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The intrinsic mechanical properties of single‐grain RE‐Ba‐Cu‐O bulk high‐temperature superconductors can be improved by employing a thin‐wall geometry. This is where the samples are melt‐processed with a predefined network of artificial holes to decrease the effective wall thickness. In this study, the tensile strengths of thin‐wall YBCO disks were determined using the Brazilian test at room temperature. Compared with conventional single grain YBCO disks, the thin‐wall YBCO disks displayed an average tensile strength that is 93% higher when the holes were filled with Stycast epoxy resin. This implies a thin‐wall sample should, in theory, be able to sustain a trapped field that is 39% higher without exceeding the mechanical limit of the sample. High‐field magnetization experiments were performed by applying magnetization fields of up to 11.5 T, specifically to break the samples in order to verify the effect of increased mechanical strength (and improved cooling) on the ability of bulk (RE)BCO to trap field successfully. The standard YBCO sample failed when it was magnetized with a field of 10 T at 35 K, suffering permanent damage. As a result, the standard sample could only trap a maximum surface field of 7.6 T without failure. On the other hand, the thin‐wall YBCO sample survived all magnetization cycles, including a maximum magnetization field of 11.5 T at 35 K, demonstrating a greater intrinsic ability to withstand significantly higher electromagnetic stresses. By subsequently field‐cooling the thin‐wall sample with 11 T at 30 K, a surface field of 8.8 T was trapped successfully without requiring any external ring reinforcement.

Crystallization, mechanical properties, and workability are all important for the commercialization and optimization of silicate glass compositions. However, the inter‐relations of these properties as a function of glass composition have received little investigation. Soda‐lime‐silica glasses with Na2O‐MgO‐CaO‐Al2O3‐SiO2 compositions relevant to commercial glass manufacture were experimentally studied and multiple liquidus temperature and viscosity models were used to complement the experimental results. Liquidus temperatures of the fabricated glasses were measured by the temperature gradient technique, and Rietveld refinements were applied to X‐Ray powder diffraction (XRD) data for devitrified glasses, enabling quantitative determination of the crystalline and amorphous fractions and the nature of the crystals. Structural properties were investigated by Raman spectroscopy. Acoustic echography, micro‐Vicker's indentation, and single‐edge‐notched bend testing methods were used to measure Young's moduli, hardness, and fracture toughness, respectively. It is shown that it is possible to design lower‐melting soda‐lime‐silica glass compositions without compromising their mechanical and crystallization properties. Unlike Young's modulus, brittleness is highly responsive to the composition in soda‐lime‐silica glasses, and notably low brittleness values can be obtained in glasses with compositions in the wollastonite primary phase field: an effect that is more pronounced in the silica primary phase field. The measured bulk crystal fractions of the glasses subjected to devitrification at the lowest possible industrial conditioning temperatures indicate that soda‐lime‐silica glass melts can be conditioned close to their liquidus temperatures within the compositional ranges of the primary phase fields of cristobalite, wollastonite, or their combinations.

Hard and brittle solids with covalent/ionic bonding are used in a wide range of modern‐day manufacturing technologies. Optimization of a shaping process can shorten manufacturing time and cost of component production, and at the same time extend component longevity. The same process can contribute to wear and fatigue degradation in service. Educated development of advanced finishing protocols for this class of solids requires a comprehensive understanding of damage mechanisms at small‐scale contacts from a materials perspective. The basic science of attendant deformation and removal modes in contact events is here analyzed and discussed in the context of brittle and ductile machining and severe and mild wear. Essentials of brittle–ductile transitions in micro‐ and nano‐indentation fields are outlined, with distinctions between blunt and sharp contacts and axial and sliding loading. The central role of microstructure in material removal modes is highlighted. Pathways to future research—experimental, analytical, and computational—are indicated.

Indentation fracture models of ductile machining in hard and brittle materials are critically appraised. Relations obtained in a seminal study for critical depths of cut below which fracture is suppressed are examined and amended. Limitations inherent in any such materials-based analysis, in addition to uncertainties in empirical measurements of underpinning mechanical properties (modulus, hardness, toughness) and of threshold grinding depths, suggest that caution should be exercised in unconditional usage. Notwithstanding these limitations, the value of the indentation fracture methodology in placing ductile machining on a sound materials science footing is maintained.

This work utilized a combination of experimental evidence and fractal geometric method to assess the effect of crack extension concerning the thermal shock on residual strength of ceramics. Sintered alumina (Al2O3) ceramic slabs were bundled and quenched in water under different thermal shock temperatures. The fractal dimension of thermal shock crack patterns on the interior surface and the cooled surface was calculated by the Box-counting method. Fracture energy of a fractal pattern of microcracks in quasi-brittle solids was employed to explain the relationship between crack length and fractal dimensions. The results show that if the crack propagation has the same crack length but a larger fractal dimension, it will absorb more fracture energy. The thermal shock crack patterns of Al2O3 ceramics with different grain sizes were analyzed, and the smaller grain size ceramic had a higher fractal dimension of crack patterns than the larger one.

The effect of ion-exchange on the fracture behavior and the threshold load is investigated for radial crack initiation resulting from cube-corner indentation. Both tin and air sides of the sodium aluminosilicate float glass are considered. The threshold load and mechanical properties are experimentally measured by nanoindentation. A qualitative explanation of crack initiation is developed by analyzing the stresses at the indentation site. The ion-exchanged glasses show a lower threshold load for radial crack initiation with a cube-corner indenter than the raw glass, and this is due to a higher crack driving stress for ion-exchanged glasses. However, the compressive stress on the surface of the ion-exchanged glasses can inhibit the expanding of the radial cracks. The air side always shows higher values for the threshold load than the tin side before and after ion-exchange, which is in accordance with the calculated crack driving stress results.

A novel methodology combining multiscale mechanical testing and finite element modeling is proposed to quantify the sintering temperature-dependent mechanical properties of oxide matrix composites, like aluminosilicate (AS) fiber reinforced Al2O3 matrix (ASf/Al2O3) composite in this work. The results showed a high-temperature sensitivity in the modulus/strength of AS fiber and Al2O3 matrix due to their phase transitions at 1200°C, as revealed by instrumented nanoindentation technique. The interfacial strength, as measured by a novel fiber push-in technique, was also temperature-dependent. Specially at 1200°C, an interfacial phase reaction was observed, which bonded the interface tightly, as a result, the interfacial shear strength was up to ≈450 MPa. Employing the measured micro-mechanical parameters of the composite constituents enabled the prediction of deformation mechanism of the composite in microscale, which suggested a dominant role of interface on the ductile/brittle behavior of the composite in tension and shear. Accordingly, the ASf/Al2O3 composite exhibited a ductile-to-brittle transition as the sintering temperature increased from 800 to 1200°C, due to the prohibition of interfacial debonding at higher temperatures, in good agreement with numerical predictions. The proposed multiscale methodology provides a powerful tool to study the mechanical properties of oxide matrix composites qualitatively and quantitatively.

The strengths of four brittle materials―cordierite glass ceramic, fused silica, silicon, and polycrystalline alumina were measured after exposure to weakly-corrosive water and moderately-corrosive buffered HF (BHF) solution. Exposure to water did not alter the strengths in subsequent inert strength tests and decreased the strengths in reactive strength tests. Exposure to BHF increased the strengths in both tests, but only after an incubation exposure time. Prior to the incubation time, the BHF had the same effect as water, suggesting that the bond rupture kinetics were unaffected. Examination of strength-controlling indentation flaws after the incubation time showed clear corrosive effects on the flaw geometry indicative of reductions in the indentation residual stress fields. The implication is that moderately corrosive environments increase the strength or lifetime of a brittle component by reducing the crack driving force via flaw alteration and do not, as perhaps expected, decrease the strength or lifetime through enhanced chemical reactivity. This article is protected by copyright. All rights reserved.

Laser processing of thin flexible ceramics and glasses is challenging due to the incurred brittleness and unfavourable thermal properties of such materials. Optical properties are also typically unsuited to laser machining. We describe an alternative laser cutting method which utilizes surface stress raisers to cleave brittle substrates along a defined path. An ultrashort laser source is used to precisely pattern a plurality of aligned elliptical recesses on the material surface. The apex of an ellipse concentrates applied tensile stresses. Depending on the elliptical dimensions, the achievable stress concentration factor can be up to 50. The orientation of the ellipses defines a preferred scribing path. The technique was successfully applied to thin flexible YSZ ceramic and borosilicate glass substrates. The form and properties of the material play an important role during the fracture process. Polycrystalline ceramics were found to accurately auto cleave along the path due to stresses produced during the laser ablation. The resulting fractured surface is of higher quality and strength than surfaces cut using full body laser cutting techniques, while the crystalline phase is preserved. The optical setup is simple, low cost, and compatible with roll-to-roll manufacturing. This article is protected by copyright. All rights reserved.

The thermal shock behavior of Ti2AlC synthesized by means of self-propagating high temperature combustion synthesis with pseudo hot isostatic pressing is investigated, with a focus on the effect of the quenching temperature and quenching times. In general, Ti2AlC exhibits a better thermal shock resistance than typical brittle ceramics like Al2O3. Although the flexural strength decreases quickly in the temperature range of 300-500 °C, no discontinuous decrease in the retained strength is observed in Ti2AlC which, as with other MAX phases, differs from the behavior of typical brittle ceramics. Overall, the initial strength (grain size) plays a determining role in the thermal shock behavior of Ti2AlC and other MAX phases. On increasing quench times to 5 cycles, the retained flexural strength decreases further, however with a lower rate of decrease compared to the first quench. Quenching at 300 °C and above, voids after the pullout of grains and cracks are present, which however are absent in the un-quenched samples, indicating the weakening of bonding among grains and the induced damage around the grain boundary during the thermal shock. This article is protected by copyright. All rights reserved.

This communication addresses some common misconceptions about weakest link theory and Weibull statistics as they pertain to strength distributions of brittle fibers. After describing the nature of the problem, the flaws in ensuing proposed models of strength distributions are highlighted and discussed. A way forward that obviates the problems is suggested.

Dynamic brittle fragmentation is typically described using analytical and computational approaches for tensile stress-states. However, most fragmentation applications (e.g., impact, blast) involve very large initial compressive stresses and deformations. In this study, the compressive fragmentation of brittle materials is investigated experimentally across a range of materials: silicon carbide, boron carbide, spinel, basalt and a stony meteorite. Analysis of our experimental results suggests that there exists two different regimes in the fragment size distributions, based on two brittle fragmentation mechanisms. The first is a mechanism that produces larger fragments and is associated with the structural failure of the sample being tested. This mechanism is influenced by the loading conditions (rate, stress state) and sample geometry. The second fragmentation mechanism produces comparatively smaller fragments and arises from the coalescence of fractures initiating and coalescence between defects in regions of large stresses and contact forces (e.g., between two fractured surfaces from the larger fragments). A framework is developed for comparing experimental compressive fragmentation results with tensile fragmentation theories. The compressive experimental results are shown to be adequately described by the theories using the new framework.

Understanding and quantifying the atomic structure, energetics, and the effect of defects on mechanics and reactivity of low symmetry oxides is an engineering challenge. This study focuses on atomistic analysis of screw dislocation in three thermodynamically reversible polymorphs of tricalcium silicate (Ca3SiO3), which is the key ingredient of cement. We employ molecular simulations to decode the interplay between the dislocation formation core energies, nonplanar core structures, displacement fields, disregistry functions, and Peierls stresses of tricalcium silicate polymorphs. By analyzing the core geometry at multiple scales, we found that the screw dislocations minimally impact the atomic bonds and angles at the vicinity of the Ca3SiO3 core dislocations, largely due to the sizeable channels and rigid body type relocation of atoms, which moderate most of the disturbances enforced by dislocations. Furthermore, we found that while the rhombohedral C3S (R-C3S) exhibits a relatively large core energy and core radius, the two monoclinic C3S structures (M3-C3S and MM-C3S) exhibited lower and almost identical core energies and core radii, likely due to the fact that they are two crystal realizations of the same polymorph. Finally, we discuss the implications of this work on brittleness and reactivity of cement crystals, and potential opportunities to better understand and modulate the grinding energy, reactivity, and mechanical properties of cement clinkers.

The in situ synthesis/consolidation of B4C–TaB2 eutectic composites by spark plasma sintering (SPS) is reported. Samples for the evaluation of bending strength were cut from specimens with diameters of 30 mm. The sample prepared for the three-point flexural strength test had fibers of tantalum diboride with diameter of 1.3 ± 0.4 μm distributed in the B4C matrix, thereby reducing composites brittleness and yielding an indentation fracture toughness of up to 4.5 MPa·m1/2. Furthermore, the Vickers hardness of B4C–TaB2 eutectics formed by SPS was as high as 26 GPa at an indentation load of 9.8 N. The flexural strength of the B4C–TaB2 system has been reported for the first time. Some steps were identified in the load–displacement curve, suggesting that micro- and macrocracking occurred during the flexural test. Ceramic composites with a eutectic structure exhibited a room-temperature strength of 430 ± 25 MPa. Compared with other eutectic composites of boron carbide with transition-metal diborides, room-temperature strength the B4C–TaB2 was 40% higher than that of B4C–TiB2 ceramics, demonstrating advantage of the in situ synthesis/consolidation of eutectic composites by SPS.

An extended Mohr–Coulomb model is proposed to represent the pressure-dependent strength of intact structural ceramics under quasi-static and dynamic loading rates at high pressures. A plot of normalized shear stress versus hydrostatic pressure for a range of structural ceramics revealed that despite the differences in material properties, test methods, and strain rates, the failure strength data fall in a narrow range, which can be represented by a unified model. The overall deformation behavior of structural ceramics and their strain rate dependence of strength in multiaxial loading at high pressure beyond Hugoniot elastic limit (HEL) can be explained by an exponential pressure dependence in the Mohr–Coulomb model. The analysis also suggests that the influence of strain rate on strength of a ceramic diminishes with increase in confinement pressure.

Si3N4-based nanocomposites containing 0–50 wt% TiC0.37N0.63 are directly consolidated at 1700°C by spark plasma sintering, and their reciprocal sliding behavior against a Si3N4 counterbody is investigated under a maximum Hertzian stress of 1.27 GPa in unlubricated conditions. The average grain widths of Si3N4 and TiC0.37N0.63 are about 85 and 90 nm, respectively. The decreasing relative densities of the as-sintered nanocomposites indicate that the nano-TiC0.37N0.63 may introduce pores and reduce the hardness and fracture resistance of the materials. The brittleness index for sliding contacts in all the samples is 25–31, indicating brittle fracture taking place on the wear surface and inducing cavities. When the mean free paths of nano-TiC0.37N0.63 are slightly greater than grain length of Si3N4, the best wear resistance is achieved in Si3N4 containing 20/30 wt% TiC0.37N0.63 due to the process of surface smoothing by triboproducts. Severe wear response can be observed in Si3N4 nanocomposites containing 0, 10, 40, and 50 wt% of TiC0.37N0.63. The wear responses are explained by considering the microstructural parameters (like grain characteristics for both phases and mean free path of nano-TiC0.37N0.63) and contact-induced fracturing behavior, as well as tribochemical reactions.

The utility of indentation testing for characterizing a wide range of mechanical properties of brittle materials is highlighted in light of recent articles questioning its validity, specifically in relation to the measurement of toughness. Contrary to assertion by some critics, indentation fracture theory is fundamentally founded in Griffith–Irwin fracture mechanics, based on model crack systems evolving within inhomogeneous but well-documented elastic and elastic–plastic contact stress fields. Notwithstanding some numerical uncertainty in associated stress intensity factor relations, the technique remains an unrivalled quick, convenient and economical means for comparative, site-specific toughness evaluation. Most importantly, indentation patterns are unique fingerprints of mechanical behavior and thereby afford a powerful functional tool for exploring the richness of material diversity. At the same time, it is cautioned that unconditional usage without due attention to the conformation of the indentation patterns can lead to overstated toughness values. Limitations of an alternative, more engineering approach to fracture evaluation, that of propagating a precrack through a “standard” machined specimen, are also outlined. Misconceptions in the critical literature concerning the fundamental nature of crack equilibrium and stability within contact and other inhomogeneous stress fields are discussed.

Today, the Mohs scale is used profusely throughout educational systems without any persuasive understanding of the fundamental principles. Why one mineral has a scratch hardness over the next culminating in a scale of 1 (chalk) to 10 (diamond) has no atomistic or structure-sensitive basis that explains this outcome. With modern computationally based atomistic and multiscale models, there is increasing promise of defining the pressure and rate-dependent parameters that will allow a fundamental understanding of the Mohs scale. This study principally addresses the combined fracture and plasticity parameters that qualitatively affect fracture at the nanoscale. A physical model wherein the crack tip under a scratch is shielded by dislocations is supported by molecular dynamics (MD) simulations in both ductile aluminum and brittle silicon carbide. Next, this model is applied to nanoindentation data from the literature to produce a ranking of Mohs minerals based on their fundamental properties. As such, what is presented here is a first step to address the flow and fracture parameters ultimately required to provide a figure of merit for scratch hardness and thus the Mohs scale.

Refractory carbides of transition metals (Zr, Ta, etc.), owing to a mixture of ionic, covalent, and metallic bonding, exhibit high hardness (10s of GPa), high elastic moduli (100s of GPa), good resistance to wear, ablation, and corrosion, high-temperature mechanical strength, along with good electrical conductivities. They are attractive as high-temperature structural components in aerospace vehicles. Although these materials are exceptionally hard, their structural applications at low temperatures have been limited because of their brittleness. The design of ceramic materials possessing both high hardness and enhanced ductility has been a long-standing challenge. Various research studies have focused on this topic with limited success. Here, we present a brief review of our recent results obtained from direct observations of mechanical deformation during uniaxial compression of group IV and group V transition-metal carbide (ZrC and TaC) single crystals inside a transmission electron microscope. Our studies provide new insights into the mechanical deformation mechanisms and help to identify strategies for enhancing room-temperature plasticity in this class of materials.

Microelectromechanical Systems (MEMS) devices typically need to be designed against a very low failure probability, which is on the order of 10−4 or lower. Experimental determination of the target strength for such a low failure probability requires testing of tens of thousands of specimens, which can be cost prohibitive for the design process. Therefore, understanding the probabilistic failure of MEMS devices is of paramount importance for design. Currently available probabilistic models for predicting the strength statistics of MEMS structures are based on classical Weibull statistics. Significant advances in experimental techniques for measuring the strength of MEMS devices have produced data that have unambiguously demonstrated that the strength distributions consistently deviate from the Weibull distribution. Such deviations can be explained by the fact that the Weibull distributions are derived based on extreme value statistics, which is inapplicable to MEMS devices where the dimensions of the material microstructure are not negligible compared to the characteristic structural dimensions. This paper presents a robust probabilistic model for strength distribution of polycrystalline silicon (poly-Si) MEMS structures that could be extended to other brittle materials at the microscale. The overall failure probability of the structure is related to the failure probability of each material element along its sidewalls through a weakest-link statistical model. The failure statistics of the material element is determined by both the intrinsic random material strength as well as the random stress field induced by the sidewall geometry. Different from the classical Weibull statistics, the present model is designed to account for structures consisting of a finite number of material elements, and it predicts a scale effect on their failure statistics. It is shown that the model agrees well with the measured strength distributions of poly-Si MEMS specimens of different sizes, and the calibrated mean strength of the material element is consistent with the theoretical strength of silicon. Meanwhile, it is shown that the two-parameter and three-parameter Weibull distributions cannot provide optimum and consistent fits of the observed size-dependent strength distributions, and thus have very limited prediction capability. The present model explicitly relates the strength statistics to the size effect on the mean structural strength, and therefore provides an efficient means of determining the failure statistics of MEMS structures.