ROLE OF UST ON INDIAN RAILWAYS

OBJECTIVES

To:

• Understand the principle of ultrasonic testings

• Know the technique of testing.

• Become familiar with standard & codes.

• Learn how to interpret the indications.

INTRODUCTION • Ultrasonic: The science of using high frequency

sound waves for inspection of metal pieces or components in IR

• UST of axles has been introduced sometimes in 1950s as a tool for preventive maintenance to ensure safety in running of trains

• An excellent NDT for detecting & locating internal defects

• Provides quantitative information regarding thickness, depth & size of a discontinuity present in a: – metallic materials – non-metallic materials

• Has restricted applications on ASS & CI

CLASSIFICATION OF WAVES

SOUND WAVES

SUBSONIC (˂ 20 Hz)

SONIC (20-20,000 Hz) [Audible Range]

SUPERSONIC ˂ 20,000 Hz

ULTRASONIC 20 KHz- 500 MHz (20 KHz-15 MHz in practical use)

HYPERSONIC ˂ 500 MHz

Ultrasonic wave: Sound waves of frequency more than 20,000 Hz (20 KHz) & less than 500 MHz

PRODUCTION OF U- WAVES

• Some natural & artificial crystals produce U- waves when alternating voltage is applied on the faces of suitably cut crystal.

• The crystals vibrate with a frequency equal to that of the applied electrical pulses.

• These vibrations are the sources of the U- waves • Commonly used crystals producing U-waves: • Natural crystal:

– Quartz – Sodium potashium tartrate

• Artificial crystal: – Barium titanate – Lithium sulphate – Lead zirconate titanate (PZT).

NEED FOR U- TESTING OF AXLE • Mainly employed for detection of internal

flaws where as the service failure of axles is mainly due to fatigue cracks originating from the surface.

• The surface crack can be detected by MPI, DPT, etc. But in axles with press fitted wheels, gears & bearings, etc, the location of wheel seat, gear seat & bearing seat respectively remain inaccessible to any other method of testing & as such U-method of testing is most suited.

RECOMMENDED FREQUENCY FOR DIFFERENT APPLICATION: UST

Recommended Frequency

Different Application

0.5 MHz - 1 MHz Examination of CI & steel casting & materials of coarse grained structure

2 MHz Examination of steel plates

2MHz – 4 MHz Examination of forging, weldment && materials of fine grained structure

6 MHz – 10 MHz

For fine grained material where higher resolution to detect flaws is required

7MHz – 10 MHz Special application

TRANSDUCER/PROBE A search unit Transducer/Probe: A search unit containing piezo electric device that converts electric energy in to mechanical energy (sound) & then converts sound back to electric signals displayed on CRT

PRINCIPLE: ULTRASONIC TESTING

• A beam of U-waves, generated by means of piezo electric effect, is set up on the surface of the component through a couplant.

• The waves propagate into the material to the opposite surface with some attenuation & reflected & returns to the transducer

• Any discontinuity in the path of waves scatters the waves & are reflected back sooner from the defect free part

PRINCIPLE: ULTRASONIC TESTING

• The transducer at once converts U-waves into electrical energy.

• The energy reflected from the flaw and that from the back surface, having travelled different path lengths, are indicated on the CRT screen. CRT screen is divided in to 10 equal divn along X-axis & 5 equal divn along Y-axis

PRINCIPLE: ULTRASONIC TESTING

COUPLANT

• The couplant couples the transducer to the surface of the test specimen to ensure efficient sound transmission from transducer to the specimen

• Done by smoothing out the irregularities on the test surface & by excluding the air between the transducer & the test specimen

• Commonly used couplants are Water, Glycerine, Machine oil, Grease, Paraffin etc.

SELECTION OF COUPLANT

• For testing purposes, usually various grades of oil, grease, glycerine are used:

Surface Couplant

Very good surface Thin m/c oil

Rough surface Mixture of machine & cylinder oil

Horizontal & Vertical surface

Heavy oil/ Grease.

PROPERTIES OF A GOOD COUPLANT

• Easy available

• Easily applicable

• Good adherence

• Anti toxic

• Anti corrosive

• Free from foreign particles

• Homogeneous

TESTING METHOD:U-TESTING

Probing being done:

0 2 4 6 8 10 plate

crack

initial pulse

crack echo

back surface echo

SKETCH OF U-SCAN

CALIBRATION STANDARDS

IIW BLOCK IIW BLOCK

CALIBRATION OF UFD • UFD is calibrated for the following parameters:

• Linearity of time base: – Position of successive bottom peaks obtained from a std test

block of known length for all ranges (up to 500 mm)

• Linearity of amplitude: – Linearity for each frequency range by comparing echo

amplitude obtained with different gain settings & the gain is re-set to obtain echo at half the previous scale height

• Checking of resolution: – The probe is placed on IIW block using a time base range of

100 mm & the resolution is calculated by taking the echoes from the surface at 85 mm & 91 mm

CALIBRATION OF UFD • Max penetrating power:

– The probe is placed on the surface of the perspex disc inserted in IIW block to obtain the no. of multiple echoes from the bottom of the perspex & amplitudes of the echoes is determined

• Checking of dead zone: (Dead zone-This is the distance below the surface of a material in which the defect can not be detected) – The probe is placed on the reference block & corresponding

echoes at a distance of 5 mm or 10 mm of steel

• Checking of sensitivity: – The probe is placed on opposite end of a reference block of

406 mm long having a flat bottom of 3.2 mm dia at a mid radius to depth of about 25 mm perpendicular to one end face & the gain is adjusted to obtain the echo of 12.5 mm amplitude & the amplitude of the bach echo is seen whether it is at 25 mm by scanning on the axle end faces

DIFFERENT TYPES OF AXLE

• Various type of axles used in railways: – Loco

– Carriage

– Wagon

– EMU

– Tower wagon

– Crane, etc

CLASSIFICATION OF AXLE

On Indian Railways the different type of axles in use have been classified into four following categories for U- testing purposes with regard to different techniques:

• Type ‘A’

• Type ‘B’

• Type ‘C’

• Type ‘D’

RECOMMENDED TIME FOR U-TESTING OF AXLES Type of axle Testing technique Rec time per

axle in mts Class

Mandatory Conf Mand Conf

Type ‘A’ Axles with wheel seat outer

most or higher or equal in dia

than other portion of the axle

FE FE NELA TD HA

14 80 WP. WG,YP/YG, WDS4B Loco axle

Type ‘B’ Axles with journal outermost & lower dia than wheel seat & body of the axle

FE NELA

FE NELA TD HA

30 75 C & W, EMU

Type ‘C’

Same as type ‘B’ but axles

with gear

FE NELA

FE NELA TD HA

35 80 WDM1, WDM2, Electric loco

Type ‘D’

same as type ‘B” but with

protrusion at journal end face

FE (15-20mm dia probe) NELA

FE NELA TD HA

40 85 BEML, ICF coach axle

PERIODICITY OF U-TEST • All types of axle passing through the shops for POH

or for further attention are tested in shops as per different periodicity stipulated by RDSO

• In every code of procedure some standard peaks are specified in all the techniques

• While testing of an axle, several peaks in some of the axles & a few in other axles are also appearing in addition to the standard peaks

• It becomes a problem for the operator to take a decision in such cases

• In this case the operator follows the directives given in the respective code of procedure that an axle showing a prominent signal other than those shown in the pattern has to be withdrawn from service.

PERIODICITY OF U-TEST

Type of Axle Periodicity (month)

Carriage axle 18

Wagon axle 48

EMU coaches 6

Diesel & Electric

Locomotives

12 & 6 months depending

on run

C & W At the time of dropping &

ROH in sick line

DIFFERENT TESTING TECHNIQUES There are three basic techniques applied for UST

of axles recommended by RDSO:-

• FE scanning/straight beam technique (using normal

probe): Through scanning of whole length of the axle. Mandatory for all type of axles

• NELA (scanning from axle end & in to the nearer wheel seat

using angular probe lower than 200). Mandatory for B, C & D type of axles

• HA (scanning across the dia using angular probe with an angle

higher than 350). Mandatory where indicated by RDSO & confirmatory for the findings in FE & NELA

• An additional technique i.e.TD. (using normal probe).

Not mandatory in any case. Confirmatory technique in case of doubt

CONTROL SIGNAL • One signal is selected for a particular scanning

technique & is called ‘Control Signal’

• Testing is done at specified height of control signal as mentioned in the code

• Control signal is taken from a far end fillet for FE & inner fillet for NELA & HA

FAR END SCANNING • The main purpose of FE is to test the whole

length of axle by probing from each end face • Employed for type ‘A’ axle • May be applied for type B, C,& D axle but this will

not be fool proof method for such type of axles • UFD: Calibration for the particular range:

– The time scale is calibrated to 250/300 mm per msd acc to the length of the axle using a std bar of 500 mm

• Probe used: 2.5 MHz; 20/25 mm dia • The probe is placed perpendicular on the cleaned

end of the axle keeping it 2-5 mm away from outer periphery with slight rotatory movement using soft grease as couplant & height of the control peak as given in the code of procedure supplied by RDSO is adjusted

FAR END SCANNING • U-waves travel through the whole length of the axle &

are reflected from its end, change of sections or from defects, if present

• The probable reflection in axle may be as follows:

– End of the axle

– End of the journal

– Wheel seat radius

– Stress relieve groove

– Gear seat radius

• All the peaks are observed carefully & compared all the peaks given in the respective codes

• Scanning of the full axle is done from both the end faces

TESTING OF AXLE IN SERVICE

Far end scanning

FAR END SCANNING

FAR END SCANNING TRACE PATTERN OF WAGON AXLE (DRAWING NO. SK-69601)

NEAR END LOW ANGLE • Useful in checking cracks in type

B, C & D axles • Used:

– to conform the findings in FE – to detect crack in raised wheel seat

& gear seat etc of an axle

• UFD: the time scale is calibrated for 100 mm per msd using normal probe.

• Probe used: 2.5 MHz; 20/25 mm dia

• The normal probe is fitted with perspex wedge of angles (between 50 to 200) as mentioned in the respective codes

NEAR END LOW ANGLE

• The perspex wedge is placed on the axle end face directing the central beam towards the inner fillet of wheel seat

• In flaw free axle, one back peak is obtained which is taken as reference peak (control peak)

• By this technique, crack in the inner wheel seat radius is detected.

• Crack appearing at the end of near wheel seat radii from the probing end can not be detected as the same will be under shadow zone.

LOW ANGLE SCAN TRACE PATTERN FOR WDG2 DIESEL LOCO AXLE TO DRG NO SK.VL-034 PROBING FROM THE GEAR END

HIGH ANGLE • Used to:

– check the findings obtained in FE & NELA – find out cracks which is not accessible by NELA

• UFD: The time scale is calibrated for 50 mm per msd of shear wave using normal probe.

• Probe used: 2.5 MHz; 20/25 mm dia • The perspex wedge is placed on the journal & body of

the axle with probe index marked at distances given in the respective codes from collar fillet & wheel seat inner fillet

• The probe is moved forward & backward longitudinally from the mean position up to a distance given in the code

• The probe is moved circumferentially (curved surface) on the axle directing the central beam towards the wheel seat inner & collar fillet

HA SCANNING

HIGH ANGLE SCAN PATTERN

TRACE DELAY • Used for conforming the findings in FE • Probe used: 2.5 MHz; 20/25 mm dia • UFD: Calibrated for 500 mm

• 0-500 mm: 1st Range • 0-500 mm: 2nd Range • 0-500 mm: 3rd Range • 0-500 mm: 4th Range • 0-500 mm: 5th Range

• In FE scanning long testing range (about 2500-3000 mm) is required & as such peaks from various reflectors from the axle appear closer. The operator may face difficulties in evaluating the oscillograph pattern. Therefore, this technique is applied to examine the axle in parts of 500 mm or less as desired.

TRACE DELAY • The tested axle is examined in

parts (4-5 parts depending on its length) by longitudinal wave using normal probe for confirmation of the flaw signals obtained during FE

• The couplant is applied on its end face & probing is done

• The first back peak is shifted to 10th & CRT is observed for any additional peak

• Similar test is done on all the parts.

DIFFICULTIES FACED DURING UST OF AXLES IN SERVICE • Surface of the axle: End faces with sharp edges, dent &

hammer marks etc must be removed by filing or by using emery paper before probing

• Body of the axle: Body of the axles(C&W axle) containing/contaminated with rust, scales, dirt, grit, paint, pit must be cleaned by using k.oil/filing/emery paper before conducting HA technique

• Extra holes drilled on the end face: Sometimes extra holes are drilled on the end face of the axle after aftre plugging the oversized holes which minimises testing areas. Scanning by lower dia probe to be used

• Geometry of the axle: – Electric & diesel axles have no. of fillets & std peaks from the

fillet locations appear very closer & as such TD technique must be employed.

DIFFICULTIES FACED DURING UST OF AXLES IN SERVICE – Axles with protruded ends (BEML axle) have limited

probing area & as such lower dia probe to be used for FE & perspex wedge to be kept carefully on the narrow portion for NELA

• Coarse grain axle: – Coarse grainedaxles (due to improper HT) to be tested

with 2.5 MHz. Absence of depleted back peak amplitude associated with greasy patterns received from all the available probing positions makes the axle suspected to be coarse grained

– Such axles to be tested with 1.25 MHz probe. If bak peak amplitude improves with absence or decreasing greasy pattern, the coarse grained structure is confirmed & axles to be drawn from service

CRITERIA FOR ACCEPTANCE & OTHERWISE

• The axle is rejected in which permanent flaw signal is found during scanning by FE, NELA & HA technique

STAMPING ON TESTED AXLES

• All tested axles are stamped on the inner face of the hub with following details:

– UT.02.1.01. MM/YY/XXXX

• UT: Ultrasonic test

• 02: Code for Railway

• 1: Code for type of workshop

• 01: Code for place of workshop

• MM/YY: Month & year of testing

• XXXX: Initial of person testing the axle

REPORTING & RECORDING OF TESTED AXLE

• The report of tested axles (Passed/Rejected) is recorded in the shop/shed register

ADVANTAGE:UST • Most powerful tool for ensuring safety

• Fast & reliable method

• Extremely sensitive at high frequency

• Locates the position & size of the flaw

• Gives indication that the proper HT has been given or not

• In-situ adaptability

• Portable unit

• Both ferrous & non ferrous components can be tested

• Can be penetrated up to 10 meters length at a time

LIMITATION & APPLICATION LIMITATION:

• Irregular & complicated design is difficult to test

• Testing surface must be cleaned & flattened

• High skillness & experienced men required to operate UFD

APPLICATION:

• Inspection of castings, forgings, weldments, rolled products & components in service

• Routine inspection of locomotive

axles, wheel, crank pin, suspension

bearing, etc

• Inspection of rail without dismantling

UST OF L0COMOTIVE WHEEL:IRS-R-34/99; CLAUSE-

15, APPENDIX-’A’ IN INDIAN RAILWAYS

WHEEL

INTRODUCTION • For detecting internal discontinuities in Rim &

Hub of the wheel

• Reference standard used for calibration & sensitivity setting:

– Shall be 3.2 mm dia flat bottom hole, drilled perpendicular to the Rim face & to a depth of 25±2 mm at the used thickness of the Rim

REFERENCE STANDARDS FOR UST OF RIM Reference standards Alternate Reference standards

UST: CALIBRATION FOR RIM • Probe: 2.0-2.5 MHz ; 20 mm dia

• Equipment: RDSO approved UFD

• Calibration: • 200 mm for longitudinal wave (i.e. 20

mm/msd)

• using ref std of a portion of a wheel rim containing readymade defects

• UFD sensitivity adjusted to produce half of full scale reflection(50% height) from either of the reference standards

50% height adjustment from ref std

SCANNING • Wheel is inspected

axially by probing it from both sides by manual or automatic scanning

REJECTION STANDARDS: RIM • Any wheel disc with flaw indication equal or higher

than ref defect i.e. 50% or higher height. If more than three defects of amplitude 30% or above are observed: rejected

UST: Hub

• Wheel passing in rim test: subjected to hub testing

• No reference standards used

• Probe: 2.0-2.5 MHz; 20 mm dia

• Adjust back echo by any gain at 100% by keeping probe on flawless hub.

UST: HUB- REJECTION STANDARD

• Suppression of back echo less than 25% of the full height although there may be no flaw echo

UST: HUB- REJECTION STANDARD

• When the flaw echo more than 50% of the back echo

UST: HUB- REJECTION STANDARD • Flaws coming at more than three locations &

closer than 50 mm from each other although the echo heights are less than 50% of the back echo.

MARKING

• Wheel confirming to the standard should be punch marked as ‘UT’ on the back plate at least 25 mm in height or as mentioned in the drawing