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Autonomous healing of fatigue cracks via cold welding

Nature volume 620, pages 552–556 (2023)Cite this article

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Abstract

Fatigue in metals involves gradual failure through incremental propagation of cracks under repetitive mechanical load. In structural applications, fatigue accounts for up to 90% of in-service failure1,2. Prevention of fatigue relies on implementation of large safety factors and inefficient overdesign3. In traditional metallurgical design for fatigue resistance, microstructures are developed to either arrest or slow the progression of cracks. Crack growth is assumed to be irreversible. By contrast, in other material classes, there is a compelling alternative based on latent healing mechanisms and damage reversal4,5,6,7,8,9. Here, we report that fatigue cracks in pure metals can undergo intrinsic self-healing. We directly observe the early progression of nanoscale fatigue cracks, and as expected, the cracks advance, deflect and arrest at local microstructural barriers. However, unexpectedly, cracks were also observed to heal by a process that can be described as crack flank cold welding induced by a combination of local stress state and grain boundary migration. The premise that fatigue cracks can autonomously heal in metals through local interaction with microstructural features challenges the most fundamental theories on how engineers design and evaluate fatigue life in structural materials. We discuss the implications for fatigue in a variety of service environments.

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Fig. 1: Still images documenting key stages of crack propagation, healing and regrowth.
Fig. 2: Detailed observations of the healing process taken from dynamic video.
Fig. 3: An atomistic model confirms boundary migration, crack flank contact and healing.

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Data availability

The datasets generated and/or analysed during the current study are available from the corresponding author on reasonable request.

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Acknowledgements

We thank D. Medlin, P. Lu, R. J. Parrish, W. M. Mook, D. D. Stauffer, C. Kunka and R. O. Ritchie for valuable interactions regarding this work. We thank R. Schoell for compiling data collected by Z.M. C.M.B., D.C.B., N.M.H., D.P.A., K.H. and B.L.B. were supported by the US Department of Energy (DOE) Office of Basic Energy Science, Materials Science and Engineering Division. The contributions of T.D. and M.J.D. were supported by the Department of Energy, National Nuclear Security Administration (DE-NA0003857). A.M. and A.S. acknowledge support from the National Science Foundation, Division of Civil, Mechanical, and Manufacturing Innovation (1663130). This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the US DOE Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the US DOE National Nuclear Security Administration (DE-NA-0003525). The views expressed in the article do not necessarily represent the views of the US DOE or the US Government.

Author information

Author notes

  1. Khalid Hattar

    Present address: Department of Nuclear Engineering, University of Tennessee, Knoxville, TN, USA

Authors and Affiliations

  1. Sandia National Laboratories, Albuquerque, NM, USA

    Christopher M. Barr, Daniel C. Bufford, Zachary Milne, Nathan M. Heckman, David P. Adams, Khalid Hattar & Brad L. Boyce

  2. Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, NM, USA

    Christopher M. Barr, Zachary Milne, Nathan M. Heckman, Khalid Hattar & Brad L. Boyce

  3. Department of Materials Science and Engineering, Texas A&M University, College Station, TX, USA

    Ta Duong, Abhilash Molkeri, Ankit Srivastava & Michael J. Demkowicz

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Contributions

C.M.B., D.C.B., Z.M., N.M.H. and K.H. designed and performed the in situ transmission electron microscope nanocrystalline fatigue investigations. C.M.B., Z.M., N.M.H. and B.L.B. performed the formal analysis of the experimental data and data curation. D.P.A. oversaw the deposition of the nanocrystalline thin-film depositions. T.D. and M.J.D. performed and analysed the molecular dynamic simulations. A.M., A.S. and M.J.D. performed and analysed the finite element simulations. M.J.D. developed the theoretical model of the effect of crack healing on fatigue thresholds. Writing of the original draught was performed by C.M.B., K.H., M.J.D. and B.L.B. All co-authors discussed the results and performed activities in writing, reviewing and editing the manuscript. B.L.B. supervised the project.

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Correspondence to Michael J. Demkowicz or Brad L. Boyce.

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Extended data figures and tables

Extended Data Fig. 1 Additional still images after crack healing.

(a) crack at new location after monotonic unload after crack healing event, (b–e) crack tip blunting without significant advance at same crack tip location in (a); there is no observation (either from still frames from the video or during monotonic loading/unloading) of the deflected crack being present. In each subfigure, the arrow points to the location of the crack tip. In (e) the apparent appearance of a daughter crack is actually a visual illusion due to transmission imaging, and the video evidence more clearly shows that this crack is connected to the main crack.

Extended Data Fig. 2 Additional atomistic model results.

Two different realizations of GB34 models: (a) ideal alignment with {111}-type planes in adjacent grains, leading the perfect coherency along the GB plane; (b) GB plane normal to sample surface and ~6.2 degrees offset from ideal {111}-type alignment, resulting in an array of Shockley partial twinning dislocations along the GB plane. (c) Stress-strain curve for a triple junction model with the imperfect CTB shown in (b) loaded in biaxial tension. Tractions on the top and right loading surfaces were computed by adding up the forces on the atoms with fixed y- and x-coordinates in the top and right loading layers, respectively. The response to mechanical loading of the model where the GB34 plane is normal to the sample surface, i.e., ~6.2 degrees offset from ideal {111}-type alignment: (d) as-constructed structure and (e) mechanically-driven GB34 migration after 3% tensile loading in the y-direction (and corresponding level of biaxial loading in the x-direction).

Extended Data Fig. 3 Effect of twinning defects on boundary migration and crack healing.

Example of delayed crack healing that occurs when the defected twin boundary is replaced by a perfectly coherent twin boundary: (a) immediately after application of 0.2% tensile loading, (b) onset of closure at the crack tip 100 ps after application of loading, (c) propagation of a front of crack closure, (d) complete crack healing 160 ps after application of loading.

Extended Data Fig. 4 Polycrystal finite element analysis of stress heterogeneities.

Hydrostatic stresses induced by the triple junction under 0.2% applied tensile strain. This model contains no crack.

Extended Data Fig. 5 Polycrystal finite element analysis indicating crack closure at the crack tip.

(a) Continuum-level version of the triple junction model shown in main text, Fig. 3a. The arrows on the top and bottom surfaces indicate the direction of the tensile axis; (b) the difference in normal displacement of the top and bottom crack faces induced by the non-uniform triple junction stresses in (a). The red contour shows the locus of zero relative crack face displacement. Near the crack tip, the displacement is negative, indicating crack closure.

Extended Data Fig. 6 Polycrystal finite element model including the effect of boundary migration.

Once the bonds along GB24 are severed, then migration of GB34 causes a shear in the part of G4 through which the migration has occurred, as shown in (a). The shape of the resulting gap between G2 and G4 is shown in (b). The resultant difference in normal displacement of the top and bottom crack faces induced by migration of GB34 in (a–b). The red contour shows the locus of zero relative crack face displacement. The displacement is negative in a broad region at the crack tip and near one of the free surfaces, indicating a strong tendency toward crack closure.

Extended Data Fig. 7 In situ TEM fatigue observation in nanocrystalline Cu.

Observations of crack healing were similar to those observed in nanocrystalline Pt. The sample was loaded with mean load of 152 µN and amplitude of 101 µN at 200 Hz for a 100 s interval. (a) quasi-static image of a mode I crack in the sample after being subjected to 276,000 previous loading cycles, (b) rapid crack deflection by nearly 90° downward from mode I at approximately 278,000 cycles followed by retreat by 288,000 cycles, then (c) propagation at approximately 45° upward from mode I then retreat, followed by (d) propagation by approximately 60° downward again before a third observation of crack retreat before finally (e) returning to normal mode I crack propagation. Outlines of the crack flanks at the maximum extent shown in (b–d) before retreat and propagation in a new direction appears in panel (e). Video of crack deflection, retreat with no signs of crack existing after return to mode-I loading can also be observed in Supplementary Video 3.

Extended Data Fig. 8 Overview of starting Pt microstructure and in situ TEM method.

(a) Bright field TEM image of free standing nanocrystalline Pt; (b) Nanocrystalline Pt grain size distribution; (c) Overview of experimental push-to-pull device with a nanocrystalline thin film with (d) localized view of an example sample over the push-to-pull gap; (e) example loading sequence where (f) quasi-static loading, (g) dynamic loading at 200 Hz, and (h) quasi-static unloading bright-field TEM images are shown.

Extended Data Fig. 9 Additional details of crack initiation and early fatigue-crack propagation.

(a–b) Observation of fatigue crack initiation where highlights crack path direction; (c) propagation to first grain boundary between G1 and G2 (GB12) where arrow indicates location of crack path; (d) Inverse pole figure coloured orientation map collected at 124,000 total fatigue cycle where the fatigue crack is temporarily arrested.

Extended Data Fig. 10 Crystallographic interpretation of intragranular crack growth.

Crack propagation directions appear commensurate with slip plane traces in Grain 2 (G2).

Supplementary information

Supplementary Information

Supplementary Notes 1–13, Figs. 1–5, Tables 1 and 2, and References.

Peer Review File

Supplementary Video 1

In situ TEM video of fatigue crack deflection and healing in G2 occurring between fatigue cycles 604,000 and 684,000. The video consists of two full fatigue sequences: dynamic loading between 604,000 cycles to 644,000 cycles and 644,000 to 684,000 cycles. The video also contains the static unloading and loading between these fatigue sequences. In the first sequence, the crack deflects towards the triple junction in G2. In the second sequence, the crack heals. For each fatigue loading sequence, the sample was tested at 200 Hz with 200 seconds of real time cyclic loading. Video is 10× real time speed.

Supplementary Video 2

This video provides an animation of the process shown in Fig. 3. Under a static, 0.2% tensile load, the atomistic model containing an imperfect CTB undergoes complete healing.

Supplementary Video 3

In situ TEM video of fatigue crack deflection and healing in a nanocrystalline Cu tension sample previously subjected to ≈ 276,000 cycles. Sample was loaded at 152 µN mean load and 101 µN amplitude load at 200 Hz for 100 second dynamic segment (plus quasi-static load and unload segments). Video is in real time.

Supplementary Video 4

Monotonic tensile loading of our atomistic model leads to crack tip dislocation emission and crack tip blunting. No crack healing is observed.

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Barr, C.M., Duong, T., Bufford, D.C. et al. Autonomous healing of fatigue cracks via cold welding. Nature 620, 552–556 (2023). https://doi.org/10.1038/s41586-023-06223-0

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