If the yield stress
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is defined at zero offset strain. Hence the true stress-true strain behaviour is written to be Swaddiwudhipong et al, 2005:
= ,
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n
¡ s
¢
o
_ _
E
3.4
In these indentation FE models, the Young’s modulus ‘E’ of the work piece materials was set to 200 GPa and Poisson’s ratio was set to 0.3, which were commonly used value for
steel Chen et al, 2007.
3.2. FE modelling of the Vickers indentation and effects of material properties
Figure 3.2.a show the FE Models of the Vickers indentation. Only a quarter of the indenter and material column was simulated as a result of plan symmetric geometry. The
sample size is more than 10 time the maximum indentation depth, which is sufficiently large to avoid any sample size effect or boundary effect Johnson, 1985. The bottom face
of the material volume was fixed in all degrees of freedom DOF and two side faces A and B were symmetrically fixed in z and x direction. The element type used is C3D8R
reduced integration element used in stressdisplacement analysis. Contact was defined at indenter facet. The material of interest was allowed to move and the contact between the
indenter surface and the material was maintained at all the time. The mesh in the regions with large deformation such as that directly under the indenter tip was refined with high
mesh density in order to obtain more accurate results. The Vickers indenter has the form of the right pyramid with a square base and an angle of
136 between opposite face. It is normally made of diamond with Young’s modulus of over 1000 GPa, which is significantly stiffer than steel E=200 GPa. The indenter was
considered as the rigid body to improve the modelling efficiency. A predefined displacement was applied in a ramp mode and the reaction force is recorded on the
reference point, representing the overall load on the indenter. Figure 3.2 b shows a typical P-h curve Force Vs Indentation depth during loading and unloading phase
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=100 MPa, n=0,01. The loading curve represents the resistance of material to indenter
penetration, while difference between the loading and loading curve represents the energy loss Swaddiudhipong et al, 2005. The unloading part of the P-h curve is primarily
influenced by E, as it is essentially an elastic process, while the loading part of the P-h curve correlates with
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and n Taljat, 1998. This work is focused on the studies of plastic parameters, so only the loading curve was utilised to predict the plastic material
parameters. Figure 3.3 b illustrated the effect of material properties on the P-h curves of Vickers indentation
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=300 MPa with E set at 200 GPa for Steel. It is clearly shown that both the yield strength and work hardening coefficient influence the indentation
curves.
3.3. FE modelling of the spherical Indentation and effects of material properties
