4901 - ASSESSING FATIGUE CRACK GROWTH IN RAILWAY AXLES

Railway axles are safety critical components. Designing in failsafe mechanisms is very difficult and the safety of the component is determined though a good understanding of the structural integrity and through effective management policies. This paper first reviews from a historical viewpoint the development of the design and management of railway axles, and then outlines state of the art methodologies to be employed in the successful management of railway axles. Advancements in fatigue fracture mechanics have permitted the development of statistical techniques which enhance the understanding of axle failures which occur relatively infrequently. Because of the extremely low number of in-service failures, there exists a possibility to increase the NDT inspection interval, and to even abandon certain inspection procedures, such as far-end ultrasonic scans, completely. There is some evidence to suggest that inspection procedures which involve a degree of disassembly of the axle actually introduce a risk which offsets the benefit associated with crack detection. 1 HISTORICAL PERSPECTIVE Railway axles were one of the first components which were to subjected to large numbers of repeated cycles. Because of the loading geometry the axle is in approximately 4 point bending, and each time the axle rotates, an element of material on the surface of an axle goes from a compressive state to a tension state of equal magnitude. The large number of rotations that were experienced by early axles led to the first reported fatigue failures in which failure was observed at stress levels well below the yield strength of the material. These failures inspired the work of Wohler [1] who discovered that below a limiting stress level the material could survive repeated cycles indefinitely. This stress level is commonly termed the fatigue or endurance limit. Current design standards all share common origins traceable back to Reuleaux, who was a German engineer and Professor. In 1861, he published, in German, The Constructor: A Hand-Book of Machine Design, which was enlarged in three subsequent editions. The forth edition was translated by Henry Harrison Suplee, and published in Philadelphia, USA in 1894. More recently, axle design guides have begun to converge. Europe has adopted common standards, EN 13103:2001 & EN 13104:2001, respectively for trailing and driven axles. These design guides are very general and accommodate allowances for a wide range of designs. Newer designs employing features such as inboard journal bearings and hollowed out axle centres also need careful attention when assessing the design for its susceptibility to fatigue. Fatigue failures in railway axles are generally extremely rare. In the UK for example, axles fail at a frequency of 1-2 per year (average taken over the last 30 years). When 1 Corresponding author: s.Hillmansen@imerial.ac.uk compared with the number of rail breaks, which are of the order of several hundred in the UK per year, the investigation of failure mechanisms of railway axles rightly commands a low priority. Even though axles are statistically very safe, an industry exists to inspect axles at regular frequencies using ultrasonic and magnetic particle inspection techniques. Ultrasonic inspections occur relatively frequently and involve passing an ultrasonic sound wave into the axle and then measuring the reflections. The results are compared with a standard reflection trace measured in a structurally sound axle and an assessment is made of any deviations. The more sensitive magnetic particle exams are performed at major wheelset overhauls in which wheels and other components such as brake disks are completely removed from the axle allowing a thorough exam of the axle’s surface to take place. Ultrasonic inspection frequencies are determined by computing the time taken for a crack, which can be detected with a good degree of certainty, to grow to failure. The inspection interval must be less than this, and usually is a fraction (1/3) of this time to allow the next inspection an opportunity to detect the crack should it be missed during the first inspection in which it becomes visible. Because of the nature of the problem, the probability of a fleet of axles containing even a single defective axle is quite low. The operators of the detection equipment are therefore presented with a large number of axles with only a very small percentage with defects. There are added human factors which put the operator in a disadvantage when faced with the large number of axles which presumably pass the ultrasonic exam. Furthermore, because some of the ultrasonic probes require that the axle box cover is removed, additional risk is induced through the possibility of failing to correctly reassemble the axle box, or through the introduction of contamination into the bearing housing. Axle box failures are also very serious and occur more frequently than axle failures. The safety benefit of ultrasonic inspections could therefore be completely countered by the additional risk introduced due to the procedures followed during the inspection. The magnetic particle exams which occur at major wheelset overhauls are very sensitive to the detection of surface cracks. However there is a case for advocating by default the withdrawal of the axle from service. This is especially the case when the axle design is simple. The cost of the replacement axle may be of the same order as the cost of performing the magnetic particle exam. In summary, because of the safety critical nature of railway axles, considerable experience has been developed over many years in the design, operation and management of axles. In the forum of this conference it is acceptable to advocate a relaxation in inspection methodology, but in reality, the possibility of railway administrations adopting such a measure, especial so in the safety conscious UK, is unlikely in the foreseeable future. 2 FATIGUE DESIGN OVERVIEW Successful fatigue design is dependent on an understanding of the material properties, the input loads, and how the structure responds to those input loads. Because of the interrelationship between each of these three design inputs, the degree of certainty to which fatigue behaviour may be predicted is dependent on the design input with the greatest degree of uncertainty. The first necessary task in any attempt to improve the fatigue design is the identification of the design input of which least is known. Refining a stress analysis computation in a finite element package may improve the precision of the result from ±5% to ±1%. This improvement is unproductive if the uncertainty of the boundary conditions (input loads) is ±10% for instance [2-3].