ABSTR ACT
This paper presents an experimental prototype
developed for rail flaw imaging. This capability
can help obtain quantitative information on
detected flaws during manual flaw verification.
Ultrasonic synthetic aperture focus (SAF) imaging
has advantages over phased-array imaging for both
speed and accuracy. The prototype developed is
hosted in a portable and battery-powered carry-on
size case. The probe is a linear ultrasonic array
mounted on a wedge and with a position encoder to
build 3D point clouds from 2D beamformed images.
The prototype includes several advances over the
basic SAF technique, including sparse subarray
firing that allows fast imaging speeds (e.g., 25 Hz)
without sacrificing image accuracy. Validation results
are presented from scans performed on rail sections
from the FRA rail defect library, which contains
natural transverse defects and artificial end-drilled
hole defects. The tests showed good accuracy in
defect size and shape, as compared to the available
ground truth information, for defects located away
from the railhead corners. Additional developments
are required to properly cover the head corners, and
especially in the case of heavily worn rails.
KEYWORDS: nondestructive testing, ultrasonic imaging,
synthetic aperture focus, SAF, phased arrays, rail flaws
Introduction
Internal rail flaws are a significant cause of train accidents.
According to FRA’s Safety Statistics data shown in Figure 1, in
the past five years (2018–2022) detail fractures were responsi-
ble for as many as 222 derailments and damage cost of US$79
million (the highest cost of any other cause within the category
of Track, Roadbed, and Structures). Transverse/compound
fissures (TF) were responsible for 77 derailments and US$21
million in damage, and vertical split head (VSH) defects
caused 83 derailments and ~US$20 million in damage. These
three defects combined, therefore, caused as many as ~80
derailments per year and ~US$25 million in damage per year.
The detection and quantification of these flaws is clearly of
importance to railroad safety and efficiency.
The current manual verification of detected flaws consists
of a simple ultrasonic pulse-echo test conducted using a
handheld ultrasonic transducer with a wedge that is manually
moved around the flaw in attempt to estimate the flaw size
through a –6 dB threshold technique (Lanza di Scalea 2007).
This process yields rail flaw sizing results that are highly sub-
jective to the operator’s judgement. An improved flaw verifi-
cation would allow the generation of 3D ultrasound images of
the internal flaw for an objective determination of flaw size and
orientation. Knowledge of the correct flaw size can inform the
most appropriate remedial actions, which can largely reduce
the cost of rail maintenance and improve safety.
Current OEM portable systems exist for manual flaw
imaging in structural components using ultrasonic techniques.
These systems are based on phased array (PA) technology
(Witte and Poudel 2016). As schematized in Figure 2, in PAs
the transmission is sent to all channels that are appropri-
ately delayed for physical focusing and steering at various
depths. This means that (a) the PA hardware is fairly compli-
cated because of the multiple digital-to-analog (D/A) output
channels required (b) the PA imaging speed is limited by the
need to physically focus at different locations in the medium
and (c) the classical PA beamforming is only achieved in trans-
mission through focused beams, which limits the lateral reso-
lution. Conversely, synthetic aperture focus (SAF) techniques
have been considered for defect imaging for various benefits
over the PA methods (Drinkwater and Wilcox 2006). In a tradi-
tional SAF scheme, the transmission is sent to a single channel
NDTTECHPAPER
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ME
RAIL FLAW IMAGING PROTOTYPE BASED
ON IMPROVED ULTRASONIC SYNTHETIC
APERTURE FOCUS METHOD
BY CHENGYANG HUANG* AND FRANCESCO LANZA DI SCALEA*†
*Experimental Mechanics &NDE Laboratory, Department of Structural
Engineering, University of California at San Diego, La Jolla, CA 92093
† flanza@ucsd.edu
Materials Evaluation 82 (1): 51–59
https://doi.org/10.32548/2024.me-04371
©2024 American Society for Nondestructive Testing
J A N U A R Y 2 0 2 4 • M A T E R I A L S E V A L U A T I O N 51
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at a time, and the focusing is done in post-processing both in
transmissions and in receptions (two-way synthetic focusing)
(Flaherty et al. 1967). This means that (a) the SAF hardware
can be much simpler since only a few D/A output channels
are required (b) the SAF imaging speed can be increased
by limiting the output channels and (c) the SAF focusing is
achieved in both transmission and reflection, leading to better
resolution in a large inspection area.
It should be also mentioned that modern PA technology
is utilizing elements of SAF in the ability to focus in reception
through time backpropagation delay laws.
The objective of this paper is to present an experimental
prototype system for 3D imaging of internal rail flaws using
ultrasonic SAF techniques. An improved SAF beamforming
scheme is proposed based on sparse subarray firing to provide
high-contrast images in quasi real time (Huang and Lanza di
Scalea 2022). A sophisticated post-processing routine is devel-
oped to enable automatic rail flaw quantification without the
user’s judgement. The prototype’s hardware is packaged in a
battery-powered storage case for portability and ruggedness.
Validation tests were performed on a number of flawed rail
sections from the FRA rail defect library managed by MxV Rail
(formerly TTCI). The flaw images generated by the imaging
prototype showed a good match compared to the ground
truth established from rail break tests, especially in the case of
natural transverse-type defects.
The SAF Imaging Prototype
A portable imaging prototype was designed, assembled, and
tested to enable handheld ultrasound imaging of rail flaws
based on an enhanced SAF technique. As shown in Figure 3a,
the hardware components of the imaging prototype were
a multiplexer, a 12 V battery, a host computer, and a probe
comprised of a transducer array, a wedge, and an encoder
wheel. All the hardware components were screw fixed inside a
carry-on size storage case. The multiplexer (a high-speed data
acquisition system) that allowed multichannel data acquisi-
tion controlled the pulsed emission and reception to/from the
array. A 12 V battery was used to support the multiplexer for up
to 8 h of autonomous operation. The probe was composed of
a transducer array, a shear wedge, and an encoder, as shown
in Figure 3b. The transducer was a 64-element longitudinal (L)
wave linear array with a central frequency at 2.25 MHz. The
array was attached to a 55-degree wedge to generate direc-
tional shear (S) waves in the rail steel. The encoder recorded
ME
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RAILROADS
Figure 1. Examples: (a) FRA
safety statistics data for all
track, roadbed, and structures
(2018–2022) (b) detail
fracture (DF) (c) transverse
fissure (TF) and (d) vertical
split head (VSH).
DF TF
VSH
All channels in transmission
with applied time delays
(physical focusing in transmission)
Complicated and expensive hardware
Slow imaging through focused scans
Selected channels in transmission
(synthetic focusing in both
transmission and reception)
Simpler hardware
Fast imaging possible through subarrays
PA SAF
Figure 2. Ultrasound imaging technology: (a) conventional phased arrays vs.
(b) synthetic aperture focusing.
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