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

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 µm 3 and a second one of 160 µm 3 -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.

Metallic fractures assessments: OCT versus SEM

“…Our studies approached the analysis of broken, ductile, fragile and fatigue surfaces of alloys (steel, cast iron, non-ferrous alloys) obtained both via SEM and OCT [7,8]. Following these analyses, we have demonstrated that OCT can replace SEM for the breakage surface analysis.…”

“…The latter aspect implies that it cannot be utilized to perform in situ assessments, as it is required for example in forensic investigations-for example in the case of plane, train, or car accidents, as well as of other incidents, such as blade ruptures of turbines or pipe ruptures. Given the above, we have proposed, for the first time to our knowledge, the use of an IR-based imaging method, Optical Coherence Tomography (OCT) [5,6], to replace SEM in such investigations [7,8]. A low coherence interferometry method, OCT systems have (much) lower costs (roughly by a factor of 20), it does not require trained operators, and it can be utilized in situ, as mobile systems have been constructed.…”

“…In the present paper we present a few analyses of the structure of metallic materials (their fracture surfaces) subjected to tests for ductile, brittle, or fatigue fractures. Due to space limitations, only a few elements of the extended studies we have been carrying on in [7,8] are pointed out.…”

Modelling of the Influence of Tensile Test Speed on the Fracture Load

“…Engineering materials are classified in accordance to their deformation at fracture (i.e., brittle or ductile), to the form and appearance of fracture where ductile fracture produces less serious problems than brittle fracture under the same operating conditions, and to the crystallographic manner in which the fracture occurs (i.e., sliding, cleavage) (Hutiu et al, 2018). Among the various failure mechanisms introduced (Fan et al, 2017), investigating those associated with a brittle and ductile fracture in metallic alloys is important.…”

Temperature variations in sintering ovens for metal ceramic dental prostheses: Non-destructive assessment using OCT

“…OCT is a biomedical imaging techniquebased on the principles of low coherence interferometry, that has been initially developed for ophthalmology, especially for studies of the retina and cornea [10][11][12][13]; it has further on expanded towards imaging the skin [14], as well as the oral cavity, for both hard and soft tissue [15][16][17][18][19]. While OCT is being utilized in an increasing area of such biomedical applications, it has also begun to be used extensively in Non-Destructive Testing (NDT) of materials, both refractive (for which cross sections/B-scans can be provided -a major advantage of OCT [2026]) and reflective (i.e., metallic, for which the topography of material surfaces can be obtained [27,28]). …”