Surface Failure Mechanisms Research Articles

Spatio-temporal evolution law of anisotropic shear damage on rock mass joint surface affected by joint morphology

The spatial–temporal evolution law of shear damage on the joint surface is closely related to its morphological characteristics. Understanding the heterogeneous failure behavior of the joint surface is essential for revealing the controlling role of the joint morphology. The diorite from the Guanshan Tunnel is investigated through anisotropic direct shear tests in this study. Based on shear damage quantification and acoustic emission (AE) localization techniques, the heterogeneous damage evolution law of joint surfaces under different joint compression strength (JCS), normal stress (σn), and morphology parameter (M) is studied. The results indicate that the joint surface morphology, shear damage, and AE parameters exhibit similar anisotropic characteristics in different shear directions. An increase in the morphology parameter leads to an increase in shear failure, with shear fracture energy and the number of fracture events showing an increasing trend. Meanwhile, the occurrence of a large number of fracture points shifts from the pre-peak nonlinear period to the post-peak period. However, the evolution process of fracture events exhibits certain similarities, though the morphology parameters are different in two opposite shear directions. Additionally, shear damage volume and fracture energy demonstrate consistency in spatial distribution and quantitative characteristics, while the linear scale factor between cumulative shear fracture energy and damage volume decreases with increasing joint surface strength. These findings assist in a better understanding of the anti-sliding property of joint surfaces and provide crucial clues for further exploration of joint surface failure mechanisms.

Study on shear surface morphology and failure mechanism of soil-rock mixture under wet-dry cycle action

The present study investigates the failure modes and formation mechanisms of shear surfaces in soil-rock mixtures from various perspectives. Firstly, through in-situ direct shear tests, two main shear failure modes, namely planar and non-planar, are identified. Subsequently, using PFC 2D numerical simulation, an in-depth exploration of the characteristics and causes of these two typical failure modes is conducted. The findings reveal that in the natural state, the material is relatively dry, and the matrix suction within the soil-rock mixture is significant. During shearing, the inter-particle force chains are prone to rupture, exhibiting characteristics akin to brittle failure. This leads to nearly planar shear surfaces, with force chain ruptures primarily localized near the planar regions adjacent to the shear surface. However, after multiple dry-wet cycles, the plastic enhancement of the soil-rock mixture reduces the matrix suction to almost zero. The continuous rupture and reorganization of force chains deepen the shear band under their influence, resulting in non-planar shear surfaces. It is noteworthy that the characteristic point fitting curve of non-planar shear surfaces exhibits a nonlinear trend. In summary, our study elucidates the evolution process and causes of shear surface morphology in soil-rock mixtures, which holds significant implications for understanding their mechanical properties and engineering behavior.

Performance of eccentrically loaded strip footings on geocell-reinforced soil

In this pioneering study, the performance of an eccentrically loaded strip footing on geocell-reinforced sand was assessed with instrumented laboratory model tests in terms of pressure-settlement response, surface displacement profiles, failure mechanisms and ultimate bearing capacity considering load eccentricity, geocell height, geocell material stiffness and the relative density of the soil. The results indicated that strip footings on the geocell-reinforced sand outperformed those on unreinforced soils, with up to a 6.5-fold increase in the bearing capacity and significant improvements in the initial slope of the pressure-settlement curve. Furthermore, the strip footing under centric loading on the geocell-reinforced loose and dense sand exhibited either only punching or local shear failure while load eccentricity on the strip footing could lead to the shear failures including punching, local and general. In this research, both a design chart for predicting failure modes of geocell-reinforced strip footings and a new interpretation method to evaluate ultimate bearing capacity were proposed. Increasing the relative density of the soil and material stiffness enhanced the ultimate bearing capacity of geocell-reinforced strip footings under both centric and eccentric loading conditions, with stiffer materials resulting up to 25% increase. However, increased geocell height had no significant impact on bearing capacity.

Large-scale toppling slope under water level fluctuation of reservoir: A case of Yunnan Province, China

Landslides induced by reservoir inundation are common in Southwest China, negatively influencing hydropower stations. The Wunonglong hydropower station dam was constructed in the upper reaches of the Lancang River, accordingly causing the water level at the Lajinshengu slope to increase by 30 m. A tension crack with a visible depth of 8 m was observed in the upper sector of the Lajinshengu slope after reservoir impoundment for 170 d. In the following days, numerous cracks appeared on the surface of the slope, and the maximum displacement of the slope reached 3.22 m. Then, a large-scale active deformation body within the Lajinshengu slope formed with an area of 2.62 × 105 m2 and a volume of 1.65 × 107 m3. Detailed field investigations, on-site monitoring, and centrifugal model tests were carried out to analyze the surface features, deformation characteristics, and failure mechanism of the Lajinshengu slope. The results show that the slope is an ancient landslide, divided into two parts (i.e. zone A and zone B) by the gully. Zone B is a traction landslide caused by the displacement of zone A. The long-term inundation weakens the soft rock at the slope foot, intensifying the toppling of bedrock and consequently triggering the sliding of the overburden in zone A. The failure mode of the Lajinshengu slope is a typical case of toppling-sliding failure, and the underlying rock toppling drives the overlying sliding. In addition, early identification methods for toppling deformation covered by overburdened soil were proposed based on monitoring data and deformation signs.

Ti/RuO2-IrO2 anodic electrochemical oxidation composting leachate biochemical effluent: Response surface optimization and failure mechanism

In this work, the electrolytic process conditions for the electrochemical oxidation (EO) of composting leachate biochemical effluent (CLBE) were optimized via the response surface methodology (RSM). Meanwhile, a comparative study had been done on the failure characteristics of Ti/RuO2–IrO2 anodes in a single electrolyte solution system (H2SO4 and NaCl) and real wastewater (CLBE) by accelerated life tests, respectively. The RSM optimization results showed that the COD, NH3–N and TN removal rates were 50.53%, 100% and 95.61% at 30 min, respectively, with a desirability value of 0.993. In parallel, the electrochemical and material characterizations were carried out on the electrodes before and after failure, by which the failure mechanism of Ti/RuO2–IrO2 anodes was clarified. On the whole, the true failure in the H2SO4 solution was attributed to coating dissolution and Ti substrate oxidation. In contrast, the electrode exhibited “apparent failure” due to the “bubble effect” in both NaCl and CLBE solutions, and the “effective roughness” formed compensated for the loss of activity caused by the absence of the coating. Besides, additional dissolution of the Ti substrate occurred in the CLBE solution due to the current edge effect and the presence of organic matter. This paper takes the actual wastewater as the research object and reveals its electrode failure mechanism, which provides a theoretical basis and reference for the subsequent optimization of the actual electrode service life.

Mechanical properties of TiO2/carboxylic-acid interfaces from first-principles calculations.

Nature forms structurally complex materials with a large variation of mechanical and physical properties from only very few organic compounds and minerals. Nanocomposites made from TiO2 and carboxylic-acids, two substances that are available to nature as well as materials engineers, can be seen as representative of a huge class of natural and bio-inspired materials. The hybrid interfaces between the two components are thought to determine the overall properties of the composite. Yet, little is known about the atomistic processes at those interfaces under load and their failure mechanisms. The present work models the stress-strain curves of TiO2/carboxylic-acid interfaces in the slow deformation limit for different facets and binding modes, employing density functional theory calculations. Contrary to former hypotheses, the interface rarely fails through a de-bonding of the molecule, but rather through a surface failure mechanism. Furthermore, a stress-release mechanism is discovered for the bi-dentate binding mode on the {101} facet. Deriving mechanical properties, such as the interface strength, strain at interface failure, and the elastic modulus, allows a comparison with experimental results. The calculated strengths and elastic moduli already agree qualitatively with properties of nanocomposites, despite the simplifications in the model consisting of periodic sandwich structures. The results presented here will help to improve these materials and can be directly integrated in multi-scale simulations, in order to reach a more accurate quantitative description.

Experimental Study on the Dry Drilling Nickel-Based Superalloy of CrAlYN Coated Carbide Bit.

Nickel-based superalloy is regarded as one of the materials with the poorest cutting and drilling performance. Additionally, there is much less research on the drilling of it. This paper aims to study the drilling performance of dry drilling nickel-based superalloy with uncoated and CrAlYN coated carbide bit. First of all, the primary and secondary factors influencing the machining performance of dry drilling nickel-base superalloy uncoated carbide bit were explored through an orthogonal test. Secondly, the self-prepared CrAlYN coated carbide drills, and uncoated drills were compared and analyzed from perspectives of service life, drilling force, drilling temperature, drill surface topography, failure mechanism, and machining surface quality. The research results are as follows: the drilling temperature is the primary factor affecting the drilling performance under dry drilling conditions. CrAlYN coating can obviously prolong the service life of tools, reduce the drilling force and drilling temperature, and improve the machining surface quality at lower rotational speeds. Moreover, the coated cemented carbide bit has a similar failure mode to the uncoated cemented carbide bit after the CrAlYN coating falls off in the wear zone of cemented carbide bit, which is mainly bonding wear on the rear tool surface and the front tool surface.

Ground Surface Displacements and Failure Mechanisms induced by EPB Shield Tunneling Method and Groundwater Levels Changes

Ground Surface Displacements and Failure Mechanisms induced by EPB Shield Tunneling Method and Groundwater Levels Changes

Failure analysis of shot-piston used in squeeze casting process equipment

The failure investigation has been conducted on shot-piston used in injection system of squeeze casting equipment to clarify its failure mechanism. The macroscopic morphology of failed shot-piston showed there were many network cracks on its upper-end surface while the cylindrical surface was seriously worn. Further studies were carried out on the two surfaces respectively. Firstly, the shot-piston upper-end surface was sampled. And the crack distribution and depth were observed. Then, the crack micromorphology was investigated by scanning electron microscope (SEM), and the sampling points inside cracks were analyzed by energy dispersive spectrometer (EDS). The results show the shot-piston upper-end surface produces thermal stress cracks due to the high-temperature thermal cycle, and oxidation occurs in the cracks. The same method was also used for the cylindrical surface failure mechanism analysis. Above all, the cylindrical surface was sampled and the wear morphology was measured. Afterward, the material composition of the bulge on the cylinder surface and the micromorphology were confirmed by SEM and EDS. The results demonstrate the main failure forms are abrasive wear and adhesive wear. The inconsistent thermal deformation of the shot-piston and shot sleeve caused changes in the fit clearance and resulted in friction and wear. Simultaneously, the metal solution and oxygen erode the shot-piston matrix through the furrow, accelerating wear and failure. According to the failure mechanism of different surfaces, shot-piston service life can be extended by optimizing the squeeze casting process, controlling the temperature gradient, and selecting the appropriate fit clearance, which has important engineering significance.

Effect of shot peening on notched fatigue performance of powder metallurgy Udimet 720Li superalloy

The tensile and stress-controlled LCF tests of a powder metallurgy (PM) superalloy, Udimet 720Li have been conducted at 650 °C in order to investigate the effect of shot peening (SP) on notched low cycle fatigue (LCF) performance. The measurements of surface tomography, Micro-Vickers hardness, residual stress and failure mechanism analysis were conducted in detail. The results show that there are many regularly shaped machining traces on as-received (AS) surface while lots of highly disorganized dimples are mainly distributed on SP surface. A larger microhardness gradient is produced along the path from SP surface to the bulk of material compared with AS specimen. The maximum compressive residual stress of SP specimen is not more located at surface like AS one but at a depth of approximately 50–90 μm from surface. Compared with the applied stress, the effective stress curve of SP specimen becomes “flat” and the effective stress gradient decreases accordingly. The LCF lifetime of the notched specimen is significantly improved after SP process and the LCF lifetime increment is between 2.05 and 7.66 times. Finally, there are obvious differences on the failure mechanisms, such as fatigue crack initiation, propagation and final fracture between AS and SP specimens.

A molecular dynamics study of silicene reinforced cement composite at different humidity: Surface structure, bonding, and mechanical properties

As the silicon counterpart of graphene, silicene has a great potential in reinforcing cement-based materials due to its outstanding physical properties. In this paper, the surface structure, bonding, mechanical performance and failure mechanism of silicene reinforced calcium-silicate-hydrate composite (C-S-H/silicene) with different humidity were investigated by molecular dynamics simulations. It has been demonstrated that the incorporation of silicene nanosheet can effectively bridge the upper and lower layers of C-S-H and enhance the strength and stability of the interlayer region of C-S-H via various chemical bonds (e.g. Si-Os-Si covalent bonds). This bridging effect provided by silicene nanosheet significantly improves the strength and plasticity of C-S-H. However, the penetration of water molecules may weaken the stability of these chemical bonds, leading to the degradation of tensile strength. Besides, C-S-H/silicene breaks in the internal area rather than interlayers in a dry state during the failure process. This can be explained that Si-Os-Si bonds are strong enough to connect the silicene sheet and C-S-H layers and hinder the propagation of cracks. Hopefully, the reinforce mechanism of silicene on C-S-H could provide new insights on understanding and predicting performance of silicene under specific conditions.

Effects of foliation on deformation and failure mechanism of silty slates

The objective of this study was to investigate the deformation and failure mechanism of silty slates loaded with an angle 0 and 90° between foliation and direction of uniaxial force. For this purpose, mineral composition and microstructure test, uniaxial compression test, acoustic emission (AE) monitoring and scanning electron microscope (SEM) were carried out on Silurian silty slates. Based on the results of tests to analyze the deformation and failure mechanism as well as the macroscopic failure modes, microscopic rupture surface characteristics and acoustic emission characteristics of silty slates under different test conditions. Tests results showed that the silty slates are mainly composed of clay minerals (chlorite and illite) and brittle minerals (quartz and feldspar) and in a layered structure. Under the two angles, silty slates specimens exhibited large differences in the deformation and damage process, macroscopic failure modes, microscopic rupture surface characteristics, deformation and failure mechanism. Besides, there are large differences in the relative parameters of AE event. These results reveal that there is a certain relationship between mechanisms of microscopic fractures and macro failure of silty slates.

An approach for predicting failure mechanism in rough surface rolling contact fatigue

A finite element model was developed to investigate the influence of near surface orthogonal shear stress (OSS) on the competitive failure mechanism between surface originated pitting (SOP) and subsurface originated spalling (SOS), which is intrinsic to rolling contact fatigue (RCF). Surface roughness in heavily loaded non-conformal contacts causes competition between SOS and SOP. In this investigation, tribo-surface roughness has been represented as sinusoidal waveform based on surface measurements of rolling element bearings. These measurements outlined the range of roughness frequency and amplitude. The effects of these surfaces on the contact were investigated and the resulting pressure distributions were used in a finite element model in order to quantify the effects of pressure distribution on near surface orthogonal shear stress concentration. The resulting pressure distributions obtained from rough surfaces were also used in a continuum damage mechanics finite element model (CDM-FEM). The results indicate that a contact with a low frequency surface roughness (pressure distribution) is more susceptible to surface failure, whereas the contact with high frequency surface roughness frequency will resist surface failure. To quantify surface originated failure for a given surface roughness, the probability of surface failure parameter (πsf), which is defined as the ratio of contacts exhibiting SOP characteristics to the total tested is proposed. The near surface stress analysis and failure mechanism results were used to establish a relation between the near surface OSS concentration and πsf. This relation is described by a 2-parameter Weibull cumulative distribution function (CDF). The results indicate that roughness frequency and half contact width are the main parameters controlling the probability of surface failure.

Polymer-Mediated Adhesion: Nanoscale Surface Morphology and Failure Mechanisms

We present coarse-grained molecular dynamics simulations of polymer-mediated adhesion between chemically heterogeneous surfaces. Our surface models exhibit weakly and strongly absorbing sites in 1:1 proportion but are characterized by different degrees of segregation among these sites. When the surfaces are pulled apart, we observe systematic variations in the stress–strain curves, indicating a significant weakening of the adhesive layer on moving from finely interdispersed to more segregated morphologies. In our model systems, the macroscopic failure of the sandwiched polymer films always appears to be cohesive but, at the nanoscale, there is, in fact, a gradual transition from a cohesive to a mixed cohesive–adhesive mechanism.

Microscale intrinsic properties of hybrid unidirectional/woven composite laminates: Part I experimental tests

Understanding of the failure of composites is critical for evaluating their safety and reliability when they are in service. In this research, a combined image analysis technique using computed microtomography (micro-CT) and scanning electron microscopy (SEM) is proposed to study the microstructures, damage and failure of a hybrid unidirectional/woven composite laminate (HUWCL). Micro-CT is used to extract the 3D microscale morphology and identify the internal failure behaviours of the HUWCL. A series of material parameters, such as fiber volume fraction, void ratio and layer mode, are acquired by the statistical analysis of the micro-CT images. In addition, microscale surface damage and failure mechanism of woven and unidirectional (UD) laminae are further investigated by SEM. It is found that the matrix damage around the interface always occurs in both woven and unidirectional (UD) laminae, and distinct delamination is found between the UD laminae and the woven composites. The study shows a great potential of combining micro-CT with SEM in revealing the local damage and intrinsic failure mechanism in HUWCLs at microscale.

Superhydrophobic surfaces for extreme environmental conditions.

Superhydrophobic surfaces for repelling impacting water droplets are typically created by designing structures with capillary (antiwetting) pressures greater than those of the incoming droplet (dynamic, water hammer). Recent work has focused on the evolution of the intervening air layer between droplet and substrate during impact, a balance of air compression and drainage within the surface texture, and its role in affecting impalement under ambient conditions through local changes in the droplet curvature. However, little consideration has been given to the influence of the intervening air-layer thermodynamic state and composition, in particular when departing from standard atmospheric conditions, on the antiwetting behavior of superhydrophobic surfaces. Here, we explore the related physics and determine the working envelope for maintaining robust superhydrophobicity, in terms of the ambient pressure and water vapor content. With single-tier and multitier superhydrophobic surfaces and high-resolution dynamic imaging of the droplet meniscus and its penetration behavior into the surface texture, we expose a trend of increasing impalement severity with decreasing ambient pressure and elucidate a previously unexplored condensation-based impalement mechanism within the texture resulting from the compression, and subsequent supersaturation, of the intervening gas layer in low-pressure, humid conditions. Using fluid dynamical considerations and nucleation thermodynamics, we provide mechanistic understanding of impalement and further employ this knowledge to rationally construct multitier surfaces with robust superhydrophobicity, extending water repellency behavior well beyond typical atmospheric conditions. Such a property is expected to find multifaceted use exemplified by transportation and infrastructure applications where exceptional repellency to water and ice is desired.

Nanoscale serration characteristics of additively manufactured superalloys

Structural elements made of nickel-based superalloys usually operate at high temperatures. Many surface failure mechanisms such as creep, fatigue, fretting fatigue, corrosion, and stress corrosion cracking start from the surface. Thus, studying the surface mechanical properties and deformation behavior of materials is vital in order to draw a correlation between the surface properties and failures. Herein, a nanoindentation technique was implemented to characterize surface properties of a widely used additively manufactured (AM) superalloy, i.e., Inconel 625. The specimens were tested in a range of temperatures in a vacuum chamber. Serrated plastic flow characterized by pop-in events during nanoindentation, known as Portevin-Le Chatelier (PLC) effect, was observed in all three tested elevated temperatures. This phenomenon depicts itself as bursts of plasticity in the loading section of the load-displacement curves. These bursts were studied comprehensively to explore incipient plasticity. Hertzian contact mechanics were implemented to extract the maximum shear stress beneath the indenter from the pop-in loads. The results show that the initial pop-in load increases as the temperature increases. The average initial pop-in load increases by almost four times from 300 °C to 650 °C. This was attributed to the formation of strengthening precipitates. The average size of the serrations and the magnitude of dislocation nucleation increase from 300 °C to 500 °C and decrease from 500 °C to 650 °C. The serrations are attributed to the dislocation generation and movement as well as their interaction with the solute atoms and precipitates.

Contact Stress Analysis for a Functionally Graded Half-Plane at Subsonic, Transonic and Supersonic Sliding Speeds

This paper investigates frictional dynamic contact mechanics of a functionally graded half-plane subjected to moving contact by a rigid flat punch possessing subsonic, transonic and supersonic speeds. The shear modulus along the half-plane is expressed by an exponential function, and the Poisson’s ratio is assumed constant along the graded half-plane. Governing partial differential equations are derived based on the planar theory of elastodynamics. Boundary conditions are applied, and displacement fields in the graded half-plane are determined analytically. Formulation for the contact problem is reduced to a singular integral equation involving Cauchy singularity and a Fredholm kernel. Singular integral equation is solved numerically utilizing a suitable collocation technique. Contact stresses and normalized punch stress intensity factors are calculated for prescribed subsonic, transonic and supersonic speeds of the moving punch. It is expected that the results obtained by this study will help to understand the contact behavior and the surface failure mechanisms of functionally graded materials, especially at transonic and supersonic sliding speeds.

Brittle or Ductile? Abrasive Wear of Polyacrylamide Hydrogels Reveals Load-Dependent Wear Mechanisms

Cartilage and hydrogels are composed of an elastic network that retains a large volume of water allowing them to efficiently maintain smooth sliding interfaces while under high compressive loads. For hydrogels to be viable candidates to replace osteoarthritic cartilage, a study of their robust long-term use and surface failure mechanism is necessary. In this work, a sandpaper covered probe attached to a microtribometer with a reciprocating stage was used to wear 7.5 wt% polyacrylamide hydrogels under a range of speeds (1 mm/s, 2 mm/s, and 3 mm/s) and normal loads (1 mN, 5 mN, 10 mN, and 20 mN). The subsequent wear scars were imaged using a 3D laser scanning confocal microscope. For all sliding speeds of the 1 mN and 5 mN loading conditions, microcutting, which is characteristic of brittle materials, contributed more to the measured wear volume. For the 10 mN and 20 mN loading conditions, microplowing, which is characteristic of ductile materials, contributed more to the measured wear volume. We found that the mechanical wear of hydrogels is a competition between ductile fracture and brittle fracture, and the dominant mechanism is dependent upon load, but not speed for the range of speeds tested. The different wear behavior between the lower loads (1 mN and 5 mN) and higher loads (10 mN and 20 mN) suggests that there is a critical load between 5 and 10 mN that marks the shift from more brittle fracture to more ductile fracture. This work is the beginning of developing more accurate predictions of the wear behavior of hydrogels and cartilage based on the nature of the materials.

Ballistic impact performance and surface failure mechanisms of two-dimensional and three-dimensional woven p-aramid multi-layer fabrics for lightweight women ballistic vest applications

This paper investigates the influences of woven fabric type, impact locations and number of layers on ballistic impact performances of target panels through trauma dimension and panel surface damage mechanisms for lightweight women ballistic vest design. Three panels with 30, 35 and 40 layers of two-dimensional plain weave and another two panels with 30 and 40 layers of three-dimensional warp interlock fabrics were prepared. The three-dimensional woven fabric was manufactured using automatic Dornier weaving machine, whereas the two-dimensional fabric (with similar p-aramid fibre type (Twaron®)) was received from the Teijin Company. The ballistic tests were carried out according to NIJ Standard-0101.06 Level IIIA. Based on the result, woven fabric construction type, number of layers and target locations were directed an upshot on the trauma measurement values of the tested target panels. For example, 40 layers of two-dimensional plain weave fabric panels show lower trauma measurement values as compared to its counterpart three-dimensional warp interlock fabric panels with similar layer number. Moreover, 40 layers of two-dimensional fabric panels revealed 47% and 39% trauma depth reduction as compared to panels with 30 layers of two-dimensional fabric panel in moulded (target point 1) and non-moulded (target point 6), respectively. Due to higher amount of primary yarn involvement, two-dimensional plain weave fabric panel face higher level of local surface damages but less severe and fibrillated yarns than three-dimensional warp interlock fabrics panels. Moreover, three-dimensional warp interlock fabric panels required higher number of layers compared to two-dimensional plain weave aramid fabrics to halt the projectiles. Similarly, based on the post-mortem analysis of projectile, higher projectile debris deformation was recorded for panels having higher number of layers for both types of fabrics at similar target locations.