Main Toughening Mechanisms Research Articles

Morphology, toughness mechanism, and thermal properties of hyperbranched epoxy modified diglycidyl ether of bisphenol A (DGEBA) interpenetrating polymer networks

AbstractNew hyperbranched poly(trimellitic anhydride‐triethylene glycol) ester epoxy (HTTE) is synthesized and used to toughen diglycidyl ether of bisphenol A (DGEBA) 4,4′‐diaminodiphenylmethane (DDM) resin system. The effects of content and generation number of HTTE on the performance of the cured systems are studied in detail. The impact strength is improved 2–7 times for HTTE/DGEBA blends compared with that of the unmodified system. Scanning electron microscopy (SEM) of fracture surface shows cavitations at center and fibrous yielding phenomenon at edge which indicated that the particle cavitations, shear yield deformation, and in situ toughness mechanism are the main toughening mechanisms. The dynamic mechanical thermal analyzer (DMA) analyses suggest that phase separation occurred as interpenetrating polymer networks (IPNs) for the HTTE/DGEBA amine systems. The IPN maintains transparency and shows higher modulus than the neat epoxy. The glass transition temperature (Tg) decreases to some extent compared with the neat epoxy. The Tg increases with increase in the generation number from first to third of HTTE and the concentrations of hard segment. The HTTE leads to a small decrease in thermal stability with the increasing content from TGA analysis. The thermal stability increases with increase in the generation number from first to third. Moreover, HTTE promotes char formation in the HTTE/DGEBA blends. The increase in thermal properties from first to third generation number is attributed to the increase in the molar mass and intramolecular hydrogen bridges, the increasing interaction of the HTTE/DGEBA IPNs, and the increasing crosslinking density due to the availability of a greater number of end hydroxyl and end epoxide functions. Copyright © 2008 John Wiley & Sons, Ltd.

A positron annihilation study on the microstructure of cyanate ester resin toughened by carboxyl‐terminated butadiene‐acrylonitrile rubber system

AbstractThe toughness of cyanate ester (CE) resin matrix improves significantly with the addition of carboxyl‐terminated butadiene‐acrylonitrile rubber (CTBN). The curing behavior of the system was studied by differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. The results show that carboxyl groups on the CTBN chain have a slight activation effect on the CE curing reaction at the beginning of the curing process. Phase separation was found to be the main toughening mechanism for CE/CTBN composites. The existence of macro‐size pores induced by the decomposition of a small amount of the low weight molecular part of CTBN might be another toughening mechanism. It is confirmed that positron annihilation lifetime spectroscopy (PALS) is still valid in such a system where macropores filled with gas molecules exist. When a high weight percentage of CTBN (>8%) was added to CE, free‐positron annihilation was found to be the dominant annihilation process in the macropores. For CTBN weight percentage higher than 8%, the contribution of ortho‐positronium (o‐Ps) annihilation in the macropores to τ3 and I3 was found to be insignificant. It is effective to use PALS as a probe of free‐volume properties in such systems by determining the changes in the τ3 and I3 of the composite. The compatibility and interfacial adhesion of the composites can be estimated from the changes in the free‐volume properties of the composites. Copyright © 2008 John Wiley & Sons, Ltd.

Fracture toughness, strength and slow crack growth in a ceria stabilized zirconia–alumina nanocomposite for medical applications

Mechanical properties and slow crack growth (SCG) behavior of a 10Ce-TZP/Al 2O 3 nanocomposite currently developed as a biomaterial are considered. Fracture toughness is determined for sharp, long (double torsion) and short (indentation) cracks and a good agreement is found between the two types of cracks. The main toughening mechanism in the nanocomposite is the tetragonal to monoclinic phase transformation of the ceria-stabilized zirconia (Ce-TZP) phase. Transformation at the surface of ground specimens leads to surface compressive induced stresses and an increase in strength. Crack velocity curves ( V– K I curves) are obtained under static and cyclic fatigue using the double torsion method. The static V– K I curve in air reveals the three stages characteristic of stress corrosion with a threshold K I0 ∼ 4.5 MPa m 1/2 and a fracture toughness of 8.8 MPa m 1/2 significantly higher than those of currently used inert bioceramics (i.e. alumina and Y-TZP). A crack growth accelerating effect is shown under cyclic loading, correlated with a decrease in the threshold. However, the cyclic fatigue threshold (4 MPa m 1/2) still stands above that of current biomedical grade alumina and zirconia.

Fabrication, property characterization and toughening mechanism of HA-ZrO2(CaO)/316L fibre composite biomaterials

HA-ZrO2(CaO)/316L fibre composites were successfully fabricated with vacuum sintering method and their properties and toughening mechanism were studied. The results showed that HA-ZrO2(CaO)/316L fibre biocomposite having 20 vol% fibres had optimal comprehensive properties with bending strength, Young’s modulus, fracture toughness and relative density equal to 140.1 MPa, 117.8 GPa, 5.81 MPa·m1/2 and 87.1%, respectively. The research also addressed that different volume ratios of the composites led to different metallographic microstructures, and that metallographic morphologies change regularly with volume ratios of the composites. 316L fibres were distributed randomly and evenly in the composites and the integration circumstance of the two phases was very well since there were no obvious flaws or pores in the composites. Two toughening mechanisms including ZrO2 phase transformation toughening mechanism and fibre pulling-out toughening mechanism existed in the compsites with the latter being the main toughening mechanism.

Fracture behaviour of microcrack-free alumina–aluminium titanate ceramics with second phase nanoparticles at alumina grain boundaries

Alumina + 10 vol.% aluminium titanate composites were obtained by colloidal filtration and reaction sintering of alumina and titania. The materials were dense with aluminium titanate grains of average sizes 2.2–2.4 μm located mainly at alumina triple points. The reaction sintering schedule promoted the formation of additional nanometric grains, identified as aluminium titanate using STEM–EDX analysis between the alumina grains. This special microstructure led to a change of the toughening mechanism from the typical crack bridging reported for microcrack-free composites fabricated from alumina and aluminium titanate powders to microcracking. The identification of microcracking as the main toughening mechanism was done from the analysis of stable fracture tests of SENVB samples in three points bending and fractographic observations. Monophase alumina materials with similar grain sizes were used as reference. Different fracture toughness parameters were derived from the load–displacement curves: the critical stress intensity factor, K IC, the critical energy release rate, G IC, the J-Integral and the work of fracture, γ WOF, and the R curves were also built. The comparison between the linear elastic fracture parameters and the non-linear ones revealed significant toughening and flaw tolerance.

Effects of coupling agents on the impact fracture behaviors of T-ZnOw/PA6 composites

The impact fracture behaviors of polyamide 6 (PA6) composites with tetra-needle-shaped zinc oxide whisker (T-ZnOw) are studied in this work. Two different silane coupling agents, KH-550 with amino groups and KH-560 with epoxy groups, are used to treat T-ZnOw. The results show that the coupling agent types, content as well as the T-ZnOw content have much influence on the impact fracture behaviors of T-ZnOw/PA6 composites. Compared with the composites filled with untreated T-ZnOw, in which the impact strength decreases linearly with the increasing content of filler, chemical treated T-ZnOw improves the impact strength of the composites apparently due to the improved interfacial interaction between T-ZnOw and matrix. Further results show that the main toughening mechanisms of treated T-ZnOw in PA6 composites include the pullout of T-ZnOw from matrix, the anchoring role of other three needles when one of the four needles of T-ZnOw is pulled out from matrix, and the local yielding or deformation of matrix around the T-ZnOw.

Changes in Fracture Micromechanism with Increasing Reinforcement Volume Fraction in Glass Matrix Composite

Fracture behaviour and mechanical properties are the key features when a material for given application is supposed to be selected. Advanced glass ceramics composites are perspective structural materials for many applications due to their low production expenses and satisfactory properties even at elevated temperatures. Borosilicate glass matrix composite reinforced by alumina platelets was investigated to describe toughening mechanisms and their changes in a wide range of temperatures (from room temperature up to glassy transition temperature Tg). The dissipation of energy by bridging and/or deflection of propagating crack by alumina platelets uniformly dispersed in the glass matrix were the main toughening mechanisms observed. The alumina platelets have a higher ability to deflect propagating crack in comparison with spherical or rectangular particles having the same volume. Three and four point bend test for Young’s modulus and flexural strength determination was used. Fracture toughness determination was conducted using chevron notch technique. More than 100% increase of fracture toughness was observed when 30% of alumina platelets were added in to borosilicate matrix.

Effect of TiC whisker addition on properties of YG10F alloy

The cemented carbide material (YG10F) with different additions of TiC whisker (0%, 0.3%, 0.6%, mass fraction) was prepared by different techniques. The effect of TiC whisker addition on the density, microhardness and toughness of the experimental material was investigated. The results indicate that after the wet-milling for 8 h and sintering in vacuum at 1 440 °C, the toughness of YG10F is remarkably improved and meanwhile higher microhardness is obtained by 0.3% TiC whisker addition. Preliminary analysis suggests that the main toughening mechanism of TiC whisker in YG10F is whisker pull-out and bridging phenomena.

Effect of MoSi2 Particles on the Fracture Toughness of AlN-, SiC-, and Si3N4-based Ceramics

The effect of 30 vol% MoSi2 addition on the fracture toughness of AlN-, SiC-, and Si3N4-based composites is investigated. To isolate the contribution of MoSi2 particulate, the toughness of the composite is compared to that of the reference AlN, SiC, and Si 3N4 matrixes. It turns out that the main toughening mechanisms are residual stress and crack deflection. The residual stress is evaluated through the strain measure in the reinforcing particles by X-ray diffraction, and the crack deflection by analysis of the crack paths. The overall toughening effect is calculated as the sum of these two contributions compared to the experimental values of the toughness of the composites.

Effect of Different Stabilizer Addition on Preparation and Hydrothermal Stability of ZrO<sub>2</sub>-TiN Composites with Varying TiN Content

Fully dense ZrO2-TiN composites containing 1.75-2 mol %Y2O3, 1 mol% Y2O3 and 1 mol% Nd2O3 stabilizers, small amounts of Al2O3, and electrical conductive TiN particles (40-70 vol%) have been produced by hot pressing and spark plasma sintering at 1550°C. Although the intrinsic hardness of TiN (1400 kg/mm²) is higher than that of t-ZrO2 (1200 kg/mm²), the decreasing hardness trend can be attributed to the larger TiN grain size with the higher TiN content. Since TiN is more brittle, the fracture toughness decreases with increasing TiN content. Transformation toughening has been attributed as the main toughening mechanism as a result of fracture toughness decreasing with the transformability. Spark Plasma sintering temperature was too high for mechanical properties and hydrothermal stability of the mixed stabilized composites. The transformability decreases so hydrothermal stability increases linearly with increasing TiN content as a result of smaller volume fraction of t-ZrO2 grains becoming susceptible to hydrothermal transformation due to the shielding effect of the present TiN grains.

Tensile properties of 3D needled C/SiC friction materials produced by chemical vapor infiltration and reactive melt infiltration

Cost effective and high performance 3D needled carbon/silicon carbide friction materials were prepared by a combination of chemical vapor infiltration and reactive melt infiltration.The microstructures and tensile properties of the composites were studied.Results indicated that the composites consisted of carbon,silicon and silicon carbide.The density and porosity of the composites were about 2.1g·cm-3 and 4.4% respectively,and the tensile strength and Young's modulus were about 114-154MPa and 40-63GPa respectively.The composites exhibited excellent pseudo-plastic behavior similar to that of metal.The main toughening mechanism of the as-prepared composites was fiber pull-out,interfacial debonding,deflection of cracks,bridging of fibers and bifurcation of cracks.

Effect of ethylene–acrylate–(maleic anhydride) terpolymer on mechanical properties and morphology of poly(ethylene terephthalate)/polyamide‐6 blends

AbstractBackground: Poly(ethylene terephthalate) (PET)/polyamide‐6 (PA‐6) blends are promising for engineering and food‐packaging applications. However, their poor toughness limits their use. In this study, an ethylene–acrylate–(maleic anhydride) terpolymer (E‐AE‐MA) was added to PET/PA‐6 blends in order to improve the toughness.Results: Izod impact tests indicated an excellent toughening effect of E‐AE‐MA. E‐AE‐MA particles were observed to be selectively dispersed at the interface between PET and PA‐6 phases and in the domain of the PA‐6 phase. Fourier transform infrared spectroscopy and differential scanning calorimetry results demonstrated that the formation of E‐AE‐MA layers around PA‐6 particles cut off the interaction between PET and PA‐6, resulting in an enlarged PA‐6 phase domain.Conclusion: Based on the experimental results, a core–shell microstructure, with PA‐6 as a hard core and E‐AE‐MA as a soft shell, could be suggested. The formation of this core–shell microstructure, along with the increased PA‐6 phase domain size, is the main toughening mechanism of E‐AE‐MA in PET/PA‐6 blends. Copyright © 2007 Society of Chemical Industry

Influence of AlN addition on thermal and mechanical properties of cordierite-based glass/ceramic composites

Cordierite-based glass/ceramic composites doped with different volume fractions of AlN particles were successfully prepared by hot-pressing process. Thermal and mechanical properties of the composites were also investigated. Results show that the thermal conductivity, the flexural strength and fracture toughness of cordierite-based glass/AlN composites increase with increasing AlN content. The thermal conductivity of the composite doped with 50 vol.% AlN reaches 6.5 W/m K. By incorporation of 40 vol.% AlN, the flexural strength and fracture toughness of the composites increase 81 and 139% compared to the unreinforced cordierite-based glass matrix, respectively. XRD analysis indicated that no chemical reaction occurred between cordierite-based glass and AlN. The main strengthening and toughening mechanisms is load transfer and crack deflection.

Micro- and Nano-Sized Calcium Carbonate Toughened Polystyrene

Polystyrene (PS)/CaCO3 micro- and nanocomposites were prepared by melt mixing in a twin screw extruder using four particles with average diameters of 55 nm, 70 nm, 240 nm, and 10 /microm in various loadings. It was found that PS can be effectively toughened with 1 wt% cubic shaped CaCO3 particles (55 nm, 70 nm, 10 microm) achieving significant increases of impact strength by 52%, 95%, and 190% respectively relative to neat PS. However, the needle shaped nanoparticles (240 nm) were not effective for improving the toughness of the matrix. SEM analysis of the fractured surfaces indicated good fillers dispersion at the optimum filler loading of 1 wt%, and the cavitations-induced shear yielding in the polymer matrix are shown to be the main toughening mechanism.

Toughening and reinforcing alumina matrix composite with single-wall carbon nanotubes

The authors report an efficient way of incorporating single-wall carbon nanotubes (SWNTs) into alumina matrix with strong interfaces by heterocoagulation. The fracture toughness of SWNTs/Al2O3 composite reaches 6.40±0.3MPam1∕2, which is twice as high as that of unreinforced alumina. The flexure strength of the composite also increases by 20%. The main toughening mechanism is crack bridging of SWNTs, and SWNT pullout takes effect also.

Rubber toughening of phenolic resin by using nitrile rubber and amino silane

The aim of this study was to investigate rubber toughening of resol type phenol–formaldehyde resin. For this purpose, phenolic resin was first modified by acrylonitrile butadiene rubber alone, and then by using nitrile rubber together with 3-aminopropyltriethoxysilane. Test specimens were prepared by mixing and casting of liquid phenolic resin in three groups. In the first one, neat phenolic resin specimens were produced. In the second group, phenolic resin was modified with 0.5, 1, 2, and 3 wt% nitrile rubber, while in the last group modification was carried out by using 0.5 wt% nitrile rubber together with 1, 2, and 4 wt% amino silane (with respect to nitrile rubber). All specimens were heat cured in an oven. In order to observe behavior of the specimens, three-point bending, Charpy impact, plane-strain fracture toughness, and dynamic mechanical analysis tests were conducted according to the related ISO standards for all specimen groups. Scanning electron microscopy (SEM) was also used for the fractographic analysis of some samples. It was concluded that although there were problems in mixing and casting of liquid resol type phenolic resin, its toughness could be improved by using nitrile rubber and amino silane. Modification by using nitrile rubber and amino silane together was much more effective than by using only nitrile rubber. For instance, in this synergistic case Charpy impact strength and fracture toughness values of the neat phenolic specimens were increased 63 and 50%, respectively. SEM studies indicated that the main rubber toughening mechanism was shear yielding observed as deformation lines, especially initiated at the domains of nitrile rubber and amino silane.

Correlation between fracture toughness and leucite content in dental porcelains

To determine the correlation between fracture toughness and leucite content in dental porcelains. The mechanisms by which leucite influences the fracture toughness of dental porcelains were also investigated. Six porcelains were tested: A (Ceramco I/Dentsply), B (Ceramco II/Dentsitply), C (Finesse/Dentsply), D (d.Sign/Ivoclar), Cb (Cerabien/Noritake) and V (Vitadur Alpha/Vita). Bar-shaped specimens were produced, and their fracture toughness was determined by means of the single-edge precracked beam (SEPB) method. The test consisted of fracturing the specimen after a precrack was generated by a bridge-anvil device. KIc was calculated based on fracture force and size of the precrack. Microstructural analysis and determination of the leucite volume fraction were performed on polished specimens etched with 2% HF for 15s by means of scanning electron microscopy. Fractographic analysis was performed on fracture surfaces. Porcelains A and B presented the highest leucite contents (22%) and similar KIc values (1.23 and 1.22 MPa m1/2, respectively), significantly higher than the other materials. Porcelains C and D presented similar K(Ic) values (0.81 and 0.93 MPa m1/2, respectively), but different leucite contents (6 and 15%, respectively). Porcelain D presented higher KIc compared to porcelains Cb and V (0.71 and 0.75 MPa m1/2, respectively), which presented similar values and the lowest leucite contents (0%). Fractographic analysis showed that porcelains with higher leucite content presented higher incidence of crack deflection. For the materials evaluated in this study, the leucite content was directly related to KIc. The main toughening mechanism observed was crack deflection around leucite particles and clusters.

Preparation of MWNTs/Al2O3 composites and their mechanical and electrical properties

The mechanical and electrical properties of MWNTs/Al2O3 composite prepared fabricated by hot pressing are investigated. The relation between properties and microstructure of composites is also discussed. With 4wt% MWNTs as reinforcement, the fracture toughness of composite obtained reaches 5.55 MPa·m1/2, which is 80% higher than that of pure Al2O3 obtained in the same conditions. The main toughening mechanism is CNTs’ pinning alumina grain boundaries, and the pullout of CNTs takes effect also. Through adding 2wt% MWNTs and altering the mix method, the fracture toughness of the composite obtained is 3.97 MPa·m1/2. Furthermore, the electrical resistivity is as low as 8.4×10−3 Ω·m, decreasing by 14 orders of magnitude compared with pure alumina ceramics. The function of CNTs in composite is related to the distribution state of CNTs in composite, and the distribution state of CNTs in matrix is dependent on preparation procedures.

Fracture and toughening behavior of aramid fiber/epoxy composites

The effect of short Aramid fibers on the fracture and toughening behavior of epoxy with high glass transition temperature has been studied. Fine dispersion of the fibers throughout the matrix is evidenced by optical microscopy. Compared with neat epoxy resin, the fracture toughness (KIC) of the composites steadily increases with increasing fiber loading, indicating that addition of Aramid fibers has an effective toughening effect to the intrinsically brittle epoxy matrix. Scanning electron microscopy (SEM) indicates that formation of numerous step structures for fiber-filled epoxy systems is responsible for the significant toughness improvement. SEM and transmitted optical microscopy show that fiber pullout and fiber breakage are the main toughening mechanisms for the Aramid fiber/epoxy composites. POLYM. COMPOS. 26:333–342, 2005. © 2005 Society of Plastics Engineers.

Mechanical properties, morphological structure, and thermal behavior of dynamically photocrosslinked PP/EPDM blends

AbstractA dynamically photocrosslinked polypropylene (PP)/ethylene–propylene–diene (EPDM) rubber thermoplastic elastomer was prepared by simultaneously exposing the elastomer to UV light while melt‐mixing in the presence of a photoinitiator as well as a crosslinking agent. The effects of dynamic photocrosslinking and blend composition on the mechanical properties, morphological structure, and thermal behavior of PP/EPDM blends were investigated. The results showed that after photocrosslinking, tensile strength, modulus of elasticity, and elongation at break were improved greatly. Moreover, the notched Izod impact strength was obviously enhanced compared with corresponding uncrosslinked blend. Scanning electron microscopy (SEM) morphological analysis showed that for uncrosslinked PP/EPDM blends, the cavitation of EPDM particles was the main toughening mechanism; whereas for dynamically photocrosslinked blends, shear yielding of matrix became the main energy absorption mechanism. The DSC curves showed that for each dynamically photocrosslinked PP/EPDM blend, there was a new smaller melting peak at about 152°C together with a main melting peak at about 166°C. Dynamic mechanical thermal analysis (DMTA) indicated that the compatibility between EPDM and PP was improved by dynamic photocrosslinking. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3371–3380, 2004