Spalling Behavior Research Articles

Molecular dynamics simulations on shock response and spalling behaviors of semi-coherent {111} Cu-Al multilayers

Shock response and spalling behaviors of semi-coherent {111} Cu-Al multilayer are investigated by molecular dynamics simulations. Firstly, the influences of interfaces on shock wave propagation are studied. The simulations show that reflection-unloading happens when the shock wave propagates from Cu to Al and reflection-loading occurs when the shock wave propagates from Al to Cu. Strong discontinuities of stress components at the interfaces are revealed as well, the discontinuities are correlated to mismatches of Hugoniot curves and elastic-plastic properties of the two materials adjacent to the interfaces. Secondly, shock induced dislocation activities are studied. It is found that the stress barrier offered by the interface Cu-Al is smaller than that offered by Al-Cu. In fact, the resistance for dislocations to transmit through interfaces is determined by the coherency stresses, Koehler stresses and misfit dislocations at the interface. In addition, spalling fracture of Cu-Al multilayers after stress wave reflection at the free surface is studied. It is observed that ductile damage only nucleates in Al layer, rather than in Cu layers. This observation agrees well with the experimental results of Han (Acta Mater., 2014 (63) 150–161). Due to complex reflection and transmission of the stress waves at the interfaces, the profile of the free surface velocity histories of the multilayer target is quite different from that of the single-crystalline counterpart. In order to study thermal dissipation process during cavitation, analysis on thermal dissipation in the spalling damage zones is also provided in this paper. Moreover, contour plots and profiles of stress are illustrated to understand the spallation process of the multilayer target deeply. Tensile stress emerges in Al1 layer first, causing voids to nucleate; then the tensile stress wave attenuates rapidly and propagates to Cu1 and Cu2 layers. With the dissipation effect of damage and plastic deformation in the target, the tensile stress wave dissipates ultimately.

Spalling behavior of metakaolin-fly ash based geopolymer concrete under elevated temperature exposure

Fire-induced spalling is a serious risk to concrete structures, especially for high strength concrete structures. This paper presents results from high temperature spalling tests on geopolymer concrete. The effect of moisture content, concrete strength, heating rate and temperature level on the spalling behavior of geopolymer concrete is studied. The temperature-induced spalling mechanism in geopolymer concrete is investigated through the measurement of residual compressive and splitting tensile strength, variation in permeability (by sorptivity test) and chemical composition (by X-ray diffraction test) of geopolymer concrete after elevated temperature exposure up to 700 °C. The test results indicate that geopolymer concrete exhibit a good spalling resistance as compared to that of OPC concrete. The lower spalling risk in geopolymer concrete under high temperature exposure is facilitated from the highly connected pore structures and lower strength degradation with temperatures. Further results indicate that the pore structure (permeability) of geopolymer concrete gets a significant evolution with the exposure temperature, especially above 500 °C range. This is related to the sintering reaction in geopolymer binders at high temperatures.

Shock and Spall in the Low-alloy Steel AF9628

The shock and spall behavior of tempered AF9628, an ultra-high strength (UHSS) martensitic low-alloy carbon steel, is investigated using gun-driven, symmetric, planar impacts. The study utilized laser velocimetry to obtain transmitted wave velocity histories that were analyzed to obtain the Hugoniot Elastic Limit (HEL), the shock Hugoniot, and the spall strength. The work was limited to the stress range below 13 GPa (below the e phase transition), and an HEL of 2.35 GPa and a spall strength of 6.8 GPa are reported. The HEL and spall strength of AF9628 are both above the typical values for inexpensive low-alloy steels, and only slightly below those for a much more expensive Aermet steel. The dynamic compressive yield strength (as calculated from the HEL) is only slightly higher than the quasi-static compressive yield strength when compared using a similar yield assignment convention, indicating a small, but positive, value of strain-rate sensitivity. The Hugoniot for AF9628, US = 0.67uP + 5.01 km/s, is in good agreement with other low-alloy martensitic steels for which data is available, and distinctly different than the Hugoniot for ferritic steels. The Hugoniot extrapolates to a bulk sound speed significantly higher than what is measured at ambient conditions, an incongruity that is unusual among metals and that the authors attribute to the unsteady nature of the shock waves generated in this work. Finally, it is suggested that there may be a relationship between dynamic compressive yield strength (calculated from the HEL) and dynamic tensile failure strength (the spall strength).

Damage behavior and maintenance design of tunnel lining based on numerical evaluation

The cracking and spalling of lining will adverse impact on the serviceability and durability of the tunnel. Thus, corresponding maintenance work to repair this damage in the tunnel structure is pertinent to its longevity. In this study, the cracking and spalling behaviors of the lining within an operational tunnel structure are examined. An integrated field investigation is conducted to characterize the spatial distribution of the damage, and the lining failure is speculated to be caused by insufficient lining thickness. To further understanding of the influence of insufficient lining thickness on the lining structure performance, extended finite element method (XFEM), is used to conduct a reliable numerical simulation. According to the maximum tension strain, failure evolution, safety factors of the failure region obtained by XFEM, the simulation results generally agree with the actual lining damage forms. This work demonstrates that serious lining damage is directly caused by insufficient lining thickness. Ultimately, a specific maintenance method is developed and simulated based on the deficiencies in tunnel construction. The proposed maintenance method is confirmed to be efficient and safe according numerical results.

Multiphysics Lattice Discrete Particle Model for the simulation of concrete thermal spalling

Explosive thermal spalling behavior during fire exposure is one of the major issues in the design of modern reinforced concrete structures. Previous experience on fire disasters indicates that spalling of concrete can have serious structural and economic consequences and must be taken into account in the design for fire. However, spalling mechanisms and their interaction still remain in dispute in the scientific community. In order to shed some light on this phenomenon, a discrete hygro-thermal model of concrete at high temperature called DTemPor3 is proposed and a full coupling scheme between DTemPor3 and the Lattice Discrete Particle Model (LDPM) is performed. The proposed multi-physical coupled model features the effect of pore pressure and temperature on the mechanical response as well as the impact of cracking on moisture mass transport and heat transfer. Simulations of typical spalling experiments show good agreements with data gathered from the literature for both high-performance concrete and ordinary concrete, demonstrating the accuracy of the proposed approach. Cracking localization is found to significantly impede the local pore pressure build-up due to the increase of pore or crack volume. The numerical simulations demonstrate that the spalling phenomenon can be successfully reproduced, only when the effect of thermal stresses is taken in account along with the effect of pore pressure on crack initiation.

Effect of residual stress on the spallation of the thermally-grown oxide formed on NiCoCrAlY coating

In this study, the effect of residual stress in the NiCoCrAlY coating on the oxidation and spallation behavior of the thermally grown oxide (TGO) has been investigated. The surfaces of the NiCoCrAlY coatings were polished to a 0.1 μm finish before oxidation. The oxidation performed at 1150 °C shows that coating with compressive stress induced by grit blasting exhibits a higher oxidation rate, higher surface roughness and lower spallation resistance compared with coating with tensile stress. This is attributed to the significant interfacial rumpling induced by the relaxation of the compressive stress in the coating at the initial oxidation stage. The interfacial rumpling leads to the formation of the fast-growing spinel oxides and interfacial cracks, which are detrimental to the interfacial adherence.

Effect of elevated temperatures on properties of sustainable concrete composites incorporating waste metalized plastic fibres

The utilisation of industrial waste in the production of sustainable construction materials has attracted much attention recently due to the saving of vital places for landfills, low-cost of waste materials and also an improvement in the concrete properties. Exposing concrete structures to elevated temperatures causes progressive failure of the macro- and micro-structures of cement paste and, therefore, severe deterioration and damages in the load-bearing capacity. This study explored the effect of waste metalized plastic (WMP) fibres and palm oil fuel ash (POFA) on the performance of concrete exposed to high temperatures of 200, 400, 600 and 800 °C. Four concrete mixes comprising 0 and 0.5% WMP fibres, and 0 and 20% POFA content were cast. Properties studied include mass loss, compressive strength, and ultrasonic pulse velocity. The results showed that the adding of WMP fibre to the concrete mixes significantly improves the concrete performance at elevated temperatures with the lower rate of strength loss along with eliminating the explosive spalling behaviour as compared to those of plain concrete mixes. Furthermore, in comparing the results of compressive strength losses at a high temperature of 800 °C, strength losses were lower for specimens containing 0.5% WMP fibres than those of plain specimens. Moreover, green concrete decreases waste materials, the diminution of harmful impacts on the environment, and leads to sustainable and green cement and concrete industries.

Influence of the Base Element on the Thermal Properties of Non- Ferrous Chromium-Rich TaC-Containing Alloys Elaborated by Conventional Casting: Part 3: Surface and Cross-Sectional Metallographic Characterization of the Oxidized Alloys

The six alloys the thermal properties of which and the tendency to oxide spallation of which were studied in the first two parts of this work, were here characterized after oxidation for 70 hours at 1250°C. The external chromia scale, and also the CrTaO4 subsurface oxide, formed for all the alloys, almost independently of the Co and Ni proportions in the base element content. But, because of the formation of more CrTaO4 for the nickel-richest alloys probably due to the higher availability of Ta in the matrix and its easier diffusion towards the neighbourhood of the oxidation front, the adherence of chromia was weakened and spallation, suggested by the thermogravimetric curves in the second part of this work, is here really observed and the denuded part of alloys clearly seen. The degradation of the subsurface, which can be in a first time summarized by the development of a carbide-free zone and a {Cr, Ta}-depleted zone, depends on the Co and Ni proportions. The microstructure of the bulk is differently affected by long exposure at elevated temperature. The changes in carbide population characteristics are stronger for the nickel-based alloys than for the cobalt-based ones. Finally, the isothermal oxidation behaviour is best for the nickel-richest alloys but the oxide spallation behaviour and the potential mechanical properties are the best for the cobalt-richest alloys.

Understanding and predicting damage and failure at grain boundaries in BCC Ta

Understanding the effect of grain boundaries (GBs) on the deformation and spall behavior is critical to designing materials with tailored failure responses under dynamic loading. This understanding is hampered by the lack of in situ imaging capability with the optimum spatial and temporal resolution during dynamic experiments, as well as by the scarcity of a systematic data set that correlates boundary structure to failure, especially in BCC metals. To fill in this gap in the current understanding, molecular dynamics simulations are performed on a set of 74 bi-crystals in Ta with a [110] symmetric tilt axis. Our results show a correlation between GB misorientation angle and spall strength and also highlight the importance of GB structure itself in determining the spall strength. Specifically, we find a direct correlation between the ability of the GB to plasticity deform through slip/twinning and its spall strength. Additionally, a change in the deformation mechanism from dislocation-meditated to twinning-dominated plasticity is observed as a function of misorientation angles, which results in lowered spall strengths for high-angle GBs.

An experimental fire-spalling assessment procedure for concrete mixtures

In this paper recommendations for an assessment and ranking method to evaluate the fire spalling risk and extent of concrete mixtures is presented. Particular emphasis is placed on the influence of the permeability, the porosity and the moisture content on spalling. X-ray diffraction and simultaneous thermal analysis provides insight into the mineralogical composition during heating. Fire tests on specimens with varying saturation degrees allow the identification of key parameters and their influence on explosive fire spalling. With the obtained dataset, correlations between the key parameters and the spalling behavior are established and a ranking and evaluation method for the spalling risk, spalling volume and maximum spalling depth of concrete mixtures is introduced.

Effects of Waste Ceramic as Cement and Fine Aggregate on Durability Performance of Sustainable Mortar

In the last 3 decades, the attention in consuming substitute materials such as solid wastes in construction has grown continuously. An extensive amount of waste ceramic is being generated all around the world. These wastes are mostly sent to the landfill without considering recycling option. Such waste ceramic in the powder and fine particle forms has good potential in the infrastructure industry. In this study, the strength and durability properties of a mortar comprising ceramic waste powder as supplementary cementing material and ceramic particles were investigated. Properties studied include workability, compressive and tensile strengths, chloride and sulfate resistance. The effect of waste ceramic was also assessed by using scanning electron microscopy and X-ray diffraction analysis. It was observed that the utilization of waste ceramic in both forms of binder and fine aggregate significantly improved the compressive and splitting tensile strengths and higher resistance against chloride and sulfate attacks. The microstructure of the mortar was further enhanced by replacing ceramic waste powder and fine aggregates. It, therefore, caused in the higher crystalline formation and reduction in porosity and cracks in addition to eliminating the spalling behavior of mortar specimens exposed to chloride and sulfate attacks.

On the effect of pore-space properties and water saturation on explosive spalling of fire-loaded concrete

In this paper, the influence of concrete properties such as permeability and environmental conditions (water content) on the spalling behavior of concrete subjected to fire loading is investigated. For this purpose, a special fire-test setup is presented, allowing an improved reproduction of temperature histories observed, e.g., in tunnel fires. Moreover, the developed test setup enables a continuous monitoring of spalling, giving access to the spalling history and the final level of damage of concrete specimens.The obtained results, considering different water/cement-ratios and saturation degrees, are related to the required initial tensile strength determined by means of a numerical assessment tool. Finally, correlations between the identified parameters governing fire-spalling were established and allowed an evaluation of the spalling risk of concrete mixtures. These correlations revealed that – in contrast to requirements given in national and international standards – the combination of water content and permeability may serve as proper key and design parameter, defining the risk of spalling.

DUCTILITY EVALUATION OF REINFORCED CONCRETE COLUMN MADE OF NORMAL- TO HIGH-STRENGTH CONCRETE UNDER CONSTANT AXIAL LOAD LEVEL COMBINED WITH FLEXURAL LOADING USING NONLINEAR SECTIONAL FIBER BASED MODEL

This study presents the ductility evaluation of reinforced concrete column made of normal-strength material using various empirical stress-strain model with nonlinear sectional fiber based analysis. The purpose is to evaluate the confinement requirement for reinforced concrete column under high axial load level. The concrete strength considered in the analysis are varies from 30 to 70 MPa while the steel reinforcing bar yield strength considered is only 400 MPa. The ductility is evaluated by using the customized ductility index measurement. The ratio of the concrete cover to the concrete core is set to 0.1 but not more than 40 mm. Attard and Setunge’s concrete constitutive model is used in this investigation. Cover spalling behavior is considered in the analysis by including the restrained shrinkage effect on the concrete strength and the softening behavior. From this study, it was found that extra confinement is necessary to maintain the expected minimum level of ductility.

Microstructure evolution of Mg/Cu dissimilar metal jointed by ultrasonic-induced transient liquid phase bonding with Zn interlayer

Ultrasonic-induced transient liquid phase bonding of Mg/Cu dissimilar alloys with Zn interlayer was carried out. The effects of pressure and temperature on the microstructure and mechanical properties of the weld seam were investigated. The formation of the MgZn solid-liquid interface, α-Mg solid solution spalling and merging growth behavior were analyzed. With the increase of pressure, the interface metallurgical reaction between the base metal and the interlayer was intensified, and the width of the weld seam decreased gradually. Solid solution, MgZn, MgCu, CuZn, and CuMgZn intermetallic compounds in the weld seam were detected. The fine granular eutectic structure has a tendency to transform into a layered eutectic structure gradually. A tendency to merge and grow for the adjacent α-Mg solid solution was observed. CuMgZn formed on the Cu side was embedded on the surface of Cu5Zn8, and remained in the weld seam. The thickness of CuMgZn increased with the increase of temperature. The fracture occurred between CuMgZn and Cu5Zn8, and the fracture manner was a brittle fracture. The highest average strength was 80.75 MPa.

Fatigue Investigation on Railway Wheel Steel with White Etching Layer

Spalling is one common defect generally observed in railway wheels. The spalling phenomenon usually starts from the white etching layer (WEL) of the wheel due to cyclic loading. So far, the research work has not been systematically done on how the crack orientation influences the spalling behaviour. In our current study, the fatigue behaviours of the wheel steel with a surface crack under cyclic loading are investigated using extended finite element method. The significant parameters affecting the spalling behaviours such as the crack orientation and loadings are studied. Two key indices are used: the number of cycles required for the crack to reach the interface between the WEL and wheel steel material, and the crack propagation rate. The fatigue propagation trend in the WEL is validated by relevant experimental studies. Moreover, it is concluded from our results that a crack vertical to the surface has a greater propagation rate compared to other crack orientations.

On the explosive spalling behavior of ultra-high performance concrete with and without coarse aggregate exposed to high temperature

Experimental investigations are conducted on the explosive spalling behavior of two types of ultra-high performance concrete (UHPC), i.e., UHPC with coarse aggregate (UHPC-CA) and UHPC without coarse aggregate (UHPC-no-CA) under high temperature slow heating conditions. The influences of polypropylene (PP) fiber and moisture content on the spalling behavior of UHPC are studied. The inner vapor pressure and temperature in the UHPC specimens are measured to analyze the driving force of explosive spalling of UHPC during heating. Results show that UHPC-CA exhibits a slightly better resistance to explosive spalling than UHPC-no-CA and coarse aggregate has a beneficial effect on minimizing the explosive spalling of UHPC. Also, Moisture content of UHPC specimen remarkably affects the explosive spalling behavior of both UHPCs, i.e. the higher the moisture content, the severer the explosive spalling is. Hybrid fibers (steel fiber and PP fiber) significantly improve the resistance to explosive spalling of UHPC and can prevent some UHPC specimens from explosive spalling. These results prove that vapor pressure is the primary driving factor on the explosive spalling of UHPC under the heating conditions in this work. Additionally, layered explosive spalling occurs to both UHPCs. The temperature gradient in UHPC-no-CA is much higher than that in UHPC-CA. Many coarse aggregates without cracking damage peel off from the UHPC-CA matrix. These results indicate that thermal stress is an important factor driving the explosive spalling. However, when the UHPC specimens are fully saturated (100% moisture content), the “pressure releaser” function of PP fiber may not play the most important role in alleviating the explosive spalling in UHPCs.

Estimation of spalling strength of sandstone under different pre-confining pressure by experiment and numerical simulation

Spalling failure is a common underground engineering disaster, particularly in deep environments with high geo-stress and strong stress disturbances. Therefore, the further investigation of the spalling behaviour of rock under different pre-confining pressures is of considerable importance. In the presented work, a modified split Hopkinson pressure bar (SHPB) system is modified with a pre-confining loading device that can apply pre-static loads to Φ50 mm × 300 mm specimens. The system is employed to study the spalling characteristics of rock specimens subjected to full pre-confining pressure. Spalling tests of Φ50 mm × 300 mm rock specimens under different pre-confining pressures were conducted. The experimental results indicated that spalling failure was influenced by pre-confining pressure. Furthermore, numerical simulations with the same loading conditions as the experiments were conducted using the finite element software, LS-DYNA, and the spalling strengths of the rock specimens under different pre-confining pressures were obtained based on the improved stress wave analysis method. The results indicated that the stress wave analysis method in view of numerical simulation can effectively be used for calculating the spalling strength of rock and rock-like material under pre-confining pressure. The spalling strength of the rock specimens increased first and then decreased as the pre-confining pressure increased.

Modeling cover spalling behavior in high strength reinforced concrete columns using a plasticity-fracture model

The cover spalling behavior in reinforced concrete (RC) columns can greatly affect the axial peak load carrying capacity. For RC columns with normal-strength concrete, the concrete cover still carries some load in the post-peak regime. However, for high-strength concrete, the concrete cover can spall prematurely, eliminating the cover capacity from the peak axial capacity of the column. The strength reduction due to premature cover spalling in high strength concrete columns may be recovered once confinement of the core increases and results in a second peak axial load. Hence, in nonlinear analysis a proper strategy is required to quantify any premature cover spalling for high- or very high-strength reinforced concrete columns. This paper discusses the strategies in modeling the concrete cover region spalling at the constitutive level. These strategies involve: (1) Adjusting the material properties for the concrete cover elements because of restrained shrinkage caused by the steel reinforcement; (2) Extending the plastic dilation rate into the tensile pressure region and adjusting the concrete dilatancy in presence of the tensile pressure; (3) Adding a tension cut-off failure surface to distinguish cracks caused by pure tensile stresses and concrete crushing (tensile splitting cracks) and (4) Assigning the cover spalling material properties only for cover elements outside the reinforcement cage or weak planes. To verify the proposed strategies, numerical validations against experimental published results are undertaken using a 3D-NLFEA finite element package. From the comparisons, it is shown that the proposed model is in good agreement with the experimental results and can accurately predict the location of the cover spalling in the load-deflection behavior.

Void collapse and subsequent spallation in Cu50Zr50 metallic glass under shock loading by molecular dynamics simulations

Void evolution at the microscopic scale is an important part of the shock response of porous metallic glasses (MGs). Here, large-scale molecular dynamics simulations are used to investigate the shock loading of Cu50Zr50 MG, including thermodynamic quantities, shock-induced void collapse, and spall behavior. The results show that the shear transformation zone nucleation and growth around the void is the main plastic deformation mechanism for the shock-induced void collapse in MGs. The stress around the void is analyzed to reveal the evolution of the void shape and the relationship between the critical stress for the void collapse and the Hugoniot elastic limit stress. A model is proposed to predict the void collapse time in MGs. Softening occurs at around the location of the void after the void collapse due to a local temperature increase. Consequently, spallation is colocated with the high temperature region, rather than at the position associated with maximum tensile stress. Void growth and nucleation of tension transformation zones compete with each other as the shock intensity increases. At a high strain rate, the Cu50Zr50 MG shows more brittle fracture behavior with a larger number of voids and smaller average void size.

An atomic view on spall responses of release melted lead induced by decaying shock loading

Knowledge of spallation behaviors of partially melted metals, as well as the ones of solids and liquids, is critical for understanding the complete spall responses of metals to a wide range of loading strengths. However, compared with traditional spallation in solids, the spallation of liquids, especially of partially melted metals, receives considerably less attention despite some advancements in recent years. In the present work, spallation behaviors of partially melted/completely melted Pb under various decaying shock loadings are investigated. The effects of peak free surface velocity on the incipient spall strength of the partially melted Pb are uncovered, which is attributed to the difference of melting degree. Our results suggest that the incipient spall strength decreases with the growing melting degree for the partially melted Pb. Besides, incipient spall criterion, as well as damage evolutions at the later stage, is discussed. It is found that, after the incipient spall takes place, the total void volume linearly increases with time evolutions regardless of the detailed void mechanism for the volume increment. The increasing rate grows with the promoting melting degree even under the same tensile strain rate and initial temperature. For a liquid Pb, the increasing rate is governed by the tensile strain rate. Physical reasons for the different behaviors between partially and completely melted metals are analyzed.