Shear Strength Degradation Research Articles

Shear Behaviors of the Intersurface between Rice Straw Rope and Dredger Fill Silt: Experimental and Mechanism Studies

The further development of land reclamation, port waterway, and wharf construction brings about proper treatments of dredger fill silt, while huge amounts of rice straw set aside in China argument rational disposal every year. Therefore, rice straw is bundled up as ropes, which represent as drainage body and reinforcement, to make eco-friendly treatment for dredger fill silt. This paper investigates the mechanical properties and validity of rice straw rope as certain treating material of dredger fill silt through a series of pull-out test, mass loss test, and tension test on specimens with different water contents and dry densities. The results reveal that peak value of interfacial shear strength rises with the increase of normal stress at the same immersion time, and in particular, it rises by up to 250.0% when the normal stress is 40 kPa. The tensile force of rice straw rope increases slowly with the rise of tensile displacement, and the failure mode changes from brittle to ductile with the rise of immersion time, which witnesses first rapid back slow degradation trend. The proper interfacial shear strength, tensile force, and reasonable degradation rate of rice straw rope make it ideal in drainage and consolidation of dredger fill silt.

Effect of Elevated Temperature Exposures on Shear Properties of Sheathing Panels

Abstract Structural wall sheathing such as oriented strand board (OSB) and plywood have been heavily used in residential and commercial timber frame construction. The response of these wood-based composites under elevated temperatures between 100°C and 200°C (herein referred to as elevated temperatures) and exposure time needs to be characterized to assess residual strength of the materials in the existing structures. The main objective of this work is to study the effect of temperature and exposure time on shear strength and shear modulus of plywood and OSB. A total of 110 test specimens was tested in shear after exposure to five different temperatures and two exposure durations, followed by cooling to ambient temperature. The results indicated that the plywood and OSB behaved differently after exposure to elevated temperatures and exposure duration. Plywood showed a consistent degradation of shear strength with elevated temperature and time, while OSB did not exhibit a clear picture of thermal degradation. The results further indicated that the shear modulus of plywood and OSB remained unaffected after exposure to elevated temperatures.

Dynamic Shear Strength of Rock Joints and Its Influence on Key Blocks

Shear strength deterioration of rock joints and its induced instability of key blocks subjected to dynamic disturbance constitute the mechanism of many geological disasters such as rockburst, landslide, and rockfall. In this study, the influence of dynamic disturbance on rock joints was quantified in the form of stress, of which model was established by quasistatic method. The dynamic shear strength of rock joints was analyzed theoretically following the Coulomb-Navier criterion and further investigated experimentally by split Hopkinson pressure bar (SHPB) tests with impact velocities of 4.850 m/s, 8.722 m/s, 12.784 m/s, and 16.935 m/s. The effect of engineering disturbance on shear strength of rock joints was measured, and a degradation coefficient was used to describe it quantitatively. Considering the degradation of dynamic shear strength of rock joints and its influence on block stability, the method for determining key blocks under dynamic disturbance was given. Implementing this method into the program of Geotechnical Structure and Model Analysis-3D (GeoSMA-3D) developed by the authors’ team, the visualization of key blocks was achieved. From theoretical analysis and experimental investigation, it was figured out that the shear strength of rock joints is degraded under dynamic disturbance. The degradation of friction angle becomes more sensitive than that of cohesion. Additionally, numerical results show that the number of key blocks was increased with the increasing impact velocities.

Runout simulation of seismic landslides using discontinuous deformation analysis (DDA) with state-dependent shear strength model

The reliable numerical simulation of the landslide process contributes to the establishment of evidence-based disaster mitigation measures in seismically active zones. To achieve this goal, a simple and unified state-dependent shear strength model of discontinuities is presented to describe the shear strength degradation in a seismic landslide process. The proposed model establishes a relationship between the shear strength parameters and the global safety factor of the slope by assuming that the slope instability (or landslide initiation) is accompanied by an instantaneous shear strength degradation of discontinuities. To realize the model numerically, the algorithms for the computation of global safety factor and the modification of shear strength parameters were incorporated into the discontinuous deformation analysis (DDA). Subsequently, the kinematic accuracy of the improved DDA method was validated by comparisons with theoretical solutions for the dynamic sliding of a block on an inclined plane. Numerical simulations of the Daguangbao landslide triggered by the Wenchuan earthquake were performed using the improved DDA method. The results illustrate that the shear strength degradation of discontinuities affect the evolution process, travel distance, and post-failure shape of the seismic landslide significantly.

Modelling the degradation of penetration resistance during cyclic T-bar tests in a Gulf of Mexico clay

Installation and repetitive movement during the operation of a pipeline causes remolding and softening of the surrounding soil. Similar effects can occur around foundations during their operating life. The degradation of the undrained shear strength of soft clays near the seabed is a critical component of the design of subsea facilities, including pipelines and shallow foundations. This paper presents a two-stage strength degradation model based on the framework developed by Hodder et al. (2010) for repeated vertical movement of a cylindrical object (T-bar penetrometer) embedded in a soft Gulf of Mexico (GOM) clay. The model is compared with the behavior observed in four box cores sampled in the Gulf of Mexico, in which 8 cyclic T-bar penetrometer tests were performed. By varying the parameters in the strength degradation model, its applicability for GOM clay is examined, and the effects of the location of cycling within the box core and parameter variability between box cores relative to the average parameters are explored.

Experimental study on the undrained shear strength of deep-sea soft soil using improved T-bar penetrometer

This paper presents an experimental investigation on the ultra-soft marine soil recovered from the seabed in deep-water areas in South China Sea (SCS). An improved T-bar penetrometer is made and employed in laboratory to measure the undrained shear strength of the soils and its degradation with the penetration cycles. A further investigation on the microstructure of the marine soils during the cyclic penetration has been conducted. The findings indicate that there is a kind of “crust zone” existing in the upper layer of seabed sediment. The reasons of this “crust zone” are further interpreted. By comparing the intact and remoulded samples, it is found that the former exhibits a higher strength degradation degree than that of the latter. This discrepancy in shear strength degradation between the two kinds of samples can be attributed to the microstructure changes. During the cyclic penetration process, the natural flocculated structure of intact soil samples collapses with the breakage of particle aggregates, and a further destruction of large-sized inter-particle pores occurs. As far as the remoulded soil samples, their microstructures experience a continuing fracturing course starting with a dense packing of soil particles and followed by a progressive development of the loose packing state.

Investigation of damage behaviors of ECC-to-concrete interface and damage prediction model under salt freeze-thaw cycles

This paper reveals an investigation of damage behaviors of ECC-to-concrete interface with various ECC strength grades and interface roughness of concrete substrate subjected to salt freeze-thaw cycles. The test results showed that salt freeze-thaw cycles had significant negative influences on bonding behaviors of ECC-to-concrete interface. The occurrence of interface fracture and degradation of interface shear strength under salt freeze-thaw cycles could be significantly influenced by ECC strength grade and interface roughness of substrate concrete. The lower ECC strength and the smoother interface always caused earlier occurrences of interface fracture under salt freeze-thaw cycles. Besides, the lower ECC strength and the smoother interface always caused earlier occurrences of a sharp decline of interface shear strength and more decrease of interface shear strength. The area erosion rate of interface under salt freeze-thaw cycles could be significantly influenced by interface roughness of substrate concrete, and the smoother interface always caused more area erosion rate of interface. Based on the micro-damage mechanism of ECC-to-concrete interface under salt freeze-thaw cycles, a damage prediction model of ECC-to-concrete interface was proposed in this paper. The model is expected to be used to predict the damage degree of ECC-to-concrete interface subjected to salt freeze-thaw cycles.

A low-cycle fatigue approach to predicting shear strength degradation in RC joints subjected to seismic actions

Reinforced concrete (RC) structures realised in earthquake-prone regions are exposed to seismic shakings that may result in significant damage and even lead them to collapse. As well known, high stress concentration occurs at the end of both beams and columns of RC frames, making those members extremely prone to damage. Moreover, beam-to-column joints are particularly sensitive to brittle failure, especially in the cases of unreinforced and unconfined joints. This paper investigates the cyclic response of RC beam-to-column joints. Specifically, it is intended at demonstrating that the decay in shear strength due to cyclic actions can be interpreted in the light of the well-known low-cycle fatigue approach. To do so, cyclic experimental tests on RC joints reported in the scientific literature are collected and analysed. The obtained results show that the parameters governing the shear strength degradation are clearly influenced by both the layout of the RC joints and their actual design criteria. This finding highlights that low-cycle fatigue curves can be considered for describing the decay in shear strength due to cyclic actions; however, further well-documented experimental results are needed to completely identify the relationship between the relevant properties of the RC joint and the resulting low-cycle fatigue curve.

Large-deformation finite-element modelling of earthquake-induced landslides considering strain-softening behaviour of sensitive clay

Large-scale landslides in sensitive clays cannot be explained properly using the traditional limit equilibrium or Lagrangian-based finite-element (FE) methods. In the present study, dynamic FE analysis of sensitive clay slope failures triggered by an earthquake is performed using a large-deformation FE modelling technique. A model for post-peak degradation of undrained shear strength as a function of accumulated plastic shear strain (strain-softening) is implemented in FE analysis. The progressive development of “shear bands” (the zone of high plastic shear strains) that causes the failure of a number of soil blocks is simulated successfully. Failure of a slope could occur during an earthquake and also at the post-quake stage until the failed soil masses come to a new static equilibrium. Upslope retrogression and downslope runout of the failed soil blocks are examined for varying geometries and soil properties. The present FE simulations can explain some of the conditions required for different types of seismic slope failure (e.g., spread, flowslide or monolithic slides) to be triggered, as observed in the field.

Thixotropic Hardening of Fao Clay

Fao region is characterized by weak soft silty clay to clayey silt thick layer which extends to a depth of about 20 m. The construction of some structures on such soils may needs piles. During the installation of driven piles, the soil geotechnical properties are exposed to significant changes result due to shearing under large shear strains. These changes significantly decrease the shear strength of the virgin soil due to the destruction of soil structure caused by remolding. The degradation of shear strength is usually followed by strength regaining which is called “Thixotropy”. In this study, the thixotropic effect on Fao clay was investigated. Many disturbed and undisturbed soil samples were brought from Fao region. Some of the soil samples were thoroughly remolded in the laboratory in its natural water content and molded as direct shear specimens. More than 180 specimens were prepared and tested over 36 weeks in order to accurately assess the percentage of strength regaining with time passing. The results show that soil remolding causes about a 45 % reduction in shear strength in comparing with the shear strength of the intact soil. This reduced percent in shear strength was almost fully regained after 25 weeks due to thixotropic effect.

Characterization of the Reduction in Undrained Shear Strength in Fine-Grained Soils due to Cyclic Loading

AbstractTo evaluate the effect of plasticity characteristics and mineralogical composition on postcyclic shear strength degradation, 18 soils, prepared as different proportions of kaolinite, montmo...

A degrading shear strength model for R.C. columns with hollow circular cross-section

This study aims contributing to the investigation about the shear strength of reinforced concrete members with hollow circular cross-section and ordinary concrete strength. First, a proper experimental database of tests carried out in the literature on hollow circular members is collected and analysed. Then, the few existing shear strength models from the literature or seismic codes considered suitable for hollow circular sections are discussed and compared with the experimental results. Starting from the lonely specific degrading model existing in the literature, new equations are proposed on mechanical bases for concrete, transverse reinforcement and axial load contributions to the shear strength and for the shear strength degradation due to increasing ductility demand. The new proposed model is finally validated by means of the collected experimental data. Numerical-versus-experimental comparisons show very good results in terms of predicting capacity of both maximum (not-degraded) and degraded shear strength. The relative percentage prediction error always results lower than 4% with a very limited dispersion. The proposed equations can be thus considered as a reliable improvement of existing shear strength models for the investigated structural typology.

Cyclic Load Test and Shear Strength Degradation Model for Columns with Limited Ductility Tie Details

AbstractFor existing buildings in low and moderate seismic zones, various nonseismic ties not conforming to special seismic requirements have been used in columns for convenience in reinforcing bar...

Mechanics of the earthquake-induced Hongshiyan landslide in the 2014 Mw 6.2 Ludian earthquake, Yunnan, China

The Hongshiyan landslide, the largest landslide (volume ~ 12.24 Mm3) triggered by 2014 Mw 6.2 Ludian earthquake, blocked the Niulan river forming a rockslide dam; the failure potential of the dam created an elevated risk to population and infrastructure downstream. We provide insight into the failure mechanics of the Hongshiyan landslide using data obtained by means of remote sensing techniques and traditional in-situ surveys. Geological data obtained by these methods was then used for kinematic and numerical analyses. Our study shows that failure of the Hongshiyan landslide involved a high rock slope with an average slope of 55° and aspect of 185° SSW, in which the geological setting consists of an upper strong limestone slab and an underlying weak silty mudstone. The geological setting allowed the landslide to develop, as follows: i) the silty mudstone, due to its poor mechanical properties, was subject to ductile deformation under the compressive loading from the overlying strong limestone slabs; ii) a large number of release fractures in the limestone developed as a consequence of pre-failure progressive deformation in the underlying weak mudstone; iii) the oxidation and solution along steep joint walls in the upper limestone indicates that the joints are pathways that allow water ingress into the mudstone, further promoting the degradation of shear strength in this layer. Distinct element modelling (UDEC) was used to back analyse the failure mechanism and develop the geological model that best reproduced the Hongshiyan failure geometry. Results show that the failure surface consisted of two elements. The upper steeply dipping release surface formed along the slope-parallel steep cross joints defining a jointed, strong limestone slab, and the toe of the failure formed a curved failure surface through the ductile weak silty mudstone, underlying the limestone slab, independent of any discontinuities within it. The pre-failure observations and numerical analysis suggest that the Hongshiyan slope was in a state of marginal static stability prior to the earthquake as a result of pre-failure progressive deformation controlled by the shear strength of the underlying mudstone. Overall, the marginal pre-earthquake stability of the Hongshiyan slope, coupled with the strongest seismic loading, which the Hongshiyan slope has been historically subjected to, are the reasons why this moderate earthquake induced a large-scale catastrophic rock slope failure.

Thermal and mechanical properties of micro Cu doped Sn58Bi solder paste for attaching LED lamps

The junction temperature is a key factor in determining the reliability of light emitting diode (LED) package, which can be lowered using high thermal conductive soldering material. The effects of Cu micro particles on the thermal and mechanical properties as well as microstructures of Sn58Bi solder paste were investigated in this paper. The results indicated that the thermal conductivity of Sn58Bi-5 wt% Cu solder layer is 26.60 W/(m·K), which is much higher than the Sn58Bi solder layer of 18.89 W/(m·K). According to the thermal simulation analysis, the temperature on chip is the highest area inside the whole LED package. The maximum junction temperature of soldered LED package decreases with 5 wt% Cu particles addition and then increases. The maximum junction temperature of LED package increases as the input power increases from 1 to 3 W. The lowest junction temperature is obtained in Sn58Bi-5 wt% Cu soldered package. There is no significant degradation in shear strength of 5 wt% Cu micro particles doped Sn58Bi solder joint when comparing with the pure Sn58Bi soldered ones. The shear strength of solder joint decreases approximately 50% when the content of added Cu micro particles increases from 5 to 15 wt%, and weak bonding presents in 20 wt% Cu doped solder joint. Comparing with the Sn58Bi solder, the solder layer with 5 wt% Cu micro particles exhibited more uniform microstructure and the number of coarse Bi-rich phases is reduced. The Cu micro particles facilitate the heat dissipation process of Sn58Bi solder layer by adding highly conductive cores in the layer. The 5 wt% Cu micro particles doped Sn58Bi solder exhibits remarkable thermal and mechanical performance in attaching LED packages.

Influence of hygrothermal aging on the durability and interfacial performance of pultruded glass fiber-reinforced polymer composites

The influence of the fiber/matrix interface of pultruded glass fiber-reinforced polymer (GFRP) composites exposed to hygrothermal environments, including deionized water immersion and saltwater immersion at 20 ± 5 °C, 30 ± 1 °C, and 60 ± 1 °C for 180 days, was investigated. The effect of moisture absorption on tensile properties was discussed. After 180 days of immersion in deionized water, tensile strength and modulus of specimens decreased 25.7% and 26%, whereas the equivalent respective losses were 2.1% and 18.2% for specimens immersed in saltwater. The short-beam-shear test and the single-fiber fragmentation test were selected to reflect the degradation of macro- and micro-interfacial properties, respectively. After 180-day immersion in deionized water and saltwater at 60 ± 1 °C, the inter-laminar shear strength of specimens decreased 28.8% and 18.5%, respectively, and the corresponding interfacial shear strength decreased 53.2% and 23.5%, indicating that the diffusion rate of micro-interface was higher than that of macro-interface in the fiber direction. Immersion in all media leads to pronounced degradation in tensile strength, modulus, and inter-laminar shear strength. Furthermore, based on the change of interfacial strength and Weibull distribution, a prediction model was proposed to describe the degradation trends and temperature effects on ultimate bearing capacity of pultruded GFRP composites in hygrothermal environments.

Shear strength degradation of steel plate shear walls with optional located opening

Regarding high ductility and potential of steel plate shear walls in energy absorption, they are required to be very thin in thickness especially in upper floors of the building in order to resist lateral loads and sometimes it will reach to a fraction of millimeter in calculations. Since preparation of such thin steel plates is not simply possible, a thicker plate with an opening can be used to reduce stiffness. On the other hand, the existence of opening is inevitable due to architectural considerations such as lighting. In the present paper, shear strength of steel plate shear wall with openings in different zones has been studied by finite element method. As a result, an empirical simple dimensionless equation has been presented to estimate accurately the amount of decrease of shear strength of the wall with an arbitrary opening position in any zone of the plate. To validate the accuracy of suggested relation, numerous finite element models have been simulated with different geometric properties such as shape, diameter, location of opening, thickness and span to height ratio of plate. Comparing results with that of suggested relation and corresponding values of theoretic relations shows the accuracy of the proposed relation for applying in a wide range of steel plate shear walls with different geometric specifications.

Interlayer engineering for titanium clad steel by hot roll bonding

Hot roll bonding was carried out between commercially pure titanium TA2 and high-strength low-alloy steel Q390 using pure Nb or Mo interlayer at 950 °C with a total reduction ratio of 86.7%. Interfacial microstructure and bonding properties of titanium clad steel plates were investigated by electron microscopy and mechanical tests. The results showed β-Ti, TiC and Fe2Ti reaction phases were generated at Ti/steel interface for the clad plates with no interlayer. Inserting Nb or Mo interlayer can effectively suppress the formation of brittle phases, while the weak bonding joint transferred to Nb/steel or Mo/steel interface. And some micro-voids were found at the interface of Nb/steel and Mo/steel. The improved shear strength of clad plates with Nb interlayer might be attributed to the elimination of brittle phases at bonding interface. The small size and little quantities of the micro-voids at Nb/steel interface had a relatively weak effect on shear strength. However, the large number and big size of micro-voids were responsible for the degradation of shear strength for the clad plates with Mo interlayer.

Transient liquid phase (TLP) bonding of Ti-6Al-4V/UNS 32750 super duplex stainless steel

Dissimilar metal joining between Ti-6Al-4 V and UNS 32750 super duplex stainless steel (SDSS) alloys was carried out by TLP bonding with a Cu interlayer at 890, 920, 950 and 980 °C for a bonding time of 60 min. In addition to microstructural survey, the microhardness and shear strength of different bonded specimens were also investigated. The results confirmed that the isothermal solidification zone (ISZ) was extended with the rising temperature up to 980 °C. Moreover, the formation of intermetallic compounds (IMCs) like TiCu, Ti2Cu, Cr2Ti, Fe2Ti and FeTi was demonstrated in TLP joints by SEM/EDS examinations. It was found that increasing the bonding temperature promoted the formation of IMCs by increasing the diffusion of Ti, Cu and Fe, which resulted in degradation of shear strength of the joints. Mechanical characterizations indicated that the joint made at 890 °C had the highest shear strength (271 MPa), which was equal to 0.49 strength of Ti-6Al-4 V, as well as 0.47 of UNS 32750. Furthermore, the average hardness of the bonded zone was superior than that of the base metals. Also, the SDSS side exhibited high hardness values in the range of 420–460 HV, while the Ti-6Al-4 V side revealed lower hardness values (330–370 HV).

Degradation of a discrete infilled joint shear strength subjected to repeated blast-induced vibrations

This paper presents the results of a series of numerical modeling experiments aimed at quantifying blast-induced degradation of shear strength of discontinuities. Near-field vibration history of a single-row production blast in a limestone quarry was used as input to the numerical model. For this purpose, two rock blocks, representing a stiff massive sulfide rock and a weaker limestone rock, were simulated using the 2D Particle Flow Code (PFC2D). Rock mass containing a single inclined joint was modeled as Mohr-Coulomb. The results show that the crack generation rate is increased in both samples after repetitive vibration loading. Joint shear strength degradation rate in the stiffer massive sulfide rock sample is higher than the softer limestone rock, which is attributed to the higher seismic impedance mismatch. The results show that even low-amplitude blasting vibrations (<80 mm/s), when repeated as in multi-hole blasts, can significantly degrade joint shear strength in the nearby pit walls.