Fracture Traces Research Articles

Grouting Process Simulation Based on 3D Fracture Network Considering Fluid–Structure Interaction

Grouting has always been the main engineering measure of ground improvement and foundation remediation of hydraulic structures. Due to complex geological conditions and the interactions between the grout and the fractured rock mass, which poses a serious challenge to the grouting diffusion mechanism analysis, fracture grouting has been a research hotspot for a long time. In order to throw light on the grout diffusion process in the fractured rock mass and the influence of grout on the fracture network, and to achieve more realistic grouting numerical simulation, in this paper a grouting process simulation approach considering fluid–structure interaction is developed based on the 3D fractured network model. Firstly, the relationship between fracture apertures and trace lengths is used to obtain a more realistic value of fracture aperture; then a more reliable model is established; subsequently, based on the 3D fracture network model, different numerical models are established to calculate fluid dynamics (grout) and structure deformation (fractured rock mass), and the results are exchanged at the fluid–structure interface to realize the grouting process simulation using two-way fluid-structure interaction method. Finally, the approach is applied to analyze the grouting performance of a hydropower station X, and the results show that the grouting simulation considering fluid–structure interaction are more realistic and can simultaneously reveal the diffusion of grout and the deformation of fracture, which indicates that it is necessary to consider the effect of fluid–structure interaction in grouting simulation. The results can provide more valuable information for grouting construction.

Tracking of geological structures and detection of hydrothermal intrusion by geo-electrical methods in the highlands of Bolivia

Oruro city in the Bolivian highlands depends solely on groundwater to supply domestic consumption and irrigation. The top porous aquifer currently exploited is not fully understood in aspects like geometry, hydrogeological properties and interaction with other aquifers in the region. Recent studies detected traces of fractures in the bedrock beneath the porous aquifer; these geological structures seem to be part of a fractured aquifer in contact with thermal sources. The present study aims to fill the gap between those recently detected fractures and the well-mapped fault system to the east of the study area and identify hydrothermal flows by using geo-electrical methods like Electrical Resistivity Tomography and Transient Electromagnetic soundings. Thirteen tomographic lines, placed transversely to the direction of three main faults, were meant to identify prolongations of those structures by tracking distinctive low resistivity in sectors where saline water saturates the subsoil. This type of water is also present in some hot springs near Capachos, where hydrothermal flows discharge under artesian conditions. Two of the investigated faults seem extending to the northwest, in agreement with the expected linkage towards the recently detected fractures. These two faults appear to reach a volcanic formation since the hydrothermal flows, going mainly upwards, align with their strikes. The remaining fault seems not to be connected to any hydrothermal source. The study presents new information, data and interpretations intending to improve the knowledge about the geological structures in a sensitive part of the local aquifer system.

EFFECT OF FRACTAL FRACTURES ON PERMEABILITY IN THREE-DIMENSIONAL DIGITAL ROCKS

The fracture has great impact on the flow behavior in fractured reservoirs. Fracture traces are usually self-similar and scale-independent, which makes the fractal theory become a powerful tool to characterize fracture. To obtain three-dimensional (3D) digital rocks reflecting the properties of fractured reservoirs, we first generate discrete fracture networks by stochastic modeling based on the fractal theory. These fracture networks are then added to the existing digital rocks of rock matrixes. We combine two low-permeable cores as rock matrixes with a group of discrete fracture networks with fractal characteristics. Various types of fractured digital rocks are obtained by adjusting different fracture parameters. Pore network models are extracted from the 3D fractured digital rock. Then the permeability is predicted by Darcy law to investigate the impacts of fracture properties to the absolute permeability. The permeability of fractured rock is subject to exponential increases with fracture aperture. The relationship between the permeability and the fractal dimension of fracture centers is exponential, as well as the relationship between permeability and the fractal dimension of fracture lengths.

Experimental analysis of ILSS of glass fibre reinforced thermoplastic and thermoset textile composites enhanced with multiwalled carbon nanotubes

In this study experimental investigation of interlaminar shear strength of glass fibre reinforced thermoplastic polyurethane (TPU) and epoxy based thermosets composites enhanced with multi walled carbon nanotubes (MWCNTs) is presented, and comparison is made between thermoplastic and thermosets composites. Suspension of MWCNTs in TPU and epoxy matrix was prepared using magnetic stirring and sonication technique. Both thermoplastic reinforced glass fibre and thermosets reinforced glass fibre composites were manufactured using hand layup technique. Carbon nanotubes were added in the concentrations of 0.1 %weight, 0.3 %weight and 0.5 %weight in both types of composites. Results showed that as the concentration of CNTs increases, the ILSS of the nanocomposites was also improved. With an addition of 0.5 % weight CNTs, there was improvement of 24.37 % in ILSS in epoxy based composites and 10.05 % enhancement in thermoplastic polyurethane reinforced glass fibre composites. The average ILSS obtained for thermoplastic polyurethane composites was less than that of epoxy composites. The TPU based composites also demonstrated inelastic deformations without any trace of brittle fracture. The pristine epoxy based composites on the other hand did show inelastic deformations followed by brittle fracture. Higher concentrations of MWCNTs led to an absence of brittle fracture during the tests, owing to the crack bridging effect of the CNTs.

A Novel Conceptual Approach to Objectively Determine JRC Using Fractal Dimension and Asperity Distribution of Mapped Fracture Traces

The understanding of fractures in hard rock is important for topics such as geomechanics, rock mechanics and groundwater flow and solute transport. One key aspect is the roughness of the fracture, often described as the joint roughness coefficient, JRC. JRC is often subjectively interpreted by one geologist comparing a fracture trace with different type traces. It has been shown that several geologists are needed to get reliable interpretations of JRC. There are numerous attempts in the literature to develop objective methods to estimate JRC from digital traces. Some methods are not applicable to fractures, which give arbitrary results while other methods are sensitive to the resolution of the digitalisation and hence need a new relationship for each resolution. Another way of describing the roughness is by the two parameters fractal dimension and magnitude distribution of the asperities. These parameters can be objectively inferred using algorithms and act as input for a model to estimate JRC. Using several evaluation methods, the uncertainty can be decreased and, hence, more robust results achieved. A multilinear model is developed, JRC = − 4.3 + 54.6σδh(1mm) + 4.3H, that estimates JRC, of the classic ten type curves by Barton and Choubey, with standard deviation ± 1 unit. Despite the simplicity of the model it explains 96.5% of the variance in JRC. The developed model is benchmarked against an ensemble of geologists, using nine synthetic fracture traces. The median difference of JRC is 0.2 units and the model shows 40% smaller spread compared to the geologists.

Impact of scaling break and fracture orientation on effective permeability of fractal fracture network

Fractures and fracture networks determine the permeability of many natural rocks. In this study, new solutions are developed to calculate the effective permeability of fractal fracture network with non-universal scaling of fracture aperture and trace length. One component of the permeability tensor is derived by assuming that the hydraulic gradient is perpendicular to the trace direction. A new solution is also derived to examine the reduction in the effective permeability of fracture network due to the influence of random fracture orientation. The fracture lengths in the network are assumed to follow the fractal scaling law. The orientation dispersion of factures in the fracture network varies according to the Fisher distribution. The effective permeability is derived by treating the fractal fracture network as a porous medium and using Darcy’s law. Results demonstrate that the effective permeability decreases with the scaling exponent before the break when the scaling break occurs early. However, the effective permeability increases with the scaling exponent before the break if the scaling break occurs late. The effective permeability increases with the scaling exponent after the break. The extent of increase varies with the location of the scaling break. If the scaling break occurs early, the increase in the effective permeability is more significant. The effective permeability of fractal fracture network increases with the location of scaling break. The reduction in the effective permeability due to fracture orientation dispersion varies from 2/3 to 0 depending on the degree of orientation dispersion as quantified by the precision parameter.

Comprehensive apparent permeability model coupled shale gas transfer mechanisms in natural fractures and matrix

This paper presents a method to quantitatively characterize natural Fractures Intensity (FI) and proposes a Comprehensive Permeability Model (CPM) coupling gas transfer mechanisms in natural fractures and matrix for shale gas reservoirs. Firstly, the Scanning Electron Microscope (SEM) images of natural shale cores are used to quantitatively characterize FI, such as the aperture, length, spacing, general fracture distribution described by Fisher orientation dispersion value (K), and the Angle between the mean pole of Fractures and the Scan-line (AFS). Secondly, the corresponding relationship between the AFS and K, which can be used to evaluate the length of fracture traces per unit area of trace plane and the area of fractures per unit volume of rock, is built based on the numerical simulation results. Then, we can obtain the natural fracture distribution state and quantitatively distinguish the natural fracture volume from the matrix. Finally, the CPM is proposed by combining the FI and matrix pore parameters based on the series-parallel circuit theory. The CPM is not only verified by the numerical simulation in which the Boundary Element Method (BEM) is adopted to simulate the fractures’ influence, but also in good agreement with the lab measurements of shale samples from the Cambrian Niutitang formation in South Sichuan Basin, China. The FI evaluation method and the CPM proposed in this paper provide a new approach to quantitatively understand the natural fracture distribution and the contribution to the shale reservoir percolation capability. In turn, this will enable engineers better understand the shale gas transport mechanisms and develop the simulation in a more practical way.

Finish form grinding of thermally sprayed nano-structured coatings

ABSTRACTThermally sprayed Molybdenum coatings have excellent high temperature strength and wear resistance. The as-sprayed components (surface roughness 5–15 µm) need surface finishing before operation. Because of high hardness and typical coating architectures, grinding is considered as the best surface finishing operation for thermally sprayed coatings. In the present work, nano-structured coatings of tin-bronze and molybdenum are deposited and characterised. Diamond reinforcement is added during ball milling to improve mechanical properties. Both conventional and ball-milled feedstock of bronze was deposited using a HVOF spraying facility. Molybdenum coatings were deposited using an Air-Plasma-Spraying facility. Coatings were ground in precision grinding facility using super-abrasive grinding wheels in plunge grinding mode. Both tangential and normal grinding forces increased in case of nano-structure coatings. Scanning electron microscopy reveals traces of ductile chip formation mode in grinding of conventional coatings. On the contrary, the nano-structured coatings revealed traces of micro-crack formation, brittle fracture and grain spallation upon grinding. An additional compressive residual stress was added to intrinsic stress level of the as-coated samples upon grinding. An increase in residual stress upon grinding is higher in case of nano-structured coatings.

A case of bone fracture with callus on the right femur of a chicken (Gallus gallus domesticus, L. 1758) from the ancient site of Dharih, Jordan

Archaeozoology provides bones, which quite regularly present traces of fractures. These fractures are more or less at an advanced level of healing and bear witness to traumas or pathologies. These cases of palaeopathology are not always the subject of publications, which further restricts our knowledge about them. This short note allows the scientific community to be aware of an original case from an archaeological context in Jordan of a fracture on a hen’s femur, consolidated by a callus and with displacement of the distal ends. Beyond the "anecdotal" aspect, and without imagining the circumstances in which the fracture occurred, the animal survived.

Geological Analysis of Martian Rover‐Derived Digital Outcrop Models Using the 3‐D Visualization Tool, Planetary Robotics 3‐D Viewer—PRo3D

AbstractPanoramic camera systems on robots exploring the surface of Mars are used to collect images of terrain and rock outcrops which they encounter along their traverse. Image mosaics from these cameras are essential in mapping the surface geology and selecting locations for analysis by other instruments on the rover's payload. 2‐D images do not truly portray the depth of field of features within an image, nor their 3‐D geometry. This paper describes a new 3‐D visualization software tool for geological analysis of Martian rover‐derived Digital Outcrop Models created using photogrammetric processing of stereo‐images using the Planetary Robotics Vision Processing tool developed for 3‐D vision processing of ExoMars PanCam and Mars 2020 Mastcam‐Z data. Digital Outcrop Models are rendered in real time in the Planetary Robotics 3‐D Viewer PRo3D, allowing scientists to roam outcrops as in a terrestrial field campaign. Digitization of point, line, and polyline features is used for measuring the physical dimensions of geological features and communicating interpretations. Dip and strike of bedding and fractures is measured by digitizing a polyline along the contact or fracture trace, through which a best fit plane is plotted. The attitude of this plane is calculated in the software. Here we apply these tools to analysis of sedimentary rock outcrops and quantification of the geometry of fracture systems encountered by the science teams of NASA's Mars Exploration Rover Opportunity and Mars Science Laboratory rover Curiosity. We show the benefits PRo3D allows for visualization and collection of geological interpretations and analyses from rover‐derived stereo‐images.

Study and Application of Structure Failure Simulation of Fracture Tracing by Manifold Method

By improving the simulation method of contact and contact forces, the function of tracing the propagation of Multi-cracks, the Numerical Manifold Method proposed by Genhua- Shi is developed which can simulate the discontinuity of block system, the tensile or shear failure, and the crack propagation. The least-square method to calculate SIF is presented in this paper for broken cracks. Also the calculation method of contact stiffness matrix is modified. For edge- to-edge contact, correct distribution of contact stress is guaranteed, namely the contact stiffness and contact length make direct proportion. The structure failure problems inducing by crack propagation in different engineering domains are simulated. It shows that the method put forward and the program developed in this paper can effectively simulate the structure failure and can give scientific suggestion for engineering supporting.

Graph theory and the analysis of fracture networks

Two-dimensional exposures of fracture networks can be represented as large planar graphs that comprise a series of branches (B) representing the fracture traces and nodes (N) representing their terminations and linkages. The nodes and branches may link to form connected components (K), which may contain fracture-bounded regions (R) or blocks. The proportions of node types provide a basis for characterizing the topology of the network. The average degree <d> relates the number of branches (|B|) and nodes (|N|) and Euler's formula establishes a link between all four elements of the graph with |N| - |B| + |R| - |K| = 0.Treating a set of fractures as a graph returns the focus of description to the underlying relationships between the fractures and, hence, to the network rather that its constitutive elements. Graph theory provides a wide range of applicable theorems and well-tested algorithms that can be used in the analysis of fault and fracture systems. We discuss a range of applications to two-dimensional fracture and fault networks, and briefly discuss application to three-dimensions.

Characterizing Fracture Geometry from Borehole Images

The use of borehole image logs in natural-fracture evaluation has traditionally focused on mapping the strikes and dips of the individual fractures that intersect the borehole, estimating their apertures, and from these computing fracture attributes such as fracture density. These fracture attributes are commonly used to build three-dimensional fracture-network models. However, these attributes do not directly constrain an important aspect of these models—fracture geometry—specifically, the size, shape, and rotation of three-dimensional fractures within the model. The published formulas of Ozkaya estimate the radius of circular fractures from the statistics of fracture intersections observed in a borehole image. These formulas are shown to be approximate, only applying to circular fractures with shallow relative dip, and are replaced with exact formulas for elliptical fractures that are valid over the entire range of relative dip. A new statistic is defined, the measured distribution of relative azimuths for fracture-trace midpoints, that is sufficient for estimating both the elongation ratio and the rotation angle of elliptically shaped fractures. As estimation accuracy for this fracture geometry is dependent on the number of observed fracture traces, practical methodologies are provided for computing the uncertainty of these estimates. Finally, since natural fracture size is typically not constant, but follows a distribution such as a power law, solutions are provided for using these same borehole image statistics to estimate the fracture-size distribution. Numerical examples with exponential and Pareto distributions demonstrate that a single parameter of a parametric size distribution can be estimated along with its uncertainty.

Estimation of the REV Size and Equivalent Permeability Coefficient of Fractured Rock Masses with an Emphasis on Comparing the Radial and Unidirectional Flow Configurations

A method to estimate the representative elementary volume (REV) size for the permeability and equivalent permeability coefficient of rock mass with a radial flow configuration was developed. The estimations of the REV size and equivalent permeability for the rock mass around an underground oil storage facility using a radial flow configuration were compared with those using a unidirectional flow configuration. The REV sizes estimated using the unidirectional flow configuration are much higher than those estimated using the radial flow configuration. The equivalent permeability coefficient estimated using the radial flow configuration is unique, while those estimated using the unidirectional flow configuration depend on the boundary conditions and flow directions. The influences of the fracture trace length, spacing and gap on the REV size and equivalent permeability coefficient were investigated. The REV size for the permeability of fractured rock mass increases with increasing the mean trace length and fracture spacing. The influence of the fracture gap length on the REV size is insignificant. The equivalent permeability coefficient decreases with the fracture spacing, while the influences of the fracture trace length and gap length are not determinate. The applicability of the proposed method to the prediction of groundwater inflow into rock caverns was verified using the measured groundwater inflow into the facility. The permeability coefficient estimated using the radial flow configuration is more similar to the representative equivalent permeability coefficient than those estimated with different boundary conditions using the unidirectional flow configuration.

High-resolution stratigraphic characterization of natural fracture attributes in the Woodford Shale, Arbuckle Wilderness and US-77D Outcrops, Murray County, Oklahoma

Natural fracture aperture-size, spacing, and stratigraphic variation in fracture density are factors determining the fluid-flow capacity of low-permeability formations. In this study, several facies were identified in a Woodford Shale complete section. The section was divided into four broad stratigraphic zones based on interbedding of similar facies. Average thicknesses and percentages of brittle and ductile beds in each stratigraphic foot were recorded. Also, five fracture sets were identified. These sets were split into two groups based on their trace exposures. Fracture linear intensity (number of fractures normalized to the scanline length [[Formula: see text]]) values were quantified for brittle and ductile beds. Individual fracture intensity-bed thickness linear equations were derived. These equations, along with the average bed thickness and percentage of brittle and ductile lithologies in each stratigraphic foot, were used to construct a fracture areal density (number of fracture traces normalized to the trace exposure area [[Formula: see text]]) profile. Finally, the fracture opening-displacement size variations, clustering tendencies, and fracture saturation were quantified. Fracture intensity-bed thickness equations predict approximately 1.5–3 times more fractures in the brittle beds compared with ductile beds at any given bed thickness. Parts of zone 2 and almost entire zone 3, located in the upper and middle Woodford, respectively, have high fracture densities and are situated within relatively organic-rich (high-GR) intervals. These intervals may be suitable horizontal well landing targets. All observed fracture cement exhibit a lack of crack-seal texture. Characteristic aperture-size distributions exist, with most apertures in the 0.05–1 mm (0.00016–0.0032 ft) range. In the chert beds, fracture cement is primarily bitumen or silica or both. Fractures in dolomite beds primarily have calcite cement. The average fracture spacing indices (i.e., bed thickness-fracture spacing ratio) in brittle and ductile beds were determined to be 2 and 1.2, respectively. Uniform fracture spacing was observed along all scanlines in the studied beds.

Demarcation of Structural Domains in Fractured Rock Masses Using a Three‐Parameter Simultaneous Analysis Method

A structural domain represents a volume of a rock mass with similar mechanical and hydrological properties. To demarcate structural domains (or statistically homogeneous regions) in fractured rock masses, this study proposes a three‐parameter simultaneous analysis method (3PSAM) that simultaneously considers rock fracture orientation, trace length, and aperture to evaluate statistical homogeneity between two regions. First, a 102‐patch three‐dimensional Schmidt net, which represents a new comprehensive classification system, is established to characterize rock fractures based on their orientation and aperture. Two populations of rock fractures can then be projected to the corresponding patches. Second, the Wald–Wolfowitz runs test is used to measure the similarity between the two populations by considering the fracture trace lengths. The results obtained by applying the 3PSAM to seven simulated fracture populations show that the homogeneity is influenced by both the distributions of the fracture parameters and the sequences of the fracture parameters. The influence of a specific combination sequence makes it impractical to analyze the rock fracture parameters individually. Combined with previous methods, the 3PSAM provides reasonable and accurate results when it is applied to a fractured rock slope engineering case study in Dalian, China. The results show that each fracture population should be identified as an independent structural domain when using the 3PSAM. Only the 3PSAM identifies the west exploratory trench 2 and the east exploratory trench as being nonhomogeneous because the difference in the aperture of the two fracture populations is considered. The benefit of the 3PSAM is that it simultaneously considers three parameters in the demarcation of structural domains.

Evaluation of the impact of rock fracturing on the reservoir properties on the example of the hydrocarbon-bearing sandstones in the Polish part of the Western Carpathians - a case study

So far, the largest documented reserves of oil in the Skole unit are found in the Wańkowa-Łodyna-Brzegi fold, where the reservoir rock is composed of the sandstones belonging to layers of the menilite series. The oil inflow from Inoceramian, Hieroglyphic and Krosno layers was also obtained from this fold. These layers characterized by a low level of reservoir parameters, are associated with the presence of a system of fractures. The impact of fracturing on the permeability of these sandstones is of interest to the oil industry. The aim of the research was to carry out measurements of the basic geometric parameters of fractures, i.e: the length of the fracture trace, the assessment of the degree of mineral fill, the openness and the determination of the angle and azimuth of fracture dip. Then, linear densities, surface coefficients of fracture and the fracture permeability coefficient were calculated for each measuring field. Obtained research results in the form of diagrams, tables and diagrams showed that inoceramian sandstones belong to rocks of low-class reservoir capacity. The analyzed sandstones are characterized by low permeability and have a low effective porosity. The results of the analyzes carried out simultaneously confirmed the effect of the fracturing of inoceramian sandstones on their potential permeability.

Detection of Rock Discontinuity Traces Using Terrestrial LiDAR Data and Space-Frequency Transforms

Part of the rockmass assessment and its application in numerical modelling, within the geotechnical engineering field, is acquiring information such as discontinuity number, density, intensity, size etc., which can be obtained by mapping fracture traces on exposed rockmass surfaces and processing of the recorded field data. Moving past from traditional mapping techniques in the field, fracture traces can be extracted from terrestrial light detection and ranging (LiDAR) point-clouds or LiDAR-derived surface models. However, similarly to field mapping, the extraction of fracture-traces is often done manually. This is an arduous and timely task in most cases. The automatic-detection of such traces is an emerging topic in geotechnical engineering; however, existing methods focus solely on the spatial domain. Space-frequency representations are ideal for detecting singularities due to their localization in space and frequency. Furthermore, they allow multiscale analysis, which is important for isolating LiDAR-data noise and weak traces in lower scales. In this study, three space-frequency transforms are evaluated, namely, (1) wavelet, (2) contourlet, and (3) shearlet. In addition, the well-known methods of Sobel, Prewitt, and Canny for edge detection are used for comparison purposes. The performance of the different edge-detection methods is tested using data collected from the Brockville Tunnel in Ontario, Canada. Numerical and visual assessment show that contourlets and shearlets achieve the highest agreement with manually-extracted traces that are used for validation. The two methods, along with minimal user interaction, can be used in order to increase the efficiency of rockmass mapping and geometric modelling in stability assessment of tunnels, mines, slopes, and related applications.

Platelet-rich plasma and its effect in bone regeneration in mandibular fractures. Controlled clinical trial

Currently there is great interest in developing clinical applications of platelet-rich plasma to enhance bone repair. To assess bone regeneration in mandibular fractures, with the application of this adjuvant. Twenty patients with mandibular fractures were included in a randomized clinical trial. Patients of the experimental group (n = 10) were submitted to internal fracture reduction and administration of platelet-rich plasma, and patients of the control group (n = 10) were submitted to the same surgical procedure without plasma application. Radiologic assessment was made before and at 1 and 3 months after surgery. X-rays were digitalized for analyze intensity and density as reflection of bone regeneration. The average age was 32 ± 11.3 years and 31.2 ± 8.48 years respectively (p = 0.76). The radiographic intensity and density in the experimental group at the 1st and 3rd month were higher in contrast to the control group (p < 0.005). Bone regeneration time was 3.7 ± 0.48 and 4.5 ± 0.52 weeks respectively (p = 0.002). There was no morbidity related to the application of the platelet-rich plasma. The platelet-rich plasma increased the bone intensity and density in the fracture trace allowing bone regeneration and recovery in shorter time than patients in which it was not used.

Structural Changes in the Vanadium Sample Surface Induced by Pulsed High-Temperature Deuterium Plasma and Deuterium Ion Fluxes

The structural changes in the vanadium sample surface are studied as functions of the conditions of irradiation by pulsed high-temperature deuterium plasma and deuterium ion fluxes in the Plasma Focus installation. It is found that processes of partial evaporation, melting, and crystallization of the surface layer of vanadium samples take place in the plasma flux power density range q = 108–1010 W/cm2 and the ion flux density range q = 1010–1012 W/cm2. The surface relief is wavelike. There are microcracks, gas-filled bubbles (blisters), and traces of fracture on the surface. The blisters are failed in the solid state. The character of blister fracture is similar to that observed during usual ion irradiation in accelerators. The samples irradiated at relatively low power density (q = 107–108 W/cm2) demonstrate the ejection of microparticles (surface fragments) on the side facing plasma. This process is assumed to be due to the fact that the unloading wave formed in the sample–target volume reaches its irradiated surface. Under certain irradiation conditions (sample–anode distance, the number of plasma pulses), a block microstructure with block sizes of several tens of microns forms on the sample surfaces. This structure is likely to form via directional crack propagation upon cooling of a thin melted surface layer.