ABSTRACT There is a need to analyze locomotive wheels for flank cracks in a non-destructive manner in order to prevent catastrophic failures. Flaw, shape, and size are desired parameters in estabfishing the cp/ality of cormnercial tires. A variety of defects such as voids, inclusions, surface and internal cracks, or the like, must be discerned in order to prevent failure. This paper exhibits and compares the benefits of a number of different techniques used for flaw detection. Non-destructive evaluation (NDE) techniques used are magnetic particle inspection, dye penetrant, eddy current, electro-magnetic acoustic transducer (EMAT). The techniques vary in their ability to ascertain the flaw characterishcs. Using a non-contact sensor such as the EMAT, to scan the wheels in an automated manner offers greater inspection speed at lower manpower. This paper reviews the basic concept of EMATs, introduces a recently developed technique for simulating EMAT performance by Finite Element calculation and features bench top results of waveform acquisition and signalto-noise ratio dependence on lift-off. Next presented are calibration results for spark -eroded flaws in wheel sections for a variety of locations and sizes. Finally data are on flaw detection in a railroad service facility on several locomotives with wheels spinning at speeds up to 40 meters/minute. Results for both artificial and actual flaws are shown

B R Tittmann

S Jayamman

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PVP-Vol. 484, Recent Advances in Nondestructive Evaluation 
Techniques for Materiel Science end Industries 
July 25-29 ,  2004, San Diego, California USA 
PVP2004-2833 
Locomotive Wheel Inspection with EMAT Technology 
B.R. Tittmann and S. Jayamman 
Department of Engineenng Science and Mechanics, Pennsylvania State University, 
University Park, PA 16802 
ABSTRACT 
There is a need to analyze locomotive wheels for flank 
cracks in a non-destructive manner in order to prevent 
catastrophic failures. Flaw, shape, and size are desired 
parameters in estabfishing the cp/ality of cormnercial tires. A 
variety of defects such as voids, inclusions, surface and 
internal cracks, or the like, must be discerned in order to 
prevent failure. This paper exhibits and compares the benefits 
of a number of different techniques used for flaw detection. 
Non-destructive evaluation (NDE) techniques used are 
magnetic particle inspection, dye penetrant, eddy current, 
electro-magnetic acoustic transducer (EMAT). The techniques 
vary in their ability to ascertain the flaw characterishcs. Using 
a non-contact sensor such as the EMAT, to scan the wheels in 
an automated manner offers greater inspection speed at lower 
manpower. This paper reviews the basic concept of EMATs, 
introduces a recently developed technique for simulating 
EMAT performance by Finite Element calculation and 
features bench top results of waveform acquisition and signal- 
to-noise ratio dependence on lift-off. Next presented are 
calibration results for spark -eroded flaws in wheel sections for 
a variety of locations and sizes. Finally data are on flaw 
detection in a railroad service facility on several locomotives 
with wheels spinning at speeds up to 40 meters/minute. 
Results for both artificial and actual flaws are shown. 
INTRODUCTION 
Trains carry milfions of tons of dangerous chemical and 
nuclear wastes every year. In the last year alone there were 
1,757 derailments in the United States. Overall, the current 
methods for wheel crack detection are archaic, time 
consuming and require highly skilled operators to perform. 
None of the current methods can detect cracks beneath the 
surface. All methods require at least 20 hours of work per 
locomotive, as the wheels must be removed and carefully 
cleaned. Considering the critical role the integrity of the 
locomotive wheel plays in the safety of trains, it is vital that 
these methods be improved. The dirtiness of the surface of the 
wheel and the need to remove the wheel from the locomotive 
provide the biggest challenges for the new methods to detect 
defects in locomotive wheel sections.. We have examined 
locomotive wheel tire sections with various techniques for 
flaw detection, including magnetic particle, dye penetrant, 
eddy current and, electro-magnetic acoustic transducer 
(EMAT). It was determined the current methods worked with 
limited success. Eddy current method worked very well for 
surface flaws but however, due to skin depth constraint, did 
not detect sub-surface flaws. Surface acoustic wave (SAW) 
EMAT signals could determine the position of a defect at 
sim~ificant lifloff Finite element simulations showed good 
convergence with experimental and theoretical results. A 
Prototype was used in an railroad service center and it 
detected manufactured as well as fatigue flaws in an actual 
locomotive wheel. A computer display system was also 
designed that would aid the operator in making decisions 
regarding thedefects. 
REVIEW OF LOCOMOTIVE WHEEL TESTING 
Most locomotive wheels used for high-speed locomotives 
at Austrian OEBB are heat-treated; curved plate wheels made 
of a single block of metal, called mono-block wheels. A 
photograph of a locomotive axis is shown in figure 1. Many 
defects can occur in these wheels including: quarried out parts 
and cracks at raft contact surfaces, radial and tangential cracks, 
loose felly-to-tire shrinking, loose snap ring, and felly rotated 
against the fire, where signs do not overlap. The maximum 
size of the crack allowed depends on the type and position of 
the crack, as well as the individual requirements of the railroad 
company. For example, for tires to remain operable, OEBB 
has determined that detected crack size at inspection must not 
exceed a maximum length of 15 nun for tangenlial cracks, and 
6 nun for radial cracks. In some areas, the radial crack must be 
smaller than 3 mill. There are currently three major methods of 
measuring creeks in locomotive wheels: Visual Inspection, 
Hammer Hit, and Magnetic Particle Evaluation. For all three 
methods, the wheel must be removed from the locomotive. 
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Figure 1: Photo of a locomotive axis 
Visual Inspection 
Visual inspection is the most common method used today 
to detect cracks in wheels. As the name implies, a mechanic 
ca~fully cleans the wheel and then visually inspects it. This 
method is extremely dependent on the skill of the operator, 
and is very time consuming. Also, visual inspection can not 
detect cracks beneath the surface. 
Hammer  Hit 
Hammer hit can be explained as when a skilled worker 
hits the wheel with a hammer and listens to the sound. If  the 
wheel has poor contact to the felly, the sound generated by the 
blow will differ from a properly attached wheel. This method 
is even more dependent on the skill of the operator than the 
visual inspection method. Not surprisingly, the hammer-hit 
method is often done in conjunction with the visual inspection 
method. 
Magnetic Particle Evaluation 
In magnetic particle evaluation, the wheel is cleaned, 
magnetized, and then magnetic particles are either dusted over 
the wheel, or suspended in a liquid (often kerosene), and 
poured over the wheel. A dye is added to the magnetic 
particles, which absorb ultraviolet lighL and emit a yellow- 
green fight A surface defect forms a magnetic anomaly, 
which attracts and holds magnetic particles more than the 
other parts of the surface. Therefore, by shining ullraviolet 
fight on a wheel treated with the magnetic particles, a yellow- 
green light is seen at the cracks. 
This method is limited in thai the wheel has to be made of a 
ferromagnetic material, which is not possible on most stainless 
steels. Also, magnetic particle evaluation is very dependent 
on the qnality of the cloning of the surface, and the skill of 
the operator. If a crack happens to appear along the grooves 
on the surface of a wheel, it is harder to detect it than if it lies 
perpendienlar to the grooves. Finally, the wheel often has to 
be demagnetized after inspection, which can be time 
consuming and expensive. 
Overall, the current methods for wheel crack detection are 
lime cons-ming and require highly skilled operators to 
perform. 
Also, none of these methods can detect cracks beneath the 
surface. Moreover, all methods require the wheel to be 
removed from the locomotive and carefully cleaned and 
require, at least, 20 hours of work per locomotive. 
Considering what an important role the integrity of the 
locomotive wheel plays in the safety of trains, it is vital that 
these methods are improved. 
OVERVIEW OF NON-CONTACT METHODS 
Most non-destructive methods to measure anomalies 
require contact with a surface either directly or through a 
medium (often water). There are only a few viable methods 
that can be used without contact; we will focus on Eddy 
Currents and EMATs (Electromagnetic Acoustic 
Transmitters). 
Eddy Current Method 
Eddy currents are volumetric electrical currents in an 
electrically conducive material created by an exterior current. 
For example, when an alternating current flows through a coil 
close to a conducting surface, the magnetic field of the coil 
will induce circulating eddy currents in that surface (Fig. 2). 
The magnitude and phase of the eddy currents in the ~mple 
change the loading on the external coil, and its impedance. 
Therefore, a crack in the surface will affect the magnitude and 
phase of the eddy current thus changing the impedance of the 
external coil. It is therefore possible to detect changes in the 
integrity of a material by monitoring the voltage across the 
coil in such an arrangement. 
G 
Figure 2: AC current in coil creating FEddy Currents in Sample 
Note that cracks must interfere with the surface eddy current 
G Eddy Current Strength 
- -  0 3 7 %  1 0 0 %  
flow m be dete~ed. Cracks lying parallel m ~ e  ctmem path 
will not cause significant interruption and are harder, if not 
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impossible, to detect. The eddy current and the impact of a 
defect decrease when moving into the sample (Fig 3). As a 
result, this method is only effective for surface cracks. 
Figure 3: Decrease in eddy current strength vs. depth 
EMAT Tedmique 
EMATs are ultrasonic transmitters that do not need to 
have contact with the material to create strong acoustic waves. 
First, a permanent magnet is placed near the surface of the 
sample, creating a magnetic field within the sample. A wire 
with an AC current is then placed near the surface of  the 
sample, creating an alternming field within the sample, which 
creates eddy currents. These eddy currents interact with the 
permanent magnetic field and create deformations in the 
material. Finally, the deformations create acoustic waves (Fig. 
4). The type of acoustic wave created in the sample depends 
on the shape of the external coils and permanent magnet above 
the sample. Currently, there are three major types of EMAT 
systems used: Meander Coil (Fig. 5), Pancake Coil, and 
Racetrack Coil. 
AC CCuzr~t b J 
Coil 
I ~ y  curmat J W 
Figure 4: EMAT method to create acoustic waves 
S.,w,p .: i :: 
SV SV 
Figure 5: Meander Coil - Excites surface, longitudinal and vertical 
polarized shear waves. 
FINITE ELEMENT SIMULATIONS 
Numerical ~imulatious and especially finite element 
method are widely used tools that can facilitate and speed up 
the design process of new acoustic transducers by reducing the 
number of protetypes. A modeling scheme was developed 
using the CAPA finite element software for the 2-D simulation 
of the wave generation and reception process with the 
electromagnetic acoustic transmitters based on the Lorentz 
force mechanism. Simulation and experiments were carried 
out for a 0.24-mm thick Aluminum plate (figure 5) with 
through-transmission setup of the EMATs to qualitatively 
compare waveforms from the simulation with those obtzined 
with real EMATs (figure 6). 
m 
/ "7: 
Figure 5: Setup of a Plate Wave EMAT. 
tl ttt ~.~! ........................ i . . . . . . . . . . . .  i .................. ~ ........................ 
~.4 . . . . . . . . . . . . . . . . . . . . . . . .  i . . . . . . . .  ; . . . . . . . . . . . . . . . . . .  ~ ........................ 
~, ~20 ................. ~llll}IIIl ~ ! i. . . . . . .  ,!; ,....... i ~................. 
10 15 20 2~- 
t h ~ 3  
Figure 6. Comparison of Signzls from Simulation and 
Experiments 
The group velocity dispersion curves (Fig. 7) calculated 
using CAPA agree well with the theoretical and experimental 
results. 
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10 
91 
a! 
7 
3 
0' 
SO 
1 2 3 4 5 6 7 8 
fd IMHz "ram] 
Figure 7. Measured, Theoretical and Simulated Group 
Velocity Dispersion Curves 
LIFTOFF DEPENDENCE IN SAW EMAT$ 
The EMAT system was modified to have adjustable 
cylindrical roller bearings to create various air gaps between 
the EMAT coil and the sample surface and to allow the EMAT 
system to move smoothly (Fig. 8). To accurately measure the 
lifloff, a feeler gauge was employed. A RITEC (RAM-1000) 
Pulser system was used with a Gated RF S i sa l  output at 10 
its delay. The breakthrough pulse at the beginning was gated 
out. A signal with a frequency of 1.01 MHz and amplitude of 
1.2 V was used as input to the EMAT system. Figure 9 shows 
the driving s i sa l  where the intensity was damped by - 40 dB 
(or 10 -4 less intensity) so that the signal could be recorded 
without overloading the oscilloscope. 
Figure 8: Positioning SAW EMATs for lifloff dependence 
Figure 9. Input signal 
'rngg~l ] m~-sm,,.t 
=++ I 
Figure 10. Omline of the Measurement System 
The signal was then recorded at a distance of  about 32- 
nun with several different lifloff positions. For low lifloff, the 
signal was clearly visible without much signal processing. 
However, for high liftoff, the signal was much clearer for 
more simml snmmafion averages. Therefore, for a low lifloff 
of 0.53 mm the sir, hal was only snmmed over 7 signals (Fig. 
11), while for a lifloff of 1.2 mm the signal was snmmed for 
100 signals (Fig 12). Note that there is a factor of 10 in the 
amplitude of the reflection from the crack between figures 12 
and 13 caused by the different liRoff. Also, in figure 13 the 
amplitude of the echo signal is only 2.2 times larger than 
noise. Higher liRoff values lhan 1.2 mm did not pro~de 
distinct peaks even with summations of sirTnalg. 
O A  
O.g 
-0 .  • . . . . .  : 
, 0 . ~  . . . . . . . . .  - . . . . . . . . . . . .  
. . . . . . . .  !i . . . . . . . . . .  
. 0 . 5  
-OA 
0 
'++ . . . . . . . . . . . . . . .  F r o m  C r a o k  . . . . . . . .  
+ .............. ] + i  . . . . . . . .  
Lii .... A ,_ .  . . . . .  
I + i , l l t r+  i 
From EMAT 
i 
2 4 
Figure 11. Signal at 0.53-ram Lift-off 
$ 
X I 0  a 
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• ~ . . . . . . .  7 -  ' 
| 
i 
• ~ i  . . . . . .  = . . . . . .  
~ J M  
~ J 3 6 '  
. . . . . . . . . . . . . . . .  2m.Crack . . . . . . . . .  
From EMAT 
.... L[ . . . . .  . . . . .  i 
i I I 
o ;I 4 
Figure 12. Signal at 1.2-ram Lift-off 
A sit, hal processing technique using auto-col lat ion was used 
to determine the amplitude of both the reflection from the 
crack and the direct signal fxom the driving EMAT as a 
function of lifloff (Fig. 13). 
3 
L8 t
t .13 
1.4 , 
~,le I" 
% 
• .  . ~ . . . . . . . . . . .  • . . . . . . . .  : . . . . . . . . . . . .  
: \  i " . .  
1.4 
Figure 13. Amplitude of direct echo (danhed) and crack echo 
(solid) versus liftoff. 
EXPERIMENTS AT AUSTRIAN OEBB 
Experiments were conducted on locomotive wheels at the 
Austrian OEBB. A jack was used to raise the locomotive 
wheel (Fig. 14) and an EMAT array was mounted on the 
wheel. Tests were performed to detect any flaws including 
fatigue cracks. Tests were conducted with some mamffactured 
flaws (3-ram and 1.5-ram boreholes and simulated notches) to 
investigate the effectiveness of  the EMAT system, as well 
(Fig. 15). The EMAT system indicated the presence of  the 
manufactured flaws as well as the fatigue crack (Fig. 16) in 
the locomotive wheel, very well. 
Figure 14. EMAT Array on the Wheel and the Jack to Raise 
the Wheel 
m .  . . . . . . . . .  . . . . . . .  
I 
. 1 
q 
2 
- - / " ' h  i $ 1 m a t l e m r  I . . . . . . . . . . . . . .  t . . . . .  
Rm .... . . . . . . . . . . . . . . . .  i ....... ! 2 e  . . . .  ~ i ~ -  . - - - ~  . . . . . . . . . . . . . . . . . . . .  
r ,  , ~ ~ 
Figure 15. Mannfactured Flaws Detected by the EMAT Array 
v. z -  
_r 
r l U m  
Io . . . . . . . . . . . . . . . .  i . . . . . . . . . . .  J - -  J l t ~  
. _  t L . . . . . . . .  I 
• w e  :m ~ tea 
Figure 16. Fatigue Crack Detected by the EMAT Array 
COMPUTER DISPL.AY SYSTEM 
A computer display system was developed at Sonic 
Sensors, CA that utiliTes the information obtained from the 
sensors while scanning the locomotive wheels to provide a 
guideline to the operator to detect defects. The computer 
displays the RF signal ampfitude and the result o f  a numerical 
algorithnL The adjustable algor/lhm interprets the RF 
amplitude for the detection of cracks. The algorithm has two 
purposes: to detect a crack and to remove or minimize any 
anomalous indications. The inspection is based on the 
adequate amplitude of  an ultrasonic wave traveling over a 
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given path. When the wave confronts a crack, energy is 
retlected or scattered, and the ampfimde is diminished. The 
signal amplitude can also change from alignment or a surface 
irregularity. These small variations need to be ignored and the 
signal drop out from a crack needs to be enhanced. The 
algorithm was designed to ignore small effects and be very 
sensitive to crack type effects. The algorithm converts a 
"good" signal into a small number and a "crack" indication 
into a large value. The algorithm result is compared to two 
thresholds, a "yellow Caution" and a "Red Rejection" 
threshold. These adjustable thresholds provide a simple 
accept/reject criterion for the engineer to set for the operator. 
A Green - Yellow - Red display (Fig. 17) provides a clear 
visual indicator of the condition of the wheel. Including all 
this information on one display provides the operator a sense 
of understanding of the quality of the scan he just performed, 
and provides an easy to interpret, reliable, rejection criteria. 
. e ,  
t 
Figure 17. An Example Scan Display 
CONCLUSIONS 
Several relevant experiments have been conducted. We 
have shown that the SAW EMAT signals can determine the 
position of a defect at significant lifloff. The Finite element 
modeling using CAPA proved that simulation results compare 
very well with theory and experiment which helped in 
reducing the number of prototypes. The EMAT array system 
worked well when tested on the actual locomotive wheel with 
both manufactured as well as fatigue defects. Finally, A 
computer display system has been developed that can aid the 
operator in making decisions on the health of the locomotive 
wheel. 
References 
[1] Bray, D.E. & Stanley, R.K. 1997. Nondestructive 
Evaluation: A Tool in Design, Manufacturing, and 
Service, CRC Press, pp. 98-99. 
[2] Thompson, D. O. & Chimenti, D.E. 1998. High-Speed 
Monitoring of Surface Defects In Rail Tracks Using 
Ultrasonic Doppler Eff~t, Review of Progress in 
Qualitative Nondestructive Evaluation, Vol 17 pp. 
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PVP2004-2833 Locomotive Wheel Inspection with EMAT Technology

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PVP2004-2833 Locomotive Wheel Inspection with EMAT Technology

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