Back when we were looking at static loading situations, we spent some time talking about stress concentration factors. When you have a part whose shape contains a discontinuity of some kind (a hole in a plate, or a step in a shaft, or some other abrupt change in geometry), the effects of stress are magnified in the immediate region of the discontinuity.
This kind of behavior is still a factor for us in dealing with fatigue loading, but we’ll have to adjust for fatigue instead of static loading. The basic equation we’ll use is this one:
Where K_t is the static stress concentration factor, K_f is the fatigue stress concentration factor, and q is the notch sensitivity. Notch sensitivity is a measure of how sensitive a material is to stress concentrations. It roughly correlates to ductility - more ductile materials are less notch sensitive than brittle materials - but also depends on notch radius. A small notch results in less notch sensitivity. Like the static stress concentration factor, notch sensitivity isn’t something you’d have to calculate out for yourself normally (although there are ways to do it). Instead, you’d usually get it from a chart like this one for steels:
(Image from Machine Design: An Integrated Approach, 4th Ed., by Robert L. Norton.)
So, knowing the static stress concentration factor for a part and its notch sensitivity, you can get a stress concentration factor for fatigue loading. You’d use it just like you use the static stress concentration factor: you multiply your nominal stress by that factor and figure that that’s the maximum stress your part will have to withstand.
A theory confirmed by experiments explains what has been an unpredictable cutting process.
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Slicing through a solid can turn out a continuous band of liberated material or a disorderly sequence of fractured shards. Now researchers have shown that there is a universal transition between these two different ways materials come apart when cut [1]. Others have noted that the depth of the cutting tool determines the material’s response, but the new theory explains why the transition occurs and predicts the depth at which the transition takes place for any relevant material. The theory is supported by new experiments with several different plastics as well as previous data from experiments on other materials. This new understanding may allow manufacturers to design tools that achieve a better surface finish.
Glass windows often endure external environmental factors such as wind, rain, and humidity, which lead to the formation of microcracks in their surface. For instance, a gust of wind can propel sand onto a window and create microscale surface cracks due to the impact of sharp-edged sand particles. These microcracks then grow in size when aggravated by water droplets and humidity.
When water comes in contact with these surface flaws, it penetrates the microcracks and slowly dissolves the silicon-oxygen bonds. This chemical attack breaks the glass networks which gradually degrades the mechanical strength and optical properties of glass structures. This environmentally enhanced crack growth process is known as stress corrosion or subcritical crack growth (SCG). The depreciation of mechanical performance due to SCG raises safety concerns for skyscrapers and high-rise buildings.
Recently, scientists have taken an interest in the effect of SCG on two widely used types of soda-lime glass: annealed and tempered glass. However, most studies have focused on establishing crack growth rates, and not much is known about the effect of water on the propagation of crack flaws. Additionally, many of the studies investigated the microcrack propagation dynamics of annealed glass and not tempered glass.
A small team of researchers with the Technical University of Denmark and Drexel University in the U.S. has found that it is possible to cause a liquid to crack under the right conditions. In their paper published in the journal Physical Review Letters, the team describes their experiments, what they learned and outline ways in which their findings may be suitable for use in industrial applications in the future.
A lot of study has been done regarding cracksinsolid materials to learn what causes them, how to prevent them, etc. Doing so has helped to create materials that better suit our purposes and which are, in some cases, safer for us to use, e.g. for airplane components. But up till now, little work has been done to study cracks in liquids, in part because it would seem logical to assume that they don’t exist. But as the researchers with this new effort found, they not only do exist, but take on many of the same properties as cracks that occur and propagate through solid materials.
If you’ve never had the plumber to your house, you’ve been lucky. Pipes can burst due to a catastrophic event, like subzero temperatures, or time and use can take a toll, wearing away at the materials with small dings and dents that aren’t evident until it’s too late.
But what if there were a way to identify those small, often microscopic failures before you had to call for help?
The Autonomous Materials Systems (AMS) Group at the Beckman Institute for Advanced Science and Technology has recently found a new way to identify microscopic damage in polymers and composite materials before total failure occurs.
“Autonomous indication of small cracks has exciting potential to make structures safer and more reliable by giving time to intervene and repair or replace the damaged region prior to catastrophic failure,” said Nancy Sottos, professor of materials science and engineering, and one of the authors of “A Robust Damage-Reporting Strategy for Polymeric Materials Enabled by Aggression-Induced Emission,” recently published in ACS Central Science. The paper is part of a research project selected as a finalist for the Institution of Chemical Engineers (IChemE) Global Awards 2016.
It’s nearly impossible to break a dry spaghetti noodle into only two pieces. A new MIT study shows how and why it can be done.
If you happen to have a box of spaghetti in your pantry, try this experiment: Pull out a single spaghetti stick and hold it at both ends. Now bend it until it breaks. How many fragments did you make? If the answer is three or more, pull out another stick and try again. Can you break the noodle in two? If not, you’re in very good company.
The spaghetti challenge has flummoxed even the likes of famed physicist Richard Feynman ’39, who once spent a good portion of an evening breaking pasta and looking for a theoretical explanation for why the sticks refused to snap in two.
Feynman’s kitchen experiment remained unresolved until 2005, when physicists from France pieced together a theory to describe the forces at work when spaghetti — and any long, thin rod — is bent. They found that when a stick is bent evenly from both ends, it will break near the center, where it is most curved. This initial break triggers a “snap-back” effect and a bending wave, or vibration, that further fractures the stick. Their theory, which won the 2006 Ig Nobel Prize, seemed to solve Feynman’s puzzle. But a question remained: Could spaghetti ever be coerced to break in two?
Intergranular fractures (shown through SEM images in the four pictures above) are when materials fail along the grain boundaries, often due to imperfections or inclusions at the grain boundaries or hostile environmental or usage conditions.
