Crack opening and closure detection through coupled DCPD and non-continuous DIC method -- Application to LCF tests
Pith reviewed 2026-06-26 21:45 UTC · model grok-4.3
The pith
Cracks in low-alloy steel under low-cycle fatigue do not remain fully closed during part of the compressive cycle.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Coupling DCPD and non-continuous H-DIC on low-alloyed steel specimens in low-cycle fatigue establishes that the crack does not remain completely closed during a portion of the compressive part of the fatigue cycle for both tested strain amplitudes. The crack opening strains combined with the cyclic stress-strain curve produce an equivalent cyclic opening stress that decreases with increasing applied maximum stress in tension-compression loading.
What carries the argument
Coupled direct current potential drop (DCPD) and non-continuous high-resolution digital image correlation (H-DIC) measurements that locate crack opening and closure points within each fatigue cycle.
If this is right
- Equivalent cyclic opening stresses decrease with rising applied maximum stress under tension-compression loading.
- The crack remains partially open during part of the compressive portion of each fatigue cycle at both strain amplitudes.
- Both DCPD and non-continuous H-DIC produce consistent crack opening results when applied to the same tests.
- Opening strains can be converted to stresses via the material cyclic stress-strain curve.
Where Pith is reading between the lines
- Fatigue crack growth models that assume complete closure under compression may need adjustment to account for the observed partial opening.
- The combined measurement approach could be extended to other alloys or loading ratios to test whether the decrease in opening stress with maximum stress holds more generally.
- Life prediction methods for components experiencing tension-compression cycles might improve by incorporating measured opening levels rather than nominal zero-load assumptions.
Load-bearing premise
The two measurement techniques accurately capture true crack opening strain without major artifacts from electrical contact resistance or the discontinuous correlation procedure.
What would settle it
A third independent technique such as in-situ optical microscopy or acoustic emission recording full crack closure throughout the entire compressive half-cycle on the same specimens would contradict the reported opening strains.
read the original abstract
The crack closure effect of a low-alloyed steel subjected to low-cycle fatigue loading has been characterized at two different imposed strain amplitudes. Two techniques (non-continuous DIC H-DIC and DCPD) have been employed in this aim, leading to similar conclusions. Thus, it is shown that the crack does not remain completely closed during a part of the compressive portion of the fatigue cycle for both applied loadings. The crack opening strains, combined with the cyclic stress-strain curve of the material allowed to determine an equivalent cyclic opening stress. This confirmed that the crack opening stresses decrease with the applied maximum stress when subjected to tension-compression loading, as commonly found in the literature.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports an experimental investigation of crack closure in low-cycle fatigue of a low-alloyed steel at two imposed strain amplitudes. Two independent techniques—direct-current potential drop (DCPD) and non-continuous high-resolution digital image correlation (H-DIC)—are applied to detect crack opening and closure. The central claims are that the crack remains open during part of the compressive half-cycle for both loadings and that the equivalent opening stress decreases with increasing maximum stress, consistent with prior literature.
Significance. If the measurement accuracy in the compressive regime is confirmed, the work supplies direct experimental evidence from two orthogonal techniques that crack opening persists into compression under tension-compression LCF and that opening stress scales with peak stress. Such data are useful for refining closure models in fatigue life prediction. The use of coupled DCPD and discontinuous DIC is a methodological strength that could be adopted more widely if the validation steps are made explicit.
major comments (2)
- [Abstract / Results] Abstract and Results sections: the claim that the crack is open during part of the compressive cycle rests on the assertion that DCPD and non-continuous H-DIC yield similar conclusions, yet no uncertainty budgets, cross-validation metrics, or checks against known contact-resistance or subset-discontinuity artifacts are reported. These omissions are load-bearing because both techniques can produce spurious opening signals precisely when displacements are small and faces are in partial contact.
- [Methods] Methods section: the non-continuous H-DIC procedure is described only at a high level; no information is given on subset size, discontinuity handling, noise floor, or how opening strain is thresholded from the displacement field under compression. Without these details the agreement with DCPD cannot be evaluated as independent confirmation.
minor comments (2)
- [Figures / Results] Figure captions and text should explicitly state the strain amplitudes used and the number of cycles at which opening measurements were taken.
- [Results] The term “equivalent cyclic opening stress” is introduced without a clear definition or reference to the cyclic stress-strain curve equation employed.
Simulated Author's Rebuttal
We thank the referee for the constructive comments, which identify areas where additional methodological detail will strengthen the manuscript. We respond to each major comment below and will revise accordingly.
read point-by-point responses
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Referee: [Abstract / Results] Abstract and Results sections: the claim that the crack is open during part of the compressive cycle rests on the assertion that DCPD and non-continuous H-DIC yield similar conclusions, yet no uncertainty budgets, cross-validation metrics, or checks against known contact-resistance or subset-discontinuity artifacts are reported. These omissions are load-bearing because both techniques can produce spurious opening signals precisely when displacements are small and faces are in partial contact.
Authors: We agree that the original manuscript lacks explicit uncertainty budgets, quantitative cross-validation, and artifact checks. In the revision we will add (i) uncertainty estimates for both DCPD voltage and H-DIC displacement fields, (ii) direct comparison metrics (e.g., opening-strain differences and correlation) between the two techniques, and (iii) discussion of contact-resistance effects in DCPD and subset-discontinuity handling in H-DIC, including any mitigation steps performed. These additions will appear in a new subsection of Results and an expanded Methods section. revision: yes
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Referee: [Methods] Methods section: the non-continuous H-DIC procedure is described only at a high level; no information is given on subset size, discontinuity handling, noise floor, or how opening strain is thresholded from the displacement field under compression. Without these details the agreement with DCPD cannot be evaluated as independent confirmation.
Authors: The original submission indeed presented the H-DIC procedure at a summary level. The revised manuscript will specify: subset size and shape, the discontinuity-detection algorithm (including how crack faces are segmented), noise-floor evaluation from unloaded reference images, and the precise thresholding rule applied to the opening-strain field in both tension and compression. These parameters will allow readers to assess the independence of the two techniques. revision: yes
Circularity Check
No circularity: purely experimental confirmation of literature trend via independent measurements
full rationale
The paper performs experimental measurements of crack opening using two techniques (DCPD and non-continuous H-DIC) on low-cycle fatigue specimens. It reports that the techniques yield similar conclusions about partial opening in the compressive regime and that the derived opening stresses decrease with maximum stress, matching prior literature. No equations, derivations, fitted parameters presented as predictions, self-definitional constructs, or load-bearing self-citations appear in the provided text. The central claim rests on empirical data collection and cross-method agreement rather than any reduction of outputs to inputs by construction. This is a standard experimental measurement study with no mathematical chain that could exhibit circularity.
Axiom & Free-Parameter Ledger
Reference graph
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