Spalling Phenomena Research Articles

Investigating the interface cracking mechanism of CMAS-corroded thermal barrier coatings based on the cohesive zone model

This study enhances the cohesive zone model to describe the propagation of interfacial cracks in thermal barrier coatings (TBCs) corroded by calcium-magnesium-alumino-silicates (CMAS) at high temperatures. The finite element method is used to build a multi-layer 2D plate model that simultaneously simulates stress distributions, infiltration of CMAS, corrosion of ceramic coat and interface crack growth in TBCs. Furthermore, this study provides a detailed elaboration of the growth mechanism of interface cracks and the performed corrosion experiment. The theoretical model presented here validates the spalling phenomena of TBCs caused by CMAS corrosion at high temperatures.

Experimental Study by Visualization of the Behavioral Properties of the Vortex Structures and the Ecltements on the Upper Surface of a Revolution Warhead

Scientific research of the flow visualizations, developed on the upper surface of the cones and warhead of revolution, the delta or Gothic wings and that of the warheads of the revolution, were performed in the wind tunnel of the aerodynamics and hydrodynamics laboratory of the Polytechnic University of Haut de France. This research has made it possible to better analyze and understand the phenomena of vortex structures and vortex spalling see photos2,3,4,5 and 6. angles, thus defined, over a wide range of Reynolds. The study concerns visualization analysis of the behavioral properties at the top surface of a revolution ogive having an apex angle of 74.5 ° at low angle of attack and performed at variable speeds. It has been noted that speed variations have no influence on the behavioral properties of the development of vortex structures whereas, on the other hand, changes in angles of incidence strongly influence this development. The study of the rise of the vortex rupture at the angles of attack has revealed original behavioral properties which find their expression notably in the discontinuous evolution, in terms of angle at the summit, of the angles of attack which define the beginning and end of the ascent of this vortex failure. These properties probably reflect those already observed in similar studies carried out on the delta and Gothic wings and on the cones. However, no current theory seems to be able to provide a direct explanation of these phenomena of the splitting of the whirling structures.

Experimental study of the mechanical properties of basalt fibre-reinforced concrete at elevated temperatures

Generally, achieving a proper mixing ratio of basalt fibre in concrete can improve the mechanical properties of concrete and enhance its anti-corrosion ability. Although basalt fibre exhibits excellent resistance to high temperatures, the mechanical behaviour of basalt fibre-reinforced concrete (BFRC) exposed to high temperatures is still unclear. In addition to the basic compressive and tensile strength testing for ordinary concrete and BFRC, the core temperature, mass loss, mechanical properties and synergistic mechanism of basalt fibre and concrete matrix under different temperatures were evaluated. The results show that BFRC provides excellent thermal insulation and can mitigate the concrete spalling phenomenon. Therefore, using basalt fibre can effectively improve the mechanical properties of concrete exposed to high temperatures.

Concrete plates with externally bonded FRP under contact detonations

Local damage on concrete elements subjected to a contact detonation include spalling and cratering of the back and front face, respectively. This paper presents the results of a detailed experimental and numerical methodology aiming to analyse the influence of externally bonded fibre reinforced polymers (FRP) on the spalling phenomena of concrete when it is subjected to contact explosions. The influence of externally bonded reinforcement (EBR) on the diameter and depth of the spalling is experimentally investigated, as is also the total momentum transferred to the ejected fragments when their mass increases. The experimental observations enable the development of a computational framework, where a finite element (FE) model is validated and used to perform a through-thickness analysis focused on the stress wave propagation and associated damage mechanisms. It is found that the extent of concrete volumetric damage is highly influenced by the mass of the bonded material, as well as increased bonded areas leading to enlarged surface damage. Additionally, it was verified that the compressive strength of concrete plays a major role in the spall damage.

Development of a real-time spalling measurement system for ball-type constant velocity joints

This study proposes a measurement system to detect spalling phenomena of ball-type constant velocity joints (CVJ). The spalling phenomena that usually occur in the inner race of a CVJ are a significant factor to the CVJ’s life, but it is very difficult to measure the spalling because the inner race is mechanically enclosed by an outer race and a cage. The developed system can not only detect the spalling but also measure the depth of adhesive wear in real time by implementing kinematics and applying force to an individual ball-type CVJ.The kinematics of a CVJ was analyzed to ensure that the measurement system operates the same as the actual CVJ. Especially, reciprocal yaw motions in the ball-type CVJ were perfectly implemented to the system. The system consists of three subsystems. The first system applies normal force to the contact points. Ball screws, spur gears attached to the motor amplify the force. The second system creates the reciprocal yaw motion with the same profiles as an actual CVJ. The third system is the measurement part, consisting of an incremental rotary encoder and a load cell. The resolution of wear depth is amplified and achieves up to 0.18°/μm.For validation, a ball-type CVJ was chosen and its spalling depth was measured. The results were verified by comparing the system measurement data with the profilometer measurement data.

A new spalling mechanism of intermetallics from the adhesion layer in the terminal-stage reaction between Cu and Sn

Three-dimensional integrated circuits provide a promising approach to extend Moore's law by vertically stacking multiple functional chips with microbumps. However, with miniaturization, depletion of the wetting layer of under bump metallurgy becomes a worrisome possibility. Upon depletion of the wetting layer, the adhesion layer will be directly exposed to intermetallics at the terminal stage of solid-state reactions. In this study, the resulting spalling phenomenon is investigated for the first time. The microstructure evolution of ultrathin Cu–Sn microbumps shows that spalling of Cu6Sn5 and Cu3Sn from the Cr adhesion layer occurs when Cu is depleted. The driving force for spalling is the high interfacial energy between intermetallics and the Cr adhesion layer. To prevent such spalling, which deteriorates the integrity of the bumps, the Cu wetting layer must be at least 1.3 times thicker than that of Sn.

Fabrication of micro holes in Yttria-stabilized zirconia (Y-SZ) by hybrid process of electrochemical discharge machining (ECDM)

In the present work, the electrochemical discharge machining (ECDM) process with an integrated approach of electrolyte flow and pressurized feeding is demonstrated to produce micro-holes in Yttria-stabilized zirconia (Y-SZ). The mechanism of material removal reveals that the material is primarily removed due to the thermal spalling phenomenon over the melting and evaporation. Unlike the other glass based ceramic materials, the contribution of chemical etching phenomenon is found to be almost negligible in removing the material from Yttria-stabilized zirconia work substrate. The incorporation of electrolyte flow in the ECDM process increases the machining efficiency by improving hole depth by 42% and reducing diameter by 33% for the same processing durations. The improved frequency of electrochemical discharges (ECD’s) due to the supply of electrolyte is responsible for removing the material in controlled manner without causing any fracture at the hole edges. Moreover, the supply of electrolyte reduces the tool wear, which results in consistent machining conditions for longer machining durations. The maximum penetration speed obtained in this investigation was 126 μm/min.

Spalling of concrete cover induced by reinforcement

This paper investigates the phenomenon of cover spalling in reinforced concrete induced by bond or by the action of an inner pressure. This research is based on an experimental programme comprising a series of bond tests and a series of tests with inner-pressure on cylindrical openings. The inner-pressure test series (aimed at representing the conditions occurring for instance due to corrosion of reinforcement) was performed with hydraulic inflator devices embedded within concrete openings near to the free concrete surface. By means of detailed surface measurements performed with Digital Image Correlation, the mechanisms triggering spalling failures and the associated resistance are discussed and analysed thoroughly. This series investigates in addition a number of phenomena relevant to spalling failures, such as the influence of the casting position, group and size effects. The observed response, analysed by means of a mechanical analogy, is later used to investigate a series of structural tests performed on pull-out specimens. This analysis highlights the analogies and differences between the two types of tests (subjected to an imposed pressure or to bond stresses). On that basis, a comprehensive approach for treatment of bond-related cases failing by cover spalling is proposed, showing consistent agreement to the experimental evidence.

Fire performance of composite columns made of high strength steel and concrete

The use of high strength steel and concrete materials for composite column construction is partly impeded by the lack of research on their fire performance. In EN1994-1-1, the highest concrete grade allowed for designing composite columns is capped at C50. This paper investigates the behaviour of concrete encased steel (CES) composite columns made of C120 concrete and S500/690 steel section under ISO834 standard fire through a series of experimental and numerical analyses. Fire tests were carried out on five high strength CES column specimens subject to varying eccentricity of loading to study their behaviours in fire. Polypropylene fibres were added into the high strength concrete mix to prevent the explosive spalling so that explicit modelling of cover spalling phenomenon can be omitted in the numerical analysis. A unified method is proposed to determine the transient strain of high strength concrete at elevated temperatures, allowing the stress-strain curves in EN1992-1-2 to be extended to high strength concrete up to C120. A numerical model, which can capture the strain reversal of concrete caused by the flexural deformation of columns at elevated temperatures, is proposed to enable a good prediction of axial displacement responses of CES columns in comparison with the test results. Finally, based on the validated numerical model and parametric studies, a new tabulated data method is proposed for the fire resistance design of CES columns with concrete class up to C90 concrete and steel section grade up to S550.

Melting characteristics and dynamic evolution of microstructure of single-crystal gold film irradiated by laser under the non-Fourier effect

The process of ultra-thin single crystal gold film irradiated by femtosecond laser is simulated by the combination of improved two-temperature model (TTM) and molecular dynamics (MD). The results indicate that under the non-Fourier effect, the gold film shows special melting characteristics and spalling mechanism. The phenomena of homogeneous melting and pore spalling of gold film are successfully distinguished by curing rate method and atomic density method respectively. Due to the tiny thickness of the gold film studied, the damage mechanism of the gold film irradiated by laser has also changed.

Benefits of curvilinear straight steel fibers on the rate-dependent pullout resistance of ultra-high-performance concrete

This study examines the effectiveness of using curvilinear steel fibers to enhance the pullout resistance of conventional straight steel (S-) fiber for an ultra-high-performance concrete (UHPC) matrix. Because structures composed of UHPC are subjected to various loading conditions and have random fiber orientations, three different loading rates ranging from static (0.018 mm/s) to impact (up to 906.2 mm/s) and two inclination angles of 0° and 45° were adopted. To fabricate the novel curvilinear steel fibers, five different curvatures were also utilized. Test results indicated that the curvilinear fibers provided a clearly higher pullout resistance than that of the conventional S-fiber, and the effectiveness increased with the curvature, regardless of the inclination angle and loading rate. Higher bond strengths were obtained for the S- and curvilinear fibers when they were inclined at 45° rather than aligned. In general, the bond strengths and pullout energies of the S- and curvilinear fibers in UHPC were improved by increasing the loading rate, and a higher loading rate sensitivity on the bond strengths was observed in aligned fibers than in inclined fibers. The matrix spalling phenomenon was only observed in an inclined state and became more significant with an increasing curvature and loading rate. The developed curvilinear fibers provided noticeably smaller matrix spalling areas than the conventional deformed steel fibers under both static and impact conditions, owing to the mitigated stress concentration. Thus, these may be worthy of attention as a novel reinforcing fiber type for UHPC composites.

The Dynamic Response and Failure Model of Thin Plate Rock Mass under Impact Load

The layered rock mass widely exists in mining, construction, transportation, and water conservancy projects, and the damage phenomena of plate crack and spalling often occurs in the process of coal and rock dynamic disaster in deep mining. Therefore, the rock mass nearby excavation surface is usually considered to be composed of layers of thin plate rock mass to reveal the damage and failure mechanism of rock mass. In the whole dynamic process of mining and coal and rock dynamic disaster, rock mass would bear the dynamic disturbance from mine earthquake, and at present, the mechanical characteristics of rock mass are mainly studied under static load, while dynamic mechanical response characteristics and the mechanisms of dynamic damage, failure, and disaster‐causing are still unclear. This study mainly focused on the dynamic response characteristic and failure mechanism of rock mass based on a rectangular thin plate model. The frequency equations and deflection equations of the thin plate rock mass with different boundary conditions (S‐F‐S‐F, S‐C‐S‐C, and C‐C‐C‐C) were established under free vibration by the thin plate model and the dual equation of the Hamilton system, and the deflection equations under impact load were derived based on the Duhamel integral. And then, the effective vibration modes of the thin plate rock mass with different boundary conditions and their natural frequencies were obtained by Newton’s iterative method. Based on the third‐strength theory and the numerical simulation results by LS‐DYNA, the maximum shear of the effective vibration modes and the processes of damage and failure under impact load were analyzed. The research results showed that the initial position of damage and failure may be determined by effective vibration mode with the lowest frequency; the develop tendency of which by the combined actions of other effective vibration modes and the effective vibration modes with lower frequency could have greater influence on the process of damage and failure of the thin plate rock mass, which are beneficial to revealing the mechanism of coal and rock dynamic disaster.

Spallation Analysis of Concrete Under Pulse Load Based on Peridynamic Theory

Spallation analysis is one of important research directions in impact dynamics. By combining the newly developing Peridynamics (PD) theory, the spallation phenomenon of concrete is numerically simulated using C language and MATLAB programming. The factors that may affect the spalling are verified: the type of pulse load, the geometric size of the model and the action time of pulse load. The dynamic response of spallation of three-dimensional concrete columns under different pulse loading forms (rectangular pulse, triangular pulse and exponential pulse) is analyzed. (1) Under the same impulse effect, only one spalling occurs in the rectangular pulse, and no multiple spallation occurs when the pulse amplitude increase. Exponential and triangular pulses can produce multiple spallation phenomena, and the time for the first spallation phenomenon is rectangular pulse < triangle pulse < exponential pulse. (2) The effect of the same linear triangle pulse on spalling of concrete columns with different lengths (100 mm, 200 mm and 300 mm) is analyzed. The triangle pulse can cause single or multiple spallation, which is related to the length and size of the model. (3) Finally, by changing the number of time steps of the pulse load, the different spalling phenomena of triangular pulses are analyzed. The thickness of the first layer increases significantly with the increase of the action time.

Physical Model Experimental Study on Spalling Failure Around a Tunnel in Synthetic Marble

Based on the frequent occurrence of stress-induced hazards in deep rock tunnels, a physical model experimental study of the development process, failure mechanism, and onset conditions of deep hard rock spalling in horseshoe-shaped tunnels is carried out. The test process mainly includes model preparation, loading, excavation, and monitoring. A high-speed camera, distributed fiber optic sensing, acoustic emission (AE) system, and visual imaging correlation-2D (VIC-2D) techniques are integrated to monitor the mechanical response of the surrounding rocks. The test results indicate that the physical model test successfully simulates the spalling phenomena of the deep hard rock. The integrated monitoring system successfully captures the information on deformation, AE signals, and crack propagation in the instantaneous process of the surrounding rock spalling failure, which provides a sound basis for a more comprehensive understanding of the mechanism. Microcracks and dilation appear first on the sidewall surface; the cracks then continue to propagate and coalesce, forming thin rock flakes and spalling in a lamellar manner from shallow to deep. The quantitative relationships between the onset stresses, the critical strains, and the initial boundary stresses of the spalling failure are established. At the same depth from the excavation boundary, the radial strain of the sidewall is larger than that of the crown, and it decreases away from the excavation boundary to the interior. The increasing loading in vertical direction leads to the stress concentration in the shallow part at the sidewall and a sharp increase of deformation. There is a positive correlation between the spalling failure degree and the initial boundary stresses.

High performance computing simulations of spall phenomenon in a submicron thick nanocrystalline aluminum

The spall failure phenomenon is investigated at nano-scale in a plate-on-plate impact configuration using large-scale molecular dynamics (MD) high performance computing (HPC) simulationson three multi-billion 0.5 μm thick nanocrystalline aluminum (nc-Al) systems, each with an average grain size in the range of 18–100 nm and each under impact velocities in the range of 0.7–1.5 km s−1. Using a material conserving atom section-averaging process, distributions of several mechanical responses were obtained and the extremely transient spall failure phenomena were located in the interaction zones of the stress release waves released from the impacted and free ends of the atom systems. The spall zones’ thicknesses were estimated along with the spall strengths. For the nc-Al atom systems, the spall strength was observed to increase as the impact velocity increased from 0.7 to 1.5 km s−1, but only showed a slight decrease as the grain size increased from 18 to 100 nm. The spall strengths thus estimated from the stress release waves’ interaction zones were found to be conservative when compared to the traditional estimates obtained from the pullback velocity signatures in the free-end velocity evolutions. The MD results were further analyzed using a crystal analysis algorithm and a twin dislocation identification method to obtain the densities of the atomistic microstructures evolving under the interaction of the stress release waves. High-fidelity large-scale HPC simulation results showed that certain dislocation partials strongly influenced the spall response. The Stair-rod type dislocation partials increased by more than a factor of 10 during the interaction of the stress release waves as the spall failure commenced.

Mechanical Properties of Concrete with Steel and Polypropylene Fibres at Elevated Temperatures

Addition of steel fibres to concrete is known to have a significant positive influence on the mechanical properties of concrete. Micro polypropylene (PP) fibres are added to concrete to improve its performance under thermal loads such as in case of fire by preventing the phenomena of explosive spalling. An optimum mixture of steel and micro PP fibres added to concrete may be utilized to enhance both the mechanical and thermal behaviour of concrete. In this work, systematic investigations were carried out to study the influence of elevated temperature on the mechanical properties and physical properties of high strength concrete without and with fibres. Three different mixtures for high strength concrete were used, namely normal concrete without fibres, Steel fibre reinforced concrete and Hybrid fibre reinforced concrete having a blend of hooked end steel fibres and micro PP fibres. The specimens were tested in ambient conditions as well as after exposure to a pre-defined elevated temperature and cooling down to room temperature. For all investigated concrete mixtures the thermal degradation of following properties were investigated: compressive strength, tensile splitting strength, bending strength, fracture energy and static modulus of elasticity. This paper summarizes the findings of the tests performed.

Dynamic Mechanical Properties and Failure Mode of Artificial Frozen Silty Clay Subject to One‐Dimensional Coupled Static and Dynamic Loads

The dynamic stress‐strain relationship of artificial frozen silty clay under one‐dimensional coupled static and dynamic loads is obtained using modified split Hopkinson pressure bar (SHPB) equipment. The variation in dynamic compressive strength, dynamic deformation modulus, energy dissipation, and failure mode of artificial frozen silty clay with axial precompressive stress ratio are studied in this research. Experimental results indicate that the dynamic stress‐strain curves under uniaxial state and one‐dimensional coupled static and dynamic loads can be divided into four stages, i.e., compaction stage, elastic stage, plastic stage, and failure stage. The dynamic compressive strength, first‐stage deformation modulus, second‐stage deformation modulus, and absorbed energy density of artificial frozen silty clay present a trend of first increase and then decrease with the increase of axial compressive stress ratio, and the axial compressive stress ratio corresponding to the peak value is 0.7 in this test. In addition, there is a very similar effect of axial precompressive stress ratio on dynamic compressive strength and second‐stage deformation modulus of artificial frozen silty clay. At 0.4 axial compressive stress ratio, spall phenomenon appears at circumferential direction and center position of the frozen soil specimen has no obvious failure. Shear failure appears at 0.7 to 0.9 axial compressive stress ratio, and the larger the axial compressive stress ratio applies, the more obvious the shearing surface appears; moreover, comminution failure mode appears at 1.0 axial compressive stress ratio.

Dynamic Damage Mechanism of Coal Wall in Deep Longwall Face

The stability of coal wall in deep longwall face has always been a research hotspot. In this study, pure vibration signals in the coal wall during the operation of mining machinery were obtained for the first time, and their energy is mainly concentrated in 7–12 Hz. Besides, based on the law of stress wave propagation, with the coal wall of deep longwall taken as the research object, the theory of dynamic damage in coal wall was put forward from the perspective of dynamics. The results show that the loading and unloading waves generated by the mining machinery disturbance will be reflected and transmitted at the interface with different impedances, resulting in the formation of multiple unloading and loading waves and multiple tensile stress zones and stress concentration zones. These stress concentration zones tend to induce tensile stress generation and coal failure. As a result, the coal undergoes zonal failure and spalling. Through the vibration test of coal, it is found that the crack development of the coal sample can be divided into five stages, and the phenomena of zonal failure and spalling occur, which is consistent with the theory. At the same time, the sample that has gone through a large disturbance cannot be further damaged by a small disturbance, which is verified by the damage statistical constitutive model based on the isotropy hypothesis.

Improvement on sintering property of the face coat of yttria mould shells for investment casting

In order to explore methods for improving the sintering property of yttria face coat to prevent the thermal spalling phenomenon during investment casting, without significantly increasing the cost，the effects of electrode types in the vacuum furnace and the doping of oxides on the sintering character of yttria face coat at high temperature have been investigated. By contrast with the results of traditionally sintered pure yttria face coat at 1700 ℃, the promotion in densification of the yttria face coat of the mould shells by doping La2O3 + ZrO2 is the most efficacious, whereas doping CeO2 + ZrO2 can impede the densification of the yttria face coat. After sintering at 2000 ℃ in a vacuum furnace with graphite electrodes, the yttria face coat can get complete densification, while the yttria face coat shows incomplete densification in a furnace with tungsten electrodes, whatever doped or undoped. But yttria grains in the face coats doped with oxides are larger than those in the un-doped one. Both CaO + ZrO2 doping and MgO + ZrO2 doping can make greater acceleration of the yttria grain growth than La2O3 + ZrO2 doping and CeO2 + ZrO2 doping.

Experimental analysis of the spalling phenomenon in precast reinforced concrete columns exposed to high temperatures

Abstract Among the processes that involve the degradation of concrete structures subject to the high temperatures of a fire there is the spalling phenomenon. Its mechanisms are related to the thermal stress of the materials dilatations and pore pressure the process of vaporization of water during heating. The factors that influences in its occurrence are related to concrete properties, structural member characteristics or the exposure conditions, and their parameters are not clearly known yet. This paper aimed to study the influence of three concrete mixtures, four coating thicknesses and two bars diameters of longitudinal reinforcement in the spalling phenomena exposed to ISO 834 fire curve. The characterization of concrete were performed either of the axial compression strength tests, water absorption by capillary and mercury intrusion porosimetry, besides the fire resistance tests in real-scale specimens. It was concluded that the diameter of the bar does not have influence, while the mixture and the concrete cover thickness does. More spalling was recorded for the columns with thicker concrete cover and concrete compressive strength at 61,9 MPa, and although higher strength concrete have less permeability, this characteristic can be balanced with the higher tensile strength of this type of concrete.