Dominant Toughening Mechanism Research Articles

Fracture and Toughening of Mycelium-based Biocomposites

This study presents a combined experimental and analytical study of the fracture behavior and toughening mechanisms of bioprocessed mycelium-based biocomposites. The composites comprise hemicellulose hemp ducts (as nutritional and reinforcing components) intertwined with increasing weight percentages of laterite particles. Single-edge notched fracture experiments and in-situ observations of crack growth were used to explore the effects of varying proportions of laterite on the composite resistance-curve behavior. The toughening mechanisms, fracture modes, and crack-microstructure interactions were also elucidated. Since crack-bridging and crack-deflection were observed to be the dominant toughening mechanisms, they were modeled using fracture mechanics approaches. Crack-bridging was shown to dominate the toughening at lower weight fractions of laterite (0–20 wt%). However, as the laterite content increases (20–40 wt%), a combination of crack-bridging and crack-deflection was observed. Finally, at higher laterite weight fractions (>40 wt%), crack-tip shielding occurred primarily via crack deflection. The fracture mechanics predictions of resistance-curve behavior are shown to be consistent with the experimental measurements. The results suggest that mycelium-based and mycelium-laterite composites can be engineered with tunable fracture toughness. The implications of the results are also discussed for the development of sustainable building materials.

Experimental investigation on translaminar fracture behavior of cross-laminated bamboo

Cross-laminated bamboo (CLB) is a bamboo-based construction product with bamboo's advantages, which are green, environmentally protective, and sustainable. However, bamboo is easy to crack. Structural elements in bamboo constructions are assembled onsite, making splitting cracks easily form at joints and bolt connections. Therefore, it is essential to assess cracked structure safety with fracture mechanics. However, rare studies are addressed on fracture properties of bidirectional laminated bamboo, especially the translaminar fracture resistance, which represents the ability to arrest a catastrophic crack in structures. This paper studies the translaminar fracture properties of CLB composite using three-point bending beam tests. Load-displacement and load-CMOD curve characteristics, fracture process, and the failure modes of bidirectional laminated bamboo are analyzed. The R-curve expressed by the energy release rate is calculated by three methods. Results demonstrate that the R-curves obtained by the three different methods show similar results. The methods to determine R-curves employed in this study are simple and can describe the translaminar fracture toughness of laminated bamboo. The methods are validated by comparing the traction-separation relations calculated from the R-curves to those measured by double-edge notched tests (DENT). A comparison of the bidirectional laminated bamboo's energy release rate to the matrix-dominated energy release rate of unidirectional laminated bamboo reveals that bidirectional fiber arrangement has a remarkable improvement on the toughening of laminated bamboo at and after crack initiation. Moreover, the lay-up effects of CLB are analyzed by relating the energy release rate results to the failure pattern of fractured specimens. It is found that fiber pullout/bridging caused by 90°direction crack propagation around fiber bundles is the dominant toughening mechanism of CLB. The lay-up sequence of 0° and 90° plies also has an effect on 90°direction crack propagation; therefore, it will influence the toughness of CLB.

Evaluation of tensile and flexural properties of woven glass fiber/epoxy laminated composites oriented in edgewise and flatwise directions

In this study, the tensile and flexural properties of skin-core-skin woven glass fiber/epoxy laminated composites were conducted. Six plies (laminae) for the skins and twenty-four plies for the core were hand-laid up. All plies used had the same dimensions and the same elastic strength where the skins’ plies kept in flatwise direction while the core’s plies oriented in edgewise and flatwise to study the effect of plies alignments and hence loading direction i.e., edgewise (in-plane) and flatwise (out-of-plane) on the materials characteristics. It was observed that the composites oriented in edgewise position had the highest strength of 186 MPa (enhancement 59%) and deformed at higher loads with minor displacements compared to the flatwise strength of 117 MPa. The SEM analysis of the tensile’s fracture surface showed two dominant toughening mechanisms are debonding (fiber pullout) and fiber breakage, while interlaminar/intralaminar delaminated yarns was observed in the flexural fracture surface.

Effects of Al addition and cryogenic cyclic treatment on impact toughness of phase-transformable Ti-based bulk metallic glass composites

• Al addition can significantly affect impact toughness of BMGCs at 298 and 77 K • Impact toughness of phase-tranformable BMGCs is sensitive to β-Ti phase stability • Deformation-induced martenstic transformation of β-Ti is largely restricted at 77 K • Cryogenic cyclic treatment affects toughness at 298 and 77 K in different ways • Impact toughness is dominated by crystals at 298 K but by glass matrix at 77 K Developing bulk metallic glass composites (BMGCs) with high toughness is vital for their practical application. However, the influence of different microstructures on the impact toughness of BMGCs is still unclear. The effects of Al addition and cryogenic cyclic treatment (CCT) on the Charpy impact toughness, a K , at 298 and 77 K of a series of phase-transformable BMGCs are investigated in this work. It is found that deformation-induced martensitic transformation (DIMT) of the β-Ti dendrites is the dominant toughening mechanism in the phase-transformable BMGCs at 298 K, but at 77 K, the toughness of BMGCs is primarily determined by the intrinsic toughness of the glass matrix. The addition of Al can moderately tune the β-Ti phase stability, which then affects the amount of DIMT and impact toughness of the BMGCs at 298 K. However, at 77 K, Al addition causes a monotonic decrease in the toughness of the BMGCs due to the embrittlement of the glass matrix. It is found that CCT can effectively rejuvenate the phase-transformable BMGCs, which results in an enhanced impact toughness at 298 K. However, the toughness at 77 K monotonously decreases with increasing the number of CCT cycles, suggesting that the rejuvenation of the glass matrix affects the toughness at both 298 and 77 K of BMGCs, but in dramatically different ways. These findings reveal the influence of microstructures and CCT on the impact toughness of BMGCs and provide insights that could be useful for designing tougher BMGs and BMGCs.

Improved fracture resistance and toughening mechanisms of GNPs reinforced ceramic composites

The R-curve behavior and toughening mechanisms of graphene nano-platelets (GNPs) reinforced ceramic composites are investigated. A toughening model is developed with the consideration of interface debonding, crack bridging and pull-out of GNPs, which can be used to quantify the contribution of different mechanisms to the improved toughness of ceramic composites. The theoretical results agree well with the experimental data when GNPs homogeneously dispersed in ceramic matrix. All prepared GNPs/ceramic composites exhibit a raising R-curve behavior owing to the toughening mechanisms induced by GNPs, and the curve becomes steeper with increasing GNPs content, indicating that the fracture resistance and flaw tolerance are improved. The dominant toughening mechanism is GNPs pull-out, which is followed by crack bridging and interface debonding. Furthermore, the analytical model suggests that improving GNPs properties, interfacial sheer strength and reducing GNPs thickness can improve the fracture toughness of ceramic composites.

Fabrication of (TixZr1−x)B2-(ZrxTi1−x)N composites by reactive spark plasma sintering of ZrB2-TiN

In this study, microstructural and mechanical properties of (ZrxTi1−x)N-(TixZr1−x)B2 composite fabricated by reactive spark plasma sintering (SPS) of ZrB2 and TiN was investigated. ZrB2 and TiN starting materials with equimolar ratio were sintered at 2000 °C for 8 min under a pressure of 30 MPa. An expansion (about 4%) was observed during the sintering process of the composite in the temperature range between 1290 °C and 1625 °C, which was attributed to the reaction of the starting materials. XRD pattern of the fabricated composite revealed the formation of TiB2 and ZrN phases after the sintering process. It was found that (Ti0.91Zr0.09)B2 and (Zr0.92Ti0.08)N phases are formed after sintering. High values of Vickers microhardness (28 ± 0.2 GPa), flexural strength (706 ± 51 MPa) and fracture toughness (8.49 ± 0.92 MPa m1/2) were achieved for this composite. Fracture of the (ZrxTi1−x)N and (TixZr1−x)B2 phases and crack path deflection mechanisms was founded as dominant toughening mechanisms in this composite.

Effect of Co addition on microstructural and mechanical properties of WC–B4C–SiC composites

In this study, the effect of Co addition on microstructural and mechanical properties of WC-B4C–SiC composites sintered by spark plasma sintering (SPS) method was investigated. For this purpose, three batches of WC-B4C–SiC with different contents of Co (10 vol%, 15 vol%, and 20 Vol %) were sintered at 1400 °C. The results of X-ray diffraction (XRD) analysis of the samples indicated the formation of W2B5, W3CoB3 as well as the remained C phases and unreacted SiC phase. It was observed that by increasing the Co content, the amount of W2B5 phase reduces and W3CoB3 and C contents increase. Therefore, W2B5 peaks were not detected in the sample containing 20vol% Co. Relative density values above 97% were obtained for all the composites. However, a decrease was observed in relative density by increasing the Co content in the composites. The highest flexural strength (510 ± 42 MPa), fracture toughness (10.34 ± 0.82 MPa m1/2), and hardness (20.63 ± 0.75 GPa) were also obtained for the sample containing 10vol% Co compared to the other samples. In addition, Transgranular fracture of SiC as well as pulling out of W3CoB3 and W2B5 particles were observed in the fracture surface micrographs of the samples. The presence of micro-cracks in the SiC grains, fracture of W3CoB3 grains, and crack deflection was reported as dominant toughening mechanisms.

Influence of multi-walled carbon nanotubes on the hydration products of ordinary Portland cement paste

We elucidate the mechanisms by which multi-walled carbon nanotubes (MWCNTs) influence the microstructure, fracture behavior, and hydration of cement paste. We disperse MWCNTs using a multi-step approach that involves high-energy pre-dispersion using ultrasonic energy followed by low-energy dispersion using un-hydrated cement particles. In turn, the low-energy dispersion step involves high-shear mixing and mechanical stirring. High-resolution environmental scanning electron microscopy of cement+0.2 wt% MWCNT, cement+0.5 wt% MWNCT, and of cement+1 wt% MWCNT show that MWCNTs bridge air voids, thereby refining the pore size and strengthening the C-S-H matrix. The fracture toughness increased by 9.38% with the addition of 0.2 wt% multi-walled carbon nanotubes, and by 14.06% with the addition of 0.5 wt% multi-walled carbon nanotubes and ligament bridging was the dominant toughening mechanism. Moreover, for all reinforcement levels, MWCNTs induced a conversion of low-density C-S-H into high-density C-S-H along with a drastic drop in the capillary porosity: adding 0.1–0.5 wt% MWCNT resulted in a 200% increase in the volume fraction of high-density C-S-H. Thus, our experiments show that MWCNT enhances the mechanical properties and transport properties by: (i) promoting high-density C-S-H formation, (ii) promoting calcium hydroxide formation, (iii) filling microscopic air voids, (iv) reducing the capillary porosity, (v) increasing the fraction of small gel pores (1.2–2 nm in size), and (vi) by bridging microcracks.

Enhanced fracture toughness of ZrB2–SiCw ceramics with graphene nano-platelets

In this study, the spark plasma sintering technology was implemented to produce hybrid zirconium diboride ceramics comprising of silicon carbide whiskers (SiCw: 25 vol%) and various amounts of graphene nano-platelets (Gnp: 0–7.5 wt%). The effects of nano-graphene addition on the structural evolution, hardness, fracture toughness, and densification behavior of ZrB2–SiCw nanocomposites were investigated. Though all produced hybrid ceramics reached the full densification level, with relative densities of ≥99.9%, a negligible drop was observed in the relative density of the specimen reinforced with 7.5 wt% graphene nano-platelets. After the sintering step and due to the lack of any chemical interaction in the ZrB2–Gnp–SiCw system, no in-situ formed phases were detected by X-ray diffraction or electron microscopy investigations. The incorporation of nano-scale graphene additives resulted in grain coarsening inhibition significantly. The hardness slightly increased from 21.9 GPa for the specimen with no graphene to 22.2 GPa for the sample doped with 2.5 wt% graphene. However, with rising the nano-graphene amount, the hardness remarkably dropped to 16.6 GPa and 19.1 GPa for the samples doped with 7.5 wt% and 5 wt% nano-graphene, respectively. The fracture toughness remarkably enhanced, with a linear trend, by the addition of graphene nano-platelets from lower 5 MPa m½ in the carbon-free ceramic to above 6 MPa m½ in the specimen doped with 7.5 wt% nano-graphene. The identified dominant toughening mechanisms were crack bridging, crack arresting, and crack deflection.

Preparation and toughening of mechanochemically modified lignin-based epoxy

In this work, succinylated lignin (SA-KL) was prepared by reacting a kraft lignin and succinic anhydride (SA) through a solvent-free ball milling process. A bisphenol A/F epoxy resin (DER 353) cured with a commercial alicyclic anhydride and SA-KL (~38 wt% in the composition) exhibited a flexural strength of 48.8 MPa and modulus of 3.62 GPa but a low impact strength of 6.7 kJ/m2 which were comparable to the properties of many anhydride cured epoxies. Toughening of the lignin-based epoxy resin with a carboxyl-terminated butadiene acrylonitrile (CTBN) liquid rubber was investigated. Introduction of the CTBN rubber significantly increased the impact strength and fracture toughness but resulted in reduction of flexural strength. A ductile-brittle transition was observed at 5–12 wt% CTBN rubber. Examination of the fracture surfaces by scanning electron microscopy suggests that rubber particle cavitation and massive shear yielding of the epoxy matrix were the dominant toughening mechanisms. Incorporation of CTBN had little change to the thermal stability of the lignin-based epoxy. This study introduces a green method for synthesis of lignin-derived polyacid and a viable approach for achieving balanced mechanical and thermal properties for lignin-based epoxy materials.

Spatial distribution of nanodiamond and its effect on mechanical behaviour of epoxy based composite using 2D modulus mapping

The present study has taken an initiative to understand the distribution of nanophase reinforcement in polymer matrix and their behaviour during loading, using 2D modulus mapping. The composite system, being studied here, is a nanodiamond reinforced epoxy resin matrix. Modulus Mapping gives an idea of distribution of nanodiamond in matrix, in terms of spatial distribution of micron level higher modulus regions in epoxy matrix. The modulus mapping studies, carried out on composites with and without application of uniaxial tensile stresses, reveals the active role being played by nanodiamond in modulating the mechanical behaviour of the epoxy matrix and the evolution of distribution of the former with application of stress. Hardness and elastic modulus of epoxy is improved by ∼470% and ∼94% with 1 wt% nanodiamond. The stiffening effect of nanodiamond dispersed in epoxy matrix, evaluated through experiment is also in concurrence with results simulated using finite element method (FEM) of the representative volume element (RVE). Tensile strength and fracture toughness also showed an improvement of ∼56% and ∼85%, respectively, with 0.3 wt% ND. Interestingly, % strain (at break) also increased by ∼66%. TEM showed good dispersion and strong interaction of NDs with epoxy matrix. SEM analysis validates crack pinning and crack deflection as dominant toughening mechanism, while, efficient load sharing between ND and epoxy is found as main strengthening mechanism. As a whole, this study develops 2D modulus mapping technique as a generalized tool for evaluating the quality of dispersion of the nanoscale reinforcement phases and their role in mechanical behaviour of the macro-scale composite structures.

Filler size effect on fracture behavior of milled-fiber composites

Quasi static flexural and fracture tests are conducted on anhydride rich epoxy system reinforced with three different sizes of milled glass fibers, 1/32”, 1/8” and 1/4”. The matrix is prepared by mixing DGEBA resin and MTHPA hardener in equal ratio by weight whereas the 16 μm diameter slender fillers are reinforced at 3% volume fraction. It is observed that the flexural modulus and fracture toughness increase monotonically with increase in the slender filler size whereas the flexural strength of composites decreases with increasing filler lengths. The largest fillers enhance the flexural modulus and fracture toughness of neat epoxy system by ~ 1.3 and ~ 3 times, respectively. It is suggested that in the presence of large slender fillers, fiber fracture is the dominant toughening mechanism whereas the smaller size fibers are subjected to pull-out in the fracture process

Toughening effect of mullite whisker within low-density ceramic proppants

Main crystal phases of low-density ceramic proppants prepared by bauxite and feldspar are granular corundum and whisker-shaped mullite. Mullite whiskers are interlocked with one another and piled up inside the pores. High aspect ratio of mullite whiskers inside the pores can greatly enhance the fracture toughness. The dominant toughening mechanism for the proppants is attributed to crack bifurcation and deflection and pulling out and bridging effect of mullite whiskers.

Effects of short carbon fiber content on microstructure and mechanical property of short carbon fiber reinforced Nb/Nb5Si3 composites

The short carbon fiber reinforced Nb/Nb5Si3 composites were innovatively fabricated by ball milling and spark plasma sintering. The effects of short carbon fiber contents (0.50 wt. %, 1.0 wt. %, 1.5 wt. % and 2.0 wt. %) on the microstructure and mechanical property of short carbon fiber reinforced Nb/Nb5Si3 the composites were investigated. The results show that the content of interfacial phase Nb4C3 increases with the increase of short carbon fiber content. Compared to the matrix Nb/Nb5Si3, the mechanical properties of the composites are improved significantly owing to the addition of short carbon fiber. It is found that the increase of short carbon fiber content contributes to the enhancement of Vickers hardness, ultimate tensile strength and fracture toughness of the composites. The fracture surfaces are characterized by interface debonding and fiber pull-out, which indicate that a medium bonding strength of the interface between fiber and matrix was obtained in this composite. The interface debonding and fiber pull-out behavior play the important role in the fracture process, which are the dominant toughening mechanisms of the short carbon fiber reinforced Nb/Nb5Si3 composites.

Tough and Processable Hydrogels Based on Lignin and Hydrophilic Polyurethane.

Lignin is a low-cost, natural polymer with abundant polar sites on its backbone that can be utilized for physical cross-linking of polymers. Here, we use lignin for additional cross-linking of hydrophilic polyether-based polyurethane (HPU) hydrogels, aiming to improve their mechanical properties and processability. Without reducing the swelling, simple addition of 2.5 wt % lignin increases the fracture energy and Young's modulus of HPU hydrogels from, respectively, 1540 ± 40 to 2050 ± 50 J m-2 and 1.29 ± 0.06 to 2.62 ± 0.84 MPa. Lignin also increases the lap shear adhesiveness of hydrogels and induces an immediate load recovery of 95%. We further confirm that hydrogen bonding is the dominant toughening mechanism and elucidate the toughening mechanism by applying the Lake-Thomas and a recently developed sequential debonding theory. We show that unlike the Lake-Thomas theory, the latter model is able to capture the impact of lignin on toughening of hydrogels. Moreover, the lignin-loaded HPU hydrogels are easily processable by various techniques, such as fiber spinning, casting, and 3D printing and are biocompatible with primary human dermal fibroblasts.

Toughening mechanisms in cellulose nanopaper: the contribution of amorphous regions

Cellulose nanopaper is a strong and tough fibrous network composed of hydrogen bonded cellulose nanofibres. Upon loading, cellulose nanopaper exhibits a long inelastic portion of the stress–strain curve which imparts high toughness into the material. Toughening mechanisms in cellulose nanopaper have been studied in the past but mechanisms proposed were often rather speculative. In this paper, we aim to study potential toughening mechanisms in a systematic manner at multiple hierarchical levels in cellulose nanopaper. It was proposed that the toughness of cellulose nanopaper is not, as is often assumed, entirely caused by large scale inter-fibre slippage and reorientation of cellulose nanofibres. Here it is suggested that dominant toughening mechanism in cellulose nanopaper is associated with segmental motion of molecules facilitated by the breakage of hydrogen bonds within amorphous regions .

Tensile properties and deformation mechanisms of PU/MWCNTs nanocomposites

The aim of this study was to investigate the mechanical properties and deformation mechanisms of thermoset polyurethane reinforced with different contents of multi-walled carbon nanotubes, i.e. 0.05–5 wt%, fabricated through a solution casting method. Tensile test was done at constant strain rate and different temperatures and Halpin–Tsai equation was used to compare the experimental results with theoretical predictions. Results showed that modulus, strength and toughness of polyurethane depended strongly on the amount of CNTs loading. Tensile strength of PU was enhanced approximately 122% by the contribution of 1 wt% CNTs at room temperature. Predictions of tensile strength by modified Halpin–Tsai equation were in good agreement with the experimental data. Microscopic observations showed that MWCNTs dispersed homogeneously throughout the PU matrix up to 1 wt% CNTs; but evidences of agglomeration were found at higher contents. Finally, dominant toughening mechanisms of PU/CNTs nanocomposites were considered.

Sintering behavior and mechanical properties of spark plasma sintered β–SiAlON/TiN nanocomposites

In this study, fully dense β-SiAlON/TiN composites were produced by Spark Plasma Sintering (SPS) method. Si3N4, Al2O3, AlN and TiO2 powders were used as precursors. Starting powders were mixed with high energy ball milling and then were sintered by SPS method (at 1750°C under pressure of 30MPa for 12min.). The milled powders had an average particle size of below ~155nm. The XRD patterns of SPS-ed composites showed that the entire β-SiAlON phase constituent was in the form of Si4Al2O2N6 phase and cubic TiN phase can be formed by the phase transformation of TiO2 in relation with other precursors. FESEM micrographs confirmed that TiN particles were distributed homogeneously throughout β-SiAlON matrix. Mechanical properties evaluation revealed that by adding micro sized TiO2, optimal mechanical properties with a hardness ~14.6GPa and a fracture toughness ~6.3MPam1/2 were obtained. The improvement in the fracture toughness was attributed to the presence of the crack deflection as the dominant toughening mechanism in the SPS-ed β–SiAlON/TiN composites.

Microstructure and mechanical properties of multiwalled carbon nanotube toughened spark plasma sintered ZrB2 composites

ZrB2 based composites containing 10 vol.-% carbon nanotubes (CNTs) are synthesised by spark plasma sintering at temperatures ranging from 1600 to 18008C and at an applied pressure of 25 MPa. The effects of sintering temperature on densification behaviour, microstructural evolutions and mechanical properties are presented. Results indicate that ZrB2-CNT composites fabricated at 16508C have the optimal combination of dense microstructure and properties. The fracture toughness is sensitive to the temperature change and reaches 7.2 MPa m1/2 for the CNT toughened ZrB2 ceramics, which is higher than the measured result for monolithic ZrB2 (3.3 MPa m1/2). The crack deflection and CNT pullout are the dominant toughening mechanisms.

Microstructure and mechanical properties of multiwalled carbon nanotube toughened spark plasma sintered ZrB2composites

ZrB2 based composites containing 10 vol.-% carbon nanotubes (CNTs) are synthesised by spark plasma sintering at temperatures ranging from 1600 to 18008C and at an applied pressure of 25 MPa. The effects of sintering temperature on densification behaviour, microstructural evolutions and mechanical properties are presented. Results indicate that ZrB2-CNT composites fabricated at 16508C have the optimal combination of dense microstructure and properties. The fracture toughness is sensitive to the temperature change and reaches 7.2 MPa m1/2 for the CNT toughened ZrB2 ceramics, which is higher than the measured result for monolithic ZrB2 (3.3 MPa m1/2). The crack deflection and CNT pullout are the dominant toughening mechanisms.