#ductility

Come visto, edifici con periodi diversi rispondono in modo diverso allo stesso sisma. Analogamente, uno stesso edificio può reagire diversamente a terremoti differenti. Per la progettazione è quindi fondamentale disporre di una rappresentazione grafica della risposta dell’edificio in funzione delle frequenze di moto del suolo: questa è lo spettro di risposta del sito. Lo spettro di risposta è un…

Balancing the seesaw: Simultaneously enhancing strength and elongation in metallic materials

Just as one side of a seesaw rises while the other falls, in the realm of metallic materials, "strength" and "elongation" typically conflict with each other. However, a collaborative team from POSTECH and Northwestern University has recently introduced a groundbreaking technology that enhances both properties. A research team, consisting of Professor Hyoung Seop Kim from the Graduate Institute of Ferrous & Eco Materials Technology and the Department of Materials Science and Engineering, Professor Yoon–Uk Heo from the Graduate Institute of Ferrous & Eco Materials Technology, and Ph.D. candidate Hyojin Park from the Department of Materials Science and Engineering at POSTECH, collaborated with Dr. Farahnaz Haftlang from Northwestern University's Department of Materials Science & Engineering. Together, they have tackled a long-standing issue in metals research: the tradeoff between strength and elongation. Their breakthrough involves designing an alloy that boasts both high strength and high elongation.

An overview of ductility and brittleness and how different materials behave under loading. Understanding these material properties can help you make better decisions in engineering, construction, and manufacturing.

Ductile to Brittle Transition Temperature

Of the two general types of fracture (ductile or brittle), not all materials fall neatly into one category or the other. Some materials experience what is known as the ductile to brittle transition temperature, or DBTT. Ferritic steels in particular, among other BCC metals, have two failure modes, switching from ductile failure at higher temperatures to brittle failure at lower ones. While the transition is perhaps more well known in metals, it can occur in other materials such as polymers and ceramics as well. Different forms of impact testing are used to determine the temperature.

Researchers develop a unique quantum mechanical approach to determining metal ductility

A team of scientists from Ames National Laboratory and Texas A&M University developed a new way to predict metal ductility. This quantum-mechanics-based approach fills a need for an inexpensive, efficient, high-throughput way to predict ductility. The team demonstrated its effectiveness on refractory multi-principal-element alloys. These are materials of interest for use in high-temperature conditions, however, they frequently lack necessary ductility for potential applications in aerospace, fusion reactors, and land-based turbines. Ductility describes how well a material can withstand physical strain without cracking or breaking. According to Prashant Singh, a scientist at Ames Lab and leader of the theoretical design efforts, there are currently no robust ways to predict metal ductility. Additionally, trial-and-error experimentation is expensive and time-consuming, especially in extreme conditions.

New criteria to determine whether shear bands are beneficial or harmful to crystalline materials

Shear band formation is not typically a good sign in a material—the bands often appear before a material fractures or fails. But materials science and engineering researchers at the University of Wisconsin–Madison have found that shear bands aren't always a negative; under the right conditions, they can improve the ductility, or the plasticity, of a material.

Led by Izabela Szlufarska, a professor of materials science and engineering at UW–Madison, the researchers published details of their work in the journal Nature Materials.

Using a combination of experimental characterization and simulations, the team identified potential strategies for encouraging shear bands. This could lead to new ways of increasing the toughness of a wide array of materials.

"In a previous paper, we demonstrated that shear bands in a material called samarium cobalt could actually be beneficial," says Szlufarska. "That led to the questions, "When do shear bands form?" and "When do they support plasticity versus fracture? When do you want to avoid them and when do you want to promote them?'"

In the 1900s it was discovered that ceramic materials, at least in principle, can be permanently deformed without fracture at room temperature. Since then, materials researchers have dreamed of making ceramics that can be bent, pulled, and hammered without fracture. In his article Dr. Erkka J. Frankberg comments on recent research results on ductile ceramics and ponders whether they could be scaled for commercial use.

Making of ductile ceramics is a hard task. Plasticity in ceramics is rarely observed and typically requires special conditions such as extreme temperatures to be plausible. Therefore, instead on denting, your ceramic coffee mug will fracture into pieces when dropped on a hard floor.

In his article, Dr. Erkka J. Frankberg, a Finland based expert on plasticity of ceramics, comments some of the latest findings regarding room temperature plasticity in ceramics, reported by J. Zhang et al. in the Science378, 371 (2022). In his commentary, Frankberg paints a broader view on the potential benefits if such ductile ceramics could be made possible and scaled for commercial use, possibly ushering in a new stone age.

Why would it be important to discover ceramics that are ductile at room temperature? It is due to the atoms themselves and the bonding between them. Ceramics have ionic and covalent bonding between the atoms that significantly differ from, for example, bonds in metal alloys. One major difference is that the ionic and covalent atom bonds are among the strongest we know. As a result, in theory, ceramics should be among the strongest engineering materials that exist.

Creating stronger and more ductile microlattice materials with reduced unit sizes

Projection micro stereolithography (PμSL) has emerged as a powerful three-dimensional (3D) printing technique for manufacturing polymer structures with micron-scale high resolution at high printing speed, which enables the production of customized 3D microlattices with feature sizes down to several microns. However, the mechanical properties of as-printed polymers were not systemically studied at the relevant length scales, especially when the feature sizes step into micron/sub-micron level, limiting its reliable performance prediction in micro/nanolattice and other metamaterial applications.

Based on self-developed in situ micro-mechanical platform, Prof. Yang Lu from City University of Hong Kong demonstrates that projection micro-stereolithography (PμSL)-printed microfibers could become stronger and significantly more ductile with reduced size ranging from 20 μm to 60 μm, showing an obvious size-dependent mechanical behavior, in which the size decreases to 20 μm with a fracture strain up to ~100% and fracture strength up to ~100 MPa.

Such size effect enables the tailoring of the material strength and stiffness of PμSL-printed microlattices over a broad range, allowing to fabricate the microlattice metamaterials with desired/tunable mechanical properties for various structural and functional applications.