Increase In Fracture Toughness Research Articles

Fracture mechanics behavior of coarse-grained MgO–C at room and high temperature

The fracture mechanical properties of coarse-grained MgO–C were investigated in four-point bending tests at single-edge V-notched beams (SEVNB) at room and high temperature. For measurement of the crack mouth opening displacement (CMOD) an optical system was used. Room temperature tests were accompanied with microstructural observations of the crack path with the use of digital image correlation to investigate the mechanisms of crack propagation and to determine the crack length. MgO–C showed stable crack propagation at room temperature as well as up to 1500 °C. Increased fracture toughness KIc and work of fracture were observed in the temperature range from 700 °C to 1200 °C. This behavior was correlated with crack closure, quasi-plastic deformation as well as the activation of visco-plastic creep mechanisms above 1200 °C. Additionally, the results were discussed in terms of Hasselman's thermal shock parameters.

Quench and Tempered Embrittlement of Ultra-High-Strength Steels with Transition Carbides

The effect of tempering after water quenching on the strength and fracture toughness of two steels with chemical compositions of 0.34%C-1.77%Si-1.35Mn-0.56%Cr-0.2%Mo-0.04%Nb-0.03Ti-0.002B and 0.44%C-1.81%Si-1.33%Mn-0.82%Cr-0.28%Mo was examined. The last steel exhibits quenching embrittlement in an as-quenched condition. At a tempering temperature of 280 °C, the precipitation of transition η–Fe2C carbides in martensitic matrix leads to increasing fracture toughness and eliminates quench embrittlement in the steel with 0.44 wt.%C. Tempered martensite embrittlement at 400 °C appears as decreased values of the Charpy V-notch impact energy, ductility and the product of strength and elongation, σB×δ (MPa×%) and is attributed to increased effective grain size for fracture, mainly. The precipitation of boundary cementite takes place at tempering at 500 °C and provides increased ductility and fracture toughness despite a decohesion along carbide/ferrite interfaces. The low severity of TME in Si-rich low-alloy medium carbon steels is attributed to the suppression of boundary cementite precipitation at tempering temperatures ≤400 °C.

Improved mechanical properties of reaction-bonded SiC through in-situ formation of Ti3SiC2

Reaction-bonded SiC is a ceramic with excellent thermal properties, good corrosion resistance and the characteristic of near-net-shape manufacturing. However, the poor fracture toughness of free Si limits the applications of reaction-bonded SiC. In this study, TiC was added to reaction-bonded SiC and reacted with free Si to form Ti3SiC2. The effects of TiC and carbon black on the mechanical properties of reaction-bonded SiC were investigated. The results demonstrated that the in-situ formation of Ti3SiC2 and decrease in the content and size of free Si improved the mechanical properties of reaction-bonded SiC ceramics. The mechanical properties of TiC-added reaction-bonded SiC with 17.5 wt% carbon black were superior to those of TiC-added reaction-bonded SiC with 15 wt% carbon black. Moreover, increasing the TiC content of reaction-bonded SiC with 17.5 wt% carbon black from 0 to 7.5 wt% caused an increase in its bending strength from 183.92 to 424.43 MPa and an increase in fracture toughness from 3.7 to 5.24 MPa m1/2.

Engineered Interleaved Random Glass Fiber Composites Using Additive Manufacturing: Effect of Mat Properties, Resin Chemistry, and Resin-Rich Layer Thickness.

Standard lay-up fabrication of fiber-reinforced composites (FRCs) suffer from poor out-of-plane properties and delamination resistance. While advanced manufacturing techniques (e.g., interleaving, braiding, and z-pinning) increase delamination resistance in FRCs, they typically result in significant fabrication complexity and limitations, increased manufacturing costs, and/or overall stiffness reduction. In this work, we demonstrate the use of facile digital light processing (DLP) technique to additively manufacture (AM) random glass FRCs with engineered interleaves. This work demonstrates how vat photo-polymerization techniques can be used to build composites layer-by-layer with controlled interleaf material, thickness, and placement. Note that this engineering control is almost impossible to achieve with traditional manufacturing techniques. A range of specimens were printed to measure the effect of interleaf thickness and material on tensile/flexural properties as well as fracture toughness. One important observation was the ≈60% increase in interlaminar fracture toughness achieved by using a tough resin material in the interleaf. The comparison between AM and traditionally manufactured specimens via vacuum-assisted resin transfer molding (VARTM) highlighted the limitation of AM techniques in achieving high mat consolidation. In other words, the volume fraction of AM parts is limited by the wet fiber mat process, and engineering solutions are discussed. Overall, this technique offers engineering control of FRC design and fabrication that is not available with traditional methods.

Effect of medium-high temperature conditioning on the mechanical properties of single quartz fibres

This work adopted multi-scale characterizations to investigate the effects of exposure to medium-high temperatures (600–800 °C) on the mechanical properties of single quartz fibres, focusing on whether bulk property changes, i.e., Young’s modulus, density and fracture toughness, occur and can be related to the thermal strength loss, herein quantified as 75% up to 86% with increasing temperature. Investigation of the fracture surfaces through scanning electron microscopy revealed that failure originated from the fibre surface regardless of heat treatment. Neither bulk crystallization was highlighted through X-ray diffraction nor relevant changes or gradients of Young’s modulus and hardness were disclosed over the fibre cross-sections through high-speed nanoindentation mapping. A 9% increase in fracture toughness measured through micro-pillar splitting revealed a slightly improved crack propagation resistance that cannot compensate for the drastic effect responsible for strength reduction, which is discussed in terms of surface-controlled mechanisms involving the development and growth of surface flaws.

Effect of phase transformation of CoCrFeNiAl high-entropy alloy on mechanical properties of WC-CoCrFeNiAl composites

WC-CoCrFeNiAl composites were fabricated via SPS using commercial CoCrFeNiAl and WC powders, with the optimal addition content of CoCrFeNiAl determined. Furthermore, the influence of phase transformation in CoCrFeNiAl high-entropy alloy on the mechanical properties of WC-CoCrFeNiAl composites was investigated. The results indicate that the hysteresis diffusion effect of CoCrFeNiAl HEA can significantly impede the growth of WC grains. Moreover, during the sintering process, a BCC-to-FCC phase transformation occurs in CoCrFeNiAl HEA. The phase transition of HEA can be regulated by adjusting the sintering temperature, resulting in a decrease in hardness and an increase in fracture toughness of WC-CoCrFeNiAl composites as HEAs undergo phase transformation. The Vickers hardness and fracture toughness values of WC-10CoCrFeNiAl composites sintered at 1250 °C are comparable to those of WC-10Co hard alloy, with respective values of 17.64 GPa and 12.3 MPa m1/2.

Fracture toughness anisotropy of Ti17 disk fabricated by through-transus-processed forging and the role of toughening mechanisms

The fracture toughness anisotropy of the Ti17 disk fabricated by through-transus-processed forging was discussed. For the specimens in different directions, the fracture initiation toughness Ki was calculated through the crack tip opening displacement, and the contribution of the crack tip blunting and dimple formation to the fracture initiation toughness was distinguished. Furthermore, the origin resulting in the fracture toughness anisotropy was analyzed according to the energy criterion for fracture. The results reveal that the specimen directions have a significant influence on the global fracture toughness and the crack propagation path. Meanwhile, the fracture initiation toughness occupies a significant portion of the global fracture toughness, and it reflects the ability of the crack tip plasticity. In addition, a schematic illustration is applied to demonstrate the toughening mechanism. This reveals that the different specimen directions result in the crack deflection or shearing off the ligament, leading to increased fracture toughness compared with the weak specimen direction.

Observed reduction in fracture toughness of AlN and AlN–Mo ceramic matrix composites with carbon additive

The fracture toughness of AlN ceramic matrix composites containing carbon (0.5 vol%), yttria (5.0 vol%), and Mo (0.0–4.0 vol%) was evaluated using Vickers indentation. The results of these measurements were compared to a carbon-free, commercial AlN (ST-200 ALN) as well as to AlN and AlN–Mo compositions from the literature. The presence of added carbon was found to correlate with a 21% reduction in fracture toughness of the 0.0 vol% Mo carbon-containing sample, compared to commercial ST-200 AlN, in both the A-orientation (indentation parallel to the pressing direction) and the B-orientation (indentation perpendicular to the pressing direction). Mo additions at small loading fractions (~ 0.25 vol%) were found to exhibit greater-than-expected increases in fracture toughness in the A-orientation, when compared to literature data on AlN–Mo composites. This increase in fracture toughness correlates to the removal of elemental carbon in the AlN matrix through reaction with the Mo additive, forming Mo2C, localized at the Mo particle sites. Further increasing Mo loading was observed to result in generally increasing fracture toughness values, as would be expected from literature data.Graphical abstract

Вязкость разрушения мелкозернистых бетонов, модифицированных минеральными добавками на основе термоактивированных глинистых и карбонатных пород

The use of modifying additives in cement concrete compositions is the most affordable way to improve their technological, physical, and mechanical properties. However, the high cost and territorial limitation of the most effective modifiers (microsilica, metakaolin) production do not meet the growing customer’s demand. The development of mineral additives based on local raw materials will bridge deficit the most common additives, as well as reduce the consumption of Portland cement. The paper presents the results of determining the fracture toughness (crack growth resistance) of fine-grained concretes obtained from self-compacting concrete mixtures modified with thermally activated clay (Nikitskoye deposit, Saransk, Republic of Mordovia) and thermally activated mixture of clay and limestone (Atemar village, Republic of Mordovia). The paper contains the assessment of crack resistance of fine-grained concretes conducted in accordance with GOST 29167-2021 during equilibrium bending tests of type I samples. The paper found a 9 38% increase in fracture toughness of modified fine-grained concretes with increasing binder consumption due to the introduction of mineral additives in the composition under study

Impact of low temperature and NaCl attack on the fracture properties of engineered cementitious composites (ECC)

Engineered cementitious composites (ECC) is a potential reinforcement and repair material for hydraulic structures due to its excellent mechanical properties. However, the fracture properties of ECC under the coupling effect of low temperature and chloride ions attack are still unclear. In this study, fracture experiments were performed on ECC specimens under different working conditions (immersed in water and NaCl solutions for different time (0, 30, 60, and 90d) and undergoing different freeze-thaw cycles (0, 25, 50, and 100 cycles) in NaCl solution) at experimental temperatures of 20 °C and −20 °C. The relationship between the depth of chloride ions erosion and the nominal fracture energy was analyzed. The effect of the coupled of temperature and chloride ions attack on the fracture performance of ECC under different working conditions was investigated. The results showed that: Under the loading condition of −20 °C, the fracture parameters of ECC under NaCl solution immersion were all lower than those of the water immersion group, indicating that extravasated chloride ions negatively impacted ECC's fracture properties. When ECC was loaded at −20 °C, the initiation fracture toughness and unstable fracture toughness of ECC under NaCl solution immersion were lower than those of the water immersion group, but the fracture energy was larger than that of the water immersion group, and when the specimens were immersed for 90 days, the drop in the fracture energy of ECC loaded at −20 °C compared to 20 °C for the NaCl solution immersion group was 59.1% less than 62.7% for the water immersion group; While loading at 20 °C revealed a linearly declining relationship between erosion depth and fracture energy, loading at −20 °C revealed no discernible pattern. The crack initiation fracture toughness of ECC gradually decreased with the increase of the freeze-thaw cycles, but the specimens' crack initiation fracture toughness following NaCl freezing at −20 °C was consistently larger than that at 20 °C. Following 100 freeze-thaw cycles on the specimens, the increase in initiation fracture toughness at −20 °C compared to 20 °C for the water freeze-thaw group was 128.4% less than that of the salt freeze-thaw group, which was 135.8%, while the decrease in fracture energy was 19.9% less than that of the salt freeze-thaw group, which was 46.3%. It shows that the repair effect of ice on ECC after NaCl freeze-thaw cycles is stronger than that of water freezing at the low temperature.

Blending hydrogen in existing natural gas pipelines: Integrity consequences from a fitness for service perspective

Blending hydrogen in existing natural gas pipelines compromises steel integrity because it increases fatigue crack growth, promotes subcritical cracking and decreases fracture toughness. In this regard, several laboratories reported that the fracture toughness measured in a hydrogen containing gaseous atmosphere, KIH, can be 50% or less than KIC, the fracture toughness measured in air. From a pipeline integrity perspective, fracture mechanics predicts that injecting hydrogen in a natural gas pipeline decreases the failure pressure and the size of the critical flaw at a given pressure level. For a pipeline with a given flaw size, as shown in this work, the effect of Hydrogen Embrittlement (HE) in the predicted failure pressure is largest when a failure occurs by a brittle fracture. The HE effect on failure pressure diminishes with a decreasing crack size or increasing fracture toughness. The safety margin after a successful hydrostatic test is reduced and therefore the time between hydrotests should be decreased. In this work, all those effects were quantified using a crack assessment methodology (level 2, API 579-ASME FFS) considering literature values for KIH and KIC reported for an API 5L X52 pipeline steel. To characterize different scenarios, various crack sizes were assumed, including a small crack with a size close to the detection limit of current in-line inspection techniques and a larger crack that represents the largest crack size that could survive a hydrotest to 100% of the steel Specified Minimum Yield Strength (SMYS). The implications of a smaller failure pressure and smaller critical crack size on pipeline integrity are discussed in this paper.

Predicting the Effect of Hardener Composition on the Mechanical and Fracture Properties of Epoxy Resins Using Molecular Modeling

Improving the toughness of brittle epoxy while keeping its high strength-to-weight ratio is challenging, as these two properties work against each other. Fracture processes are difficult to ascertain with experiments, as they occur at nanoscopic lengths and time scales and require higher efficiency than what can be attained with atomistic simulations. To overcome this challenge, we utilize a recently developed chemistry specific coarse-grained model to examine two different hardeners, diamine 4,4′-methylenebis(cyclohexylamine) (PACM) and propylene oxide diamine (Jeffamine), to cure bisphenol A diglycidyl ether (DGEBA) at varying stoichiometries and understand how hardener composition influences the elasticity, yield strength, and fracture toughness of epoxy resins. The results indicate that PACM mainly contributes to the modulus and that long Jeffamine chains increase fracture toughness by making the epoxy ductile, whereas short Jeffamine chains significantly improve the yield strength. Longer Jeffamines also lead to larger voids and the formation of fibrils that carry a significant amount of stress and contribute to toughness. Interestingly, the Ashby plots reveal that epoxies with intermediate-length Jeffamine chains (D800 and D2000) outperform other systems, as the toughness enhancement from flexible Jeffamine chains and the stiffness due to PACM help to overcome the strength–toughness trade-off. Our modeling framework and findings establish a path toward resin design from predictive multiscale models with no empirical input and reveal new insights into the molecular failure mechanisms of epoxy resins.

Methods for increasing fracture toughness of thermosetting polyester polymers with vinyl ester mixtures as raw materials for automotive components

Methods for increasing fracture toughness of thermosetting polyester polymers with vinyl ester mixtures as raw materials for automotive components

Bioinspired fracture toughness enhancement of a fully bio-based epoxy resin

In this work, the concept of fracture toughness improvement via spatial variation of mechanical properties is applied to a composite with 100% bio-based carbon content based on epoxidized linseed oil. By proper selection of the hardener, either citric acid or sebacic acid, the mechanical properties were adjusted in a way that a bio-composite exhibits a stiff-soft-stiff layer architecture. Samples with two different thicknesses of soft interlayer (approx. 0.1 mm and 1.3 mm) were subsequently analyzed regarding their final performance. Specimen characteristics, owing to the curing and manufacturing process, were analyzed by means of local Fourier-transform infrared spectroscopy and differential scanning calorimetry. Fracture mechanics tests were performed to verify if the soft interlayer acts as a crack arrester. The results propose a high chemical compatibility between the used epoxy resins. Embedding a soft thin interlayer into a stiff resin matrix led to an increase in fracture toughness of 13 times, compared to the pure stiff resin. An increase in interlayer thickness led to a further increase in fracture toughness of 24 times. However, the stiffness decreased by 44% and 67%, respectively.

Preparation, aging behavior and interface bonding of cordierite glass-ceramics as veneer porcelain for Si3N4 ceramics

In this work, aiming at developing veneer porcelain for Si3N4 dental ceramics, the cordierite glass-ceramics were prepared via the sintering method and the effects of glass powder particle size and accelerated aging time on the structure and properties of cordierite glass-ceramics were investigated. With the milling time extension, the glass powder particle size decreases and becomes more uniform, the sintering property, bending strength and fracture toughness increase and then decrease. The cordierite glass-ceramics prepared from glass powder with ball milling time of 10 h exhibit the optimum performance which meets the standard requirements for dental ceramics. Accelerated aging test was conducted in distilled water at 134 °C and 2 atm. After accelerated aging for 10 h, the performance of the samples still can meet the standard requirements, suggesting good stability and anti-aging properties. Effects of HF acid etching and oxidation treatment of Si3N4 ceramics on the microstructure, bonding strength and failure mechanism of the cordierite glass-ceramics/Si3N4 ceramics interface were assessed. HF acid etching can effectively improve the surface roughness of Si3N4 ceramics, and the interface exhibits a mixed failure mode with a bonding strength of 8.49 MPa.

Effect of nano-reduced graphene oxide (NRGO) on long-term fracture behavior of Warm Mix Asphalt (WMA)

Recent studies have shown that in addition to ambient temperature, time-dependent factors such as temperature cycles and freeze–thaw cycles (FTCs) affect the speed of creation and growth of low-temperature cracks (LTCs) and intermediate-temperature cracks (ITCs) by creating the aging process in the asphalt mixture. Therefore, it is necessary to study the cracking behavior of asphalt mixtures by considering these factors. In the present study, an attempt was made to evaluate the long-term fracture behavior of Warm Mix Asphalt (WMA) modified with nano-reduced graphene oxide (NRGO) containing vertical and angular cracks under mode I loading conditions at low and intermediate temperatures using Semi-circular bend (SCB) geometry. In order to simulate long-term performance, two conditions of 3 FTC and aging process (for 6 days) were applied to the mixtures with and without NRGO, and the results were compared with samples subjected to 0 FTC (short-term performance). The results showed that adding 0.3% and 0.6% NRGO to the WMA mixture increased fracture toughness (KIC), fracture energy (GF), and stiffness under mode I at low and intermediate temperatures. At +15 °C and for the geometry containing the vertical crack, the NRGO-modified WMA mixture had better flexibility under 3 FTC than 0 FTC; while, the flexibility of the NRGO-modified WMA mixture decreased under aging process. For geometry containing angular crack, flexibility indices improved for NRGO-containing mixtures under 3 FTC and aging process (at +15 °C temperature). On the other hand, the results showed that the addition of NRGO to the WMA mixture, while increasing the maximum load at the moment of fracture, caused a decrease in the displacement at the fractured moment at low and intermediate temperatures. This result was obtained for the mixtures that were conditioned under aging process. However, the results showed that applying 3 FTCs decreased the maximum load at the moment of fracture and increased the displacement at the fractured moment. Finally, the results showed that the reduction of fracture indices was higher for WMA and NRGO-modified WMA mixtures under 3 FTCs (at temperatures ±15 °C); therefore, applying 3 FTC instead of aging process can be used to evaluate the long-term performance of asphalt mixtures.

Investigation on the effect of interface properties on compressive failure behavior of 3D woven composites through micromechanics-based multiscale damage model

In this paper, the effect of interface properties on the compressive failure behavior of 3D woven composites (3DWC) is investigated by incorporating a micromechanics-based multiscale damage model (MMDM). The correlation between the mesoscopic stress of yarns and microscopic stress of constituents is established by defining a stress amplification factor. With the microscopic stresses, the fiber breakage and matrix failure can be separately evaluated at the microscale, without assuming the yarns as transversely isotropic homogeneous materials. Especially, the interfacial debonding between yarns and matrix is also a dominant damage mode within 3DWC. Given that there is still a lack of studies on the influence of interfacial properties on the compressive failure behavior of 3DWC, it is meaningful to perform numerical parametric studies to reveal how the interface properties contribute to the damage mechanisms of 3DWC under compressions. The predicted results indicate that with the increase of interface strengths and fracture toughness, the compressive resistance of 3DWC can be significantly improved, resulting in higher strength and failure strain. Additionally, the studied 3DWCs with weak, medium and strong interfaces exhibit different damage development processes.

Steady-State Crack Growth in Nanostructured Quasi-Brittle Materials Governed by Second Gradient Elastodynamics

The elastodynamic stress field near a crack tip propagating at a constant speed in isotropic quasi-brittle material was investigated, taking into account the strain gradient and inertia gradient effects. An asymptotic solution for a steady-state Mode-I crack was developed within the simplified strain gradient elasticity by using a representation of the general solution in terms of Lamé potentials in the moving framework. It was shown that the derived solution predicts the nonsingular stress state and smooth opening profile for the growing cracks that can be related to the presence of the fracture process zone in the micro-/nanostructured quasi-brittle materials. Note that similar asymptotic solutions have been derived previously only for Mode-III cracks (under antiplane shear loading). Thus, the aim of this study is to show the possibility of analytical assessments on the elastodynamic crack tip fields for in-plane loading within gradient theories. By using the derived solution, we also performed analysis of the angular distribution of stresses and tractions for the moderate speed of cracks. It was shown that the usage of the maximum principal stress criterion within second gradient elastodynamics allows us to describe a directional stability of Mode-I crack growth and an increase in the dynamic fracture toughness with the crack propagation speed that were observed in the experiments with quasi-brittle materials. Therefore, the possibility of the effective application of regularized solutions of strain gradient elasticity for the refined analysis of dynamic fracture processes in the quasi-brittle materials with phenomenological assessments on the cohesive zone effects is shown.

Short- and medium-range structure synergistically control fracture toughness of densified aluminoborate glasses

Methods to improve the fracture toughness of oxide glasses are needed since low fracture toughness is a major bottleneck for their applications. To overcome this, it is critically important to investigate the effect of both short- and medium-range structural features on fracture toughness. Recent work reported a record-high fracture toughness for a bulk lithium aluminoborate glass subjected to hot compression. Here, we further explore the structural origin of this high fracture toughness by subjecting different alkali aluminoborate glasses to hot compression. Through a combination of x-ray total scattering experiments and atomistic simulations, we find that hot compression causes significant changes to both the short- and medium-range order structure of the glasses, e.g., increased coordination numbers (CNs) of network forming species and decreased average size of ring-type structures. To this end, we reveal positive correlations between the pressure-induced increase in fracture toughness and (i) the increase in average CN of network forming species and (ii) the area of the first sharp diffraction peak in the structure factor. Our study thus improves the understanding of which structural features benefit intrinsic toughening of oxide glasses.

Microstructure and mechanical properties of (Zr,Ti)B2–SiC composites obtained by pressureless sintering of ZrB2–SiC–TiO2 powder mixtures

Multiphase ceramics have been highlighted due to the combination of different properties. This work proposes to obtain the multiphase composite of (Zr,Ti)B2–SiC based on the mixture of ZrB2, SiC, and TiO2 sintered without pressure. The effect of TiO2 addition on solid solution formation with ZrB2, densification, microstructure, and mechanical properties was investigated. For this, 2.0 wt% TiO2 was added to ZrB2–SiC composites with 10–30 vol% SiC and processed by reactive pressureless sintering at 2050 °C with a 2 h holding time. Sinterability, crystalline phases, microstructure, Vickers hardness, and indentation fracture toughness of these composites were analyzed and compared to the non-doped ZrB2–SiC samples. The XRD analysis and EDS elemental map images indicated the incorporation of Ti atoms into the ZrB2 crystalline structure with solid solution generation of (Zr,Ti)B2. The addition of TiO2 resulted in matrix grain size refinement and a predominant intergranular fracture mode. The relative densities were not significantly modified with the TiO2 addition, though a higher weight loss was detected after the sample sintering process. The composites doped with TiO2 showed an increase in fracture toughness but exhibited a slightly lower Vickers hardness compared to composites without TiO2 addition.