The mechanisms of ductile rupture Noell, Philip J.; Carroll, Jay D.; Boyce, Brad L.
Acta materialia,
12/2018, Letnik:
161, Številka:
C
Journal Article
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One of the most confounding controversies in the ductile fracture community is the large discrepancy between predicted and experimentally observed strain-to-failure values during shear-dominant ...loading. Currently proposed solutions focus on better accounting for how the deviatoric stress state influences void growth or on measuring strain at the microscale rather than the macroscale. While these approaches are useful, they do not address a significant aspect of the problem: the only rupture micromechanisms that are generally considered are void nucleation, growth, and coalescence (for tensile-dominated loading), and shear-localization and void coalescence (for shear-dominated loading). Current phenomenological models have thus focused on predicting the competition between these mechanisms based on the stress state and the strain-hardening capacity of the material. However, in the present study, we demonstrate that there are at least five other failure mechanisms. Because these have long been ignored, little is known about how all seven mechanisms interact with one another or the factors that control their competition. These questions are addressed by characterizing the fracture process in three high-purity face-centered cubic (FCC) metals of medium-to-high stacking fault energy: copper, nickel, and aluminum. These data demonstrate that, for a given stress state and material, several mechanisms frequently work together in a sequential manner to cause fracture. The selection of a failure mechanism is significantly affected by the plasticity-induced microstructural evolution that occurs before tearing begins, which can create or eliminate sites for void nucleation. At the macroscale, failure mechanisms that do not involve cracking or pore growth were observed to facilitate subsequent void growth and coalescence processes. While the focus of this study is on damage accumulation in pure metals, these results are also applicable to understanding failure in engineering alloys.
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Additive manufacturing offers unprecedented opportunities to design complex structures optimized for performance envelopes inaccessible under conventional manufacturing constraints. Additive ...processes also promote realization of engineered materials with microstructures and properties that are impossible via traditional synthesis techniques. Enthused by these capabilities, optimization design tools have experienced a recent revival. The current capabilities of additive processes and optimization tools are summarized briefly, while an emerging opportunity is discussed to achieve a holistic design paradigm whereby computational tools are integrated with stochastic process and material awareness to enable the concurrent optimization of design topologies, material constructs and fabrication processes.
Metamaterials, otherwise known as architected or programmable materials, enable designers to tailor mesoscale topology and shape to achieve unique material properties that are not present in nature. ...Additionally, with the recent proliferation of additive manufacturing tools across industrial sectors, the ability to readily fabricate geometrically complex metamaterials is now possible. However, in many high-performance applications involving complex multi-physics interactions, design of novel lattice metamaterials is still difficult. Design is primarily guided by human intuition or gradient optimization for simple problems. In this work, we show how machine learning guides discovery of new unit cells that are Pareto optimal for multiple competing objectives; specifically, maximizing elastic stiffness during static loading and minimizing wave speed through the metamaterial during an impact event. Additionally, we show that our artificial intelligence approach works with relatively few (3500) simulation calls.
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•The geometry of metamaterials can be automatically designed to maximize their performance against multiple objectives.•The pragmatic approach combines a genetic algorithm with a convolutional neural network to efficiently invent manufacturable architectures.•The convolutional neural network performs best when the input is a 3x3 tiled lattice rather than a single unit cell.•Active learning enables a significant reduction in the number of example designs needed to train the convolutional neural network.
While lattice metamaterials can achieve exceptional energy absorption by tailoring periodically distributed heterogeneous unit cells, relatively little focus has been placed on engineering ...heterogeneity above the unit-cell level. In this work, the energy-absorption performance of lattice metamaterials with a heterogeneous spatial layout of different unit cell architectures was studied. Such multi-morphology lattices can harness the distinct mechanical properties of different unit cells while being composed out of a single base material. A rational design approach was developed to explore the design space of these lattices, inspiring a non-intuitive design which was evaluated alongside designs based on mixture rules. Fabrication was carried out using two different base materials: 316L stainless steel and Vero White photopolymer. Results show that multi-morphology lattices can be used to achieve higher specific energy absorption than homogeneous lattice metamaterials. Additionally, it is shown that a rational design approach can inspire multi-morphology lattices which exceed rule-of-mixtures expectations.
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•Multi-morphology lattices are composed using face-centered cubic (FCC) and body-centered cubic (BCC) unit cell architectures.•Different multi-morphology lattice designs are inspired by mixture rules and a developed rational design framework.•The topological arrangement of unit cell architectures plays a prominent role in determining energy-absorption performance.•Primary collapse mode can be controlled through design so that it persists across different base materials.•Improvements over both homogeneous lattice metamaterials and rule-of-mixtures expectations are driven by design.
Interlocking metasurfaces Bolmin, Ophelia; Young, Benjamin; Leathe, Nicholas ...
Journal of materials science,
2023/1, Letnik:
58, Številka:
1
Journal Article
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Interlocking metasurfaces (ILMs) are architected arrays of mating features that enable joining of two bodies. Complementary to traditional joining technologies such as bolts, adhesives, and welds, ...ILMs combine ease of assembly, removal, and reassembly with robust mechanical properties. Structural in nature, they act in a quasi-continuous manner across a surface and enable joining of complex surfaces, e.g., lattices. In this perspective, we define an ILM, begin exploring the design domain and illustrate its breath, and pragmatically evaluate mechanical performance and manufacturability. ILMs will find applications in various fields from aerospace to micro-robotics, civil engineering, and prosthetics.
Recent work suggests that thermally stable nanocrystallinity in metals is achievable in several binary alloys by modifying grain boundary energies via solute segregation. The remarkable thermal ...stability of these alloys has been demonstrated in recent reports, with many alloys exhibiting negligible grain growth during prolonged exposure to near‐melting temperatures. Pt–Au, a proposed stable alloy consisting of two noble metals, is shown to exhibit extraordinary resistance to wear. Ultralow wear rates, less than a monolayer of material removed per sliding pass, are measured for Pt–Au thin films at a maximum Hertz contact stress of up to 1.1 GPa. This is the first instance of an all‐metallic material exhibiting a specific wear rate on the order of 10−9 mm3 N−1 m−1, comparable to diamond‐like carbon (DLC) and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments are either higher or comparable to that of the Pt–Au alloy, despite the substantially higher hardness of the ceramic probe materials. High‐resolution microscopy shows negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Mitigation of fatigue‐driven delamination enables a transition to wear by atomic attrition, a regime previously limited to highly wear‐resistant materials such as DLC.
A stable nanocrystalline alloy of Pt and Au is shown to be extremely resistant to mechanical abrasion and fatigue, having volumetric or specific wear rates comparable to diamond‐like carbon. This is the first report of a metal having such wear resistance.
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•Inserted planes increased fracture toughness in solid beams and metamaterials.•The toughening mode varied as a function of base material.•Some toughening was so substantial that ...fracture could no longer be induced.
Fracture toughness, rather than strength, is often the limiting factor of structural materials. Developing new base materials with improved fracture toughness often takes more than a decade. Alternatively, topological design has recently been expanded by additive manufacturing. In the present study, architected planes of internal porosity mimicking a weak interface were found capable of arresting and deflecting a propagating crack, delaying fracture. This concept was demonstrated experimentally in solid beams of polymeric 3D printed material, and in gyroid metamaterials constructed from either a brittle polymer or stainless steel. Improvements in fracture toughness ranged from 22% to 300% depending on the material. Especially with topological optimization, toughening via designed porosity provides an avenue for cost-effective and simple toughening across a range of materials.
Abstract The viscoelastic response of bovine corneas was characterized using in vitro inflation (bulge) experiments combined with spatially-resolved deformation mapping via digital image correlation. ...A complex fixture conforming to the limbal annulus was developed to hold the attached sclera rigid while allowing deformation only in the cornea. A statistical set of experiments was performed for a pressure range of 3.6–8 kPa (27–60 mmHg), representing nominal bovine intraocular pressure (IOP) to acute glaucoma conditions. A broader pressure range of 0–32 kPa (0–240 mmHg) was also examined to characterize the nonlinear finite deformation behavior of the tissue. Results showed that for pressures near and above IOP, the majority of the deformation was localized in the limbus and peripheral regions, which left the central cornea largely undeformed. This observation was consistent with the known preferred circumferential alignment of collagen fibrils outside of the central cornea. In general, the inflation experiments observed viscoelastic behavior in the form of rate-dependent hysteresis in the pressure–deformation response of the apex of the cornea, creep in the apex deformation at a constant inflation pressure, and relaxation in the pressure response at a constant inflation volume. The 3.6–8 kPa (27–60 mmHg) pressure range produced small viscoelastic deformations and a nearly linear pressure–deformation response, which suggests that for physiological pressure ranges, the cornea can be approximated as a linear viscoelastic or linear pseudo-elastic material.
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