Issue 78

G. Palomba et alii, Fracture and Structural Integrity, 78 (2026) 233-247; DOI: 10.3221/IGF-ESIS.78.14

Figure 6: Stress-strain curves about non-welded and welded specimens.

These findings directly support the assumptions and inputs of the finite element model, where spatially distributed mechanical properties were assigned based on Vickers microhardness profiles. Moreover, the DIC technique allowed full field strain development to be monitored throughout the test, offering a more comprehensive validation of the FEM than pointwise extensometry could provide. Hence, the DIC measurements not only validate the predictive accuracy of the hardness-derived FEM but also provide insight into the role of local mechanical mismatches in governing the strain evolution. This underscores the importance of incorporating spatial property gradients in the simulation of welded joints, especially in applications where localized failure or fatigue is of concern. Hardness testing Fig. 7 (a)shows the extracted specimens used in this study. A dog-bone specimen was sectioned to extract the central area where the welded is located. The obtained cross-section was mirror-polished as described in the previous section for metallographic inspection and microhardness measurements. Fig. 7 (b) presents the macro-etched cross-section of the welded joint, etched to distinguish metallurgical zones. The darker, finer-grained weld metal (WM) is clearly visible at the center, surrounded by a transitional HAZ on both sides. The BM appears brighter and more uniform, consistent with the parent material's undeformed microstructure. A line of micro indents (visible as small white dots) traces the Vickers hardness path used to derive the local mechanical property gradient. The curvature of the fusion boundary and the variation in etching response emphasize the heterogeneous thermal exposure during welding. These observations form the physical basis for mechanical property zoning in the subsequent finite element modeling. The etched surface clearly delineated the BM, HAZ, and WM, allowing accurate positioning of the hardness indentations. The hardness of materials and their mechanical strength are linked by the fundamental mechanisms of plastic deformation. Hardness is a measure of material resistance to localized plastic flow, which is governed by the motion and interaction of dislocations. The hardness value (H) can be related to the yield strength ( σ ᵧ ) via the Tabor relation H ≈ 3 σ ᵧ , although the relationship varies depending on the microstructure and work-hardening behavior. This simple relationship is the basis of many empirical models used to derive tensile properties from hardness tests. For metals such as austenitic stainless steels, which exhibit significant strain hardening, the hardness–strength relationship must account for the entire stress–strain curve rather than only the yield stress. The correlations proposed by Lopez &

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