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REFERENCE

SB NIST

PUBLICATIONS

NBSIR 85-3101

Assessment of Nondestructive

Evaluation Methods for

Hoop-Wrapped Cylinders

John Smith

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Metallurgy Division

Gaithersburg, MD 20899

Harold Berger

Industrial Quality, Inc.

January 1985

Final Report

Issued February 1985

This work sponsored by ~ apartment of Transportation

(JC ashington, DC 20590

100

• U %

No- 85-5101

NBSIR 85-3101

ASSESSMENT OF NONDESTRUCTIVE

EVALUATION METHODS FOR

HOOP-WRAPPED CYLINDERS

John Smith

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Metallurgy Division

Gaithersburg, MD 20899

Harold Berger

Industrial Quality, Inc.

January 1985

Final Report

Issued February 1985

This work sponsored by

Department of Transportation

Washington, DC 20590

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Disclaimer

This report was prepared under the sponsorship of the United

States Government. Neither the United States, nor the United

States National Bureau of Standards, nor any of their employees,

nor any of their contractors, subcontractors, or their employees,

make any warranty, express or implied, or assume any legal

liability or responsibility for the accuracy, completeness, or usefulness of any information or data disclosed, or represent

that its use would not infringe privately owned rights.

Certain trade names and company products are mentioned in

the text in order to specify or identify the products adequately

where generic designations would be insufficient. In no case

does such identification imply recommendation or endorsement, or conversely, disparagement or criticism by the National Bureau of

Standards, nor does it imply that any specific product is necessarily the best or worst available for the purpose.

1

INTRODUCTION :

Catastrophic ruptures and leaks have recently occurred in high pressure

cylinders used as air-breathing apparatus. These failures have all occurred

in fiberglass wrapped seamless-aluminum cylinders which have a designed service

pressure of 4500 psi. The cylinders experiencing leaks or ruptures are

designed as hoop wrapped cylinders in which the fiberglass is wrapped only

over the body of the cylinder and the neck and head is left unwrapped due to

the lower stresses present in the head and neck of the cylinder. All cylinders

are fabricated of aluminum alloy type 6351 heat treated to the T-6 temper

condition (solution treated and aged).

The cylinders in which leaks and ruptures have occurred are manufactured

to specifications given by the U.S. Department of Tranportation (DOT) exemption

number DOT-E-7235. The present work was undertaken by NBS at the request of

the DOT to determine if a suitable nondestructive evaluation procedure could

be developed to identify cylinders containing cracks and to remove these cylinders

from service prior to failure.

The particular mode of failure shown in these cylinders first occurred

in cylinders made from alloy 6351 which contained higher than normal levels

of impurities such as lead (Pb) and bismuth (Bi). A maximum level of

0.05% of Pb and Bi is permitted in alloy 6351, although the normal level of

these elements is approximately 0.01%. In the first cylinders in which rupture

and leaking was observed, the level of Pb and Bi approached 0.05%.

The ruptures and leaks observed in these cylinders were caused by cracks

in the threads in the neck of the cylinder. The cracks were oriented parallel

to the longitudinal axis of the cylinder and started from interior of the

cylinder at the point where the lowest thread intersected the neck of the

cylinder. Both single and multiple cracks (usually located 180° apart)
2

were observed. The cracks propagated through the cylinder neck in an intergranular

mode. The cracking occurs over time when the cylinder is stressed due to the

internal gas pressure and is believed to be a stress-time phenomena due to

a form of creep cracking mechanism.

As a result of the rupture of two cylinders and the leaking of several

others, the following steps have been taken to reduce the occurence of failures

and to increase the probability that any failures will be leaks rather

than catastrophic ruptures:

1. The service pressure has been reduced from 4500 psi to 4000 psi.

2. Visual inspection of the threaded necks of all cylinders is being

performed.

3. Manufacture of new cylinders has been suspended except for the manufacture

of a limited number as replacements for those removed from service due

to cracking. These new cylinders must be fabricated from alloy 6351

containing less than 0.01% of Pb and Bi.

In addition an extensive research and development program has been initiated

to: 1) determine the cause of failure through analysis; 2) determine the

mechanism of cracking through controlled laboratory tests; 3) modify the

shape of the cylinder head to reduce the stress level in the neck; 4) modify

the manufacturing process for forming the cylinder heads to produce an improved

microstructure in the head; and 5) design a "neck ring" which can be retrofitted

to the cylinder to cause failure by leaking instead of by rupture.

In order to permit the cylinders presently in service to continue to be

used and at the same time to reduce the probability of catastrophic rupture,

the need for a suitable and reliable nondestructive inspection procedure

became apparent. A reliable nondestructive evaluation (NDE) procedure
3

is required for the following reasons:

1. To immediately inspect all existing cylinders and remove from service

any containing existing cracks;

2. To provide a method for periodic reinspection of cylinders in the future

to locate any cracks which develop during service, and

3. To inspect existing cylinders and remove any containing cracks before

retrofitting "neck rings" to permit the service pressure to be raised

back to 4500 psi from the presently restricted 4000 psi.

The requirements for an acceptable NDE procedure were determined to be the

following:

1. All cracks larger than a specified minimum size must be located.

As an initial acceptance level a crack size that has a depth

equal to the crest to root depth of the thread and a length

equal to the distance between the crest of two threads was

chosen as the minimum size that the NDE procedure should be

able to locate;

2. A minimum number of "false readings" should occur due to seams and

folds, threads, or noise from the NDE instrumentation;

3. No contamination of the cylinder should occur that would require

special cleaning before the cylinder could be refilled with

breathable quality air;

4. The NDE procedure should be readily usable under field conditions

such as cylinder retest or refill facilities;

5. The inspection cost should be moderate relative to the initial cost

of the cylinder;

6. Reliable results should be obtained with minimal inspector trainin'.!

and the NDE procedure should be relatively insensitive to variat:
4

by different inspectors;

7. Currently available, commercial NDE equipment must be used with a

minimal amount of adaptation or optimization of the equipment; and

8. It is desirable to be able to determine the approximate size

(length and depth) of any cracks found.

I

General Description of Non Destructive Evaluation Methods

Several nondestructive evaluation (NDE) methods (1) were considered for this

application and, for most of these, preliminary tests were made to determine

feasibility. The methods that appeared feasible included the following: visual/optical

liquid penetrant, radiographic, eddy current, ultrasonic and acoustic emission.

The visual/optical methods has the virtue of simplicity. The methods depend

on the changing reflection of light from a sound and a cracked surface. There are

difficulties in getting light into the small cylinder neck while leaving room for

an observer to look at the threaded walls. Small, directed light sources, angled

magnifying mirrors and borescopes that can provide a light and visualization path

in one assembly by the use of fiber optics appear useful for this application.

. The liquid penetrant method (2) was considered in conjunction with the visual/

optical approach since penetrants are used to improve the visibility of surface

defects such as cracks. The method depends on liquids that will readily enter

a surface opening when put on the material to be tested. The liquids contain a

dye (either a high contrast visual dye or a dye that fluoresces upon ultraviolet

excitation; the latter is a fluorescent penetrant). When the material surface is

cleaned of excess penetrant only the dye-containing liquid in the discontinuities

remains. This is drawn up to the surface by the blotting action of a developer

to yield an enlarged and high contrast visual indication of the discontinuity.

Penetrants appeared useful for this application because improved visibility
5

of cracks would result. However, dye penetrants have the disadvantage that

any residue from the penetrant may contaminate the cylinder.

Radiography (3,4) indicates discontinuities by the changing radiation transmission through volumes that differ in density. An opening, such as a crack, will

transmit more radiation than solid material. This difference in transmitted

radiation is normally detected with radiographic film. Possibilities for radiography included (1) placing the source outside the cylinder and shooting across

the threaded opening to a film around the back side of the threaded neck, (2)

placing a source inside the threaded neck and making a panoramic radiographic

exposure to film wrapped around the outside of the threaded neck and (3) placing

the source outside and shooting along the cylinder axis from bottom to top to a

film perpendicular to the radiation beam (along the cylinder axis) at the

threaded end. Option 1 would require several exposures to detect radiallyoriented cracks since the radiation beam must be reasonably oriented with the

crack. Option 2 would yield a result with a single film exposure but would

require the use of an extremely small focal spot radiation source in order to

provide a radiograph with sufficient resolution to observe cracks. Option 3

would yield a result with a single film exposure. The radially-oriented cracks

expected tend to be parallel to the axis so radiography in this mode should show

the cracks.

Eddy currents (5,6) are well suited for the detection of surface and nearsurface discontinuities in metallic (electrically conducting) materials. A

varying magnetic field interrogates the material, creating circulating electrical

currents in the material. A discontinuity will perturb the current flow and cause

a change in the magnetic field the current creates. These changes can be detect.'.

i

by detectors such as coils or Hall probes. For this problem, it will be neress.irv

to place the interrogating coil and detector (which are often the same coil;
6

close to the material surfaces in the threads and at a consistent distance in

order to minimize signal variations due to a changing distance between coil and

material (known as a lift-off effect). Embedding an eddy current coil in a

threaded bolt that can be turned through the threads appears attractive.

Ultrasonic methods (7) involve the use of high frequency acoustic energy

to interrogate material. The ultrasonic wave is changed as it encounters a

discontinuity in the material. Cracks in metals present a significant change in

accoustic impedance and a large part of the ultrasonic energy striking an interface

or a crack is reflected. Detection of this signal - or the change in the transmitted

signal - is indicative of a discontinuity in the material. Since ultrasonic methods

are particularly useful for crack detection, this technique must be considered for

this application. However, the close proximity of the threads to the region being

inspected, the changing thickness and curvature of the cylinder and the large grain

structure present problem areas. Ultrasonic signals will be received from the

threads making it difficult to observe a defect in that region. The surface

curvature makes it difficult to couple the ultrasonic signal into the material

and to locate a receiving transducer to detect the return signal. The large

grain structure approaches the size of the ultrasonic wave-length and leads to

high attenuation of the signal and confusion due to the scattering from the

grains themselves.

In acoustic emission testing (8,9) one tries to detect stress waves emitted

from a propagating defect. A stress is necessary so the material under test

should be subjected to stress due to pressure, mechanical or other means. This

approach is attractive for the cylinder problem because one can visualize a

test in which one "listens" to the cylinder as it is being pressurized. Typically in acoustic emission several transducers are used so the source of the

emission can be located by triangulation. In principle this is possible in the
7

case of the gas cylinders but one would have to locate series of transducers

around the diameter. Further, the stress waves emitted from a growing crack

in the neck would be subject to multiple reflections from the threads and walls.

There are also strong possibilities for stress wave emissions due to breaking

of the filaments around the body of these filament-wound cylinders and from

movements of the pressure valve in the threads. One can, therefore, expect

signals from several sources. This, along with the fact that aluminum usually

yields relatively low intensity acoustic emission signals, will tend to complicate

the situation.
8

NBS EXPERIMENTAL RESULTS:

Optical/Visual Methods :

1. Dentist Mirror - _lx with lighting by Burtron micro light set

Procedure described below was followed: major cracks rarely found even by experienced inspector.

2. Dentist Mirror - 2x with lighting by Burtron micro light set

a. Tested: full cylinders

cracked

uncracked

b. Cleaning prior to examination

ultrasonic bath with solvent

to remove oil/grease/dirt

c. Inspection procedure:

darken room manually inspect with mirror and light

d. Results:

Operator skill/experience:

At least three people examined each cylinder.

Several tries (~ 5 or 6) before each person could

find even major cracks (~ entire thread length

cracks)

Generally it was required that a severely cracked

cylinder be inspected and the crack pointed out to

the operator-in-training before unknown cylinders

could be inspected and all major cracks found.

A set of three cracked and three uncracked cylinders were inspected

and repeated inspections were conducted on each cylinder. The major

cracks (one or two per cylinder) were reliably found only after

several trials by the inspector and being pointed out by a previously

trained inspector. After this demonstration, the inspector could

generally find all the major cracks on a repeatable basis. The

size (length) of the crack could be determined approximately from the

number of threads cut by the surface crack. No assessment of the
9

reliability of this length measurement could be made. No estimate

of the crack depth is possible with this method. The seams/folds in

the neck near the lowest thread often are mistaken for small cracks

(i.e. represent false readings), even when inspection is done by

an experienced inspector.

e. Assessment of this method:

No quantitative estimate can be made of the ability of this method

to find all cracks of a size beyond a specified size (i.e. the

probability of missing cracks). The method is strongly dependent on

the skill and diligence of the inspector. An independent check of

the inspector’s performance is possible, for example, through the

use of standard test specimens. False readings are frequent in

the lowest threads in the neck due to forming seams and folds.

This method is useful as a rapid, inexpensive procedure for

removing from service cylinders with significant, major surface

cracks (~ 5 or more threads in length) but with a substantial

probability of missing even these cracks if the inspector

is not experienced or extremely diligent.

3. Fiber Optics Borescope

a. Inspection Equipment

A fiber optics instrument consisting of a bundle of optical

fibers for imaging surrounded (concentricity) by a bundle of optical

fibers for illumination was used to inspect the cracked and uncracked cylinders after cleaning and preparation as described ibThis apparatus (American Optical Fiberscope Model FS100) cou : b
10

I

*

I be used with a direct (straight-on) or right angle viewing

attachment to inspect the threads in the cylinder neck. This

j apparatus had a working distance of 1/2 to 2 inches and the

eyepiece provided a magnification up to lOx. The illumination

was provided by a high intensity halogen light source.

i b. Results:

Three cracked and three uncracked cylinders were inspected by

three different people on numerous trials after substantial

familiarization and practice with the apparatus. The cracks

in the cylinders could not be consistently and repeatedly

found even when the inspectors knew of the presence of the

cracks prior to the inspection. Inspections of unknown cylinders

(where the operator did not know whether the cylinder contained

cracks or not) were generally unsuccessful in that major cracks mmmBm .. ..i, ,, ^ sui

were frequently missed.

c. Assessment of the method:

i This particular fiber optics borescope did not provide as reliable

or consistent results as the simple optical examination with the

I magnifying "dentist" mirror. This procedure did not demonstrate

sufficient reliability to be considered to- be an acceptable inspection

method. Improved fiber optic inspection devices are available that

incorporate magnifying lens. These devices may be expected

to improve the reliability of the visual inspection procedure.

However, the limited availibility of the more advanced fiber

optics devices and the relatively high cost of such devices

(approaching the cost of the electronic inspection apparatus described
11

below) makes such devices limited in their applicability for

inspecting cylinders.

Liquid Penetrant Inspection

1. Background:

All cracks identified to date in failed or inspected cylinders have

been found to be exposed to the inner surface of the threads in the

cylinder neck. Because all cracks are open to the surface, it was

expected that the use of a liquid penetrant would substantially

enhance the ease of identifying cracks in the cylinder.

2. Inspection Procedure:

The cylinders were thoroughly cleaned in a degreasing solution in an

ultrasonic cleaning bath rinsed in water and dried prior to use of the

liquid penetrant. A visible dye type of penetrant (Spot chek manufactured by Magnaflux Corporation) was used for the inspection. The

procedure recommended by the manufacturer consisting of cleaning, application of the penetrant, removal of the penetrant, and application of the

developer was carefully followed. Observation following the application

of the dye penetration was done by naked eye as well as by using the

magnifying "dentist" mirror described above.

3. Test Results:

Numerous applications of the dye penetrant using applications

by spraying, swabbing, wiping on with a fine cloth, and brushing ver>’

tried. In all cases it was not possible to adequately remove the

excess penetrant from the root of the threads so that excessive

"bleeding" could be avoided and the cracks could be readily r u

general, the entire threaded area turned pink (using a red iv<-
12

penetrant and a white developer) so that the cracks did not stand

out distinctly from the background. These attempts demonstrated

that the threads made removal of the dye penetrant so unreliable and

difficult that the use of liquid penetrants for this inspection is

not likely to be successful. These tests were so unsuccessful that

additional liquid penetrant testing, for example using water washable

fluorescent penetrants, was not attempted.

4. Assessment of the Method:

The difficulty in easily removing the excess liquid penetrants from

the internal threads in the cylinder makes the use of this method

unreliable and not feasible for the specific application considered

here. In addition, the cylinders would require thorough cleaning after

using the penetrant test before they could be refilled with breathable

quality air. This cleaning would add substantially to the time and

cost required to use this inspection method.

Radiographic Examination

1. Background:

The relatively thin wall of the aluminum in the unwrapped portion of

the cylinder neck and the expected brientation of the cracks indicated

that relatively deep cracks (>20% of the wall thickness) should be

readily detected by conventional radiography without the need for

further development of the radiographic procedures. Radiography

also offers the advantage of not requiring any special cleaning or

preparation of the cylinders prior to inspection and does not result

in any contamination of the cylinder that requires special cleaning

prior to refilling with air for breathing.
13

2. Inspection Procedure:

The radiography was conducted at NBS using a constant potential x-ray

source set at an energy of 150 kv, with a source to film distance of

2 meters to improve the radiographic image definition. An exposure time

of 2 to 3 minutes was required when using fine grained film processed by

normal, manual film processing procedures. The cylinders to be inspected

were placed vertically, upside down (sitting on the neck of the cylinder)

on the unexposed film and the radiographic source was placed above the

bottom of the cylinder and the radiation passed through the bottom of the

cylinder as well as through the shoulder and neck of the cylinder

before reaching and exposing the film. Any cracks in the neck of

the cylinder showed up as a radial indication on the film.

3. Test Results:

Six cylinders were inspected using the procedure described above. Three

of these cylinders, that were known to have cracks, showed distinct cracks

extending perpendicularly from the hole in the neck of the cylinder.

Single cracks were found in cylinders No. 43690 and 44030 and two cracks

(180° opposite) were found in cylinder No. 43743. Three cylinders that

were found not to have cracks when visually inspected and from cylinder

lots not expected to exhibit cracking were inspected. Two of these

cylinders did not show cracks when examined by radiography. However,

a third cylinder (No. 46248) did show a single crack in the neck in the

cylinder.

The forming seams and folds near the neck of the cylinders were

visible on the radiograph. However, using the procedure describe:

here of having the x-rays pass vertically through the botton of
14

the cylinder, these seams or folds can easily be distinguished from

true cracks. The true cracks extend for a considerable depth into

the wall of the cylinders, whereas, the forming seams or laps extend only

a very small distance (significantly less than the depth of the threads)

into the cylinder wall. Not only can the cracks be distinguished from

the seams or folds by this radiographic procedure but the depth (in the

through-wall direction) can be determined from the radiographs.

Sections taken from the neck of cracked cylinders were radiographed in

several orientations and at a closer distance to the film. These tests

showed that the cracks were actually a closely spaced series of parallel or

branched cracks and that the width of the cracked area could be as much as

approximately 1 /16-inch.

4. Assessment of the Method:

The radiographic method used for this inspection proved to be feasible

and reliable for finding and measuring the depth of cracks in the

threaded area of the neck of these cylinders and did not give significant false indications. The reliability of this procedure is considered

to be acceptable when normal radiographic procedures are used and does

not require additional development or modification. This procedure

can be used to determine the depth (through the wall dimension) of

the crack, but no estimate of the length of the crack can be made from

the radiographs alone. This procedure has the further advantage that

the cylinder does not suffer any contamination requiring additional

cleaning prior to refilling with breathable quality air. The only

limitation of this procedure is that it requires specialized
15

radiographic equipment and highly trained and qualified personnel. The

procedure would generally be done by bringing the cylinders to the

radiographic laboratory. However, portable equipment and procedures

are readily available to permit this type of inspection to be done

anywhere in the field.

Further attempts at radiography of cylinders were made by Luxter USA, Inc.

and by NASA. In these tests, multiple exposures were made of the neck

area of the cylinders from various angles to locate the crack regions.

The results of these tests indicate that the cracks can be found but

that a significant number of false indications are caused by the seams

and folds in the neck of the cylinder. These false indications can not

readily be distinguished from the true cracks and therefore, a significant

number of uncracked cylinders would be unnecessarily rejected when this

procedure is used.

Ultrasonic Inspection

1. Background:

The presence of known sharp cracks oriented perpendicular to the surface

of the neck of the cylinder suggests that ultrasonic inspection methods

should be feasible to detect and possibly determine the size of the cracks.

The commercial availability of relatively inexpensive and portable

ultrasonic test equipment for doing either longitudinal or shear wave

type ultrasonic inspection made these techniques appear to offer substar, ti

promise for inspecting the cracked cylinders.

2. a. Contact Tests

A direct contact transducer, approximately 1/8 inch in diameter,

which transmitted a longitudinal wave perpendicular to the outer
16

surface of the cylinder was used in a frequency range of 5 to 15

MHz. No attempt was made to custom fit a transducer to the contour

of the cylinder neck.

b. Immersion Tests - Longitudinal Waves

Cylinders were immersed in a water bath to ensure positive coupling

of the ultrasonic signal to the cylinder and optimum transmission of

the ultrasonic waves into the cracked neck of the cylinder. Longitudinal

waves with frequencies of 10 to 15 MHz were passed from the outside of

the cylinder wall through the cylinder wall to detect cracks. Two

cylinders known to have cracks in the neck were examined.

c. Immersion Tests With Shear Waves

Cylinders were immersed in the water bath and longitudinal waves

were angled to the cylinder to convert the longitudinal waves to 45°

shear waves in the cylinder wall. Test frequencies of 10 to 15 MHz were

used. Two cylinders known to have cracks in the neck were inspected

by this procedure.

3. Test Results

Two cylinders with known cracks in the cylinder necks were examined

using the three procedures described above. None of the three

procedures could distinguish between cracks and the threads when

the cracks were near the threads. Using all three procedures, known

cracks that were several threads long and which extend through a

significant portion of the wall thickness could not be detected

with any reasonable degree of confidence or reproducibility. The

only crack that could be found with any reliability was a large
17

crack that extended entirely through the wall (the cylinder was a

"Leaker"). This crack could be located only with the direct contact

transducer procedure.

It was concluded that the very large and highly variable grain size

in the neck of the cylinder seriously interfered with the ultrasonic

signals and resulted in many false signals as well as uncertainty in

detecting the presence of cracks in the cylinder. In addition, the

geometric reflections due to the threads interfered with the ultrasonic signals and made the detection of known cracks difficult.

4. Assessment of the Method:

None of the three ultrasonic procedures tried, proved to be suitable

for detecting even known large cracks in the necks of the cylinders in

the vicinity of the threads. All procedures tried also produced an

unacceptable number of false readings in areas of the cylinder neck where

cracks were known to be nonexistent. From the lack of success with these

initial ultrasonic tests, it was concluded that additional ultrasonic

test method development was not warranted.

Eddy Current Test Methods

1. Background:

Eddy current methods are well developed for use in detecting surface

or near surface cracks in conducting materials and are especially well

suited to locating cracks exposed to the surface in non-raagnetic material

Therefore, the eddy current method seemed to have a high potential • r

detecting cracks in the threaded neck of the composite-aluminum v!

In addition, the eddy current methods have the following advanta/<*s

for the specific problem being studied here. 1) No contamination
18

the cylinder results from the examination method, 2) the test equipment is widely available commercially and is relatively inexpensive

and 3) the test method is readily adapted to field use by inspectors

without extensive training or experience. The three specific eddy

current testing procedures described below were evaluated for detecting

cracks in the cylinders.

|

2. Inspection Procedures

a. Single frequency test procedure without lift-off compensation.

i (High frequency amplitude sensitive).

The initial tests were conducted using a single frequency

3 MHz (amplitude sensitive) eddy current test instrument (the

HALEC MK II, manufactured by Hocking Electronics, Inc.). This test

instrument was chosen because of its simplicity, low cost, and ease of

I signal read out (by meter or audio signal). Specially made eddy

i current probes were required to permit examination of the threaded

area of the cylinder. The first probe made consisted of a threaded

lucite plug containing an eddy current coil that could be screwed into

the threaded cylinder. A modification was made to the probe in a

second probe design which consisted of a split plug to produce a

| spring loading effect and thereby more accurately control the

"stand-off" distance (lift-off) between the probe and the threaded wall of

the cylinder.

|

b. Impedance-Plane Variable Frequency Test Procedure

A test instrument that permitted the test frequency to be varied and

that permitted the signals from the bridge circuit to be displayed

in such a way that the phase and amplitude are displayed separately
19

was tried using: only the split "spring" probe described above. This

procedure makes it possible to balance the instrument to distinguish between

"defects" indications that are displayed on the vertical axis of the

oscilloscope and the lift-off signals that can be displayed on the

horizontal axis of the oscilloscope. The lift-off signals are all

of a similar phase, whereas, the defect signals are of a different

phase. Therefore, signals that represent defects can be separated

from signals that represent changes in lift-off. By separating

the amplitude associated with each phase, it is possible to reliably

identify only signals associated with defects and therefore

eliminate "false indications” caused by changes in lift-off distance.

These tests were first run with the split spring probe described above

using a 1 MHz operating frequency even though the probe was designed

to operate at a 3 MHz frequency. A third probe was designed with an

impendence such that it operated at 500 KHz for maximum sensitivity

and the tests described above were repeated.

3. Test Results:

a. High Frequency Amplitude Sensitive Test Instrument

Six full cylinders and heads cut from six cylinders were examined using

this procedure using both the rigid and spring loaded high frequency

probes at a single operating frequency of 3 MHz. In all cases, a

strong indication (change in amplitude) was obtained at the point

in the cylinders or cylinder heads where cracks were known to exist.

However, due to changes in lift-off distance, numerous "false

readings" as indicated by a change in amplitude occurred in posit::

where no cracks where found. These results are unacceptable bec.uis.-
20

these false readings will cause the unncessary rejection of cylinders

which do not contain cracks.

b. Impedance-Plane, Variable Frequency Test Procedure.

These tests permitted the amplitude change due to a defect to be

separated from the amplitude change due to a change in lift-off

distance by associating the amplitude change with the change in phase.

This procedure was used with both the 3 MHZ probe (at an operating

frequency of 1 MHz) and the 500 KHz probe (operated at 500 KHz) to

examine six full cylinders and six cut cylinder heads. Both heads

and cylinders known to contain cracks as well as ones known to be free

of cracks were examined. Cracks were reliably and reproducibly

found in all tests and false signals due to changes in lift-off

distance could be clearly distinguished from signals from defects.

The 500 KHz probe worked significantly better than the 3 MHz probe

in that stronger and more reproducible signals were produced from

cracks and the signal due to changes in lift-off distance were more

clearly distinguished from the crack signals.

4. Assessment of the Eddy Current Method:

The eddy current test method has been demonstrated to be feasible for

finding cracks with a high degree of reliability when specially designed

probes are fabricated for this application and the signals are processed

through an electronic circuit that separates changes in amplitude

and phase. It is necessary that changes in signal amplitude be

separated into changes due to the presence of a crack and changes

due to variation in lift-off spacing. Using spring loaded probes

designed to fit into the cylinder threads, the inspection procedure
21

is rapid and simple and no special cleaning of the cylinder is required

before or after the inspection. The operation of the test apparatus and

the signal interpretation from the eddy current probe are relatively

simple and should require only modest training of the inspector to achieve

acceptable results. False readings should be easily kept to a low

and acceptable level. The test apparatus required is readily portable,

moderately expensive (less than $10,000), and the test procedure can

be satisfactorily used in a production or retest facility environment.

With this inspection procedure, cracks in the cylinder threads can be

readily and reliably located but can not be precisely sized. An

approximate length of the crack can be determined from the number of

threads which the crack cuts. The depth of the crack can not be

determined with this method with any measureable degree of precision.

The minimum depth of crack that can be detected with this method was

not determined experimentally but is believed to be less than the

distance between the root and crown of the threads.

A simpler eddy current procedure in which only changes in signal

amplitude and not changes in signal phase are measured was attempted

and found to be not feasible for measuring cracks in the cylinder

threads. Although all known cracks were readily found with this

procedure, an unacceptable number of false indications occurred due

to changes in lift-off distance causing a change in signal amplitude

which was indistinguishable from changes in amplitude due to the

presence of a crack. Use of this procedure would result in an excessive number of cylinders being rejected during inspection vivthey did not contain cracks.
22

Summary

Of the methods considered those that appear to have the greatest potential

are eddy currents, radiography and visual/optical testing. In the tests made

at NBS the eddy current approach was shown to be capable of detecting known

cracks consistently and giving some size (length) information by counting the

number of threads over which a signal was obtained. The phase and amplitude

sensitive, scope-type instrument gave good defect signals and discriminated

against lift-off effects. Field use of the eddy current method presents little

problem.

Radiography through the bottom of the cylinder was shown to be useful

for detection of cracks having some depth and for discriminating against false

signals due to folds and seams. The radiographic technique can be performed

in the field but would require trained operators familiar with radiation

equipment and procedures.

The visual/optical method is relatively simple to use in the field. It

certainly offers the potential for detecting large cracks. The method is

obviously highly dependent on the operator.

A summary of the NDE methods tried and considered is given in Table I
23

CONCLUSIONS :

Several non-destructive evaluation methods were tried and evaluated for

use in finding cracks in the necks of composite, aluminum, hoop-wrapped

cylinders. The following assessment of these methods was made:

1. Eddy Current Procedures - this method was shown to be feasible and

reliable when low frequency probes (~500 KHz) were used and the

signal was processed to separate changes in signal amplitude from

the presence of cracks and from changes in lift-off distance. These

procedures were judged to be the most reliable of all NDE methods

evaluated and adaptable to inspection in the field by inspectors

with a minimum level of expertise.

2. Radiography - this method was shown to be reliable when the source

was positioned to shoot throught the bottom of the cylinder in a

single exposure. The depth of the crack can be estimated with this

procedure. This procedure can be conducted with portable equipment

in the field under most conditions but would require a well qualified

operator and an authorized inspection procedure.

3. Visual Inspection - this method was demonstrated to be feasible for

finding major cracks in the cylinder necks when proper lighting and

optical magnification mirrors were used to assist the visual inspection.

No assessment of the accuracy of this method could be made and the

results are strongly dependent on the skill and diligence of the

inspector. This method is simple and readily adapted to use in field

sites
24

4. Dye Penetrant - the use of dye penetrant procedures to enhance the

visual inspection was shown to be not feasible due to the difficulty

in adequately removing the dye penetrant from the threads in the

cylinder.

5. Ultrasonic Procedure - these procedures were shown to be not feasible

due to many false readings and to the inability to find cracks known

to be in the cylinders. The presence of threads and large grains

was believed to be the cause of the lack of success with ultrasonics.

6. Acoustic Emission Procedures - these procedures were not attempted

experimentally but were judged to be not feasible because of the

location of the cracks, the presence of the glass wrapping on the

cylinder, and the lack of clear acoustic signals from the intergranular cracks present on the cylinder.
25

RECOMMENDATION FOR FUTURE WORK

1. Fabricate an optimum design for a spring-loaded, low frequency ( ~500 KHz

eddy current probe.

2. Develop and document a specific test method and operator qualifications for

performing the eddy current inspection.

3. Develop and document a specific test method and operator qualifications for

performing visual/optical inspection.

4. Perform an inspection on a sufficiently large sample of cylinders (~500)

by both the optimal eddy current and visual inspection procedures to

accurately access the reliability of these two methods.

5. Establish requirements and procedures for inspection of all cylinders currently

in use. Requirements should be established for immediate inspection, inspection

during periodic retest, and at the time of any modification (i.e. retrofit

of neck rings).
26

ACKNOWLEDGEMENTS

Assistance is gratefully acknowledged to Dr. Robert Placious of the NBS

Radiation Physics Division for conducting the radiographic examination, to

Dr. Gerald Blessing of the NBS Mechanical Production Metrology Division for

conducting the ultrasonic examination, and to Mr. T. J. Flaherty and A. G.

Julier of Detek, Inc. for providing the eddy current probes and conducting

the eddy current examination. Mr. Gerald Waite of Luxfer-USA generously

provided samples of cracked and uncracked cylinders for use as test specimens.
27

REFERENCES

1. R. C. McMaster, ed. , "Nondestructive Testing Handbook," 2 volumes, Ronald

Press, New York, 1959.

2. R. C. McMaster, ed. , "Nondestructive Testing Handbook-Liquid Penetrant Tests,"

Vol. 2, Am. Soc. for NDT, Columbus, 1982.

3. R. Halmshaw, "Industrial Radiology-Theory & Practice," Applied Science Publ.,

London, 1982.

4. L. E. Bryant, ed. , "Nondestructive Testing Handbook-Radiography & Radiation

Testing,” Vol. 3, Am. Soc. NDT, Columbus, 1984.

5. H. L. Libby, "Introduction to Electromagnetic Nondestructive Test Methods,"

McGraw-Hill, New York, 1971.

6. G. Birnbaum & G. Free, eds. , "Eddy Current Characterization of Materials &

Structures," ASTM STP 722, Am. Soc. for Testing and Materials, Philadelphia,

1981.

7. J. Krautkramer & H. Krautkramer, "Ultrasonic Testing of Materials," Springe rVerlag, Berlin, New York, 3rd Ed., 1983.

8. J. C. Spanner, "Acoustic Emission-Techniques and Applications," Intex Publ.

Co., Evanston, IL , 1974.

9. W. F. Hartman & J. W. McElroy, eds., "Acoustic Emission Monitoring of

Pressurized Systems," ASTM STP 697, Am. Soc. for Testing & Materials, 1979.

Table

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NBS-1 ] 4A rev. 2-ev.i U.3. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See / nstructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 85-3101

2. Performing Organ. Report No. 3. Publication Date February 1985

4. TITLE AND SUBTITLE

/?.

Eval uation- of Nondestructive Evaluation Methods For Hoop-Wrapped Cylinders

5. AUTHOR(S)

John H. Smith

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, O.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

£>£-T10. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gn i fi can t bibliography or literature survey, mention it here)

Cracks have been found in the threads in the neck of hoop-wrapped composite

cylinders used as part of air breathing apparatus. These cracks have led

to leaking and rupture of the cylinders during normal service operation.

Therefore, a reliable non-destructive evaluation method was required to be

developed to identify and remove from service any cracked cylinders. Several

standard nondestructive evaluation methods were used and evaluated for the

inspection of these cylinders. The eddy current techniques were found to

be the most reliable and practical for this application. Radiography was found to be reliable but judged to be too complex and expensive for wide

scale use in the field. Ultrasonic and acoustic emission techniques were

found to be unsuitable.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by ;emic = composite cylinders; crack detection; eddy current; Non-destructive Evaluation;

pressure vessels; ultrasonics

13. AVAILABILITY

| [ Unlimited

r^^For Official Distribution. Do Not Release to NTIS

| Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

|

! Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGE'

15. P-


t