Pull-out Toughening Research Articles

Maximum utilization of nacre-mimetic composites by architecture manipulation and interface modification towards critical damage state

Diverse ultrahigh-strength platelets offer immense promise for high-efficient energy-absorbing design of nacre-mimetic composites. Yet, incompatible interfacial bonding has greatly restricted the pull-out toughening efficiency to a superior level, resulting in insufficient utilization of platelet strength advantages. Herein, we demonstrate an ingenious critical damage state (CDS) control method to utterly exert the energy absorption potential of nacre-mimetics by pursuing the most efficient toughening mode, which is a universal maximum utilization strategy applicable to platelets with arbitrary strengths. Architecture manipulation results suggest that small aspect ratio platelets can generate superb toughness amplification, and stiffening effect is favored for large-size platelets with dense content and high offset ratio, which is consistent with experimental results. More importantly, by identifying the upper limit of shear strength and the lower limit of plastic deformability, we elucidate the interface modification principle imposed on optimized structure to achieve perfect compatibility. Interestingly, we find the uniform staggered pattern is more recommended for superior energy-absorbing design owing to its pronounced strengthening-toughening effect and looser compatibility restriction compared with the random staggered pattern. Furthermore, the proposed CDS control method should not only be generally feasible for nacre-mimetics, but also shed light on other hard/soft layered composite designs targeting utmost synergistic toughening.

Bimodal microstructure toughens plasma sprayed Al2O3-8YSZ-CNT coatings

Current work pursues generating controlled bimodal microstructure by plasma spraying of micrometer-sized Al2O3 and nanostructured spray-dried agglomerate with reinforcement of 20 wt% of 8 mol % yttria stabilized zirconia (8YSZ) and 4 wt% carbon nanotube (CNT) as potential thermal barrier coating (TBC) on the Inconel 718 substrate. Composite coatings exhibit bimodal microstructure of: (i) fully melted and resolidified microstructured region (MR), and (ii) partially melted and solid state sintered nanostructured regions (NR). Reinforcement with 8YSZ has led to an increase in hardness from ∼12.8 GPa (for μ-Al2O3) to ∼13.9 GPa in MR of reinforced Al2O3-YSZ composite. Further, with the addition of CNT in Al2O3-8YSZ reinforced composite, hardness of MR has remained similar ∼13.9 GPa (8YSZ reinforced) and ∼13.5 GPa (8YSZ-CNT reinforced), which is attributed to acquiescent nature and non-metallurgical bonding of CNT with MR. Indentation fracture toughness increased from 3.4 MPam0.5 (for μ-Al2O3) to a maximum of 5.4 MPam0.5 (8YSZ- CNT reinforced) showing ∼57.7% improvement, which is due to crack termination at NR, retention of t-ZrO2 (∼3.3 vol%) crack bridging, and CNT pull-out toughening mechanisms. Modified fractal models affirmed that the introduction of bimodal microstructure (NR) i.e., nanometer-sized- Al2O3, nanostructured 8YSZ and CNTs in the μ-Al2O3 (MR) contributes ∼44.6% and ∼72% towards fracture toughness enhancement for A8Y and A8YC coatings. An enhanced contribution of nanostructured phases in toughening microstructured Al2O3 matrix (in plasma sprayed A8YC coating) is established via modified fractal model affirming crack deflection and termination for potential TBC applications.

Room and high-temperature mechanical properties of ZrB2-based composite alloyed with Ti and refractory metal Nb

ZrB2 and ZrB2-based composites have extensive applications; however, their application is hindered by their low fracture toughness at room temperature. A ZrB2/(NbTi) composite with 60 wt% ZrB2 was densified to 97.1% through hot-pressing sintering at 1600 °C under 30 MPa. The phase, microstructure, and room and high-temperature (800–1200 °C) mechanical properties of the composite were investigated. The composite mainly consisted of a (Ti,Nb,Zr) solid solution (ss), Nb-rich (Ti,Nb)B, Ti-rich (Nb,Ti)B, and ZrB2. The room-temperature fracture toughness of the composite was up to 6.9 MPa·m1/2 due to crack bridging, crack defection, and pull-out toughening mechanisms. The high temperature deformability of ZrB2/(NbTi) improved as the test temperature increased from 800 °C to 1200 °C due to the plastic rheological behaviour of the ductile (Ti,Nb,Zr)ss phase and the localised temperature rise. The room and high-temperature compressive deformation processes and failure mechanisms were discussed in detail.

Investigation on strengthening and toughening mechanisms of Nb-Ti-ZrB2 metal matrix ceramic composites reinforced with in situ niobium and titanium boride

Nb-Ti-ZrB2 metal matrix ceramic composites with a fixed atomic ratio Nb/Ti = 2/1 and ZrB2 volume fraction changing from 0, 11 vol%, 23 vol% to 36 vol% were hot pressed at 1600 °C under 30 MPa. The influence of ZrB2 content and Ti addition on the phase constitution, microstructure evolution, toughening mechanisms and strengthening mechanisms were investigated. It was shown that the formation of in situ Nb-rich (Ti,Nb)B and Ti-rich (Nb,Ti)B was attributed to a high mutual solubility of monoborides and the amount of niobium and titanium borides increased with increasing ZrB2 content. The needle-shaped (Ti,Nb)B phase weakened the damage to fracture toughness caused by ZrB2 particle fracture due to crack bridging, crack defection and the pull-out toughening mechanisms. The highest fracture toughness of the Nb-Ti-ZrB2 composites was 12.0 MPa·m1/2. The stiff (Nb,Ti)B phase acted as a strong obstacle to the dislocation motion, leading to dislocation pile-up and enhancing the strength of the Nb-Ti-ZrB2 composites during compression tests. However, stress concentration around the needle-shaped (Ti,Nb)B phase easily leads to crack initiation and extension, resulting in decreased strength. The yield strength of Nb-Ti-ZrB2 composites ranged from 657.3 MPa to 1783.0 MPa owing to the combined influence of the strenghening mechanism caused by (Nb,Ti)B and the weakening mechanism caused by (Ti,Nb)B. The compressive deformation and failure process were also discussed in detail in this study.

Toughening effect of non-periodic fiber distribution on crack propagation energy of UHTC composites

Different configurations of continuous carbon fiber-reinforced ultrahigh temperature ceramics (UHTCs), by combining coatings and matrix, were produced via electrophoretic deposition (EPD) and slurry infiltration. The toughening of non-periodic fiber distribution induced by the EPD process was investigated through work of fracture analysis. The results show that a non-periodic fiber distribution results in toughness increase from 8 MPa√m to 11 MPa√m with respect to a periodic fiber distribution. This toughness improvement does not strongly affect the flexural strength, which is mainly related to the fiber volumetric amount. It is shown that the assembling of carbon fibers into bundles (i.e. by dispersing the fibers with a non-periodic distribution) increases the crack propagation energy dissipated on the crack-wake from 0.5 kJ/m2 to 1 kJ/m2, which can be mainly ascribed to the fiber/bundle pull-out. On the other hand, the energy dissipated on the crack-tip (as fiber/matrix debonding) is fiber distribution-independent and increases from 0.3 kJ/m2 to 0.4 kJ/m2 with increasing the fiber amount from 33 vol% to 40 vol%. Finally, WoF analysis is proposed as test to evaluate pull-out toughening instead of push-in and push-out tests.

Effects of glass fiber reinforcement and thermoplastic elastomer blending on the mechanical performance of polylactide

Abstract The purpose of this study was to investigate how optimum mechanical properties (strength-modulus-toughness) of inherently very brittle polylactide (PLA) could be obtained by reinforcing with E-glass fibers (GF) and blending with thermoplastic polyurethane elastomer (TPU). Composites and blends were compounded by twin-screw extruder melt mixing, while specimens were shaped by injection molding. SEM analyses revealed that 15 wt% GF and 10 wt% TPU domains, alone and together, could be uniformly distributed in the PLA matrix leading to significant improvements in properties. Mechanical tests indicated that use of TPU blending alone resulted in enormous increases in the ductility and fracture toughness values, while GF reinforcements led to significant increases in strength and elastic modulus values. When GF and TPU were added together, it was observed that crack deflection, debonding and fiber pull-out toughening mechanisms of GF reinforcements were as effective as the rubber toughening mechanism of TPU blending.

Characterization of GNP-Containing Al2O3 Nanocomposites Fabricated via High Frequency-Induction Heat Sintering Route

In this paper, we present alumina (Al2O3) nanocomposites reinforced with various graphene nanoplatelets (GNPs) concentrations (0.75 and 1.25 wt.%) and fabricated by rapid high frequency-induction heat (HF-IH) sintering route. The influence of the GNP on the microstructures, mechanical properties, and interfacial connections of the resulting nanocomposites were thoroughly investigated. GNPs were synthesized using combined chemical oxidation and thermal exfoliation processes and dispersed homogenously into base Al2O3 ceramic matrix using colloidal chemistry technique. Pressure-assisted HF-IH sintering rapidly consolidated nanocomposites close to theoretical densities (~99%) without damaging the GNP intrinsic nanostructures and electron microscopy revealed firmly bonding of the nanocomposite constituents at interfaces. Nanocomposite samples containing 0.75 wt.% GNP demonstrated 60% finer microstructure with 45% higher fracture toughness (KIC) and 9% improvement in hardness against benchmarked monolithic Al2O3. However, nanocomposites loaded with higher GNP contents (1.25 wt.%) showed deprived properties due to GNP accumulations. Homogenous dispersions and two-dimensional features allowed GNP to interact wide area of the matrix grains thus refined the microstructure and gave rise the grain anchoring mechanism thereby led nanocomposite to superior mechanical properties following GNP crack-bridging and pull-out toughening mechanisms.

Morphology and interdiffusion control to improve adhesion and cohesion properties in inverted polymer solar cells

The role of pre-electrode deposition annealing on the morphology and the fracture properties of polymer solar cells is discussed. We found an increase in adhesion at the weak P3HT:PCBM/PEDOT:PSS interface with annealing temperature, caused by increased interdiffusion between the organic layers. The formation of micrometer sized PCBM crystallites, which occurs with annealing above the crystallization temperature of PCBM, initially weakened the P3HT:PCBM layer itself. Further annealing improved the cohesion, due to a pull-out toughening mechanism of the growing PCBM clusters. Understanding how the morphology, tuned by annealing, affects the adhesive and cohesive properties in these organic films is essential for the mechanical integrity of OPV devices.

Strength-toughening model of eutectic ceramic composite with inherent defects

Strengthening and toughening mechanisms in composite ceramics is complex. A change in a single parameter induces multiple property variations. The multiple changes in properties are often incompletely represented in theoretical models. This incompleteness in the parameter chosen fails to explain the mechanism of failure in composite ceramics. The exponential toughness function is used to represent the pull-out toughening mechanism, which dominates the crack growth resistance curve(R-curve). The strengthening-toughening model is established based on the Mori-Tanaka method(M-T method). The influence of inherent defects on toughness function and strength is analyzed by using this model. The theoretical result is compared with the experiment data. This model exactly reflects the change in strength. The theoretical result indicates that defects change the toughness function. Moreover, micro-cracks increase toughness size a c, and the strength of crack instable extensions acutely decreases as defect content increases. This presented model establishes the relationship among the important mechanical parameters of defect, strength, elastic modulus, and the R-curve.

Characterization of multiwalled carbon nanotube-reinforced hydroxyapatite composites consolidated by spark plasma sintering.

Pure HA and 1, 3, 5, and 10 vol% multiwalled carbon nanotube- (MWNT-) reinforced hydroxyapatite (HA) were consolidated using a spark plasma sintering (SPS) technique. The relative density of pure HA increased with increasing sintering temperature, but that of the MWNT/HA composite reached almost full density at 900°C, and then decreased with further increases in sintering temperature. The relative density of the MWNT/HA composites increased with increasing MWNT content due to the excellent thermal conductivity of MWNTs. The grain size of MWNT/HA composites decreased with increasing MWNT content and increased with increasing sintering temperature. Pull-out toughening of the MWNTs of the MWNT/HA composites was observed in the fractured surface, which can be used to predict the improvement of the mechanical properties. On the other hand, the existence of undispersed or agglomerate MWNTs in the MWNT/HA composites accompanied large pores. The formation of large pores increased with increasing sintering temperature and MWNT content. The addition of MWNT in HA increased the hardness and fracture toughness by approximately 3~4 times, despite the presence of large pores produced by un-dispersed MWNTs. This provides strong evidence as to why the MWNTs are good candidates as reinforcements for strengthening the ceramic matrix. The MWNT/HA composites did not decompose during SPS sintering. The MWNT-reinforced HA composites were non-toxic and showed a good cell affinity and morphology in vitro for 1 day.

Toughening Analysis for Eutectic Composite Ceramic Containing Lamellae

On basis of the residual-stress-field of the composite ceramic, the frictional force on the boundary of the lamellae can be computed. Then the frictional work is determined. The pull-out toughening mechanism is gotten. The result shows that the toughening value is dependent on the size of lamellae: the pull-out toughening value will increase as the thickness of the lamellae increase.

Study of the energy absorption capabilities of laminated glass using carbon nanotubes

Laminated glass (LG) typically consists of two or more glass plies bonded together with a transparent thermoplastic elastomeric interlayer, often composed of polyvinyl butyral (PVB). This interlayer primarily serves as a means of preventing splintering and of absorbing energy upon blast/impact. This research attempted to enhance the impact resistance of LG by increasing PVB interlayer energy absorption by embedding carbon nanotubes (CNTs). Interlayers were formed by electrospinning aligned PVB fibers mat embedded with various concentrations of CNTs. Subsequently, the fiber mat was hot-pressed between two glass plies forming a composite film. The composite fibers were characterized using optical, SEM, and TEM microscopy. The mechanical and thermo-mechanical properties of fibers were determined by dynamic mechanical analysis (DMA), and the energy absorption capacities of the modified LGs were measured by applying the Charpy impact test of un-notched samples. A∼30% increase in composite fiber (CNT 1.5wt.%) strength was observed, along with a∼70% increase in elastic modulus, measured at a strain rate of 0.1min−1, in comparison to CNT-free fibers. Increased CNT loading restricted the segmental motion of polymer macromolecules and provided the geometrical confinement effect to neighboring macromolecules in the nanoscale fiber. The energy absorption of a double-layered LG embedded with carbon nanotubes increased by nearly 341%, where experimental results demonstrated the role of the CNTs pull-out toughening mechanism. In parallel, transmission of visible light decreased by 60%.

Preparation and Properties of Solidified TiC-TiB<sub>2</sub> with Hypoeutectic, Eutectic and Hypereutectic Microstructures

By using combustion synthesis under high gravity, TiC-TiB2 fine-grained composite ceramics with hypoeutectic, eutectic and hypereutectic microstructures were prepared through rapid solidification. XRD, FESEM and EDS results show that with increasing TiB2 content, TiC-TiB2 composite ceramics transform the microstructures consisting of fine TiB2 platelets from ones composed of fine TiC spherical grains, whereas when TiB2 content reaches 50mol%, the ceramics develop the eutectic microstructures that small aspect-ratio TiB2 platelets were embedded in TiC matrix. Mechanical properties show that relative density, Vickers hardness and flexural strength of TiC-50mol%TiB2 all reach the maximum values (respectively as 98.6 % ,18.4 GPa, 840 MPa) due to eutectic reaction during solidification, meanwhile TiC-50mol%TiB2 has the maximum fracture toughness of 11.5 MPa∙m0.5 due to the cooperative action of crack deflection, crack-bridging and pull-out toughening by fine TiB2 platelets.

Finite Element Simulations of Fiber Pullout Toughening in Fiber Reinforced Cement Based Composites

Abstract The role of fibers in fiber reinforced cement based composites was studied by means of finite element method. The study was conducted in two steps. The first step simulated the fiber pullout from a cementitious matrix and resulted in pullout force vs. slip displacement response. The pullout response was used in the second step as the bridging pressure applied over the crack length in composite specimen. The contribution of fiber’s closing pressure was quantitatively measured through calculation of the J-integral and the effective stress intensity factor. The interfacial zone was characterized as a third phase with a lower stiffness and strength as compared to matrix and fiber. The debonding criterion was based on a yield surface defined by normal and shear strength of the interface. After debonding, Coulomb friction was introduced in the debonded zone. Effects of interfacial adhesional strength, clamping pressure, and fiber length on the fiber pullout response were studied. In the composite response simulations, the fibers across a prescribed crack length were modeled as nonlinear spring elements. The pullout force vs. slip displacement was used for the stiffness of the spring elements. J-integral was evaluated for the two cases of with and without fibers, and the difference between the two was used as the toughening contribution of fibers. The fiber toughening effect was studied for different fiber lengths and interface parameters. Results were compared with analytical simulations of crack growth using R-curves and a simplified approach based on linear crack opening–closing pressure relationship.

Postirradiation fiber debonding and pull-out in SiC-SiC composites

The toughness of ceramic matrix composites is contributed by crack bridging, matrix crack deflection, fiber debonding and pull-out and other minor effects. Crack bridging relies on fibers being intact close to the crack tip, while pull-out toughening relies on the debonding and frictional characteristics of the fiber-matrix interface. The interface friction depends on the interface pressure (i.e., on misfit strains) and interface roughness. In this paper, a calculational model for postirradiation fiber debonding and pull-out toughness in SiC-SiC composites is presented. It is shown that fiber debonding and pull-out toughness in SiC-SiC composites vary significantly with neutron fluence and irradiation temperature, which is a direct consequence of the dependence of the misfit strain on these irradiation variables.

Preparation of biologically active glass ceramics with rod-shaped crystals dispersion by hydrothermal hot-pressing

Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, 2-5-1 Akebono-cho, Kochi 780, Japan Biologically active glass (Bioglass) in the system Na20-CaO-SiO2-P2Os, is used as implant mater- ials because of the ability to bond chemically to living bone [1-3], but its mechanical properties, especially toughness, are insufficient for hard tissue. Therefore, their usefulness is limited to areas with- out mechanical requirements. Recent studies have demonstrated that dense ceramics can be toughened by rod-shaped particles dispersed in them, due to the crack deflection and pull-out toughening mechan- isms [4, 5]. Fully dense glass ceramics could be prepared by mechanical pressing under hydro- thermal conditions by the hydrothermal hot-pressing technique [6-10]. The present work deals with the preparation of reinforced Bioglass ceramics with rod-shaped crystals dispersion by hydrothermal hot- pressing. The autoclave for hydrothermal hot-pressing used in this work was a steel cylinder. Bioglass powders (45S5, Nikon Co., Japan) pulverized into grains under 200 mesh (smaller than 75 gm) were kneaded with 20 wt % distilled water, then transferred into the autoclave. The starting sample in the chamber was compressed uniaxially by cast rods from above and below. The details of this autoclave [11] and related autoclaves [12] were described in the previ- ous paper. The Bioglass ceramics were prepared by hydro- thermal hot-pressing at temperatures from 100 to 350 °C under 50 MPa mechanical pressing for 2 h. The produced phases were identified by powder X-ray diffractometry (XRD, Rigaku RAD-RC) with Ni-filtered CuKo~ radiation, operating at 40 kV and 100 mA. The microstructure of the glass ceramics was observed by scanning electron microscope (SEM, Hitachi $530) for the polished surface. According to the XRD analysis, the solidified Bioglass hot-pressed hydrothermally at 100 °C had no crystalline phases. This solidified Bioglass with- out crystalline phases had about 86% theoretical density. The Vickers hardness was determined to be 1.0 GPa by the micro-Vickers indentation method with 1.96 N loading for 15 s (Akashi MVK-EIII). The compressive strength of this glass was found to be 150 MPa by uniaxial compression perpendicular to the base of the compact in cylindrical shape with crosshead travel rate of 0.02 cmmin -1 (Shimadzu REH-100). XRD analysis (Fig. 1) revealed that the glass ceramics hot-pressed hydrothermally at above 200 °C had crystalline phases identified as NaCa2 0261-8028 © 1992 Chapman & Hall