Nature of defects in rails

  Nature of defects in rails

(a) Horizontal crack in head:  These cracks run usually parallel to the rail table at a depth of 10-20 mm and may finally split the material layer.  Crushing of the railhead may also be observed in the vicinity of the crack.  Clusters of non-metallic inclusions and abnormal vertical service stresses are the factors responsible for this defect.  USFD can easily detect such flaws.

(b) Vertical-longitudinal split in head:  These cracks run parallel to the longitudinal axis of the rail and are caused by presence of non-metallic inclusions, poor maintenance of joints and high dynamic stresses.  It cannot be easily detected in early stages by USFD due to their unfavourable orientation.

(c) Horizontal crack at head web junction: Such flaws may lead to rail head separation. Contributory causes are wheel flats, bad fish-plated joint, inclusions and high residual stresses.  USFD is sensitive to such defects and can easily detect them.

(d) Horizontal crack at web-foot junction:  Such cracks develop both towards head and foot. They are caused by high vertical and lateral dynamic loads, scoring and high residual stresses.  USFD can easily detect these flaws.

(e) Vertical longitudinal splitting of the web:  It is primarily due to heavy accumulation of non-metallic inclusions and wheel flats.  USFD conducted from rail top can detect it only if the defect is severe and in an advanced stage. Vertical longitudinal defects of minor nature are not amenable to USFD examination conducted from rail top. Probing from railhead sides can detect such defects for which hand probing may be essential.

(f) Bolt hole crack:  Such cracks often run diagonally and may run towards head or the foot. They result from inadequately maintained joints and unchamfered fish boltholes and stress concentration.  USFD (37o probe) can easily detect these cracks. Normal probes provide indication as diminished back wall echo.

(g) Transverse fracture without apparent origin:  These fractures occur suddenly, especially during winter and may emanate from microscopic flaws (embedded or on surface) and are generally very difficult to detect by USFD.  These minute flaws manifest suddenly under severe service conditions or when the fracture toughness values are comparatively low.

(h) Transverse fatigue crack in head:  They resemble a kidney in shape in the railhead and USFD is ideally suited for detecting them.  They are generally inclined at the angle of 18deg-23deg and originate at a depth of 15-20 mm below the running surface.  Mainly hydrogen accumulation and non-metallic inclusions cause this defect.  These cracks are easily detected by 70deg probe.

(i) Horizontal crack at top and bottom fillet radius:  These cracks are caused by accumulation of non-metallic inclusions and high residual stresses introduced at the time of rail straightening.  These are difficult to be detected by USFD.

(j) Vertical ± longitudinal crack in foot:  Such cracks develop from sharp chamfers on the bottom surface of the rail foot.  Cracks occurring in this way are the points of origin of transverse cracks in the foot.

(k) Transverse cracks in rail foot:  Due to localized overheating during FB welding, structural changes in the bottom surface of the rail material takes place which result in a minor crack.  These cracks under the tensile loading give rise to brittle fracture.  Such defects are not detectable by USFD. Transverse cracks originated from AT welds in the rail foot grow as half moon and are detectable by 45oprobe.