Frictional bond

Castagnetti, D., Dragoni, E. (2013). Experimental Assessment of a Miero-Mechanical Model for the Static Strength of Hybrid Friction-Bonded Interfaces. The Journal of Adhesion, 89,642-659. 

Hydrogen and friction bonded cellulose wherein carded, air laid or wet laid fiber web is hydroentangled. By controlling the process conditions, it should be possible to develop a structure that is dispersible in water imder the suggested conditions. 

Anchors without end plates (Fig. 11) consist of a metallic profile that is embedded in the masonry and can transmit loads to the substratum by mechanical locking, friction, bond, or a combination of these three (EUgehausen et al. 2006), rather than use of an element such as a plate, a key, or a peg. Mechanical locking can be obtained, for instance, by undercut, namely, shaping the end part of the hole and introducing an anchor with an end shape so as to result larger than the rest of the shaft. Friction systems, instead, consist of torsion or displacement-... 

Discussion of the results reveals that a tensile and bending strengthening and toughening is possible, and that the NPL-theory for continuous fibres with frictional bond is valid, if one takes into account the different behaviour of the fibres before and after matrix cracking. 

Three stages of behaviour can be distinguished, corresponding to the NPL-theory for continuous aligned fibres with frictional bond between the fibres and matrix [ 2] ... 

On the basis of above mentioned properties, the melt extract fibres are prefered because of their high surface area/volume ratio, irregular contour and rough surface which improve the adhesive and frictional bonds, and the optimal content is about 1.5 percent by volume (1/d Vf=.75). But considering the cost, the cut wires made from waste steel wire rope are also acceptable, and the economic content of fibres may be 1 to 1.5 percent by volume (1/d Vf=.75—1.00). 

The coupling between shrinkage (i.e. misfit value in Eq. 3,19) and the modulus of elasticity, as predicted from Eq. 3.20, has been the subject of several experimental studies to determine the frictional bond, reported by Pinchin and Tabor [21,22], Beaumont and Aleszka [23] and Stang [24],... 

Table 3.1 Predicted clamping stresses and frictional bond In fibre-cement interface with two different matrices Portland cement (PC) and Portland cement with 10% microsilica (MS) (after Stang [24])...

Fibre Modulus (GPa) fJ- Clamping stress (MPa) Frictional bond (MPa) ... 

The load N can be calculated assuming fibre-matrix frictional bond r, and taking a pull-out length of /yv, which is only the right term in Eq. 3.36, O,5/acos0 (the other term in Eq. 3.36, is reiated to the part of the fibre which separates from the matrix on bending, Eigure 3.27) ... 

In the case of conventional fibres, with diameter of the order of 0.1 mm or bigger (called sometimes macrofibres), the adhesional and frictional bond with the cementitious matrix is not sufficient for developing adequate reinforcing efficiency. This is true for steel as well as polymeric fibres. To overcome this limitation, it is common to induce deformations in the fibre to provide it with a complex shape which will provide anchoring effects. The bonding achieved by the anchoring mechanism has been shown to be much greater than the one achieved by interfacial effects. 

Figure 3.35 Modelling of the pull-out of hooked end steel fibre when being extracted from the matrix (a) hooked steel fibre at the onset of complete debonding, (b) hooked steel fibre during mechanical interlock with two plastic hinges, (c) mechanical interlock with one plastic hinge, (d) hooked steel fibre at onset of frictional bond (after Alwan etal. [68]) PH is plastic hinge.

In view of the presence of the special ITZ microstructure several models were developed, assuming a three phase material bulk, ITZ and fibre. In these models specific characteristics (i.e. modulus of elasticity) were assigned to the ITZ. Li et a/ [75] calculated a parameter related to the stiffness of the ITZ to resolve its value for different systems. Mobasher and Li [76] extended a fracture mechanics model to describe the pull-out curves in terms of adhesional and frictional bond, stiffness of the interface and interface toughness. They demonstrated that all of these parameters influence the pull-out behaviour, and their change with age could account for the variation in the pull-out behaviour, as discussed further in Chapter 5. 

The combined effect of orientation (aligned and random arrays) and length on the overall efficiency, as calculated by Laws [4] is shown in Figure 4.6. She demonstrated that the efficiency is dependent on the frictional bond before slip (static) and the frictional bond after fibre slip (dynamic). The higher efficiency values for the systems where dynamic slip is present (dashed line in Figure 4.5) are the result of the contribution of this type of frictional resistance to the strength of the short fibre composite. 

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