Investigation of Fatigue Failure of Roll Shafts in a Tube Manufacturing Line
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CASE HISTORY—PEER-REVIEWED
Investigation of Fatigue Failure of Roll Shafts in a Tube Manufacturing Line G. Pantazopoulos • S. Zormalia • A. Vazdirvanidis
Submitted: 20 April 2010 / in revised form: 27 May 2010 / Published online: 15 June 2010 Ó ASM International 2010
Abstract A failure analysis of steel shafts operating as roll components in a tube drawing machine was conducted. Visual inspection, optical microscopy, SEM fractography, and hardness testing were used as the principal analytical tools for the failure investigation. The findings showed that the failure mechanism is rotating bending fatigue under low stress–high stress concentration conditions. The primarily inadequate component design resulted in high stress concentration factors interacted with the soft annealed material used for the component and caused fatigue under the presence of fluctuating stresses. Keywords Fatigue Fractography Roll shafts Stress concentration
Introduction and Background Information Fatigue damage is a progressive failure process that occurs under cyclic loading and is comprised of three distinct stages [1]: stage I is related to crack nucleation which generally occurs at 45° to the stress direction (slip plane fracture) [2]; stage II is progressive crack growth which occurs almost perpendicular to the stress up to the point that the remaining cross-sectional area is no longer able to sustain the applied load; and [3] stage III which is sudden overload fracture of the reduced cross section. The crack propagation rate during stage III fracture is almost equal to of speed of sound in the material. Industrial parts and engine components, transmitting or receiving rotational G. Pantazopoulos (&) S. Zormalia A. Vazdirvanidis ELKEME Hellenic Research Centre for Metals S.A., 252 Pireaus Street, 17778 Athens, Greece e-mail: [email protected]
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motion, suffered from torsional and fatigue-type failures [2–5]. Undoubtedly, the majority of the mechanical failures of machine elements and components can be attributed to fatigue loading. As a rule of thumb, the time necessary for fatigue crack nucleation corresponds to 80–90% of the total component life. Recent improvements in materials and in component design have improved the fatigue resistance of machine elements and resulted in high performance machine operation and enhanced productivity indicators. Unfortunately, not all components experience the recent improvements. In this study, broken shafts from a roller unit used in a copper tube drawing machine were brought for failure investigation (Fig. 1). During operation of the drawing machine, the shaft rotates moving the roller-way and accommodating the principal working stresses developed during power transmission. High frequency failures of the shafts (1 shaft failure about every 5 days when operations were going for three shifts per day) led to increased downtime and high maintenance costs which resulted in poor productivity of the entire manufacturing process. An immediate failure analysis was then assigned in o
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