Improved Fracture Toughness Research Articles

Effect of Sr2Nb2O7 addition on fracture toughness and thermal conductivity of Y2O3-stabilized HfO2 ceramics

Low fracture toughness limits the application of cubic-phase HfO2 based ceramics as a novel thermal barrier coating material. In this study, Sr2Nb2O7 was introduced into the 24 mol% Y2O3-stabilized HfO2 (YSH24) to improve the fracture toughness. A series of dual-phase composite ceramics with a formula of xSr2Nb2O7/(1-x)YSH24 (x = 0, 0.01, 0.02) were prepared by solid-state sintering. The effect of Sr2Nb2O7 addition on microstructure, fracture toughness, and thermal conductivity of the dual-phase composite ceramics was investigated. The results showed that the perovskite (P) phase with a simple cubic structure of Nb2O5-doped SrHfO3 was generated after Sr2Nb2O7 was introduced. The compositions of x = 0.01 and 0.02 consisted of a mixture of the matrix fluorite (F) phase and a small amount of layer-like P phase. The maximum fracture toughness of 2.47 MPa m1/2 was achieved for xSr2Nb2O7/(1-x)YSH24 dual-phase composite ceramics when x = 0.01, which is approximately 98% higher than that of YSH24. It was surprising that the thermal conductivity of the x = 0.01 composition was as low as 1.63 W m−1 K−1 at 1200 °C, and the radiative thermal conductivity was reduced by 86% compared to YSH24. The findings should provide insight into the simultaneous improvement of fracture toughness and thermal conductivity of ceramic materials and contribute additional assistance in selecting the materials for advanced thermal barrier coatings.

Effect of transition metal carbides on mechanical properties of polycrystalline diamond by HPHT sintering

In this study, W-MeC-Si/PCD (Me = Ti, Zr, Hf, V, Nb and Ta) composite materials were prepared via high pressure and high temperature (HPHT, 5.5 GPa and 1600 °C) sintering, the effect of the addition of transition metal carbides on the micromorphology, phase composition and mechanical properties of PCD has been systematically studied, and various mechanical data of W-TaC-Si/PCD have been provided. The results show that the addition of MeC is conducive to the improvement of the mechanical properties of PCD. The sample with TaC has the best mechanical properties, and its flexural strength, relative density, hardness and fracture toughness reached 743.5 MPa, 99.75%, 63.4 GPa and 8.1 MPa m1/2, respectively. Finally, the toughening mechanism of W-MeC-Si/PCD is also discussed. The improvement of fracture toughness can be attributed to the change of fracture mode and the pinning effect of MeC dispersion in the binder.

Mechanical properties and indentation-induced phase transformation in 4H–SiC implanted by hydrogen ions

This paper aims to improve machining efficiency, suppress surface cracking, and reduce subsurface damage of silicon carbide (SiC). Hydrogen ions were implanted into SiC to study mechanical properties at nano and macro scales. Nanoindentation experiments were conducted using a Berkovich indenter. Firstly, the effect of ion implantation on the load-displacement curves at different indentation depths was investigated using molecular dynamics (MD) simulations. Elastic-plasticity at nanoscale was analyzed, and the values of material properties were obtained. Secondly, variability of surface morphology, phase transformation, and coordination number induced by nanoindentation with and without ion implantation was evaluated. Although ion implantation induced damage to the SiC model, the damage after nanoindentation was lower than that without ion implantation. Additionally, nanoindentation experiments were performed for small loads and high loads, respectively. The small load experiments were employed to derive material properties of the ion-implanted SiC. Improvement mechanisms of ion implantation on crack extension, fracture toughness, and elastic recovery rate were investigated under the high-load experiments. The results indicate that the amorphous structure induced by ion implantation can successfully prevent crack propagation and improve fracture toughness. The modification technology of SiC by ion implantation significantly improves the machining efficiency and the non-damage of its surface and subsurface.

Mechanical and thermal shock properties of Cf/SiBCN composite: Effect of sintering densification and fiber coating

AbstractIn this work, resin‐derived carbon coating was prepared on carbon fibers by polymer impregnation pyrolysis method, then silicoboron carbonitride powder was prepared by mechanical alloying, and finally carbon fiber‐reinforced silicoboron carbonitride composites were prepared by hot‐pressing process. The effects of sintering densification and fiber coating on microstructure, mechanical properties, thermal shock resistance, and failure mechanisms of the composites were studied. Fiber bridging hinders the sintering densification, causing more defects in fiber‐dense area and lower strength. However, higher sintering temperature (1800–2000°C) can improve mechanical properties significantly, including bending strength, vickers hardness, and elastic module, because further sintering densification enhances matrix strength and fiber/matrix bonding strength, while the change of fracture toughness is not obvious (2.24–2.38 MPa·m1/2) due to counteraction of higher debonding resistance and less pull‐out length. However, fiber coating improves fracture toughness greatly via protecting carbon fibers from chemical corrosion and damage of thermal stress and external stress. Due to lower coefficient of thermal expansion, lower fiber loading ratio, less stress concentration at the fiber/matrix interface, and better defect healing effect, lower sintering temperature favors thermal shock resistance of composites, and thermal shock recession mechanisms are the damage of interface.

Ultratough conductive graphene/alumina nanocomposites

High brittleness is one of the Achilles’ heels that limit wider application of structural ceramics. The toughness of ceramic matrix can be improved by introducing nano reinforcements randomly, however, its improvement is quite limited. Here, we propose a novel layer-by-layer stacking structure (graphene/alumina) by a molecule-level assembling method, in which the graphene nanolayers orientation could be easily tuned in ceramic matrix. As a result, the layer-by-layer ceramic nanocomposite has a dramatically improved fracture toughness of 13.71 MPa m1/2, ∼ 3.2 times of the monolithic alumina (∼4.34 MPa m1/2). At the same time, the layer-by-layer ceramic nanocomposite shows excellent electrical conductivity of 630 S/m. It is interesting that the electrical conductivity is highly sensitive to the fracture crack, which could be used as artificial intelligent helmet with self-structural integrity-monitoring and fast thermal response capabilities. This strategy paves a facile way to prepare the ultratough ceramic nanocomposites with multifunctional properties.

Fracture Behavior of Polyrotaxane-Toughened Poly(Methyl Methacrylate).

The fracture behavior of polyrotaxane (PR)-modified poly(methyl methacrylate) (PMMA) was investigated. PR is a supramolecule with rings threaded onto a linear backbone chain, which is capped by bulky end groups to prevent the rings from de-threading. The ring structure is α-cyclodextrin (CD), and it can be functionalized to enhance its affinity with the hosting polymer matrix. Adding only 1 wt % of PR containing methacrylate functional groups (mPR) at the terminal of some of the polycaprolactone-grafted chains on CD promotes massive crazing, resulting in a significant improvement in fracture toughness while maintaining the modulus and transparency of the PMMA matrix. Dynamic mechanical analysis and atomic force microscopy studies reveal that mPR strongly interact with PMMA, leading to higher molecular mobility and enhanced molecular cooperativity during deformation. This molecular cooperativity may be responsible for the formation of massive crazing in a PMMA matrix, which leads to greatly improved fracture toughness.

Jig-less end-effector system for automating exhausting composite fuselage assembly tasks

In this study, the jig-less end-effector system developed to assemble components of a full-scale multifunctional integrated composite thermoplastic lower fuselage section is tested and validated. To offset the environmental impact of higher volume of air transport, the aviation industry wants to design lighter and more environmentally friendly aircraft. To achieve this, there is a need to exploit novel materials and technologies. Advanced thermoplastic composites provide an excellent material option thanks to their weldability, low density, low overall production cost, improved fracture toughness and recyclability. However, to fully appreciate their capabilities and benefits, new manufacturing approaches and techniques are needed. Hence, projects such as TCTool, “innovative tooling, end-effector development and industrialisation for welding of thermoplastic components”, aim to develop innovative tooling and end-effector systems for the assembly of a multifunctional thermoplastic fuselage. This study presents the development, operation, and testing of the jig-less end-effector system used in the TCTool project for picking, placing, and temporary welding and fixing fuselage’s clips and stringers.

Assessing shear, tensile and fracture properties of macroporous nanocomposites using the Arcan test

Macroporous polymers and nanocomposites remain popular materials due to their low densities and unique interconnected pore structures. The determination of mechanical properties in relevant loading conditions, such as shear, remains largely underexplored due to the comparative complexity of such analysis compared to simple compression tests. We investigated the use of the Arcan test to induce uniform biaxial stress in such materials and provide insight into the effect of porosity and silica particle reinforcement on shear properties and fracture toughness of high/medium internal phase emulsion templated macroporous polymer nanocomposites, called polyHIPE/MIPEs. Increasing foam densities were associated with greatly improved tensile and shear properties. Improvements in shear strength and modulus were also noted in polyMIPEs reinforced with as little as 5 wt.% silica nanoparticles, although any increase in tensile properties was lacking. Fracture toughness was investigated using an assumed Poisson's ratio. Decreasing the porosity of macroporous polymers and increasing particle loading resulted in considerable improvements in fracture toughness. These results illustrated the potential of the Arcan test as a simple and effective method for investigating otherwise rarely reported loading conditions in macroporous polymers and nanocomposites.

Graphene dispersed by pyrene‐terminated polyethylene glycol for reinforced epoxy composites

AbstractIncorporating graphene into epoxy composites is a facile strategy to improve their mechanical performances. However, the uniform and reliable dispersion of graphene within epoxy matrix remains a large challenge owing to the intrinsic difference in chemical properties. Here, pyrene‐functionalized polyethylene glycol (Py‐PEG‐Py) was easily synthesized and utilized as a dispersant to homogenize graphene. The effect of graphene loading on mechanical performances of reinforced epoxy composites was also investigated. As 0.01 to 0.05 wt% of graphene were incorporated in epoxy resin, the resulted composites exhibited excellent improvement in flexural strength and fracture toughness compared with neat epoxy. Typically, the flexural strength increased from 63.57 ± 1.5 MPa for neat epoxy to 108.36 ± 1.9 MPa (about 70.5%) for composite, and the fracture toughness (KIC) increased from 1.25 MPa m1/2 for neat epoxy to 2.15 MPa m1/2 (around 72%) for composite modified with only 0.04 wt% of graphene and 4 wt% of Py‐PEG‐Py. Crack pinning caused by well dispersed graphene was contributed to enhancement of the fracture toughness. This work reveals a tiny amount graphene can also largely improve the mechanical properties of composites, providing a feasible approach to disperse graphene and fabricate high performance graphene composites.

Effect of Y2O3 content on the microstructural characteristics and the mechanical and thermal properties of Yb-doped SiAlON ceramics

SiAlON ceramics are primarily employed in ceramic cutting tools, which exploit their stable physical properties in high-temperature cutting environments due to their excellent mechanical properties. Here, Yb/Y-co-doped SiAlON ceramics are prepared by adding Yb and Y rare-earth (RE) ions in the RExSi12-(m+n)Alm+nOnN16-n (m = 0.4, n = 1.0) composition. Yb2O3, the RE oxide, is the main sintering additive. For REx composition design with (Yb1-y + Yy)x, Yb2O3 is replaced by Y2O3 (y = 0.00, 0.25, 0.50, 0.75, 1.00). While Yb2O3 has excellent high-temperature stability, it is limited by its microstructural characteristics, that is, the β-SiAlON morphology and limited fracture toughness due to the small cation sizes. Thus, the changes in the above properties are investigated for various Y2O3 additive contents substituting for Yb2O3. The average grain width decreases, and the equiaxed β-SiAlON grains are elongated with increasing Y2O3 content. Regarding the mechanical properties, the hardness and fracture toughness are evaluated using the indentation fracture method. The hardness decreases with increasing Y2O3 content; however, the fracture toughness exhibits a significant increase (∼53.6%) from 4.6 to 7.0 MPa⋅m1/2. Regarding crack propagation, intergranular fracture is mainly observed in the Yb/Y-co-doped SiAlON ceramics, whereas transgranular fracture is primarily observed in the Yb-single-doped SiAlON ceramic. Y2O3 substitution increases the α/β-SiAlON phase ratio, and the grain boundary phase exhibits increasing vitrification with increasing Y2O3 content. Moreover, the thermal properties of the Yb/Y-co-doped compositions are analyzed and discussed regarding intrinsic properties such as phonon scattering. The microstructural characteristics and improved fracture toughness derived from the Yb/Y-co-doped system designed in this study suggest considerable potential for the future composition design of ceramic cutting tools.

Single-phase rare-earth high-entropy zirconates with superior thermal and mechanical properties

Emerging of high-entropy ceramics has brought new opportunities for designing and optimizing materials with desired properties. In the present work, high-entropy rare-earth zirconates (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 and (Yb0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 are designed and synthesized. Both high-entropy ceramics exhibit a single pyrochlore structure with excellent phase stability at 1600 °C. In addition, the Yb-containing system possesses a high coefficient of thermal expansion (10.52 × 10−6 K-1, RT∼1500 °C) and low thermal conductivity (1.003 W·m-1 K-1, 1500 °C), as well as excellent sintering resistance. Particularly, the Yb-containing system has significantly improved fracture toughness (1.80 MPa·mm1/2) when compared to that of lanthanum zirconate (1.38 MPa·mm1/2), making it a promising material for thermal barrier coatings (TBCs) applications. The present work indicates that the high-entropy design can be applied for further optimization of the comprehensive properties of the TBCs materials.

High-temperature mechanical properties of ZrB2/SiC/Si3N4 ceramic tool materials with dual composite architectures

ZrB 2 /SiC ceramic granules with uniform a size and spherical shape were prepared by spray drying technology. ZrB 2 /SiC ceramic granules were added into ceramic matrix, and ZrB 2 /SiC/Si 3 N 4 ceramic tool materials with double composite architectures were prepared by spark plasma sintering technology. The mechanical properties from room temperature to 1100°C were measured. The hardness, flexural strength and fracture toughness at room temperature are 18.0 GPa, 589.0 MPa and 5.93 MPa m 1/2 , respectively. While the fracture toughness at 1100°C reach 6.1 MPa m 1/2 increase by 74.3% compared with the ceramic tool material without double composite architectures. At high temperature, the improvement of fracture toughness is due to the addition of ZrB 2 /SiC ceramic granules that deflect the cracks and propagates along the boundary between the granules and the matrix, which greatly increase the path during the crack propagation process, thereby improving the fracture toughness of the ceramic tool material.

Extraordinary toughening enhancement in nonstoichiometric vanadium carbide

• VC 1- x composites have a quasi-monophasic fcc structure with coherent precipitates. • A doubling of fracture toughness is obtained in VC 1- x without any second phase. • VC 1- x show an unadulterated transgranular fracture feature, leading to toughening. • Coherent toughening and amorphous bridging toughening make a prodigious toughening. Improving fracture toughness, which has gone through decades, is a long-standing topic and is particularly important for safety-critical applications. In refractory transition metal carbides (RTMCs), remarkable toughening is usually achieved by adding metallic binders, however, resulting in a drastic deterioration of hardness and thermal stability. Here, we report a novel self-toughening strategy for synthesizing high-toughness RTMCs. Using mechanical alloying (MA) and spark plasma sintering (SPS), we synthesized nonstoichiometric VC 1- x (0.5 ≤ 1- x ≤ 0.6) with a quasi-monophasic microstructure containing a carbon-rich matrix and carbon-poor precipitates. Significantly, the VC 0.5 sintered at 1400°C shows a good trade-off of high hardness of 20.5±0.5 GPa and fracture toughness of 7.1±0.2 MPa m 1/2 . The fracture toughness of VC 0.5 increases by more than 100% accompanied by 7% hardness loss, compared with that of stoichiometric VC. The microstructure characterization and fracture behavior analysis demonstrate that the extraordinary toughening enhancement is attributed to a self-toughening strategy combined with coherency toughening and amorphous bridging toughening, which may offer an efficient pathway for developing high-performance structural ceramics.

Experimental and numerical study on Mode I and Mode II interfacial fracture toughness of co-cured steel-CFRP hybrid composites

Interfacial fracture toughness of co-cured steel-carbon fiber reinforced plastic (CFRP) hybrid composites were investigated in this paper. To illustrate the effect of the interlayer on the fracture toughness, steel-CFRP hybrid composites were prepared by different manufacturing processes based on steel surface treatment (abrasion or grit blasting) and adhesive-bonding process. The experimental results of double cantilever beam (DCB) tests and end notched flexure (ENF) tests demonstrate that, the Mode I and Mode II interfacial fracture toughness of the hybrid composites can be improved by using a grit blasting surface treatment on steel and introducing an adhesive layer at the steel/CFRP interface. The hybrid composites mainly show fiber/epoxy interfacial failure of CFRP under Mode I loading conditions, while it mostly exhibits adhesive failure of steel/CFRP interface under Mode II loading condition. Moreover, the interfacial tensile strengths of steel-CFRP hybrid composites are predicted by finite element analysis, and both experimental and numerical results confirm the improvement of interfacial fracture toughness.

Evaluation of fracture toughness in graphene-based cementitious nanocomposites via electrical impedance

The present work investigates experimentally the improvement of fracture toughness and electrical conductivity in the hardened cementitious matrix after the addition of graphene nanoplatelets (GnPs), at four different concentrations varying between 0.05 wt% and 0.4 wt% per cement. The experimental investigation is conducted through flexural and electrical impedance spectroscopy (EIS) tests on prismatic specimens. The study is oriented towards establishing a correlation between the electrical response of the nanocomposites under alternate current and the exhibited fracture toughness values (KIc). The flexural test results show that the incorporation of the GnPs in the matrix can increase the average KIc values and can decrease the average resistivity (ρ) when compared with the reference matrix. The maximum increase in KIc (+ 29 %), and the decrease in ρ (- 68 %) were found in the mixture with the lowest amount (0.05 wt%) of GnPs. Moreover, the comparison of the ρ values and the KIc values, as a function of the GnPs concentration, reveals a reverse relation between the two parameters (KIc and ρ). The functional correlation between these parameters was also confirmed by linear regression analysis, resulting from the experimental data fitting. The analysis provides evidence that EIS can be used as a non-destructive tool to assess the fracture toughness of cementitious nanocomposites.

Effect of interleaved fiber mats on the manufacturability and mechanical properties of carbon/epoxy composites

Fracture toughness is a critical property of composites, especially in the case of materials manufactured by liquid composite moulding processes because of the brittleness of the thermoset polymeric matrix. The use of thermoplastic interleaving veils can improve the fracture behaviour affecting only slightly other mechanical properties. Also, the veils can affect the manufacturability of the composites. This work presents a study of the influence of commercial and homemade veils on the manufacturability process and mechanical properties of carbon fibre reinforced composites. Results obtained show a significant improvement of fracture toughness without affecting manufactubarility.

Improvement of fracture toughness based on auxetic patterns fabricated by metallic extrusion in 3D printing

This study aims to evaluate the effect of auxetic patterns on the fracture toughness of 3D printed metallic specimens manufactured by metallic extrusion of 17-4PH stainless steel. When it is pulled in the horizontal direction, the auxetic material expands in the vertical axis. This occurs since it has a negative Poisson's ratio. In this work auxetic cell structures have been employed in fracture mechanics characterization of Single Edge Notch Bending SENB specimens in order to improve their fracture toughness dynamically during strain.SENB specimens were designed with different cell patterns for fracture toughness assessment. (i) Auxetic pattern at 0° parallel to the notch direction, (ii) at 90° perpendicular to the notch direction and (iii) a conventional honeycomb pattern is defined with equivalent cell density on the specimen surface. Three-point bending tests have been carried out to evaluate the fracture toughness of each specimen. The mechanical performance ratio of specimens was evaluated by dividing the absorbed energy till fracture over the specimen weight. The ratios were also compared to a solid specimen ratio taken as a reference in this study.The results show that all specimens with cell patterns absorb more strain energy than the solid specimen does. Although the solid specimen is the stiffest, both cell patterns (auxetic and honeycomb) have higher performance indices considering the important weight reduction. For around 35 % of reduction in the maximum measured load, the auxetic pattern at 0° can absorb up to 88% more energy compared to the solid specimen. This proves its capability of delaying the crack initiation and enhancing the fracture toughness.

Austempering of PM HSS ASP2030 for improved fracture toughness

This study presents the fracture toughness improvement of the powder metallurgical high speed steel ASP2030 by replacing the martensitic matrix with a bainitic one. For this purpose, the fracture toughness of the steel is compared in both the austempered and marquenched states. The heat treatment of the samples was carried out by austenitizing in the range of 1150–1185 °C followed by either austempering at 235 °C or marquenching processes. Triple tempering at 560 °C for 2 h was also performed on all specimens. Fracture toughness measurements were performed on circumferentially notched tensile specimens. The microstructural features of the specimens were investigated using X-ray diffraction, optical and electron microscopy, and electron backscatter analysis. The results showed that the fracture toughness is sensitive to small changes in austenitizing temperature at both heat treatment conditions. By increasing the austenitizing temperature from 1150 to 1170 °C, the fracture toughness increases and then decreases with a further increase in temperature to 1185 °C. The results suggest that the changes in fracture toughness are due to the simultaneous effects of the volume fraction of undissolved carbides and the lattice microstrain values at different austenitizing temperatures. It can be concluded that the fracture toughness increases when the martensitic matrix is replaced by a bainitic structure. This improvement is related to finer bainitic laths width and also to the larger spacing between the particles of the undissolved carbides.

Binders Used for the Manufacturing of Composite Materials by Liquid Composite Molding.

Binders, or tackifiers, have become widespread in the production of new composite materials by liquid composite molding (LCM) techniques due to their ability to stabilize preforms during laying-up and impregnation, as well as to improve fracture toughness of the obtained composites, which is very important in aviation, automotive, ship manufacturing, etc. Furthermore, they can be used in modern methods of automatic laying of dry fibers into preforms, which significantly reduces the labor cost of the manufacturing process. In this article, we review the existing research from the 1960s of the 20th century to the present days in the field of creation and properties of binders used to bond various layers of preforms in the manufacturing of composite materials by LCM methods to summarize and synthesize knowledge on these issues. Different binders based on epoxy, polyester, and a number of other resins compatible with the corresponding polymer matrices are considered in the article. The influence of binders on the preforming process, various properties of obtained preforms, including compaction, stability, and permeability, as well as the main characteristics of composite materials obtained by various LCM methods and the advantages and disadvantages of this technology have been also highlighted.

Collaborative enhancement of luminous efficacy and fracture toughness based on interface design of Al2O3/YAG:Ce3+ eutectic phosphor ceramic grown by laser floating zone melting

A solid-state light source of directionally solidified Al2O3/Y3Al5O12:Ce3+ (YAG:Ce3+) eutectic phosphor ceramic with superior luminous efficacy (∼116.48 lm/W) and high fracture toughness (∼4.0 MPa·m1/2) is fabricated by laser floating zone melting (LFZM) for white light emitting diodes (WLEDs). The CeO2-doped Al2O3/YAG eutectic ceramic presents improved fracture toughness and prevents from the pore formation as compared to the undoped eutectic ceramic. Impressively, the doping of Ce element and the diffusion coefficient play dominant roles in accelerating the formation of cellular structure and causing the enlargement of the eutectic lamella spacing, respectively. Moreover, the optimal phase interface design act as scattering center and enhance the light conversion efficiency. The maximum luminous efficacy is achieved under the comprehensive investigation of the eutectic structure, phase interface as well as Ce3+ dopant. Moreover, the eutectic phosphor ceramic also exhibits excellent capability of thermal self-managing. The as-designed Al2O3/Y3Al5O12:Ce3+ eutectic phosphor ceramics have great potential as structural and functional materials in the high-brightness application of WLEDs.