Utilizzo della Caduta di Potenziale a Corrente Elettrica Diretta (DCEPD) per la Valutazione della Tenacità a Frattura e la Caratterizzazione della Curva J-Integrale vs. Crescita della Cricca = Utilisation of Direct Electrical Current Potential Drop (DCEPD) for Assessing Fracture Toughness and the Characterisation of J-Integral vs. Crack Growth Curve

In the field of materials testing, Potential Drop (PD) techniques have been widely employed for decades in the petrochemical, aerospace, and power generation industries, particularly for monitoring crack growth and wall thickness variations caused by corrosion or erosion in critical structures. Traditionally, both Direct Current (DC) and Alternating Current (AC) PD methods have been applied, each with its own advantages and limitations. This thesis focuses on the Direct Current Potential Drop (DCPD/DCEPD) method as a means of accurately monitoring crack extension and deriving fracture toughness values. The principle of PD measurement is straightforward: a constant current is injected into a specimen, and the voltage drop across two electrodes placed on the ligament is monitored. As a crack propagates, the reduction in the effective cross-sectional area increases electrical resistance, producing a measurable potential difference. This relationship allows real-time crack sizing when properly calibrated. However, practical implementation presents challenges, including current stability, sensitivity to electrode contact resistance, and thermal drift at elevated temperatures. This work applies the DCEPD technique to derive the J–Integral vs. crack extension resistance curve (J–R curve) for compact tension (C(T)) specimens and compares its performance against the conventional Elastic Unloading Compliance (EUC) method recommended in ASTM E1820. A broad experimental programme was carried out on two alloys — G130 cast alloy and 26NiCrMoV16.8 steel — at room and elevated temperatures (400°C and 800°C). Tests were benchmarked against commercial PD systems and validated with compliance-based crack length measurements. At room temperature, EUC and DCEPD yielded consistent and ASTM-qualified toughness values (JIC ≈ 282–300 kJ/m2,  KJIC ≈ 238–227 MPa√m), demonstrating the reliability of both techniques. At 400 °C, EUC suffered from creep and time-dependent relaxation during unloading, leading to overestimated values and failed size validity checks. In contrast, DCEPD delivered valid toughness results (JIC ≈ 250–260 kJ/m2,  KJIC ≈ 265–273 MPa√m), with robust early crack-growth capture. At 800 °C, DCEPD enabled continuous crack monitoring, but the reduced yield strength of the alloys imposed stricter size requirements (B, b0 ≥ 30-40 mm) that were not met, resulting in non-qualified JQ, KQ values. These results illustrate the method’s capability but also underline the importance of specimen geometry for plane-strain toughness qualification. The findings show that DCEPD is highly suitable for high-temperature fracture toughness testing, where EUC becomes unreliable. It avoids the need for unloading cycles, improves resolution in the initiation regime, and produces continuous crack-extension data. Nevertheless, EUC retains value at room temperature as a baseline reference and for comparability with historical datasets. Running both methods in parallel at baseline conditions provides an effective calibration benchmark. Overall, this thesis demonstrates that DCEPD can serve as a reliable substitute for EUC in elevated-temperature applications and as a complementary tool for structural integrity assessment in line with ASTM E1820. Recommendations for future work include the use of larger specimens to satisfy high-temperature size validity, improved high-temperature probe systems, and further cross-validation with alternative crack measurement methods such as DIC.