Analysis of deep crack formation and propagation in railway brake discs

Highlights

• A steel brake disc was analyzed after a deep crack was discovered.

• Finite element and extended finite element models were used in the simulation.

• Service induced distortion could have a strong effect on crack behavior.

• The transient temperature differences help to define the crack behavior.

Abstract

While surface crack formation and propagation in Electric Multiple Unit (EMU) brake discs has been an active research topic, relatively few studies have considered similar behavior in deep cracks which become unstable. Moreover, most analyses assume nominal design conditions which does not reflect changes which may have been occurred during service. In the present study, a steel brake disc was analyzed after a deep crack was discovered during a maintenance check. The objective was to identify the impetus for deep crack propagation and characteristic semi-elliptical shape. Finite element (FEM) and extended finite element (XFEM) models which included residual stress effects from service were used to analyze the thermo-mechanical history and resulting crack propagation. The simulated and actual fracture surfaces demonstrated satisfactory agreement and showed that the crack preferentially initiated at the bolt hole corners and that transient temperature differences between adjacent hot spots help to define the crack orientation. It was also found that unstable propagation along the axial direction was due to a steep temperature gradient. Service induced distortion was also found to indicate the presence of internal stresses which can have a strong effect on crack behavior. It is expected that the findings will contribute toward a better understanding of how braking conditions and service effects influence unstable crack behavior in EMU brake discs.

Introduction

Railway brake discs (Fig. 1) are required to rapidly transform large amounts of kinetic energy into heat and are thus subjected to severe thermal loading. Due to the uneven heat distribution that develops during braking, surface cracks routinely appear after several thousand kilometers. While small cracks are generally not deleterious to safety, mechanical integrity can be compromised if a fissure exceeds a critical size and undergoes unstable propagation. Because of this, EMU brake discs are periodically monitored during service and immediately replaced once a crack exceeds a specified length. Although the root causes are generally known, the discontinuous nature of crack propagation and complex thermo-mechanical loading history during/after braking necessitate further study to understand how these events influence deep crack behavior and create the characteristic semi-elliptical shape. Based on the cost and difficulty associated with physical testing, finite element (FE)-based models have been used to study crack initiation and propagation in steel brake discs. However, due to the constraints of Lagrangian-based FE models, simulating crack front trajectories has been somewhat problematic and consequently, the ability to study deep crack propagation has been limited. It has only been recently that a more detailed analysis of transient crack behavior has become possible with the introduction of the extended finite element method (XFEM).

The topic of surface cracks development and propagation in steel railway brake discs has received considerable attention from researchers who have analyzed forged components [1], [2], [3] though less is known about the corresponding behavior of cast material. Panier [4] performed an experimental study using radiography and subsequently proposed a classification scheme for hot spots. While noting that different forms of thermal distortion could result during service, the effects of deformation on braking performance was not considered. Afzal [5] studied the thermal deformation resulting from an individual braking cycle and showed that distortion on the friction surface was a primary cause of brake jitter and cracking. Shallow reticular cracks and deep radial main cracks represent the main form of brake disc failure according to Li [6], [7] who also showed that the latter have a tendency to become unstable under complex stress conditions. Li also postulated that this behavior could be interpreted in terms of energy release. Wu [8] was one of the earliest workers to employ a linear-elastic XFEM model and attempted to develop theoretical fatigue life and crack length prediction curves based on cumulative braking cycles. However, it should be noted that an analysis of crack propagation in ductile materials also requires consideration of plastic behavior.

In the present study, the evolution of an unstable radial crack was analyzed in order to identify the mechanism responsible for deep crack propagation and resulting semi-elliptical shape. The cast steel brake disc used in the study was taken from a commercial EMU vehicle after two in-line radial cracks were discovered during a maintenance inspection (Fig. 2). The cracks were of interest for two reasons. The first is that the cracks showed signs of unstable propagation based on the size of the fissures. The second is that CRH maintenance records showed that replacement occurred at 550,000 km and was well below the expected 1,200,000 km service life for brake disc. After the brake disc had been removed and examined, an FE model was developed to study the thermomechanical history in the vicinity of the cracks. A non-linear XFEM model was then validated and used to simulate unstable crack propagation and analyze the effect of geometric distortion that was observed in the component. It is expected that the results will help to further an understanding of how deep cracks form and propagate in EMU brake discs.