Emulating nature for better engineering

UK researchers describe a novel approach to making porous materials, solid foams, more like their counterparts in the natural world, including bone and wood in the new issue of the International Journal of Design Engineering.

According to Carmen Torres-Sanchez of the Department of Mechanical Engineering, at Heriot-Watt University, Edinburgh and Jonathan Corney of the Department of Design, Manufacture and Engineering Management, at the University of Strathclyde, Glasgow in the natural world, the graduated distribution of porosity has evolved so that nature might transfer forces and minimise stresses to avoid whole structure failure. For instance, a crack in the branch of a tree will not lead to the felling of the tree in the same way that a broken ankle will not lead to collapse of the whole leg. “Porosity gradation is an important functionality of the original structure that evolution has developed in a trial and error fashion,” the team explains.

It is not just tree trunks and bones that have evolved graduated porosity, beehives, marine sponges, seashells, teeth, feathers and countless other examples display this characteristic. Researchers would like to be able to emulate the way in which nature has evolved solutions to the perennial issues facing engineers. In so doing, they will be able to develop structures that use the least amount of material to gain the lowest density structure and so the maximum strength-to-weight ratio.

“Many engineering applications, such as thermal, acoustics, mechanical, structural and tissue engineering, require porosity tailored structures,” the team says. If materials scientists could develop porous materials that closely mimic nature’s structural marvels, then countless engineering problems including bridge building and construction in earthquake zones, improved vehicle and aircraft efficiency and even longer-lasting more biocompatible medical prosthetics might be possible.

Unfortunately, current manufacturing methods for making porous materials cannot mass-produce graduated foams. The collaborators in Scotland, however, have turned to low power-low frequency ultrasonic irradiation that can “excite” molten polymers as they begin to foam and once solidify effectively trap within their porous structure different porosity distributions throughout the solid matrix. This approach allowed the team to generate polymeric foams with porosity gradients closely resembling natural cellular structures, such as bones and wood. The technology opens up new opportunities in the design and manufacture of bio-mimetic materials that can solve challenging technological problems, the team adds.

The researchers anticipate that using more sophisticated ultrasound energy sources as well as chemical coupling agents in the molten starting material will allow them to fine tune the formation of pores in the material. This is an area of current interest because it would facilitate the design of novel texture distributions or replicate more closely nature porous materials, the team concludes.

Sanchez, C., & Corney, J. (2011). A novel manufacturing strategy for bio-inspired cellular structures International Journal of Design Engineering, 4 (1) DOI: 10.1504/IJDE.2011.041406

Between a rock and a fluid place

US researchers have found a way to monitor geological faults deep in the Earth that could help them predict an imminent earthquake more precisely than with other methods. This is the first time that scientists have been able to detect temporal changes in fault strength at seismogenic depth from the Earth’s surface.

The late Paul Silver and Taka’aki Taira of the Carnegie Institution’s Department of Terrestrial Magnetism, working with Fenglin Niu of Rice University and Robert Nadeau of the University of California, Berkeley, used highly sensitive seismometers to detect subtle changes in earthquake waves that travel through the San Andreas Fault zone near Parkfield, California, over a two-decade time span.

“Fault strength is a fundamental property of seismic zones,” explains Taira, who has moved to the Berkeley since the research was done. “Earthquakes are caused when a fault fails, either because of the build-up of stress or because of a weakening of the fault. Changes in fault strength are much harder to measure than changes in stress, especially for faults deep in the crust. Our result opens up exciting possibilities for monitoring seismic risk and understanding the causes of earthquakes.”

Seismologists have focused the San Andreas Fault near Parkfield, the “Earthquake Capital of the World,” for years. The site has a sophisticated array of borehole seismometers, the High-Resolution Seismic Network, as well as other geophysical instruments in situ. Researchers consider it a natural laboratory for seismology because of the frequent quakes that occur there.

Earlier studies have suggested that there are fluid-filled fractures within the fault zone and that these shift. When this happens, “repeating” earthquakes apparently become smaller and more frequent, which researchers say is indicative of a weakened fault. “Movement of the fluid in these fractures lubricates the fault zone and thereby weakens the fault,” says Niu.

“The total displacement of the fluids is only about 10 metres at a depth of about three kilometres, so it takes very sensitive seismometers to detect the changes, such as we have at Parkfield.” Niu further explains that it seems to be distant earthquakes that cause the fluids to shift, such as the 2004 Sumatra-Andaman Earthquake, which led to tsunamis throughout the Indian Ocean that year.

It is San Andreas fault (Adapted from Wikipedia image)
It is San Andreas fault (Adapted from Wikipedia image)

The authors speculate that such large events should produce a temporal clustering of global seismicity, a hypothesis that appears to be supported by the unusually high number of large earthquakes occurring in the three years following the 2004 earthquake. The team presents additional evidence that a similar phenomenon occurred following the 1992 Landers earthquake.

LINKS

Nature, 2009, 461, 636-640
Department of Terrestrial Magnetism at the Carnegie Institution of Washington
Northern California Earthquake Data Center (NCEDC)

Earthquake fuses could save lives and buildings

Steel “fuses” that distort when a building shakes during an earthquake to dissipate the energy could allow multi-story buildings hold themselves together during even violent earthquakes and then return to standing plumb straight afterwards. The fuses would then be replaced once the aftershocks die down.

The system has been tested in Japan, will not only help a multi-story building hold itself together during a violent earthquake, but also return it to standing up straight on its foundation afterward, true and plumb, with damage confined to a few easily replaceable parts.

Researchers at Stanford University and the University of Illinois in the US designed the system to protect buildings from irreparable structural damage even in earthquakes of magnitude 7 of higher, such as the recent event in Indonesia. The team has successfully tested the system on an enormous “shake table” in Japan.

A previously flat steel fuse deformed by the shake test (Credit: Deierlein/Stanford et al)
A previously flat steel fuse deformed by the shake test (Credit: Deierlein/Stanford et al)

“This new structural system has the potential to make buildings far more damage resistant and easier to repair,” explains Stanford’s Greg Deierlein, “so people could reoccupy buildings a lot faster after a major earthquake than they can now.”

The steel frames, or fuses, would be situated around the building’s core or along exterior walls and could be made part of the building’s initial design or incorporated into an existing building undergoing a seismic refit. The materials employed are commonly used in the construction industry and can be easily made using standard fabrication methods.

“The idea of this structural system is that we concentrate the damage in replaceable fuses,” Deierlein explains. The fuses are built to flex and distort, which dissipates the vibrational energy of the earthquake. “What is unique about these frames is that, unlike conventional systems, they actually rock off their foundation under large earthquakes,” Deierlein adds.

Schematic showing the earthquake simulator (Credit: Deierlein/Stanford et al)
Schematic showing the earthquake simulator (Credit: Deierlein/Stanford et al)

The fuses support rocking frames in steel “shoes” secured at their base and with steel tendons running down their centre. These tendons are made of high-strength steel cables twisted together, as the earth moves, they flex and then rebound to their normal length, which pulls the building back into proper vertical alignment afterwards.

Deierlein and his colleagues tested the system at the Hyogo Earthquake Engineering Research Center in Miki City, Japan. They used various configurations to find the most resilient setup.

“This is the first time we’ve put this whole system together to see how it would respond dynamically in a building as if it were subjected to an earthquake,” says Deierlein. It performs well under extreme earthquake shaking.” He adds that even simulating earthquakes above 7 left the rocking frame virtually undamaged on the test rig, which had three 100-tonne “storeys”.

Most seismically designed buildings are self-sacrificial, which means the occupants are saved, but the building must be demolished afterwards. The steel fuse system means that the building would also be saved and when the fuses blow they are simply replaced

Professor Gregory Deierlein homepage