Shear Fracture Toughness Research Articles

Testing the influence of filler type and content on thermal cracking of mastic

Low-temperature cracking significantly affects durability of asphalt pavements. This research addresses the role of the mastic phase by experimentally evaluating the low-temperature cracking performance of selected bitumen and mastics with different filler types and contents. Their thermal contraction coefficient (αT), low temperature viscoelasticity, and strength properties are measured using standard tests like the dynamic shear rheometer (DSR) and fracture toughness tests (FTT). An enhanced laboratory technique, the annular restrained cold temperature induced cracking (ARCTIC) test, is employed to study combined thermal and mechanical effects on low-temperature cracking. The alignment of FTT with ARCTIC results highlights a good correlation between these methods for mastics with nearly the same αT. However, the insensitivity of FTT to αT raises concerns about its applicability to materials with significantly different αT, as it may not capture accurately their performance. The ARCTIC test is free of such a problem and even shows a higher sensitivity to the mastic composition. The findings demonstrate that the addition of filler significantly affects the resistance of mastic to low-temperature cracking by altering its αT. Additionally, the filler content, type, and gradation distinctly impact the thermal and mechanical characteristics of mastics, enhancing their ability to withstand lower temperatures more effectively than bitumen. In particular, adding 50% by volume of different filler types reduces the αT of mastic by 45 to 60% and results in 3 to 10 °C reduction in cracking temperature (Tcr) measured with the ARCTIC test.

Polyurethane-modified room-temperature curing epoxy super adhesive for artifact restoration and light emitting diode encapsulation

Epoxy adhesives are widely used for their strong adhesion but face challenges due to high curing temperatures and brittleness, limiting their application in precision manufacturing and fine industries. In an innovative endeavor to address these limitations, we have synthesized a novel adhesive system by incorporating epoxy-functionalized polyurethane into the conventional bisphenol A-based epoxy resin. Upon subjecting the adhesive to a 24h curing period at ambient temperature, the resulting material exhibited remarkable mechanical properties. Specifically, the shear strength, tensile strength, and fracture toughness of the cured adhesive were 20.6 MPa, 36.7 MPa, and 2.87 MPa·m1/2, respectively, while the unmodified epoxy resin had values of only 1.9 MPa, 15.5 MPa, and 1.0 MPa·m1/2. Adhesive system unique crosslinked structure has been discovered to confer blue aggregation-induced emission fluorescent properties. We have successfully applied this interesting characteristic in the restoration of cultural relics and the encapsulation of light emitting diode. The research has yielded a groundbreaking adhesive formulation that not only overcomes the traditional limitations associated with epoxy materials but also introduces novel functionalities that extend its utility into diverse and sophisticated applications.

Investigation of two sandwich-structured nanohybrid coating derived from graphene oxide/carbon nanotube on interfacial adhesion and fracture toughness of carbon fiber composites

Designing stronger interphase towards solving the long-standing dilemma of interfacial delamination is critical for stable application of carbon fiber composites. Herein, nano-scale sandwich-structured coatings, where carbon nanotubes (CNTs) were uniformly anchored on both sides of graphene oxide (GO) layer (abbreviated as C/GO/C) and its reverse, that is double GO layers encapsulated CNT network (G/CNT/G in short), were reported around fiber periphery via vacuum filtration method. The effects of surface structure differences on interfacial shear strength (IFSS) and fracture toughness were compared in epoxy matrix. Impressively, composite incorporating G/CNT/G modified fiber delivered prominent IFSS and interfacial fracture toughness of 114.6 MPa and 137.0 J/m2, 105.7 % and 279.5 % increases over control fiber composite. This strategy was also superior to C/GO/C and other reported GO and CNT related works. The main factors for maximal IFSS offered by G/CNT/G are that two GO panels enrich active sites to tightly bridge fiber and epoxy, as well as its layered feature and large surface area provide a stable “skeleton” at interphase for stress transfer. Additionally, the G/CNT/G “skeleton” is closer to sandwich structure of iris leaf, in which the porous CNT intermediate network creates larger deformation and adsorb more energy, leading to peak interfacial fracture toughness.

Layer-by-layer assembling boron nitride/polyethyleneimine/MXene hierarchical sandwich structure onto basalt fibers for high-performance epoxy composites

Interfacial adhesion directly affects the mechanical properties of basalt fiber (BF)-reinforced polymer composites. To construct a more superior interphase between BFs and epoxy resin (EP) than a weak interphase of the unmodified BF/EP, we propose a hierarchical sandwich structure consisting of sodium hydroxide–activated boron nitride (BNOH), polyethyleneimine (PEI), and MXene (MX, Ti3C2Tx) through facile layer-by-layer self-assembly. The fabricated BNOH/P/MX sandwich structure (P denoting “PEI”) can synergistically improve the interface adhesion by enhancing the mechanical interlocking and chemical bonding of the composites. When the composites reinforced by BF–BNOH/P/MX subject to the external loading, flexible PEI molecules allow two-dimensional (2D) rigid BNOH and MX nanosheets to slip at the interface by uncurling the molecular chains, dissipating a great amount of energy during the fracture progress. Meanwhile, the hierarchical BNOH/P/MX sandwich structure acts as an excellent interface and possesses multistage gradient modulus and wider thickness, uniformly and efficiently transferring the stress from the EP matrix to BFs. The interfacial shear strength, impact strength, and fracture toughness of BF–BNOH/P/MX-reinforced EP composite are substantially improved by 45.9 %, 60.6 %, and 148.9 %, respectively, compared with bare BF–based composites. This study can provide valuable references and inspirations for designing and constructing high-quality interfaces for high-strength and high-toughness BF structural materials, taking advantage of 2D materials.

The effect of swelling/deswelling cycles on the mechanical behaviors of the polyacrylamide hydrogels

In practical applications, hydrogels experience swelling or deswelling in response to external stimuli, showcasing large deformation, tunable mechanical properties, and excellent biocompatibility. Previous studies focused on the effect of individual swelling or deswelling on the mechanical behaviors of hydrogels, neglecting the combined effects of both processes. The underlying mechanism of how the combined processes of swelling and deswelling affect the mechanical behaviors of hydrogels has not been systematically explored. To address this gap, polyacrylamide hydrogels with diverse initial compositions are prepared and subjected to controlled swelling-deswelling or deswelling-swelling cycles to achieve the same water content state. The mechanical properties of the hydrogel samples exposed to these cycles, including shear modulus, stretchability, Mullins effect, and fracture toughness, are experimentally evaluated and compared with those of as-prepared hydrogel samples without undergoing such cycles. Our findings reveal a significant reduction in stretchability, the Mullins effect, and fracture toughness of hydrogels exposed to either cycle. The extent of these reductions varies depending on the initial compositions of the hydrogels. This reduction in properties is primarily attributed to the rearrangement of hydrogel networks during these cycles, which becomes noticeable in hydrogels with network inhomogeneity. We also found that although the shear modulus of hydrogels varies after swelling-deswelling or deswelling-swelling cycles, it remains at relatively consistent levels compared to the as-prepared hydrogel. These findings suggest that the mechanical properties of hydrogels can be adjusted by optimizing the initial compositions and regulating the swelling-deswelling processing method. By studying the effects of swelling/deswelling cycles on the mechanical properties of hydrogels, we provide valuable insights for the application of hydrogels under dynamic environmental changes.

Effect of heat treatment on the shear fracture and acoustic emission properties of granite with thermal storage potential: A laboratory-scale test

The exploitation of geothermal resources enhances the energy structure but poses challenges related to thermal–mechanical issues. This study utilizes short core in compression (SCC) specimens of granite to investigate thermo-mechanical properties. We examined the effects of heat treatment temperature on physical properties, shear fracture toughness, as well as acoustic emission (AE) characteristics. Fracture surface morphology and microstructure of the heat-treated SCC specimens were also analyzed using 3D laser scanning and scanning electron microscopy. Results indicate that both mass loss and P-wave velocity decrease with increasing heating-treated temperature, and similar trends are observed in mechanical properties. Specifically, fracture energy and shear fracture toughness decline by 51.48 % and 61.27 %, respectively, as temperature rises from 25 °C to 750 °C. The b-value and proportion of shear cracks initially increase before falling, with an inflection point at 300 °C, linked to thermal-induced microstructural deterioration. Fractal dimension (D) and joint roughness coefficient (JRC) show significant increases with higher heat treatment temperatures. Microstructural degradation, including dehydration and thermal-induced microcracking, drives the reduction in shear properties of heat-treated granites. Overall, our findings offer valuable insights into determining various methods of heat storage reconstruction with great application potential.

Simultaneously improving interfacial adhesion and toughness of carbon fiber reinforced epoxy composites by grafting polyethyleneimine on the carbon fiber surface to construct a rigid‐flexible interface

AbstractChemical grafting is commonly employed to functionalize carbon fiber (CF) surface to enhance the interfacial properties of CF reinforced polymer composites (CFRP) by forming a robust interface. However, an overly rigid interface layer can result in relatively poor interface toughness. To balance interfacial strength and toughness, this paper proposes an effective method to simultaneously strengthen and toughen the interface of CF/epoxy composites by grafting branched polyethyleneimine (PEI) on CF surface using hexachlorophosphazene (HCCP) as active intermediate, which features a rigid ring structure and six active P‐Cl groups. The introduction of PEI provided a large number of amine and imine groups on CF surfaces, which were contributive to improving surface wettability and reactivity of CF, thus resulting in strong chemical bonding between CF and epoxy matrix. Additionally, the long and flexible molecular skeleton of PEI constructed a rigid‐flexible composite interface, which could effectively reduce stress concentration and absorb impact energy. After surface modification with PEI, the interfacial shear strength and fracture toughness of CF/epoxy composite were increased to 89.9 MPa and 200.3 J m−2, by 64.3% and 149.7%, respectively, in compared with desized CF reinforced epoxy composites.Highlights A rigid‐flexible interface was constructed between CF and epoxy resin. The interfacial strength and toughness of CFRP were synergistically improved. IFSS and interfacial toughness increased by 64.3% and 149.7%, respectively.

Effect of curing process on shear and interlaminar fracture toughness of vinylester urethane-glass fibre composites

The influence of the curing process on the interlaminar shear strength and mode II toughness of Vinylester-Urethane resin composites with high density glass fibre plain fabric is shown in this work. There is a shorter curing treatment that achieves similar mechanical properties compared to the treatment proposed by the resin manufacturer. This is of industrial interest as shortening time leads to savings in production costs.

Progress in adhesive-bonded composite joints: A comprehensive review

Among the myriad joining techniques, the adhesive bonding technique is widely used to join complex large-scale composite structures because of its numerous advantages compared to traditional joining techniques. This article profusely analysed the various techniques for ameliorating the performance of composite joints, such as bonding methods (secondary bonding, co-bonding, co-curing, and multi-material bonding), surface modification techniques (plasma, laser surface treatment, surface grinding, etc.), additional reinforcement techniques (Z pin, wire mesh, nanofiller, etc), and different joint geometries (stepped joints, half-stepped joints, balanced joints, and scarf joints). Also, the effect of various adhesives and fabrication techniques on the static and dynamic performance of CFRP and GFRP-based joints was studied in detail. Moreover, this review addresses the finite element modelling and optimisation techniques on adhesively bonded joints. It has been observed that the bonding methods, surface modification to enhance the roughness of the adherend, addition of nanofillers, and variations in joint geometry greatly influence the shear strength, fracture toughness, fatigue, and vibration behaviour of FRP composite joints.

A novel fracture criterion for elastic‐brittle cracked body under compression and shear conditions

AbstractWhen the elastic‐brittle cracked body is subjected to compressive‐shear loading, not only does the phenomenon of wing crack initiation and propagation occur, but there is also a notable incidence of secondary crack initiation and propagation due to shear stress. Moreover, the shear fracture toughness (KIIc) predicted by the fracture criterion in the framework of classical fracture mechanics is usually smaller than the tensile fracture toughness (KIc), which is inconsistent with the actual observations. Therefore, this research firstly establishes a mechanical model of an elastic‐brittle cracked body under biaxial compression considering three kinds of crack parameters and materials' mechanical properties. Then, the stress field characteristics at the crack tip are analyzed. Next, a compression‐shear mixed fracture criterion is proposed. Finally, the effects of the lateral pressure coefficient (λ), critical plastic zone size (rc), friction coefficient (f), and deformation parameters of the crack surface on the failure modes at the crack tip are investigated.

Desirable Strong and Tough Adhesive Inspired by Dragonfly Wings and Plant Cell Walls.

The production of wood-based panels has a significant demand for mechanically strong and flexible biomass adhesives, serving as alternatives to nonrenewable and toxic formaldehyde-based adhesives. Nonetheless, plywood usually exhibits brittle fracture due to the inherent trade-off between rigidity and toughness, and it is susceptible to damage and deformation defects in production applications. Herein, inspired by the microstructure of dragonfly wings and the cross-linking structure of plant cell walls, a soybean meal (SM) adhesive with great strength and toughness was developed. The strategy was combined with a multiple assembly system based on the tannic acid (TA) stripping/modification of molybdenum disulfide (MoS2@TA) hybrids, phenylboronic acid/quaternary ammonium doubly functionalized chitosan (QCP), and SM. Motivated by the microstructure of dragonfly wings, MoS2@TA was tightly bonded with the SM framework through Schiff base and strong hydrogen bonding to dissipate stress energy through crack deflection, bridging, and immobilization. QCP imitated borate chemistry in plant cell walls to optimize interfacial interactions within the adhesive by borate ester bonds, boron-nitrogen coordination bonds, and electrostatic interactions and dissipate energy through sacrificial bonding. The shear strength and fracture toughness of the SM/QCP/MoS2@TA adhesive were 1.58 MPa and 0.87 J, respectively, which were 409.7% and 866.7% higher than those of the pure SM adhesive. In addition, MoS2@TA and QCP gave the adhesive good mildew resistance, durability, weatherability, and fire resistance. This bioinspired design strategy offers a viable and sustainable approach for creating multifunctional strong and tough biobased materials.

Holistic Characterization of MgO-Al2O3, MgO-CaZrO3, and Y2O3-ZrO2 Ceramic Composites for Aerospace Propulsion Systems

Aerospace propulsion systems are among the driving forces for the development of advanced ceramics with increased performance efficiency in severe operation conditions. The conducted research focused on the mechanical (Young’s and shear moduli, flexural strength, hardness, and fracture toughness), thermal (thermal conductivity and coefficient of thermal expansion), and electric (dielectric properties) characterization of MgO-Al2O3, MgO-CaZrO3, and stable YSZ ceramic composites. The experimental results, considering structural and functional traits, underscore the importance of a holistic understanding of the multifunctionality of advanced ceramics to fulfill propulsion system requirements, the limits of which have not yet been fully explored.

Bio-inspired copper ion-chelated chitosan coating modified UHMWPE fibers for enhanced interfacial properties of composites

Ultra-high molecular weight polyethylene (UHMWPE) fibers are broadly applied in lightweight and high-strength composite fiber materials. However, the development of UHMWPE fibers is limited by their smooth and chemically inert surfaces. To address the issues, a modified UHMWPE fibers material has been fabricated through the chelation reaction between Cu2+ and chitosan coatings within the surface of fibers after plasma treatment, which is inspired by the hardening mechanism, a crosslinked network between metal ions and proteins/polysaccharides of the tips and edges in arthropod-specific cuticular tools. The coatings improve the surface wettability and interfacial bonding ability, which are beneficial in extending the application range of UHMWPE fibers. More importantly, compared to the unmodified UHMWPE fiber cloths, the tensile property of the modified fiber cloths is increased by 18.89% without damaging the strength, which is infrequent in modified UHMWPE fibers. Furthermore, the interlaminar shear strength and fracture toughness of the modified fibers laminate are increased by 37.72% and 135.90%, respectively. These improvements can be attributed to the synergistic effects between the surface activity and the tiny bumps of the modified UHMWPE fibers. Hence, this work provides a more straightforward and less damaging idea of fiber modification for manufacturing desirable protective and medical materials.

Fracture toughness response of glass/epoxy composite in symmetric and asymmetric design layup

Composites are used in transportation industries for their exceptional strength, reduced weight, and cost. Their performance depends upon many factors, such as stacking sequence, fiber volume fraction (FVF), etc. In the present work, glass/epoxy composites are prepared in symmetric (ɸn/θ2n/ɸn//ɸn/θ2n/ɸn) and asymmetric (θn/ɸ2n/θn//ɸn/θ2n/ɸn) configuration with FVF of 48.13 and 61.34%. The laminate performance was evaluated using interlaminar shear strength, flexural, and mode-II interlaminar fracture toughness tests. The results indicate that symmetric layup has better mechanical strength than asymmetric layup. The interlaminar shear strength, flexural, and fracture toughness of asymmetric layup were lower by 14.72–16.39, 12.54–16.72, and 11.92–17.98%, respectively. Further, low FVF specimens had better mechanical strength, and high FVF specimens showed lower strength by 3.67–5.56, 10.75–15.17, and 23.53–28.79%, respectively,

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE MECHANICAL BEHAVIOR OF THE MODIFIED METAL AUXETIC STRUCTURE

Humans have always sought the optimal use of materials around them and, in this field, inspired by nature, have succeeded in inventing various structures. As one example, lattice structures, which are lightweight, strong, and stiff, are used widely in various applications, including energy absorbers. Lattice structures with a negative Poisson's ratio have been developed as a new type of lattice structure. As a result of this feature, auxetic structures have unique properties like shear strength, penetration resistance, fracture toughness, crack resistance, and high energy absorbability. In this paper, the mechanical behavior of the auxetic panels made using the 3D metal printer method is investigated by experimental tests and finite element methods. Experiments are used to verify the accuracy of the numerical model. Using the DMLS method, samples were prepared from metal-based AlS10Mg Aluminum composition. The 3D printing method was used to fabricate samples. Afterwards, experimental tests were made and the mechanical properties of these materials were determined by tensile test and used in finite element simulations. Following the confirmation of the model's accuracy, the finite element simulation results are used to perform a parametric study and determine the appropriate geometry. The numerical analysis is conducted using ABAQUS software, which uses the nonlinear finite element method.

Fabrication of strong and tough multifunctional soy protein adhesives via constructing catechol-iron ion fiber-reinforced hierarchical structure inspired by mussel byssus

It is challenging to prepare strong and tough soy protein (SP) adhesives with multifunction to replace formaldehyde-based adhesives. Herein, inspired by the catechol-iron ion (Fe3+) fiber-reinforced hierarchical structure of the mussel byssus, the tannic acid-functionalized cellulose nanofiber (CNF-TA) was used to mimic the fibrous core and catechol derivative, ferric chloride mimicked the Fe3+, and the SP mimicked the protein matrix in the mussel byssus to fabricate a multifunctional SP adhesive with high strength and toughness. Due to the construction of a stable fiber-reinforced covalent crosslinked network, along with the energy dissipation of sacrificial coordination and hydrogen bonds, the dry shear strength and fracture toughness increased by 130.6% and 247.1% to 2.49 MPa and 1.18 MJ m−2, respectively. Furthermore, the water-resistant wet shear strength rose by 264.1% to 1.42 MPa benefiting from the stable crosslinked structure. The Fe3+-induced organic-inorganic hybrid structure imparted the adhesive with outstanding flame retardance. Moreover, the adhesive also exhibited excellent antibacterial properties, mildew resistance, and oxidation resistance. This research offers a viable biomimetic method for fabricating multifunctional high-performance bio-based adhesives, hydrogels, and composites.

Mechanical and Viscoelastic Properties of Carbon Fibre Epoxy Composites with Interleaved Graphite Nanoplatelet Layer

The use of interleaving material with viscoelastic properties is one of the most effective solutions to improve the damping capacity of carbon fibre-reinforced polymer (CFRP) laminates. Improving composite damping without threatening mechanical performance is challenging and the use of nanomaterials should lead to the target. In this paper, the effect of a nanostructured interlayer based on graphite nanoplatelets (GNPs) on the damping capacity and fracture toughness of CFRP laminates has been investigated. High-content GNP/epoxy (70 wt/30 wt) coating was sprayed on the surface of CF/epoxy prepregs at two different contents (10 and 40 g/m2) and incorporated at the middle plane of a CFRP laminate. The effect of the GNP areal weights on the viscoelastic and mechanical behaviour of the laminates is investigated. Coupons with low GNP content showed a 25% increase in damping capacity with a trivial reduction in the storage modulus. Moreover, a reduction in interlaminar shear strength (ILSS) and fracture toughness (both mode I and mode II) was observed. The GNP alignment and degree of compaction reached during the process were found to be key parameters on material performances. By increasing the GNP content and compaction, a mitigation on the fracture drop was achieved (−15%).

Self-assembled nano-polymers modified water-based sizing agent for enhancing the dual interfacial properties of carbon fibre/epoxy resin composites

The dual interface-enhanced sizing agents are prepared to simultaneously exhibit high surface roughness and high interfacial wettability through self-assembled nano-polymers. Five self-assembled nano-polymer modified water-based sizing agents (NPSAs) with various cross-linking structures are synthesized in this paper. It is demonstrated strong characteristics of homogeneous particle size in the 90–200 nm range and heat stability below 200 °C. NPSAs significantly improve the CF surface properties of both surface roughness and surface energy, with reached high values of 13.89 nm and 40.22 mN m-1. Notably, the interlaminar shear strength (ILSS), flexural strength, and fracture toughness (KIC) of CF/ER composites sized by NPSAs are enhanced to 70%, 51%, and 120% compared with the unoptimized composite, respectively. The NPSA reinforcing mechanism, enhancing interfacial properties of CF/ER composites in the interfacial interlocking and adhesion ability, was clarified by the coefficient of thermal expansion (CTE1). Additionally, NPSAs have the advantage of being applicable for simple one-pot synthesis methods, and secondly, no toxic solvents were used.

Temperature- and pressure-induced structural transformations in NbN: A first-principles study

Structural transformations at high temperatures and pressures, elastic moduli, hardness, fracture toughness, Debye temperature, stress-shear strain relations, electronic structure, and lattice dynamics in the experimentaly observed NaCl–NbN (δ, Fm-3m), anti-TiP-NbN (ε, P63/mmc), anti-NiAs-NbN (δ′, P63/mmc), WC-NbN (η, P-6m2), and TiP–NbN (ε′, P63/mmc) phases and hypothetical new tP4-129 (P4/nmm), hP6-189 (P-62m), oP8-25 (Pmm2) and cP2-221 (Pm-3m) structures are studied by using first-principles calculations and molecular dynamics simulations. The possible mechanisms of the phase transitions between these structures based on the condensation of a certain phonon mode with subsequent spontaneous strains are suggested. The ε, η and δ′, and cP2-221 structures are brittle materials and exhibit highest shear moduli (200.5–216.8 GPa), Young moduli (492.4–528.8 GPa), Vickers hardness (23.9–27.1 GPa), fracture toughness (4.54–4.72 MPa m1/2), and Debye temperatures (730.5–767.0 K). It is found that the main slip systems should be (0001)<10-10> for ε and η, and both (0001)<10-10> and (0001)<-12-10> for δ’.

Fabrication and Experimental Estimation of Mechanical Properties of Kevlar-Glass/Epoxy Interwoven Composite Laminate

Hybrid composites made of natural and synthetic fibers are stronger, lighter, cheaper, biodegradable, and greener than conventional metals, and they are replacing conventional metals. The primary objective of the study was to examine the mechanical properties of interwoven hybrid composite laminates. Kevlar and glass fiber are used as reinforcement for this work. The fibers are woven together using various weaving techniques. 1 × 1, 3 × 3, and 5 × 5 weaving patterns are considered to explore the properties of the laminates. The composites are woven using a conventional handloom method. As a matrix, LY556 resin and HY951 hardener are combined at a ratio of 10 : 1. The composites are cured using compression molding. The cured composites are assessed for their tensile strength, flexural strength, compressive strength, interlaminar shear strength, impact strength, and fracture toughness. The highest tensile, compressive, and flexural strength were found in the 1 × 1 pattern, shear strength and fracture toughness were found in the 5 × 5 pattern, which finds applications in aerospace and defense sectors, and 3 × 3 dominated in impact strength; as a result, it can be used in bulletproof applications. At last, a scanning electron microscope (SEM) was used to visualize the matrix-reinforcement bonding. The microscopic images show the ripped-out fibers because of the tensile test. The shards in SEM are evident that impact force breaks the matrix elements in a brittle manner.