Debonding and pull-out

In the fiber pull-out test, a fiber(s) is partially embedded in a matrix block or thin disc of various shapes and sizes as shown in Fig 3.6. When the fiber is loaded under tension while the matrix block is gripped, the external force applied to the fiber is recorded as a function of time or fiber end displacement during the whole debond and pull-out process. There are characteristic fiber stresses that can be obtained from the typical force (or fiber stress). The displacement curve of the fiber pull-out... 

Gray, R..1. (1984). Analysis of the effect of embedded fiber length on the fiber debonding and pull-out from an elastic matrix. J. Mater. Sci. 19, 861-870. 

Kim, J.K., Zhou, L.M. and Mai, Y.W, (1993a), Interfacial debonding and pull-out stresses part 111, Interfacial properties of cement matrix composites. J. Mater. Sci. 28, 3923-3930. 

Weibull. W. (1951). A statistical distribution function of wide applicability. J. Appl. Mech. 18, 293-297. Wells, J.K. and Beaumont, P.W.R. (1985). Debonding and pull-out processes in fibrous composites. J. Mater. Sci. 20, 1275-1284. 

Mai Y.W. and Castino F. (1985). The debonding and pull-out properties of coated Kevlar fibers from an epoxy resin matrix. J. Mater. Sci. Lett. 4, 505-508. 

Other toughening mechanisms microcracking, inclusion debonding and pull-out... 

Other toughening mechanisms are related to the debonding and pull-out of the inclusions during fracture. These mechanisms are especially effective in whisker and short-fibre reinforced glass composite materials [35]. 

Bowling, J. and Groves, G.W. (1979) The debonding and pull-out of ductile wires from abrittle matrix. Journal of... 

Hartness [100], working with XAS and HMS fibers in a PEEK matrix showed similar behavior. The similarities between PEEK and PP are probably greater than the differences in their crystalline structure. Beaumont [101] has shown that with HMS (treated) fiber, there is almost no pull-out, whereas with HMU (untreated) fiber, extensive debonding and pull-out take place. The pull-out lengths can be measured and using an analytical technique outlined by Phillips [102], values can be obtained for the nylon/fiber interfacial bond strength and fracture energies. 

CY Yue, WL Cheung. Interfacial properties of fibrous composites. Part 1. Model for the debonding and pull-out process. J Mater Sci 27 3173-3180, 1992. 

Figure 3.35. Schematic representation of microsphere cross sections in matrix under plane strain for circled areas in Figure 34(b) (a) partially debonded and broken with tom marks, solid line circles (b) fully debonded and pulled out from matrix, dashed line circles and (c) fuUy bonded but broken without torn marks, dash-dotted line circles [37]...

Figure 3.498. Schematic of possible failure mechanisms near T- oriented fibers (1) tendency to form zig-zag crack profiles and crack bifurcations (2) short-circuiting of cracks between cracks at fiber ends or broken fiber sites (3) formation of crazes and cracks at fiber ends and along their well bonded interfaces at the onset of fiber pull-out (4) as (2) in more detail (5) fiber debonding and pull-out in case of a poorly bonded system [1267],...

Processes of debonding and pull-out of steel fibres from cement matrix ... 

The strain necessary for initiation of multiple cracking and crack propagation is lower for a cyclic load than for a static and constant load. The process of fatigue in fibre-reinforced cement composites is similar to the static increase of load in the sense that debonding and pull-out also occur gradually. The fibres used as reinforcement for cement matrices usually do not show fatigue at all because the stress in fibres is maintained at quite a low level as related to their strength, in steel or carbon fibres, for example. 

The subject of debonding and pull-out has been studied extensively and a variety of models have been presented [5-14]. Many of them are based on common concepts which were reviewed in references [1,5,6]. A significant portion of this section will be based on these three papers. 

The special interfacial microstructures of the matrix in the vicinity of the fibre and the complex geometry of some fibres and their surface properties can cause the mode of debonding and pull-out to be quite different from that predicted by the models based on the simple pull-out geometry presented in Figure 3.1, which is characterized by symmetrical pull-out and debonding at the actual fibre-matrix interface. 

Y. Geng and C.K.Y. Leung, A microstructural study of fiber/mortar interfaces during fibre debonding and pull-out , J. Mater. Sci. 31,1996,1285-1294. 

For L < the matrix bond will fail in shear when t = of the matrix, and the fiber will puU onL This will also occur when there is less than perfect bonding, and the fiber debonds and pulls out. Contrary to what we might initially presume, less-than-perfect bonding is desired for better impact strength. Use of short fibers, designed to slip and pull out, gives a better polymer for energy absorption in impact situations. See Chapter 5 for impact properties. 

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