Formation Of Crack Networks Research Articles

Blast wave interaction during explosive detonation in a variable cross-sectional charge

Purpose. The research aims to assess the efficiency and performance of solid media destruction in directed blasting of a charge with variable cross-sectional shape. Methods. Numerical modelling of the blast wave interaction process is performed using the finite element method based on the Euler-Lagrange algorithm. The Johns-Wilkins-Lee equation of state is used to determine the pressure-volume dependences of medium destruction. Assessment of solid medium destruction mechanism during a directed blasting of a charge with variable cross-sectional shape is carried out based on polarization-optical method on models made of optically active material. Findings. Experimental studies of solid medium destruction by the action of directed blasting with a variable cross-section charge made it possible to determine the direction of blast wave propagation and its amplitude in stress wave, influencing the intensity of radial crack network formation in superposition areas, and directed perpendicularly to explosive cavity. At the same time, the average peak pressure in collision zone of two shock waves in centre of spherical cavity is approximately 1.48 and 1.84 times higher than that in weakly blast-loaded areas. It has been found that when two shock waves collide and superimpose on each other, the intensity of their impact increases. Moreover, the shock wave velocity in collision zone is higher than that of the radial shock wave. Originality. It has been determined that the maximum pressure values on the explosive cavity wall at the initiation points sharply increase and then gradually stabilize as the blast stress waves propagate and have an arbitrary distribution pattern. Three areas should be considered: not superimposed, weakly superimposed, and strongly superimposed. At each point of detonation, the pressure on explosive cavity wall will be minimal, while in the charge centre in the spherical insert zone, on the contrary, it will be maximal. In this case, the pressure in the central superposition area is about 2.84 times greater than at the initiation ends, and the nature of distribution changes according to a linear dependence. Practical implications. The performed research findings can serve as a basis for development of effective parameters of resource-saving methods for stripping hard rocks of complex structure in the conditions of ore mines.

Damage Evolution and Fractal Characteristics of Granite under the Influence of Temperature and Loading

To study the damage characteristics and evolution law of granite under the influence of temperature and loading rate, uniaxial compression tests at different loading rates were performed for granite samples at 30 °C, 40 °C, 60 °C, and 80 °C. The damage characteristics and evolution law of granite were studied in conjunction with acoustic emission ringing, acoustic emission b values, acoustic emission signaling constellations, and fractal dimensions. The results showed the following: (1) At a low loading rate (0.005 mm/s), the peak stress of the granite increases gradually as the temperature increases. At high loading rates (0.01 mm/s and 0.02 mm/s), the peak stress of the granite first decreases, then increases as the temperature increases. The elastic modulus at different loading rates decreases first, then increases as the temperature increases. The elastic modulus of the granite is the lowest at 40 °C; (2) At the same temperature, specimens subjected to higher loading rates exhibit accelerated internal damage progression, with a precipitous decrease in the b-value occurring earlier. At a loading rate of 0.005 mm/s, the total number of acoustic emission ring-down events is at least 1.7 times greater than that at other loading rates, indicating a more complex development of internal cracks in the granite specimens. Moreover, as the temperature increases under a constant loading rate, the concentration of acoustic emission ring-downs intensifies during the unstable crack development and failure phases, accompanied by progressively larger fluctuations in the b-value; (3) Under the same temperature conditions, the acoustic emission signals in the specimens loaded at 0.005 mm/s and 0.01 mm/s propagate from one end to the other. At 0.02 mm/s, emissions appear simultaneously at both ends of the specimen and converge toward the center. The proportion of damage and energy associated with compression differs with temperature; at 30 °C and 40 °C, damage and energy are concentrated in the early phase of compression, whereas at 60 °C and 80 °C, they are more significant during the later stages; and (4) The fractal dimension of the granite specimens generally decreases, then undergoes a phase of fluctuating growth, followed by a decline. With increasing loading rates at a constant temperature, the compaction of the granite specimens decreases; the orderliness of the crack patterns gradually increases. As the temperature increases, the distribution of cracks during the compaction and failure phases becomes more orderly. During the crack initiation and development phases, the dispersion of cracks and the formation of crack networks are more pronounced. These findings provide a theoretical reference for understanding the microscale damage and failure mechanisms of granite under varying loading rates and temperatures.

Relating the microstructural and glass transition temperature evolution of a carbon–carbon composite precursor during pyrolysis using thermomechanical analysis

AbstractPolybenzoxazine thermoset resins exhibit significant potential as precursors for carbon–carbon composites, owing to their high char yield upon pyrolysis. During pyrolysis, the resin is converted to an amorphous carbon‐rich material through a series of mechanisms, resulting in a linear increase of the glass transition temperature (TG) from 133 to 396°C at 0.48 pyrolytic conversion. To better understand this relationship, a non‐isothermal kinetic model was integrated, enabling the prediction of the TG onset, inflection, and endpoint based on the heat treatment protocol (process temperature (TP) vs. time) and the total volume of pyrolytic gases generated. By considering the TG behavior and anticipated gas effects, three distinct mechanisms were identified: (1) gas accumulation and matrix expansion leading to increased sample thickness (TG < TP), (2) crack network formation caused by gas‐pressure‐induced brittle failure (TG > TP), and (3) sample thickness reduction due to gas venting through the crack network (TG < TP). To validate this theory, thermal mechanical analysis was performed during pyrolysis, and changes in thickness (±ΔL/L0) were attributed to gas pressure‐related matrix deformation. Conversely, no change in ΔL/L0 indicated the formation of an interconnected crack network through which gases could vent without building a measurable internal pressure.

Experimental Investigation on Failure Mechanism of Hot Dry Rock under Microwave Irradiation

Hot dry rock (HDR) geothermal energy is clean energy that can meet people’s demand for low carbon. The traditional hydraulic fracturing method is challenging to break HDR dominated by granite, mainly because of HDR’s high temperature and hard characteristics. Microwave irradiation is considered a proven approach to breaking granite. This study investigates the failure mechanism of HDR at 500°C–800°C under microwave irradiation. The experiments cover preheating, uniaxial compression, granulometric analysis, binocular vision monitoring (BVM) technique, and X-ray powder diffraction. The result shows that the uniaxial compression strength of granite decreases the most at 500°C–600°C, which is 62.77% on average. The failure form is from the brittle failure (untreated) to ductile failure (800 °C) with the rise of the microwave irradiation temperature. The microwave makes a particle size in the range of 0.6–2.36 mm the most apparent particle size of granite after uniaxial compression. The BVM technique reconstructs the surface deformation with a 0.7% error. Microwaves cause heat accumulation near the rock near the magnetron, resulting in the formation of crack networks and a molten cavity. The possibility of microwave-assisted fracturing of HDR is discussed at the end of the article.

Doping and stress induced failure of Ti(Al/Pt)N films on Ti6Al4V plate in a simulated PEMFC environment

Ti(Al/Pt)N films prepared on Ti6Al4V (TC4) plates were soaked in a simulated proton exchange membrane fuel cell (PEMFC) anode environment for three weeks. The dense Ti(Al/Pt)N films exhibited various preferred orientations: TiN and TiAlN films with (111) and TiAl(Pt)N film with (200) orientations. The inner stresses of the Ti(Al/Pt)N films changed from −13 MPa for TiN, −115 MPa for TiAlN, and −66 MPa for TiAl(Pt)N. The corrosion current densities of TiN, TiAlN, and TiAl(Pt)N were of the order of 10−8, 10−7 and 10−6 A·cm−2, respectively. Their corresponding contact resistance values decreased sequentially from 29.85, 15.25 and 9.85 mΩ cm2. The appearance of second EIS impedance circles after soaking the films in the simulated PEMFC environment for 3 weeks was indicative of the corrosion failure of the Ti(Al/Pt)N films. The relatively high inner stress in the TiN film led to the formation of oxides and corrosion pits. Peeling of the TiAlN film off the TC4 occurred owing to the high inner stress. The micro-potential difference between the TiAlN columns and Pt particles that were distributed along the column boundary led to the formation of crack networks in the TiAl(Pt)N film. Al/Pt in TiN deteriorated the durability of the films, although it enhanced their conductivity.

An analytical solution to the stress fields of kinked cracks

Propagating cracks may deflect due to dynamic instability, running into pre-existing weak regions of heterogeneous media, or encountering variation in driving forces. The mechanical analysis of a kinked crack is of engineering significance for safety control and crack-network formation. Existing theories for kinked cracks relied on the perturbation method, as befit small kinks. The stress intensity factors (SIFs) are valid in the close proximity of the primary crack tip. As to the stress field of a kinked crack, it remains unsolved so far. In this work we develop an analytical solution to the stress fields of kinked cracks. By employing the conformal mapping and the Muskhelishvili approach, the close-form solution works for arbitrarily sized kinked cracks. The analytical theory is then validated using finite-element simulations. With this prior knowledge, we analyze the dependence of crack deflection on loading conditions, critical energy release rate, and the geometry of a kinked crack. We further demonstrate that such an analytical approach paves the way to obtain the solution of multiple-kinked cracks.

Effect of helium pre-implantation on the thermal shock performance of tungsten

• He plasma and ion irradiation improve the thermal stability and damage threshold of W. • Annealing decreases the cracking threshold of W, while not affects the roughening threshold. • He ion irradiation has a stronger influence than He plasma exposure. In ITER and future tokamaks, high heat flux loads will lead to recrystallization and decrease in toughness and thermal shock resistance of tungsten plasma facing materials, resulting in crack network formation and possible failure. In this study, thermal shock tests of blank, helium-plasma-exposed and helium-ion-irradiated rolled tungsten with/without isothermal annealing are conducted. Without annealing, cracks start to form in blank tungsten at the power density of 0.35 GW/m 2 , while no cracks are observed on the loaded surface of helium-plasma-exposed and helium-ion-irradiated ones. After annealing at 1473 K for 1 h, the recrystallization fractions of the three kinds of tungsten are around 50%, 18% and 8%, respectively. The following thermal shock tests of the annealed samples show that the cracking thresholds of blank and helium-plasma-exposed ones reduce to 0.3 GW/m 2 , while cracks do not appear in the helium-ion-irradiated one at this power density. Surface roughening thresholds are also improved in helium pre-implanted samples and do not change after annealing. It indicates that both helium plasma and ion irradiation improve the thermal stability and damaging threshold of tungsten, and helium ion irradiation has a stronger influence. This suggests a potential benefit of helium to extend the service lifetime of tungsten.

Low velocity drop weight impact behaviour of Al2O3-Ni-ZrO2 and Al2O3-Ni-Cr2O3 ceramic composites

Particulate Al2O3 matrix nanocomposites containing 1 vol.%Ni were prepared by the heterogeneous precipitation method and the addition of 5 vol.% ZrO2 (ANZ) or 1 vol.% Cr2O3 (ANC). The prepared samples were subjected to the low energy drop weight impact tests to compare the behaviour of the composites under low energy impact and to investigate the damage mechanisms. The pure Al2O3, Al2O3/Ni, Al2O3/ZrO2 and Al2O3/Cr2O3 compositions with the same additive ratios were also produced to make the comparison systematically. Also, the Vickers hardness measurements were carried out and a significant increase in hardness was attained for both ANZ and ANC composites. The average hardness value around 24.8?1.0GPa was measured for the ANZ and ANC composites which means ~15% improvement compared to the pure Al2O3. Between all the compositions, the maximum force (Fmax) value was obtained for the ANZ (for 12 J impact energy level Fmax = 26617N) according to the low energy drop weight impact test results. Tensile radial crack network formation, cone formation, fracture and crushing of the cone structure were observed as damage mechanisms for all compositions. The volume of conical frustum structure was evaluated for each composition and the effect of microstructure on possible ballistic performance was also discussed.

Network of thermal cracks in meteorites due to temperature variations: new experimental evidence and implications for asteroid surfaces

ABSTRACT In recent years, several studies have shown the importance of thermal fracturing of rocks due to temperature variations, on The Earth and Mars. Rock thermal cracking might also be a process at play on the lunar surface. These temperature variations as well as change rates can reach important amplitude on bodies without an atmosphere, in particular on those that reach small perihelion distances such as near-Earth asteroids. On the other hand, the formation, geometry, and extension of cracks on these bodies have not been fully investigated yet. Here, we show results of thermal cracking laboratory experiments on chondrite meteorites, which develop networks of cracks when subjected to rapid temperature cycles with amplitudes similar to those experienced by asteroids with low perihelion distances. The depth of the cracks can reach a few hundred of microns in some hundreds of temperature cycles, in agreement with theoretical studies. We find that dehydration of hydrous minerals enhances the cracking process. The formation of crack networks increases the porosity both at the surface and in the sub-surface of our specimens. We propose that this process could help explaining the recent finding of the very highly porous surfaces of most of the boulders on the asteroids Ryugu and Bennu.

Crack propagation patterns and factors controlling complex crack network formation in coal bodies during tri-axial supercritical carbon dioxide fracturing

Supercritical carbon dioxide (SC-CO2) fracturing, as an environmentally-friendly waterless fracturing mining technology, has attracted more and more attention during the safe mining of coal resources. The propagation pattern of fracturing cracks, including the type, number, length, and direction of crack propagation, is the key basis for the design of coal seam fracturing. The direct observation of the crack propagation pattern in fracturing coal body at the engineering site is quite problematic, so laboratory tests are used to study the coal body fracturing process. Based on the tri-axial SC-CO2 fracturing test system, the crack propagation pattern in the SC-CO2-fractured coal body (SC-CO2-FCB) was studied under thermo-hydro-mechanical coupling conditions. SC-CO2-FCB sample tests were conducted at different crustal stresses, fracturing fluid injection rates, and fracturing fluid temperatures. The injection pressure evolution was analyzed; the type, number, length, and propagation directions of the sample surface cracks were determined, and the crack distribution in SC-CO2-FCB was obtained under the effects of temperature field-seepage field-stress field coupling. From three aspects of fracturing media, fractured substances, and their interactions, the formation mechanism of complex cracks in SC-CO2-FCB was revealed, which provided theoretical guidance for applying and promoting SC-CO2 fracturing in coal mines.

Evaluation of wear mechanisms of PVD and CVD coatings deposited on cemented carbide substrates applied to hard turning

A good understanding of the coating wear mechanism is essential in relation to tailoring the coating properties to the application, with a focus on increasing the tool performance. When a coated cemented carbide tool is applied in processes with high thermal and mechanical loads (e.g., hard turning process), the cutting tool can suddenly collapse with the deterioration of the coating. The focus of this research was to track the sequence of events that leads to the deterioration of coating materials (PVD TiAlN and MT CVD TiCN/Al2O3/TiN), to reveal the real wear mechanisms associated with coated cemented carbide cutting tools applied to hard turning. The tool wear was evaluated by focus variation microscopy (FVM) and scanning electron microscopy (SEM), before and after cutting edge collapse, during the hard turning of quenched and tempered AISI 4340 steel. The mechanisms associated with the progression of wear on MT CVD coatings involve abrasion, crack nucleation, propagation, the formation of crack networks, delamination, detachment between coating layers, and spalling. The deterioration of PVD coatings is related to the abrasion wear mechanism and the high deformation values at the cutting edge, which leads to the nucleation of cracks in the coating, reducing the bonding strength between the coating and the tool substrate, leading to spalling of the coating. The tool life of the PVD coating was three times longer than that of the CVD coating. The wear mechanisms acting on the coating is the main factor that influences the end of tool life.

Laser machining of different diameter holes in alumina ceramic: Thermal stress analysis

ABSTRACTLaser machining of different diameter holes into alumina tiles is carried out. Temperature and stress fields are predicted by using the finite element code. Surface temperature and residual stress predictions are validated through the thermocouple data and X-ray diffraction measurements. Morphological changes in the cutting section are examined by incorporating optical and scanning electron microscopes. It is found that the predictions of surface temperature and the residual stress formed at the cut section agree well with the experimental findings. In general, cut sections are free from large asperities; however, local dross attachments at the kerf edge and crack network formation at the kerf surface are observed.

Computational and experimental studies of the causes of crack network formation in the area of the heat exchanger tube sheet in the BN'600 reactor

In order to examine the condition of intermediate sodiumtosodium heat exchangers (IHX) and to substantiate their operation life extension to 45 years at the BN600 reactor plant at Beloyarsk NPP Unit 3, one of the six heat exchangers was removed from the reactor in April 2006. Inspection revealed cracks with 7 maximum depth of mm on the outer surface of the upper tubesheet (UTS) and adjacent shell.To predict durability of substantially fatigued metal, verification of the existing relationship for the threshold stress intensity factor range for the 10Cr18Ni9 steel was needed. To this end, specimens of two structural elements in the IHX – upper tube sheet and protection block were tested.To identify failure mechanisms, fractographic studies were performed on the surface of cracks detected in the tube sheet and produced in the specimens. The studies led to the conclusion that the failure mechanism for cracks detected on the tube sheet was identical to the mechanism generated in the test specimens. In both cases the intergranular failure prevailed that is typical for the material stress level indicative of crack growth termination. This result makes it possible to speak about crack initiation and propagation in the IHX tube sheet in the high-cycle fatigue region at low-level strain ranges and stress intensity factor ranges.An analysis of causes of crack formation showed that the cracks could have formed as a result of the temperature pulsation effect produced by mixing of sodium flows having different temperatures – sodium entering the IHX inlet and sodium coming from the reactor vessel cooling system.Computational analysis results showed that for all thermal pulsation conditions and crack propagation cross-sections under consideration, the leak-tightness condition is met for the upper tube-sheet and IHX outlet chamber shell that divide the primary and secondary coolant circuits.The computational and experimental studies have proved that presence of these cracks does not limit the potential service life extension to 45 years for the IHX in the BN600 reactor plant.

Расчетно-экспериментальные исследования причин образования сетки трещин в районе верхней трубной доски теплообменника реактора БН-600

Расчетно-экспериментальные исследования причин образования сетки трещин в районе верхней трубной доски теплообменника реактора БН-600

The effect of the heat treatment on the structure and properties of sintered TiC-Ti-Ni, TiC-Ni-Fe alloys

The features of heat treatment effect on the phase composition and crack resistance of sintered tool metastable-bunch alloys, hardened with TiC, were considered in the paper. Due to the fact that one of the major failure mechanisms of the tool, for the manufacture of which these alloys (forming rolls, bandages) were designed, is the crack network formation, the problem of decreasing brittleness of materials is of great scientific and practical value. Alloys with compositions 23 % Ti-27 % Ni and 39 % Ti-11 % Ni (+50 % TiC); as well as 19 % Ni+46 % Fe (+35 % TiC); 10%Ni+40 % Fe (+50 % TiC) and 40 % i+10 % Fe (+50 % TiC) were considered. Crack resistance indicators of alloys along the length of the radial crack from hardness indentations in the as-sintered and as-annealed condition at t=800...1100oC were defined. It was shown that the reduction in crack resistance parameters provokes annealing at t=800..1000 oC for the alloy 19 % Ni+46 % Fe (+35 % TiC), t=800 oC, 900 oC and 1100 oC for 40 % Ni+10 % Fe (+50 % TiC), 1100 °C – 10 % Ni+40 % Fe (+50 % TiC), respectively. This condition is accompanied by the presence of intermetallic compounds in the microstructure. The data obtained allow to recommend the use of alloys in the as-sintered condition, and avoid prolonged exposure of embrittlement temperatures during operation of products from them.

Effect of the casting process on microstructure and lifetime of the Al-piston-alloy AlSi12Cu4Ni3 under thermo-mechanical fatigue with superimposed high-cycle fatigue loading

For the present research work, the well-known Al-piston-alloy AlSi12Cu4Ni3 was manufactured in three different processes (gravity die casting – GDC, low pressure die casting – LPDC, and high pressure die casting – HPDC) and T5 heat treated. The microstructure of the material from each process was analyzed, specimens were tested in OP-TMF loading with and without superimposed high-frequency fatigue and lifetimes were compared.The microstructure of GDC specimens shows a homogeneous distribution of primary Si and intermetallic phases. The LPDC material also shows a homogeneous microstructure over the whole sample. However, the Al-mixed-crystal formed bigger dendrite arms compared to the GDC material. The HPDC material shows a gradient in the microstructure getting finer from the center to the outer shape. In the very fine microstructure in the outer regions of the specimen no primary Si was formed and the Al-mixed-crystal built a globular–dendritic structure, surrounded by a eutectically solidified melt. Porosity was higher in the LPDC and HPDC compared to the GDC material.To simulate the thermally induced loading at the “hot side” of a piston during start–stop, strain controlled out-of-phase thermal–mechanical fatigue (TMF) tests with superimposed high-cycle fatigue (HCF) loading were performed. The TMF-cycles were carried out with a minimum temperature Tmin=200°C and a maximum temperature Tmax=440°C. The mechanical strain amplitude εa,tme of the TMF cycles was kept equal to 50% of the thermal strain amplitude εath and the amplitude of the superimposed HCF cycles εa,tHCF was varied between 0.03% and 0.05%.The lifetime of the specimens produced in the GDC-process represents the current state of the art. Compared to that, the specimens manufactured in LPDC and HPDC reached nearly the same TMF lifetimes as the GDC samples. The maximum stress of the LPDC specimens is approximately equal to that observed at the GDC material. However, during the very first TMF cycles, HPDC-specimens show higher maximum stress than the reference material. This is attributed to the very fine, nearly defect free microstructure at the outer shape of the HPDC material. Afterwards, the maximum stress of the HPDC samples is decreasing faster than that of the GDC material due to early formation of crack networks starting from fine pores in the HPDC microstructure. In TMF/HCF-testing the HPDC material shows the same effect. Furthermore, the HPDC samples show pronounced swelling during temperature cycling and TMF testing. The root cause was identified as the high internal pressure of air encased in the pores formed during HPDC.

Effect of 16.3 dpa neutron irradiation on fatigue lifetime of the RAFM steel EUROFER97

Low cycle fatigue specimens of the reduced-activation martensitic/ferritic steel EUROFER97 were neutron irradiated at 250 °C up to an accumulated dose of 16.3 dpa. After irradiation, the specimens were push–pull fatigue tested under strain–controlled conditions at 250 °C to determine the impact of irradiation on lifetime, fracture behavior, and microstructure. The typical cyclic softening of martensitic/ferritic steels was observed. Furthermore, a considerable increase of lifetime after irradiation and subsequent cycling at lower strain amplitudes was remarkable. This behavior was attributed to the homogeneous distribution of stable irradiation-induced dislocation loops and small precipitates acting as barriers for the cyclic motion of dislocations, thereby influencing substantially crack initiation and crack network formation. While in the un-irradiated material push–pull fatigue sweeps the dislocations to the boundaries, a significant fraction of dislocations was fixed at irradiation-induced defects after irradiation and fatigue testing.

Stage I surface crack formation in thermal fatigue: A predictive multi-scale approach

A multi-scale numerical model is developed, predicting the formation of stage I cracks, in thermal fatigue loading conditions. The proposed approach comprises 2 distinct calculation steps. Firstly, the number of cycles to micro-crack initiation is determined, in individual grains. The adopted initiation model depends on local stress–strain conditions, relative to sub-grain plasticity, grain orientation and grain deformation incompatibilities. Secondly, 2–4 grains long surface cracks (stage I) is predicted, by accounting for micro-crack coalescence, in 3 dimensions. The method described in this paper is applied to a 500 grains aggregate, loaded in representative thermal fatigue conditions. Preliminary results provide quantitative insight regarding position, density, spacing and orientations of stage I surface cracks and subsequent formation of crack networks. The proposed method is fully deterministic, provided all grain crystallographic orientations and micro-crack linking thresholds are specified.

3D crack network analysis during a scratch test of a polymer: A combined experimental and multigrid X-FEM based numerical approach

The scratch behavior of a thermoset solid polymer exhibiting brittle behavior in tension is investigated. The surfaces are scratched and an imaging system is used to record real time photographs. The 3D crack pattern is analyzed using fluorescence confocal laser scanning microscopy. A finite element simulation is performed to determine the indenter/specimen contact conditions. They are used as input data for the 3D crack network analysis based on combined 3D localized multigrid and X-FEM/level set techniques. The computed stress distributions within the 3D cracked specimen are of great interest to understand the crack network formation observed during the experiments.

Material removal mechanisms of monocrystalline silicon under the impact of high velocity micro-particles

Abrasive jet micromachining is a novel non-conventional machining process with applications in the opto-electrical and semiconductor industries. To understand the process, it is important to investigate the material removal mechanisms starting from a single particle impact. Different types of impressions or erosions produced by single particle impact on monocrystalline silicon and the effect of erosion parameters are presented and discussed. It is found that particle velocity, particle size and erosion angle all play an important role in the material removal process. Three types of impressions are broadly identified which include craters, scratches and microdents. Among these types of impressions, craters with brittle cleavage fractures cause instant material removal by crushing upon micro-particle impact. On the other hand, scratches are mainly responsible for crack network formation, which decreases material strength. The radial and lateral crack networks, which cause material spalling, are present in both surface and sub-surface region as shown by electron microscopy investigation. A transmission electron microscopy study shows the retention of crystalline structure below the crater impressions. It is also found that increasing the particle velocity, particle size and erosion angle increases the formation of crater-type impressions.