New Ways of Analyzing Mechanical Stress

A big component of designing and building a structure is understanding the stress the components will be under. This can be somewhat hard to measure and harder to monitor. Some materials don’t show the damage that is occurring until it’s too late. To help create safer and better materials, researchers are doing what they can to learn more about stress and develop materials that respond to stress in more helpful ways.

Glass Ceramic to Monitor Stress Using Light 

Researchers have developed a new material that emits light when under stress. Called mechanoluminescence, this ability to emit light can be extremely useful when monitoring structures. This material could be used to visually see the stress in buildings and give early warning about dangerous stress or fractures.

“We designed a glass-ceramic material with mechanoluminescence, which allows glass-like processing approaches to be used to form virtually any shape — including fiber, beads or microspheres — that can be incorporated into various components and devices,” research team leader Lothar Wondraczek from Friedrich Schiller University Jena in Germany told Science Daily.

The new glass ceramic consists of “chromium-doped zinc gallate (ZGO) crystals embedded in a potassium germanate glass matrix”. The crystals are the component that emit the light. They are small enough that they won’t impact the transparency of glass.

Typically mechanoluminescence materials are powders which aren’t very versatile. The powders also need extra steps for production, like encapsulation in a matrix material.

This is why researchers turned to glass ceramics. “Glass-ceramics are a relatively new type of material that consists of a crystalline material embedded into a glass matrix.”

Published in a special issue of Optical Materials Express, the researchers tested their newly developed material using a ball-drop test. Using this test, the researchers concluded that the “mechanoluminescence response was reproducible and rechargeable and that it exhibited a direct correlation with the impact energy”.

The next step is to develop the material into different compositions such as sheet-like objects, optical fiber and microscale spherical beads.

Seeing Stress Using Super Computers

Researchers at the University of Texas at Austin have determined that material stress isn’t symmetrical. That stress behaves the same even if the stressed object is turned or flipped has been an assumption made while running calculations.

Using supercomputer simulations, the researchers have determined that at the atomic level this assumption doesn’t appear to be true. Classical continuum mechanics has always assumed that a “material is infinitely divisible such that the moment of momentum vanishes for the material point as its volume approaches zero.” 

Using the definition of stress as the force per unit area acting on three rectangular planes, researchers have “simulated the force interactions of Lennard-Jones perfect single crystal of 240,000 atoms”.

The authors made two main conclusions based on these simulations. One, “the commonly accepted symmetric property of a stress tensor in classical continuum mechanics is based on certain assumptions, and they will not be valid when a material is resolved at an atomistic resolution.” Two, “the widely used atomic Virial stress or Hardy stress formulae significantly underestimate the stress near a stress concentrator such as a dislocation core, a crack tip, or an interface, in a material under deformation.” 

Using this new way to look at stress, researchers could develop new materials like metal or glass that doesn’t ice up.