Issue 30

D. Nappini et alii, Frattura ed Integrità Strutturale, 30 (2014) 394-402; DOI: 10.3221/IGF-ESIS.30.47 397 d 2 δ (D δ)     in which D indicates the indenter diameter. - Some parameters calculated by the standard Brinnell formulation:   max max 2 2 0.102 P 0.102 2P H  A πD D D d       in which P max indicates the maximum value of the applied force (N), D and d are expressed in mm. Replacing d with the previous expression, the relationship obtained is:   max 2 0.102 2P H πD D D 4(δ (D δ)        Using different values for the displacement δ three separate parameters H 1 , H 2 , H 3 were defined. In particular in H 1 the variable δ were set to δ max value, in H 2 δ = δ f , whereas in H 3 the value δ = δ i were used. Figure 2: Parameters evaluated from load-indentation depth curves. Note that because these definitions of “hardness” are not based on the contact area after unloading, they deviate from the traditional hardness measurement. Moreover, during the test the maximum value of the load is not maintained for a specific dwell time as the ASTM standard testing requires. So any evaluated hardness value cannot be in agreement with the standard, however the evaluated H parameters could be indicative of the material hardening. In all cases the penetration depth is calculated starting from the value of displacement at the first contact, assumed in correspondence of P = 1 N (Fig. 3). Figure 3 : Estimation of penetration value δ at the first contact for P = 1 N used to calculate the parameters.