PSI - Issue 15

Raasti Naseem et al. / Procedia Structural Integrity 15 (2019) 51–54 Naseem et al. / Structural Integrity Procedia 00 (2019) 000–000

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the material surface. However, the use of pyramidal indentation on biomaterials of a complex nature can be problematic and leads to a wide variation in the data obtained due to the polymer chains and their malleability. There are a number of contributing factors which can explain the variation of properties across the sample, including indentation size effect (ISE), local variation of material properties and a bi-modal response of the material (Iqbal et al., 2013). Alisafaei et al. (2014) investigated the indentation size effect using two different geometrical tips in epoxy, with indentation depth ranging from 30 to 3000 nm. It was seen that with the three-sided Berkovich tip, elastic modulus increased with decreasing indentation depth as opposed to no effect seen with indentation using a spherical tip. Magnitude of higher order displacement gradient is the rationale behind this, and there is an increase in magnitude with a decreasing penetration depth for sharper tips (i.e. Berkovich). This, however, remains constant when a spherical tip is used. With spherical tips, strain gradient depends solely on the radius of the indenter tip and not on the indentation depth (Alisafaei et al., 2014). The application of blunter tips for indentation (i.e., spherical) allows for an extended elastic-plastic deformation. Additionally, load-displacement data is capable of being converted into indentation stress strain curves, which can allow for the determination of material yield points (Fischer-Cripps, 2011). Behaviour of the material during spherical indentation is dependent on the ratio between actual strain and yield strain. Low ratios are equivalent with elastic behaviour as opposed to high ratios representing plastic behaviour. Strain increases with progressive indentation depth, and therefore, the material behaviour, from purely elastic to plastic, can be seen with gradually increasing loads through the converted stress-strain curves (Paar, 2006). In this paper, spherical indentation has been carried out to assess the property of polymer tubing from which stents are laser cut, in comparison with those obtained from pyramidal indentation. 2. Methodology A Poly-3-hydroxybutyrate (P3HB) tube, from which stents are laser cut, was sectioned into samples for nanomechanical testing. Poly-3-hydroxybutyrate (P3HB) is a highly crystalline and linear polyester. The polymer is naturally biodegradable but synthesized by microorganisms. Two indenter tips were used for experimentation, 3 sided Berkovich and a 10 µm-radius spherical tip (Micromaterials, Ltd). A loading rate of 0.5 mN/s and an unloading rate of 5 mN/s were chosen for both testing methods, with a 40-second dwell time at the level of maximum loading. To allow for correction of indentation data with regards to the thermal drift, a hold phase of 60 s was implemented at 80% of unloading. The elastic modulus of the materials was determined using the Oliver-Pharr theory, based on the slope of the unloading curve (top portion; 20%). Offset between indents was maintained at 40 µm. 3. Results As shown in Figure 1, there is a significant difference between the modulus obtained with the two indenter tips. For the same indentation depths, pyramidal indentation provides a higher modulus when compared to spherical indentation. Due to the consistency with regards to experimental test set up, i.e., loading and unloading rates, the observation can be attributed to the indenter tip geometry. With spherical indentation, there is a clearer elastic plastic transition of the material due to the blunter tip geometry, when compared to pyramidal indentation where plastic deformation is induced quickly. This is reflected in the load-displacement curves and the slopes of unloading curves, therefore translating to the difference in Young’s modulus. This trend is also observed when single-cycle indentations were run on the tubing at 23 mN. With the Berkovich tip, an average modulus of 2.24 GPa was obtained for 60 indents. For both methods, there is a trend of declining modulus with increasing load and indentation depth. This could be representative of an indentation size effect (ISE). However, ISE is much less for the modulus obtained with spherical indentation. In addition, the stress-strain behavior of the polymeric tube was also assessed with spherical indentation, demonstrating typical yielding and plastic-deformation behavior of the polymer as shown in Figure 2. It should be noted that the numbers 1-4 in Figure 2 represent the 4 different indents and the black symbols represent an average response of the 4 indents.