(PMMA) Based Bone Cement - Tribology in Industry

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3. EXPERIMENTAL TESTWhere Ac is the developed contact area and can becalculated from the projected contact area A p andthe indenter geometry as:Where α is the angle between the axis of thediamond pyramid and its faces, α = 68° for aVickers indenter (Fig. 3) and α = 65.27° for amodified Berkovich indenter.For Vickers indenter: HV = 0.0945 H ITFor modified Berkovich indenter: HV = 0.0926 H ITBone cement prepared from commercially availabletype of mixture was used as a sample material forindentation tests. PMMA is isotropic, showing nocrystallinity, elasto-viscoplastic solid andamorphous polymer. Several indentations havebeen performed on each sample using the CSMMicro Indentation Tester (Fig. 4). Indentation testlasted for 300 cycles. Tests were performed withmaximum normal load of 15 N, minimum load of 5N and approach speed of 5000 nm/min.Vickers indenter was applied with quadraticloading type. There was no pause at maximum loadbefore unloading. Displacement measurementswere realised using LVDT sensor which is a part ofthe tester. Maximum possible indentation depth waslimited to 200 µm with displacement resolution of0.3 nm.Figure 3. CSM Micro instrumented indentationtesting: a) Optical micrograph of a Vickersindentation into a standard steel reference sample;b) Detail of Vickers indenter mounted abovesampleIndentation Modulus E ITThe reduced modulus of the indentation contact, E r ,is given by:Where β is a geometric factor depending on thediamond shape (circular: β = 1, triangular: β =1.034, square: β = 1.012). The Young’s modulus ofthe sample, E IT , can then be obtained from:Figure 4. a) CSM Micro Indentation Testerb) Schematic view of the main elements of MicroIndentation TesterWhereν i is the Poisson’s ratio of the indenter.ν s is the Poisson’s ratio of the sample.E i is the modulus of the indenter.4. RESULTS AND DISCUSSIONIndentation curve, that is, diagram of the normalforce applied during micro indentation as a functionof the number of cycles is shown in Fig. 5. This isclassical fatigue mode of indentation testing,Tribology in industry, Volume 33, No. 4, 2011. 149
whereas the force is loaded and unloaded constantlyduring indentation without a pause. Material issubjected to constant load/unload pattern and is notleft to rest, such as in case of indentation mode oftesting on creep.Figure 5. Normal force versus penetration depthcurve during micro indentationImprint on bone cement sample after indentationlasting for 300 cycles, is shown in Fig. 6. Typicalpattern produced by Vickers indentation is clearlyseen. Detailed view of the left lower angle is shownin Fig. 6b. The imprint is well shaped with clearlydeveloped edges and no microcracks around angles.This is in consistence with the fact that amorphousmaterials usually present an elastic behavior up tofailure [16]. Permanent deformation occurs due tohigh shear and compressive stresses which are bothpresent in highly localized area.Calculated values of hardness H IT and elasticmodulus E IT of bone cement sample after 300cycles obtained by the indentation tester is shownin Table 1.Figure 6. a) Micrograph of the indentation imprinton bone cement after 300 cycles, b) Detail of theindentation imprint on PMMA after 300 cycles (leftbottom angle)Table 1. Values of hardness H IT and elasticmodulus E IT of bone cement sample after 300cycles obtained by the indentation testerValue for PMMAsample after 300 cyclesHardness, H IT [MPa] 162.84Elastic modulus, E IT [GPa] 2.74Hardness profile and elasticity profile duringmulticycling loading, unloading and reloading areshown in Fig. 7. Values given in Table 1 areautomatically calculated by the tester according toOliver and Pharr approach, by analysing theunloading curves (Fig. 5). It can be clearly seen thatE IT value is rather uniform, whereas hardness hasslightly decreasing trend.Figure 7. Hardness, H IT and elasticity, as a functionof the number of cycles during micro indentationLoosening is recognized as one of the primarysources of total hip replacement failure. One of themain reasons is considered to be fatigue failure ofthe implant-bone cement and bone cement - boneinterface. That is the reason why characterization of150Tribology in industry, Volume 33, No. 4, 2011.
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(PMMA) Based Bone Cement - Tribology in Industry

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