Spalling Behavior Research Articles

Defect and damage evolution during spallation of single crystal Al: Comparison between molecular dynamics and quasi-coarse-grained dynamics simulations

The links between loading orientation of single crystal Al and the dynamic evolution of defects (dislocations, twins, stacking faults etc.) during spallation are investigated using molecular dynamics (MD) simulations. The microstructural evolution during the shock compression of single crystal Al is observed to be primarily guided by the nucleation and evolution of Shockley partials and twin partials. The shock response and spallation behavior of single crystal Al is observed to be anisotropic and is influenced by the density of various types of dislocations during shock compression and void nucleation at the spall plane. The capability of the “quasi-coarse-grained dynamics” (QCGD) method to reproduce the MD predicted nucleation, interaction and evolution of dislocations during shock compression and spallation of single crystal Al is discussed. The QCGD method retains the relative contribution of different types of dislocations during propagation of shock compression wave, interactions of the wave and spallation of single crystal Al as predicted by MD using a fraction of representative atoms allowing the modeling of of larger sized systems for larger times.

Concrete propensity to fire spalling: testing and observations

This article presents results of fire spalling tests on small concrete slabs and studies of material parameters that may increase its occurrence. Experimental techniques enabling to study and determination of material features are presented and discussed. Experimental studies on spalling behaviour of elements were carried out on seven different concrete mixes with constant content of cement paste and mortar. Research aimed at determining influence of the following parameters: w/c ratio (0.30; 0.45; 0.60), cement type (CEM I, CEM III) and type of aggregates (riverbed gravel, granite, basalt) on fire concrete spalling. Paper discusses also the influence of cold rim that forms while testing slab-like element is subjected to one-side heating.

Effectiveness of Polypropylene and Steel Fibers in Enhancing Fire Resistance of High-Strength Concrete Columns

Fire-induced spalling is one of the concerns with the use of high-strength concrete (HSC) in structural applications. Some recent studies have recommended addition of polypropylene and/or steel fibers to overcome such spalling. This paper presents results from fire resistance tests on the comparative fire performance of HSC columns with and without fibers. Four reinforced concrete (RC) columns made of HSC with plain (no fibers), polypropylene, steel, and hybrid fibers, as well as one RC column made of conventional normal-strength concrete (NSC), were tested under different fire conditions. Data from these tests are utilized to evaluate comparative fire behavior of RC columns made of plain and fiber-reinforced HSC. In addition, an analytical study is carried out to evaluate relative changes in porosity and temperature-induced pore pressure in NSC and HSC, and its effect on spalling behavior and fire resistance of HSC columns. Results from fire resistance experiments and numerical studies show that hybrid-fiber-reinforced HSC columns exhibit better fire performance compared to polypropylene- or steel-fiber-reinforced or plain HSC columns.

The characteristics of void distribution in spalled high purity copper cylinder under sweeping detonation

The effects of different peak compression stresses (2–5 GPa) on the spallation behaviour of high purity copper cylinder during sweeping detonation were examined by Electron Backscatter Diffraction Microscopy, Doppler Pins System and Optical Microscopy techniques. The velocity history of inner surface and the characteristics of void distributions in spalled copper cylinder were investigated. The results indicated that the spall strength of copper in these experiments was less than that revealed in previous reports concerning plate impact loading. The geometry of cylindrical copper and the obliquity of incident shock during sweeping detonation may be the main reasons. Different loading stresses seemed to be responsible for the characteristics of the resultant damage fields, and the maximum damage degree increased with increasing shock stress. Spall planes in different cross-sections of sample loaded with the same shock stress of 3.29 GPa were found, and the distance from the initiation end has little effect on the maximum damage degree (the maximum damage range from 12 to 14%), which means that the spallation behaviour was stable along the direction parallel to the detonation propagation direction under the same shock stress.

Shock wave propagation and spall failure of nanocrystalline Cu/Ta alloys: Effect of Ta in solid-solution

The role of the concentration and distribution of the Ta solute in a solid solution in the shock response and spall failure of a bulk nanocrystalline Cu (nc-Cu) system is investigated using large scale molecular dynamics (MD) simulations. The nanocrystalline Cu/Ta (nc-Cu/Ta) microstructures comprise a 16 nm grain size Cu matrix with distributions of 3.0%, 6.3%, and 10.0% Ta atoms either along the grain boundary or randomly in the Cu matrix. The shock response is investigated by identifying the modifications in the dynamic evolution of defect structures (dislocation nucleation and interactions), as well as the nucleation and evolution of voids. The MD simulations reveal the complex role of Ta in altering the spall behavior of the nc-Cu system. The presence of Ta is observed to improve the spall strengths of the nc-Cu system, and the strengthening behavior is achieved by limiting the capability to nucleate dislocations during shock compression and under tensile pressures. The highest values for spall strength are observed for a random distribution of Ta at a concentration of 6.3% Ta. While the increase in strength for a 16 nm grain sized nc-Cu system is reported to be ∼8%, the role of the Ta solute is observed to be more pronounced at a larger grain size of 30 nm with a ∼15% increase in the spall strength for randomly distributed Ta with a concentration of 6.3%. The links between the concentration and the distribution of Ta in the solid solution and the evolution of microstructures under shock loading conditions are discussed.

Time-dependent spalling behavior of thermally grown oxide induced by room temperature interfacial deformation

The room temperature spalling of oxide scale is an important phenomenon for the application of high temperature alloys or coatings. In this study, the mechanism of the time-dependent spalling behavior of the thermally grown oxide (TGO) formed on a Pt-diffused bond coat was investigated. To detect the interfacial deformation at room temperature, the surface roughness of the TGO was measured in situ as function of time immediately after cooling, which reflected the change of interfacial morphology. Finite element analysis was employed to simulate the deformation and crack nucleation at the interface. It was proposed that the room temperature plastic deformation could occur at the TGO/bond coat interface, leading to the crack nucleation. Furthermore, the effect of the interface geometry on the crack length was also studied. It was inferred that the time-dependent spalling behavior of the TGO might be associated with the room temperature deformation of the interface.

Spall behaviour of single crystal aluminium at three principal orientations

A series of plate impact experiments have been conducted to study the spall strength of the three principal crystallographic orientations of single crystal aluminium ([100], [110] and, [111]) and ultra-pure polycrystalline aluminium. The samples have been shock loaded at two impact stresses (4 GPa and 10 GPa). Significant differences have been observed in the elastic behaviour, the pullback velocities, and the general shape of the wave profiles, which can be accounted for by considerations of the microscale homogeneity, the dislocation density, and the absence of grain boundaries in the single crystal materials. The data have shown that there is a consistent order of spall strength measured for the four sample materials. The [111] orientation has the largest spall strength and elastic limit, followed closely by [110], [100], and then the polycrystalline material. This order is consistent with both quasi-static data and geometrical consideration of Schmid factors.

Oxidation Kinetics and Spallation Model of Oxide Scale during Cooling Process of Low Carbon Microalloyed Steel

AbstractThe spallation behavior of oxide scale on the surface of low carbon microalloyed steel (510L) is investigated during the laminar cooling of hot rolling strip. Surface, cross-section morphology and phase composition of oxide scale in different laminar cooling rate are observed by scanning electron microscopy (SEM) and X-Ray Diffraction (XRD). Moreover, a spallation mathematic model is established based on empirical formula to predict the critical thickness of oxide scale and the test of high temperature oxidation kinetics at different temperatures between 500 °C to 900 °C provides oxidation rate constant for the model calculation. The results of heat-treatment test and model calculation reveal that laminar cooling rate plays an important role in controlling the thickness of oxide scale and suppressing spallation behavior.

Spall response and failure mechanisms associated with a hot-extruded AMX602 Mg alloy

Normal plate impact experiments were conducted to study the spall behavior of AMX602 magnesium alloy fabricated via Spinning Water Atomization Process (SWAP), which was followed by cold compaction and hot extrusion. Incipient spall damage in the specimens was measured at different shock conditions using 51mm and 105mm diameter bore gas guns. The Hugoniot Elastic Limit (HEL) was measured to be approximately 187 ± 11MPa. The spall strengths extracted from the free surface velocity profiles of the shocked specimens was found to increase by 8% for shock stresses ranging from 1.61 to 4.53GPa. The experimental data also revealed some inherent scatter in the spall strength data due to microstructural heterogeneity. Post-test fractographic analysis of the shock-recovered specimens suggests that the initiation of spall was most likely associated with intermetallic Al2Ca-bearing precipitates forming during the hot extrusion process. The subsequent spall failure propagated via cavitation events that involved the nucleation of nano- and micrometer-sized voids with very limited plastic growth.

Experimental investigation of the effect of silica fume on the thermal spalling of reactive powder concrete

The distinct spalling performances of reactive powder concrete (RPC) specimens with various silica fume (SF) contents exposed to high temperatures were observed via high-resolution photography. The RPC microstructures and pore structures after high-temperature exposure were characterized using scanning electron microscopy and mercury intrusion porosimetry. The results provide experimental evidence of the high-temperature spalling mechanism of RPC. Increasing the SF content in RPC increases its compressive strength and compactness, offering greater mitigation of devastating spalling behaviour, but also producing more pulverized spalling remnants. This is attributed to the post-heating cracked microstructure and refined pores, which promote localized rather than entirely explosive spalling.

Investigations on the spall and delamination behavior of UHMWPE composites

Composite materials in armor applications are commonly subjected to different deformation and failure mechanisms, which result in delamination of the solid laminate. Direct planar plate impact experiments were executed in order to study spall behavior of Dyneema® HB26. For the first time, the spall behavior of UHMWPE composites was determined by evaluating the free surface velocity measured via Photonic Doppler Velocimetry (PDV). High-speed camera images from lateral view revealed that spall failure of UHMWPE composites, unlike e.g. metals or ceramics, is not restricted to a single crack plane. Instead, the spallation of the composite plates in through-thickness direction caused by the first release wave is accompanied by several delaminations at ply interfaces, which are caused by intersections of subsequent release waves.

Shock-induced spall in single and nanocrystalline SiC

Shock-induced spall in SiC is investigated via high strain-rate loading molecular dynamics simulations. The dynamic response under different shock intensities is characterized along the low-index 3C-SiC crystallographic directions, [001], [110], and [111], and in a nanocrystalline sample with 5 nm average grain size. The simulation results show that all single crystal samples generate elastic, plastic, and structural phase transformation waves for increasing particle velocities in good agreement with previous investigations. However, crystal anisotropy effects affect the exact shock response and the corresponding wave structures. The Hugoniot elastic limit is significantly higher along the [111] and [110] directions while the patterns of plastic deformation, based on deformation twinning, contrast along the three crystallographic directions. The spall behavior, both for single and nanocrystalline samples vary from classical to micro-spall. The predicted spall strength is at maximum along the [111] direction, at 34 GPa, followed by the [110], and [001] directions, at 32 and 30 GPa, respectively. Nanocrystalline SiC displays a spall strength over 66.7% lower than single crystals. Spall strengths from direct and indirect methods agree well for both classical and micro-spall regimes after applying an elastic-plastic correction and considering the change in sound velocity, in particular for the case where the structural phase transformation occurs.

Effect of Crystal Orientation on Femtosecond Laser-Induced Thermomechanical Responses and Spallation Behaviors of Copper Films

Ultrafast thermomechanical responses and spallation behaviours of monocrystal copper films irradiated by femtosecond laser pulse are investigated using molecular dynamics simulation (MDS). Films with 〈100〉, 〈110〉 and 〈111〉 crystal orientations along the thickness direction were studied. The results show that the crystal orientation has a significant effect on femtosecond laser-induced thermomechanical responses and spallation behaviors of monocrystal copper films. The discrepancy between normal stresses in copper films with different crystal orientation leads to distinct differences in lattice temperature. Moreover, the copper films with different crystal orientations present distinct spallation behaviors, including structural melting (atomic splashing) and fracture. The melting depth of 〈100〉 copper film is lower than that of 〈110〉 and 〈111〉 copper films for the same laser intensity. The dislocations and slip bands are formed and propagate from the solid-liquid interface of 〈110〉 and 〈111〉 copper films, while these phenomena do not appear in 〈100〉 copper film. Additionally, numerous slip bands are generated in the non-irradiated surface region of copper films due to reflection of mechanical stress. These slip bands can finally evolve into cracks (nanovoids) with time, which further result in the fracture of the entire films.

Specimens size, aggregate size, and aggregate type effect on spalling of concrete in fire

SummaryThis paper attempted to isolate variables that govern concrete spalling when exposed to a hydrocarbon fire. The influence of specimen size was investigated by studying 4 specimen sizes consisting of cylinders, columns, and panels. Three aggregate sizes, 7 mm, 14 mm, and 20 mm were used in the concrete mixes to determine their effect on concrete spalling. Influence of aggregate type on concrete spalling was also investigated. Forty‐two different specimens were considered in this investigation. Concrete spalling was quantified as nominal spalling depth, which has been presented as a new way of quantifying the degree of concrete spalling. The results indicated that specimen size did have an effect on the spalling of concrete under hydrocarbon fire exposure and that nominal spalling depth of concrete increases as the specimen size increases. Aggregate size effect was evident when the maximum aggregate size increased from 7 mm to 20 mm, and explosive spalling was more severe for specimens with small size aggregates. Specimens with 14‐mm aggregate size showed inconsistent results and the spalling behavior witnessed was more random and sporadic. The type of aggregate used has no clear bearing on concrete spalling given both aggregates had similar linear expansion profiles.

Effect of alloyed Lu, Hf and Cr on the oxidation and spallation behavior of NiAl

The influences of alloyed Lu, Hf and Cr on the oxidation and spallation behavior of NiAl were investigated. Although Lu has a “block effect” of Al as Hf does, the oxidation rate of Lu-doped NiAl is significantly higher, due to: (1) inhibitation of θ- to α-Al2O3 transformation in early oxidation stage; (2) stronger scale grain refinement effect and enhanced oxygen transport rate. Segregation of S and C occurs not only at the alloy/scale interface, but also within the scale. Both Lu and Hf can significantly improve the scale adherence. However, This beneficial effect can be easily overcompensated by Cr.

Behavior of structural tunnel elements exposed to fire and mechanical loading

PurposeIncidents like the fire in the Channel Tunnel, where severe concrete spalling was determined, have led to requirements in limiting the spalling depth and involved zone to local and compatible magnitudes. Because the prevention of critical concrete spalling was also significant for the validity of the load-carrying capacity calculation for an existing railway tunnel, this paper aimed to investigate the spalling behavior of two contemplable concrete mixtures. The large-scale tests should show the load-carrying capacity over the whole duration of the fire exposure respecting all thermal and mechanical loads considered in the calculations.Design/methodology/approachIn this paper, the fire behavior of two concrete mixtures for an existing railway tunnel are investigated. Small-scale tests prior to the main tests were conducted to identify an appropriate concrete mixture for the large-scale tests. During the large-scale tests, a tunnel segment is loaded with horizontal and vertical loads derived from a calculation taking into account the existing boundary conditions. Resulting restraint forces were calculated using the soil stiffness and tunnel fire design curve as fire scenario and applied via hydraulic jacks. To avoid additional restraint forces during the experiment, thermal strains due to fire exposure were allowed.FindingsThe results of the small-scale tests did not allow for a clear statement whether one concrete mixture would perform better regarding the spalling behavior. The two large-scale tests showed different results regarding the spalling behavior. Over the whole duration of fire exposure, the first test specimen remains nearly undamaged. During the test of the second specimen, spalling started about 3 min after burner activation. Because of the results, a suggestion for the concrete mixture of the first test was made, and this mixture was then used for the redevelopment of the existing railway tunnel.Originality/valueThe test setup was capable of incorporating all relevant boundary conditions for the analysis of an existing railway tunnel as part of an important north – south connection. The results have shown that a fire-proof construction is possible by adding polypropylene fibers to the concrete mixture. Additionally, it was possible to avoid the mounting of expensive and time-consuming fire protection measures like the installation of thermal insulation boards.

Numerical analysis of heating rate effect on spalling of high-performance concrete under high temperature conditions

High-performance concrete (HPC) is vulnerable to spalling under high temperature conditions and it has been found that the heating rate can exert a tremendous effect on spalling of HPC. To prevent HPC from spalling, the heating rate effect should be understood. However, quantitative analyses are still lacking and the heating rate effect has not been well interpreted so far. In this paper, a numerical analysis of the heating rate effect on spalling of HPC is presented. Based on the experimental results reported in the literature, the spalling behavior of cubic HPC specimens under fire heating and slow heating with a heating rate of 5°C/min is modeled. With a meso-level thermo-chemo-hydro-mechanical analysis, the temperature gradient induced thermal stress and the mechanical effect of build-up vapor pressure are investigated. The results show that, at different heating rates, the spalling mechanisms are different. Finally, possible manners and mechanisms of spalling are discussed.

Effects of Elevated Temperatures on Residual Properties of Concrete Reinforced with Waste Polypropylene Carpet Fibres

In this study, the effect of waste polypropylene carpet fibres and palm oil fuel ash (POFA) on the mechanical and microstructural properties of concrete exposed to elevated temperatures was investigated. Concrete samples were exposed to high temperatures up to $$800\,{^{\circ}}\hbox {C}$$ then cooled to ambient temperature before tests. Four mixes containing carpet fibres (0 and 0.5%) and POFA (0 and 20%) were prepared. Mass loss, residual ultrasonic pulse velocity, compressive strength, scanning electron microscopy, X-ray diffraction and differential thermal analysis were performed to investigate the effects of carpet fibres and POFA on the performance of the concrete at elevated temperatures. The results showed that both carpet fibres and POFA were associated with a significant enhancement in the fire resistance and residual compressive strength and also eliminating the explosive spalling behaviour of the concrete at elevated temperatures. Furthermore, the role of carpet fibres and POFA is discussed through the microstructural analysis and fibre–matrix interactions as function of heat treatment.

Modeling the thermodynamic behavior and shock response of Ti systems at the atomic scales and the mesoscales

The ‘quasi-coarse-grained dynamics’ (QCGD) method is extended to model the thermodynamic behavior and the shock response of HCP Ti systems at the mesoscales by coarse-graining the atomistic microstructure using representative atoms (R-atoms) and scaled interatomic potentials. To demonstrate the capability of the QCGD method, the melting behavior of a single-crystal slab of HCP Ti and the dynamic failure (spallation) behavior of nanocrystalline systems under shock loading conditions are first investigated using molecular dynamics (MD) simulations using an embedded atom method interatomic potential for Ti. The melting simulation suggests an interplay between the nucleation and propagation of the surface-induced heterogeneous melting and the nucleation and propagation of bulk homogeneous melting of the system. In addition, the spall strengths calculated using MD at strain rates of 1010 s−1 allow the development of improved models for the strain rate dependence of the spall strength determined experimentally at 105 s−1. The QCGD method is observed to be capable of reproducing the MD-predicted kinetics of melting and the shock response and spall failure of nanocrystalline Ti systems using a coarse-grained microstructure comprising of representative atoms (R-atoms). The QCGD simulations demonstrate the ability to model the mesoscale behavior of Ti systems by modeling the shock deformation and failure due to spallation of a 1 µm × 1 µm × 2 µm sized system at strain rates of 108 s−1 to bridge the gap between MD simulations and experiments.

Influence of initial texture on the shock property and spall behavior of magnesium alloy AZ31B

In this work, flyer plate impact experiments are conducted on a commercially available magnesium alloy, AZ31B. Equal channel angular pressing (ECAP) process is an established procedure for fabrication of ultra-refined grain structures in metals and alloys. The microstructure of commercially available AZ31B is refined through ECAP process and two sets of specimens with different initial textures and microstructure are considered here. One set of the specimens is machined from a hot rolled AZ31B Mg alloy plate and the others are processed by ECAP of four pass after samples are cut from the initial material. The Hugoniot elastic limit (HEL) and spall strength are determined by analyzing the measured free surface velocity profiles of the specimens. Scanning electron microscope (SEM) is used to observe the microvoids produced along the spall plane and electron back scatter diffraction (EBSD) is employed to examine the twinning distribution on the cross-section of the recovered specimens cutting along the thickness direction. It is found that elastic precursor decay appears in the as-received (without ECAP process) specimens but it is absent in ECAPed specimens. The HEL of the as-received specimens is higher than that of ECAPed specimens over various impact velocities and showed strain rate sensitivity. The spall strength of both sets of specimens is almost identical and enhanced by increasing the impact velocity. The SEM results showed that in the as-received specimens the spall is dominated by coalescence of microvoids emerged along the grain boundaries in a brittle manner. However, in the ECAPed specimens spall was dominated by coalescence of microvoids characterized by inner dimples formation by the pull out of grains in a ductile mode.