W.R. Tyfour

Deterioration of rolling contact fatigue life of pearlitic rail steel due to dry-wet rolling-sliding line contact

This study is aimed at the deterioration of rolling contact fatigue (RCF) life of pearlitic rail steel, under rolling-sliding conditions, where the wet phase of the test is preceded by different numbers of dry cycles. It is shown that initial dry cycles above a critical number causes sudden and significant deterioration in RCF life. This effect has been explained using the argument of plastic strain accumulation (ratchetting) in the surface layer during the dry phase when the coefficient of friction is above 0.25. A strong correlation was found between the degree of ratchetting and the deterioration in RCF life. An empirical relationship to estimate this deterioration was concluded.

The steady state wear behaviour of pearlitic rail steel under dry rolling-sliding contact conditions

Abstract The present study is aimed at studying the onset of steady state wear behaviour of pearlitic rail steel. Wheel-rail contact is simulated by a rolling-sliding line contact. The results show that steady state wear rate prevails after a certain number of rolling-sliding cycles. The effect of strain hardening and uni-directional plastic strain accumulation on the wear behaviour has also been studied. It has been shown that the start of the steady state wear rate coincides with the cessation of plastic strain accumulation and additional strain hardening. The ratchetting failure mechanism has been employed to explain this coincidence.

On the Analysis of Short-Pulse Laser Heating of Metals Using the Dual Phase Lag Heat Conduction Model

Transient heat conduction in a thin metal film exposed to short-pulse laser heating is studied using the dual phase lag heat conduction model. The initial heat flux distribution in the film, resulting from the temporal distribution function of the laser pulse, together with the zero temperature gradients at the boundaries normally used in literature with the presumption that they are equivalent to negligible boundary heat losses is analyzed in detail in this paper. The analysis presented here shows that using zero temperature gradients at the boundaries within the framework of the dual phase lag heat conduction model does not gcutrantee negligible boundary heat losses unless the initial heat flux distribution is negligibly small. Depending on the value of the initial heat flux distribution, the presumed negligible heat losses from the boundaries can be even way larger than the heat flux at any location within the film during the picosecond laser heating process. Predictions of the reflectivity change of thin gold films due to a laser short heat pulse using the dual phase lag model with constant phase lags are found to deviate considerably from the experimental data. The dual phase lag model is found to overestimate the transient temperature in the thermalization stage of the laser heating process of metal films, although it is still superior to the parabolic and hyperbolic one-step models.

The effect of rolling direction reversal on the wear rate and wear mechanism of pearlitic rail steel

Abstract The effect of different single and multiple rolling direction reversal (RDR) regimes on wear rate and mechanism is studied in this paper. Changes in structure deformation morphology and accumulated plastic strain are also analysed. Evidence that unidirectional rolling sliding contact can result in directional mechanical properties of the deformed layer is given. Results obtained under the test conditions used show that RDR has a beneficial effect on the wear rate of pearlitic rail steel. Multiple short RDR resulted in the lowest wear rate, less than half the unidirectional value.

The effect of rolling direction reversal on fatigue crack morphology and propagation

Abstract The effect of repeated rolling direction reversal on crack morphology, propagation and RCF life of rail steel is investigated in this paper. It has been established that reversal is of beneficial effect on RCF life. This effect is a function of the reversal regime. An optimum reversal is achieved when the number of rolling cycles per reversal is between 25% and 37% of the uni-directional RCF life cycles of the material. Under such a reversal regime, the RCF life is almost doubled. Rolling direction reversal appears to affect RCF cracking propagation as well as morphology. A new mechanism, the ‘variable crack face friction mechanism’, is proposed to explain this effect.

The effect of strain hardening on shakedown limits of a pearlitic rail steel

Abstract The load limit below which a material remains elastic for the first application of load is known as the elastic limit. However, material in a railway rail is loaded not once but many times and the load limit below which the material remains elastic in the steady state is known as the elastic shakedown limit. This limit is higher than the elastic limit, and is the basis of tribological design. Such limits are known for solids which have hardness unvarying with depth. However, techniques of heat treatment or coating lead to a variation in hardness with depth for which the effect on the shakedown limit has only recently been established. In the present work the influence of strain hardening on the shakedown limit of a pearlitic rail steel has been analysed. The data used were taken from an experimental study by Tyfour et al. (1) in which wheel-rail contact was simulated by rolling-sliding line contact. The results indicate that the load bearing capacity increases to about four times that without strain hardening in line contact, and even more in point contact.

Role of impact angle reversal on impact wear of mild steel

Impact wear of mild steel has been studied in light of impact angle reversal. An in-house built, specially designed test rig has been used to facilitate test conditions, including impact angle reversal. Metallographic examination and scanning electron microscopy were used to analyze the morphology of impact surfaces. Results showed lower impact wear under condition involving impact angle reversal for the whole range of impact angles. Furthermore, multiple impact direction reversal showed significant impact wear reduction. Behavior has been explained in terms of failure due to plastic deformation accumulation.

Effect of moving sand as a ballast contaminant on rail corrugation: field experience

Research work on rail corrugation phenomena is mostly focused at trying to explain the mechanism by which these corrugations are formed and to some extent, the environmental consequences represented by the noise of trains passing corrugated sections of track. In this work, it is the effect of environment as a corrugation catalyst that is studied. In certain countries of the Middle East, sections of some railway lines pass through dry desert areas where the dominant sand storms keep fine sand particles in continuous motion. The work presented in this paper describes rail table corrugation phenomena observed in sections of Aqaba Railway Company track in light of the effect of moving sand as a ballast contaminant. Sand fine particles hinder the motion of ballast gravels by filling the voids between these gravels to result in increased track stiffness and damping.

Sliding wear mechanism of ductile materials – Effect of sliding direction reversal

The work presented in this paper tries to shed more light on the mechanism by which ductile surfaces fail and leave the contact surface during loaded pure sliding contact. An extensive experimental program was designed aimed at exploring the role of plastic shear strain accumulation in surface failure. Reversing the direction of strain during testing was the main variable which was facilitated by reversing the sliding direction. Changes in structure deformation morphology and accumulated plastic strain were analyzed. The effect of different sliding direction reversal regimes during testing, compared to unidirectional sliding to the same sliding distance, was thoroughly investigated. Results came to support that plastic strain accumulation is responsible for contact surface failure and, as a result, material loss from the ductile surface during sliding. It was evident, under the test conditions used, that reversing the sliding direction at different predefined sliding distances has resulted in delaying surface failure, resulting in lower wear loss compared to that found under unidirectional sliding. Multiple strain direction reversals resulted in higher beneficial effect in delaying failure. Furthermore, the earlier the sliding reversal is carried out, the better its effect of delaying failure. Findings have been explained in terms of plastic strain accumulation that leads to failure of the surface layer after reaching a certain strain to failure limit.

On the models of erosive wear of ductile materials

As the mechanism by which material is lost from ductile surfaces during solid particle erosion is still a matter of scientific debate, the work presented in this paper is aimed at trying to shed more light on the mechanism by which material is detached from ductile surfaces during solid particle erosion. Moreover, validating some of the most widely accepted models that predict erosive wear rate will form part of the paper. A specially designed test rig was used to facilitate test condition of an extensive experimental program. Results of the test program showed that plastic strain accumulation is largely responsible for material loss from ductile surfaces, even at cute impact angles. The key to this finding is the drop of erosive wear upon impact angle reversal indicates. It has been shown that none of the most widely accepted models of erosive wear could explain the result obtained under condition of impact angle reversal.