Deformation micro

Concerning the above-mentioned critical quantities the authors have in fact established (i) that irrespective of stress level damage is apparently initiated at a critical creep strain ec of 3 to 3.5% (ii) that a notable deviation of creep data from the potential law starts just at this strain level and (iii) that although the strain rate dev/df is a function of stress, the minimum in the Sherby-Dorn plot also occurs (for the tubular specimens) at ec. The postulated changes in sample morphology at about the time when the strain values started to deviate from Findley s equation, were in fact seen by these and other authors [42,52], who detected in U PVC deformed micro-cavities later... 

The found coincidence of deformation micro-jumps in creep with the sizes of spherulites may be associated with the fact that creep occurs initially preferably through micro-shears along the boundaries of spherulites. Of course, at larger strains the transformation of the sPucture can result in the manifestation of new structural units and another deformation jumps on the meso-scale level. 

Melting the end of the fibre shapes the exit surface of optical fibres. An almost spherical surface will be created by the surface tension of liquid quartz. Its form is determined by the volume of melted quartz, which is equivalent to the amount of absorbed energy. Several techniques have been used to achieve this deformation micro furnace, Bunsen micro-burner, electrical arc and a CO2 laser beam. 

The solid friction is calculated by an energetic approach which is described in [1] and in more depth by Bartel [10]. The components of the solid friction are deformation and adhesion. The first one can be calculated from the elastic-, plastic deformations of the macro and micro geometry. To calculate the adhesion force, some assumptions have to be taken into account at the moment Adhesive bonding can only occur if plastic deformation of the material takes place and secondly if local temperatures exceed a critical value which lead to a desorption of the fluid film. Since the simulation program is not able to determine local temperatures yet, the second assumption cannot be tested. But it seems reasonable that the plastically deformed micro asperity also experience local temperature peaks. Since it is quite sophisticated to get reliable values for the shear strength in adhesive bonding, in this work one sixth of the universal hardness or plastic pressure limit of 100Cr6 (SAE 52100), respectively, is used ... 

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

Pashley, M.D. and Tabor, D., Adhesion and deformation properties of clean and characterized metal micro-contacts. Vacuum, 31(10-1), 619-623 (1981). 

It is necessary to consider the micro-mechanical processes of polymer glasses and elastomers separately as their mechanical properties are so different. In addition, cross-linking profoundly affects the deformation processes in glasses but very little is known about the micro-mechanical processe.s that occur in single phase cross-linked glasses so the latter materials will not be discussed further. 

Micro-mechanical processes that control the adhesion and fracture of elastomeric polymers occur at two different size scales. On the size scale of the chain the failure is by breakage of Van der Waals attraction, chain pull-out or by chain scission. The viscoelastic deformation in which most of the energy is dissipated occurs at a larger size scale but is controlled by the processes that occur on the scale of a chain. The situation is, in principle, very similar to that of glassy polymers except that crack growth rate and temperature dependence of the micromechanical processes are very important. 

The mode of action of plasticizers can be explained using the Gel theory [35 ]. According to this theory, the deformation resistance of amorphous polymers can be ascribed to the cross-links between active centres which are continuously formed and destroyed. The cross-links are constituted by micro-aggregates or crystallites of small size. When a plasticizer is added, its molecules also participate in the breaking down and re-forming of these cross-links. As a consequence, a proportion of the active centres of the polymer are solvated and do not become available for polymer-to-polymer links, the polymer structure being correspondingly loosened. 

Figure 2 Representation of TLCP deformation process in die exit zone (micro scale). Source. Ref. 33.

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In TPs, craze zones can develop that are important microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction... 

Glad [37] studied the micro deformations of thin films prepared from DGE-BA/MDA by electron microscopy. His results are also shown in Fig. 7.5. The deformation of the sample with high strand density was small and consequently its image in the EM rather blurred. Therefore, the result on Mc = 0.5 kg/mol should perhaps have been omitted. 

In the design of MOEMS components, various parameters have to be tuned. These parameters differ according to the functionality of the component. We will consider two different family of devices, programmable slits for Multi-Object Spectroscopy, including Micro-Mirror Arrays (MMA) and Micro-Shutters Arrays (MSA), and Micro-Deformable Mirrors (MDM) for Adaptive Optics systems. 

By integration of the loeal slopes, we have reconstructed the micro-mirror surface. An example is shown in Fig.4, along the line indicated by an arrow on the slope map. The surface deformations do not exceed 1 nm along the studied profile. Although surface shapes vary from mirror to mirror, deformations in the nanometer range demonstrate the remarkable quality of this device. 

Since September 2000, we have engaged an active collaboration with a French laboratory expert in micro-technologies, the Laboratoire d Analyse et d Architecture des Systfemes (LAAS) in Toulouse, France, for the conception and the realization of micro-deformable mirror (MDM) prototypes. Our design is based on three elementary buildings blocks (Fig. 6). From top to bottom ... 

Figure 6. Schematic view of our Micro-Deformable Mirror architecture.

Micro-Opto-Electro-Mechanical Systems (MOEMS) will be widely integrated in new astronomical instruments for future Extremely Large Telescopes, as well as for existing lOm-class telescopes. The two major applications are programmable slit masks for Multi-Object Spectroscopy (see Ch. 12) and deformable mirrors for Adaptive Optics systems. Eirst prototypes have shown their capabilities. However, big efforts have stiU to be done in order to reach the requirements and to realize reliable devices. 

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