Two-phase nanolaminates are known for their high strength, yet they suffer from loss of ductility. Here, we show that broadening heterophase interfaces into “3D interfaces” as thick as the individual ...layers breaks this strength-ductility trade-off. In this work, we use micropillar compression and transmission electron microscopy to examine the processes underlying this breakthrough mechanical performance. The analysis shows that the 3D interfaces stifle flow instability via shear band formation through their interaction with dislocation pileups. To explain this observation, we use phase field dislocation dynamics (PFDD) simulations to study the interaction between a pileup and a 3D interface. Results show that when dislocation pileups fall below a characteristic size relative to the 3D interface thickness, transmission across interfaces becomes significantly frustrated. Our work demonstrates that 3D interfaces attenuate pileup-induced stress concentrations, preventing shear localization and offering an alternative way to enhanced mechanical performance.
We herein report a unique scientific strategy in engineering green composites with excellent balanced performance of stiffness, toughness and heat deflection temperature (HDT). To achieve such ...target, a high melt strength bioplastic blend of poly (butylene succinate), PBS and poly (butylene-adipate-co-terephthalate), PBAT with super toughness could take-up stiff natural fibres like inexpensive purpose grown Miscanthus fibre and Oat hull (a co-product from food industry) in the green composite structure through reactive extrusion. The novelty of this work is to allow the free radical reaction to occur within polymer matrix and between the blend and natural fibres by introducing biomass from a side feeder, aiming to achieve matrix toughening and polymer/fibre interface enhancement simultaneously. Tensile modulus of the bio-blends was improved by maximum 650% with the reinforcement of 40 wt% agricultural by-products. For equal amounts of 40% biomass fibres, the Miscanthus fibre is more suitable to strengthen the biopolymers, in terms of higher modulus (3.0 GPa), higher HDT (110 °C) and lower water absorption (3.4%); however, the oat hull composites show much higher impact strength of 83 J/m. A systematical structure-properties relationship study indicates that higher aspect ratio, higher cellulose contents and stronger interfacial actions account for the higher stiffness and HDT of the Miscanthus fibre composites compared to the oat hull-filled counterparts. This abundant biomass has potential to fabricate sustainable composites to substitute certain fossil fuel-based plastics in terms of mechanical and thermal properties, as well as cost and energy-saving via using the industrial friendly technology as proposed here.
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•Stiffness-toughness-HDT balanced green composites were fabricated.•Blend matrix and polymer/filler interface improvement were achieved simultaneously.•Two different abundant biomass – miscanthus fiber and oat hull were evaluated.•Structure-properties relationship in green composites were clarified.
Powder Bed Fusion (PBF) techniques constitute a family of Additive Manufacturing (AM) processes, which are characterised by high design flexibility and no tooling requirement. This makes PBF ...techniques attractive to many modern manufacturing sectors (e.g. aerospace, defence, energy and automotive) where some materials, such as Nickel-based superalloys, cannot be easily processed using conventional subtractive techniques. Nickel-based superalloys are crucial materials in modern engineering and underpin the performance of many advanced mechanical systems. Their physical properties (high mechanical integrity at high temperature) make them difficult to process via traditional techniques. Consequently, manufacture of nickel-based superalloys using PBF platforms has attracted significant attention. To permit a wider application, a deep understanding of their mechanical behaviour and relation to process needs to be achieved. The motivation for this paper is to provide a comprehensive review of the mechanical properties of PBF nickel-based superalloys and how process parameters affect these, and to aid practitioners in identifying the shortcomings and the opportunities in this field. Therefore, this paper aims to review research contributions regarding the microstructure and mechanical properties of nickel-based superalloys, manufactured using the two principle PBF techniques: Laser Powder Bed Fusion (LPBF) and Electron Beam Melting (EBM). The ‘target’ microstructures are introduced alongside the characteristics of those produced by PBF process, followed by an overview of the most used building processes, as well as build quality inspection techniques. A comprehensive evaluation of the mechanical properties, including tensile strength, hardness, shear strength, fatigue resistance, creep resistance and fracture toughness of PBF nickel-based superalloys are analysed. This work concludes with summary tables for data published on these properties serving as a quick reference to scholars. Characteristic process factors influencing functional performance are also discussed and compared throughout for the purpose of identifying research opportunities and directing the research community toward the end goal of achieving part integrity that extends beyond static components only.
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•Review of academic and industrial contributions over the last 25 years.•Practice and evaluation methodologies are explored.•The capabilities of PBF processes and the resulting material properties are linked.•Opportunities for further research are stated to help direct the research community.•A benchmark of mechanical properties obtained to date for PBF are provided.
The bipartite toughnesstB(G) of a non-complete bipartite graph G=(X,Y) is defined as tB(G)=min{|S|c(G−S)}, in which the minimum is taken over all proper subsets S⊂X (or S⊂Y) such that G−S is ...disconnected and c(G−S)>1. A non-complete bipartite graph G is tB-tough if |S|≥tBc(G−S) for every proper subset S⊂X (or S⊂Y) with c(G−S)>1. By incorporating the toughness and spectral conditions, we provide spectral radius and edge conditions for 2-factors in 1-tough balanced bipartite graphs.
Asphalt concrete pavements containing top-down cracks can be subjected to combined opening-out of plane sliding (i.e. mixed mode I/III) deformations due to traffic loads. However, lack of a suitable ...laboratory test specimen for fracture toughness testing of asphalt composites under mixed mode I/III, is one of the main shortcomings of this field. Hence, a new and simple test configuration is proposed in this research for determining mixed mode I/III fracture toughness of these composite materials. The specimen which is called the Edge Notched Disc Bend (ENDB), is a disc containing an edge crack created through one side of specimen along the diameter and is loaded by a three-point bend fixture. It was demonstrated that the complete range of mode mixity from pure mode I to pure mode III can be achieved by this specimen simply by changing the crack inclination angle relative to the loading support direction. The practical ability of the suggested ENDB specimen, was then examined for obtaining the mixed mode I/III fracture toughness of a typical hot mix asphalt (HMA) composite for different mode mixities including pure modes I and III. Corresponding values of KIc and KIIIc were determined for each mode mixity and it is shown that the obtained results are in quite well agreement with the predictions of a mixed mode I/III fracture theory. Consequently, the ENDB specimen can be used as a favorite candidate specimen for mixed mode fracture toughness study of asphalt concrete materials.
In materials of all types, hysteresis and toughness are usually correlated. For example, a highly stretchable elastomer or hydrogel of a single polymer network has low hysteresis and low toughness. ...The single network is commonly toughened by introducing sacrificial bonds, but breaking and possibly reforming the sacrificial bonds causes pronounced hysteresis. In this paper, we describe a principle of stretchable materials that disrupt the toughness–hysteresis correlation, achieving both high toughness and low hysteresis. We demonstrate the principle by fabricating a composite of two constituents: a matrix of low elastic modulus, and fibers of high elastic modulus, with strong adhesion between the matrix and the fibers, but with no sacrificial bonds. Both constituents have low hysteresis (5%) and low toughness (300 J/m²), whereas the composite retains the low hysteresis but achieves high toughness (10,000 J/m²). Both constituents are prone to fatigue fracture, whereas the composite is highly fatigue resistant. We conduct experiment and computation to ascertain that the large modulus contrast alleviates stress concentration at the crack front, and that strong adhesion binds the fibers and the matrix and suppresses sliding between them. Stretchable materials of high toughness and low hysteresis provide opportunities to the creation of high-cycle and low-dissipation soft robots and soft human–machine interfaces.
•True mode II fracturing is compared to the conventional mode II fracturing.•A criterion is suggested to predict the fracturing type (tensile- or shear-based).•The superiority of the DNBD test over ...the existing ones is discussed.•True mode II DNBD tests are conducted on three different rock types.•True mode II values of the fracture toughness are compared to the mode I ones.
This paper discusses the use of the double-edge notched Brazilian disk test (DNBD) for measuring true mode II fracture toughness of rocks. The term true emphasises that in this test, not only is the crack tip loading shear-based, but also the material failure is shear-induced. Conventional mode II tests typically experience dominantly tensile failure. We introduce a fracture growth criterion that explains where and how a shear-based fracture extension occurs. Our theoretical analysis demonstrates that large values of compressive T-stress in the DNBD specimen significantly help inducing a true mode II fracturing. Crack tip parameters are computed by finite element analyses for various notch lengths and loading angles. These values are then employed to determine the geometry and loading condition for the optimal performance of the test. We also compare the DNBD with two other available tests for measuring true mode II fracture toughness, and show that the DNBD test typically has a lower contribution of mode I loading than the two alternative approaches and, therefore, better approximates the true mode II condition while at the same time being the experimentally simplest. Three types of rocks (limestone, marble and granite) were tested using the new approach and their true mode II fracture toughness is reported for two different crack lengths. The measured true mode II fracture toughness is compared with the mode I fracture toughness obtained from the semi-circular bending test.
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Counting for approximately 3.13 million tons of products, epoxy resins (EPs) have been extensively employed as matrix resins of composites and applied in aerospace, aviation, wind, ...nuclear power, high-speed rail, as well as many other manufacturing industries. However, certain inherent limitations severely impede further applications of these advanced materials, such as poor fatigue resistance, low impact resistance, hard to recycle, and a “seesaw” between toughness and strength. Here, we especially focused on the recent progress in toughening methods and associated mechanisms for these epoxy resins and analytic techniques for characterizing toughness, which highlighted the applicable approaches to generate homogeneous structures. As a typical homogeneous toughness material, hyperbranched polymers, especially hyperbranched epoxy resins (HERs), are ideal candidates to solve the knotty problem of EPs because HERs can homogeneously reinforce and toughen diglycidyl ether of bisphenol-A (DGEBA), promoting the degradation of DGEBA, as well exhibiting strong interfacial interactions among components, due to their excellent compatibility, wettability and low viscosity. Last but not least, we advance the objectives and challenges of epoxy resins in the future. Overall, this review presents an up-to-date overview of toughening methods and mechanisms for EPs, and guidance of emerging research on the sustainable development of EPs in versatile high-tech fields.
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