Materials and methods
3.8 Tensile testing of microtensile test specimen
The microtensile test specimens were cut from the sheets obtained from the compression moulding machine and tested according to ASTM D 1708-10 standard. The sample dimension, composite and T blend test specimens are shown in Figure 3.5a, 3.5b and 3.5c, respectively. The test was performed in a digitally controlled closed loop servo hydraulic universal testing machine, INSTRON 8801, having the dynamic load capacity of ± 100 kN with load cell accuracy of 0.005 % and it is shown in Figure 3.5d. The test was conducted at a cross head speed of 1 mm/min. in the temperature range of 20 – 24 0C. In order to ensure the repeatability of the results, five samples were tested per material type and the average of the results is reported.
(a)
(b)
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All dimensions are in mm
Figure 3.5. a) ASTM D1708 microtensile test specimen b) Microtensile test specimens of composites obtained from the consolidated sheets c) Microtensile test specimens of T blends obtained from the consolidated sheets d) INSTRON-8801 universal testing machine with closed
loop servo hydraulic system 3.9 Nanoindenter
The Nanoindentation experiments were carried out using a CETR, UMT-2 Nanoindenter.
The diamond Berkovich tip, which is a 3-sided pyramid with a total included angle of 136o, was used in this study. The indenter setup is inbuilt with a capacitive transducer having the load resolution of ~ 150 nN, and the depth resolution of ~ 1 Å. As the tip defect may greatly affect the evaluation of the mechanical properties of the test surface, the accuracy of the instrument was tested by performing the tip shape and frame stiffness calibration using the fused Silica sample, which has the constant elastic modulus at different depth, Oliver et al. [1992].
The load was applied at a rate of 0.5 mN/s to the maximum load of 10 mN, where it was held for 10 s (creep time) to avoid the ‘nose problem’ and then unloaded to 10 % of the maximum load at the same rate as that of loading and maintained for 10 s (thermal drift time) to compensate the error, which arises due to thermal drift within the indenting system and then finally it was unloaded. Allowing creep at the maximum load aids in mechanical stabilization of the indent and hence improves accuracy of the contact area and depth used for the calculation of hardness and
(c) (d)
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A2 A1
Loading Unloading
B
CI C
Displacement
Load
hmax hc
hr
Perfectly plastic material
Viscoelastic plastic material
O
S
contact stiffness
Young’s modulus, Briscoe et al. [1998]. The load as a function of the indenter displacement is obtained during loading and unloading conditions, which are recorded for data analysis. Figure 3.6 shows the Nanoindenter setup used in the present study.
Figure 3.6. Nanoindenter
Figure 3.7. Schematic representation of a typical load- displacement curve obtained from the Nanoindentation test (hr – residual depth, hc – contact depth, hmax – maximum depth of
penetration, A1- plastic work done, A2- elastic work recovered)
Figure 3.7 shows the typical load vs. displacement curve, which includes both loading and unloading data for a general viscoelastic–plastic material. The curve OBCO represents the loading, unloading and thermal drift segments to elucidate the calculations involved in estimating the plasticity index of the material. The data obtained by the unloading curve provides information about elastic, visco-elastic and plastic behavior of the material. The load corresponding to the point B in the plot is the maximum load, Pmax, and the corresponding displacement is obtained at C1, maximum displacement, onthe X-axis as hmax. The residual displacement after removing the indenter tip is denoted as hr. The intercept of the tangent line drawn from the first part of the unloading curve describes the elastic deformation effects, Oliver et al. [1992], and the intercept in the displacement axis is represented as hc. The difference between hc − hrrepresents the viscoelastic recovery, which follows the immediate elastic depth recovery. The slope of the line evaluated at the maximum displacement represents the contact stiffness, S, Oliver et al. [2004]. In case of a perfectly plastic material, there is no displacement of the material during the unloading condition and thus the curve assumes a vertical line and intersecting at C1 in the displacement axis. In case of a visco-elastic material, the curve traces along BC during the unloading segment and the material rebounces to hr at C. The plastic work done in the visco-elastic material is represented by area A1 (OBC), and the elastic work recovered during the unloading condition corresponds to area A2 (CBC1).
The relationship between stiffness and elastic modulus at the maximum penetration depth for any indenter geometry is defined as follows, Pharr et al. [1992]:
… 3.1
where Amax is the projection of contact area between the material and indenter tip at hmax, is the parameter related to indenter geometry (1< < 1.034) and E* is the reduced elastic modulus of the contact. The hardness and plasticity index of the test sample were calculated using the equation 3.2 and 3.3, respectively, Briscoe et al. [1998].
… 3.2
… 3.3 where Pmax is the load at the maximum displacement and A is the projected area supporting the load. A1 and A2 have been described earlier with reference to Figure 3.7. The tests were conducted on the AS Processed samples (ASP W hereafter, where W stands for wt. % of the MWCNTs) and the surface layer removed samples (SLR W hereafter), where the layer was removed upto 100 µm.