Indentation anisotropy

From the foregoing it is clear that indentation anisotropy is a consequence of high molecular orientation within highly oriented fibrils and microfibrils coupled with a preferential local elastic recovery of these rigid structures. We wish to show next that the influence of crystal thickness on AMH is negligible. The latter quantity is independent on 1 and is only correlated to the number of tie molecules and inter-crystalline bridges of the oriented molecular network. 

Since the increase of the elastic modulus of oriented polymers with draw ratio is, to a large extent, the consequence of the tie molecules or intercrystalline bridges interconnecting crystal blocks within the fibrils 72) one may anticipate a correlation to exist between indentation anisotropy and modulus. Recent data 23) illustrating the... 

H measurements of PBT under strains of up to 20% relative deformation (e) were performed using a stretching device. The strain s is defined ass = ( — o) /f o where q and are the starting and stretched lengths of the sample, respectively. The indentation anisotropy AH = 1 — (d /dj ) was also derived (see eq. (2.6)). In order to evaluate the contribution of each polymorphic phase to the total H it is necessary to know their mass fractions at the different deformation stages as required by the additivity law (eq. (4.3)). For this purpose the data of Tashiro et al. (1980) obtained by the infrared study of the a-fi transition have been used. The same authors have shown that just in the transition deformation interval (e = 4-16%) the relationship between s and the amount of the phase is linear. 

Table 6.1. Microhardness perpendicular to the chain orientation (strain) direction H , indentation anisotropy at room temperature (25°C) AH, X-ray crystallinity Wc(WAXS), percentage of a and modifications (according to Tashiro et al, 1980) and crystal hardness of homo-PBT He stretched at various strains, s. 

Figure 7.2 schematically shows the geometry of an injection-moulded polymer bar in which z is the injection direction. Indentations were made on the yz plane. In all cases an indentation anisotropy arises because the microhardness is maximum when the indentation diagonal is parallel to the injection direction (Hj) and minimum when the diagonal is normal to it (Hy). The large value corresponds to... 

Figure 7.3. Indentation anisotropy A//, and hardness values Hi and Hy in the yz plane (Fig. 7.2) at the centre of the PE bar (x = 2 mm) measured along the y axis for various processing temperatures Tp. (After Rueda et a/., 1989.)...

Measurements at the surface of the injected bars give values of about H 60 MPa, which are close to the values obtained for linear isotropic PE with a density p = 0.96 g cm . The low orientation at the surface of the mouldings explains the low indentation anisotropy (AH = 10%) shown in Fig. 7.3. 

Fig. 7. Plots of H and H as a function of annealing temperature for cold-drawn PET. The inlet illustrates the plot of indentation anisotropy AH vs Ta for the same material.

Uniaxial mechanical deformation produces a conspicuous anisotropic shape of the residual indentation (4). The anisotropy depends on the orientation of the diagonals of indentation relative to the axial direction. Two well-defined hardness values emerge. One value (maximum for a Vickers indenter) can be derived from the indentation diagonal parallel to the fiber axis, d. The second one (minimum) is deduced from the diagonal perpendicular to it, d . The former value responds to an instant elastic recovery of the fibrous network in the draw direction. The latter value defines the plastic component of the oriented material. It is useful to define the indentation anisotropy as AH = 1 - (d /d ). 

A AH increase with increasing draw ratio k has been found for solid-state extruded and highly drawn pol5miers, and a correlation between indentation anisotropy and Young s modulus has been foimd (4,31). Indentation anisotropy values of several carbon-fiber composites have been reported (32). Recently, the indentation anisotropy of cold-drawn PET, annealed at different temperatures, has also been examined (33). 

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