Spall Damage Research Articles

Spall damage in single crystal Al with helium bubbles under decaying shock loading via molecular dynamics study

The mechanism and characteristics of spall damage in single crystal Al with randomly distributed helium bubbles under decaying shock loading are studied by molecular dynamics simulations. We find that He bubble plays important role in the dynamic process. The mechanism of spall damage is dominated by He bubble expansion-merging-absorption, followed by void nucleation-growth-coalescence. During spalling process, He bubbles impede the nucleation of surrounding voids, they expand at a higher rate than that for void growth, adjacent He bubbles merge together, and small voids are absorbed by nearby large He bubble. These behaviors finally lead to a very different morphology of the damaged zone from that for the perfect crystal, whose spall damage mechanism is void nucleation-growth-coalescence. Obvious differences in temperature profile are observed between the crystal with He bubbles and the perfect crystal because the compression of surrounding atoms induced by He bubble expansion results in higher local temperature, especially in solid state. In melted state, the contribution of He bubbles becomes comparatively small since more voids are nucleated followed by faster voids growth and coalescence that lead to higher temperature rising. Additionally, He bubble significantly reduces the spall strength of Al in solid state, but this effect becomes negligible when the crystal is melted.

Experimental and numerical study on the behaviour of CFDST columns subjected to close-in blast loading

The dynamic behaviour of concrete-filled double-skin steel tube (CFDST) columns under close-in blast loading was investigated using experimental and numerical approaches in the present study. Field test results on three large scale CFDST column specimens subjected to different blast loading conditions were presented. High-fidelity physics-based numerical models were developed utilizing the Arbitrary-Lagrangian-Eulerian (ALE) formulation coupled with Fluid-Structure Interaction (FSI) algorithm available in the nonlinear dynamic analysis program LS-DYNA. The numerical models were verified with the experiment results and were then used to investigate the dynamic response, damage mechanism and energy absorption capacity of CFDST columns subjected to close-in explosions. The results indicated that the typical failure pattern of CFDST columns under close-in blast loading is dominated by localized denting of the cross-section directly facing the explosion with minor global deformation of the column. The concrete core suffers severe damage while the steel tubes remain almost undamaged except in a limited region in the vicinity of the detonation. The energy absorbed by each part of the CFDST columns demonstrates that the severe plastic damage of concrete core plays a significant role in the energy absorption mechanism of CFDST columns under blast loading from close-in explosion. The primary function of the steel tubes is to provide confinement to the concrete core therefore prevent concrete spall damage. In addition, the influence of explosive charge setups and level of axial loads on the structural response and damage was assessed and discussed.

Atomistic simulations and modeling analysis on the spall damage in lead induced by decaying shock

This study reveals some dynamic properties of the spall damage under decaying shock loadings based on atomistic simulations of lead, where a series of shock peak stresses and decaying profiles are introduced. Firstly, different failure modes from the typical spallation to cavitation fragmentation are analyzed with the wave system and relevant statistics. The velocity and mass distributions in the cavitation region approximately satisfy a linear function, where the velocity of the outermost spall layer can be estimated from the surface velocity peak. The spall strengths derived from surface and from bulk are compared. The former is distinctly higher than the latter and too sensitive to the high strain rate, while the latter reduces linearly with the shock breakout stress until the melting slows down this variation law. Furthermore, we propose approximate descriptions of the spall layer thickness and the damage depth under triangular waves, using the strength from bulk and propagation rules of simple waves. Besides, the characteristic size of the voids or fragments at the extreme strain rate is found to approximately satisfy the energy-based fragmentation theory.

Simulation of Spall Damage of Uranium at Various Temperatures in a Region of Polymorphic Phase Transitions

Simulation of Spall Damage of Uranium at Various Temperatures in a Region of Polymorphic Phase Transitions

Image Processing-Based Recognition of Wall Defects Using Machine Learning Approaches and Steerable Filters.

Detection of defects including cracks and spalls on wall surface in high-rise buildings is a crucial task of buildings' maintenance. If left undetected and untreated, these defects can significantly affect the structural integrity and the aesthetic aspect of buildings. Timely and cost-effective methods of building condition survey are of practicing need for the building owners and maintenance agencies to replace the time- and labor-consuming approach of manual survey. This study constructs an image processing approach for periodically evaluating the condition of wall structures. Image processing algorithms of steerable filters and projection integrals are employed to extract useful features from digital images. The newly developed model relies on the Support vector machine and least squares support vector machine to generalize the classification boundaries that categorize conditions of wall into five labels: longitudinal crack, transverse crack, diagonal crack, spall damage, and intact wall. A data set consisting of 500 image samples has been collected to train and test the machine learning based classifiers. Experimental results point out that the proposed model that combines the image processing and machine learning algorithms can achieve a good classification performance with a classification accuracy rate = 85.33%. Therefore, the newly developed method can be a promising alternative to assist maintenance agencies in periodic building surveys.

Blast Vulnerability Assessment of Road Tunnels with Reinforced Concrete Liners

Tunnels are a critical component of our transportation infrastructure, and unexpected damage to a tunnel can significantly and adversely impact the functionality of a transportation network. Tunnel systems are vulnerable to potential threats of intentional and accidental blast events because of their relatively unrestricted public access. These events can lead to spalling and breach of the tunnel liner which, depending on the surrounding media, can result in local damage and progressive collapse of the tunnel. Current approaches for evaluating blast-induced damage to a tunnel liner either require significant computational effort or oversimplification such that accurate spatial distributions of damage cannot be obtained. This study presents an effective approach to predict and map the damage to a reinforced concrete liner of a roadway tunnel from various explosive threat sizes and tunnel geometries. A literature review of existing studies is conducted, and potential scenarios of blast events are examined with varying charge position and size. Rectangular, horseshoe, and circular tunnel geometries, each with the same traffic throughput, are evaluated. An efficient analytical approach to determine the spatial distribution of blast-induced spall and breach damage is presented and shows good agreement with numerical models analyzed in LS-DYNA. The proposed approach is then used to examine the relationship between increasing blast hazard intensity and the extent of spall and breach damage. Inflection points in this relationship can be used to identify hazard levels at which a progressive collapse evaluation would be warranted.

Quantifying heterogeneous deformation in grain boundary regions on shock loaded tantalum using spherical and sharp tip nanoindentation

Grain boundaries play an important role in the overall mechanical performance of metals and alloys; however, isolating the effects of individual grain boundaries remains rather challenging experimentally. In this work, wire-feed, electron beam additively manufactured tantalum is studied under shock loading conditions generating incipient spall damage. Three grain boundaries aligned parallel to the shock direction were isolated inside a single sample. Postmortem metallography showed voids preferentially appeared on two of the three grain boundaries which had high misorientation angles > 30° compared to the third grain boundary with a relatively lower misorientation angle < 10°. Nanoindentation in grain boundary regions for both pyramidal and spherical tips showed characteristically different strain hardening trends for each grain boundary. Significant local strain hardening is present on both sides of one high angle grain boundary, while strain softening is captured at the other high angle grain boundary. A negligible trend of either hardening or softening is shown in the vicinity of the single low angle grain boundary. This leads to the conclusion that the trend of strain hardening/softening in grain boundary regions is not monotonically correlated to the susceptibility of specific grain boundaries to an increased propensity for void damage.

Effect of peak stress and tensile strain-rate on spall in tantalum

Materials subjected to dynamic environments experience a complex and wide range of stress, strain, and strain-rate conditions. To have confidence in material models, an accurate, predictive capability is required. In this study, we present a series of flyer-plate impact tests on well characterized, high purity tantalum. The shock-waves generated at impact release from the free-surfaces, reflect, and interact to produce incipient spall fracture. By varying the flyer-plate material and impact velocity, both the peak stress and the strain-rate in the samples were controlled independently. Velocimetry was used on the rear free-surface of the samples to measure the shock-response and the spall strength. While this measurement provided the same spall strength for all cases, at approximately 5.1 GPa, when the samples were sectioned during post-mortem, the quantity and distribution of internal damage was markedly different. For the high-strain rate cases, voids remained small and isolated, whereas in the lower strain-rate experiments, the spall damage was far more localized, with a well-defined continuous spall plane. With the use of hydrocode simulations, this was discovered to result from how the different release rates affect the interaction volume inside the sample. These results highlight the importance of careful sample recovery, and the risks of relying solely on free-surface velocity measurements.

The void nucleation mechanism within lead phase during spallation of leaded brass

The incipient spall behaviours of Cu-34%Zn-3%Pb leaded brass samples with annealed and cryogenic-treated conditions were loaded using one-stage light gas gun experiments. The effect of Pb-phase on dynamic damage nucleation in leaded brass specimens was investigated by means of optical microscopy, scanning electron microscopy and x-ray computer tomography. It was found that the voids of incipient spall were mainly nucleated in the interior of the lead (no tensile stress would be produced within lead according to the impact theory) instead of nucleated at the phase interface as expected by quasi-static damage fracture theory. A nucleation model is proposed in the present work that is the asymmetry high compression zones in the centre of the lead-phase were formed by the rarefaction wave convergence effects of matrix/quasi-spherical lead interface, which caused adiabatic temperature rise that exceeded melting point of lead due to severe plastic deformation, finally led to local melting and void nucleation. In addition, the spall strength and damage rate increased with the increase in the Pb-phase number.

Spall damage repair using 3D printing technology

Abstract Concrete road surfaces may crack due to shrinkage from drying. Often, cracks are induced at predetermined locations to solve this problem. However, the impact loads from vehicles are concentrated on these pre-induced transverse joints, often resulting in palm-sized spall damage. Repairing this damage with cast-in-place concrete leads to indirect losses from road blockages that occur during curing. This research presents a new method of repairing spall damage that minimizes indirect loss. A three-dimensional concrete patch is prepared in advance and placed over the damaged area. When using cast-in-place concrete to repair spall damage, road use must be restricted for a minimum of seven days. The proposed method reduces road closure to 2 h. According to the US Department of Transportation guidelines, the proposed method is estimated to reduce indirect losses from road repair from USD $140,000 to USD $1700. A three-dimensional concrete patch attached to a damaged surface can withstand a shear load of up to 15.7 MPa. The proposed method can withstand at least 91% of this load. Moreover, given the friction between the patch and the damaged surface, the structural stability of a concrete patch in response to shear loads may be even greater than 91%. This study presents the proposed methodology and discusses its economic value and structural practicality.

Deformation and spallation of shock-loaded graphene: Effects of orientation and grain boundary

We perform molecular dynamic simulations to investigate the effects of orientation and grain boundary (GB) on shock response of graphene, including shock-induced slip and spall. For compression under different loading orientations, the critical resolved shear stress for slip is constant along a given growth direction (armchair or zigzag) in single-crystal graphene, and spall initiates in elastically deformed graphene via tension-induced debonding. For bicrystal graphene, the locations of compression-induced slip and initial spall are influenced by preexisting stress field at GB and the interactions between GB and shock waves, respectively. Both of these two factors are related to GB structure, including the density of pentagon-heptagon pairs at GB. However, bicrystals show negligible anisotropy in shock-compression response. For polycrystalline graphene at high shock strength, slips initiate at GBs and triple junctions under shock compression, develop along GBs or toward grain interior, and serve as nucleation sites for spall damage.

Experimental and numerical study on damage mechanism of CFDST bridge columns subjected to contact explosion

The blast performances of concrete-filled double-skin steel tube (CFDST) columns subjected to contact explosion were investigated experimentally and numerically in the present study. Field tests were carried out on three circular CFDST column specimens under contact explosion, and different damage modes were observed from the tests. Arbitrary-Lagrangian-Eulerian (ALE) formulations coupled with Fluid-Structure Interaction (FSI) algorithms that are available in the explicit nonlinear dynamic analysis program LS-DYNA were utilized to develop the numerical models. The feasibility and accuracy of the numerical models were verified against the experimental results. After that, the numerical models were used to further investigate the dynamic response, damage evolution, and energy absorption mechanism of CFDST columns subjected to contact explosion. The results indicate that CFDST columns under contact explosion are likely to suffer a localized damage while exhibits almost no global responses. The localized damage concentrates in the vicinity of the detonation zone, including cratering around the detonation point, fracture or rupture failure of the outer steel tube, and breaching failure in the front sandwich wall. The core concrete failure plays a dominant role in the energy absorbing mechanism of CFDST columns owing to the confinement effect provided by the steel tubes. In addition, the presence of the outer steel tube can effectively prevent the concrete spall damage thus mitigating the potential damage to personnel and equipment caused by ejected fragments.

Ultrasonic Detection of Spall Damage Distribution Subjected to Plate Impact Test with Different Thickness

Plate impact test on medium carbon steel were carried out to the target plate by impacting the flyer plate with one-third and two-thirds thickness of the target plate. The spall damage within the target plate was evaluated non-destructively with a low frequency scanning acoustic microscope as well as ultrasonic velocity and backscattering intensity. We observed the spall damage distribution by the B-and Cscan images. The distribution of spall damage through the plate thickness depends on the tensile stress area within the target plate. The difference of spall damage distribution was investigated by the plate impact test by flyer plate with different thickness. In the plate impact test by the flyer plate with 1/3 target plate thickness, the spall damage was generated by tensile stress area which superposed in the back surface side. On the other hand, in the case of 2/3 target plate thickness, the spall damage was detected at the impact surface side. By generating the spall damage in the different position through the target thickness, it will be possible to evaluate the accumulation of spall damage by reflection and transmission of the stress wave at the internal damage subjected to repeated impact.

Investigating the Dynamic Tensile Response of Lean Duplex Stainless Steel and the Effects of Radial Waves Using the Recovered Plat-Impact Experiment

Lean Duplex Stainless Steel 2404 (LDX2404) is the newest commercial type of duplex stainless steels, recently introduced by Outokumpu®. This paper investigates the dynamic tensile failure (spall strength) of this new alloy under different peak stresses (4.5 GPa to 8.2 GPa) via the recovered plate-impact experiment using a single stage gas gun. The microstructural investigation for the as-received and soft recovered post-shock material conditions were accomplished using Electron Backscatter Diffraction (EBSD) and optical microscopy. The effect of the radial release waves and the effectiveness of using momentum trap rings on the spall strength and the spall damage during the impact experiment were studied. Results indicate that LDX2404 experienced spall damage for the entire range of tested peak stresses. The spall strength was not affected by the implementation of momentum trapping rings whereas; the damage content was highly influenced. The spall damage of LDX 2404 exists parallel to the phase boundary interfaces, which are normal to the impact direction. Grain orientation, based on Taylor factor values, was not controlling the voids nucleation and voids growth.

Deformation and Spallation of Explosive Welded Steels under Gas Gun Shock Loading**Supported by the National Basic Research Program of China under Grant No 2014CB845904, and the National Natural Science Foundation of China under Grant Nos 11627901, 11372113 and 11672110.

We investigate deformation and spallation of explosive welded bi-steel plates under gas gun shock loading. Free surface histories are measured to obtain the Hugoniot elastic limit and spall strengths at different impact velocities. Pre- and post-shock microstructures are characterized with optical metallography, scanning electron microscopy, and electron backscatter diffraction. In addition, the Vickers hardness test is conducted. Explosive welding can result in a wavy steel/steel interface, an ultrafine grain region centered at the interface, and a neighboring high deformation region, accompanied by a hardness with the highest value at the interface. Additional shock compression induces a further increase in hardness, and shock-induced deformation occurs in the form of twinning and dislocation slip and depends on the local substructure. Spall damage nucleates and propagates along the ultrafine grain region, due to the initial cracks or weak interface bonding. Spall strengths of bimetal plates can be higher than its constituents. Plate impact offers a promising method for improving explosive welding.

Stress and Strain Rate Effects on Incipient Spall in Tantalum

Spall fracture is a high-rate tensile damage phenomenon associated with impulsive and shock-load events. Typically, the material undergoes a sequence of compression followed by release into high rate (on the order of 104 s-1 and up) tension, causing voids to nucleate and grow, which can then coalesce into a crack and the material fails. We present a series of experiments on high purity, well characterized tantalum samples subjected to shock-loading via gas-gun plate impact. Through careful selection of the flyer-plate velocity and material we have independent control over the peak compressive stress and the tensile strain rate in the sample. At all times, the spall damage remains incipient, i.e. in the early stages of void formation and the material does not fully fracture. Velocimetry was used on the rear of the sample to record the wave-profiles and determine spall strength. Soft recovery and sectioning of the samples allowed the internal damage to be observed, quantifying the damage amount, distribution, and relationship to microstructural features with both optical and electron based microscopy.

Strengthening, rehabilitation and widening of the existing arch bridge on national route 7 over the Olifants River, South Africa

This paper describes the rehabilitation of the existing bridge over the Olifants River. The bridge is on National Route 7 near Klawer and dates back to 1957. The defining feature of this elegant bridge is a concrete arch spanning 92.7 m. The existing bridge displayed extensive cracking, failed expansion joints and spall damage to beam soffits. Some columns showed vertical cracks while some foundations presented with horizontal cracks. The arch presented with longitudinal cracks on both the upper and lower surfaces and a previous attempt to fill the cracks proved unsuccessful, as the cracks continued to widen. This paper presents the investigations undertaken to determine the cause for the observed defects as well as the methods employed to rehabilitate the bridge. These measures include strengthening with carbon fibre, coatings, patch repair, replacement of reinforcement, replacement of expansion joints and replacement of parapets. The work commenced early 2016 and was substantially completed mid-2018.

Numerical analysis of damage kinetics in aluminium under axisymmetric loading

Based on the analysis of the results of two R-Damage experiments, this paper provides a variant of a relation to simulate the spall damage recollection under the effect of weak compression waves. This relation closes the kinetic fracture model, considering both growth and recollection of damage. Using this model makes it possible to simulate adequately (with good quantitative agreement) the sequence of the processes “shock-wave compression – spall fracture – convergence of spall layer – recollection of damaged medium”, including the dynamics of the compacting wave arrival to the internal surface of the hollow cylindrical targets since it passed through the damaged media.

Deformation and damage of sintered low-porosity aluminum under planar impact: microstructures and mechanisms

Plate impact experiments are conducted to study compaction and spallation of 5% porosity aluminum. Free surface velocity histories, the Hugoniot elastic limit (HEL), and spall strengths are obtained at different peak stresses and pulse durations. Scanning electron microscopy, electron backscatter diffraction, and X-ray computed tomography are used to characterize 2D and 3D microstructures. 3D void topology analyses yield rich information on size distribution, shape, orientation, and connectivity of voids. HEL decreases/increases with sample thickness/impact velocity and approaches saturation. Its tensile strength increases with increasing peak stress and shock-induced densification. With the enhanced compaction under increasing impact velocities, spall damage modes change from growth of original voids to inter-particle crack propagation and to “random” nucleation of new voids. Such a change in damage mechanism also gives rise to a distinct decrease in damage extent at high impact velocities. Compaction induces strain localizations around the original voids, while subsequent tension results in grain refinement, and shear deformation zones between staggered cracks.

Spall damage of a Ta particle-reinforced metallic glass matrix composite under high strain rate loading

We investigate deformation and damage of a Zr-based bulk metallic glass (BMG) and its Ta particle-reinforced composite (MGMC) under impact loading, as well as quasi-static tension for comparison. Yield strength, spall strength, and damage accumulation rate are obtained from free-surface velocity histories, and MGMC appears to be more damage-resistant. Scanning electron microscopy, electron back scattering diffraction and x-ray computed tomography, are utilized for characterizing microstructures, which show features consistent with macroscopic measurements. Different damage and fracture modes are observed for BMG and MGMC. Multiple well-defined spall planes are observed in BMG, while isolated and scattered cracking around reinforced particles dominates fracture of MGMC. Particle–matrix interface serves as the source and barrier to crack nucleation and propagation under both quasi-static and impact loading. Deformation twinning and grain refinement play a key role in plastic deformation during shock loading but not in quasi-static loading. In addition, 3D cup-cone structures are resolved in BMG, but not in MGMC due to its heterogeneous stress field.