Pull-out mechanism

Fig. 6.2-5 The activated pull-out mechanism, exposing the shaft and plow mixing elements of a mixer/agglomerator for easy cleaning (courtesy Lodige, Paderborn, Germany)...

The influence of speed is obvious (contrary to the macroscopic results), with an increase in friction with speed for both grades of PDMS. This effect is similar to the influence of speed observed in macroscale adhesion, for which a higher separation speed induces an increase in the tack energy. Chain motions during nano-friction and pull-out mechanisms, which are speed-dependent, could explain this behavior. 

The pull-out mechanism of failure (figure 7.6a) is most likely to occur for block copolymers in which one block is relatively short. The fracture energy can then be estimated as the work done against the frictional force which resists the chain being pulled out. If the block is not fully entangled we might expect... 

Apart from pinch-offs detachment, fiow may also speed up the pull-out of copolymers from the interface and their subsequent dispersion as micelles in the continuous blend phase. This pull-out mechanism depends on the frictional shear force exerted on the interfacial copolymer chains, which is determined by the thermodynamical interaction of the interfacial copolymer chain with the blend chains at each phase and by the molecular weight and structure of the copolymer chains [77[. The experimental studies of Inoue et al. [77-79] corroborate the frictional theory and show that the pull-out tendency depends on the structure of the interfacial copolymer, on its molecular weight, and on the intensity of shear stress during fiow ... 

The fibrillated crazes grow through the continuous stretching of new material at the craze boundaries surface drawing or pull-out mechanism). The situation at a craze/bulk interface is illustrated in Fig. 1.7. Stretching of the fibrils occurs up to an elongation that depends on the parameters f and d of the entanglement network. The transition zone (active zone g) forms the craze interphase with a characteristic thickness g. 

The fracture surface of an as-received sample is shown in Figure 2. The fracture surface exhibits the characteristic fiber pull-out typical of quasi-ductile glass matrix composite materials [13,15]. However, inspection of the fracture surfaces reveals that the average pull-out lengths are not uniform across the composite section but depend on the relative orientation of the fibre bundles and the fracture propagation plane. Areas exhibiting fewer fibres are observed when these were oriented parallel to the fracture surface. This behaviour explains qualitatively the lower Kic values determined in this material in comparison to unidirectional fibre reinforced composites, as mentioned above, where all fibres contribute equally to toughening by the pull-out mechanism [13,15]. 

With respect to the "first order" approximation outlined in [4] the changes are marginal. The gain is nevertheless that in addition to the pulling out mechanism, the shearing over the crack edges and the anchoring effect are encompassed. 

In practice, most FRC composites are made with fibres having a more or less random orientation, and therefore, many of the fibre bridging across the crack assume an angle different from 90° with respect to the crack surfaces. The pull-out mechanisms invoked with randomly oriented fibres can be quite different from those with fibres normal to the crack. Orientation effects will be discussed in Section 3.3. 

A.E. Naaman and S.P. Shah, Pull-out mechanism in steel fibre reinforced concrete , J. Struct. Div.ASCEWZ, 1976,1537-1548. 

Takamizawa, K., Shoda, K., Matsuda, T. (2002). Pull-out mechanical measurement of tissue-substrate adhesive strength endothelial cell monolayer sheet formed on a thermoresponsive gelatin layer. Jourrud of Biomaterials Scierwe Polymer Edition, 13, 81-94. 

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