CHAPTER – 8

 

(Chapter  revised vide ACS NO. 11 OF 2002)

 

LIMITATIONS OF ULTRASONIC FLAW DETECTION OF RAILS

 

PREAMBLE

 

          Every scientific method/technique/equipment functions on certain principles and its applicability depends upon fulfillment of preconditions necessary to be satisfied.  It accordingly implies that USFD examination is based on certain guiding principles and its flaw detection success depends upon thorough understanding of the governing factors.

 

          Thus, the limitations being mentioned are not per-se the deficiency of the USFD technique rather in the existing arrangement under field conditions the equipment utilised incorporates facility only for specified defects.  This aspect may be kept in view and the technique is to be pursued accordingly.

 

          Limitations in respect of rail examination considering the various arrangements presently available have been elaborated below.  It may also be mentioned here that limitations with regard to AT welding i.e. defects in web foot junction half moon cracks, vertical defects in web portion etc. are no longer limitations of the USFD equipments since in the newly developed equipment special probes have been provided for detection of these defects.

 

8.1 (i) To detect the defect efficiently, ultrasonic beam is to be directed towards the flaw perpendicularly, otherwise, the reflected beam may not be received by the receiver crystal, resulting in absence/reduction in amplitude of flaw signal in the CRT.  Cracks normally have facets and hence even under misorientation provide reflecting surfaces leading to flaw indication.

 

(ii)             For detection of defects in rails, probes having incidence angle 0°, 70° (F), 70° (B), 37° (F), and 37° (B) have been provided in the USFD trolley.  The angles have been chosen in a manner so as to detect defects which are generally observed during service and have been the cause of rail fractures.

 

The section of rail which is scanned by each type of probe has been indicated in Fig. 2(A), 2(B), 2(C) and 2(D)

 

For detection of defects originating from Gauge Face Corner, a dedicated test set-up has been developed.  This set-up incorporates three 70° probes covering approx.  The full width of the rail head and a set of two 45° probes.  The area scanned by this arrangement is shown in Fig.2(E). A defect located at 5mm from the corner is detectable using this equipment.

 

                    All commonly observed defects in rails are detectable by the above arrangement.  In the event of gross mis-orientation of defect at times it may not be amenable for detection, however such situations are rare.

 

(iii)           Severe pipe in the rail may give indication of flaw echo by 0° probe, But in case of hairline or fine central shrinkage (pipe), negligible drop occurring in bottom signal may remain unnoticed by the USFD operator. (Ref Fig-23).

 

(iv)            For detection of bolt hole cracks, 37° probe have been provided.  This is because the cracks emanating from bolt holes are generally oblique and propagate in the zig-zag manner.  However, bolt hole cracks are also detectable by using 0° probe since they obstruct the path of sound waves and lead to drop/loss of back wall echo.

 

37° probes have been provided both in forward and backward direction.  Forward probes detect defects in second and fourth quadrant where as back ward probes detect cracks in 1st and 3rd quadrant.  At fish plated joint, as shown in figure 16, if the cracks are not favourably oriented detection may not be possible.  Similarly, if the cracks are propagating vertically downwards or upwards, detection is not possible.  In case of double rail testers which do not have 37° probes, detection is possible by 0° probe.

 

Under such situations if the cracks are so located that they are unable to be scanned by 0° probes, initially due to smaller size, such cracks may not be detected.

 

(v)               If sensitivity of the machine is poor or battery gets discharged the operator may miss the flaw signal.  Hence, it is essential to ensure full charging of the battery.

 

(vi)            The ultrasonic probes used    in the rail testers have a frequency of 4MHz (longitudinal wave) and 2 MHz (transverse waves).  Therefore, cracks lesser than 0.8mm size cannot be detected by the present arrangement.

 

The sizes & frequency of the probes employed in the single rail tester/double rail tester are as under.

 

S. No.

Probe type

Size of crystal

Shape of crystal

Frequency

1.

0° (Double crystal)

18mm or

18mm x 18mm

Circular square

4MHz

2.

70° (Single crystal)

20mm or

20mm x 20mm

Circular sqaure

2MHz

3.

37° (Single crystal)

20mm or

20mm x 20mm

Circular sqaure

2MHz

 

(vii)          Rails having rust, pitting, hogging, battering of rail end, misalignment of joints, scabs, wheel burns and other surface imperfections restrict proper acoustic coupling between probe and rail table and may not permit detection of flaws.

 

When ever such defects are encountered, loss of back wall echo or an alarm signal is obtained.  This indicates that defects if any below these patches may remain undetected.  Under such circumstances hand probing may be done.

 

 

 

 

 

8.2            Testing of SEJs, CMS crossing, points and crossings

 

           Testing of these components is accomplished in the same way as rails.  However due to its specific shape near the nose, it is difficult to move the trolley for testing and achieve accustic coupling.  Therefore except the stock rail, the balance portion is not amenable for detection by USFD trolley.  Under such circumstances, hand probing is required to be carried out according to the procedure laid down in the manual for points and crossing or in the USFD manual (chapter 11 & 12).

 

 

Since the testing of SEJ, Points and crossings specially in the nose portion poses practical problems during USFD examination of the track, it is essential to carry out thorough testing of these items before they are put in to service and also before they are taken up for reclamation.

 

 

8.3            Testing on sharp curves gradients, slack gauge etc.

 

The USFD trolley has been designed to operate under normal conditions of gauge.  In the event of dimensional variations in the gauge and also at sharp curves it is possible that the probes are not properly contacting the rail surface.  This is indicated by loss of backwall echo or also by alarm provided in DRTs for backwall drop.  Wherever it is not possible to ensure proper acoustic coupling due to these reasons, testing by hand probing or by single rail tester may be resorted to.  Acoustic coupling needs to be ensured under all circumstances to detect the flaws.

 

II      The existing figure no. 2 given in the list of  figures may be replaced by figure 2(A), 2(B), 2(C), 2(D) & 2(E).  Descriptions of these figures is given below:

 

(i)               Figure 2(A) Area covered during USFD examination by normal (0°) probe.

(ii)             Figure 2(B) Area covered during USFD examination by 70° probe.

(iii)          Figure 2(c) Area covered during USFD examination by 37° probe.

(iv)            Figure 2(D) Area covered during USFD examination by 70°, 1.25 MHz probe in flange testing of AT welds.

(v)              Figure 2 (E) Area covered during USFD examination by gauge face corner detection equipment.