- The paper demonstrates that coupling DCPD and H-DIC enables high-resolution detection of crack opening and closure in nuclear-grade steel under LCF conditions.
- It reveals that crack opening begins during the compressive phase, challenging traditional assumptions in fatigue and fracture mechanics.
- The integrated methodology provides actionable insights for calibrating fatigue crack growth models by combining electrical and optical measurement techniques.
Coupled DCPD and Non-Continuous DIC for Crack Closure Detection in LCF: Methodological Advancements and Insights
Introduction
The investigation centers on the crack closure phenomenon in a nuclear-grade low-alloy steel subjected to low cycle fatigue (LCF), focusing on both the precision of detection methods and the mechanics of crack behavior in reversed loading scenarios. Accurate characterization of crack opening and closure is critical for fracture mechanics-based life assessment methodologies of pressure vessel components operating under variable thermal-mechanical loads. The work delineates the integration of Direct Current Potential Drop (DCPD) and Heaviside-enhanced Digital Image Correlation (H-DIC) for rigorous monitoring of crack closure and opening, advancing the state-of-the-art in experimental fracture mechanics, particularly for large-scale yielding regimes.
Experimental Methodology
The studied material, 18MND5 low-alloy steel, representative of reactor pressure vessels, was characterized both chemically and mechanically to ensure standard compliance with RCC-M specifications. Rectangular specimens, notched via EDM to favor controlled crack initiation and propagation, were subjected to LCF with imposed total strain amplitudes of 0.2% and 0.6%. The experimental protocol carefully controlled image acquisition during static segments of cyclic loading, triggered to synchronize with DCPD signals to ensure both temporal and spatial correspondence of mechanical and electrical data.
For crack size and shape monitoring, the DCPD method—calibrated both numerically (finite element using Cast3M) and experimentally (ink markings and liquid nitrogen fracture)—enabled continuous measurement of crack depth and propagation with sub-millimetric accuracy. The H-DIC method, utilizing a shape function incorporating a Heaviside jump, delivered direct measurement of surface displacement discontinuities associated with crack opening. This direct approach circumvents the limitations of conventional DIC, especially in scenarios where displacement field continuity fails near cracks.
Results and Key Numerical Findings
Both DCPD and H-DIC provided consistent identification of crack opening and closure, but with varying spatial sensitivity:
- H-DIC demonstrated higher sensitivity for small cracks (usable for cracks as shallow as ~1 mm), while DCPD required deeper cracks to register an unambiguous nonlinear response to opening.
- Both methods show that the onset of crack opening occurs at negative strain levels, indicating that part of the compressive phase contributes to crack opening—a direct contradiction to the common engineering assumption that compressive load excursions are crack-arresting.
- Normalized crack opening/closure ratios (Uop/Ucl) increase with crack depth at both strain amplitudes, implying that as cracks advance, the portion of the load cycle where the crack remains open increases.
- Quantitatively, opening and closure strains decrease with increasing crack depth and with increasing strain amplitude, highlighting the sensitivity of closure phenomena to plasticity and load history.
- The DCPD-derived opening strains are lower than those from H-DIC, attributed to DCPD averaging over the crack front (including plane strain-dominated interior), while H-DIC is surface-focused (plane stress-dominated zone).
The crack opening stress, estimated by correlating opening strains with cyclic stress-strain data, monotonically decreases with the applied maximum stress and can become negative at high maximum stresses—again emphasizing that compressive portions of the cycle cannot be neglected for crack-driving force estimates. These results align with Newman's model and literature on multiple steel and Al grades, validating the methodology and confirming material-specific but predictable trends.
Theoretical and Practical Implications
This work provides robust experimental evidence refuting the prevailing paradigm that the compressive phase of tension-compression LCF cycles is non-contributive to crack propagation in ferritic steels. The findings necessitate adjustment of fatigue crack growth models, particularly for nuclear and high-integrity components under reversed load cycles where closure effects are non-negligible. The demonstrated coupling of DCPD and non-continuous DIC (specifically H-DIC) offers a high-fidelity, cross-validated toolkit for studying three-dimensional aspects of crack opening, applicable to both small- and large-scale yielding where closure is neither uniform nor trivially defined. The explicit quantification of crack opening stress relative to material flow stress also provides actionable parameters for numerical simulation calibration in engineering critical assessments.
For materials presenting significant strain hardening, this methodology enables detailed investigation of the correlation between hardening behavior and closure mechanics. Recognizing that crack opening and closure behavior are highly path-dependent and sensitive to microstructural and geometric parameters, this dual-approach enables better integration of micro-mechanical phenomena into continuum-level fatigue models.
Future Directions in AI-Assisted Fracture Mechanics
Advancements in AI, computer vision, and DIC methodologies are expected to yield enhanced automation in crack tip localization and displacement measurement, as well as more reliable, real-time closure detection under complex multiaxial and thermomechanical loading. The presented integration of electrical and optical methods sets a foundation for multimodal data fusion, where AI could accelerate identification of critical closure events and optimize experimental routines based on learned fracture behaviors. Additionally, the improved physical understanding of closure under LCF is poised to inform data-driven predictive models that require validated, high-resolution experimental input for training and validation.
Conclusion
By employing coupled DCPD and H-DIC, the study achieves high-resolution, validated measurement of crack closure phenomena under realistic LCF conditions in a reactor-grade steel, revealing that crack opening events extend into the traditionally neglected compressive portion of cyclic loading. This directly challenges existing assumptions in fracture mechanical modeling and confirms the necessity of integrated, multimodal diagnostic methods for accurate fatigue life prediction. The methodological paradigm is extensible and informs both practical assessment and theoretical modeling, providing a path forward for increasingly sophisticated, AI-augmented analysis frameworks in structural integrity assessment.
Reference: "Crack opening and closure detection through coupled DCPD and non-continuous DIC method -- Application to LCF tests" (2606.18007).