Up to 80% of an airplane is covered in an aluminumĚýalloy, making these alloys the most important aerospace materials. Understanding their mechanical and physical properties, however, remains a challenge for engineers.ĚýĚý

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The aerospace industry is constantly evolving, with new materials emerging that can revolutionize the sector. These materials must be lightweight to make air transport more economical and greener, but must also be rigid and strong enough to withstand intense mechanical stress. As aluminum alloys are increasingly used for their high damage tolerance and toughness, quantifying this ‘rigidity’ is important to understand how effective the material is.Ěý

To determine the stiffness of a material—to find its Young’s modulus—Dr. Dalaver Anjum, Assistant Professor of Physics, has collaborated with Dr. Muna Khushaim from Taibah University, Saudi Arabia, to develop a method of ‘seeing’ how the alloys are impacted at the nanometer scale by adding different metals. They published their findings in .

The Young’s modulus of a material is a fundamental property that cannot be changed within the elastic limit of that material. It is the stiffness of a material and states how easily it can bend or stretch, dependent upon temperature and pressure. When a material reaches a certain stress point, it will begin to deform. Consider a rubber band: when a rubber band is pulled, it is stretched, but not deformed. Stretch too far, however, and the band will begin to deteriorate or deform until it inevitably breaks. For engineers across all domains, the Young’s modulus is a critical constant for research and design as every material responds differently to stress.

One of the ways to enhance the mechanical properties of metals involves synthesizing their alloys by mixing one metal with another. For example, the properties of aluminum can be dramatically enhanced using small amounts of copper or lithium. These added metals are known to exist in aluminum metal as precipitates and can make materials stronger by impeding the movement of metal atoms across the crystal defects in the host metal’s lattice structure. The stiffer a material, the higher its Young’s modulus.

“The stresses and elasticity of the strengthening precipitates in aluminum-based alloys play an important role in improving their mechanical and structural properties,” explained Dr. Anjum. “Using precipitates is known as strain hardening or precipitate hardening, with this method affecting the Young’s modulus of the host metal in question.”

Determining the Young’s modulus of the new alloy can be difficult. It is usually determined experimentally at macro or bulk scales and can only provide average values of the mechanical qualities. It would be preferable to determine it at nanoscale, simply because the precipitates adding to the Young’s modulus result are present at the nanoscale; they’re like nanoparticles in the host metal.

“More than 15 years ago, transmission electron microscopes were first used in conjunction with electron energy loss spectroscopy (EELS) detectors to estimate the Young’s modulus of metals,” explained Dr. Anjum. “But this only provides the average value on a rough scale. Ideally, we would like to be able to show how the precipitates impact the stiffness of the metal at the nanoscale and even sub-nanoscale. We could then see how the strain field is distributed around the precipitates in the metal lattice structure.”

To better ‘see’ the strain field and Young’s modulus simultaneously, Dr. Anjum and Dr. Khushaim developed a new method to generate spatially resolved maps of metal alloys by combining dark-field scanning transmission electron microscopy (DF-STEM) and EELS data and then processing these datasets with specific software algorithms.

The TEM results show the structure of the precipitates in the aluminum metal matrix, while the EELS data shows whether or not the electrons that pass through the material lose some of their energy or have their paths deflected by the atoms in the matrix. The amount that the electron’s path is deflected can be measured with DF-STEM and gives information about the dispersion of the atoms within the structure. The relative dispersion difference between the matrix and precipitate regions can then be expressed as ‘strain.’ Similarly, the electron beam also forces the freely roaming electron gas or plasmons to oscillate around their equilibrium and this can be measured with EELS. The frequency of these oscillations is related to the stiffness of materials.Ěý

The researchers then feed this data into their algorithms to map and ‘see’ the strain fields in the alloy matrix. Using this data, they can then calculate the Young’s modulus of the material in question.ĚýĚýĚý

This research has great potential in the UAE as the method can be used to develop the mechanical properties of metal alloys with applications in the aerospace industry and other areas important to the nation.

“Since it is based on measuring the fundamental properties of metals at the nanoscale, it offers a window to developing next-generation metal alloys based on fundamental materials science,” explained Dr. Anjum. “Therefore, these alloys are expected to be more durable, while also providing the opportunity of producing next-generation scientists and engineers in the UAE.”

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Jade Sterling
Science Writer
24 January 2021

The post A New Method to Determine the Strength of Aerospace Materials appeared first on Khalifa University.

Faculty Asserts Frequencies Can Be Tested Experimentally to Advance Unifying Physics Theory

New types of wave oscillations in black holes have been discovered that can be probed experimentally by gravitational-wave detectors, which in turn could advance scientific understanding of the key elements of a grand unifying theory for physics.

A black hole is formed through the collapse of a star, which causes a massive gravitational force to pull in all objects around it, including light, dust, and gas, thus causing the black hole to grow. These massive and incredibly dense objects have in general three ‘layers’– the singularity at the center, then the inner event horizon, and finally the outer event horizon, where phenomenon take place that challenge the laws of General Relativity. Our galaxy – the Milky Way – is estimated to have several black holes. Moreover, recent research in astrophysics indicates that a supermassive black hole should sit at the center of every galaxy. The mass of such astrophysical objects should be typically of the order of several million solar masses.

Black holes pull objects towards them and they can also attract each other. Like two whirlpools in the ocean, the black holes orbit around each other, radiating gravitational waves as they draw nearer. Eventually they lose energy in the gravitational radiation as their revolutions speed up and get closer, allowing their event horizons to merge. The last phase, before they merge, is called the ‘ringdown’, where the unified black hole system is still ringing and radiating, but progressively less so.

This ringdown phenomenon was first detected in 2016, when the Laser Interferometer Gravitational-Wave Observatory (LIGO) operated by Caltech and the Massachusetts Institute of Technology detected gravitational wave signals from a pair of inspiralled black holes as they merged and underwent the ringdown – discoveries thatĚýled to the Nobel Prize in 2017.

“In the ringdown phase, the black hole starts vibrating after interacting with matter. ĚýThese vibrations get translated into gravitational waves, in the same way a guitar string translates being plucked into sound waves. It also happens that independently on how you ‘pluck’ the black hole, for example if it is fed by a scalar particle, a photon, or an electron, the resulting gravitational wave will have the same frequency, much like the string,” explained KU Assistant Professor in the Department of Applied Mathematics and Statistics Dr. Davide Batic.

The waves are sent out during the ringdown phase and are composed by many frequencies, called quasinormal modes. Their oscillations become smaller and smaller as time goes by.

“Despite all the knowledge we have on the quasinormal spectrum of black holes, there has been no actual explicit formula to compute them. All computations have been done using numerical methods,” Dr. ĚýBatic added.

Dr. Batic has co-published a paper on the new black hole oscillations he believes he has discovered. The paper titled ‘Some exact quasinormal frequencies of a massless scalar field in Schwarzschild spacetime’, was published in the journal Physical Review D with co-authors Dr. Marek Nowakowski from the Universidad de los Andes, Columbia, and the master student Karlus Redway, from the University of the West Indies.

The team’s research results may also advance the development of a grand unified physical theory, which has a been an ongoing challenge in physics for decades. Such a grand unified theory should merge two of the main pillars of modern physics – General Relativity and Quantum Mechanics. Furthermore, when General Relativity is pushed to the limits, like inside the event horizon of a black hole, it makes an ‘unphysical prediction’ that the core of a black hole would have infinite curvature. Ěý

In Einstein’s General Theory of Relativity, gravity is caused by the curvature of space-time. However, the theory cannot account for ‘unphysical predictions’ — calculations not in accordance with the laws or principles of physics — when applied to what happens inside the event horizon of a black hole.

“Apart from trying to describe how quantum fields interact with black holes – this is what we call quantum field theory in curved space-times – results in this area are of paramount importance in the development of a unified physical theory such as Quantum Gravity because every candidate theory of topics such as String Theory and Loop Quantum Gravity will need to pass a fundamental test, namely it must be able to reproduce on a certain scale all predictions arising from quantum field theory in curved space-times,” Dr. Batic explained.

He is now working to derive a formula to compute the numerical values of the quasinormal wave modes from black holes. This, combined with the experimental data collected by LIGO and the European Virgo interferometer experiment, may be able to show the existence or absence of black holes inspired by noncommutative geometry, thus helping us to better understand the key ingredients of Quantum Gravity.

“We already know that General Relativity is not able to reliably explain what happens inside the event horizon of a black hole. This suggests that we need a better theory unifying General Relativity with Quantum Mechanics, and at the same time black holes may contain the deepest secrets of the universe and its beginnings. Many things can be benefited by further study into black holes, as they provide a unique opportunity to test all of the physical extremes – very large distances, very small distances, very high energies, etc.,” Dr. Batic explained.

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