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Interesting lectures ...

19-01-2005

of Prof. Daniel Wagner from Weizmann Institute of Science (Israel) will be held Thu, 27th Jan 2005, 10.00-12.00, in room 305 IM.

MECHANICS AT A SMALLER SCALE: FROM CARBON NANOTUBES TO BONE

and

HOW DO FIBROUS MATERIALS FAIL?

see summaries below:

MECHANICS AT A SMALLER SCALE: FROM CARBON NANOTUBES TO BONE

Some of our recent experimental and theoretical results regarding materials mechanics at the nanoscale will be briefly reviewed. The main theme includes carbon nanotubes and nanotube-based composite materials. Carbon nanotubes hold great promises as a possible reinforcing phase in composite materials of a new kind. Such developments still present, however, enormous practical challenges, in particular when attempting to probe the properties of individual nanotubes, for which most studies consist of computer simulations. We report promising results regarding polymer-nanotube wetting and interfacial adhesion. The potential use of nanotubes as sensors in composite matrices is briefly outlined. Comments about nanoscale biological composites such as hydroxyapatite-collagen composites (bone) and dentin will also be given, and the possible importance of the nanoscale in the properties of both inert and biological composites will be examined.

HOW DO FIBROUS MATERIALS FAIL?

Fibrous arrays and composites are among the strongest structures created by man or found in nature. Examples of strong synthetic structures range from very large cables such as those found in suspended bridges or radio telescope facilities, to high-performance composites used in aerospace components. Bamboo, bone and nacre are representative tough counterparts found in nature. A major function of the unidirectional brand among such structures is to resist axial forces. Although fracture of an isolated fiber is often of a brittle, catastrophic nature, the fibrous array in composite materials most often prevents the occurrence of rapid structural collapse: fibrous composites preferentially fail in a slow, cumulative fashion. Two decades ago, theoretical schemes have led to mathematical correlations between the stochastic strength distributions of single fibers and of unidirectional composites based on the same fibers. These distributions were found to be approximately of the Weibull type, and the size (N*) of a critical cluster of adjacent broken fibers, which inevitably leads to final failure, was calculated to depend only on the ratio of average strengths of both distributions. We used synchrotron X-ray tomography at high resolution (2 mm) to monitor the nucleation and growth of damage and fracture in three-dimensional unidirectional composites under in-situ axial tension. The experimental value of N* is found to be 3 to 5 times larger than the value predicted by stochastic theory. We propose a straightforward fracture mechanics argument that rationalizes the experimental data and credibly relates the critical fiber cluster size to the material strength.

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