameters that can predict the macroscopic damage of binary media are obtained: the AE b value, the ratio of shear crack signals, and the number of signals with a peak frequency of 220 Hz to 320 Hz.

Shear failure process of rectangular tunnel: Physical experimental test and numerical back-analysis

Shear failure is a typical failure mode in underground engineering and has resulted in a large number of instability issues. To study the shear failure mechanism, physical shearing tests for rectangular tunnel of sandstone, including single rectangular tunnel (SRT) and double rectangular tunnels (DRT) specimens, were carried out under three normal stress states. During the shearing tests, the internal failure process and the external cracks evolution of tunnel specimens were recorded by video camera, and the acoustic emission (AE) events of specimens were also monitored for exposing the mesoscopic failure characteristics. In addition, the discrete element method was also applied to analyze the corresponding shear experiment of rectangular-tunnel specimens. The tested results showed that the shear strength, shear modulus of the samples were positively correlated with the normal stress. For example, the shear strength, and shear modulus of SRT increased from 5.18 MPa to 8.70 MPa, and 1.02GPa to 1.88GPa, respectively when the normal stress increased from 2.0 MPa to 5.0 MPa. More importantly, the shear failure of rectangular tunnel model can be generally divided into four stages: (i) Shearing compression stage characterized by loading contacting and compressing; (ii) Shearing micro-crack propagation stage dominated by inner particles ejection of tunnel, tensile crack initiation and shear crack propagation of specimen; (iii) Shearing fracture stage dominated by the specimen’s crack propagation, coalescence, penetration, and corresponding rock ejection; (iv) Shearing friction stage dominated by shear damage of tunnel specimen. Corresponding back analysis by numerical simulation and scanning electron microscope results indicated that the shear failure of rectangular-tunnel specimen can be classified as tensile-shear mixed failure mode. The research results provide a new cures for shear prevention and support design of rectangular tunnels in underground engineering.

Geosynthetic Interface Friction at Low Normal Stress: Two Approaches with Increasing Shear Loading

The evaluation of geosynthetic interface friction is a key parameter for the stability of coupled geosynthetics, as in landfill capping liner. At the present time, few types of tests are suitable for measuring the interface friction at low normal stress: one of these is the inclined plane, usually carried out under a vertical stress of 5 kPa. This type of test is not without critical aspects, mainly due to the nonuniform normal stress state induced by the inclination of the plane, but, on the other hand, the most widespread direct shear test generally cannot be performed at such low values of normal stress. After a short discussion on the pros and cons of these two types of test, the paper presents a comparison of the interface friction angles obtained, for three interfaces, by means of an inclined plane and an unconventional direct shear apparatus, under the same low normal stress. The peculiarity of this latter device is of ensuring a gradual increase of the mobilized strength, in a way similar to what occurs during the inclined plane test. The good correspondence of the results of the two types of tests confirmed the validity of both the test approaches.

Theoretical Analysis for Stability Evaluation of Rock Mass Engineering Structure under Combined Compression-Shear Loading: A Case Study of Inclined Pillar

An inclined pillar is a typical support structure under compression and shear loads for underground mining. The shear load caused by the inclination of the ore-body affects the bearing capacity of the pillar. At present, there is no systematic investigation on the influence of shear load on the stress state evolution and bearing capacity of the inclined pillar. Additionally, there is still a lack of effective evaluation of the bearing capacity of the inclined pillar in the presence of additional shear load. In this research, the theoretical analysis method is used to solve these problems. First, the compressive and shear load components on the inclined pillar were calculated by the tributary area method, and the average stress state of the inclined pillar, considering the influence of the shear load, was characterized by a series of generalized stress circles. The factors that affect the shear load, such as the area extraction ratio, the inclination of the ore-body, and the in-situ stress ratio, were analyzed, and it reveals that there are three kinds of stress paths of the inclined pillar and their trajectories are straight line, circle, and curve, respectively. Then, a shear strength model was proposed to evaluate the bearing capacity of inclined pillars. The expression of this model is multiplied by a vertical pillar strength model and a dimensionless coefficient that is named the contribution factor of shear load (CFSL). Some cases of inclined pillars were employed to verify the rationality of this model. Finally, the factors that affect the bearing capacity of pillars were analyzed. This investigation presents that the shear load affects the stress path and determines the bearing capacity of the pillar. Therefore, the shear load should not be neglected in pillar design and stability analysis.

The effect of the graded bilayer design on the strain depth profiles and microstructure of Cu/W nano-multilayers

The properties and thermal stability of thin films and nano-multilayers (NMLs) are generally governed by the in-depth stress (strain) gradients rather than the average stress state. The effect of strain gradient variation in Cu/W NMLs on the thermal stability between 400 and 800 °C was investigated. The strain distribution in the NML stacks was varied by combining Cu/W bilayers with different Cu and W thicknesses of either 3 or 10 nm. A recently developed method based on in-plane grazing X-ray diffraction was adopted to extract the strain depth profiles. In addition, the evolution of the average stress in the Cu/W NMLs during growth was monitored by an in-situ wafer curvature technique. The mean residual stresses in Cu and W were found to be independent of the disposition of the different Cu/W bilayer substacks. On the contrary, the strain depth profile of the W nanolayers was found to strongly depend on the disposition of Cu/W bilayer substacks in the Cu/W NML, which resulted in different Cu outflow characteristics upon annealing. Moreover, application of different Cu/W bilayer units within the NML stack also provides an innovative pathway for producing Cu/W nanocomposites with graded thermal and mechanical properties.

Residual stress analysis of aluminum nitride piezoelectric micromachined ultrasonic transducers using Raman spectroscopy

In this study, the Raman biaxial stress coefficients KII and strain-free phonon frequencies ω0 have been determined for the E2 (low), E2 (high), and A1 (LO) phonon modes of aluminum nitride, AlN, using both experimental and theoretical approaches. The E2 (high) mode of AlN is recommended for the residual stress analysis of AlN due to its high sensitivity and the largest signal-to-noise ratio among the studied modes. The E2 (high) Raman biaxial stress coefficient of −3.8 cm−1/GPa and strain-free phonon frequency of 656.68 cm−1 were then applied to perform both macroscopic and microscopic stress mappings. For macroscopic stress evaluation, the spatial variation of residual stress was measured across an AlN-on-Si wafer prepared by sputter deposition. A cross-wafer variation in residual stress of ∼150 MPa was observed regardless of the average stress state of the film. Microscopic stress evaluation was performed on AlN piezoelectric micromachined ultrasonic transducers (pMUTs) with submicrometer spatial resolution. These measurements were used to assess the effect of device fabrication on residual stress distribution in an individual pMUT and the effect of residual stress on the resonance frequency. At ∼20 μm directly outside the outer edge of the pMUT electrode, a large lateral spatial variation in residual stress of ∼100 MPa was measured, highlighting the impact of metallization structures on residual stress in the AlN film.

Loading conditions constrained wrinkling behaviors of thin-walled sheet/tube parts during metal forming

• Initial geometric imperfections (GI) make the wrinkling prediction more sensitive. • The wrinkling is more prone to occur under tension-compression stress state. • Increasing normal constraints can effectively reduce wrinkling instability. • Different loading conditions cause the inconsistent effects of the material role on wrinkling. With the increasing usages of high strength and lightweight materials, how to accurately predict and fully understand the wrinkling instability of thin-walled parts forming under various loading conditions, viz. stress states and normal constraints, is still a challenge for free-defect design and manufacturing. In this study, taking tension-compression stress state w/wo normal constraints (Yoshida test and deep drawing of sheets) and compression stress state w/wo normal constraints (axial compression and rotary draw bending of tubes) as the comparative cases, two kinds of geometric imperfections (GI), viz., random thickness imperfection (RTI) and geometric deflection imperfection (GDI) are introuduced and implemented into dynamic explicit finite element (FE) models to realize more sensitive prediction of the wrinkling instability. And then, a systematic comparative study on the critical wrinkling behavior and final wrinkling morphology under varous loading conditions has been studied. It is found that the wrinkling is more prone to occur under tension-compression stress state, and the wrinkling of the processes without normal constraint are severer than those with normal constraints. Furthermore, the effects of material parameters on the wrinkling are inconsistent in different processes, the reason proved to be the different loading conditions. When the degree of normal constraint increases, the influence of material properties becomes negligible. Combining the geometric and material parameters, the key issues affecting the wrinkling of thin-walled parts from the perspective of loading condition have been expounded, laying the foundation and theoretical guidance for the study of wrinkling behavior under different mechanical states.

Calculation of Stress-Strain State and Contact Stresses in the Process of Chip Formation

The reported paper presents a field pattern of sliding lines in a plastic zone of flow chip formation in the process of cutting metals considered for front and rear angles on the cutting blade other than zero. Equations of sliding lines for these conditions are proposed. Using calculation data, plasticity zone border lines are plotted for positive and negative front angles. The paper suggests methods and calculation data on average stress and plane stress state components in nodal points of the plastic zone of chip formation. The study provides data for plotting distribution diagrams of normal and tangential stresses of contact stresses on work surfaces of a cutting blade.