FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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EVALUATION OF ULTRASONIC METHODS FOR THE DETECTION AND SIZING OF
REAL DEFECTS IN THE BASE OF RAILS
l. Picke, and J.F.
National Resea-tc/i Council Canada, Bsucii^ iv iHe , Qunhcc
J.-P. Monchalin
Encïgij, M-ôiei and Reiou-ïcciJ,
Ottawa, Canada
Abstract
The presence of a seam or a crack in the base of rails is not uncommon and
can, in some cases, lead to catastrophic failure if their depth should exceed
= 0.5 mm (0.020 in). Such flaws should be detected during the production
stage and it is required that the evaluation be done automatically in a
continuous way and, of course, be nondestructive. We have carried out a
survey of the different ultrasonic techniques which were applicable to this
specific problem. In particular, the possibility of using surface (Rayleigh)
waves was investigated, and compared to the more classical bulk wave
pulse-echo techniques. Both artificial (machined) flaws and real flaws were
studied. In the case of real flaws, results were found to be sensitive to
whether the crack was closed and/or filled, especially when Rayleigh waves are
used.
1 INTRODUCTION
An important task of NDE is that of detecting and sizing surface breaking
cracks. Such defects act as stress concentrators and depending on their
origin, nature, and shape may lead to catastrophic failure by fast fracture.
An example is giv&n by railways in Canada where the danger is enhanced since
the rails are submitted to particularly harsh weather conditions. For this
reason Canadian railroads impose strict specifications on newly manufactured
rails; the head and the web [1] are inspected but also the base, where
experience shows that the presence of surface flaws can be highly hazardous.
Defects in the base occur usually in the form of seams and cracks, along the
principal axis of the rail and located near the center. The main criterion
for acceptability is that their depth be less than 0.020 in (0.5 mm).
Different techniques are used and surface flaws are detected by visual,
dye-penetrant, magnetic particles and ultrasonic techniques. Ultrasonics has
often proven to be a very powerful tool for NDE, however, for surface flaws, a
reliable [2] inspection procedure is not available as yet. Various methods
have been proposed which may give accurate results for artificial defects but
their relevance [3] in characterizing real defects can be debated. Here, we
describe the work that we have done to appraise different techniques which use
standard NUT equipment applicable to plant conditions and purposely leave
aside more sophisticated but less easily applicable techniques. In