Pullout energy

A brief review of microfracture processes and the energy absorption mechanics of fiber reinforced composites is given by Miyajima et al [239]. Fiber pullout is considered to be the most important toughening mechanism. They describe an experimental technique to determine fiber pullout energy, using a 3-point bend specimen. From measurements of fundamental fracture parameters, fracture mechanisms for the fiber pullout processes of carbon fiber reinforced carbon composites are discussed. 

Geometry optimization for each of the composite systems (i.e., both the CNT and the polymer matrix) was performed with a SWNT fully embedded in the matrix, and its potential energy was obtained. The SWNT was then pulled out from the polymer matrix in a stepwise manner, as shown in Fig. 13.9. The pullout energy, W, work required for CNT pullout, is related to the interfacial shear stress, 77, between the CNT and the polymer matrix by ... 

Fig. 10. Cantilever beams used to measure the fracture energy of nail pullout from wood. Top plan of beam showing nail heads. Bottom method of loading beams with a load P after [58].

This solution assumes that the beams behave elastically and that all the energy dissipation is associated with the pullout process. Typically for rigid incompatible interfaces, this fracture energy is quite low, ca. 1-5 J/m [1,20,21,61,59]. 

Crack bridging is also complemented by the contribution of pullout of the failed whisker reinforcements. The pullout operation consumes energy which would otherwise contribute to the advancement of the crack front and thus enhances toughness. 

The reduced hardness and improved machinability are attributed primarily to the crack deflection process. It can be seen in Fig. 13.8 that the composite showed obvious particle pullout and significant crack deflection along interphase boundaries due to the weak interface bonding. The crack deflection mechanism (absorbing fracture energy and blunting crack tip) could lead to an increase in machinability. As described above, the thermal expansion... 

Figure 33. Scanning electron micrographs of the fracture surface of a 3D carbon/carbon composite (59) Fiber pullout causes increased energy consumption in fracture.

Numerically, this estimate gives fmmw = 6.3x 10 12 N/monomer. This result should be compared with the estimate obtained with Eq. (8) which is four times higher. The discrepancy between the two results is due to the viscoelastic dissipation that was not taken into account in the energy calculation. Clearly, excess dissipation takes place near the interface even in the straight pullout regime. 

Pullout test is also a method, providing useful information about fabric tearing, its ability to absorb energy especially in ballistic applications, lllnishing efCbiency, bending, and shearing hysteresis of the fabric and Qially the frictional behavior of the fabric. 

Badrossamay et al. [1] introduced an oscillation model which was capable of anticipating yam pullout force displacement proQe. And Qially Kirkwood et al. [4] and their coworkers studied yam pullout process as an energy absorbent and characterized the yam pullout force and energy as a function of pullout distance through a semi empirical model. 

Kirkwood, K. M., et al. Yarn Pullout As a Mechanism for Dissipating Ballistic Impact energy in Kevlar KM-2 Fabric, Part 1 Quasi Static Characterization of Yam Pullout. Tex. Res. J., 74, 920-928 (2004). 

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