Toughening Mechanisms Of Composites Research Articles

Experimental and numerical investigation on crack propagation in biomimetic nacreous composites with gradient structures

Combining interlocking structures and gradient design, this paper proposes a 2D biomimetic nacreous composite material with a gradient interlocked structure. Experimental and numerical simulation methods are employed to investigate the initiation and propagation behavior of cracks in the regular interlocking and gradient interlocked biomimetic nacreous composite materials. Samples of biomimetic nacreous composite materials with pre-existing cracks are manufactured using 3D printing technology, and uniaxial tensile tests are conducted to explore their crack propagation behavior. For biomimetic nacreous composite materials with periodic interlocking structures, the interlocking angle is found to play an important role in the toughening mechanism of composites. In biomimetic composite materials with large interlocking angles, crack inhibition effects are achieved through structural gradient design, effectively preventing catastrophic failure. Within the numerical analysis framework, a cohesive zone modeling approach is employed to represent the softer phase of the material, and the numerical simulation are validated by direct comparison with experimental results. In addition, a comprehensive numerical analysis is carried out to comprehend how the structural gradient affects the spread of cracks in these nacreous composites. This research not only illuminate the underlying deformation and toughening mechanisms intrinsic to interlocking nacreous composites but also pave the way for innovative strategies in the design of biomimetic materials through the incorporation of structural gradients.

Comparison of toughening mechanisms in natural silk-reinforced composites with three epoxy resin matrices

Natural silk fibre can reinforce the polymer matrix for composites with improved toughness and impact strength. However, the optimum selection of matrices for the mechanical performance of silk fibre reinforced plastics (SFRPs) is still unclear, especially with respect to toughness. Here Bombyx mori silk is applied to reinforce three epoxy resin matrices with various crosslinking structures. The epoxy resin from bisphenol-A epoxy and aliphatic diamine precursors that is the most ductile and the toughest leads to the SFRPs with the highest tensile breaking energy (14.1 MJ·m−3) and impact strength (110 kJ·m−2). Examination of the toughening mechanisms in these composites, which haveeither a brittle or ductile matrix, indicated the predominance of crack propagation and fibre pull-out to increase specific energy dissipation during the fracture processes. The rationale for this study is to provide guidelines for the design of matrices in SFRPs with optimal toughness, by identifying the salient toughening mechanisms in these tough and ductile fibre-reinforced composites.

Microstructure and mechanical properties of B4C matrix composites prepared via hot-pressing sintering with Pr6O11 as additive

High-performance B4C-PrB6 composites were prepared via hot-pressing sintering with matrix phase B4C and with 2–5 wt% Pr6O11 as additive. The effects of different sintering processes and Pr6O11 content on the microstructure and mechanical properties of the composites were studied in detail. It is found that increasing sintering temperature and pressure will contribute to the densification of B4C-PrB6 composites. Coarse grains are formed in B4C without additives at high temperature conditions, resulting in the decrease of the densification. Pr6O11 can effectively hinder the formation of coarse grains and finally promote the densification of the composites. The main toughening mechanisms of composites was crack deflection. The composites with 4 wt% Pr6O11 prepared at 2050 °C and 25 MPa had the best comprehensive mechanical properties. The relative density, hardness, flexural strength and fracture toughness reached to 98.9%, 37.6 GPa, 339 MPa and 4.4 MP am1/2, respectively.

Well ordered-microstructure bioceramics

It is a great challenge to endow the bioceramics with the bone-matched mechanical performances. Here, capturing inspiration from well ordered-microstructures in the high-performance natural composites, we propose a universal biomineralization-assembly-sintering approach for the fabrication of well ordered-microstructure meta-hydroxyapatite. Owing to the well ordered-microstructure and a new metal ions-related interface toughening mechanism, this well ordered-microstructure hydroxyapatite exhibits remarkable mechanical performances, which are far beyond those of existing hydroxyapatite-based bioceramic and totally matched with the human cortical bone. Furthermore, the ordered microstructure plays a significant role in modulating the cell fates and finally stimulating bone defects regeneration in vivo. This work not only provides a strategy for the fabrication of well ordered-microstructure bioceramic but also broadens the horizons for the interface toughening mechanism of composites and microstructure design of biomaterials for bone regeneration.

Effect of holding time and pressure on the densification, microstructure and mechanical properties of hot pressed Al2O3-CA6-ZrO2/Ni multi-phase composites

Densification, microstructure and mechanical properties of Al2O3-CA6-ZrO2/Ni multi-phase composites depend on different parameters, particular the holding time and pressure during the hot pressing sintering process at 1650 °C. In this study, the fine grain size, low porosity and dense structure of composites obtained for a holding time of 30 min under a pressure of 30 MPa. Meanwhile, the wettability between the Ni particles and the matrix was significantly improved owing to the homogeneous distribution of Ni phase. In addition, the systematically optimized sintering parameters resulted in the achievement of a relative density of 98.07% with an excellent Vickers hardness of 16.2 GPa, combining a flexural strength of 458.24 MPa with an acceptable fracture toughness of 7.20 MPa m1/2 for a holding time of 30 min under a pressure of 30 MPa. The toughening mechanisms of composites are also discussed in detail around the sintering parameters while the dominant ones are the transgranular fracture and crack deflection.

High-temperature tensile behavior of 3D SiC/SiC composites

The high-temperature tensile properties and stress-strain behavior of a 3D SiC/SiC composites prepared by the PIP method were investigated. Based on the FESEM analysis of the fracture morphology, the tensile failure mechanism of the composite at elevated temperature was discussed. The results show that the tensile strength of SiC/SiC composites showed a significant decline for specimens from a room temperature value of 482.5 MPa, to 167 MPa and 138 MPa at 1100 °C and 1300 °C, respectively. The toughening mechanism of composites at 1100 °C, including interface debonding, long pull-out fibers, and fracture mirror characteristics, is consistent with the pseudo-plastic fracture behavior implicated in the stress-strain curve. As a result of more significant oxidation, the fracture morphology of the specimen tested at 1300 °C was characterized by the signs including fiber-to-fiber bonding, larger size glassy phase, and shorter fiber pull-out length.

SiBCN ceramic aerogel/graphene composites prepared via sol-gel infiltration process and polymer-derived ceramics (PDCs) route

The SiBCN ceramic aerogel/graphene composites were synthesized by combining a simple sol-gel infiltration process with CO2 supercritical drying technology and polymer-derived ceramics route. In order to select the best preceramic sample for sintering, the micromorphology of PSNB aerogel/graphene composites fabricated with different graphene oxide solution concentrations were investigated. The microstructure evolution of the prepared SiBCN ceramic aerogel/graphene composites and phase composition were studied by SEM, TEM and XRD, the pore structure of the preceramic composites pyrolyzed at 1200 °C was tested by specific surface area and pore size analyzer. Furthermore, the compressive strain-stress curve and toughening mechanisms of composites were also investigated in detail. The results showed that all the preceramic composites and obtained ceramic aerogel composites possessed the mesoporous structure. The basic structure of SiBCN aerogel network changed from the initial spherical particles accumulation to the nanowires lapping with the sintering temperature increased from 800 °C to 1200 °C. After pyrolyzing at 1200 °C, the specific surface area and pore volume for the sample were 101.61 m2 g−1 and 1.43 cm3 g−1, respectively, and a small amount of β-SiC crystalline phases were formed in amorphous ceramic matrix and had an relatively uniform distribution. Moreover, the paepared ceramic aerogel composites possessed a certain degree of toughness, the toughening mechanisms of composite samples mainly included the crack deflection, graphene pull-out, graphene bridging and graphene crumpling.

Microstructure and Properties of Nb/Nb5Si3 Composites Strengthened with Multiwalled Carbon Nanotubes by SPS

Multiwalled carbon nanotubes (MWCNTs) were modified noncovalently with cetyltrimethylammonium bromide. The Nb/Nb5Si3 in situ composite material that was strengthened with MWCNTs was prepared by SPS. The effect of MWCNT content on the microstructure and properties of Nb/Nb5Si3 in situ composites was investigated. The results showed that the composites consisted of Nb, α-Nb5Si3 and β-Nb5Si3 phases. With the addition of over 2 wt% MWCNT, a new Nb4C3 phase was formed in the composites. The properties of the Nb/Nb5Si3 in situ composites were affected by MWCNT addition. The relative density, Vickers hardness and fracture toughness increased with increasing MWCNT addition. When the MWCNT addition was 2 wt%, the relative density and Vickers hardness reached a maximum and increased by ~ 1.4% and ~ 18%, respectively (relative to 0 wt% MWCNT addition). When the MWCNT content exceeded 2 wt%, the relative density and the Vickers hardness decreased. However, the fracture toughness of the Nb/Nb5Si3 composites with 3 wt% MWCNTs reached a maximum (an increase of ~ 68% relative to 0 wt% MWCNT addition). Scanning electron micrographs of the fractography showed brittle cleavage and partial intercrystalline composite fracture. The composite toughening mechanisms arose mainly because of MWCNT removal and bridging.

Microstructure and mechanical properties of nacre-like alumina toughened by graphene oxide

Graphene oxide (GO) is one of the most important ceramic-toughening candidates due to its outstanding physical and chemical properties. In this paper, nacre-like alumina ceramics reinforced with GO have been prepared by freeze casting followed by hot pressing. The experimental results showed that a laminated structure with homogeneous distribution of GO was obtained in the composites. Microstructural analysis, relative densities, flexural strength and fracture toughness of the samples were investigated to reveal the effect of GO. Higher flexural strength and fracture toughness (~380 MPa and ~7.5 MPa m1/2, respectively) were achieved and the multiple length-scale toughening mechanisms of composites contributed to the improved mechanical properties. The results show that it is a promising method to enhance the mechanical properties of nacre-like alumina by the addition of GO.

Ceramic composites: A review of toughening mechanisms and demonstration of micropillar compression for interface property extraction

Abstract

The Fabrication and Properties of the Squeeze-Cast TiN/Al Composites

Co-continuous TiN/Al composites with different volume fractions of Al phase have been fabricated by the squeeze-casting method. TiN porous ceramics with different porosities were fabricated through carbothermal reduction by changing the content of TiN and were used as preforms. The outstanding mechanical properties were attributed to the absence of excessive interface reaction between TiN and Al for the co-continuous TiN/Al composites. With the increase of Al content in the composites, the flexural strength and the microhardness decreased, and the fracture toughness increased. The strengthening and toughening mechanism of composites included dislocation strengthening, ductile rupture, crack deflection, and secondary cracks.

Study on Mechanical Properties and Toughening Mechanism of MWCNTs/Ti(C, N) Cermets-Based Composites

MWCNTs/Ti(C, N) cermets-based composites were prepared by vacuum hot-pressing sintering method. The mechanical properties of samples were examined. XRD and SEM were used to investigate the crystal structure of the composites and microstructure of fractures surface, respectively. The toughening mechanism of composites was discussed particularly. The experimental results showed that the optimum comprehensive mechanical properties of samples were obtained by adding 1wt% MWCNTs in the composites. The bending strength, Vickers hardness and fracture toughness of the composites were 1275.14MPa, 22.75GPa and 10.60MPa•m1/2, respectively, which were improved by 16.89%, 17.15% and 25.59%, respectively, compared to the Ti(C, N)-based cermets without MWCNTs. Bridging and pulling out of MWCNTs, crack deflection, residual stress toughening and micro-voids toughening were attributed to the toughening mechanism of the composites.

Investigation of the mechanical properties of Mo-reinforced glass-matrix composites

One approach for reducing the inherent brittleness and flaw sensitivity of glass is to fabricate a composite material by incorporating particles or fibers in the glass matrix. In order to fully understand the toughening mechanisms in composites, it is useful to develop predictive models able to describe the mechanical behavior and its dependence on microstructure. To this aim, numerical models can be used in order to assess the effect of the glass-matrix composite microstructure on the effective macroscopic mechanical and fracture properties. An innovative microstructure-based finite element code is used to describe the mechanical performance of glass matrix composites. This code is able to convert digitized images of the material microstructure into a finite element grid, so that all microstructural details such as inclusion size, morphology and volume fraction can be incorporated in the model. In this study, a borosilicate glass matrix composite reinforced with molybdenum particles is characterized using the aforementioned microstructure-based FEM approach in terms of residual stresses and elastic properties.

Combustion-synthesized β′-SiAlON reinforced with SiC monofilaments

Dense composites of combustion-synthesized β′-Si 3Al 3O 3N 5 (β′-SiAlON) reinforced with 20 vol.% SiC monofilaments (AVCO SCS-6) were hot pressed at a temperature of 1600 °C for 2 h under a pressure of 34 MPa. The mechanical properties of as-fabricated composites were investigated in the three-point flexure mode. The composites exhibited significant improvement in the work of fracture as well as in the ultimate strength, in comparison with monolithic β′-SiAlON. Ultimate flexure strength values between 682 and 793 MPa for the composite and between 398 and 528 MPa for the monolithic β′-SiAlON were obtained. A work of fracture of 13–20 kJ m −2 was obtained for the composite, compared with 1.7-3.1 kJ m −2 for the monolithic material. Optical microscopy and scanning electron microscopy (SEM) examinations of the fractured specimens showed the usual composite toughening mechanisms of microcracking. interfacial debonding, filament bridging and pull-out. The interfacial shear strength as well as frictional stress were also investigated with a fiber push-out technique. The push-out load-deflection curves revealed a moderate interfacial bonding strength of 26 MPa and a frictional sliding stress of 24 MPa. Transmission electron microscopy interfacial characterization was correlated with SEM observation of the interfacial debonding site. It revealed the presence of definite but weak physical bonding between the outermost carbon-rich layer of the SiC filament and the matrix. It appeared that the filament and the matrix were compatible with each other both physically and chemically, despite the fact that the matrix contained 20 wt.%Al 2O 3 as a secondary phase.

X-Ray Stress Measurement of Alloy Steels X-Ray Study of Elastic Deformation for Alloy Steels with Composite Microstructures

Residual stress is inevitably introduced into composites because of the mismatch of the coefficient of thermal expansion, and it is different for each phase. The x-ray method can detect separately the stress in each phase, so will yield useful information for analyzing the toughening mechanisms of composites.