Issue 35

G. Gobbi et alii, Frattura ed Integrità Strutturale, 35 (2016) 260-270; DOI: 10.3221/IGF-ESIS.35.30 263 The damage initiation criterion, by which the process of degradation starts, is the maximum nominal strain criterion according to the definition given in [20]. It implies that the damage initiates when the nominal strain ε evaluated at each i- th increment of the analysis, reaches the user defined maximum allowable strain value ε 0 . After that, to describe the rate of the degradation process a damage variable D is used. It represents the overall damage in the material and it is related to the stress components as it follows [20]:   1 i i D     (1) where σ i is the stress tensor computed in the current increment considering absence of damage, and i  is the stress predicted by the elastic traction-separation in the same increment. The values of D run monotonically over a range from zero to one: zero represents the starting condition of damage; one describes a situation of full damage in which the element cannot hold up the applied load anymore. In order to assign the TSL shape, a tabular function of the effective displacement δ i beyond damage initiation is used. The formulations of damage D as a function of the effective displacement δ i is reported below: Elastic part: 0 D  Plastic part: 0 1 1 1 i D      (2) Failure part: 0 1 1 i A D      where F i F N A        The values of these parameters are chosen in order to fit the experimental crack propagation data. Therefore, a calibration process of TSL for this specific steel without considering the hydrogen effect is necessary as starting point to develop the more complex model including the presence of hydrogen. Figure 2 : Scheme of the specimen and the F - V LL quantities used for the calibration process. Calibration process of TSL parameters for AISI 4130 steel Generally, the cohesive law can be calibrated by a comparison with experimental crack propagation data or it can be theoretically modeled using a numerical method that resolves the fracture process. In this case, we selected the experimental approach. A calibration procedure is developed to estimate the cohesive parameters more suitable to represent the characteristic fracture behavior of AISI 4130, in hydrogen non-contaminated conditions. This calibration