Subsurface Voids Research Articles

Adhesive bond strength enhancing between carbon fiber reinforced polymer and aluminum substrates with different surface morphologies created by three sulfuric acid solutions

Optimized porous aluminum (Al) substrates chemically etched by three electrolytes have been prepared in this study. They are used to place multi-walled carbon nanotubes (CNTs) via resin pre-coating (RPC) technique for stronger adhesive bonding with carbon fiber reinforced polymers (CFRP). Shallower channels (~1–2 μm) were etched by 20 wt% H2SO4 solution, deeper channels (~5 μm) were etched by 20 wt% H2SO4 + 0.5 wt% C2H2O4 (or + 0.5 wt% FeSO4) solution. The former has less sub-surface void defect by directly adhesive bonding, its bonded composite has higher shear strength. Through RPC, the latter promotes effective interface behaviors including increasing contact area, reducing void defect and improving mechanical occlusion. Moreover, they have sufficient pore volumes to embed and form quasi-Z-directional CNTs, leading to around 135% improvements in shear strength. Therefore, Al substrate surface with deeper channels potentially contributes to improving adhesive interface and positioning vertical CNTs via RPC to further reinforce adhesive joint.

Flow pathways in multiple-direction fold hinges: Implications for fractured and karstified carbonate reservoirs

Caves developed in carbonate units have a significant role in fluid flow, but most of these subsurface voids are below seismic resolution. We concentrated our study on four caves to determine the roles of fractures and folds in the development of karst conduits that may form flow pathways in carbonate reservoirs. We performed structural field investigations, petrographic analyses, and geometric characterization using Light Detection and Ranging (LIDAR) for caves in Neoproterozoic carbonates of the Salitre Formation, central part of the São Francisco Craton, Brazil. We found that the conduit shape, usually with an ellipsoidal cross-section, reflects the tectonic features and textural variations. Carbonate layers containing pyrite and low detritic mineral contents are generally karstified and appear to act as favorable flow pathways. Our results indicate that the development of the karst system is related to fracture corridors formed along parallel and orthogonal sets of fold hinges, which provide preferential pathways for fluid flow and contribute to the development of super-K zones. This study provides insights into the prediction of subseismic-scale voids in carbonate reservoirs, with direct application for the hydrocarbon and hydrogeology flow and storage.

Effect of nano-void position on surface integrity in nanomachining of single crystal copper

In this paper, molecular dynamics simulations were performed to study the effect of nano-size voids in different locations relative to the surface on nanomachining characteristics and surface integrity. Using EAM potential, MD simulations were performed with two approaches: material removal and relaxation of the work material, to find a low defect permanent configuration after the rigid tool has traversed the machined area. Results show that high hydrostatic pressure induced by tool edge diffuses atoms into the void during the machining process. Investigation of residual stress for subsurface void workpiece represents that higher compressive stress remains in the surface due to the tendency of void for spring back. Besides, in the nanomachining process, surface atoms are greatly disordered and dislocation loops glide deep into substrate atoms. After relaxation, if void pitting contains no other atoms, the machined surface is reorganised as FCC lattice similar to a pure workpiece.

Detection of Subsurface Void from Radar Images by Three-dimensional Convolutional Neural Network and Finite Difference Time Domain Method

Detection of Subsurface Void from Radar Images by Three-dimensional Convolutional Neural Network and Finite Difference Time Domain Method

Semi real-time detection of subsurface consolidation defects during concrete curing stage

Subsurface consolidation defect is a common issue in concrete pavement construction, and the hidden defects often require costly repairs after project delivery. Therefore, being able to identify this type of defect during construction will enable contractor to conduct quick repairs to avoid costly post-construction rework. In this paper a semi real-time detection approach using infrared thermography is introduced. The developed approach utilizes the hydration heat during curing time to identify the subsurface voids based on the regional temperature contrast. Experimental studies were conducted using artificial void-defect in different sizes and depths. The thermographic analysis is employed to locate the void-defects during the first 12 h of curing time. Two field experiments were conducted to validate the proposed approach. The outcomes show that the defects could be identified and located within certain time windows during the curing process, even under adversary outdoor environmental conditions. In conclusion, infrared thermography can be used as a feasible and reliable non-destructive-detection method to conduct real-time or semi real-time monitoring of subsurface defects during concrete pavement construction. The influential environmental factors, practical considerations, and the future study are also discussed.

Monitoring and Studying Audible Sounds Inside Different Types of Soil and Great Expectations for its Future Applications

A majority of seismological studies are concerned with soil properties in low frequencies (1–10 Hz), but little is known about these properties in the audible sound domain (20–20,000 Hz). This is probably due to the high attenuation of the high frequencies within the soil, resulting in a minimal effect on buildings. For this study, 172 stations were recorded over different types of soils using variable types of P-wave and S-wave geophones to examine the variation of soil properties in the range of audible sound (20–3000 Hz). High resolution 32 bit spectrograms for the sounds recorded within every soil sample were analyzed. Moreover, a model for empty room was built in the subsurface to study changes in sound caused by the existence of large voids or cavities in the subsurface. The sound wave was able to differentiate between rigid, hard soil and softer, weaker soil. While high-strength rocks or soils tend to show sharp sound pitches (300–3000 Hz), weaker soils show lower sound pitches (20–100 Hz). The existence of subsurface voids or cavities tend to make sound pitches more regular, higher and sharper than those in the surrounding soils. This is most probably due to resonance of sound in closed places (e.g. a violin). Soil energy levels and how they change due to the soil’s excitation were studied. Soil research in the field of audible sounds is considered an emerging field with several applications (e.g. geological hazards, water exploration, and oil exploration and so on). There is a need for special high-resolution equipment to be developed for the same. This equipment should be capable of recording wide range of sound frequencies preserved in the soil and directly producing high-resolution spectrograms.

Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part II: NaCl-KCl-MgCl2 ternary system

Molten chlorides are promising heat transfer fluids (HTF) and thermal energy storage (TES) materials for third-generation concentrated solar power (CSP) plants. Despite their low cost and wide operating temperature ranges (400–800 °C), structural materials experience severe corrosion in commercially available chloride salts. The understanding of the corrosion mechanisms is therefore critical for successful plant design to ensure constant power generation over the 30 years of expected lifetime. This work investigates the corrosion behavior of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten NaCl-KCl-MgCl2 (24.5-20.5-55.0 wt%) under isothermal conditions at 700 °C in Ar. For both alloys, the corrosion attack is associated with the selective dissolution of Cr and the removal of Cr-rich carbides from the alloy matrix leaving subsurface voids. The initial “impurity-driven” corrosion mechanism associated with the cathodic reduction of MgOH+ impurities leads to the formation of insoluble MgO on the substrates’ surface. Compositional and microstructural features of the alloys, especially the distribution of Cr-rich carbides as well as the solubility and mobility of carbon within the metallic matrix, are found to significantly affect their corrosion resistance. The experimental observations indicate, that the formation of galvanic pairs and solid-state diffusion mechanisms play a major role regarding the corrosion attack of the alloys. The systematic corrosion experiments conducted in this study indicate a lower corrosiveness of NaCl-KCl-MgCl2 ternary mixture than that of NaCl-KCl binary mixture in static conditions at 700 °C under Ar. This work gives significant insights into corrosion issues that may be expected in third-generation industrial CSP plants.

Forward modelling on GPR responses of subsurface air voids

Along with the development of urbanization, an increasing number of subsurface voids in urban cities can have serious effects on the expected functionality of key infrastructure elements such are roads. Ground penetrating radar is an effective technique for imaging the subsurface and detecting such air voids. The GPR responses of air voids with different horizontal width were quantitively investigated in this study using forward modelling: empirical experiments in both laboratory and site; as well as numerical simulations with FDTD. Two typical road constructions were modelled: reinforced concrete pavements and bituminous pavements, where the majority of underground utility networks are buried underneath. Pulse GPR signals having two centre frequencies: 600 MHz and 200 MHz were simulated. It was observed that the ratio of void size and GPR signal wavelength would result in various patterns in the GPR B-scans: they can be a hyperbola, a cross pattern, a bowl shape pattern, and a plain reverberation pattern. And GPR responses of voids under road structures were distorted by reinforcement. With signatures of air voids appeared in GPR B-scans through validation by lab experiments and ground truthing, confidence of void identification will be enhanced.

Microgravity method in archaeological prospection: methodical comments on selected case studies from crypt and tomb detection

AbstractDetailed and precise measurement of the Earth's gravity field (microgravity method) can be effectively used for the detection and quantification of subsurface voids and/or cavities. There exist a variety of successful applications of the microgravity method in near surface geophysics, namely in geotechnical, environmental and archaeological prospection. Using state‐of‐the‐art ‘microgal’ relative gravity meters, cavities of several metres in each dimension (positioned at a similar depth) can be detected and interpreted. Such objects produce negative anomalies with amplitudes of several tens of microGals (1 microGal = 10−8 m s−2). This contribution is focused on a methodological overview of the most important acquisition and processing steps in archaeological microgravimetry. In the processing of acquired gravimetrical data into a Bouguer anomaly, the so‐called building correction plays an important role, because the gravitational effect of building masses can produce false, usually negative anomalies. Several selected methods for quantitative interpretation are presented, these are based on depth estimation and density modelling. These interpretation methods give satisfactory results in the case of these type of negative anomalies that are caused by subsurface cavities. Microgravimetry can obtain good support from electromagnetic and electrical methods, mainly from ground penetrating radar and electrical resistivity tomography, respectively. Finally, we present successful case‐studies of microgravimetrical detection of crypts in various churches from the Middle Ages and period of Modern History, surveyed during recent decades in Slovakia and Czechia.

Detection of subsurface voids in concrete-filled steel tubular (CFST) structure using percussion approach

Concrete-filled steel tubular (CFST) structures are essential load-bearing components in many civil engineering structures. Subsurface voids between the contacting surface of the concrete and steel in a CFST structure reduce the load-bearing capacity of the CFST structure. This paper presents a novel, non-destructive, percussion-based approach to detect subsurface voids in CFST structures. In our approach, we exploit the contrasting sound produced by the percussion of surfaces with and without subsurface voids. Percussive acoustic signals in non-void and void zones are recorded. By analyzing the power spectrum density (PSD) of the percussion sound, nine features can be extracted. Two specimens (A and B) were fabricated in our experiment. The features of the sound signal extracted from the specimen A are used as the database for training and testing the support vector machine (SVM) model. Then, the trained SVM is applied to specimen B to determine whether or not a void between the concrete core and the outer steel tube exists. The experimental results show that the prediction precision is 94.17%. Therefore, the percussion-based approach is a simple, efficient, and accurate method to detect the void defects.

Corrosion mechanisms of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy in molten chlorides. Part I: NaCl–KCl binary system

With suitable thermophysical properties and commercial viability, molten chlorides are promising candidates to replace nitrate salts as heat transfer fluids (HTF) and thermal energy storage (TES) materials for next-generation concentrated solar power (CSP) plants. Nevertheless, structural materials including steels and nickel-based alloys experience severe corrosion in molten chlorides. The explicit understanding of the corrosion mechanisms is therefore decisive in order to propose viable technical solutions to increase the durability of these materials. In this work, the corrosion behavior of ferritic-martensitic P91 steel and Inconel 600 nickel-based alloy is investigated in molten NaCl–KCl at 700 °C under Ar. The salt mixtures after exposure as well as the corroded samples were simultaneously characterized. The systematic metallographic study after the water-free preparation indicates that electrochemical processes play a major role in the overall corrosion mechanisms. In both P91 steel and Inconel 600, the formation of micro-galvanic pairs between Cr-rich phases (anodic sites) and the alloy matrix (cathode) leads to the selective dissolution of Cr from the alloys resulting in the formation of subsurface voids. The influence of O2 and of various alloying elements on the corrosion kinetics is discussed. Based on the experimental observations, several corrosion mechanisms combining electrochemical reactions, solid-state diffusion and chloridation-oxidation processes have to be taken into account.

Mechanical and optical degradation of flexible optical solar reflectors during simulated low earth orbit thermal cycling

Multilayer thin film systems on flexible polymer substrates are used as flexible optical solar reflectors or thermal insulation of satellites and spacecraft. During one year of operation, a satellite in low earth orbit typically encounters 6000 thermal cycles of ±100 °C. Due to the different coefficients of thermal expansion between the individual layers and the substrate it is important to investigate the thermo-mechanical stability of the multilayers as a function of the cyclic heat load. Scanning electron microscopy and focused ion beam cross-sectioning revealed that Inconel-Ag bilayers on fluorinated ethylene propylene (FEP) substrate severely degrade during thermal cycling of ±150 °C in a gaseous N2 atmosphere. After only 100 cycles through thickness cracks and subsurface voids in the Ag layer form as a result of equi-biaxial thermal stresses caused by the large difference in thermal expansion between film and substrate. Transmission Kikuchi Diffraction (TKD) before and after thermal cycling also revealed grain growth and twin widening in the Ag layer. Cracking and void formation are detrimental to application relevant material properties including corrosion protection (Inconel) and reflectivity (Ag). Reflectance measurements revealed that the amount of reflected energy as well as the reflection mode (specular vs. diffuse) significantly change during the first 100 cycles. Saturation of reflection characteristics was observed after 25 cycles, which correlates to a turning point in the evolution of Ag voids. Results of this study indicate that special focus should be directed towards thermal stress control (Δα) and tailoring of the metal-polymer interface to improve resistance of versatile metal-polymer systems against thermal cycling.

Depth detection of subsurface voids in concrete-filled steel tubular (CFST) structure using percussion and decision tree

Abstract The void defects significantly threaten the integrity of concrete-filled steel tubular (CFST) structure. Along with detecting the presence of subsurface voids, the knowledge of the geometry of voids can provide meaningful information to determine the overall structural health. This study develops a method to determine the depths of subsurface voids by integrating the percussion technique with a machine learning algorithm. A decision tree model is trained to detect different depths of subsurface voids in a CFST specimen. The percussed sounds from areas with and without subsurface voids were analyzed by using the power spectrum density (PSD). The process was repeated 100 times, and the average correction ratio is up to 96.33%. The results showed that the proposed approach has great potential in subsurface void inspection and evaluation for CFST structures.

GPR pattern recognition of shallow subsurface air voids

Countless subsurface voids in urban areas of cities threaten people’s lives and property. A workflow for automatically identifying subsurface voids from ground penetrating radar (GPR) data was developed in this study. The workflow consists of 3 stages: locating voids automatically from C-scans, then verifying voids from corresponding B-scans, and finally making judgements based upon the previous 2 sets of results. This study adopted 2 (Lai et al., 2016) approaches: approach 1 quantified the GPR response of air voids using forward modelling, while approach 2 used workflow prototyping and validation with inverse modelling. Forward simulations indicated that different ratios of void size to GPR signal footprint could result in a variety of patterns in B-scans: they can be hyperbolas, cross patterns, bowl shaped patterns and reverberations. With a database of void patterns of both C-scans and B-scans established, in approach 2 the workflow uses a pyramid pattern recognition method – with pixel value or gradient being used for feature identification – to search automatically for air-filled void responses in GPR data. The workflow was tested using 2 laboratory and field experiments and the results were promising. The constraint values proposed by the 2 experiments were validated with another site experiment. Given the huge workload involved in city-scale subsurface health inspections, a standardized workflow can help improve efficiency and effectiveness of subsurface void identification.

Effect of subsurface voids on the nanoindentation of Fe crystals

Subsurface voids may strongly affect the response of materials to nanoindentation. We explore these effects for a bcc single-crystalline Fe sample using molecular dynamics simulation. Deformation occurs mainly by nucleation and propagation of dislocations. As dislocations impinge into the voids, these suffer a reduction in volume, consistent with mass transfer mechanisms. Our results show that voids act as highly efficient absorbers of dislocations, effectively limiting the extension of the plastic zone. Surprisingly, mechanical properties are marginally affected by the presence of voids in the range of sizes and spatial distributions tested, except for voids a few nanometers below the surface. Deformation twinning is observed as a transient effect in some cases; however, for voids close enough to the indentation area, no twinning was found.

Diffraction stack imaging as a potential tool for detecting underground voids – the case of the ancient copper mines of Timna Valley (Israel)

AbstractDetection of subsurface features using geophysical methods is an important component of archaeological research. Most geophysical methods suffer from loss of resolution with depth and are most successful when employed on hyper‐shallow targets. We present a novel imaging method that utilizes summation of diffracted seismic energy in the depth domain to image the subsurface and detect areas of material contrasts, which is especially useful for detecting subsurface voids. The diffraction imaging (DI) method was validated in a synthetic case and in two archaeological sites where known ancient tunnels exist and then employed in the ancient copper ore district of Timna Valley (southern Israel). The results demonstrate the feasibility of the DI method to detect ancient underground mining galleries that connected between (now blocked) mining shafts, which in turn constitutes a significant contribution to the study of ancient prospection and mining technologies and related social and historical aspects.

Nanoscale conditions for ductile void nucleation in copper: Vacancy condensation and the growth-limited microstructural state

Ductile rupture or tearing usually involves structural degradation from the nucleation and growth of voids and their coalescence into cracks. Although some materials contain preexisting pores, the first step in failure is often the formation of voids. Because this step can govern both the failure strain and the fracture mechanism, it is critical to understand the mechanisms of void nucleation and the enabling microstructural configurations which give rise to nucleation. To understand the role of dislocations during void nucleation, the present study presents ex-situ cross-sectional observations of interrupted deformation experiments revealing incipient, subsurface voids in a copper material containing copper oxide inclusions. The local microstructural state was evaluated using electron backscatter diffraction (EBSD), electron channeling contrast (ECC), transmission electron microscopy (TEM), and transmission kikuchi diffraction (TKD). Surprisingly, before substantial growth and coalescence had occurred, the deformation process had resulted in the nucleation of a high density of nanoscale (≈50 nm) voids in the deeply deformed neck region where strains were on the order of 1.5. Such a proliferation of nucleation sites immediately suggests that the rupture process is limited by void growth, not nucleation. With regard to void growth, analysis of more than 20 microscale voids suggests that dislocation boundaries facilitate the growth process. The present observations call into question prior assumptions on the role of dislocation pile-ups and provide new context for the formulation of revised ductile rupture models. While the focus of this study is on damage accumulation in a highly ductile metal containing small, well-dispersed particles, these results are also applicable to understanding void nucleation in engineering alloys.

The Effect of Material Present in the Void for Thermography Inspection in Concrete

In the previous study by the author, a numerical model to predict the thermal contrasts resulting from subsurface voids (i.e. delaminations) in concrete under a given set of environmental conditions was developed using the finite element method. The model was verified using the experimental test data, and the results indicated that the model could be an effective tool to support the thermography inspection of the concrete. In this present study, the use of the verified model to evaluate the effects of materials present in the void created by the delamination expected to influence the detectability of the subsurface voids. The effect of this parameter on the thermal contrast developed on the surface above a subsurface delamination was assessed under a specific set of environmental conditions. The results indicated that air-filled void produced a significant thermal contrast compared to water-filled void, ice-filled void, and epoxy adhesive-filled void.

Fungal resistance of nanomodifiers and corrosion inhibitor amended fly ash concrete

In this paper, we report a novel concrete composition with enhanced biodeterioration resistance against colonization and fouling by fungus Fusarium oxysporum. The new formulation (CFNI) contains 56 wt% cement, 40 wt% fly ash, 1 wt% nano-TiO2, 1 wt% nano-CaCO3 and 2 wt% sodium nitrite based corrosion inhibitor. The CFNI specimen is found to have minimal reduction in pH, weight loss and dimensional changes after three months of exposure in the aggressive fungal culture. Microscopic investigations confirmed the least biofilm thickness and minimum orange-red fluorescence intensity indicating lesser metabolizing cells on CFNI surface as compared to control concrete (CC). The distinct and compact calcium silicate hydrate (CSH) dominated surface morphology of CFNI specimen indicate the formation of more cementitious products with improved biodeterioration resistance. The nearly uniform phase intensity and faster temperature decay rate, seen in the infrared thermal images, confirm the absence of sub-surface micro-cracks, pores, and voids, and homogeneous thermal properties due to reduced biodeterioration of CFNI. These results suggest that a combination of nanomodifiers and corrosion inhibitor enable improved resistance against acid producing fungi attack on fly ash concrete.

A Potential Technology for Road Sinkhole Assessment: The Rolling Dynamic Deflectometer

Abstract In urban areas, excessive underground construction, aging of underground infrastructures (e.g., water pipe leakage), or both, can create underground cavities that ultimately lead to subsidence and sudden road sinkholes. These road collapses have often been reported in metropolitan areas around the world. A quick and accurate in situ testing method is essential to prevent human life loss and damage to civil infrastructure. This article introduces potential technology for road sinkhole assessment that can continuously assess the road subsurface condition in a nondestructive manner. The Rolling Dynamic Deflectometer (RDD) is a continuous pavement deflection profiling device that evaluates the existing structural condition of pavements. As the RDD moves along the pavement, it dynamically loads the pavement, and multiple rolling sensors simultaneously measure the induced dynamic deflection of the pavement. With a soft subgrade or subsurface voids, the RDD deflection profile clearly shows peak deflections. Three RDD case studies that demonstrate its potential as a road sinkhole assessment tool are presented: (i) delineation of stiffer subgrade conditions, softer subgrade conditions, or both, along a runway, (ii) identification of areas with poor subgrade soils, and (iii) detection of subsurface voids (or anomalies) along with ground-penetrating radar.