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Metals 2018, 8(2), 117; https://doi.org/10.3390/met8020117

Assessment of Ductile, Brittle, and Fatigue Fractures of Metals Using Optical Coherence Tomography

1
3OM Optomechatronics Group, Aurel Vlaicu University of Arad, 77 Revolutiei Ave., Arad 310130, Romania
2
Doctoral School, Polytechnic University of Timisoara, 1 Mihai Viteazu Ave., Timisoara 300222, Romania
3
Applied Optics Group, School of Physical Sciences, University of Kent, Canterbury CT2 7NH, UK
*
Author to whom correspondence should be addressed.
Received: 30 December 2017 / Revised: 2 February 2018 / Accepted: 5 February 2018 / Published: 8 February 2018
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Abstract

Some forensic in situ investigations, such as those needed in transportation (for aviation, maritime, road, or rail accidents) or for parts working under harsh conditions (e.g., pipes or turbines) would benefit from a method/technique that distinguishes ductile from brittle fractures of metals—as material defects are one of the potential causes of incidents. Nowadays, the gold standard in material studies is represented by scanning electron microscopy (SEM). However, SEM instruments are large, expensive, time-consuming, and lab-based; hence, in situ measurements are impossible. To tackle these issues, we propose as an alternative, lower-cost, sufficiently high-resolution technique, Optical Coherence Tomography (OCT) to perform fracture analysis by obtaining the topography of metallic surfaces. Several metals have been considered in this study: low soft carbon steels, lamellar graphite cast iron, an antifriction alloy, high-quality rolled steel, stainless steel, and ductile cast iron. An in-house developed Swept Source (SS) OCT system, Master-Slave (MS) enhanced is used, and height profiles of the samples’ surfaces were generated. Two configurations were used: one where the dimension of the voxel was 1000 μm3 and a second one of 160 μm3—with a 10 μm and a 4 μm transversal resolution, respectively. These height profiles allowed for concluding that the carbon steel samples were subject to ductile fracture, while the cast iron and antifriction alloy samples were subjected to brittle fracture. The validation of OCT images has been made with SEM images obtained with a 4 nm resolution. Although the OCT images are of a much lower resolution than the SEM ones, we demonstrate that they are sufficiently good to obtain clear images of the grains of the metallic materials and thus to distinguish between ductile and brittle fractures—especially with the higher resolution MS/SS-OCT system. The investigation is finally extended to the most useful case of fatigue fracture of metals, and we demonstrate that OCT is able to replace SEM for such investigations as well. View Full-Text
Keywords: metallic materials; fracture; ductile; brittle; fatigue; Optical Coherence Tomography (OCT); Scanning Electron Microscopy (SEM); surface topography; forensic investigations metallic materials; fracture; ductile; brittle; fatigue; Optical Coherence Tomography (OCT); Scanning Electron Microscopy (SEM); surface topography; forensic investigations
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Hutiu, G.; Duma, V.-F.; Demian, D.; Bradu, A.; Podoleanu, A.G. Assessment of Ductile, Brittle, and Fatigue Fractures of Metals Using Optical Coherence Tomography. Metals 2018, 8, 117.

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