Fracture Toughness Measurements Research Articles

Mechanical Properties of Silicon Carbide Composites Reinforced with Reduced Graphene Oxide

This article presents research on the influence of reduced graphene oxide on the mechanical properties of silicon carbide matrix composites sintered with the use of the Spark Plasma Sintering method. The produced sinters were subjected to a three-point bending test. An increase in flexural strength was observed, which reaches a maximum value of 503.8 MPa for SiC–2 wt.% rGO composite in comparison to 323 MPa for the reference SiC sample. The hardness of composites decreases with the increase in rGO content down to 1475 HV10, which is correlated with density results. Measured fracture toughness values are burdened with a high standard deviation due to the presence of rGO agglomerates. The KIC reaches values in the range of 3.22–3.82 MPa*m1/2. Three main mechanisms responsible for the increase in the fracture toughness of composites were identified: bridging, deflecting, and branching of cracks. Obtained results show that reduced graphene oxide can be used as a reinforcing phase to the SiC matrix, with an especially visible impact on flexural strength.

In situ TEM nanomechanical study on enhanced toughness of graphene-nanoparticle nanocomposite

Although theoretical studies and experimental research have shown outstanding mechanical properties of graphene, its brittleness and low toughness restrict its widespread industrial use. Here, we conducted a study to examine the impact of encapsulated Ag nanoparticles on the mechanical properties of graphene through in situ nanomechanical testing in an aberration-corrected transmission electron microscope (TEM). The presence of Ag nanoparticles was found to decrease crack propagation speed, enabling the TEM observation of crack deflection around the nanoparticles at nanoscale, as well as real-time measurement of the concomitant increase in stress. Using our nanomechanical testing data, we were able to quantitatively estimate the stress intensity factor, which serves as a measurement of fracture toughness for graphene, and then compared our results with previously reported values. We further discuss the role of nanoparticles in the propagation of cracks and their potential to enhance the toughness of materials. These findings have practical implications for the development of high-performance graphene composite materials.

Designing the geometry of compact tension specimens for easy fracture toughness measurement using reinforcement learning

Abstract For composite laminates, a rising R-curve is observed for their fracture toughness under Mode I stress, which is important for a comprehensive failure analysis of the materials. Since it is laborious to measure the R-curve due to its dependence on both the load and the crack extension, we put forward a novel compact tension specimen by modifying its geometry to eliminate the relation between fracture toughness and crack extension, so as to simplify the experimental process of the R-curve measurement by only recording the load history. Two machine learning models were developed for the optimum sample design based on the finite element analysis of the effect of sample geometries on the R-curve. A simple neural network model was built for designing tapered specimen and a reinforcement learning model was created for further finding the best design from a broader design space. The results showed that, in contrast to the specimens with a tapered shape, which only ensure the independence between the R-curve and crack extension in the case of a small extension, the design provided by the reinforcement learning provides such independence across a wider range of crack length and an improved accuracy.

Effects of notch width and loading rate on the dynamic mode II fracture toughness of Ti-6Al-4V

Existing work on size effects in the measurement of dynamic fracture toughness (DFT) has focused mainly on the overall geometric dimensions of the test specimens, while the effects of notch width and loading rate were not explored. In this paper, DFT experiments were conducted using Ti-6Al-4V alloy to study the effects of notch width effect and loading rate effect on its dynamic mode II fracture toughness. An extrapolation method is proposed to estimate the DFT of an ideal sharp crack, and the relationship between DFT, loading rate, and notch width is captured with a 3D spatial plane. Results revealed that the DFT decreases with a reduction of the notch width and increases with loading rate. The effects of notch width on DFT will be shown to be more sensitive compared to loading rate. The failure mode transitional behavior from ductile fracture to adiabatic shearing will be revealed through fractographic analysis of the microstructural evolution, which will elucidate the effects of loading rate on the failure mechanism of the material.

Small‐scale measurement of fracture toughness of muddy marine sediments via bubble injection

AbstractMuddy marine sediments are elastic materials in which bubbles grow and worms extend their burrows by fracture. Bubble growth and burrowing behavior are dependent on the stiffness and fracture toughness (KIc) of these muds. This article describes a custom laboratory apparatus to measure the fracture toughness of muddy, cohesive sediments using a bubble injection method. The system induces fracture in sediment samples by incrementally injecting air through a needle inserted into the sediment. The increasing pneumatic pressure is monitored until it drops abruptly, indicating bubble formation. Fracture toughness is then calculated from the peak pressure at which fracture occurred, following cavitation rheology methods developed for soft gels. The system has produced measurements that compare well to previous data but with better spatial resolution, allowing for characterization of spatial heterogeneity on small scales.

Anisotropic Fracture of Two-Dimensional Ta2NiSe5.

Anisotropic two-dimensional materials present a diverse range of physical characteristics, making them well-suited for applications in photonics and optoelectronics. While mechanical properties play a crucial role in determining the reliability and efficacy of 2D material-based devices, the fracture behavior of anisotropic 2D crystals remains relatively unexplored. Toward this end, we herein present the first measurement of the anisotropic fracture toughness of 2D Ta2NiSe5 by microelectromechanical system-based tensile tests. Our findings reveal a significant in-plane anisotropic ratio (∼3.0), accounting for crystal orientation-dependent crack paths. As the thickness increases, we observe an intriguing intraplanar-to-interplanar transition of fracture along the a-axis, manifesting as stepwise crack features attributed to interlayer slippage. In contrast, ruptures along the c-axis surprisingly exhibit persistent straightness and smoothness regardless of thickness, owing to the robust interlayer shear resistance. Our work affords a promising avenue for the construction of future electronics based on nanoribbons with atomically sharp edges.

Rapid evaluation of trade-offs between strength and impact toughness of wheel steels by instrumented spherical indentation test

In this paper, an improved size-independent instrumented spherical indentation test (ISIT) method was proposed to determine tensile strength and fracture toughness on wheel rims of two different types of wheel steels. Additionally, Charpy impact specimens were taken from three specific positions on the wheel rims and subjected to standard Charpy impact testing. The resulting Charpy impact energy (CVN) were matched with the fracture toughness measured by ISIT (KIc,IT) at the same positions. There was a clear linear correlation between CVN and the square of KIc,IT, which was consistent with the established relationship between both properties as determined by conventional Charpy impact and compact tension tests. Therefore, we expanded the ISIT method to study the impact of cooling rates on the variation of yield strength and Charpy impact energy across the wheel rim. To accomplish this, we designated additional testing points on the wheel rim using the ISIT method. The investigation found trade-offs between strength and impact toughness of wheel steels under different cooling rates. The above findings demonstrated the effectiveness of using the ISIT method, which facilitated high-resolution fracture toughness measurement, to establish accurate correlation between CVN and fracture toughness of wheel steels. Moreover, this ISIT method can be employed to regulate heat treatment of material properties to meet specific application requirements.

Unveiling the Intricacies of a Ductile‐Phase Toughened Intermetallic: An In‐Depth Exploration of a Eutectic Mo–Si–Ti Alloy and its Mechanical Behavior

Herein, the development of directional solidification for a novel high‐temperature Mo–20Si–52.8Ti (at%) ternary alloy using a modified Bridgeman type apparatus is presented. The resulting alloy exhibits a microstructure consisting of a body‐centered cubic solid solution (BCCss) and a hexagonal silicide (Ti,Mo)5Si3 with approximate volume fractions of 50% for each phase. The phases exhibit a crystallographic orientation relationship with and . Different solidification velocities are imposed, which reveal an inverse relationship to the lamellar spacing according to a Jackson–Hunt type scaling. Mechanical characterization using Vickers indentation demonstrates that the BCCss accommodates plasticity through dislocation motion, while the silicide phase exhibits high hardness and brittleness, serving as a crack initiation site. Crack propagation is arrested and deflected at the interface to the BCCss. Fracture toughness measurements via indentation yield a fracture toughness of 3.7 MPa√m for the silicide, somewhat higher than previously reported values for Nb‐, Mo‐, and Cr‐based silicides at room temperature. The directionally solidified specimens show an enhanced fracture toughness attributed to a greater BCCss length scale; thus, combining the ductile and hard phases results in a ductile‐phase toughened intermetallic composite. The findings open up new possibilities for the design of advanced intermetallic composites with improved toughness performance.

Micro‐scale mechanical properties of surface layer in ion‐exchanged glass

AbstractToday, ion‐exchanged or chemically strengthened glass is ubiquitously used all over the world, typically for scratch‐free display cover of personal mobile phones. It has been known that the strengthening is due to developments of high compressive stress in the surface layer by its internally constrained dilation . We investigated micro‐scale mechanical properties of the surface layer itself of a Na+‐to‐K+ ion‐exchanged soda lime silicate glass, revealing that the strength of the layer increased nearly 40% by the ion‐exchange with little compression effect from the interior. The fracture toughness, however, was comparable before and after the ion‐exchange, indicating that the ion‐exchange reduces the crack size of the fracture origin. Theoretical estimates of fracture surface energies verified the measured fracture toughness. It was presumed that the observed crack‐size reduction is due to crack healing effects enhanced by the ion‐exchange.

New method for determining the mode I fracture toughness of shotcrete: edge-notched partial disc test

The fracture toughness (mode I) of shotcrete specimens was obtained using edge-notched partial disc (ENPD) specimens. Notched Brazilian discs (NBDs) were also tested to validate the results of the ENPD experiments. Numerical analysis was also conducted on the ENPD results to compare the measured and numerically obtained fracture toughness values. The notch lengths in the ENPD specimens were 15, 30, 45 and 60 mm, while the notch lengths in the NBD specimens were 10, 20, 30, 40, 50 and 60 mm. It was found that the flat joint model accurately predicted the potential crack growth path and crack initiation stress as compared with the experimental results. It was also deduced that the fracture toughness was roughly the same with an increase in the notch length. The tensile strength (σt) and fracture toughness (KIC) of the shotcrete specimens were found to be meaningfully correlated (σt = 7.92KIC). The ENPD tests yielded the lowest fracture toughness values because of the pure tensile stress distribution on the failure surface. It was also found that the derived fracture extension patterns from the laboratory investigations were in acceptable agreement with the outputs of numerical simulations.

Accurate measurement of mechanical properties of soft materials by introducing transition layers into test samples

Soft materials loaded by conventional clamps are susceptible to damage caused by local stress concentrations, which leads to inaccurate measurement of strength, toughness, and other important properties. Herein, we propose a facile approach to solve the problem by introducing transition layers to test samples. The effectiveness of the proposed method is demonstrated by experiments and numerical analyses. It is found that the introduction of transition layers can stably and accurately reveal the stress-stretch relationship of soft materials by using the conventional hard clamps. What is more, the transition layers have almost no effect on the fracture toughness measurement for pure soft material samples and those with transition layers. These results should be of great significance for the measurement of various materials, and can serve as a design guidance of various soft material structures in blooming applications.

Critical evaluation of SEVNP-method to measure fracture toughness of ceramic thin plates

The fracture toughness of ceramic substrates made of alumina, ZTA and silicon nitride was determined using the single edge V-notched plate method. In order to make a statement about the measurement precision, not only 5, as required by the standard, but 30 specimens per material type were tested. In addition, a much more practical and less error-prone holding and testing device for the ceramic plates was used, which is presented in the paper. For all materials, including the comparatively fine-grained ZTA, the material-typical KIc values could be determined. However, the range of variation of the measured values is comparatively high with coefficients of variance up to 7.8 %. It is shown that a standard-compliant test on Al2O3 substrates can exhibit a fluctuation range of 0.5 MPa m0.5 without the material showing any differences. It is important to be aware of this when using the method. Especially for quality assurance, where deviations in the range of a few tenths of MPa∙m0.5 are considered problematic, the method should therefore be regarded as critical. An increase in the number of specimens should be considered, as well as the inclusion of a statement on the precision of the measurement method in the standard.

Assessment of bovine cortical bone fracture behavior using impact microindentation as a surrogate of fracture toughness.

The fracture behavior of bone is critically important for evaluating its mechanical competence and ability to resist fractures. Fracture toughness is an intrinsic material property that quantifies a material's ability to withstand crack propagation under controlled conditions. However, properly conducting fracture toughness testing requires the access to calibrated mechanical load frames and the destructive testing of bone samples, and therefore fracture toughness tests are clinically impractical. Impact microindentation mimicks certain aspects of fracture toughness measurements, but its relationship with fracture toughness remains unknown. In this study, we aimed to compare measurements of notched fracture toughness and impact microindentation in fresh and boiled bovine bone. Skeletally mature bovine bone specimens (n = 48) were prepared, and half of them were boiled to denature the organic matrix, while the other half remained preserved in frozen conditions. All samples underwent a notched fracture toughness test to determine their resistance to crack initiation (KIC) and an impact microindentation test using the OsteoProbe to obtain the Bone Material Strength index (BMSi). Boiling the bone samples increased the denatured collagen content, while mineral density and porosity remained unaffected. The boiled bones also showed significant reduction in both KIC (P < .0001) and the average BMSi (P < .0001), leading to impaired resistance of bone to crack propagation. Remarkably, the average BMSi exhibited a high correlation with KIC (r = 0.86; P < .001). A ranked order difference analysis confirmed the excellent agreement between the 2 measures. This study provides the first evidence that impact microindentation could serve as a surrogate measure for bone fracture behavior. The potential of impact microindentation to assess bone fracture resistance with minimal sample disruption could offer valuable insights into bone health without the need for cumbersome testing equipment and sample destruction.

Application of Ultrasound in Fracture Toughness Measurement of GIS Epoxy Insulators

Application of Ultrasound in Fracture Toughness Measurement of GIS Epoxy Insulators

Dynamic fracture toughness measurement of graphite material considering inertial effect

Graphite materials are widely used as moderators, reflectors, and core structural materials in high-temperature gas-cooled reactors. During operation, graphite materials in nuclear reactors may be subjected to dynamic loads caused by earthquakes or other factors. Therefore, the dynamic fracture properties of graphite materials need to be investigated. In this study, dynamic three-point bending drop hammer impact experiments were conducted on IG-11 graphite specimens with precracks to measure their dynamic fracture toughness under medium- and low-speed impacts using a digital image correlation method. Traditional methods for measuring fracture toughness based on ASTM standards do not consider the influence of the inertial effect, leading to unreasonably large measurement results. To address this disadvantage, the dynamic fracture toughness of IG-11 graphite was evaluated using a spring–mass model that considers the inertial effects of materials. Results indicated that the measured fracture toughness was significantly lower than that obtained based on the ASTM standard and increased almost linearly with increasing impact velocity. The toughening mechanism was analysed by observing the fracture morphology of the samples using a scanning electron microscope. The results showed that as the impact velocity increased, the fracture mode of the graphite samples gradually changed from intergranular to transgranular fracture, and the samples absorbed sufficient energy, resulting in a noticeable toughening phenomenon.

Interlaminar and translaminar fracture toughness of 3D‐printed continuous fiber reinforced composites: A review and prospect

AbstractFracture toughness is a critical parameter in the evaluation of a component's structural integrity and damage tolerance. For 3D‐printed continuous fiber reinforced composites (CFRCs) based on the fused deposition modeling (FDM) technique, the fracture behavior differed from that of composites manufactured by traditional processes due to the presence of voids and interfaces at different scales, receiving significant attention. Recently published research attempted to apply various testing standards (for other materials) to investigate the fracture behavior of 3D‐printed CFRCs. In these results, the fracture toughness of CFRCs was influenced by the manufacturing parameters dependent on structural porosity and interfacial bonding quality. This paper reviewed fracture toughness measurement standards compatible with CFRCs, factors influencing fracture behavior, and methods improving the fracture toughness of CFRCs. Lastly, this review explored limitations in current studies focused on fracture toughness measurement of 3D‐printed CFRCs and future perspectives for the fabrication of 3D‐printed CFRCs with improved fracture toughness.Highlights This review focuses on fracture toughness measurement standards compatible with 3D‐printed continuous fiber reinforced composites (CFRCs). The manufacturing parameters influencing fracture behavior and methods improving fracture toughness were summarized. This review explores limitations in current studies focused on fracture toughness measurement of 3D‐printed CFRCs.

Small-scale measurement of the transition in fracture behavior of marine sediments

Bubbles grow and burrows extend through cohesive, muddy marine sediments by fracture. In contrast, sands are non-cohesive, granular materials. Natural sediments comprised of heterogeneous mixtures of muds and sands are common in coastal areas and provide important habitat for infaunal animals. To explore the transition from cohesive to non-cohesive mechanical behavior of natural sediments, we modified a probe designed for measuring fracture toughness (KIc). The helical probe is rotated and translated into sediment to grip a plug of sediment, then translated upward to break off the plug while force is measured. Fracture toughness is calculated from the peak net force. The probe shows clearly distinct results in muddier sediments, in which fracture occurs, and in sandier sediments, in which no fracture occurs. The modified probe is limited to near-surface sediments, but it provides a novel method for distinguishing cohesive sediments with tensile strength from non-cohesive sediments on scales relevant for burrowing animals or bubble growth. This measurement allows for comparison of surface and subsurface cohesion and for assessing how tensile strength depends on other properties of sediments.

In-situ fracture toughness measurement of multilayer graphene

Understanding the fracture and failure properties of graphene is a critical step towards integrating graphene into electromechanical devices and flexible electronics. However, the exact fracture properties of graphene are hard to measure due to the atomic level thickness of graphene and its brittle nature which tends to cause it to break before accurate measurements of its properties can be made. In this paper, a method is presented for measuring the fracture properties of graphene with varying number of layers. This method utilizes a custom designed MEMS tensile tester and an effective and repeatable graphene transfer process that enables the transfer of patterned graphene structures onto MEMS devices. The fracture properties of graphene samples with different numbers of graphene layers were tested and the critical strain energy release rate of fracture was measured to be 22 J/m2, 71.3 J/m2, and 145 J/m2 for ∼ 7 layer, ∼12 layer, and ∼ 15 layer graphene, respectively. These results show that the fracture toughness of the graphene increases as the number of layers increases. This is likely due to the need for brittle cracks to propagate through multiple, weakly connected layers of the graphene for ultimate failure to occur in the graphene.

Edge notched disc test for evaluation of mode-I fracture toughness of brittle material

In the current investigation, the fracture toughness of plexiglass specimens was accurately measured employing Edge Notched Disc (END) type specimens. Besides, the Notched Brazilian Discs (NBD) were used in order to validate the results of the conducted END experiments. Moreover, a numerical analysis was carried out on the END tests to verify the accuracy of the measured fracture toughness values. In the first stage, the splitting tensile test was used to accurately calibrate the micro-scale properties of the Flat Joint (FJ) model. In the second phase, the numerical simulation of END and NBD models was created, in which the notch lengths in END and NBD were set to 5, 15, 25, 40 and 10, 20, 30, 40, 50, 60 in millimeters, respectively. According to the obtained results, the considered FJ model is capable of making an accurate determination of the potential crack growth path and crack initiation stress like the experimentally obtained results for plexiglass specimens. It was also derived that lengthening the notch could not have a considerable effect on the fracture toughness of the models. The Tensile stress distribution on the failure surface of END test could be the main cause of the lower fracture toughness levels of END test. Besides, the same fracture extension patterns were seen from the experimental investigations and numerical simulations.

Synthesis, microstructure, physical and mechanical properties of phase-pure entropy-enhanced (Nb0.8Ti0.05Ta0.05V0.05M0.05)4AlC3 (M = Hf, Zr) ceramics

The first 413-phase entropy-enhanced (Nb0.8Ti0.05Ta0.05V0.05M0.05)4AlC3 (M = Hf, Zr) (EEMAXHf and EEMAXZr) ceramics were successfully consolidated by spark plasma sintering (SPS) using Nb, Ti, Ta, V, Zr, Hf, Al and graphite as initial materials. The formation of solid solution with five transition metals at the M sites of hexagonal M4AlC3 unit cell was confirmed by elemental analyses. Compared with pure Nb4AlC3, both the electrical and thermal conductivities of the entropy-enhanced ceramics showed a slight decrease, which is attributed to the lattice distortion and the increasing lattice defects that prevents the transfer of electrons and phonons. On the other hand, the mechanical properties of entropy-enhanced ceramics were greatly enhanced compared to pure Nb4AlC3. The measured fracture toughness of EEMAXHf and EEMAXZr ceramics were 8.2 MPa·m1/2 and 10.0 MPa·m1/2, respectively, which were increased by 18.8% and 44.9% compared to Nb4AlC3. The compressive strength of EEMAXHf and EEMAXZr ceramics were 987 MPa and 1187 MPa, respectively, being 92.0% and 130.9% higher than that of Nb4AlC3, respectively. EEMAXHf and EEMAXZr ceramics also possessed the higher Vickers hardness of 6.8 GPa and 7.4 GPa, respectively.