Significant Improvement In Fracture Toughness Research Articles

Study of synergistic and competitive mechanisms on toughening CFRP composites with intrinsic and extrinsic multiscale interleaves

Delamination damage is a common failure mode of carbon fiber reinforced polymer (CFRP) laminates, which severely limits their application. This paper proposes a novel interlaminar toughening structure based on intrinsic and extrinsic multiscale toughening mechanisms using nanoscale multi-walled carbon nanotubes (MWCNTs) and microscale core-shell rubber (CSR). The resin sample M-0.5/C-8, containing 0.5 wt% MWCNTs and 8 wt% CSR, exhibited significant improvements in fracture toughness, with increases of 183.3 % in KIC, Resin and 412.1 % in GIC, Resin. However, its flexural strength and modulus decreased by 3.1 % and 3.3 %, respectively. Introducing MWCNTs and CSR into the interlayers of the CFRP, the interleaved toughened CFRP laminate exhibited a 123.3 % improvement in the mode-I critical energy release rate (GIC, Comp) and a 79.6 % increase in the mode-II critical energy release rate (GIIC, Comp) during the crack propagation stage, without sacrificing flexural performance. This remarkable enhancement in interlaminar fracture toughness results from the simultaneous triggering of nanoscale extrinsic toughening and microscale intrinsic toughening during crack growth. The flexural properties and fracture toughness of Epoxy/MWCNTs/CSR composites were also investigated to understand the properties transformation from resin matrix to CFRP. Morphological characterization and numerical analysis were used to analyze the synergies and constraints between two energy dissipation behaviors.

Microstructure and mechanical properties of high-pressure sintered B6O-SiC nanocomposites

Boron suboxide (B6O) is recognized as a superhard material with a low mass density, high resistance to chemical wear, and excellent wear resistance. Despite its desirable properties, the limited fracture toughness of B6O restricts its application in industrial contexts. This study presents the structural and mechanical characterization of B6O-SiC nanocomposites, which were synthesized via a high-pressure high-temperature sintering process of B6O powders and SiC whiskers. The sintering process induced fragmentation of SiC whiskers, resulting in the homogenous distribution of SiC fragments within the B6O matrix. An increase in SiC content was observed to decrease the composite's hardness, while initially reducing then enhancing its toughness. The nanocomposites containing 20 wt% and 30 wt% SiC whiskers exhibited significant improvements in fracture toughness, averaging 6.5 MPa m1/2 and 7.0 MPa m1/2, respectively—approximately threefold the toughness of polycrystalline B6O—while sustaining high hardness values of 36.3 GPa and 35.6 GPa on average. Microstructural analyses revealed that the composites’ superior mechanical performance is due to the presence of strong grain boundaries, as well as a high density of nanotwins and stacking faults. The findings demonstrate a viable method for producing B6O-based nanocomposites with enhanced hardness and toughness, potentially expanding their industrial applicability.

Investigation on mechanical properties of the green synthesis bamboo fiber/eggshell/coconut shell powder-based hybrid biocomposites under NaOH conditions

Abstract This research delves into the effects of different alkalization treatment approaches on the mechanical characteristics of epoxy matrix composites that are reinforced with natural bamboo fibers and enriched with egg and coconut shell powders as fillers. Various weight ratios of fibers and fillers were investigated, specifically at 5%, 10%, 15%, 20%, 25%, and 30%. The study assessed mechanical properties such as tensile strength, flexural behavior, microhardness, and impact resilience. Findings indicate that composites with alkali-treated fibers demonstrate superior mechanical performance (49.28 MPa of tensile, 57.33 MPa of flexural 89 HV of hardness, and 1.3 kJ·m−2 of impact) compared to untreated counterparts. Particularly noteworthy is the significant improvement in fracture toughness observed with the inclusion of 20% hybrid laminates, surpassing the performance of existing biomaterial-based composites. This heightened toughness is attributed to the optimized composition of fibers and enhanced water absorption capabilities. Conversely, the incorporation of 25% and 30% hybrid composites led to a decrease in mechanical strength (38.65 MPa of tensile, 46.7 MPa of flexural, 72 HV of hardness, and 1.19 kJ·m−2 of impact) due to the formation of additional interfacial contacts, pores, and voids within the polymeric matrix.

Enhancing the Properties of Yttria-Stabilized Zirconia Composites with Zeolitic Imidazolate Framework-Derived Nanocarbons.

Ceramic matrix composites (CMCs) reinforced with nanocarbon have attracted significant interest due to their potential to enhance mechanical, thermal, and electrical properties. Although the investigation of carbon-based materials such as graphene and carbon nanotubes as additives for advanced ceramics has been widespread, the utilization of metal-organic framework (MOF)-derived nanocarbons in CMCs remains largely unexplored. We extended our previous proof-of-concept investigations by demonstrating the effectiveness of a different type of MOF-derived carbon as a reinforcing phase in an alternative ceramic matrix. We employed spark plasma sintering (SPS) to consolidate yttria-stabilized zirconia (YSZ) and zeolitic imidazolate framework (ZIF-67) powder blends at 1300 °C and a uniaxial pressure of 50 MPa. YSZ serves as the ceramic matrix, whereas ZIF-67 serves as the nanocarbon source. The composite exhibits a highly significant improvement in fracture toughness with an increase of up to 13% compared to that of the YSZ monolith. The formation of ZIF-derived nanocarbon interlayers is responsible for the observed enhancement in ductility, which can be attributed to their ability to facilitate energy dissipation during crack propagation and inhibit grain growth. Furthermore, the room-temperature electrical conductivity of the sintered samples demonstrates a substantial improvement, primarily due to the in situ formation of nanocarbon-based fillers, reaching an impressive 27 S/m with 10 wt % ZIF-67 content. Based on the results, it can be inferred that the incorporation of in situ MOF-derived nanocarbons into CMCs leads to a substantial improvement in both the mechanical and electrical properties.

Epoxy composites integrated with functionalized wet‐milled graphitic nanoparticles: Featuring enhanced mechanical performance and in‐plane fracture resistance

AbstractComposites made of epoxy‐based systems are popular in many engineering applications because of their enhanced mechanical performance, fatigue properties, and other physical and thermal properties, but their low fracture toughness can limit their usefulness in high‐performance structural applications. This study aims to enhance the fracture properties of epoxy‐based composites by adding cost‐effective ball‐milled graphitic nanoparticles. Herein, dry ball milling for 15 and 48 h is done to produce ball‐milled graphite (BMG) nanoparticles. A significant reduction in particle size is achieved after 15 h of dry milling. Interestingly, a similar reduction in particle size is achieved for BMG (~258 nm) when wet ball milling for 3 h is performed. These BMGs were silanized (SBMG@3) and incorporated into the epoxy network. We found even at a higher loading level of SBMG@3, the resulting nanocomposites exhibited a significant improvement in fracture toughness (~2.47 MPa m1/2) and energy (~1.55 kJ/m2). This improvement was attributed to the upliftment of the in‐plane crack propagation resistance of the nanocomposites primarily offered by SBMG@3 due to coagulated interfacial interaction with the epoxy network that effectively hindered the propagation of the crack by providing numerous sites for energy dissipation, leading to improved fracture toughness. Our results demonstrate that SBMG@3 has great potential for improving the fracture toughness and energy of epoxy‐based composites, making them suitable for various industrial applications.Highlights Synthesized cost‐effective 2D carbon nanofillers via high‐energy ball milling. To overcome the re‐agglomeration effect wet ball milling is performed. Transformation of natural graphite to BMG with sizes of ~258 nm. KIC and GIC improve to ~2.47 MPa m1/2 and 1.55 kJ/m2, respectively, for ECSB1. FS and FM of the epoxy nanocomposites enhanced by ~43.69% and ~51.64%.

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.

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.

Fracture Toughness of Three-Dimensional Stereolithography Printed Polymer Reinforced with Continuous Carbon Fibers.

Additive manufacturing of fiber-reinforced polymers is one of the latest technical developments in composites manufacturing. However, there is a severe shortage of research into continuous fiber-reinforced polymers manufactured through stereolithography. For the first time, this article investigates the fracture properties of continuous carbon fiber-reinforced polymer produced by three-dimensional stereolithography printing. Compact tension (CT) specimens, both plain and fiber reinforced, were produced and tested systematically. The results showed a significant improvement in fracture toughness for fiber-reinforced specimens when compared with plain ones. The positioning of fiber bundles had a substantial effect on fracture properties, and a higher fracture toughness was reported for specimens with the fiber bundle placed closer to the crack tip. By increasing the number of fiber bundles, a significant increase in fracture toughness was reported when compared with the sample with a single fiber bundle, indicating a strong contribution of fiber volume. Also, the contribution appeared to be most effective when the fiber bundles were placed symmetrically in the thickness direction. The study is of importance and value for the development of the stereolithography technique in manufacturing continuous fiber-reinforced composites with enhanced mechanical properties.

Manipulation of Fracture Behavior of Poly(methyl methacrylate) Nanocomposites by Interfacial Design of a Metal-Organic-Framework Nanoparticle Toughener.

The interfacial region between nanoparticles and polymer matrix plays a critical role in influencing the mechanical behavior of polymer nanocomposites. In this work, a set of model systems based on poly(methyl methacrylate) (PMMA) matrix containing poly(alkyl glycidyl ether) brushes grafted on 50 nm metal-organic-framework (MOF) nanoparticles were synthesized and investigated. By systematically increasing the polymer brush length and graft density on the MOF nanoparticles, the fracture behavior of PMMA/MOF nanocomposite changes from forming only a few large crazes to generating massive crazing and to undergoing shear banding, which results in significant improvement in fracture toughness. The implication of the present finding for the interfacial design of the nanoparticles for the development of high-performance, multifunctional polymer nanocomposites is discussed.

Achieving high fracture toughness and tribological properties in high-carbon steels via sub-zero treatment and low-temperature tempering

High-carbon martensitic steel possesses extraordinary strength and hardness but inferior fracture toughness, restraining its utilization under high-performance conditions. In the present study, a significant improvement in fracture toughness and tribological properties of high-carbon steel were achieved via a conjugation of sub-zero treatment (SZT) at 233 K for 24 h and low-temperature tempering at 473 K for 2 h owing to the formation of nanoscale transition η-carbides in a well-distributed manner. Subsequently, the scratch test followed by quantitative analysis of fracture toughness (KIc) was carried out on the tiny samples in order to minimize the heterogeneous effects after treatments. SZT sample revealed high value of KIc of ∼43 MPa m, which was higher than conventional counterpart of ∼29 MPa m. Additionally, SZT sample showed outstanding nanohardness value of ∼8 GPa than that of CT of ∼6 GPa. The underlying mechanisms will be elaborated based on the microstructural analysis and fracture behavior.

The influence of different SiC amounts on the microstructure, densification, and mechanical properties of hot‐pressed Al2O3‐SiC composites

AbstractThe hot pressing process of monolithic Al2O3 and Al2O3‐SiC composites with 0‐25 wt% of submicrometer silicon carbide was done in this paper. The presence of SiC particles prohibited the grain growth of the Al2O3 matrix during sintering at the temperatures of 1450°C and 1550°C for 1 h and under the pressure of 30 MPa in vacuum. The effect of SiC reinforcement on the mechanical properties of composite specimens like fracture toughness, flexural strength, and hardness was discussed. The results showed that the maximum values of fracture toughness (5.9 ± 0.5 MPa.m1/2) and hardness (20.8 ± 0.4 GPa) were obtained for the Al2O3‐5 wt% SiC composite specimens. The significant improvement in fracture toughness of composite specimens in comparison with the monolithic alumina (3.1 ± 0.4 MPa.m1/2) could be attributed to crack deflection as one of the toughening mechanisms with regard to the presence of SiC particles. In addition, the flexural strength was improved by increasing SiC value up to 25 wt% and reached 395 ± 1.4 MPa. The scanning electron microscopy (SEM) observations verified that the increasing of flexural strength was related to the fine‐grained microstructure.

Development of Damage-Tolerant and Fracture-Resistant Materials by Utilizing the Material Inhomogeneity Effect

AbstractThe improvement of fracture strength by insertion of thin, soft interlayers is a strategy observed in biological materials such as deep-see sponges. The basic mechanism is a reduction of the crack driving force due to the spatial variation of yield strength and/or Young's modulus. The application of this “material inhomogeneity effect” is demonstrated in this paper. The effectiveness of various interlayer configurations is investigated by numerical simulations under application of the configurational force concept. Laminated composites, made of high-strength tool steels as matrix materials and low-strength deep-drawing steel as interlayer material, were manufactured by hot press bonding. The number of interlayers and the interlayer thickness were varied. Fracture mechanics experiments show crack arrest in the first interlayer and significant improvements in fracture toughness, even without the occurrence of other toughening mechanisms, such as interface delamination. The application of the material inhomogeneity effect for different types of matrix materials is discussed.

Fracture behavior of random distributed short tungsten fiber-reinforced tungsten composites

In future fusion reactors, tungsten is considered as the main candidate material for plasma-facing components. However, the intrinsic brittleness of tungsten is of great concern during operation. To overcome this drawback, tungsten fiber-reinforced tungsten composites (Wf/W) are being developed relying on extrinsic toughening principles. Tungsten (W) fibers with extremely high tensile strength and ductility are used to reinforce a tungsten matrix.In this work, field assisted sintering technology (FAST) is used to produce Wf/W material. Mechanical characterizations including Charpy impact and 3-point bending tests are performed. Based on the 3-point bending test results, the Wf/W materials can facilitate a promising pseudo-ductile behavior even at room temperature, similar to fiber reinforced ceramic composites. Fracture energy density and fracture toughness together with the crack-resistance curves (R-curves) are measured. Compared to conventional pure tungsten, Wf/W shows significant improvement in fracture toughness.

Determination of microstructure and mechanical properties of graphene reinforced Al2O3-Ti(C, N) ceramic composites

Al2O3-Ti(C, N) ceramic composites reinforced with graphene nano-platelets (GNPs) were fabricated via hot-pressing sintering (HP). The effects of GNPs content on the microstructure, mechanical properties, thermal conductivity and reinforcing mechanisms of the composites were investigated. To make GNPs dispersed homogeneously in ceramic matrix, N-Methyl-2-pyrrolidone (NMP) was used as dispersing media and alcohol solution of polyvinyl pyrrolidone was used as reference. Results show NMP is an effective dispersing media for powder mixing. The mechanical properties of Al2O3-Ti(C, N) composites are improved with the addition of GNPs, and when GNPs content is 0.4 wt%, significant improvements in fracture toughness (40.13%) and flexural strength (40.78%) are achieved in comparison with those of specimens without GNPs. However, there is no obvious effect of GNPs content change on the thermal conductivity of the composites when GNPs content is below 0.8 wt%. Many reinforcing mechanisms induced by GNPs are related to the interfacial bonding strength between GNPs and ceramic matrix. Pull-out of GNPs, crack deflection and branching are caused by weak interfaces between GNPs and matrix while crack bridging and alteration of fracture mode are caused by strong interfaces. Furthermore, interleaving distribution of weak and strong bonding interfaces between GNPs and matrix is essential for the reinforcement of ceramic composites.

Fracture toughness of additively manufactured carbon fiber reinforced composites

The fracture properties (stress intensity factor and energy release rate) of additively manufactured (AM) polylactic acid (PLA) and its short carbon fiber (CF) reinforced composites have been studied. The effects of CF reinforcement, nozzle geometry and bead lay-up orientations in fracture properties, void contents, and interfacial bonding were investigated. The fused filament fabrication (FFF)-based AM specimens using both circular and square shaped nozzle were printed and compared with the conventional compression molded (CM) samples. Compact tension (CT) specimens with different CF concentrations (0 wt.%, 3 wt. %, 5 wt.%, 7 wt.% and 10 wt.%) were printed with two bead lay-up orientations (450/-450 and 00/900) using PLA and CF/PLA composite filaments. The results show significant improvement in fracture toughness and fracture energy for CF/PLA composites in comparison to neat PLA. The fracture toughness was increased by 42% for 00/900 and 38% for 450/-450 bead orientations, respectively with 5% CF loading. The increase in fracture energy was observed to be about 77% for 00/900 and 88% for 450/-450 bead orientations, respectively for the same fiber reinforcement (5 wt. %). Such improvement in fracture properties is expected to be higher for all 900 bead orientations. The samples printed by square-shaped nozzle showed enhanced fracture toughness with less inter-bead voids and larger bonded areas in comparison to the circular-shaped nozzle. Although the fracture toughness showed very negligible differences between 00/900 and 450/-450 specimens, distinguishable variation may be seen in the case of 00 and 900 bead orientations. The crack propagation path and fracture mechanisms were studied using optical microscopy (OM) and scanning electron microscopy (SEM) examinations. Fractography revealed different modes of failure with a very high fiber orientation along the printing direction and a relatively higher void contents for 7 and 10 wt. % fiber reinforcement.

Improved tensile and fracture toughness properties of graphene nanoplatelets filled epoxy polymer via solvent compounding shear milling method

This paper investigates the effect of graphene nanoplatelets (GNP) on tensile and fracture toughness properties of an epoxy polymer. The GNP filled epoxy nanocomposites were prepared using a combination technique of solution compounding and high-shear milling with three different GNP loadings; 0.1 wt%, 0.2 wt% and 0.3 wt%. The polymer nanocomposites were characterized based on their fracture toughness and tensile properties. In order to evaluate these properties, three points bending and uniaxial tension tests were conducted on single-edge-notch bending (SENB) and dog-bone shape specimens, respectively. It was found that a significant improvement in fracture toughness and tensile properties was obtained without reducing the failure strain values when GNP was added into the epoxy polymer. The addition of 0.3 wt% GNP improved the critical stress intensity factor, critical strain energy release rate and tensile modulus of the epoxy polymer about 64%, 268 J m−2 and 32%, respectively. This can be concluded that the incorporation of GNP increased the fracture toughness as well as the mechanical properties provided a proper fabrication method was implemented. It is also demonstrated that GNP is capable to provide effective toughening mechanisms in epoxy resin as indicated by the SEM micrographs.

The fracture toughness of natural fibre- and glass fibre-reinforced SMC

ABSTRACTThe fracture toughness properties, in terms of stress intensity factor KIc and strain energy release rate GIc, of hemp fibre mat-reinforced sheet moulding compound (H-SMC) are measured using the compact tension (CT) method and compared with those of glass fibre-reinforced SMC (G-SMC). Three material parameters were considered for composite optimisation: fibre volume fraction, CaCO3 filler content and hemp fibre surface treatments using either alkaline, silane or a combination of these two treatments. The highest fracture toughness for H-SMC composites was obtained at a fibre loading of around 30 vol.-%, while it was also shown that the fracture toughness properties of H-SMC are sensitive to mineral filler content. Surface treatment of the hemp fibres using a combined alkaline-silane treatment resulted in a significant improvement in fracture toughness of H-SMC composites. Optimised H-SMC composites exhibited fracture toughness properties similar to those of G-SMC at fibre contents of 20 vol.-%, with KIc values of around 6 MPa.m−1/2.

Synergistic effect of carbon nanotube as sintering aid and toughening agent in spark plasma sintered molybdenum disilicide-hafnium carbide composite

Hafnium carbide (HfC) along with sintering aids was consolidated at a relatively lower temperature i.e. 1600°C (i.e. T=~0.41Tm) under a uniaxial load of 50MPa by spark plasma sintering. Two different sintering aids such as molybdenum disilicide (MoSi2) and carbon nanotube (CNT) were added to enhance the densification and lower the extent of grain growth in the sintered pellets. Density of the sintered pellet increased from 96.0±0.8% in HfC +5wt% MoSi2 (HM) to 99.0±0.5% with the addition of 2wt% CNT in HfC+5wt% MoSi2 (HMC) at sintering temperature of 1600°C. Further, the extent of grain growth drastically reduced from 204% in HM to 50% in HMC. Analysis of linear shrinkage during densification revealed that CNT addition increased densification rate and decreased the time required to reach the density of 99.0±0.5% at 1600°C. Increased densification and lower degree of grain growth could be due to the synergistic effect offered by the CNT, which are as follows: (i) Lubrication effect of CNT, (ii) Lower activation energy for grain boundary diffusion (iii) Reduction in liquid phase sintering temperature and (iv) Grain boundary pinning. Fracture toughness of the sintered HM and HMC composite was obtained using indentation technique. By the addition of 2wt% CNT in HM, drastic increase of 91% in fracture toughness was seen. This significant improvement in fracture toughness was due to the enhanced densification and relatively lower grain size of HMC. Also crack bridging, crack deflection, crack arrest, CNT and graphene sheet pull-out and swording played major role in toughening of HMC pellet.

Microfibrillated Lignocellulose Enables the Suspension-Polymerisation of Unsaturated Polyester Resin for Novel Composite Applications.

A new route towards embedding fibrillated cellulose in a non-polar thermoset matrix without any use of organic solvent or chemical surface modification is presented. It is shown that microfibrillated lignocellulose made from cellulose with high residual lignin content is capable of stabilising an emulsion of unsaturated polyester resin in water due to its amphiphilic surface-chemical character. Upon polymerisation of the resin, thermoset microspheres embedded in a microfibrillated cellulose network are formed. The porous network structure persists after conventional drying in an oven, yielding a mechanically stable porous material. In an application experiment, the porous material was milled into a fine powder and added to the polyester matrix of a glass fibre-reinforced composite. This resulted in a significant improvement in fracture toughness of the composite, whereas a reduction of bending strength and stiffness was observed in parallel.

Structure–Property Relationship in an Electroconductive Hydroxyapatite–Titanium Disilicide Composite

We elucidate here the structure–property relationship in a novel composite system consisting of hydroxyapatite (HA) and titanium disilicide that addresses the challenge of low electrical conductivity and fracture toughness of hydroxyapatite. Theoretical considerations indicated that the addition of 20 wt.% titanium disilicide (σ ~ 106 S/m) favorably contributed to the increase in electrical conductivity (σ ~ 105 S/m) of the HA–titanium disilicide system. As compared to theoretical value, a lower value of electrical conductivity of HA‐20 wt.%TiSi2 composite (σ ~ 67.117 ± 3.57 S/m) was observed in experimental measurement using four probe method. However, this electrical conductivity of HA‐20 wt.%TiSi2 composite was significantly higher than the HA, because no current was recorded in case of HA in the sensitivity range of the instrument. During spark plasma sintering, to raise per unit temperature, higher magnitude of current was utilized in case of HA‐20TiSi2 than HA. This resulted in higher densification of HA‐20TiSi2 during initial stage of sintering. Also, a significant improvement in fracture toughness was observed on the addition of TiSi2 to HA from 0.6 MPa.m1/2 in HA to 1.2 MPa.m1/2 in HA‐20TiSi2. The mechanism of increase in fracture toughness involved crack deflection, friction bridging, wake debonding, and elastic bridging.