Issue 35

G. Gobbi et alii, Frattura ed Integrità Strutturale, 35 (2016) 260-270; DOI: 10.3221/IGF-ESIS.35.30 266 concentration field around the crack tip, evaluated during the second step of the analysis for plane strain model. In accordance with literature [14], the peak of hydrostatic stress and then of hydrogen NILS concentration is not located at the crack tip, but slightly forward. In our simulations, this peak is located at the end of the first element ahead the tip. In this node, the model estimates a quite high C L concentration, equal to 4.43 ppm. It is around three times the initial imposed concentration at free edges (1.5 ppm). For plane stress model, similar contour is found, with a peak value for hydrogen concentration of 2.03 ppm. The last step calculates the current plastic strain, and consequently the contribution of the trapped sites, C T . Then, based on the total hydrogen concentration, NILS and traps, the decreasing factor k is applied to the previous TSL to reduce the area below the curve, thus the energy adsorbed by the cohesive elements. Thanks to this factor, it is possible to simulate a lower resistance to crack propagation and to evidence the embrittlement of the steel. In order to get F-V LL curves similar to experimental data, it results that k has to be applied to both cohesive stress and displacement of TSL. In particular, for plane strain model in presence of hydrogen, we found that the reduced TSL stress and displacement are respectively σ 0H =k · σ 0 and δ H = k 3/2 · δ , where δ is the generic displacement of the TSL curve. While the reduction in stress is in agreement with literature results [15, 16], the further reduction in displacement was quite unexpected. However, it can be justified by the high hydrogen sensitivity observed during the experimental fracture toughness tests for the current steel. In fact, it showed a more brittle behavior compared to other steels tested in the past [22]. For plane stress model, the same ratio was derived for the reduced TSL stress σ 0H = k · σ 0 , while the reduced displacement is slightly lower: δ H = k · δ . Figure 3 : Contours of C L concentration in lattice interstitial sites at crack tip region, plane strain model. Fig. 4 shows contours of the decreasing factor k around the crack tip for plane strain model. The maximum is located in front of the crack tip and it is equal to 0.26. On the tip, it is slightly lower and equal to 0.29. Instead, the maximum value of k factor for plane stress model is 0.36. This means that the embrittlement effect is more evident in case of plane strain than plane stress. Moreover, both models agree in the prediction of absence of plastic strain measured at the crack tip during the last step. Therefore, from the numerical simulations, it seems that no hydrogen is located in traps, i.e. C T =0, and the only contribution for k factor calculation is provided by the C L hydrogen amount. This means that the embrittlement is only due to hydrogen in the lattice and suggests a very brittle behavior of AISI 4130 in presence of hydrogen. As reported in literature [7, 8], AISI 4130 is supposed to have many trap sites, mainly due to its martensitic structure, resulting in high strength, rather than to its plastic strain. Although, the study should be further deepened, these numerical finding can be supported by the first experimental observations of the fracture surface of the specimens tested in hydrogen environment. In fact, for most of the steels, on the fracture surfaces vast brittle regions and smaller ductile areas appeared, likely related to the local plastic strain and therefore to hydrogen in traps [22]. Instead, in the case of AISI 4130, from the images collected after toughness tests,