08 TRACK SYSTEMSSUPPLEMENT
FIGURE 7 Comparison between stress intensity factors calculated by FEA and the RAILECT models round the crack front of web to base corner crack (left) and semicircular crack at rail base (right)
of course a time consuming and expensive approach, not suitable in field work, when a quick decision needs to be made once a flaw has been detected and sized by NDE. To address this challenge, analytical
solutions for predicting the severity of a flaw in a rail using fracture mechanics theory were developed during the RAILECT project. This would allow for the development of an algorithm, built into the RAILECT detection system, which could provide the inspector with the necessary information to immediately assess the severity of the flaw. This concentrated on the derivation of mathematical expressions for the ‘stress intensity factor’, K, around the front of a crack in a rail. The stress intensity factor (SIF) is a parameter that defines the stability of a flaw in any material and is a function of the flaw size and the applied load. Once a certain stress intensity value is exceeded, termed the ‘critical stress intensity factor’, KC
, also known as the
fracture toughness of the material, then unstable growth of the flaw takes place leading to catastrophic failure. The stress intensity factor is expressed mathematically by the equation:
and the shape and size of the fatigue cracks at failure were used to assess the accuracy of the predictive analytical solutions developed in the project. In addition, finite element analysis models were created for the tested rail with crack geometries and locations (as shown in Figure 6 on page 31) and the stress intensity factors round the crack front were calculated. Figure 7 shows a comparison of the two
predictions which show a good agreement, and indicate that the analytical solutions could be used to predict the severity of a flaw, (presuming the inspector also has information on the maximum axle weight passing over the rail). Work is underway to develop universal mathematical solutions for any shape and position of a flaw in any shape of rail. The field trials of the RAILECT system on
where, S is the remote applied stress (tensile or bending), a is the maximum crack depth, Q is the shape factor for an ellipse given by the square of the complete elliptic integral of the second kind, and Fb
is the boundary
correction factor which accounts for the influence of the various boundaries (i.e. the geometry of the component). Fatigue tests carried out on sections of rail
European Railway Review Volume 18, Issue 2, 2012
Network Rail’s test track in High Marnham were really successful and showed that the device is portable, light and can be easily clamped onto the rail track. A defective weld is identified by showing clear indications on the phased array scan images, allowing the operator to size, locate and characterise the flaws identified in the weld. The RAILECT system not only received very positive feedback from a large number of European railway companies but was also selected amongst the finalists for the IET Innovation Awards in November 2011, and also for a demonstration in front of a large audience at the Innovation Convention 2011 in Brussels. So far, the RAILECT system has and
continues to attract a lot of interest from the railway industry. Additional research projects on related topics are still being carried out beyond the original scope of the project in order to provide a commercial and marketable solution to the industry within the next few months.
George Kotsikos is Project Manager of NewRail, the Centre for Railway Research at Newcastle University. George is a Chartered Engineer with over 20 years of experience in transportation R&D, gained both in industry and academia. He has coordinated several national and
international transportation research projects, is acting evaluator for the European Commission and Accident Investigator for ERA (European Railways Agency).
Tamara Colombier is a Senior Project Leader in the Non-Destructive Testing Technology Group. Her background is material sciences and she has both an engineering diploma and a master’s degree in this area. Tamara joined TWI in 2008 and has since achieved expertise
and experience in the development of electromagnetic and ultrasonic techniques adapted to specific applications. She is also qualified in many non-destructive methods of inspection and has, within the past few years, worked closely with the railway industry. Tamara was the Project Manager for the second year of the RAILECT European Project.
Angélique Raude joined the Non- Destructive Testing Technology Group, TWI, as Project Leader in December 2005; her current position is Principal Project Leader. Her expertise lies in the research and development of electromagnetic, thermography and ultrasonic methods.
She has been involved in various projects and site inspections for power and transport industries for which she was responsible to design the inspections and apply the techniques on-site. Before joining TWI, she was awarded a post graduate degree in NDT. Angélique was the Project Manager for the first year of the RAILECT European Project.
FURTHER INFORMATION www.railect.com
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