Brittle-ductile Transition Depth Research Articles

On a Novel Modulation Cutting Process for Potassium Dihydrogen Phosphate with an Increased Brittle–Ductile Transition Cutting Depth

Potassium dihydrogen phosphate (KDP) has garnered considerable attention due to its diverse applications across various scientific and engineering domains. Although promising machining performance enhancements have been achieved in ultra-precision diamond cutting, the brittle–ductile transition (BDT) depth for KDP crystals is essentially at the nanometer range and limits the further improvement of machining efficiency. In this paper, a novel ultra-precision diamond cutting process based on tool trapezoidal modulation is proposed for the first time to investigate the BDT characteristics of KDP crystals. By intentionally designing the tool modulation locus, the uncut chip thickness and cutting direction in the cutting duty cycle are kept constant, which provides a new strategy for probing the BDT mechanism and enhancing the machining performance. The BDT depth is significantly increased compared to the conventional ultra-precision diamond cutting owing to its unique modulation machining advantages. The significance of this paper lies not only in the improvement of the machining efficiency of KDP crystals through the proposed modulation cutting process, but also in the possibility of extending the relevant research methods and conclusions to the machining performance enhancement of other brittle optical crystals.

A 3D rheological model for the Aegean Region: Mechanical layering and seismotectonic implications

We implemented a 3D thermo-rheological model for the broader Aegean Region, with special focus on determining the depth of the brittle-ductile transition (BDT) and the thickness of the brittle, potentially-seismogenic layers. In order to gain a more precise and comprehensive view on the thermo-physical characteristic of the main rheological transition, the model provides strength and temperature distribution along depth, as well as the relative values at the BDT depth. In particular, the reconstruction of a 3D model, allows to identify major trends and peculiarities, and to characterize the BDT depth, temperature and strength variations within this sector of the Mediterranean realm. The results indicate that a shallow transition (10–15 km) is common in the internal Aegean Region and most of continental Greece, while the external regions geodynamically associated to the Hellenic subduction show much deeper BDTs (around 30–35 and 40–45 km in continental and oceanic lithosphere sectors, respectively). The numerical modelling factors appearing to exert the greatest control on the BDT depth, and more in general on the spatial distribution of the rheological properties, at the scale of the whole study area, are the surface heat flow (as a proxy for the geothermal gradient) and the nature of the considered crust (continental vs oceanic). As a direct application of the results, the reconstructed 3D model of the shallower BDT is then exploited within the Aegean Region for defining the thickness of the seismogenic layer and its lateral variations. This in turn allows to independently constrain the maximum width of GreDaSS seismogenic sources and, by applying empirical relationships, to better quantify the maximum magnitudes expected on these sources.

Subduction initiation at an oceanic transform fault experiencing compression: Role of the fault structure and of the brittle-ductile transition depth

It has recently been shown that among the subduction zones initiated in the Cenozoic era, the subducting oceanic lithosphere could have any age, and was as often older than the overriding plate as younger than it. To try to explain this observation, we consider the simple setup of an oceanic transform fault and perform 2D thermomechanical experiments by applying the same convergence velocity on the two adjacent plates. We combine a non-Newtonian viscous rheology and a pseudo-brittle behavior. We focus on the influence of the fault structure at convergence onset, by testing the effects of the fault gouge depth, zwz, and of the width of the thermal transition between the lithospheres, which allows for mimicking the settings of a transform fault as well as of an old fracture zone. We investigate large ranges of plate age and of plate age offset. In most of our experiments, the mode of convergence accommodation is basically a function of the gouge depth, zwz, that we compare to the mean of the 2 plate thicknesses. We observe three main behaviors: (1) failure of subduction initiation if zwz is shallow, i.e., ≲15% of the mean depth of the 2 plate bottoms if the age offset scaled by the younger age is very low (≲1) or ≲30% otherwise, (2) old plate subduction, that is best favored for ratios zwz/zTT∼0.25 and for low offsets in relative plate ages, and (3) young plate subduction if zwz is quite deep (≳40%), whatever the age offset. The thermal transition width only has a second-order effect, that facilitates subduction initiation when the transition is wide. Compressive forces are overshooting reasonable thresholds (≳1013 N/m) if at least one of the two plates is older than ∼30 Myr. The success of a young plate subduction and, to a lesser extent, of an old plate subduction, can be predicted by comparing zwz at convergence onset to the depth of the brittle-ductile transition inside the future upper plate. The subduction success and polarity depend on the possibility to form a low shear stress plane. We predict that only the young plate subduction can initiate at an active transform fault, while the old plate subduction is restricted to the setup of an old fracture zone separating two lithospheres with a small age offset scaled by the younger age. The significant predominance in our modeling of the thin plate underthrusting results from the strong influence of the plate strength contrast when lithospheres are stiff. Our experimental results are consistent with the recent records of subduction initiation at the Mussau and Hjort Trenches, as well as at the Gagua ridge (W Pacific).

Brittle‒ductile transition behavior of 6H-SiC in oblique diamond cutting

6H-SiC is a typical hard-brittle material, and the challenge of performing ductile-regime machining in diamond cutting necessitates an in-depth understanding of the brittle‒ductile transition (BDT) mechanism. In this work, a novel machining method named oblique diamond cutting is employed to improve the BDT behavior of 6H-SiC. First, the tool edge ovalization effect is proposed, which provides deeper insight into the change in cutting edge shape evaluated in the cutting plane. An empirical model under the combined influence of the nominal tool rake angle, tool sharpness and tool inclination angle is established to effectively describe the equivalent cutting edge radius of the elliptic-shaped cutting edge in the cutting plane. When the tool inclination angle was increased from 0° to 45°, the equivalent cutting edge radius increases gradually, and the error of the prediction model remained at a low level of less than 7.5%. Second, an oblique plunge cutting experiment and finite element (FE) simulation were carried out under the consideration of tool edge ovalization effect. The simulation results indicate that the BDT depth grew gradually as the tool inclination angle increased, and shows the same changing trend as the experimental results with an average error of 14.9%. When the tool inclination angle was increased to 45°, the BDT depth obtained in experiments doubled (dc = 42.4 nm) compared to the results of traditional orthogonal cutting including from the references and this work. Such a huge improvement in the BDT behavior in cutting is of great significance for the successful execution of ductile-regime machining on a SiC optical component in practice. Finally, the reason for the increase in the BDT depth is revealed to result from changes in the stress distribution in the cutting area due to the increasing equivalent cutting edge radius of the elliptic-shaped cutting edge arc, especially the tensile stress, which is particularly sensitive to brittle materials with insufficient tensile strength. Compared to traditional orthogonal cutting, a high compressive stress zone is formed beneath the tool because more materials undergo the squeezing of the elliptic-shaped cutting edge. Thus, the tensile stress concentrated on the machined surface behind the cutting edge is suppressed, which is beneficial for promoting the plastic deformation of brittle materials with insufficient tensile strength. This is of great significance not only for SiC but also for the processing of other hard-brittle materials.

Effect of Fiber Type and Content on Surface Quality and Removal Mechanism of Fiber-Reinforced Polyetheretherketone in Ultra-Precision Grinding.

Polyetheretherketone (PEEK) is a promising thermo-plastic polymer material due to its excellent mechanical properties. To further improve the mechanical properties of PEEK, different kinds of short fibers are added into the PEEK matrix. The grinding machinability of short-fiber-reinforced PEEK varies with the effect of fiber type and content. Therefore, it is crucial to investigate the surface quality and removal mechanism of fiber-reinforced PEEK in ultra-precision grinding. In this paper, different fiber types and mass fractions of short-fiber-reinforced PEEK, including carbon-fiber-reinforced PEEK (CF/PEEK) and glass-fiber-reinforced PEEK (GF/PEEK), are employed. The grinding machinability of short-fiber-reinforced PEEK was investigated using grinding experiments with grinding wheels of different grit sizes. The effects of the fiber type and mass fraction on the surface quality and removal mechanism during grinding were discussed. The results showed that the brittle–ductile transition depth of carbon fiber was much larger than that of glass fiber, so it was easier to achieve ductile removal in grinding with the carbon fiber. Therefore, the ground surface roughness of CF/PEEK was smaller than that of GF/PEEK under the same grinding conditions. With the increase in carbon fiber mass fraction, the ground surface roughness of CF/PEEK decreased due to the higher hardness. The brittle–ductile transition depth of glass fiber was small, and it was easy to achieve brittle removal when grinding. When the glass fiber removal mode was brittle removal, the GF/PEEK surface roughness increased with the increase in glass fiber content.

Strain localization at brittle-ductile transition depths during Miocene magmatism and exhumation in the southern Basin and Range

Feldspar dominates the middle and lower continental crust. Models of crustal rheology depend on our knowledge of feldspar deformation mechanisms. To learn more about natural feldspar deformation, we conducted a multidisciplinary investigation of a brittle-ductile transition (BDT) hosted within a Cretaceous pluton in the Colorado River Extensional Corridor, southern Nevada. Here, a distinct BDT was observed at ∼10 km paleo-depth, where brittle faulting transitioned to discrete, localized ductile shearing. Field relationships confirm this BDT to have deformed and exhumed in the Miocene, temporally associated with the intrusion of two surrounding ca. 16 Ma plutons and subsequent footwall rotation in an east-directed normal-fault system. We established the overall structural framework, strain rates, and thermal histories through field traverses, aluminum-in-hornblende barometry to reconstruct paleo-depth, zircon (U–Th)/He thermochronology (ZHe) to constrain temperature-time histories, and 1D thermal models to further resolve deformation temperatures. Localized shear zones within the BDT formed ca. 16.2–15.5 Ma at temperatures of ∼500–600 °C, which continued deforming as they cooled both conductively and advectively to ∼200 °C by ca. 14 Ma, as constrained by ZHe dates. Strain was concentrated in fine-grained (7–10 μm) feldspar-rich ultramylonites that were interpreted to have primarily deformed via grain-size-sensitive diffusion creep. Grain-size reduction that allowed activation of diffusion creep likely resulted from brittle cataclasis, fluid-assisted fracturing and neocrystallization, and dislocation creep mechanisms, thus emphasizing the importance of these processes to localize deformation at relatively strong mid-crust conditions to facilitate the development of diffusion-creep shear zones. Grain-size piezometers suggest stresses of ∼50 MPa, which are significantly weaker than peak strength in quartz-rich BDTs that deform via dislocation creep. This integrated process allows relatively low viscosities (∼1018–1019 Pa s) at lower crust temperatures and explains a coherent process of strain localization at near-BDT conditions in feldspar-dominated lithologies. We suggest that this naturally deformed feldspar shear zone was preserved due to the unique geologic history with fast heating and exhumation, which froze both brittle vs crystal-plastic structures to provide valuable insights into mechanisms of feldspar deformation.

Crustal transpressional fault geometry influenced by viscous lower crustal flow

Abstract The San Andreas fault (California, USA) is near vertical at shallow (<10 km) depth. Geophysical surveys along the San Andreas fault reveal that, at depths of 10–20 km, it dips ~50–70° to the southwest near the Western Transverse Ranges and dips northeast in the San Gorgonio region. We investigate the possible origin of along-strike geometric variations of the fault using a three-dimensional thermomechanical model. For two blocks separated by transpressional faults, our model shows that viscous lower crustal material moves from the high-viscosity block into the low-viscosity block. Fault plane-normal flow in the viscous lower crust rotates the fault plane due to the simple shear flow at the brittle-ductile transition depth. This occurs irrespective of initial fault dip direction. Rheological variations used to model the lower crust of Southern California are verified by independent observations. Block extrusion due to lower crustal viscosity variation facilitates the formation of the Garlock Fault and sustains the geometric complexity of the fault.

Estimation of the maximum earthquakes magnitude based on potential brittle volume and strain rate: The Italy test case

One major critical issue in seismic hazard analysis deals with the computation of the maximum earthquake magnitude expected for a given region. Its estimation is usually based on the analysis of past seismicity that is incomplete by definition, or derived from the dimension of faults through empirical relationships with the intrinsic uncertainty in source characterization. Here, we propose a workflow aimed at providing a time-independent estimate for the maximum possible magnitude based on geological and geophysical evidence. Our estimate is also source unrelated as it is constrained by the seismic brittle volume of the crust that scales with the effective seismic energy. The seismic brittle volume is calculated considering fault kinematics and rock rheology (i.e., the brittle-ductile transition depth) over a grid that covers the entire study area. The maximum earthquake magnitude is calculated at each point of the grid based on a volume/magnitude empirical relationship. We apply this model to Italy for which we propose a map of the maximum possible magnitudes. Maximum predicted magnitudes are 7.3 ± 0.25 for thrust faulting, 7.6 ± 0.77 for normal faulting and 7.6 ± 0.37 for strike-slip faulting (± deviation from the mean value calculated at each node). These magnitudes are locally higher than the historical record. This could be due to an overestimation of the involved volumes; smaller volumes and lower magnitudes may occur where faults are detached at decollements shallower than the brittle ductile transition or where they behave aseismically. Alternatively, strong or major earthquakes could be possible, but they have longer recurrence time and they have never been recorded yet in Italy. Regardless these values are fully reliable or not, the recurrence of earthquakes with the predicted magnitude is related to current strain rates. We conclude that a large part of the Italian territory is prone to trigger Mw > 5 earthquakes.

A Self-Established "Machining-Measurement-Evaluation" Integrated Platform for Taper Cutting Experiments and Applications.

Taper-cutting experiments are important means of exploring the nano-cutting mechanisms of hard and brittle materials. Under current cutting conditions, the brittle-ductile transition depth (BDTD) of a material can be obtained through a taper-cutting experiment. However, taper-cutting experiments mostly rely on ultra-precision machining tools, which have a low efficiency and high cost, and it is thus difficult to realize in situ measurements. For taper-cut surfaces, three-dimensional microscopy and two-dimensional image calculation methods are generally used to obtain the BDTDs of materials, which have a great degree of subjectivity, leading to low accuracy. In this paper, an integrated system-processing platform is designed and established in order to realize the processing, measurement, and evaluation of taper-cutting experiments on hard and brittle materials. A spectral confocal sensor is introduced to assist in the assembly and adjustment of the workpiece. This system can directly perform taper-cutting experiments rather than using ultra-precision machining tools, and a small white light interference sensor is integrated for in situ measurement of the three-dimensional topography of the cutting surface. A method for the calculation of BDTD is proposed in order to accurately obtain the BDTDs of materials based on three-dimensional data that are supplemented by two-dimensional images. The results show that the cutting effects of the integrated platform on taper cutting have a strong agreement with the effects of ultra-precision machining tools, thus proving the stability and reliability of the integrated platform. The two-dimensional image measurement results show that the proposed measurement method is accurate and feasible. Finally, microstructure arrays were fabricated on the integrated platform as a typical case of a high-precision application.

Seismological data versus rheological modelling: Comparisons across the Aegean Region for improving the seismic hazard assessment

Compared analyses of the seismicity cutoff depth and the corresponding BDT (brittle-ductile transition) depth obtained from rheological modelling have been performed for numerous test sites all over the Aegean Region. The major goal of this research is to determine whether the rheological transition could be effectively correlated to the seismological one, so that both information can independently contribute in determining the seismogenic layer thickness and hence in constraining the maximum width of active faults affecting a region. As concerns the seismological data, only relocated events have been considered for the purpose of this paper, included either in seismic sequences associated with a mainshock, or in datasets of background seismicity. In the same areas, we carried out rheological modelling for reconstructing the local strength envelope, focusing our attention on the BDT depth. The systematic comparison at the investigated sites indicates that 90% of the relocated seismicity always occurs within the corresponding rheological transition. Moreover, for the datasets also providing magnitudes, 99% of the total seismic scalar moment is always released above the BDT depth. Such results are verified for different tectonic regimes and geological settings, both extensional, transcurrent and compressional. The achieved results confirm that the BDT depth based on accurate strength envelopes could be considered a reliable indicator for the thickness of the seismogenic layer, thus contributing to constrain the maximum widths for active faults affecting those regions. Based on these constraints, we also applied some empirical relationships to estimate the possible maximum magnitude for seismogenic sources in correspondence of the tested sites, thus potentially improving our seismic hazard assessment analyses for these areas.

Study on the crack formation mechanism in nano-cutting of gallium arsenide

Gallium arsenide is a typical brittle material, and crack formation is one of the most serious issues in the nanometric processing. This paper majors in studying crack formation mechanism in nano-cutting of gallium arsenide. In the molecular dynamics simulation of nano-cutting process, the main damage changes from dislocation to phase transformation with the increasing cutting speed. The amount of cracks increases with the higher cutting depth, and the deformation mode turns from ductile to a combination of ductile and brittle. Cracks appear at the boundary between amorphous layer and single crystal in the subsurface and tensile stress concentration is observed around the crack tip. The crack area is polycrystalline in the models with large tool radius. Brittle-ductile transition is observed by taper-cutting experiment of gallium arsenide, and the cutting depth is measured as 25 nm where the initial surface crack forms, which proves the low brittle-ductile transition depth of gallium arsenide. Micro cracks and polycrystals are found in the subsurface of transition area by cross section TEM, which is consistent with simulation results.

Sensitivity analysis for crustal rheological profiles: examples from the Aegean Region

We tested the validity of rheological profiles and their sensitivity to variations in the input parameters, with particular emphasis on the resulting BDT (brittle-ductile transition) depth and corresponding strength and temperature. For this purpose, we selected two test-sites from the Aegean Region, one in an extensional tectonic setting and the other in a strike-slip regime, and carefully realized the corresponding “reference” rheological profiles based on literature data and specific geological constraints. The reference envelopes have been then compared with a set of different profiles realized by varying the input parameters of the constitutive equations of the brittle and ductile behaviours within reasonable ranges. Firstly, tests were performed by changing the value of only one input parameter per time, with the aim of quantifying and comparing its influence on the BDT properties. The parameters exerting the greatest control are the activation energy, the power-law exponent and the surface heat flow (through its influence on the geothermal gradient), for the creep behaviour. As regards the brittle behaviour parameters, the friction coefficient and the pore fluid pressure could play a significant role especially in determining the maximum strength. In a second phase, we simultaneously varied all the input parameters in order to consider the possible synergistic effects on the resulting rheological profiles and to verify the likelihood and consistency of the reference models. For the statistical approach, one hundred thousand random combinations of the analysed parameters have been generated. The particular care spent on selecting the range of values of each parameter is reflected in the results of the statistical analyses, which show a good agreement with the reference profiles and allow estimating the overall uncertainties. Finally, the obtained strength envelopes have been compared with the accurately relocated depth distribution of recent seismic sequences that affected the two test areas. In both cases, the depth corresponding to the 95% of the total released energy nicely fits the BDT depth obtained from the rheological modelling, therefore confirming that this parameter could represent a reasonable and reliable approximation of the seismogenic layer thickness.

Seismogenic thickness of California: Implications for thermal structure and seismic hazard

The seismogenic thickness of the crust, a proxy for brittle-crust thickness, is a geometric parameter related to crustal strength, seismic hazard, and the crust's thermo-mechanical nature. We use high-resolution earthquake-location data from California to construct a topographic map of the base of the seismogenic crust by calculating the depth above which 95% of seismicity (D95) is located for fixed width bins. Seismogenic thickness is highly variable, ranging from ~5 km to >30 km, with thicker D95 values in the Great Valley-Sierra Nevada and thinner values in the Walker Lane and northern coastal California. Seismogenic thickness is inversely correlated with surface heat flow in most locations, consistent with a steady-state conductive crust, and local deviations probably reflect non-steady-state conditions related to magmatism and/or hydrothermal circulation. Such correlation suggests that, at regional scale, brittle-ductile transition (BDT) depth is mostly controlled by geothermal gradients, and the base of the seismogenic crust essentially represents a BDT isotherm (~300–350 °C for quartz-dominated lithologies). Spatial variations of D95 depths across California can be used to evaluate or constrain the locations of future seismicity, propagation direction of earthquake ruptures, and maximum depth, rupture area, and magnitude of future strike-slip earthquake events. Thicker seismogenic crust has a greater integrated strength. Seismogenic depth asperities, which represent mechanically stronger crustal patches, may focus and nucleate future earthquake events and/or impede rupture propagation.

Material removal behaviour in axial ultrasonic assisted scratching of Zerodur and ULE with a Vickers indenter

Compared to conventional grinding, axial ultrasonic vibration-assisted peripheral grinding (AUPG) has advantages in terms of the improved grinding quality, higher efficiency, as well as lowered brittle damages. However, the present studies on material removal mechanism of AUPG are still not sufficient which thereafter limit its application potential. This paper aims to investigate the material removal mechanism of Zerodur and ULE through scratching by using a Vickers indenter under two conditions, with (ultrasonic vibration-assisted scratching (UVS)) and without (conventional scratching (CS)) axial ultrasonic vibration-assisted scratching while the other scratching parameters are identical. The indenter's kinematic characteristic, scratched morphology, scratched groove dimensions and critical depth of cut are compared in between UVS and CS. The experimental results indicate that the dynamic contact length between indenter and workpiece for UVS is remarkable longer than that for CS, which is helpful to promote the crack interference and hence to increase the proportion of ductile material removal mode, resulting in the diminishment of brittle fracture size in UVS. The propagation direction of median cracks in CS are relatively consistent, while in UVS the periodic varied contact zone and contact force in between the indenter and workpiece could promote the cracks propagating in different orientations, and then to improve the material removals rate in UVS. Moreover, both the critical brittle-ductile transition depth of cut and critical brittle depth of cut in UVS are bigger than that in CS, meaning the material removed in UVS with a bigger ductile ratio leading to a decreased brittle damage depth. In addition, the different material removal behaviours between Zerodur and ULE are also investigated. This fundamental work lays a theoretical foundation for the technological development and broad application of hard/brittle material oriented AUPG, as well as poses a meaningful guidance for the ultrasonic vibration assisted grinding on low-expansion optical glasses.

Variation in vorticity of flow during exhumation of lower crustal rocks (Neoproterozoic Ambaji granulite, NW India)

The exhumation of the Neoproterozoic Ambaji granulite in the Aravalli-Delhi mobile belt, NW India, took place along NNW-SSE trending D2-shear zones. The shear zones evolved from a high temperature (>700 °C) thrust-slip shearing event in the lower-middle crust to a low temperature (450 °C) retrograde sinistral top-to-NW shearing event at the brittle-ductile-transition (BDT). The vorticity of flow (Wm) along the shear zones is estimated with the Rigid Grain Net and strain ratio/orientation techniques. The Wm estimates of 0.32–0.40 and 0.60 coincide with the high temperature event and suggests pure shear dominated deformation. The low temperature phase coincides with Wm estimates of 0.64–0.87 and ~1.0 implying two flow regimes. The shear zone was first affected by general non-coaxial deformation and gradually became dominated by simple shearing. We interpreted that the high temperature event happened in a compressive tectonic regime which led to horizontal shortening and vertical displacement of the granulite to the BDT. The low temperature event occurred in a transpressive tectonic setting that caused the lateral displacement of the granulite body at BDT depth. The Wm values indicate a non-steady strain during exhumation of granulite. This tectonic evolution is comparable with that of the Himalayas.

Evaluation of surface and subsurface damages for diamond turning of ZnSe crystal.

With single-point diamond turning (SPDT), a series of samples are processed under different cutting parameters. The brittle-ductile transition depth of ZnSe crystal is obtained, and the damages of the samples are measured from surface and subsurface damage depths as well as damage density. The effects of cutting parameters on the damages are investigated quantitatively. The results show the cutting depth has a minor while the feed has a major effects on the damages. Also, the interaction effect between feed and cutting depth is very small for surface damage depth or damage density, while it is large for subsurface damage depth. Based on the indentation mechanics and the kinetic characteristics of SPDT, a model is proposed to evaluate the surface and subsurface damage depths of ZnSe crystal by cutting parameters. The model has an average relative error less than 15.0%, which could be further used to obtain the depth and the removal characteristics of cracks in shoulder region.

Smoothed particle hydrodynamics (SPH) simulation and experimental investigation on the diamond fly-cutting milling of zirconia ceramics

This paper presents a more efficient ultra-precision diamond fly-cutting method to achieve the damage-free surface machining of hard-brittle zirconia ceramics at one time instead of the traditional abrasive-based processes. Firstly, the smoothed particle hydrodynamics (SPH) cutting simulation model of zirconia ceramics is established in LS-DYNA based on JH-2 constitutive model. The radial diamond fly-cutting milling experiments of zirconia ceramics are carried out on the three-axis ultra-precision machine tool, and the correctness of SPH simulation model is verified by the brittle-ductile transition (BDT) depth and chip morphology. Then, the effects of tool geometry and cutting parameters on stress distribution, brittle-ductile transition depth, cutting force characteristics, and chip morphology are investigated by the proposed model. Some conclusions are given as follows: the tool with more negative rake angle are benefit for hydrostatic pressure and getting greater brittle-ductile transition depth; the critical brittle-ductile transition depths of zirconia ceramics under -15 degree and -35 degree tool rake angle are 0.8μm and 1μm, respectively. Finally, by controlling the maximum undeformed chip thickness below 1μm, an application experiment is carried out and a crack-free nanometer-level surface is achieved in the axial diamond fly-cutting milling of zirconia ceramics.

Diamond cutting of micro-structure array on brittle material assisted by multi-ion implantation

Micro-structures have numerous applications for improving the surface functionability in various fields, such as surface mechanics, optics, and biology. However, machining micro-structures on brittle materials is difficult owing to the serious fractures that occur on the machined surface. To solve this problem, a method using ion implantation to modify the workpiece surface layer has been developed over the last seven years, but how to fabricate the modified layer efficiently is still unsolved and becomes a critical issue which blocks the application of this method in production. A modified layer that is thick enough for the machining is always preferred. This is difficult for the single-implantation approach studied before, owing to the large consumption in cost and time. In this study, diamond cutting assisted by multi-implantation is proposed to fabricate micro-structure array on silicon, a typical hard and brittle material. The novel method in this study shows distinct flexibility in the design of process parameters, as well as obvious improvement in efficiency. The result of surface modification is evaluated from aspects of crystal lattice and nanomechanics. The influence of channel effect is also discussed. Furthermore, chips with shear bands are discovered and analyzed in the cutting of modified silicon, which is extraordinary in the machining of brittle materials. And the brittle–ductile transition depth is significantly enhanced. Therefore, not only is multi-implantation capable of preparing thick modified layer, it also realizes further modification on material properties which is fundamental for improving the machinability. Finally, a micro-pillar array formed by 270 orthogonal cylindrical grooves (17956 micro-pillar units) is achieved in ductile mode by fly-cutting. It indicates that multi-implantation is a promising method for the micro-machining of brittle materials.

Structural modeling of lobate scarps in the NW margin of Argyre impact basin, Mars

Martian lobate scarps are prominent tectonic structures interpreted to be the topographic expression of large thrust faults generated under compression. Here, we present a structural modeling performed on three large lobate scarps (Ogygis Rupes, Bosporos Rupes and Phrixi Rupes) located in the heavily cratered highlands of Mars, specifically in Aonia Terra, between Thaumasia Montes and Argyre impact basin. These lobate scarps, formed in the Late Noachian/Early Hesperian, strike parallel to the edge of the Thaumasia Montes, and were generated by ESE-vergent thrust faults. Structural analysis of craters deformed by these lobate scarps gives minimum estimates for the faults slip of ∼1700–3100 m. We applied two structural methods in order to constrain the geometry of these thrust faults at depth, area balanced cross sections and forward mechanical dislocation modeling, obtaining a depth of faulting in this area between ∼18 and ∼45 km, and dip angles between 23° and 35°. These results are consistent with previous studies of lobate scarps on Mars. The depth of faulting gives an estimation of the depth of the brittle-ductile transition at the time of its formation giving a range of depth in which the state of the lithosphere change from brittle to ductile-dominated deformation. The heat flow values calculated from the obtained depths of the brittle–ductile transition range from 25 to 51 mW m−2. We show that the brittle-ductile transition depth in Aonia Terra is set in 18–27 km at a larger distance from the basin center, while it is deeper closer to the Argyre rim (∼33–45 km). This difference seems to indicate a thickening of the brittle domain under Argyre main rim with respect to the external area but, attending to regional geology and heat flow values calculated, this high value (∼33–45 km) might be an overestimation of the depth of faulting caused by the presence of the crater rim elevation before the formation of the lobate scarps.

Brittle-ductile transition depth versus convergence rate in shallow crustal thrust faults: Considerations on seismogenic volume and impact on seismicity

Earthquakes occur in the Earth’s crust where rocks are brittle, with magnitude increasing with the volume involved in the coseismic stage. Largest volumes are expected in convergent tectonic settings since thrust fault may be even more than 25 times larger than hypocenter depth. In general, the maximum depth of hypocenters within the crust corresponds to the brittle-ductile transition (BDT). The deepening of the BDT increases the potential seismic volume, hence raising the energy released during an earthquake. Here, by means of 2-D thermo-mechanical modelling dedicated to intraplate thrusts and thrusts within fold-and-thrust belts (shallow crust), the deepening of the BDT depth in convergent settings with variable convergence rates is investigated. Results of models characterized by shallow faults (15°–20° dip) show that BDT depth deepens by 15 km increasing the convergence rate from 1 to 10 cm/yr. Steeper thrust faults (25°–40° dip) show a lower degree of deepening of the BDT (≃5 km) as convergence rate is increased. Calculated BDT depths allow the calculation of maximum seismic volumes involved during thrust earthquakes. Deeper BDT depths obtained assuming higher convergence rates imply larger seismic volumes and an increase of 2 orders of magnitude of the stored potential energy, as effectively observed in nature.