Stress Transfer Mechanism Research Articles

Surrogate analytical models of damage localization in metal matrix composites

Several parameters entering the constitutive laws of metal matrix composites are not amenable to direct measurement. Their determination is therefore demanded to indirect identification procedures, based on the comparison between the output of some experimental works and the results of the iterative simulation of the performed tests. Largely repetitive numerical analyses can also support the optimized design of composite microstructures, the understanding of the actual stress-transfer mechanisms, and virtual testing. Surrogate analytical models can replace effectively non-linear finite element (FE) computations in parametric studies and identification procedures, reducing execution times and costs without compromising the accuracy of the results. In this context, a frequently used approach consists of the interpolation, by means of radial basis functions (RBFs), of data filtered by proper orthogonal decomposition (POD). This methodology is often used to reproduce the smooth output of different mechanical systems. This work is rather focused on the homogenized response of damaging metal matrix composites subjected to strain localization phenomena, and explores the accuracy achievable in these situations by the POD-RBF approximation of more traditional FE analyses. In all considered cases, the POD-RBF results are accurate, and able to distinguish between apparently similar situations. The approach is flexible, and the performance of the surrogate models can be tailored to the requirements of each application. In particular, various analytical approximations can be introduced to support the design of new microstructures and material couplings, and to understand the role of any material and/or geometry imperfections resulting from the production processes.

Mechanical Behavior of Fully Grouted Rock Bolts in Hydraulic Tunnels Subjected to Elevated Ground Temperatures

In this study, the mechanical behavior of fully grouted rock bolts in hydraulic tunnels subjected to elevated ground temperatures was investigated. A differential equation for axial displacement of the rock bolt was formulated, which considers the force equilibrium of infinitesimal bolt segments and the stress transfer mechanism at the anchor–rock interface. The distribution functions for axial stress within the bolt and the interfacial shear stress were obtained by solving the differential equation, which incorporated the displacement of the surrounding rock mass as a parameter. This study showed that the effectiveness of the bolt–shotcrete support system decreases over time, considering the displacement relaxation rate of the surrounding rock mass. The mechanical model’s variation laws at 20 °C, 50 °C, and 80 °C were summarized by integrating the thermal deformation equation for material parameters, and the numerical simulation results were compared and analyzed. The findings revealed that the bond strength between the rock bolt and the rock mass diminishes as the temperature of the surrounding rock increases, leading to a reduction of interfacial shear stress at both extremities of the bolt. Moreover, the maximum axial force within the bolt escalates as the neutral point migrates farther from the tunnel wall.

Simulation on pull-out performance of steel bar from 3D-printed concrete

In this study, a finite element (FE) model for the pull-out test was established and verified to examine the influence of printed concrete-to-concrete interfaces on the pull-out performance of steel bar from 3D-printed concrete. The stress transfer, debonding process, and failure mechanism of 3D-printed specimen were revealed by detailed interfacial stress analysis. Moreover, the effects of the interfacial bond strength, filament size, angle between the steel bar and the printing direction (α), and bond length on the pull-out performance were intensively investigated. The numerical results showed that the failure pattern changed from the conventional pull-out failure to interfacial bond failure induced by 3D-printed concrete-to-concrete interfaces. Compared with the pull-out specimen with α = 0°, the peak load of pull-out specimen with α = 90° increases by 9.6%, which became the weakest when angle α = 30°. Moreover, the failure mode was observed to be influenced by the relative strength of the interlayer and interstrip interfaces and their distributions.

Shear mechanical behavior and stress transfer characteristics of the bilayer casing of gob gas ventholes

Induced by multi-seam coal extraction, the casing of gob gas venthole (GGV) is prone to shear deformation or failure, which would result in substantial reduction in the gas production of GGV. The use of bilayer casing in the GGV is conducive to reduce the probability of cemented casing failure. However, the shear mechanical characteristics of the bilayer casing are not clearly understood. In this paper, the shear experiments of the single-layer casing, the bilayer casing with flexible filler (BC-FF), and the bilayer casing with rigid filler (BC-RF) were carried out. Besides, the stress distribution and its evolution of the bilayer casing during shear deformation were numerically simulated. The results showed that the shear deformation of the bilayer casing can be divided into five stages and the BC-RF demonstrated stronger shear strength, while the BC-FF exhibited a higher resistance for shear deformation. Additionally, the stress transfer coefficient was proposed to characterize the degree of stress attenuation during the stress transferring from the outer casing to the inner casing. Based on the numerical simulation results, the stress transfer coefficient of the bilayer casing was calculated. The results showed that the stress transfer coefficient of BC-RF was higher than that of BC-FF and that the peak value of stress transfer coefficient of the bilayer casing increased with the ultimate strength and elastic modulus of the filler. The stress transfer coefficient mainly correlated with both the elastic modulus and the ultimate strength of the filler before reaching the peak. However, it was only determined primarily by the ultimate strength in the fluctuation stage after it reached the peak. The results of this paper will lay the foundation for interpreting the shear mechanical behavior of the bilayer casing and optimizing the design of GGV structure.

Crimped fiber composites: mechanics of a finite-length crimped fiber embedded in a soft matrix.

Composites comprising crimped fibers of finite length embedded in a soft matrix have the potential to mimic the strain-hardening behavior of tissues containing fibrous collagen. Unlike continuous fiber composites, such chopped fiber composites would be flow-processable. Here, we study the fundamental mechanics of stress transfer between a single crimped fiber and the embedding matrix subjected to tensile strain. Finite element simulations show that fibers with large crimp amplitude and large relative modulus straighten significantly at small strain without bearing significant load. At large strain, they become taut and hence bear increasing load. Analogous to straight fiber composites, there is a region near the ends of each fiber which bears much lower stress than the midsection. We show that the stress-transfer mechanics can be captured by a shear lag model where the crimped fiber can be replaced with an equivalent straight fiber whose effective modulus is lower than that of the crimped fiber, but increases with applied strain. This allows estimating the modulus of a composite at low fiber fraction. The degree of strain hardening and the strain needed for strain hardening can be tuned by changing relative modulus of the fibers and the crimp geometry.

Soil arching evolution caused by shield tunneling in deep saturated ground

Shield tunneling is accompanied by variations of stress and displacement in soil, which could induce the soil arching effect. Although there have been many studies on the soil arching effect caused by shield tunneling, the research on deep-buried and saturated conditions is still lacking. In this study, a finite element model was firstly built and verified by field data. Then, two hypothetical finite element models were established with two different strata to investigate the stress transfer mechanisms, the development law of excess pore water pressure, and the soil arching evolution of deep-buried tunnel. The methods to determine the arch zone and loosened zone were proposed and verified, which were adopted to compare the differences of soil arching effect between saturated sandy and clayey soil layers. The results show that the heights of the arch zone and loosened zone in the saturated sandy soil layer are both higher than those in the saturated clayey soil layer after the shield tunneling. The subsequent strata consolidation resulted in a 36% reduction in the arch zone height in clayey soil, which means that time effect for the soil arching is significant. Compared with Terzaghi's loosening earth pressure theory, in deep ground, the theoretical overburden pressure is only 5.0% larger than the numerical results for sandy soil but 18.3% less than that for clayey soil. The numerical results can bring new sights about the soil arching evolution caused by shield tunneling and the corresponding tunnel overburden pressure in deep saturated ground.

Analysis on the stress mechanism of the exposed steel anchor box composite cable–pylon anchorage structure

For the exposed steel anchor box cable–pylon anchorage structure, there is a good application prospect in urban bridges of medium and small span due to its good mechanical behavior, easy construction and beautiful overall appearance. This paper reveals the stress-transfer mechanism of the exposed steel anchor box cable–pylon anchorage structure through a segmental model test with a scaled-down ratio of 1:2, and focused on the effect of prestress in the pylon. Three calculation modes for the anchorage structure are proposed, and simplified calculation equations of the horizontal load distribution relationship and the vertical shear force of the stud are derived. The effects of various design parameters on the load distribution ratio, the pylon wall tension caused by shrinkage and the vertical shear force of the studs are discussed by using the simplified calculation equation. The research results show that the exposed steel anchor box cable–pylon anchorage structure has good applicability in medium and small span bridges. Prestress can significantly reduce the tensile stress of the pylon wall concrete, and improve its crack resistance. There is no significant difference between the results of the simplified calculation equations and the finite element calculation, and the simplified calculation equations meet the design requirement.

Analysis of Non-uniform Local Strain in Corroded Reinforcing Bar in Concrete using Digital Image Correlation

Change in the mechanism of post corrosion bond behavior was investigated in a previous experimental study by analyzing the deformations in concrete using digital image correlation (DIC). Using their DIC data, this study focuses on the local strain non-uniformity in reinforcing bar during the uniaxial tensile test due to the effect of rib height reduction and corrosion layer. The results show that, the local strain below the rib tip is higher than that below the flat part between ribs, due to compressive stress resulting from mechanical interaction at the rib tip. Furthermore, the non-uniformity in local strain is less pronounced in highly corroded reinforcing bar because less compressive stress is transferred due to gentler rib slope. In highly corroded reinforcing bar, the appearance of non-uniformity in local strain is delayed since the accumulated corrosion product on the reinforcing bar surface delays the mechanical interaction and stress transfer between the concrete and the reinforcing bar.

Shear performance of high-strength friction-grip bolted shear connector in prefabricated steel–UHPC composite beams: Finite element modelling and parametric study

As a competitive alternative for accelerated bridge construction (ABC), prefabricated steel–ultra-high-performance concrete (UHPC) composite beams containing high-strength friction-grip bolt (HSFGB) shear connectors offer numerous advantages, including reduced on-site construction time and ease of replacing/removing deteriorated components. However, the failure mechanism of HSFGBs in UHPC remains unclear due to the lack of internal stress analysis, which hinders the design of these innovative composite beams. To clarify the shear performance of HSFGBs in prefabricated steel–UHPC composite beams, an effective finite element model (FEM) considering the non-linearities of materials and geometry was developed through ABAQUS. Based on the experimentally verified model, the internal stress transfer mechanisms of HSFGBs and the failure mechanism of precast UHPC were revealed. According to the extension parametric analysis results, a stronger HSFGB presented better shear performances in terms of ultimate shear strength, initial shear stiffness and slip capacity. Adopting oversized holes with appropriate bolt-to-hole clearance can improve the constructional efficiency without considerable strength and stiffness reduction. HSFGBs with low bolt pretension exhibited unfavorable initial shear stiffness, while smaller slip capacity in high bolt pretension conditions. A smaller ductility was observed as the steel beam tensile strength and slab concrete strength increased. Additionally, the ACI 318–19, Eurocode 3, and AASHTO LRFD specifications underestimated the shear strength of HSFGBs, whereas the Eurocode 4 presented acceptable predictions in determining the ultimate shear capacity of HSFGBs in prefabricated steel–UHPC composite beams.

A novel criterion for yield shear displacement of rock discontinuities based on renormalization group theory

Research on the plastic shear deformation of rock discontinuities is essential to the structural design of rock engineering, and the yield point is an important indicator for plastic shear deformation. In this paper, a conceptual model based on renormalization group theory is proposed to qualitatively explain the evolution mode of shear deformation. Then, considering the stress transfer mechanism, the mathematical description of the critical failure point is derived. Subsequently, the criterion for yield shear displacement is obtained by combining the renormalization group model with the shear constitutive model. Finally, experimental results based on the ratio of yield stress to shear strength for different rock types are provided to verify the proposed criterion. The theoretical and mathematical analysis indicates that the yield point is the phase transition point during the shear deformation of rock discontinuities, which is equivalent to the critical failure point in the renormalization group theory. The relationship between the failure probability of n-order blocks and the basic failure probability can be obtained according to the stress transfer mechanism and the conditional probability, so that a mathematical description of the critical failure point can be established. The verification by both laboratory and in-situ tests shows that the results of the proposed criterion are consistent with those of classical methods. In addition, the determination of the parameters has little influence on the criterion's performance, which means the proposed criterion could eliminate the subjectiveness in determining the yield shear displacement.

Effect of defects in adhesive layer on the interfacial bond behaviors of externally bonded CFRP-to-concrete joints

The structural strengthening technique using fiber-reinforced polymer (FRP) composites to strengthen concrete structures has been widely used. For this reinforcement technique, the stress transfer mechanism at the FRP-concrete interface is one of the key factors affecting the reinforcement effectiveness. In practice, the bond layer is prone to defects due to uneven concrete surface, environmental deterioration or external loading during service life, which can weaken the bonding performance of the FRP-concrete system and even lead to unexpected structural failure in severe cases. In this paper, an experimental study on the influence of interface defects on FRP-concrete interface bonding behavior is designed. Based on the digital image correlation (DIC) technique, the effects of defect size, shape and location on the FRP interfacial bonding behavior were investigated separately. It was found that when the defect area was less than 5% of the effective bond area, the reduction of debonding load was similar to the fluctuation of the conventional interfacial debonding process, and the effect of defects on the interfacial bonding performance was relatively limited. The results also show that the degree of influence of defects on bonding performance varies for different configurations. The higher the concentration of defects, the higher the decrease in the FRP-concrete bearing capacity, especially those defects closer to the FRP boundary. Meanwhile, the proposed loading capacity reduction model considering the defect area is compared with the experimental results to assess the severity degree of defects.

梁主筋に機械式定着工法を用いた梁段差を有する柱梁接合部の応力伝達機構

This paper discusses on R/C interior step beam-column joints with headed anchor for beam longitudinal bars, developed to streamline the works of reinforcing for R/C interior step joints. In general, beam longitudinal bars of step beam column joints are applied 90°-degree hook in step beam-column joints. In R/C buildings, step beam column joints are sometimes planed for architectural planning reasons. Nevertheless, interior step beam-column joints reinforcements are congested and 90°-degree hook of the left and right beams may be the case these hooks are overlap. In order to consider interior step beam-column joints design method, four specimens with beam-column strength ratio approximately 1.0 and 1.5 were tested. Regarding step beam-column joints, from the crack behavior, the stress transfer mechanism depends on positive and negative loading and is considered to consist of two struts which are formed by the balancing of the compressive forces concentrated at the ends of the column and beam and the balancing of the headed anchorage reaction forces. The stress transfer mechanism consisting of these two struts was modelled and proposed as a “double strut model”. A design method of R/C step interior beam-column joints based on the double strut model is also proposed. It provides sufficient satisfaction as to the interior step beam-column joints experimental test results.

Experimental investigation of FRCM under shear loading

Fabric Reinforced Cementitious Matrix (FRCM) can be used to improve the shear strength of masonry walls. The contribution of the FRCM is evaluated, accordingly to existing standard, taking into account the FRCM-to-substrate bond strength and the tensile strength of bare fabrics, neglecting the mortar matrix. Nevertheless, to date, there are no experimental tests that provide the actual shear capacity of the FRCM alone and not bonded to the substrate. To this scope, a four-hinge articulated steel frame was designed to investigate the shear behaviour of small-scale square specimens, as well as the mechanism of shear stress transfer between the fabric and the mortar. This paper presents the outcomes of shear tests performed on four FRCM composites, comprising of two basalt fabrics embedded in either lime- or cement-based mortars, in order to assess the influence of the single components. Digital image correlation (DIC) was also used to determine the displacement fields, strains and to detect the crack pattern of the specimens.

Experimental investigation on tensile and shear bond behaviour of Basalt-FRCM composites for strengthening calcarenite masonry elements

The use of Fabric Reinforced Cementitious Matrix (FRCM) composites for structural retrofit has seen an increased interest among the scientific community, during the last decade. Various studies have revealed their effectiveness as external retrofitting technique of masonry elements, offering numerous advantages respect to the well know Fibre Reinforced Polymer (FRP) in terms of compatibility with masonry support, reversibility of intervention and sustainability. Despite the growing use, the characterization of FRCM mechanical behaviour is still an open issue, due to numerous uncertainties involved in test set-up adopted and fibre-mortar combination. The proposed experimental study aims to investigate the tensile and shear bond behaviour of Basalt-FRCM for strengthening calcarenite masonry elements. Calcarenite is a natural stone with sedimentary origin and it is widely present in existing buildings of the Mediterranean areas. Direct tensile tests are performed on two types of Basalt-FRCM coupons, with cement-based and lime-based mortar, adopting two different test-set-up based on clamping and clevis grip methods. Moreover, double shear bond tests are carried out to evaluate the adhesion properties of the two types of Basalt-FRCM with calcarenite support. Experimental outcomes are compared in terms of stress-strain curves, evaluating the influence of mortar grade and test set-up on the mechanical performances of Basalt-FRCM composites. The comparisons provide information about the mechanical stress transfer phenomena that occur at the fibre-to-matrix and FRCM-to-substrate interface level and the failure modes.

Interfacial stress transfer in monolayer and few-layer MoS2 nanosheets in model nanocomposites

Understanding the stress transfer mechanisms from a polymer matrix to two-dimensional (2D) reinforcements is essential for the preparation of high performance nanocomposites. In this study, the interfacial stress transfer from a flexible polymer substrate to monolayer and few-layer molybdenum disulfide (MoS2) under tension has been investigated. Layer-dependent and strain-dependent photoluminescence (PL) spectroscopy were used to examine the stress transfer efficiency. The interlayer stress transfer efficiency of MoS2 was determined to be in the range of 0.76–0.86, higher than that of graphene. The transfer of strain from the polymer substrate to the flakes was derived through strain-dependent band shifts. With progressive loading, the strain distribution in monolayer MoS2 can be described by the shear-lag, partial-debonding and total-debonding models. The interfacial shear and frictional stresses were calculated to quantify the strength of the MoS2/polymer interface. It was found that the strength of the interface is similar to the strength of a graphene-polymer interface. Strain mapping was performed at different strain levels and it was found that the strain distribution in bilayer MoS2 is similar to the case of a monolayer sample. The interfacial shear strength remains almost unaffected, while the stress transfer length increases with increasing layer number.

Nanomechanical study of aqueous-processed h-BN reinforced PVA composites

Hexagonal boron nitride (h-BN) as a filler has significantly improved the mechanical properties of various polymers composites. Among them, polyvinyl alcohol (PVA) is particularly important for its wide range of industrial applications and biocompatibility nature. However, preparing a homogenous composite of h-BN and PVA in water is troublesome as the aqueous processing of h-BN without any additives is challenging. In this context, a pre-processing technique is used to produce an additive-free aqueous dispersion of h-BN. The uniformly dispersed composites are then prepared with different concentrations of h-BN. Free-standing thin films are fabricated using the doctor blade technique, and nanoindentation is employed to understand their deformation behaviour at smaller length scale for better understanding of micro-mechanism involved. Reduced elastic modulus and hardness of 10 wt% h-BN/PVA composite film are enhanced by ∼93% and ∼159%, respectively, compared to pristine PVA. Frequency sweep dynamic mechanical analysis is performed between 1 and 50 Hz, and the elastic properties of composite materials are found to improve significantly upon addition of h-BN nanosheets. Besides, the impact of h-BN incorporation in stress relaxation behaviour and hardness depth profiling are also investigated. The observed improvement in mechanical properties of the composites may be attributed to the uniform distribution of the nanosheets and the strong interfacial interaction between h-BN and PVA, which ensures efficient mechanical stress transfer at the interface.

A Polyurethane Organic Framework for Flexible Al–Air Batteries

Flexible Al–air batteries with hydrogels are regarded as a promising power source owing to their high specific capacity, their high ionic conductivity, and having no leakage. However, the mechanical properties of the hydrogels remain unresolved. Here, we present a polyurethane organic framework (POF) employing a polyurethane skeleton as an internal support for poly(acrylic acid) (PAA) hydrogel, where the POF can exhibit high strength and toughness, and an Al–air battery using the POF can output good electrochemical properties. The results demonstrate that the tensile stress of 30 ppi POF is 49.5 kPa owing to the stress-transfer mechanism, while that of PAA is only 3.1 kPa. Compared to that of the PAA hydrogel, the discharging capacity of Al–air batteries with 20 ppi POF can be increased by 79 mAh cm–2 at a current density of 1 mA cm–2, which can be attributed to corrosion inhibition and the surface roughness change of the POF during the discharging process. This work will deliver a selectable strategy for a trade-off between mechanical and electrochemical properties.

Elastic interaction between Mauna Loa and Kīlauea evidenced by independent component analysis

The contrasting dynamics between Mauna Loa and Kīlauea have been studied over the last 100 years from multiple viewpoints. The fact that dynamic changes of one volcano trigger a dynamic response of the other volcano indicates a connection may exist. Petrological works show a direct relationship between the magmatic systems of these two volcanoes is not possible. We analysed DInSAR data and GPS measurements of ground deformation patterns associated with the activity of Mauna Loa and Kīlauea volcanoes. The DInSAR SBAS dataset spans the interval between 2003 and 2010, and was acquired along ascending and descending orbits of the ENVISAT (ESA) satellite under different look angles. Of the 10 tracks that cover the Big Island (Hawai‘i), 4 cover both volcanic edifices. Using GPS measurements, we computed the areal strain on 15 triplets of stations for Kīlauea volcano and 11 for Mauna Loa volcano. DInSAR data was analysed by applying Independent Component Analysis (ICA) to decompose the time-varying ground deformation pattern of both volcanoes. The results revealed anticorrelated ground deformation behaviour of the main calderas of Mauna Loa and Kīlauea, meaning that the opposite response is seen in the ground deformation of one volcano with respect to the other. At the same time, Kīlauea exhibits a more complex pattern, with an additional component, which appears not to be correlated with the dynamics of Mauna Loa. The GPS areal strain time series support these findings. To corroborate and help interpret the results, we performed inverse modelling of the observed ground deformation pattern using analytical source models. The results indicate that the ground deformation of Mauna Loa is associated with a dike-shaped source located at 6.2 km depth. In comparison, the anticorrelated ground deformation of Kīlauea is associated with a volumetric source at 1.2 km depth. This excludes a hydraulic connection as a possible mechanism to explain the anticorrelated behaviour; instead, we postulate a stress-transfer mechanism. To support this hypothesis, we performed a 3D numerical modelling of stress and strain fields in the study area, determining the elastic interaction of each source over the others. The most relevant finding is that the Mauna Loa shallow plumbing system can affect the shallowest magmatic reservoir of Kīlauea, while the opposite scenario is unlikely. Conversely, the second independent component observed at Kīlauea is associated to a sill-shaped source located at a depth of 3.5 km, which is less affected by this interaction process.

Reinforcement‐concrete bond in discrete modeling of structural concrete

AbstractThe bond between concrete and reinforcement is one of the critical parameters influencing the structural behavior of reinforced concrete (RC). This research proposes a mathematical methodology to scale the reinforcement‐concrete bond‐slip relationship in a beam lattice modeling framework. A simplified, generalized approach based on stochastic analysis is proposed to model the interaction between the reinforcing bar and surrounding concrete at the macroscale. The approach considers the randomness of the lattice mesh and the mesh size and adopts an analytical model for the interface assuming the pull‐out failure of reinforcement as input, thereby including also the mesoscale geometric effect of ribs. By using the geometric configuration of Delaunay triangulation in the random lattice mesh, the interface elements can reproduce the basic conical stress transfer mechanism in concrete. Consequently, depending on boundary conditions, and without changing the interface properties, a splitting failure and bond‐slip relation for splitting failure can be predicted. The model is systematically validated in different types of pull‐out tests, through flexural and finally shear tests. With limited input (properties of the concrete and analytical equation for pull‐out failure), having a (strong) physical background, the model was shown to capture the fundamental fracture mechanisms in RC under different loading and confinement conditions.

Impact behaviour of dry granular flow against baffle structure: coupled effect of Froude and particle characteristics

The baffle design resisting dry granular flow is still impeded by the effect of the particle size distribution coupled with the influence of Froude characteristics on baffle performance – for example, the deceleration process and run-up process. Based on discrete-element method simulation, this study investigated the baffle deceleration and granular flow run-up. Baffle deceleration is controlled by the change of particle contact behaviour behind the baffle slits and is more sensitive to the particle size than the Froude characteristics of granular flow. Although the granular flow run-up height is also affected by the debris–slit interaction, it is mainly dominated by the flow climbing along the baffle, which is more sensitive to the Froude characteristics and stress-transfer mechanism. Bidisperse granular flow has more complex interaction with the baffle compared with monodisperse flow. The effect of material inhomogeneity on baffle performance depends on the particle content. For baffle design, the first array can be considered as a reference. The estimation of the baffle deceleration potential and height with consideration to unsteady dynamics and material inhomogeneity is also presented in this paper.