A Novel Methodology to Investigate Critical Depth for Ductile-to-Brittle Transition During Scratch Testing

More than two decades have passed since the introduction of the scratch testing method for rock strength determination. The test method typically involves dragging a rigid-shaped cutter across the rock surface at a fixed cutting depth. This depth determines the failure mechanism of the rock, ductile for shallow depths and brittle for deeper. In the ductile mode, intrinsic specific energy is primarily a measure of the unconfined-compressive-strength (UCS), which is pivotal for rate of penetration (ROP) during drilling and for borehole stability analysis. On the contrary, brittle failure can lead to permanent core damage and is usually not desired as it impacts interpretation of the scratch testing results. Consequently, it is imperative to identify the critical depth, and at which transition from ductile to brittle failure occurs which will help optimize rock testing and tool designs. In this study, a novel methodology is proposed utilizing micro-computed tomography (CT) imaging to determine critical depth through morphological analysis of scratch test cuttings. Scratch tests are carried out on Indiana limestone core samples with the cutter-rock interaction geometry characterized by a cutter width of 10mm and a back-rake angle of 15°. The sample is scratched in the range of 0.05mm to 0.40mm with increments of 0.05mm. Scratch powder is carefully collected after each scratch increment and stored for further analysis. This powder is then loaded into slim rubber tubes and imaged at a high resolution of 1 µm with a helical micro-CT scanner. The scans are then reconstructed using a computer program to initiate the visualization of individual grains from each cutter depth including evaluation of grain morphologies. Finally, the results from this morphological analysis are corroborated and compared with three other methods: force response analysis, force inflection point analysis, and the size effect law (SEL). Based on shape analysis, it was found that the transition from ductile to brittle regime occurs at a depth of 0.25mm. Elongation and appearance of the enhanced degree of angularity of the grains as the depth of cut (DOC) increases past 0.25mm was observed. Moreover, large grain sizes were detected and are representative of formation of chips (typical brittle regime response). Furthermore, it is illustrated that the image analysis helps eliminate the ambiguity of force signal analysis and in combination can aid in the critical depth of cut determination. The other methods involving force alone and the SEL are not able to pin-point onset of brittle regime. Using a similar methodology, creation of a database for various rock types is recommended to develop a guide for the depth of cut selection during scratch testing. This novel methodology utilizing micro-CT analysis and comparative study with other techniques will put in place an accurate strategy to determine the critical depth of cut when designing rock scratch testing programs.

Laboratory Measurements Mapping Heterogeneity and Variation of Poroelasticity for Improved Hydraulic Fracture Design in a Tight Carbonate Reservoir Onshore UAE

Reservoir X is a thin and tight carbonate reservoir with thin caprock that isolates it from an adjacent giant reservoir. An accurate geomechanical model with high precision is required for designing the optimum hydraulic fracture and preventing communication with adjacent reservoirs. The reservoir exhibits considerable variability in rock properties that will affect fracture height growth, complexity, and width and rock interaction with treatment fluids. The heterogeneity observed from the tight sections is further complicated by the variation of Biot's poroelastic coefficient, α, which is required for accurate assessment of the effective stresses. Laboratory testing was required to characterize the extensive vertical heterogeneity for key inputs in developing a geomechanics model. Approximately 120 ft of continuous core from an onshore field was provided for this study. The core material represented a potential tight carbonate reservoir interval and bounding sections. Heterogeneity mapping was performed from continuous core measurements from CT-imaging and scratch testing. CT-imaging provides an indication of the bulk density variation and compositional changes. Scratch testing provides a continuous measure of the unconfined compressive strength (UCS). Combining the two provides a means for accurate definition of rock thickness for dense, moderately dense, and lower density material coupled with corresponding compressive strength. Rock units were then subdivided based on these continuous properties for further geomechanics tests. Using log analysis combined with continuous UCS measurements from scratch testing, eight rock type classes were defined covering the target reservoir interval and bounding sections. This information was used for optimizing the sample selection process to characterize each identified rock unit. Routine core analysis measurements reveal significant vertical heterogeneity with porosity ranging from 0.1% to 18.1%. Similar variability was determined from elastic properties for each of the eight rock types. Quasi-static values for Young's modulus and Poisson's ratio determined at in-situ stress conditions ranged from 2.6 to 9.6 × 106 psi, and from 0.16 to 0.34, respectively. The Biot's poroelastic coefficient has a first-order impact on the calculated effective stress profile, which directly affects fracture stimulation model results. Testing from this study combined with previous measurements (Noufal et al. 2020, SPE-202866-MS) provides a unique correlation with porosity and bulk compressibility. In addition, rock-fluid compatibility was evaluated with proppant embedment/fracture conductivity tests. Results are dependent on a given rock type, exhibiting a wide range of fracture conductivity as a function of closure stress from 10 to 1000 md-ft. Embedment for all cases was low to moderate.

Antibiocorrosive Hybrid Materials with High Durability

We have developed novel antibiocorrosive multifunctional hybrid materials based on functionalizedperfluoroalkylmethacrylate copolymerswith epoxy groups in main chainsand selected biologically active compounds.The hybrids are transparent, showgood adhesion to various surfaces (plastic, wood),high viscoelastic recovery in scratch testing,low wear rates and glass transitions above 323 K. No phase separation is seen in scanning electron micrography. Enhanced mechanical strength and good abrasion resistance are advantages for uses of our protective and antibiocorrosive coatings in various applications including protection of cultural heritage.

Deformation Behavior of Wrought and EBAM Ti-6Al-4V under Scratch Testing

The microstructure, mechanical properties, and deformation behavior of wrought and electron beam additive manufactured (EBAM) Ti-6Al-4V samples under scratching were studied. As-received wrought Ti-6Al-4V was subjected to thermal treatment to obtain the samples with microstructure and mechanical characteristics similar to those of the EBAM samples. As a result, both alloys consisted of colonies of α phase laths within prior β phase grains and were characterized by close values of hardness. At the same time, the Young’s modulus of the EBAM samples determined by nanoindentation was lower compared with the wrought samples. It was found that despite the same hardness, the scratch depth of the EBAM samples under loading was substantially smaller than that of the wrought alloy. A mechanism was proposed, which associated the smaller scratch depth of EBAM Ti-6Al-4V with α′→α″ phase transformations that occurred in the contact area during scratching. Ab initio calculations of the atomic structure of V-doped Ti crystallites containing α or α″ phases of titanium were carried out to support the proposed mechanism.

The Effect of Multilayer Architecture and Ta Alloying on the Mechanical Performance of Ti-Al-N Coatings under Scratching and Uniaxial Tension

The present work is focused on a comparative study of the effect of Ti-Al interlayers and Ta alloying on the mechanical behavior of Ti1−xAlxN coatings under normal contact pressure and in-plane straining. The contact loading of the samples was carried out by scratch testing, while the in-plane tensile straining was performed by uniaxial tension of the coated steel substrates. The Ti0.45Al0.55N and Ti0.43Al0.45Ta0.12N monolithic coatings as well as the Ti0.45Al0.55N/Ti0.45Al0.55 multilayer coatings with different number and thickness of the layers were deposited by DC magnetron sputtering. It was found that the introduction of the ductile Ti0.45Al0.55 layers into the Ti0.45Al0.55N coating and alloying with Ta led to their significant toughening. The improved toughness of the Ti0.43Al0.45Ta0.12N coating coupled with high residual compressive stress and high hardness resulted in its strongest resistance to cracking under scratching and tensile straining among the coatings studied. The multilayer coating with the thickest metal layers exhibited the improved resistance to delamination under in-plane straining.

Experimental analysis of crack resistance indicators of demineralized tooth enamel after combined infiltration treatment

Objective. To study the indicators of crack resistance of the demineralized enamel treated with combined infiltration method using the method of scratch testing. Materials and methods. To study the elastic-strength properties of the enamel in vitro, 24 intact teeth removed by orthodontic indications were used, on the vestibular surface of the crown of which, there was modelled an artificial caries of the enamel by the patented technology. A number of multilevel studies confirmed the formation of caries. Scratch testing was performed on the sections of the intact enamel; demineralized enamel infiltrated by light composite using the modified method with a four-minute regime of conditioning; enamel laminated with bioactive hybrid glass ionomer. Results. The developed model of artificial caries corresponded to the enamel in vivo. The critical load of the start of formation of the intact enamel microcracks (Lc1) was 9.82 0.81 N; demineralized enamel 6.34 0.92 N; infiltrated by modified method 8.23 0.61 N; bioactive glass ionomer 0.82 0.17 N. The critical load of formation of the chevron cracks (Lc2) of the intact enamel was 18.21 0.68 N; demineralized 14.21 1.35 N; after infiltration 10.1 0.30 N; in bioactive glass ionomer, no parameters were registered on all the tested samples. The critical load of formation of the intact enamel chips (Lc3) was 15.73 0.73 N; demineralized enamel 5.02 0.64 N; after infiltration 22.43 0.44 N; bioactive covering 2.21 0.12 N. Conclusions. A comparative analysis of the results of scratch testing of the enamel permitted to characterize the biomaterial from the position of physical material science, determine the critical loads of the start of forming microcracks, double-helical cracks, chips.

Fracture toughness of one- and two-dimensional nanoreinforced cement via scratch testing

Cement is the most widely consumed material globally, with the cement industry accounting for 8% of human-caused greenhouse gas emissions. Aiming for cement composites with a reduced carbon footprint, this study investigates the potential of nanomaterials to improve mechanical characteristics. An important question is to increase the fraction of carbon-based nanomaterials within cement matrices while controlling the microstructure and enhancing the mechanical performance. Specifically, this study investigates the fracture response of Portland cement reinforced with one- and two-dimensional carbon-based nanomaterials, such as carbon nanofibres, multiwalled carbon nanotubes, helical carbon nanotubes and graphene oxide nanoplatelets. Novel processing routes are shown to incorporate 0.1–0.5 wt% of nanomaterials into cement using a quadratic distribution of ultrasonic energy. Scratch testing is used to probe the fracture response by pushing a sphero-conical probe against the surface of the material under a linearly increasing vertical force. Fracture toughness is then computed using a nonlinear fracture mechanics model. Nanomaterials are shown to bridge nanoscale air voids, leading to pore refinement, and a decrease in the porosity and the water absorption. An improvement in fracture toughness is observed in cement nanocomposites, with a positive correlation between the fracture toughness and the mass fraction of nanofiller for graphene-reinforced cement. Moreover, for graphene-reinforced cement, the fracture toughness values are in the range of 0.701 to 0.717 MPa m . Thus, this study illustrates the potential of nanomaterials to toughen cement while improving the microstructure and water resistance properties. This article is part of a discussion meeting issue ‘A cracking approach to inventing new tough materials: fracture stranger than friction’.