KIC Values Research Articles

(AlCrMoTiNi)1-XNX high entropy ceramic with high hardness and toughness prepared by co-filtered cathodic vacuum arc deposition

High entropy ceramic (HEC) coatings have become candidates for new protective coating due to their high strength. However, the hardness - toughness balance of the ceramic coatings is still a difficult problem to solve. The strong nitride forming elements Al, Ti, Cr, Mo, and the non-nitride forming element Ni were selected as the components of the (AlCrMoTiNi)1-XNX HEC coatings to regulate the hardness and toughness. When x = 0 and 0.29, the composite structure of amorphous and metallic Ni phase is observed in these coatings. When x = 0.37, 0.40, and 0.45, the phase structure changes to the composite structure of nanoscale face-centered cubic (FCC) phase, metallic Ni, and amorphous phase. When x = 0.45, the maximum hardness of the HEC coating is 31.25 GPa, and it also has high toughness with a KIC value of 3.25 MPa m1/2. The HEC coating with high hardness and toughness also presents excellent wear resistance and corrosion resistance. Therefore, the (AlCrMoTiNi)0.55N0.45 HEC coating has potential as a protective coating applied in various areas.

Composition optimization of PMMA base denture reinforced with different percentages of Nano Hydroxyapatite and alumina particles to obtain highest KIc and KIIc values using hybrid IWO/PSO algorithm

Composition of three-phase dental denture (Polymethyl methacrylate or PMMA + Nano Hydroxyapatite + Nano Alumina) was optimized using hybrid IWO/PSO algorithm to obtain the highest KIc and KIIc. First, initial fracture toughness tests were performed on nine groups of PMMA base dentures with different percentages of ingredients. Symmetric single edge notch beam and asymmetric three-point bend beam with inclined crack were used for KIc and KIIc testing, respectively. The initial design parameters were considered as: {75–100% PMMA, 0–10% Nano Hydroxyapatite and, 0–15% Nano Alumina}. As the initial guess values for starting the hybrid optimization algorithm, B-spline curves (as the cost function) were fitted to the experimental results of both modes I and II. Then the IWO/PSO algorithm was run to find the optimum composition and maximum fracture toughness value of each mode. The best KIc was found for the composition of {78.5% PMMA + 9% Nano Hydroxyapatide + 12.5% Nano Alumina}. Similarly, using the hybrid IWO/PSO algorithm the optimum composition of {83.4% PMMA + 8.7% Nano Hydroxyapatide + 7.9 % Nano Alumina} was obtained to gain the highest KIIc. The experimental fracture toughness values obtained from the proposed compositions via employing the hybrid algorithm were approximately 40% higher than the all tested nine groups of PMMA base dentures under both modes I and II. The fracture toughness ratio KIIc/KIc of the optimum mix-designs predicted by the hybrid algorithm was equal to 0.78 and showed good agreement with the experimental KIIc/KIc ratio. The fracture toughness for any desired mixed mode condition can also be predicted in-terms of the KIc and stress intensity factors and T-stress of the desired mode mixity.

The Tensile Properties and Fracture Toughness of a Cast Mg-9Gd-4Y-0.5Zr Alloy

Low fracture toughness has been a major barrier for the structural applications of cast Mg-Gd-Y-Zr alloys. In this work, the tensile properties and fracture toughness of a direct-chill-cast Mg-9Gd-4Y-0.5Zr (VW94K) alloy were investigated in different conditions, including its as-cast and as-homogenized states. The results show that the tensile properties of the as-cast VW94K alloy are greatly improved after the homogenization treatment due to the strengthening of the solid solution. The plane strain fracture toughness values KIc of the as-cast and as-homogenized VW94K alloys are 10.6 ± 0.5 and 13.8 ± 0.6 MPa·m1/2, respectively, i.e., an improvement of 30.2% in KIc is achieved via the dissolution of the Mg24(Gd, Y)5 eutectic phases. The initiation and propagation of microcracks in an interrupted fracture test are observed via an optical microscope (OM) and scanning electron microscope (SEM). The fracture surfaces of the failed samples after the fracture toughness tests are examined via an SEM. The electron backscatter diffraction (EBSD) technique is adopted to determine the failure mechanism. The results show that the microcracks are initiated and propagated across the Mg24(Gd, Y)5 eutectic compounds in the as-cast VW94K alloy. The propagation of the main cracks exhibits an intergranular fracture pattern and the whole crack propagation path displays a zigzag style. The microcracks in the as-homogenized alloy are initiated and propagated along the basal plane of the grains. The main crack in the as-homogenized alloy shows a more tortuous fracture characteristic and a trans-granular crack propagation behavior, leading to the improvement of the fracture toughness.

Dynamic mode II fracture mechanism of rocks using a novel double-edge notched flattened Brazilian disc specimen in the split Hopkinson pressure bar tests

The dynamic mode I fracture behaviors of rocks have been extensively investigated during recent decades, while the dynamic mode II fracture mechanism of rocks and its comparison with dynamic mode I fracture are poorly understood. In this study, a novel double-edge notched flattened Brazilian disc (DNFBD) specimen is introduced for dynamic mode II fracture toughness tests of rocks in the split Hopkinson pressure bar (SHPB) tests. First, using the finite element method (FEM), the stress distribution of the DNFBD specimen is analyzed and the stress intensity factors (SIFs) and the T-stress at the crack tips are determined. Then, dynamic mode II fracture tests are conducted on sandstone using the DNFBD specimen with proper specimen geometry configuration. High-speed photography and digital image correlation (DIC) results show that the shear crack initiates from the notch tips and propagates along the shear ligament of the DNFBD specimen, which demonstrates the reliability of the DNFBD-SHPB method for dynamic mode II fracture tests of rocks. Experimental results show that the dynamic mode II fracture toughness KIIC of rocks generally increases with increasing loading rate, exhibiting significant loading-rate dependence. For comparative investigation, dynamic mode I fracture tests using the notched semi-circular bend (NSCB) specimen are conducted on the same rocks. The results indicate that the mode II fracture toughness KIIC values are always higher than the mode I fracture toughness KIC values of rocks over a wide range of loading rates. The microscopic fracture mechanism of sandstone is interpreted based on the thin section observations. Under quasi-static loading, trans-granular (TG) fractures are more popular in the mode II fracture tests, while inter-granular (IG) fractures dominate the failure of mode I fracture. However, with the increase of loading rate, TG fracture becomes increasingly prevalent in both the dynamic mode II fracture and the dynamic mode I fracture of rocks. The results of this study are of great significance for understanding the dynamic mode II fracture mechanism of rocks.

Tensile and tear-type fracture toughness of gypsum material: Direct and indirect testing methods

In this context, four specimens, i.e. (i) circumferentially notched cylindrical torsion (CNCT), (ii) circumferentially notched cylindrical direct tension (CNCDT), (iii) edge notch disc bend (ENDB) and (iv) three-point bend beam (3PBB), were utilized to measure the modes I and III fracture toughness values of gypsum. While the CNCT specimen provides pure mode III loading in a direct manner, this pure mode condition is indirectly produced by the ENDB specimen. The ENDB specimen provided lower KIIIc and a non-coplanar (i.e. twisted) fracture surface compared with the CNCT specimen, which showed a planar mode III fracture surface. The ENDB specimen is also employed for conducting pure mode I (with different crack depths) and mixed mode I/III tests. KIc value was independent of the notch depth, and it was consistent with the RILEM and ASTM standard methods. But the mode III fracture results were highly sensitive to the notch depth. While the fracture resistance against mode III was significantly lower than that of mode I, the greater work of fracture under mode III was noticeable. ©2022 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Influence of fluorine content on crystallization and mechanical properties of SiO2–Li2O glass-ceramics

The effect of fluorine content on the crystallization, microstructure and mechanical properties of SiO2–Li2O glass-ceramics was investigated. Four parent glass samples with different fluorine content were prepared by melt quenching method. The glass-ceramic samples are obtained by heat treatment including nucleation and crystallization process. It was found that lithium disilicate (Li2Si2O5) is the main phase crystallizing in the course of heat treatment, while lithium metasilicate (Li2SiO3), petalite (LiAlSi4O10), and fluorapatite (Ca5(PO4)3F) appear in some of the samples as minor phases. The addition of fluorine increases the crystallinity and promotes the formation of Li2Si2O5 and Ca5(PO4)3F crystals, and thereby regulates the microstructure and mechanical properties. A proper amount of fluorine significantly enhanced mechanical properties. The SiO2–Li2O glass-ceramic sample with 0.5 mol% fluorine content has the highest Vickers hardness HV value of 6.64 GPa and highest fracture toughness KIC value of 0.932 MPa m1/2 after crystallization process at 670 °C for 8 h. The SiO2–Li2O glass-ceramic sample with 1.0 mol% fluorine content has the highest Young’s moduli E value of 102.1 GPa after crystallization process at 670 °C for 8 h.

Fracture mechanism of high toughness Ti2AlNb/Ti6Al4V layered metal composites

In this paper, the fracture toughness of the Ti2AlNb/Ti6Al4V layered metal composite(LMC) prepared by vacuum hot pressing was tested, and the fracture mechanism was investigated by in-situ fracture mechanics process. The results show that the KIC value of the composite is 70 MPa∙m1/2, which is more than twice higher than the fracture toughness of Ti2AlNb alloy. The layered structure, the tough Ti6Al4V layer, and the interface layer caused complex crack propagation, which played an important role in improving the fracture toughness of the composite.

Microstructure and Mechanical Characterization of Novel Al2O3-(NiAl-Al2O3) Composites Fabricated via Pulse Plasma Sintering.

The scientific goal of this paper is to study and explain the relationship between the microstructure of a ceramic-intermetallic composite fabricated by consolidating a mixture of Al2O3 and NiAl-Al2O3 using the PPS technique and its basic mechanical properties. Six series of composites were manufactured. The obtained samples differed in the sintering temperature and content of compo-powder. The base powders, compo-powder, and composites were investigated using SEM equipped with an EDS and XRD. Hardness tests and KIC measurements were applied to estimate the mechanical properties of the fabricated composites. The wear resistance was evaluated using a "ball-on-disc" method. The results demonstrate that the density of the obtained composites increases with the increased temperature of the sintering. The content of NiAl + 20 wt.% Al2O3 did not have a determining effect on the hardness of the manufactured composites. The highest hardness, contacting 20.9 ± 0.8 GPa, was found for the composite series sintered at 1300 °C and 2.5 vol.% of compo-powder. The highest KIC value from all the studied series equaled 8.13 ± 0.55 MPa·m0.5 and was also achieved for the series manufactured at 1300 °C (2.5 vol.% of compo-powder). The average friction coefficient during the ball-friction test with the Si3N4 ceramic counter-sample was between 0.8 and 0.95.

Sintering and Characterization of Alumina Reinforced Tungsten Carbide Cobalt Composite

This paper is aimed at investigating the mechanical properties of spark plasma sintered alumina (Al2O3) reinforced with tungsten-carbide-cobalt (WC-12wt%Co). Pure Al2O3 and admixed-Al2O3 powders were sintered using spark plasma sintering (SPS) technique at a temperature of 1800°C, heating and cooling rates of 100 °C/min and 200 °C/min, a pressure of 48 MPa in an argon atmosphere. The density, hardness (HV) and indentation fracture toughness (KIC) of the sintered samples were measured and compared. Between 9 and 17% increase was observed in the HV values of the additives, while the KIC value recorded an increase between 17 and 63% for all the additive as compared to pure Al. Phase analysis and identification was carried out with X-ray diffraction and microstructure was taken with scanning electron microscope. The results obtained showed the samples potential for cutting tool applications.

Anisotropic fracture toughness of a selective laser melted martensitic stainless steel

Anisotropic fracture toughness of selective laser melted AISI 420 built in two build directions was investigated. Post-process heat treatments were performed at 200 °C and 400 °C to study the tempering effect. The parallel group exhibited better fracture toughness than the perpendicular group in both as-built and tempered states. KIC value of the 400 °C tempered state was superior to that of other states due to a coarser martensite phase. Elongated cellular structure was responsible for the anisotropic behavior because of a build direction effect, while the anisotropy was barely changed by the tempering treatments.

An integrated experimental and numerical study of size effect on the mode I fracture toughness of rock

Rock is a typical quasi-brittle and heterogeneous material that shows the apparent size effect of mode I fracture toughness (KIc). To further explore the mechanism of the size effect on the KIc of rocks, a novel experimental procedure, i.e., combining a semi-circular bend test with a small-scale direct tension test, is conducted to determine the KIc and fracture process zone (FPZ) length of rock specimens of various sizes. Meanwhile, the four-dimensional lattice spring model (4D-LSM), incorporating a cohesive zone model (CZM) and the modified Mohr–Coulomb strength criterion for bond fracturing, is adopted to interpret these experimental results. After a detailed comparison between experimental and numerical results, we conclude that the size effect is dominated by the softening zone size of the mesoscale grain bond’s constitutive behaviour, rather than other mechanical parameters of rock, e.g., tensile strength, cohesion, and internal friction angle. Without introducing the size-dependent constitutive model into the discrete numerical model but only using a softening model for the mesoscale grain bond, we have successfully reproduced the change in KIc values and FPZ length of the rock specimens of various sizes with high goodness-of-it for the Bazant size effect curve, which indicates that the discrete numerical model can sufficiently reproduce or interpret the size effect in the KIc of rocks.

Production of Reactive Alumina from Domestic Sources and Its Use in Alumina-Based Self-Flowing Castable Refractories

High-quality alumina raw materials allow corundum-based brick and monolithics to perform to the full potential of alumina as a refractory material. Dense packing of the matrix to a submicron range and reduction of water demand of castables can be facilitated by a new family of multimodal reactive alumina. Dispersing aluminas ensure uniform mixing of dispersion and setting adjustment additives. In this study, fine reactive alumina powder production was investigated using Bayer gibbsite as a starting material. Experimental studies consist of two steps; in the first step, the soda content was reduced by means of boric acid and distilled water and then the powders obtained optimum conditions were ground in an attritor ball mill using distilled water for 8 hours. In the second step, physical characteristics such as rheological behaviours, bulk density, open porosity and thermal shock resistance, bending and compressive strength and fracture toughness in accordance with ASTM E399 standard were determined. Finally, KIC values of the refractory samples which was prepared with the reactive alumina produced from Seydişehir gibbsite were higher than that of the refractory mixture containing commercial reactive alumina obtained from a refractory company.

Effect of silane coupling agent and concentration on fracture toughness and water sorption behaviour of fibre-reinforced dental composites.

This paper presents the effect of silane treatment of S-2 Glass fibres on the fracture toughness and water sorption/solubility behaviour of fibre-reinforced flowable dental composites. The effect of epoxy- and methacrylate-based silane coupling agents (SCAs) on the mechanical strength and hydrolytic properties were investigated. The concentration of the selected SCAs on the mechanical and physical properties were investigated. The influence of molecular structure and concentration in the interfacial adhesion at the fibre-matrix interfaces was also studied. Short S-2 Glass fibres of 250µm in length and 5µm in diameter were etched with acid to remove any impurities and roughen the surface. The acid-etched fibres were silane treated with 3MPS, 3GPS, and 8MOTS at different concentrations by weight (%). The silane-treated fibres were incorporated at 5 % into the dental resin mixture. Untreated fibres were added at 5 % to the dental resin mixture and served as the control group. The physical properties such as water sorption, solubility, and desorption along with mechanical properties such as fracture toughness and total fracture work of the fibre-reinforced dental composites grafted with the above-mentioned SCAs were evaluated. The surface morphology of the fractured surface was studied and analysed. The fracture toughness tests showed that the dental composites grafted with optimum weight per cent (wt. %) concentration of the SCA had a better stress intensity factor (KIC) when compared to the 2.0wt. % and 3.0wt. % concentration. The KIC value of dental composites grafted with untreated surface etched glass fibres was less than the KIC values of dental composites grafted with optimum concentrations of 3MPS, 3GPS, and 8MOTS by 81.6 %, 38.6 %, and 110.5 %, respectively. A similar trend was found while investigating the total work of fracture of the dental composites, between optimum concentration, 2.0wt. % and 3.0wt. % concentration of respective SCA. The increase in silane concentration also led to an increase in the water sorption/solubility characteristics. The absorption of water was most severe in the fibre-reinforced dental composites without silane treatment (32.9µg/mm3). The ANOVA results showed that the fibre-reinforced dental composites grafted with 8MOTS at optimum concentration showed an increase in fracture toughness when compared to optimum concentrations of 3GPS and 3MPS by 51.9 % and 15.9 %, respectively. The enhanced mechanical and physical characteristics are due to the increased adhesion between the fibre and silane achieved from the optimum wt. % concentration of 8MOTS. Similarly, dental composites grafted with 8MOTS at optimum concentration showed a decrease in water sorption characteristics when compared to optimum concentrations of 3GPS and 3MPS by 18.2 % and 0.6 %, respectively. The decreased water sorption characteristics at the optimum concentration of 8MOTS could be due to the reduced availability of reactive hydroxyl groups and the hydrophobic characteristics of 8MOTS. Silane coupling agents (SCAs) are important components of dental composites. The type and concentration of SCA have a significant effect on material properties. The current study focuses on understanding the effects of different SCAs and wt. % concentrations on the interfacial fracture behaviour and the influence of different SCAs on the water sorption and solubility behaviour of S-2 Glass fibre-reinforced flowable dental composites.

High-Temperature Erosion of SiC-NiCrAlY/Cr3C2-NiCr Coating

High-temperature erosion is a detrimental phenomenon in industries where particle flow exists, in which the search for new materials and mixes to increase the lifespan of mechanical components exposed to erosion is crucial. The present work studied the erosion resistance of two coatings at 25 °C (RT) and 900 °C in a sandblast-type rig. The coatings were fabricated with cermet-type powders: (C1) commercial Cr3C2-NiCr and (C2) commercial Cr3C2-NiCr mixed with a laboratory-conditioned powder consisting of NiCrAlY (linking matrix) and SiC (ceramic phase). Both coatings were applied on an Incoloy 330 substrate using an HVOF thermal spray process. The C2 coating was 11% harder than C1 but had a 62.2% decrement in its KIC value. The erosion test results at RT and 900 °C showed better erosion resistance on C1 than C2 at both testing temperatures and the three impact angles (30°, 60°, and 90°); this was attributed to the minor KIC induced by SiC hard particles and the bigger propagation of inter-splat and trans-splat cracks in C2. The erosion mechanisms at RT and 900 °C were similar, but at high temperature, the apparent size of plastic deformation (micro-cutting, grooves, and craters) increased due to an increase in the matrix ductility. Maximum penetration depth always occurred at a 60° impact angle.

Synthesis of high-entropy carbides from multi-metal polymer precursors

Three compositions of high-entropy carbides, (TiHfVNbTa)C, (TiZrHfNbTa)C and (TiZrNbTaW)C were synthesized via a modified Pechini process, in which citric acid served as both a cation chelating agent and a carbon source. Through pyrolysis and spark plasma sintering, single phase high-entropy carbides were formed from the homogeneous precursors at a relatively low temperature of 1800 °C. The dispersion of cations in the polymer precursor facilitated shorter diffusion distances in polymer-derived materials, and thus compositional homogeneity was significantly improved relative to materials produced by a solid-state method, as quantified by a defined coefficient of variation applied to energy-dispersive X-ray spectroscopy. A finer microstructure in polymer-derived materials results in improved fracture toughness with a KIC value of 4.29 MPa·m1/2 achieved for (TiHfVNbTa)C.

Evaluation of age-hardening time on the mechanical behavior of Al-Mg-Si (Al 6063) alloy composites reinforced with alumina particles

This research attempts to evaluate the effects of age-hardening on the mechanical behavior of aluminum (6063) alloy reinforced with alumina (Al2O3) particles. Aluminum (6063)/Al2O3 composites containing 0, 5, 7, 9 and 10 wt% Al2O3 particles respectively were fabricated using double stir casting technique and representative samples from each composition were subjected to age-hardening treatment at 180 °C for 60 min, 120 min and 180 min respectively. The microstructures were evaluated using SEM while fracture toughness and tensile properties were used to evaluate the mechanical behavior of both the as-cast composites and their age-hardened counterparts. The microstructure evaluation reveals formation of coherent precipitates of Mg2Si evenly distributed in the age-hardened composites. Significant improvement in tensile strength values of 129.8, 88.6, 90.6, 137.9 and 170.6 MPa were obtained for the age-hardened composites compared to their as-cast composites with 42.2, 72.9, 81.2, 80.84 and 71.5 MPa across the series respectively. Similar trend was observed for fracture toughness with KIC values of 7.5, 5.8, 4.5, 7.8 and 9.6 MPam1/2 for age-hardened composites compared with 4.68, 4.64, 4.07, 4.06 and 3.59 MPam1/2 obtained for their as-cast composites. Age-hardening time of 180 min impart better tensile strength and fracture toughness.

An Experimental Study on the Determination of Shale KIC by Semi-Disk Three-Point Bending

In order to accurately test the KIC of the vertical stratification direction of shale, a semi-circular bending specimen with a linear chevron notch ligament (LCNSCB) was designed. The minimum dimensionless stress intensity factor (Y*min) of the LCNSCB specimen was calculated by the finite element method and the slice synthesis method, respectively. Two sets of prefabricated samples of the LCNSCB specimen under arrester and divider mode were used to conduct three-point bending loading experiments. The dispersion of the measured KIC value of the specimens was analyzed by standard deviation and coefficient of variation, and the reason that the KIC dispersion of specimens in divider mode was larger than in arrester mode was discussed. Compared with the experimental data of the existing literature, the data of this experiment shows that the LCNSCB specimen can avoid the disadvantage of lower measured KIC values due to a larger fracture processing zone featured in the CSTSCB and CCNBD specimens, combined with the merits of a shorter fracture processing zone of the SR or CR specimens, and the render measured the KIC value to be closer to the material’s true fracture toughness value. The narrow ligament of the LCNSCB specimen has a favorable crack propagation guiding effect, can generate consistent KIC values, and could be used to accurately test the fracture toughness of rock material in vertical bedding direction.

Mechanical Stability of Self-Adhesive/Ion-Releasing Resin Composites

The purpose of this study was to assess the effects of water storage on the surface microhardness (VHN) and fracture toughness (K1C) of two self-adhesive restorative materials compared to traditional resin composite and resin-modified glass ionomer cement (RMGIC) restorative materials. Methods: Two self-adhesive materials (Activa and Vertise Flow), a nonflowable composite (Filtek Z250), and an RMGIC (Fuji II) were evaluated. Hardness measurements (n = 12) were recorded at three time intervals: (i) one-hour post-irradiation; (ii) after one day of storage in water at 37 °C; and (iii) after 90 days of storage in water at 37 °C. Fracture toughness (K1C) measurements (n = 12) were conducted after one day of storage in water at 37 °C and 90 days of storage in water at 37 °C. ANOVA and Tukey post hoc tests were used for statistical analysis. Results: Baseline VHN data were 38.2–58.3, decreasing significantly to 28.8–55.6 following 90 days of water storage. The Filtek Z250 had the highest VHN before and after storage, while the Activa had the lowest. KIC values varied between 0.98–1.32 MPa·m0.5. The highest value was for the Filtek Z250 while the Fuji II showed the lowest value (after both 1 and 90 days of storage in water). However, KIC values decreased significantly after storage, except for the Fuji II. Conclusion: Self-adhesive/ion-releasing resin composites were negatively affected by water storage. Material reinforcements are possible future areas to explore.

Impact of thickness, void content, temperature and loading rate on tensile fracture toughness and work of fracture of asphalt mixtures- An experimental study using the SCB test

Asphaltic concrete mixtures are among the most common construction materials for the pavement of roads. As a multi-phase composite mixture with randomly distributed aggregates inside the mastic part, the mechanical properties of such materials can be influenced by different factors. Cracking and induced fracture is among the common degradation and failure modes in these construction mixtures that often takes place in cold regions. In this research, the effects of some influencing parameters including temperature, air void percentage and loading rate are investigated experimentally on the fracture toughness (KIc) and work of fracture (WIc) of hot mix asphalt material. Edge notched semi-circular bend (SCB) specimen was employed to conduct mode I fracture experiments. The thickness of SCB samples were considered as variable and the HMA mixtures were tested with two SCB thicknesses of 30 and 60 mm. The experimental results showed that both fracture toughness and fracture work are increased by increasing the thickness. However, the effect of thickness on the fracture work was much more significant than the KIc value. Also, the fracture and cracking resistance parameters were increased by decreasing the temperature and air void content. Both KIc and WIc values were also increased by increasing the loading rate in the investigated range of 1 to 8 mm/min. The most influencing parameters on the change of fracture resistance parameters were the temperature, loading rate, air void content and thickness, respectively.

Numerical & experimental assessment of mixed-modes (I/II) fracture of PMMA/hydroxyapatite nanocomposite

Nowadays, polymer-based nanocomposites widely used in biocompatible restorative materials and medical equipment and polymer-based nanocomposites are the most important of them. One of the most useful biocompatible polymers in this field is polymethyl methacrylate (PMMA). In this research, the experimental and numerical analysis of the fracture behaviors of PMMA nanocomposites that are reinforced with hydroxyapatite (HA) nanoparticles are investigated under pure mode I, pure mode II and mixed-mode (I/II) loading. For this purpose, notched semi-circular bend (SCB) specimens containing various Wt. % of HA nanoparticles and different notch angles were fabricated. Then, the mechanical properties of the specimens and the fracture toughness of the notched specimens were investigated. To evaluate the accuracy of nanomaterial distribution, scanning electron microscopy (SEM) images are analyzed and it was observed that at Wt. % of more than 1.5%, specimens with more cumulative masses of hydroxyapatite nanoparticles have more strength reduction. Also, to investigate the fracture behaviors of the produced specimens, the failure criteria of maximum tangential stress (MTS) and generalized maximum tangential stress (GMTS) were evaluated and it was observed that the GMTS criterion is more in line with the experimental results. In addition, the results showed that by increasing the Wt. % of HA nanoparticles from zero to 1% mass, the KIC value increased compared to the pure PMMA, whereas, by increasing this value to 1.5% mass, the fracture toughness decreased. Finally, the fracture initiation and propagation are evaluated using FEM and XFEM numerical methods and the values of stress intensity factors and crack growth path are predicted and compared with experimental values. The obtained numerical results are in good agreement with the experimental results.