Rail defects and their condition: Causes of defects, Nature of defects in rails, Horizontal crack in head, Vertical-longitudinal split in head, Horizontal crack at head web junction

Rail defects and their condition:-

Rail is the most important and critical component of the permanent way.  Most common cause of rail failure is the fatigue fracture, which is due to imperfections present in the material or due to crack formation during service.  Thus successful performance of rails is based on their resistance to crack initiation and crack propagation. It is therefore essential to identify and classify the defects in rails and in turn initiate corrective action. A brief on nature of defects, their causes and classification is presented below. 


Causes of defects 

The origin and development of such cracks is due to: 

  • Material defects originating during the manufacturing process such as clusters of nonmetallic inclusions, hydrogen flakes, rolling marks, guide marks etc. which may be present in spite of successful non-destructive tests carried out on the rails during  quality assurance examination. 
  • Residual stresses induced during manufacture (cooling, rolling, gas pressing and straightening). 
  • Defects due to incorrect handling e.g. plastic deformation, scoring, denting, etc.
  • Defects associated with faulty welding i.e. gas pores, lack of fusion, inclusions, cracks etc. 
  • Dynamic stresses caused by vertical and lateral loads particularly by vehicles with wheel flats or when the vehicle runs over poorly maintained rail joints etc. 

  • Excessive thermal stresses due to variation in rail temperature beyond specified  limits.
  • Defect location:  In order to study the fractures in rail systematically, they may be divided into the following categories based on their location of occurrence in the rail length: 
  1. Defects emanating from the rail end or reaching the end of the rail.
  2. Defects observed within fish-plated zone.
  3. Defects not covered in (a) and (b).

    Nature of defects in rails:-

 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 rail head 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.

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.


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.

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.

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.

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 can easily detect these cracks. Normal probes provide indication as diminished back wall echo.

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.

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 18-23 degree 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 70 degree probe. When such defects are nearly vertical, they can be detected using additional gain of 10db. Defects lying below scabs/wheel burns can be detected by 45 degree side probing of rail head.

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.

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.

Transverse cracks in rail foot

  Due to localised 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 45 degree probe.





Rail defects and their condition: Causes of defects, Nature of defects in rails, Horizontal crack in head, Vertical-longitudinal split in head, Horizontal crack at head web junction
Defects in Rail












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