Cork-based Energy Absorbing Composite Material With Microchannels

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Cork trees.

Cork items are made from the bark of the cork oaks. Copyright image courtesy of Dr. Bestione, used with permission. All rights reserved.

Researchers from University of Porto (Portugal) have developed a new composite material made of cork and Shear Thickening Fluid (STF), which could be used as a shock absorber.

Potentially, this new product could be employed in areas such as vibration absorption and human protection, using a natural material instead of the synthetic ones currently used.

Cork: a Material with Many Different Applications

Cork is a natural material best known for its use as bottle stoppers.

Beyond this, however, cork is also employed in many other different sectors, due to its properties and characteristics, such as sound and heat insulation, elasticity and low density. Because of this, cork is used in the construction, automotive and space industries.

Shock Absorbers

Shock absorbers are devices used to absorb the energy in an impact. Such devices are employed in many objects we use in our daily lives; their applications include absorbers for the automotive and aerospace industries, vibration absorbers and materials for human protection.

To improve the performance of these devices, researchers have been considering innovative solutions, developing composite materials with advanced energy absorbing properties.

Shear Thickening Fluids

The use of Shear Thickening Fluids (STFs) seems very promising for the manufacture of shock absorbers. These are fluids whose viscosity increases when the applied shear stress is above a critical value.

Using STFs, the increase in viscosity can be tailored by choosing the appropriate combination of components; in this way, different STF formulations for specific shock absorber applications can be considered.

STFs are usually used incorporated into materials with a porous and flexible structure, such as foams and textiles; this can lead to composite materials with enhanced shock absorbing characteristics.

New Shock Absorber: CorkSTFμfluidics

Researchers from the Faculty of Engineering of the University of Porto (Portugal) developed an innovative shock absorber system, called CorkSTFμfluidics, which combines the properties of STFs and cork.

They presented the results of their work at the Flowing Matter 2014 meeting, held in Lisbon from the 15th to the 17th of December 2014, and organized by the Flowing Matter COST Action.

Cork Laminas

Dr. Francisco J. Galindo-Rosales, one of the researchers involved in the research, explains to Decoded Science the principles of their work.

“Cork is already used for some applications as shock absorber; we thought it could be interesting to combine it with STFs and see how the composite material worked. It is something new, that nobody ever tried before.

For our study, we used cork agglomerated in laminas engraved with a particular pattern, that is circles linked together by channels. By doing this, we created a network of microchannels the fluid can go through.”

Engraved cork lamina with STF. Copyright image courtesy of Francisco Galindo-Rosales, used with permission. All rights reserved.

Engraved cork lamina with STF. Copyright image courtesy of Francisco Galindo-Rosales, used with permission. All rights reserved.

The figure here shows the pattern used; the circles and the small channels are engraved; in this way, they form a hollow space which can be filled with a fluid (the grey part in the figure).

Assembling the Device

Dr. Laura Campo-Deaño, another researcher who participated to the project, explains how they assembled the device using the cork laminas.

“We used a 2 mm thick single cork lamina and filled it with a known amount of STF; this was then sealed with a 1 mm thick self-adhesive cork lamina.

As STF, we used a water solution of corn starch (45 w/w); this is one of the most common and easily available STFs. We made these kind of devices using cork laminas with the same engraved pattern, but with three different dimensions – small, medium and large. This corresponded to a circle radius of 1, 2 and 5 mm, respectively.”

Testing the Performance

The researchers then tested the mechanical behavior of the devices; they assessed the shear stress and performed a falling weight impact test of the composites.

“What we saw was very interesting” Dr. Galindo-Rosales said, “indeed, the composites showed better performance in comparison to simple cork, with no fluid added. In particular, the system which worked best was that with the medium size dimensions (2 mm radius, connected with 200 mm channels). For instance, we saw an improvement of about 5 % in the response of this composite to the impact.

These encouraging preliminary results are due to the combination cork-fluid, but also to the microchannels. In fact, in this way, the fluid flows in a well-defined geometry, and can better absorb and dissipate the energy coming from a shock.”

Important Results

According to Dr. Campo-Deaño:

“These results are very important, as we showed as a proof of concept, at laboratory scale, that CorkSTFμfluidics can work well and has potential. In principle, these systems can be used in all the similar applications that already exist for cork, but in a more effective way.

We have already made a patent application for CorkSTFμfluidics; our idea is now to optimize the system and improve its efficiency. Ss a longer term plan, we also want to expand it to a larger scale. Hopefully, one day CorkSTFμfluidics could replace some composites which are made of synthetic materials (i.e. expanded polystyrene); this would surely be good for the environment.”

Cork, Fluids and Impact on the Environment

The work of University of Porto researchers showed that it is possible to make composites with improved shock-absorber properties using cork, a natural eco-friendly material. The principles of CorkSTFμfluidics could be potentially applied to other natural materials, widening even more the applications.

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