Unreinforced Material Research Articles

Study of tensile and compressive behaviour of the in-house synthesized al-alloy nano composite

The present study is focused on the in-house synthesis of graphene reinforced Al-alloy nanocomposites using stir casting process. The concentration of graphene nanopowder (size range: 3-8 nm) is taken as 2 wt. % and 6 wt. %. Tensile and compressive characterization study has carried out on the prepared synthesized specimens as per the ASTM standard, and further examined the microstructure of the specimens to study the behaviour of nanopowder dispersed in Al-alloy matrix. The results reveal that the tensile and compressive strength of the graphene reinforced (increasing concentration to 6 wt. %) Al-alloy nanocomposite has increased by 49% and 44% respectively as compared to un-reinforced material. Moreover, it has been observed that with increasing concentration of reinforcement, the specimen exhibits brittle character and accordingly the percentage of elongation found to be reduced. The effect of graphene nanoparticles on the tensile and compressive behaviour of the composites has also documented in this article.

Influence of particle and short-fibre reinforcement on the very high cycle fatigue behaviour of aluminium matrix composites

The fatigue behaviour of aluminium matrix composites (Al-MMCs) reinforced by alumina particles (15 vol% Al2O3) or short-fibres (20 vol% Saffil), respectively, was investigated in the very high cycle fatigue (VHCF) regime at stress amplitudes ≤140 MPa to reach fatigue lives of up to 1010 cycles using an ultrasonic fatigue testing device. Fatigue tests were complemented by in situ thermographic measurements to record the temperature of the whole specimen and to find “hot spots” indicating changes in microstructure and, therefore, the initiation or growth of cracks. Results obtained in the VHCF-region fit well with results obtained in LCF/HCF-region. A significant influence of test frequency was not found. Surprisingly, the reinforced Al-MMCs are capable to bear up to 109 cycles, although damage starts early during cyclic loading. In addition, although in the LCF/HCF regime the fatigue life and fatigue strength of the short-fibre reinforced Al-MMCs are significantly smaller compared to particle reinforced and unreinforced material, no significant differences to the particle-reinforced and unreinforced Al-MMC were observed at stresses below 140 MPa. Moreover, at stresses below 80 MPa not fatigue failure occurred at all up to 1010 cycles.

High performance cementitious composite from alkali-activated ladle slag reinforced with polypropylene fibers

Alkali-activated ladle slag (AALS) is a promising cementitious material with environmental benefits. However, the brittleness of material has been limiting the use in construction. Therefore, in this experimental investigation, different polypropylene (PP) fibers were employed as a short randomly reinforcement in cementitious matrix in order to improve mechanical performance of the AALS composites.The study reveals that the AALS composite could gain very high ductility with an appropriate fibrous reinforcement. Fracture energy and fracture toughness of PP fiber reinforced AALS mortars increased by approximately 150 and 7.6 times, respectively, compared to the unreinforced material. Additionally, the flexural strength of the composite increased by roughly 300%. Pseudo strain hardening (PSH) behavior was observed along with multiple cracks under uniaxial tensile test. Scanning electron microscope (SEM) images confirmed the local fiber bridging effect, which resulted in the high mechanical performance of the PP-reinforced AALS.

Improvement of the mechanical, tribological and antibacterial properties of glass ionomer cements by fluorinated graphene

ObjectiveThe aim of this study was to improve the mechanical properties, wear resistance and antibacterial properties of conventional glass ionomer cements (GICs) by fluorinated graphene (FG), under the premise of not influencing their solubility and fluoride ion releasing property. Materials and methodsFG with bright white color was prepared using graphene oxide by a hydrothermal reaction. Experimental modified GICs was prepared by adding FG to the traditional GICs powder with four different weight ratios (0.5wt%, 1wt%, 2wt% and 4wt%) using mechanical blending. Compressive and flexural strength of each experimental and control group materials were investigated using a universal testing machine. The Vickers microhardness of all the specimens was measured by a Vicker microhardness tester. For tribological properties of the composites, specimens of each group were investigated by high-speed reciprocating friction tester. Fluoride ion releasing was measured by fluoride ion selective electrode methods. The antibacterial effect of GICs/FG composites on selected bacteria (Staphylococci aureus and Streptococcus mutans) was tested with pellicle sticking method. ResultsThe prepared GICs/FG composites with white color were successfully fabricated. Increase of Vickers microhardness and compressive strength and decrease of friction coefficient of the GICs/FG composites were achieved compared to unreinforced materials. The colony count against S. aureus and S. mutans decreased with the increase of the content of FG. And the antibacterial rate of S. mutans can be up to 85.27% when the FG content was 4wt%. Additionally, fluoride ion releasing property and solubility did not show significant differences between unreinforced and FG reinforced GICs. SignificanceAdding FG to traditional GICs could not only improve mechanical and tribological properties of the composites, but also improve their antibacterial properties. In addition, the GICs/FG composites had no negative effect on the color, solubility and fluoride ion releasing properties, which will open up new roads for the application of dental materials.

Reinforcing effect of discontinuous microglass fibers on resin-modified glass ionomer cement.

This study investigated the reinforcing effect of discontinuous-glass fiber fillers with different loading-fractions on selected mechanical properties and wear of resin-modified glass ionomer cement (RMGIC). Experimental fiber-reinforced RMGIC (Exp-RMGIC) was prepared by adding discontinuous-glass fiber of 200-500 µm in length to the powder of RMGIC (GC Fuji II LC) with different weight ratios (15, 20, 25 and 30 wt%). Mechanical properties and wear were determined for each experimental and control material. Scanning electron microscopy was used to evaluate the microstructure of the Exp-RMGICs. Wear pattern was analyzed by a three-dimensional (3D) noncontact optical profilometer. Fiber-reinforced RMGIC (30 wt%) had significantly higher mechanical performance of fracture toughness (1.9 MPa•m1/2), flexural strength (90.3 MPa), and diametral tensile strength (31 MPa) (p<0.05) compared to unreinforced material (0.8 MPa•m1/2, 51.9 and 20.7 MPa). The use of discontinuous-glass fiber fillers with RMGIC matrix is novel reinforcement and yielded superior toughening and flexural performance compared to conventional RMGIC.

Evaluation of flexural behaviour of geosynthetic-reinforced unbound granular material beams

Nowadays, geosynthetics offer cost-effective and sustainable materials for pavement construction which enhance pavement performance. As the geosynthetic layer acts as a flexible member embedded within the pavement structure, the flexural behaviour is an important aspect to consider for the use of geosynthetics. Therefore, in this study, the flexural behaviour of geosynthetic-reinforced unbound granular materials was investigated by carrying out a series of four-point bending tests. The geosynthetic reinforcement considered involves geogrid and geocell. Silty sand and crushed aggregate were considered as infill materials. Unreinforced and geogrid/geocell-reinforced three-layered beams were constructed using polywood, plywood, selected infill materials and geogrid/geocell. Provision of the geogrid/geocell proved to provide significant resistance against the lateral spreading of the infill material when compared with the unreinforced infill materials. The resistance to lateral spread increases with an increase in the aspect ratio of geocells and the position of geogrid. The elastic modulus of the geogrid/geocell-reinforced infill materials is found to increase significantly when compared to that of the unreinforced materials with geocell reinforcement being very effective. Geogrid placed 25%H from the bottom is found to possess higher elastic modulus when compared with the geogrid placed at 50%H and 75%H from the bottom of the middle layer. The provision of geogrid/geocell within the granular sub-base of a pavement significantly improves the rutting life of the pavement. An attempt is also made to develop an empirical model for the prediction of Es of unreinforced and geocell-reinforced infill materials. There is a good agreement between the predicted and laboratory Es values with an error of 5%. The model enables the analytical design of flexible pavement incorporating geocells.

Effect of discontinuous glass fibers on mechanical properties of glass ionomer cement

Aim: This study investigated the reinforcing effect of discontinuous glass microfibers with various loading fractions on selected mechanical properties of self-cure glass ionomer cement (GIC).Method: Experimental fiber reinforced GIC (Exp-GIC) was prepared by adding discontinuous glass microfiber (silane/non-silane treated) of 200–300 µm in length to the powder of self-cure GIC (GC Fuji IX) with various mass ratios (15, 20, 25, 35, and 45 mass%) using a high speed mixing device. Flexural strength, flexural modulus, work of fracture, compressive strength and diametral tensile strength were determined for each experimental and control materials. The specimens (n = 8) were wet stored (37 °C for one day) before testing. Scanning electron microscopy equipped with energy dispersive spectrometer was used to analysis the surface of silanized or non-silanized fibers after treated with cement liquid. The results were analyzed with using multivariate analysis of variance MANOVA.Results: Fiber-reinforced GIC (25 mass%) had significantly higher mechanical performance of flexural modulus (3.8 GPa), flexural strength (48 MPa), and diametral tensile strength (18 MPa) (p < .05) compared to unreinforced material (0.9 GPa, 26 MPa and 8 MPa). No statistical significant difference in tested mechanical properties was recorded between silanized and non-silanized Exp-GIC groups. Compressive strength did not show any significant differences (p > .05) between the fiber-reinforced and unreinforced GIC.Conclusion: The use of discontinuous glass microfibers with self-cure GIC matrix considerably increased the all of the studied properties except compressive strength.

Synthesis and Dry Sliding behaviour of LM24-TiB₂ Reinforced Particulate Composite

The existing effort deals with synthesis of (LM24) composite developed by reinforcing 3wt % and 5wt % TiB2 particles in the size range of 20-60 μm in the LM24 matrix. Hardness of the alloy and the composites were evaluated to see the changes in the hardness of the materials. The sliding wear tests were performed using pin on disc machine to study the wear properties. Effect of various experimental parameters like sliding distance, and applied pressure were studied on sliding wear response of the alloy and the composite. Micro structural examination of the alloy and composite was done to see the structural changes and distribution of the particles in the matrix. Worn surfaces were examined to see the mechanism of material removal. Results show that reinforcement of TiB2 particles in LM24 alloy increases the hardness and wear resistance. Microstructural study of the base alloy LM24 and composites were performed having special attention to the dispersion of TiB2 particles in LM24 base alloy matrix and interface closeness between TiB2 particles and LM24 alloy matrix. The microstructure of aluminium alloy shows primary Al dendrites and intermetallic phases around in the inter-dendrites region. The TiB2 particles were noted to be more or less uniformly distributed within the metallic matrix and these particles were trapped within the primary aluminium dendrites instead of inter-dendritic region.The importance of this work is to study the importance of TiB2 particles in enhancing the properties of the base alloy. In general, the properties of unreinforced materials show poor mechanical as well as wear properties and there is a need to improve the properties to a certain level so that the material can be extended to a certain level for its usefulness. In the present work, it has been observed that after reinforcing TiB2 particles to the base alloy considerable amount of enhancement in the properties can be achieved. It is quite sure that this project work will certainly help in designing of the various parts.

Influence of heat treatment on the properties of AlSi10Mg-based metal matrix composites reinforced with metallic glass flakes processed by gas pressure infiltration

The reinforcement of a soft matrix material with hard particles is an established strategy to develop materials with tailored properties. In this regard, using metallic glasses with high crystallization temperatures, e.g. in the system NiNbX (X = Sn, Ta), for composites produced by liquid metal infiltration is a novel approach. The current work deals with the characterization of such metallic glass particle-reinforced AlSi10Mg-based metal matrix composites manufactured by gas pressure infiltration. Processing–structure–property relations were investigated with a special focus on the influence of an additional heat treatment on the metal matrix composite’s properties. Metallographic methods were used to investigate infiltration quality, particle distribution within the composite and the composite’s microstructure. Moreover, X-ray diffraction measurements, elastic analysis using ultrasonic spectroscopy and compression tests were performed to analyze its properties. The X-ray diffraction results indicate that there is no crystallization of the glass during processing. Metallographic investigations show that the flakes are arranged in a layered structure within the composite. The embedding of metallic glass flakes leads to an increase in Young’s modulus and compressive strength in comparison to the unreinforced material. The composite’s strength can be further increased by a heat treatment.

Sintering behaviour and mechanical characterisation of Ti64/xTiN composites and bilayer components

ABSTRACTCeramic particle reinforcement can be used to improve the surface properties of Ti6Al4V (Ti64) alloy. Powder metallurgy route is a promising method to fabricate such reinforced Ti64 components. To assess the relevance of this technique, this work investigates the effect generated by the addition of TiN particles in Ti64 powder during free sintering. TiN reinforcement particles were randomly distributed in the Ti64 matrix with three different concentrations in two configurations: completely reinforced and unreinforced–reinforced bilayer. Dilatometry was used to obtain the shrinkage kinetics of samples at 1200, 1300 and 1400°C under inert atmosphere and to investigate the impact of reinforced particles on the sintering behaviour. The microstructure of sintered materials was shown to be lamellar in the unreinforced material and equiaxed in reinforced materials. Finally, the Vickers microhardness measurement showed the huge benefit of adding TiN particles to increase the mechanical strength of the Ti64 alloy.

Metal matrix composites reinforced with carbon nanotubes by an alternative technique

Abstract The metal matrix composites (MMCs) have been used in several applications where high specific resistance and temperature are required for some uses in the aerospace and aeronautic industry. In general, these composites are manufactured using as reinforcing material particles or some fibers like glass, ceramic and carbon fibers. Recently a new manufacturing process for the production of MMCs reinforced with carbon nanotubes, known as sandwich technique has been proposed. This technique produces materials comprised of a metallic matrix and banded structured-layers of multiwall carbon nanotubes (MWCNTs). However, among other issues, the matrix-reinforcement interface and the reinforcement dispersion degree are still open questions. The present study has been aimed on the reinforcement of magnesium sheets with MWCNTs by the sandwich technique. Field emission scanning electron microscopy was used to demonstrate that the method is capable of producing a good dispersion of the MWCNTs with no evidence of damage. The mechanical properties were measured by tensile and nanoindentation tests; yield and ultimate strengths, elastic modulus and hardness were increased with respect to the unreinforced material.

Development of a 2.25%Cr Steel P23 Reinforced with Micro/Nano-Carbide Particles Produced by Self-Propagating High-Temperature Synthesis

In the present work, 1 wt.% of (Ti,Nb)C carbide particles prepared by self-propagating high temperature synthesis (SHS) were introduced into a melt of a conventional P23 steel to obtain a reinforced material with improved creep properties. The as-cast material showed a eutectic type microstucture, indicating partial dissolution of these carbides in the melt. Inside the dendritic regions, a bainitic/martensitic structure similar to that of the unreinforced material was present. A significant refinement of the prior austenitic grain size was revealed in the reinforced material. Brinell hardeness measurements reveal an increase of hardness in the reinforeced material due to the addition of the carbides. High strain rate compression tests were perfomed at temperatures in the range 950 and 1250oC to determine the optimum forming conditions. Stability maps for a wide range of temperatures and strain rates were drawn. The optimum temperature for the reinforced steel is about 77 K higher than for the non-reinforced steel.

Hollow glass fibers in reinforcing glass ionomer cements

ObjectiveThis study investigated the reinforcing effect of hollow and solid discontinuous glass fiber fillers with two different loading fractions on select mechanical properties of conventional and resin modified glass ionomer cements (GICs). MethodsExperimental fiber reinforced GIC was prepared by adding discontinuous glass fiber (hollow/solid) of 0.5mm in length to the powder of commercial GICs (GC Fuji IX and II LC) with two different weight ratios (5 and 10wt%) using a high speed mixing machine. Fracture toughness, work of fracture, flexural strength, flexural modulus, compressive strength and diametral tensile strength were determined for each experimental and control material. The specimens (n=7) were wet stored (37°C for one day) before testing. Scanning electron microscopy was used to evaluate the microstructure of the experimental fiber reinforced GICs. Fiber length analysis was carried out to investigate the fiber length distribution of experimental GICs. The results were analyzed statistically using ANOVA followed by Tukey's post hoc test. Level of significance was set at 0.05. ResultsAn increase in fracture toughness (280 and 200%) and flexural strength (170 and 140%) of hollow discontinuous glass fiber reinforced (10wt%) conventional and resin modified GICs respectively, were achieved compared to unreinforced materials (p<0.05). Compressive strength did not show any significant differences (p>0.05) between the fiber reinforced and unreinforced GICs. SignificanceThe use of hollow discontinuous glass fiber fillers with conventional and resin modified GIC matrix is a novel reinforcement. It yielded superior toughening and flexural performance compared to the particulate GICs used.

Optimization of the wear resistance of AA2024 matrix composites fabricated with hot pressing

Purpose: In the present study, the effects of B4C reinforcement volume fraction (% 5-15-20)on the abrasive wear properties of AA2024 matrix composites produced with hot pressingmethods were investigated.Design/methodology/approach: As-received samples were also used for comparison.AA 2024 powder was mixed with B4C-SiC-Al2O3 particles by a three dimensional mechanicmixer for 30 minutes. Mixed powder was pressed under 60 MPa at room temperature in thesteel mold by unidirectional. Steel mold kept in the furnace at 550ºC after the pre-pressingfor 30 minutes. Samples were pressed in heated mold under 100 Pa. The wear tests werecarried out using a pin-on-disk wear tester by sliding at sliding speeds of 1.2 m/s againstsilicon carbide paper. Normal loads of 10, 20 and 30 N at constant sliding speed at roomtemperature.Findings: The experimental result showed that B4C volume fraction significantly influencethe wear behavior of AA2024 matrix composites produced with hot pressing methods.Originality/value: It was also found that the wear resistance of AA2024 matrix compositesproduced with hot pressing methods increases with increasing B4C volume fraction.The highest weight loss was obtained in the unreinforced matrix material.

A MIM Route for Producing Ti6Al4V-TiC Composites

Discontinuous reinforced titanium matrix composites have generated significant interest due to their compelling properties such as their specific strength and wear resistance at room and elevated temperatures. For these reasons, these materials have been considered in various applications such as automotive (valve components), aerospace (engine components) and medical devices (implants). Metal injection molding (MIM) has proven to be an efficient near net-shape technology suitable for high volume manufacturing of parts having complex geometries. The MIM technology is particularly attractive for producing composites as the metallic matrix does not go through the liquid state. This helps minimizing the segregation of the hard particles. MIM also reduces the needs for machining. However, the production of titanium components with the MIM process has its own challenges and limitations, such as presence of porosities and coarser microstructures compared to wrought products. The present work introduces the results obtained during the development of a MIM route for producing Ti6Al4V-5wt%TiC composites. The feedstock developed is wax-based and incorporates a pre-alloyed metal powder. The microstructure, mechanical properties at room and elevated temperatures, the wear resistance and the thermal diffusivity of the composites have been characterised. Properties are compared with those of a Ti6Al4V MIM material produced with the same feedstock and process but without TiC as well as with those of wrought Ti6Al4V reported in the literature. The presence of a small amount of TiC promotes densification and grain size refinement and affects the surface finish of the sintered components. Tensile properties of the composites are comparable or better than those of wrought Ti6Al4V (ASTM F1472). Improved mechanical properties compared to unreinforced material are associated to the higher density, finer grain size as well as solution strengthening of the titanium matrix.

Mechanical properties and interfacial phenomena in aluminum reinforced with carbon nanotubes manufactured by the sandwich technique

Recently, a new manufacturing process for the production of metallic matrix composite materials reinforced with carbon nanotubes, known as sandwich technique has been proposed. This technique produces a material comprised of a metallic matrix and a banded structures-layers of multi-walled carbon nanotubes. However, among other issues, the matrix-reinforcement interface and the reinforcement dispersion degree are still open questions. The present study uses field emission scanning electron microscopy and high resolution transmission electron microscopy to probe that the method is capable to achieve a good dispersion of the multi-walled carbon nanotubes with no evidence of carbon nanotubes’ damage. The mechanical properties were measured by tensile and nanoindentation tests; improvements in the elastic modulus, yield and ultimate strengths were found, with respect to the unreinforced material.

Influence of molecular structure and reinforcement on fatigue behavior of tough polypropylene materials

ABSTRACTMolecular structure and reinforcement heavily influence the crack growth resistance of polypropylene materials. Aim of this study is to investigate the fatigue behavior of different unreinforced and reinforced tough polypropylene materials used for piping applications. Due to high resistance against crack growth, these materials cannot be tested in the application relevant quasi‐brittle failure mode within feasible amounts of time. In this work, the new cyclic cracked round bar test, developed for tough polyethylene materials, has been examined as a possible method to characterize this important type of failure mode in homo‐, random‐, and reinforced polypropylene. Even though molecular mass distribution, which is often used to explain differences in crack growth resistance of polymers, was similar for unreinforced materials, fatigue lifetimes differed greatly. The mismatch of molecular mass and fatigue lifetime was mainly attributed to the different buildup and morphology of the base polymer. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43948.

Tenacidad a la fractura de compuestos cermets 3Al2O3*2SiO2/Ag manufacturados por molienda de alta energía

La fabricación de materiales compuestos de matriz cerámica reforzados con partículas metálicas han propiciado la formación de nuevos materiales conocidos como compuestos CERMETS, materiales que debido a sus elementos precursores poseen propiedades distintas a las de los materiales convencionales. En este trabajo se establece la ruta de fabricación de materiales compuestos cermets base 3Al2O3*2SiO2 reforzados con partículas metálicas de Ag a partir de la formación de la composición química en peso de polvos de 3Al2O3*2SiO2 / 1% Ag en busca de un aumento en la tenacidad a la fractura con respecto al cerámico base. La composición química de polvos es sometida a un proceso de mezcla molienda de alta energía en seco en un molino tipo planetario por 2 horas a 200 rpm. Los polvos posteriormente son conformados en muestras cilíndricas de 20 mm de diámetro y 3 mm de espesor mediante la aplicación de carga uniaxial en frío de 200 MPa. Las muestras son sinterizadas a 1500°C y 1600°C por una y dos horas en un horno de resistencia eléctrica en atmósfera controlada de gas nitrógeno. Los compuestos fabricados son analizados microestructuralmente por microscopia óptica y electrónica de barrido. Se determina la densidad y las propiedades mecánicas de dureza y tenacidad a la fractura, las dos últimas por el método de indentación. Los resultados muestran la viabilidad de fabricación de materiales compuestos cermets así como los cambios en la densidad, la dureza y la tenacidad a la fractura, con respecto al cerámico 3Al2O3*2SiO2 sin refuerzo metálico.

Effect of Geosynthetic Reinforcement Inclusion on the Strength Parameters and Bearing Ratio of a Fine Soil

This paper reports an investigation on the beneficial effects of reinforcing a fine soil with a geosynthetic (reinforcement geocomposite) and their behaviour under loading. The effectiveness of the reinforcement was investigated through triaxial and California Bearing Ratio, CBR, tests. The triaxial tests showed that including the reinforcement provided additional confinement to the reinforced soil samples, causing an increase in the corresponding strength parameters. However, the reinforcement decreased the secant stiffness modulus of the composite material, particularly for low strains. The CBR tests were performed on soaked samples, compacted for different initial water content values. The influence of increasing the number of reinforcement layers was also analysed. The results showed that the reinforced samples had a maximum bearing capacity larger than the unreinforced material. The reinforcing mechanisms observed in the CBR tests were membrane tension support and bearing capacity increase. Increasing the number of reinforcement layers induced an improved response of the soil-geosynthetic composite material, particularly for a water content lower than the optimum. An increase in the initial water content induced reductions of the bearing capacity of the soil, with different values, depending on position of the initial value relative to the optimum water content.

Toughening of aluminum matrix nanocomposites via spatial arrays of boron carbide spherical nanoparticles

To enhance the toughness of metal matrix nanocomposites, we demonstrate a strategy that involves the introduction of spatial arrays of nanoparticles. Specifically, we describe an approach to synthesize a microstructure characterized by arrays of fiber-like nanoparticle-rich (NPR) zones that contain spherical nanoparticles of boron carbide (sn-B4C) embedded in an ultrafine grained (UFG) aluminum alloy matrix. A combination of cryomilling and hot-extrusion was used to obtain this particular microstructure, and the mechanical behavior and operative strengthening and deformation mechanisms were investigated in detail. When compared to an equivalent unreinforced material, the presence of the array of NPR zones contributed to a 26% increase in tensile strength. Moreover, when compared to a nanocomposite containing a homogeneous distribution of nanoparticles, a 30% increase in toughness was observed. High nanohardness values obtained for the NPR zones and the observation of “pull-out” phenomena on fracture surfaces, suggest that the NPR zones behave as “hard” fiber-like units that can effectively sustain tensile loading and thereby enhance the strengthening efficiency of sn-B4C. Also, the presence of the array of NPR zones surrounded by nanoparticle-free (NPF) zones led to an enhancement in strength with limited loss in ductility. This behavior was rationalized on the basis of a low value of the Schmid factor in regions adjacent to NPR zones, coupled with the ease of dislocation movement in NPF zones. Finally, the ratio of the plastic zone size to the size of the “hard” NPR zones is proposed as an important factor that governs the overall toughness of the nanocomposite.