Citation: Ohana, R.; Klein, R.; Shneck, R.; Bortman, J. Experimental Investigation of the Spall Propagation Mechanism in Bearing Raceways. Materials 2023, 16, 68. https://doi.org/10.3390/ ma16010068 Academic Editor: Qianhua Kan Received: 17 November 2022 Revised: 12 December 2022 Accepted: 19 December 2022 Published: 21 December 2022 Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Experimental Investigation of the Spall Propagation Mechanism in Bearing Raceways Ravit Ohana 1 , Renata Klein 2 , Roni Shneck 3 and Jacob Bortman 1, * 1 PHM Laboratory, Department of Mechanical Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 8410501, Israel 2 R. K. Diagnostics, Gilon, P.O. Box 101, D. N. Misgav 2010300, Israel 3 Department of Material Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 8410501, Israel * Correspondence: jacbort@bgu.ac.il Abstract: This article investigates the spall propagation mechanism for ball bearing raceways by focusing on an experimental investigation of cracks that evolve in the vicinity of the spall edge. Under- standing the spall propagation mechanism is an important step towards developing a physics-based prognostic tool for ball bearings. This research reflects an investigation of different spall sizes that propagate naturally both in laboratory experiments and in the field. By using a combined model of a rigid body dynamic model and a finite element model that simulates the rolling element–spall edge interaction, our results shed light on the material behavior (displacements, strains, and stresses) that creates an environment for crack formation and propagation. With the support of the experimental results and the rolling element–spall edge interaction model results, three stages of the mechanism that control fragment release from the raceway were identified. In Stage one, sub-surface cracks appear underneath the spall trailing edge. In Stage two, cracks appear in front of the trailing edge of the spall and, in Stage three, the cracks propagate until a fragment is released from the raceway. These stages were observed in all the tested bearings. In addition, other phenomena that affect the propagation of the cracks and the geometry of the fragment were observed, such as blistering and plastic deformation. We include an explanation of what determines the shape of the fragments. Keywords: rolling element bearings; spall propagation; crack detection; fatigue crack growth; dynamic model; finite element 1. Introduction This research is focused on the mechanism of spall propagation in ball bearings—the most common failure mechanism in rolling-element (RE) bearings. Despite the importance of identifying spall severity in bearings, the spall propagation process is still not clear. Understanding the spall propagation mechanism will allow physics-based prognostics, and permit safer and lower-cost maintenance. Spall formation is caused by rolling contact fatigue (RCF). This is a microscopic mechanism during which micro-cracks can propagate in two different ways: (1) near the surface, originated pitting [1,2] and (2) sub-surface, originated spalling, until metallic flakes are released from the surface of the bearing raceways and/or the REs [3–7]. Sub- surface cracks mostly generate at stress concentration sites such as non-metallic inclusions, which causing “butterfly wings” in the vicinity of inclusions [8–10]. Improper installation of a bearing, or a lack of lubrication, along with the operating conditions (load, speed, and temperature) can also cause for pitting and spall formation which affect the system vibration [11–14] and eventually affect the fatigue lifetime of the bearing [15]. The spall evolution in the raceways is divided into three stages as shown in Figure 1: (1) initiation of the spall by the RCF mechanism, (2) steady spall propagation, and (3) accelerated spall propagation until final failure. After crossing the transition between the second and Materials 2023, 16, 68. https://doi.org/10.3390/ma16010068 https://www.mdpi.com/journal/materials