In this paper an eddy current system for the detection of partially-through-the-thickness cracks in corrugated metal hose is presented. Design criteria based upon the geometry and conductivity of the part are developed and applied to the fabrication of a prototype inspection system. Experimental data are used to highlight the capabilities of the system and an image processing technique is presented to improve flaw detection capabilities. A case study for detection of cracking damage in a space shuttle radiator retract flex hoses is also presented

Hall, George

Simpson, John

Wincheski, Buzz

NASA Technical Reports Server

 
 
 
EDDY CURRENT SYSTEM FOR DETECTION OF CRACKING 
BENEATH BRAIDING IN CORRUGATED METAL HOSE 
 
Buzz Wincheski1, John Simpson2 and George Hall3 
 
 
1NASA Langley Research Center, Hampton, VA 23681 
2Lockheed Martin Space Operations, Hampton, VA 23681 
3George Washington University, Washington, D.C. 20052 
 
 
 
ABSTRACT. In this paper an eddy current system for the detection of partially-through-the-thickness 
cracks in corrugated metal hose is presented.  Design criteria based upon the geometry and conductivity 
of the part are developed and applied to the fabrication of a prototype inspection system.  Experimental 
data are used to highlight the capabilities of the system and an image processing technique is presented 
to improve flaw detection capabilities.  A case study for detection of cracking damage in a space shuttle 
radiator retract flex hoses is also presented. 
 
 
 
Keywords: Eddy Current, Fatigue Crack, Corrugated Metal Hose, Flex Hose, Space Shuttle 
PACS: 81.70.Ex, 81.40.Np, 81.05.Bx 
 
 
 
INTRODUCTION 
 
 Corrugated metal hose, also known as flex hose, poses several challenges for 
quantitative nondestructive evaluation techniques.  The complicated structure of the part is 
shown in Figure 1.  In this image the metallic braid has been pulled back to reveal the 
convoluted thin wall structure of the hose. Partially-through-the-thickness cracking in the 
corrugated metal hose can easily be masked by the irregular geometry of the part.  
Inspection of the hose through the braid, as required for any in situ application, further 
complicates the inspection by adding an irregular metallic layer between the inspector and 
the critical surface. Coupling of thermal or ultrasonic energy into the part and analysis of 
the energy flow in the system are daunting tasks, and the large percentage of the surface 
area covered by the metallic braid limits the effectiveness of even a detailed visual 
inspection.  Leak testing can be effective for through-wall cracking, but no good options 
have been available for detection of partially-through-the-thickness cracks in a pressurized 
system. In this work an eddy current technique for the detection of such damage is 
proposed.  The probe design and inspection technique, along with experimental data for the 
detection of cracking in a typical flex hose component, are presented. 
https://ntrs.nasa.gov/search.jsp?R=20090009966 2019-08-30T06:25:53+00:00Z
 
EDDY CURRENT INSPECTION OF CORRUGATED METAL HOSE 
 
 Eddy current technology provides several significant attributes important in 
overcoming the inspection challenges identified above.  Eddy current techniques are well 
recognized for crack detection capabilities in metallic structures, including thin walled 
tubing.  In addition, multiple systems for detection of flaws in lower layers of metallic 
multi-layer structures have been developed [1].  Finally, eddy current probe orientation can 
be used to couple selectively into or away from a feature of the part under test [2].  
  The initial experimentation on the development of an eddy current technique for 
the detection of partially-through-the-thickness cracking in flex hose consisted of designing 
a probe with the correct operating frequency and size to induce a sufficient current density 
into the hose convolutes.  The appropriate operating frequency based upon the required 
inspection depth and material conductivity can be estimated from the skin depth equation, 
 
!µ"## feBB xx /1,/ 0 ==
$ ,    (1) 
 
where Bx is the field strength at distance x beneath the surface, !  = the standard depth of 
penetration, 
! 
f  = excitation frequency, 
! 
µ = permeability, and 
! 
"  = electrical conductivity 
[3]. For a skin depth between 0.4 and 0.8 mm (distance through the braid and to back 
surface of hose convolute) in a stainless steel 300 series material, equation (1) estimates a 
required operating frequency of approximately 0.3 – 1.0 MHz.  For probe sizing, the coil 
diameter was targeted to match the convolute pitch such that the spatial resolution of the 
probe would correlate with the convolute spacing.  
 Optimization of the probe orientation was studied to minimize braid effects and 
maximize coupling to the underlying hose.  As can be seen in Figure 2, high pressure flex 
hose is typically braided with a linear array of stainless steel wires.  The resulting braid 
structure is highly anisotropic in conductivity.    Orienting the eddy current probe such that 
the induced electromotive force (EMF) is always non-parallel to the high conductivity 
directions of the braid minimizes the shielding effects of the braid.  To maximize coupling 
into the underlying hose, the induced EMF should also be in the plane of the hose.  
Targeting the detection of circumferential cracks adds a final criterion in that induced 
currents should be orthogonal to the crack direction for maximum eddy current response 
from the flaws.   Optimization of the probe orientation based upon these criteria led to the 
FIGURE 1.  Typical flex hose design showing convoluted thin metal wall hose and braided metallic 
overwrap.   
inspection geometry shown in Figure 2, with the coil axis tangent to the inspection surface 
and along the convolute direction.   
 
APPLICATION TO SPACE SHUTTLE RADIATOR RETRACT FLEX HOSE 
 
 The effectiveness of the eddy current inspection methodology outlined above was 
investigated based upon a target inspection of the Space Shuttle Radiator Retract Flex 
Hose.  This component is designed to carry freon to radiators attached to the payload bay 
doors.  The hose is stowed away during take-off and landing but must flex as the payload 
bay doors open and the radiators are deployed [4-6].  The component is critical for cooling 
of the orbiter during orbit and has been a source of concern for the shuttle program on 
multiple occasions [5-6].  
 A test specimen was fabricated from a spare section of radiator retract flex hose.  
After removing the flex hose braid to expose the hose convolutes, three laser cut notches 
were placed in the hose at the crown of the convolute.  The notches were each 0.005” wide 
x 0.1” long.  The first notch was cut completely through the thickness of the hose while the 
other two notches were cut to a depth approximately half way through the wall.  Figure 3 
shows a picture of the hose convolutes with close up views of the three notches. 
 Initial testing of the eddy current technique was performed by holding the eddy 
current probe stationary and scanning the hose section.  A small air gap of approximately 
0.010” was set between the eddy current coil and the flex hose, with the coil orientation 
explained above and pictured in Figure 2.  The sample was initially scanned with the 
FIGURE 2.  Probe orientation implemented to inspect for circumferential cracking in the flex hose 
convolute. 
FIGURE 3.  Radiator retract verification sample with three laser cut notches, braid removed.  
overwrap removed.  After optimizing the probe response in this configuration the braided 
metallic overwrap was placed over the hose and the sample rescanned.  In both cases a full 
360 degree scan over a 2” length along the hose axis was performed.  Figure 4 displays the 
vertical component eddy current C-scans of the raw data, along with two-dimensional 
Fourier filter processed results for the scan of the braid overwrapped hose.  In the figures 
the circumferential direction is vertical, with the annular convolutes easily identified as 
vertical banding in Figure 4a.  The three laser cut notches are clearly identified in the data 
for the unwrapped hose and are also visible in Figure 4b, although background variations 
due to braid and lift-off effects somewhat mask the flaws.  In obtaining Figure 4c, a two-
dimensional Fourier transform was applied to Figure 4b.  In the frequency spectrum, the 
regular spatial pattern of the braided overwrap produced an easily identifiable and 
separable signature. These components, along with high and low frequency variations, 
were then removed from the frequency space spectrum.  Performing an inverse Fourier 
Transform on the filtered frequency spectrum produced Figure 4c, which displays the real 
component of this inverse transform.  The automated filtering technique was seen to reduce 
braid artifacts, improving the signal to noise ratio for the three validation notches. 
 
PROTOTYPE DEVELOPMENT 
 
 Following the success of the initial testing, a scanning fixture was designed to 
enable eddy current C-scan data to be acquired on installed, pressurized flex hose 
components.   The design, pictured in Figure 5, incorporated a spring loaded probe holder 
integrated into a manual scanner.  The scanner can be coupled directly to the flex hose with 
a minimal pressure of approximately 1 psi.  An optical encoder was used to track probe 
position along the axis of the hose, with the circumferential position around the hose 
indexed at a resolution of 2.5 degrees (approximately 0.022” for 1” diameter hose).  A ball 
joint coupling between the probe holder and fixed ends allowed scanning of a hose with a 
radius of curvature of 23” or greater (hose sections with smaller radius or curvature could 
be inspected with a restricted scan length along the axis of the hose). This, combined with 
the small profile of the eddy current scanning system enabled coupling to and scanning of 
installed flex hose components in confined areas with limited access. 
a. b. c. 
FIGURE 4.  Vertical component eddy current C-scan data for (a) validation sample with braid removed, (b) 
validation sample with braided overwrap in place, and (c)  two-dimensional Fourier filter results of (b). 
 
 The flex hose scanning system was designed such that the eddy current sensor 
could be easily changed, without the need to uncouple the scanner from the hose.  Along 
with enabling rapid replacement of a damaged or questionable eddy current coil, this 
feature allowed for the evaluation of different sensors on an indication of interest.  As an 
example, probes with higher or lower operating frequencies could be applied to add 
additional information on the depth of the feature causing the indication.  In this work the 
capabilities of a differential sensor with coil spacing equal to the pitch of the corrugation 
were studied and compared to that of an absolute coil sensor.  It was reasoned that a 
relative measurement between equal areas of adjacent hose corrugations could help to 
increase the signal to noise by reducing the background levels.  Each coil of the differential 
coil set was designed as described above and pictured in Figure 2.   The two coils were 
then bonded together in a parallel axis configuration.  As with the absolute probe, testing of 
the flex hose was performed with coil axes tangent to the inspection surface and along the 
convolute direction.  Photographs of (a) the differential probe configuration in reference to 
the corrugation direction and spacing and (b) the flex scanner attached to the validation 
sample are displayed in Figure 6.  
 The performance of the flex hose scanning system with the differential eddy current 
probe is illustrated in Figure 7.  The unprocessed vertical channel C-scan of Figure 7a can 
FIGURE 6.  Differential eddy current flex hose probe and flex hose scanning system. 
 
FIGURE 5.  Flex hose scanner design for testing of pressurized, installed hoses. 
 
be compared to Figure 4b, which shows the response from the absolute probe on the same 
sample.  Note that the flex hose orientation is consistent between Figures 4 and 7, but the 
starting position of the scan is rotated approximately 135 degrees between the two figures.  
The signal to noise ratio for the system shows great improvement with the differential coil 
probe.  A pronounced maximum-minimum pair is observed at each of the three defects, as 
expected for differential coil inspection.  This characteristic response is highlighted in the 
extracted lissajous pattern of Figure 7b.  Braid effects are seen to produce a signal response 
very close to the horizontal axis, with amplitude of approximately +- 2V.  The calibration 
defect produces a strong vertical response with amplitude more than double that of the 
braid effect at approximately +- 5V.   
 Further improvement to the flaw detection capabilities of the flex hose scanning 
system was investigated through image processing of the raw C-scan data.  The processing 
routine was designed to use prior knowledge of the braid direction and differential coil 
orientation and spacing to highlight potential circumferential cracking in the convolute of 
the flex hose.  First, a differential filter along the vector separating the two coils was 
applied to the raw data.  Next, following the image processing procedure presented in 
reference to Figure 4c, two dimensional Fourier filtering was applied to remove braid 
artifacts. A morphological opening was then applied using a structural element larger than 
the coil to coil spacing.  As the result of a morphological opening is to highlight objects in 
the image larger than the structural element, this opening produced a background image of 
features larger than the coil to coil spacing.  The final result, shown in Figure 7c, was 
obtained by subtracting this background image from that obtained after the two 
dimensional Fourier filtering.  Figure 7d displays a B-scan of the processed amplitude data 
across a line containing one of the calibration notches.  The signal to noise ratio, computed 
as the quotient of the peak amplitude of the flaw divided by that of the background, is 
approximately 10 to 1. 
 
  
 
FIGURE 7. (a) Vertical component eddy current C-scan data for validation sample with braided overwrap 
in place, (b) lissajous pattern extracted from raw data for horizontal line across notch, (c) image processing 
results of (a), and (d) line scan across calibration artifact in (c). 
 
 
a. c. 
b. 
d. 
SUMMARY 
 
 In this work an eddy current technique for nondestructive evaluation of corrugated 
metal hose, or flex hose, has been presented.  Tangential eddy current sensors, both 
absolute and differential, were designed and optimized for detection of partially-through-
the-thickness circumferential cracking in the hose convolute through a metallic braid 
overwrap.  The eddy current sensor is incorporated into a flex hose scanning system to 
enable C-scan data acquisition of in service flex hose components. An image processing 
routine for the C-scan images based upon prior knowledge of the braid direction and coil 
parameters was also developed and shown to improve greatly the signal to noise ratio for 
laser cut notches placed into a validation sample.  The capabilities of the prototype system 
have been examined in reference to a target inspection of the Space Shuttle Radiator 
Retract Flex Hose.  Customization of the technique to meet inspection requirements for 
other critical flex hose components can be performed based upon the design considerations 
outlined in this work. 
 
  
               
REFERENCES 
 
1. D. Piotrowski, M. Bode, J. Bohler and D. Moore, “Assessment of Capabilities and 
Readiness of Conventional and Emerging NDI Methods for Detecting Subsurface 
Cracks in Lap Joint Structure,” 9th Joint FAA/DoD/NASA Aging Aircraft Conference, 
Atlanta GA, 2006. 
2. B. Wincheski, P. Williams, and J. Simpson, “Analysis of Eddy Current Capabilities for 
the Detection of Outer Diameter Cracking in Small Bore Metallic Structures,” in 
Review of Progress in QNDE, 27A, edited by D. O. Thompson and D. E. Chimenti, 
AIP Conference Proceedings vol. 975, American Institute of Physics, Melville, NY 
(2008), pp. 384-391. 
3. H. Libby, Introduction to Electromagnetic Nondestructive Test Methods, Robert E. 
Krieger Publishing Company, Inc., Melbourne, Florida, 1971, pp. 122-129 
4. NASA, http://www.nasa.gov/returntoflight/system/system_Orbiter.html, 2006. 
5. Chris Bergin, “Engineers evaluating radiator hose issue on the fleet,” 
http://www.nasaspaceflight.com, 2007. 
6. Chris Bergin, "Engineers find issue with Atlantis' radiator retract hose," 
http://www.nasaspaceflight.com, 2008.  
 


Eddy Current System for Detection of Cracking Beneath Braiding in Corrugated Metal Hose

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