Simultaneously enhanced cryogenic tensile strength and fracture toughness of epoxy resins by carboxylic nitrile-butadiene nano-rubber

Epoxy resins are important matrices for composites. Carboxylic nitrile-butadiene nano-rubber (NR) particles are employed to improve the tensile strength and fracture toughness at 77 K of diglycidyl ether of bisphenol-F epoxy using diethyl toluene diamine as curing agent. It is shown that the cryogenic tensile strength and fracture toughness are simultaneously enhanced by the addition of NR. Also, the fracture toughness at room temperature (RT) is enhanced by the addition of NR. On the other hand, the tensile strength at RT first increases and then decreases with further increasing the NR content up to 5 phr. 5 phr NR is the proper content, which corresponds to the simultaneous enhancements in the tensile strength and fracture toughness at RT. Moreover, the comparison of mechanical properties between 77 K and room temperature indicates that the tensile strength, Young’s modulus and fracture toughness at 77 K are higher than those at RT but the failure strain shows the opposite results. The results are properly explained by the SEM observation.     

Improvement of the fracture toughness of adhesively bonded stainless steel joints with aramid fibers at cryogenic temperatures

Adhesives should be reinforced with reinforcing fibers for the bonding of adherends at cryogenic temperatures because all the adhesives become quite brittle at cryogenic temperatures. In this work, the film-type epoxy adhesive was reinforced with randomly oriented aramid fiber mats to decrease the CTE (Coefficient of Thermal Expansion) of the adhesive and to improve the fracture toughness of adhesive joints composed of stainless steel adherends at the cryogenic temperature of −150 °C. The cleavage tests of the DCB (Double Cantilever Beam) adhesive joints were performed to evaluate the fracture toughness and crack resistance of the adhesive joints. Also, the thermal and mechanical properties of the fiber reinforced adhesive layer were measured to investigate the relationship between the fracture toughness of adhesive joints and fiber volume fraction of aramid fibers. From the experiments, it was found that the crack propagated in the adhesive with the stable mode of significantly increased fracture toughness when the film-type epoxy adhesive was reinforced with aramid fiber mats. The optimum volume fraction of aramid fibers was suggested for the film-type epoxy adhesive in the adhesive joint at the cryogenic temperature of −150 °C.     

Mode I fracture toughness of an adhesively bonded composite–composite joint in a cryogenic environment

To determine the effect of cryogenic temperature on the adhesive fracture toughness of an adhesively bonded joint with composite adherends, monotonic mode I adhesive fracture toughness tests were performed at liquid nitrogen temperature (−196 °C) and at room temperature (27 °C). From these experimental tests, the critical strain energy release rate for both test temperatures was evaluated for the selected bonded joint system constructed of carbon-BMI adherends bonded with AF-191M film adhesive. Experimental results exhibit reduced adhesive fracture toughness at the cryogenic temperature and a profound difference in fracture mode.     

The toughness of epoxy polymers and fibre composites modified with rubber microparticles and silica nanoparticles

The present paper investigates the effect of adding silica nanoparticles to an anhydride-cured epoxy polymer in bulk and when used as the matrix of carbon- and glass-fibre reinforced composites. The formation of ‘hybrid’ epoxy polymers, containing both silica nanoparticles and carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber microparticles, is also discussed. The structure/property relationships are considered, with an emphasis on the toughness and the toughening mechanisms. The fracture energy of the bulk epoxy polymer was increased from 77 to 212 J/m² by the presence of 20 wt% of silica nanoparticles. The observed toughening mechanisms that were operative were (a) plastic shear-yield bands, and (b) debonding of the matrix from the silica nanoparticles, followed by plastic void-growth of the epoxy. The largest increases in toughness observed were for the ‘hybrid’ materials. Here a maximum fracture energy of 965 J/m² was measured for a ‘hybrid’ epoxy polymer containing 9 wt% and 15 wt% of the rubber microparticles and silica nanoparticles, respectively. Most noteworthy was the observation that these increases in the toughness of the bulk polymers were found to be transferred to the fibre composites. Indeed, the interlaminar fracture energies for the fibre-composite materials were increased even further by a fibre-bridging toughening mechanism. The present work also extends an existing model to predict the toughening effect of the nanoparticles in a thermoset polymer. There was excellent agreement between the predictions and the experimental data for the epoxy containing the silica nanoparticles, and for epoxy polymers containing micrometre-sized glass particles. The latter, relatively large, glass particles were investigated to establish whether a ‘nano-effect’, with respect to increasing the toughness of the epoxy bulk polymers, did indeed exist.     

Synergistic effects of oxidized CNTs and reactive oligomer on the fracture toughness and mechanical properties of epoxy

The soft modifiers added to improve the fracture toughness of epoxies generally deteriorate their mechanical properties. Hence, oxidized multi-walled carbon nanotubes (O-CNTs) were added to the epoxy modified with reactive oligomer. The NCO terminated reactive oligomer acts as a cross-linker between the O-CNTs and the OH groups of the epoxies. The impact strengths of the 15 wt.% oligomer modified epoxy containing 0.5 wt.% of O-CNTs at room temperature (RT) and cryogenic temperature (CT) are enhanced by 23.6% and 69.5% compared to that of the unmodified epoxy. In addition to increasing fracture toughness, the tensile strength (TS) of the modified epoxy/O-CNTs at CT is found to be 91.7 MPa, which is comparable to that of the unmodified epoxy (92.1 MPa). Hence, the attachment of O-CNTs to the reactive oligomer modified epoxy can be an efficient approach to toughen epoxy resins without compromising their tensile properties.     

Microstructure and mechanical properties of an HfB2 + 30 vol.% SiC composite consolidated by spark plasma sintering

An ultra-high-temperature HfB₂–SiC composite was successfully consolidated by spark plasma sintering. The powder mixture of HfB₂ + 30 vol.% β-SiC was brought to full density without any deliberate addition of sintering aids, and applying the following conditions: 2100 °C peak temperature, 100 °C min⁻¹ heating rate, 2 min dwell time, and 30 MPa applied pressure. The microstructure consisted of regular diboride grains (2 μm mean size) and SiC particulates evenly distributed intergranularly. The only secondary phase was monoclinic HfO₂. The incorporated SiC particulates played a key role in enhancing the sinterability of HfB₂. Flexural strength at 25 °C and 1500 °C in ambient air was 590 ± 50 and 600 ± 15 MPa, respectively. Fracture toughness at room temperature (RT) (3.9 ± 0.3 MPa √m) did not decrease at 1500 °C (4.0 ± 0.1 MPa √m). Grain boundaries depleted of secondary phases were fundamental for the retention of strength and fracture toughness at high temperature. The thermal shock resistance, evaluated through the water-quenching method, was 500 °C.     

Research into mechanical properties of reaction-bonded SiC composites at cryogenic temperatures

The mechanical properties of reaction-bonded silicon carbide (RBSC) composites at cryogenic temperatures have been reported for the first time. The results show that the flexural strength and fracture toughness increase from 277.93 ± 23.21 MPa to 396.74 ± 52.74 MPa and from 3.69 ± 0.45 MPa·m^1/2 to 4.98 ± 0.53 MPa·m^1/2 as the temperature decreases from 293 K to 77 K, respectively. The XRD analysis of the phase composition reveals that there is no phase transformation in the composites at cryogenic temperatures, indicating cryogenic mechanical properties are independent of phase composition. The enhancement of mechanical properties at 77 K over room temperature could be explained by the transition of fracture mode from predominant transgranular fracture to intergranular fracture and stronger resistance to crack propagation resulting from higher residual stress at 77 K. The above results demonstrate that such composites do not undergo similar deteriorations in the fracture toughness as other materials (some kinds of metals and polymers), so it is believed that such composites could be a potential material applied in cryogenic field.     

Temperature effects on the fracture behavior and tensile properties of silane-treated clay/epoxy nanocomposites

We investigated the effects of clay silane treatment on the fracture behaviors of clay/epoxy nanocomposites by comparing the compliance, critical fracture load, and fracture toughness of silane-treated samples with those of untreated samples. The fracture toughnesses of untreated and silane-treated clay/epoxy nanocomposites were 8.52 J/m² and 15.55 J/m², respectively, corresponding to an 82% increase in fracture toughness after clay silane treatment. Tensile tests were performed at ?30 °C, 25 °C, 40 °C, and 70 °C. Tensile strength and elastic modulus were higher at ?30 °C than at 25 °C for both samples. However, the tensile properties decreased as temperature increased for both samples. In particular, at 70 °C, the tensile properties were less than 10% of the original value at room temperature, independent of surface treatment. The fracture and tensile properties of silane-treated clay/epoxy nanocomposites increased due to good dispersion of the clay in epoxy and improvement in interfacial adhesive strength between epoxy and clay layers.     

Nanosilica-toughened polymer adhesives

High-performance adhesives are of great importance to achieving strong and durable adhesive bonds in a wide range of applications. This article presents an investigation of the use of nano-sized silica particles to improve the fracture toughness of polymer adhesives, focusing on the effects of particle size, matrix ductility and adhesive thickness. The results reveal that the performance of nano-silica as a toughening additive depends strongly on the matrix’s ductility and adhesive thickness. With merely 2.1 vol% nano-silica, the fracture toughness of an epoxy has been improved from 0.19 to 1.34 kJ/m², representing a 605% improvement. Microscopy studies show that this improvement is attributed to the formation of a dilatation zone, approximately 2–3 μm thick, which dissipates energy. The nanoparticles in general produce a higher adhesive toughening effect than their micron-sized peers. A significant toughening effect has been made by dispersing the nanoparticles in a relative ductile matrix, while such an effect was not observed for the micro-sized particles.     

Fracture behaviours of epoxy nanocomposites with nano-silica at low and elevated temperatures

An investigation was conducted to characterize fracture behaviours of nano-silica modified epoxies at low and elevated temperatures. A nano-silica dispersed epoxy (Nanopox XP 22/0516, Hanse-Chemie, Germany) with 40 wt% silica nano-particles was used as modifier to toughen an epoxy resin, Araldite F (Bisphenol A based, Ciba-Geigy). Fracture toughness and other mechanical properties were measured using standard compact tension (CT), tensile and flexural specimens to elaborate the effects of nano-silica particles on fracture behaviours of epoxy nanocomposites at different temperatures, −50, 0, 23, 50 and 70 °C. Dynamic mechanical analysis (DMA) was utilized to define the glass transition temperature (T _g) upon the addition of different amounts of nano-silica particles. Fracture toughness of the nano-silica modified epoxies was clearly increased at 23 °C and 50 °C, but the role of nano-silica particles in enhancing the fracture toughness became less pronounced at 0 °C and −50 °C and disappeared at 70 °C.     

Mechanical properties of 3D fiber reinforced C/SiC composites

High toughness and reliable three dimensional textile carbon fiber reinforced silicon carbide composites were fabricated by chemical vapor infiltration. Mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.0–2.1 g cm⁻³ after the three dimensional carbon preform was infiltrated for 30 h. The values of flexural strength were 441 MPa at room temperature, 450 MPa at 1300°C, and 447 MPa at 1600°C. At elevated temperatures (1300 and 1600°C), the failure behavior of the composites became some brittle because of the strong interfacial bonding caused by the mis-match of thermal expansion coefficients between fiber and matrix. The shear strength was 30.5 MPa. The fracture toughness and work of fracture were as high as 20.3 MPa m^1/2 and 12.0 kJ·m⁻², respectively. The composites exhibited excellent uniformity of strength and the Weibull modulus, m, was 23.3. The value of dynamic fracture toughness was 62 kJ·m⁻² measured by Charpy impact tests.     

Fracture toughness and molecular structure of unfilled epoxy adhesives

The cohesive, mode I (tensile cleavage) fracture energy (or fracture toughness), G _Ic, of bulk tapered double cantilevered beam (TDCB) samples of a series of three epoxy thermoset networks was determined using a linear elastic fracture mechanics (LEFM) analysis. Networks of different crosslink density were obtained by mixing various amounts of an aromatic epoxy novolac and an aliphatic epoxy and crosslinking with an imidazole catalyst. Brittle, stick-slip fracture was observed for all formulations, with G _Ic increasing as the amount of aliphatic epoxy increased. However, fracture surface morphologies exhibited evidence of increasing plastic deformation as G _Ic increased. In the investigation of structure-property relationships for this series of thermoset networks, G _Ic was found to be inversely related to both network crosslink density and glass transition temperature (T _g). It was also found that the room temperature frequency of the glassy state transition (-transition) increased as fracture toughness increased.     

Comparison of tensile properties between NiCoCrAl/YSZ microlaminates and the monolithic NiCoCrAl foil fabricated by EB-PVD

Two NiCoCrAl/YSZ microlaminates and a monolithic NiCoCrAl foil were fabricated by EB-PVD. The two microlaminates contained 20 and 26 alternating layers of NiCoCrAl and YSZ respectively; and their metal layers were 35 and 14 times thicker than their ceramic layers (thickness ≈ 1 μm) respectively. Tensile testing was performed at room temperature, 700 °C and 1000 °C, and fracture surfaces were examined using SEM. The fracture toughness was estimated using measured data in room-temperature tensile testing. Among the three foils, the 14 μm metal-layer microlaminate displayed the greatest ductility of metal layers, tensile strength and elastic modulus; however, the 35 μm metal-layer microlaminate showed the greatest fracture toughness. The ratios of the strength of the microlaminates to that of the monolithic NiCoCrAl foil increased with increasing testing temperature. The result was discussed in terms of metal strengthening mechanism.     

Analysis of mixed-mode interlaminar fracture and damage behavior of GFRP woven laminates at cryogenic temperatures

The objective of this work is to investigate the interlaminar fracture and damage behavior of glass fiber reinforced polymer (GFRP) woven laminates loaded in a mixed-mode bending (MMB) apparatus at cryogenic temperatures. The finite element analysis (FEA) is used to determine the mixed-mode interlaminar fracture toughness of MMB specimen at room temperature (RT), liquid nitrogen temperature (77 K) and liquid helium temperature (4 K). A FEA coupled with damage is also employed to study the damage distributions within the MMB specimen and to examine the effect of damage on the mixed-mode energy release rate. The technique presented can be efficiently used for characterization of mixed-mode interlaminar fracture and damage behavior of woven laminate specimens at cryogenic temperatures.     

Performance of epoxy filled with nano- and micro-sized Magnesium hydroxide

Magnesium hydroxide (MDH) particles are often used as fillers to improve the flame retardancy of polymers. However, achieving the balance between the enhanced fire resistance and reduced mechanical properties, especially toughness, is still a challenge to the composite community. In this study, the effect of the particle size and silane surface modification of MDH particles on the flame retardant, thermal, and mechanical properties of epoxy was studied. Both nano- and micro-sized MDH particles were modified by a silanization reaction with γ-aminopropyltrietoxysilane in an aqueous solution and filled into epoxy matrix by a high-shear mixer and a three-roll mill. Results show that nano-MDH filled epoxy composites yielded better mechanical properties than their micro-MDH filled counterparts. Furthermore, the adhesion between nano-sized MDH and the matrix was improved by the silane surface modification. When comparing the flame retardant properties, enhancements in heat release rates and total heat released were observed for MDH filled epoxy composites.     

Fracture and fatigue properties of metallic alloys S275 J2 and Al7075 T6 at low temperatures

Knowledge of the fracture and fatigue behaviour of metallic alloys at extreme environmental temperature conditions is required to assess the safety of structural components operating in particular fields: aero-spatial, off-shore structures, power plants superconductors, polar Antarctic facilities, etc. Among the structural metallic alloys for civil, mechanical engineering and plant applications, steel S275 J2 is widely used, whereas aluminium alloys such as Al7075 T6 are significant especially for aero-spatial and polar Antarctic applications. In this paper, the main experimental mechanical characteristics of such metallic materials at room temperature as well as at low temperatures are examined. Three temperatures are considered: 293 K (+20 °C, room temperature RT), 243 K (−30 °C) and 193 K (−80 °C). The corresponding values of fracture toughness and endurance limit available in the literature are reported herein. Further, experimental tests have been performed to determine the unavailable mechanical properties. Then, the values of such fracture and fatigue parameters at various temperatures are critically discussed.     

Rapid densification and reaction sequences in self-reinforced Y-α-SiAlON ceramics with barium aluminosilicate as an additive

Y-α-SiAlON (Y_1/3Si₁₀Al₂ON₁₅) ceramics with 5 wt.%BaAl₂Si₂O₈ (BAS) as an additive were synthesized by spark plasma sintering (SPS). The kinetic of densification, phase transformation sequences and grain growth during sintering process were investigated. Full densification could be achieved by 1600 °C without holding and using a heating rate of 100 °C min⁻¹, but the transformation from α-Si₃N₄ to α-SiAlON is not completed simultaneously with the densification process. The equilibrium phase assemblage could be reached after SPS at 1800 °C for 5 min and the resultant material possesses self-reinforced microstructure with high hardness of 19.2 GPa and fracture toughness of 6.8 MPa m^1/2. The complete crystallization of BAS is beneficial to the high temperature mechanical properties. The obtained could maintain the room strength up to 1300 °C.     

Effect of CNT functionalization on crack resistance of a carbon/epoxy composite at a cryogenic temperature

In this study, the authors attempted to enhance the crack resistance of a carbon/epoxy composite by adding carbon nanotubes (CNTs) into the resin formulation. Prior to applying CNTs, a chemical surface treatment, amino-functionalization, was conducted to enhance the interfacial bonding between CNTs and epoxy resin. The resultant increase in tensile strength was attributed to good interfacial bonding quality. Two kinds of 3-phase carbon/CNT-epoxy unidirectional prepregs were fabricated via a hot-melting process with the addition of CNTs. The CNT-reinforcing and -functionalizing effects were investigated through DCB tests and measurement of the acoustic emission (AE) signal of a cross-ply specimen both at RT and at ?150 °C. The carbon/epoxy composite containing functionalized-CNTs showed improved interlaminar fracture toughness and transverse crack resistance at the cryogenic temperature.     

Effects of SWCNTs on mechanical and thermal performance of epoxy at elevated temperatures

A property which limits the breadth of application of thermoset polymers and their composites is their relatively low maximum operating temperatures. This work investigates the potential application of both functionalized single-walled carbon nanotubes (f-SWCNTs) based on negative charging, and unfunctionalized SWCNTs (u-SWCNTs) to increase the mechanical and thermal performance of a high-temperature aerospace-grade epoxy with a glass transition temperature of approximately 270 °C. Thermal and mechanical properties of the baseline epoxy and nanocomposites containing a low content of SWCNTs (0.2 % by weight) were characterized through thermogravimetric analyses, tensile tests, and dynamic mechanical analyses. Tensile tests were performed both at room temperature and at 80 °C. Further, room temperature tensile tests were performed on untreated and heat-treated specimens. The heat treatment was performed at 300 °C, slightly above the resin glass transition temperature. Results demonstrate that f-SWCNTs are effective in improving the mechanical and thermal performance of the epoxy. No significant improvement was observed for u-SWCNT nanocomposites. For the nanocomposite with f-SWCNTs, the ultimate tensile strength and strain to failure at room temperature (80 °C) increased by 20 % (8 %) and 71 % (77 %), respectively, as compared to the baseline epoxy. The f-SWCNT nanocomposite, unlike other examined materials, exhibited a stress–strain necking behavior at 80 °C, an indication of increased ductility. After heat treatment, these properties further improved relative to the neat epoxy (160 % increase in ultimate tensile strength and 270 % increase in strain to failure). This work suggests the potential to utilize f-SWCNTs based on negative charging to enhance high-temperature thermoset performance.     

Strength and toughness of cellular SiC at elevated temperature

Bending strength and toughness of β-SiC foams were measured from ambient temperature to 1400 °C. It was found that SiC was able to maintain the mechanical properties up to 1200 °C even after long-term exposure at this temperature. Creep deformation was not detected and the negative effects of oxidation at this temperature were balanced by the healing effect induced by the formation of SiO₂. Nevertheless, the mechanical properties were rapidly degraded at 1400 °C as a consequence of massive oxidation of SiC.