AXLES USING NON-DESTRUCTIVE TESTING
A company for the maintenance of railway vehicles in Slovenia carried out, in collaboration with the Institute of Metal Structures (IMK), Ljubljana, testing and relevant analyses of the state of carriage axles. In order to detect cracks at carriage axles and to evaluate the quality of axles surfaced, ultrasonic and magnetic particle testing methods were used. Ultrasonic testing with normal and angle probes was used to detect internal cracks in the carriage axles using the distance gain size (DGS) diagram and the distance amplitude correction (DAC) method for reference specimens. Additionally, the magnetic particle testing method, with an apparatus permitting the detection of transverse and longitudinal cracks, was used to search for surface cracks. The tests performed at numerous railway carriages provided results, which make it possible to predict a premature axle fracture in further service. The non-destructive testing methods used permitted the detection of the cracks present, causes for the crack initiation were established, and the risks of crack propagation were evaluated. A critical evaluation of the carriage axles in terms of crack initiation and size is given. The prediction of the remaining life of an axle with reference to the potential propagation of the existing cracks is given taking into account the load axle acting on the axle and the axle age.
Magnetic particle inspection equipment for testing railway wheelsets
As a matter of routine, railway axles which have been in service are periodically tested for possible surface-breaking cracks. One method of testing is the highly sensitive magnetic particle inspection technique which detects any fatigue cracks on the axle
Review of statistical methods used in quantifying NDT reliability
This paper reviews a number of statistical methods used to quantify the reliability of flaw detection and sizing by NDT from experimental data. For flaw detection, different methods are applicable according to whether the outcome of a trial is recorded as: a) a binary variable, i.e. hit/miss data (typical of enhanced visual techniques such as MPI), or b) a continuous variable, i.e. a signal amplitude relative to a given threshold (typical of ultrasonics or eddy current testing). In the former case, the method of analysis is further divided into methods that group the data, and methods that treat it as a whole to calculate a probability of detection (POD) curve. In any event there are restrictions on the data set that can make this method cumbersome for experimental application. The second method (called 'response versus size' or â versus a) requires the signal amplitude and a threshold for detection. The POD is then produced from a set of data that contains more information than the hit/miss method, and this can allow a smaller number of flaws to be used. For flaw sizing, attention is usually focussed on the amount of undersizing that can be allowed, which can then be used to set realistic acceptance criteria. However, it may also be important to quantify the amount of any oversizing, since this can cause unnecessary repairs or plant shutdown. Examples of the use of each method are given, and specific applications carried out recently by TWI to support the inspection reliability. The paper also discusses the meaning of what is meant by a ‘reliable’ inspection.