NATL INST. OF STAND & TECH

A 111 Ob 03A734

NBSIR 86-3492

Evaluation of Cracking in

Aluminum Cylinders

John H. Smith

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Institute for Materials Science and Engineering

Metallurgy Division

Gaithersburg, MD 20899

March 1987

Final Report

s work was sponsored by

partment of Transportation

ishington, DC 20590

JO. Sb-3442.

NBSIR 86-3492

EVALUATION OF CRACKING IN

ALUMINUM CYLINDERS

John H. Smith

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Institute for Materials Science and Engineering

Metallurgy Division

Gaithersburg, MD 20899

March 1987

Final Report

This work was sponsored by

Department of Transportation

Washington, DC 20590

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director

EVALUATION OF CRACKING IN ALUMINUM CYLINDERS

By

John H. Smith

Metallurgy Division

National Bureau of Standards

March 1987

Prepared For: U.S. Department of Transportation

Washington, D.C. 20590

EVALUATION OF CRACKING IN ALUMINUM CYLINDERS

INTRODUCTION : Cracking has been observed in the threaded area of high

pressure (4500 psi) fiberglass-wrapped composite-aluminum (DOT-E-

,7235) compressed gas cylinders and has resulted in leaking and

rupture of a few cylinders in the last three years (Ref. 1).

More recently, the neck and threaded area of some fiberglasswrapped composite-aluminum (DOT-E-7235) as well as some seamless

aluminum (DOT-3A1) cylinders have been visually inspected during

the normal periodic retest. As a result of this visual

inspection at the time of required periodic retesting, several

aluminum cylinders were reported to show evidence of cracking in

the neck and threaded area. A group of cylinders was obtained

for more detailed laboratory examination to determine if cracks

were actually present as reported, to determine the extent of

cracking, and to assess the significance of cracking. This

report describes the results of laboratory tests on cylinders

that were reported to have cracks.

DESCRIPTION OF CYLINDERS USED IN LABORATORY TESTS : Eight cylinders, described in Table I, were obtained

for detailed laboratory evaluation. These cylinders were

supplied to the National Bureau of Standards ( NBS ) by

Mr. Daniel Martin of the Marin Gas Products Company, San Rafael,

California. All eight of the cylinders were manufactured by

Luxfer USA Limited of Riverside, California. Each of these

cylinders was rejected during the normally required hydrostatic

retesting because -of cracks found in the neck and/or threaded

area by careful visual inspection using an intense light and a magnifying mirror for the examination. None were reported to

have experienced leaking during normal service. Five of the

cylinders were solid aluminum DOT-3A1 (or the predecessor type

SP-6498) and three were fiberglass-wrapped composite -aluminum

manufactured under exemption DOT-7235. Two of the type 3A1

cylinders were used in carbon dioxide service at a service

pressure of 1800 psi and the other six cylinders (both type 3A1

and type E-7235) were used as air breathing cylinders at a

service pressure of 2216 psi. The date of manufacture of the

cylinders (as determined from the date of the first hydrostatic

test stamped on the cylinder) ranged from May 1974 to May 1985.

The cylinders were removed from service at the time of

hydrostatic retesting during the first quarter of calender year

1986. This series of cylinders was of particular interest

because significant cracking had only been reported previously in

the higher pressure (4500 psi service pressure) fiberglass1
wrapped composite-aluminum cylinders. Verification of cracking

in lower pressure (1800 psi and 2216 psi) DOT-3A1 solid aluminum

and type DOT-E-7235 hoop-wrapped cylinders would indicate that

cracking is a far more general problem and may be present in a

large number of DOT-3A1 specification cylinders that are widely

used by industry for transporting compressed air and other gases

at service pressures below 3000 psi.

EXPERIMENTAL TESTS AND EXAMINATION ; All cylinders were photographed for documentation and

identification as initially received. Examples of typical

cylinders are shown in Figures 1 and 2. All cylinders were then

examined internally in the neck and threaded area using a fiberoptics light, a small dentist mirror and a binocular microscope

at magnifications between 10X and 30X. This inspection was

repeated several times and by different observers to verify the

presence of cracks or crack-like indications in the neck and

threaded area of each cylinder. The position of all crack-like

indications was marked on the outside of the cylinders.

Two cylinders (serial numbers T-256255 and WK-27055) that

were thought to have significant cracks were radiographed to

verify the presence of cracks. The cylinders were radiographed

by shooting the X-ray beam through the bottom of the cylinder and

placing the recording film on top of the cylinder. This was not

successful and no cracks were found by this procedure.

The heads of the cylinders were then cut off and examined in

more detail using a binocular microscope and viewed from the

inside of the head. The crack-like indications could be seen

more clearly in this way than by examination through the threaded

hole with the mirrors. All cylinder heads were photographed from

the inside at a magnification ranging from 2X to 12X to show the

exact location and appearance of the crack-like indications. In

each case, the most prominent crack-like indication was

designated the "0" position and the other locations were

designated as positions of the clock, i.e., 3, 6, 9, etc. An

example is shown in Figure 3

.

To determine the extent of cracking, each cylinder head was

cut into sections. In each cylinder, the region that had the

largest or most distinct crack-like indication by visual

examination was first examined. For cylinders numbered T-262190,

T-256255, WK-27055, and WK-31631 three-point bend specimens were

cut from the neck area at the "0" position location and broken

open to delineate the extent of the crack. The extent of

cracking can be clearly seen by visual examination of the

fracture surface of the three-point bend specimen and the length

and depth of the crack can be readily measured as shown by the

example in Figure 4

.

2
For the remainder of the cylinders, the extent of the

cracking was determined by an indirect method using measurements

made on metallographic specimens. Specimens used for

metallographic examination were mounted in epoxy, polished, and

etched (in a 10% sodium hydroxide solution). In each case, the

polished and etched specimens were examined in the metallurgical

microscope at magnifications of approximately 9x to 40X. The

specimens were successively sectioned as shown in Figure 5 so

that the extent of cracking could be determined. The top of the

cylinder was used as the reference surface and the length of the

crack on the inner surface of the cylinder was determined by

measuring the height of the metallographic specimen before

examination and then successively cutting off 0.060 to 0.100-inch

from the height of the specimen and remeasuring the height. This

process was repeated until any evidence of cracking had

disappeared.

The depth of the crack perpendicular to the inner surface of

the cylinder (designated as D in Figure 5) was determined by

first measuring the depth of the crack on the metallographic

specimen section as shown in Figure 6 (designated as ' d ’ in

Figure 5). A cross section of each of the necks of the

individual cylinders was then photographed and measured, and the

depth of the crack measured on the metallographic specimen

section (d) and the crack length (1) were plotted on the

photograph of the cylinder cross section. In this case, the

extent of the cracking was reconstructed as shown in Figure 5 and

was related to the thickness of the cylinder neck (designated as

' t ' in Figure 5). The extent of cracking in each cylinder is

described in terms of the depth of cracking perpendicular to the

inner surface of the cylinder (D) relative to the thickness of

the cylinder (t), i.e., t/D and the length of the crack (1) along

the inner surface of the cylinder as shown in Figure 5.

RESULTS

TYPE DOT- 3 Al 1800 CYLINDERS

Two cylinders (serial numbers U-53388 and U-419288) of this

type were obtained. These had been used in carbon-dioxide

service at a service pressure of 1800 psi.

Cylinder No. U-53388 ; This cylinder was manufactured to the DOT-SP-6498

specification (the predecessor specification to DOT-3A1). The

date of first hydrotest (taken as the date of manufacture) was

May 1974. From markings on the cylinder, it is shown that a

subsequent retest was conducted in October 1979. At the time of

the next retest in 1986, visual examination of the threaded and

3
neck area of the cylinder showed crack- like indications in the

neck area of the cylinder and the cylinder was removed from

service for further examination. The detailed visual examination

conducted as part of this investigation showed crack-like

indications at positions "0", "3", and "6". All the crack-like

indications appeared to be approximately 1/ 4-inch long on the

inside surface of the neck just below the straight drilled

portion of the neck. Visual examination showed that none of the

crack- like indications extended into the threads of the cylinder . An example of the most distinct crack-like indication found by

visual examination at position "0" is shown in Figure 7.

Metallographic examination of the crack-like indication at

position "0" was conducted with successive sections taken

starting from the lower part of the inside of the neck of the

cylinder. As shown in Figure 8, the crack-like indications were

identified by metallographic examination to be distinct cracks.

Multiple cracks and branching cracks were found that originatedt

the root of the metal folds in the cylinder neck. The maximum

depth of the crack measured on the metallographic specimen (d)

was approximately 0.043-inch and the metallographic examination

showed that the crack was approximately 1/ 4-inch long and did not

extend into the smooth drilled area outside of the neck area of

the cylinder. This examination did confirm that the crack-like

indications that were observed by visual examination in the neck

of the cylinder were definitely cracks and that multiple cracks

existed that could not be distinguished by visual observation

from the folds in the neck of the cylinder. The thickness of the

cylinder neck is approximately 1-1/16 inch, so as shown in Figure

9, the total depth of cracking is limited to less than 5% of the

wall thickness of the neck after 12 years of service. This

amount of crack growth is not considered to be critical or to

degrade the structural integrity of the cylinder at this time.

Due to the limited extent of cracking at position "0", the other

cracks at positions "3" and "6" were not examined

metallographically at this time.

Cylinder No. U-419288 : This cylinder was manufactured to DOT specification 3A1 and

the date of the first hydrostatic test was May 1985. The neck

and threaded area of the cylinder were examined visually in early

1986 and the cylinder was removed from service at that time due

to the appearance of crack- like indications in the neck. The

detailed visual examination conducted as part of this

investigation showed the presence of crack-like indications at

positions "0", "1", "3", M 5", ”6", "7", "8", "10" and "11". All

of the crack-like indications appeared to be approximately

1/ 4-inch long and none extended into the straight drilled portion

of the cylinder head above the neck . An example of the most

distinct crack-like indication found by visual examination at

position "0" is shown in Figure 10.

4
Metallographic examination of the crack- like indication at

position "0" was conducted with successive sections taken

starting from the lower part of the inside of the neck of the

cylinder. As shown in Figure 11, the crack- like indications were

found by metallographic examination to be distinct cracks.

Multiple cracks and branching cracks were found that originated

at the root of the metal folds in the cylinder neck. The maximum

depth of the crack measured on the metallographic specimen was

approximately 0.022 inch and the metallographic examination

showed that the crack was approximately 1/ 2-inch long but did not

extend into the smooth drilled area outside of the neck area of

the cylinder. This examination did confirm the crack-like

indications that were observed by visual examination in the neck

of the cylinder were definitely cracks and that multiple cracks

existed that could not be distinguished by visual observation

from the folds in the neck of the cylinder. The thickness of the

cylinder neck is approximately 1-inch, so as shown in Figure 12,

the total depth of cracking is limited to less than 5% of the

wall thickness of the neck. This amount of crack growth is not

considered to be critical or to degrade the structural integrity

of the cylinder at this time. Due to the limited extent of

cracking at position M 0", the other cracks at positions "1"

through "11" were not examined metallographically

.

Summary of Test Results for the DQT-3A1 1800 Cylinders : Both of the DOT-3A1 .1800 cylinders show evidence of numerous

small cracks in the neck of the cylinder below the straight

drilled part below the threads. These cracks were all less than

0.050-inch deep and were about 1/4 to 3/4 -inch long. No deep

cracks were found in either of these cylinders. It is noteworthy

that distinct but shallow cracks were found in cylinder number

U-419288 which had been in service for only about six months.

These cracks had the same general appearance as the cracks in the

cylinder number U-53388 that had been in service for

approximately 12 years. This indicates that the crack initiation

occurs at the time of manufacturing or very shortly thereafter.

There was no indication that significant crack propagation had

occurred in either of these cylinders. Careful visual inspection

using a bright light, a dentist mirror, and optical magnification

of about 10X readily found cracks in the neck of this type of

cylinders. However, it is not possible from the visual

inspection alone to determine the depth of the cracks and to

determine if they will significantly degrade the structural

integrity of the cylinders. No evidence exists which shows that

cracks of less than 0.050-inch deep (less than 5% of the

thickness of the necks) will significantly reduce the strength of

the cylinder or cause leaking unless substantial crack

propagation occurs.

5
TYPE D0T-3AL 2216 CYLINDERS

Three cylinders (serial numbers T-266763, T-262190, and

T-256255) of this type were obtained. These cylinders were used

in air breathing service and were filled with compressed air at a service pressure of 2216 psi. These cylinders were manufactured

(as indicated from the date of the first hydrostatic test stamped

on the cylinder) in December 1980 and January 1981. They were

removed from service at the time of their first hydrostatic

retest (required after 5 years service) because of the presence

of crack-like indications found on inspection of the threaded and

neck area of the cylinders. These cylinders were manufactured to

the DOT-E-6498 specification (the predecessor to the present

DOT-3A1 specification)

.

Cylinder No. T-266763 : The detailed visual examination conducted as part of this

investigation showed crack-like indications at positions M 0",

M 2", "3", "6", and "9". All of the crack-like indications

appeared to be approximately 1/4 to 1/ 2-inch long and none

extended into the threaded portion of the cylinder head above the

neck . An example of the most distinct crack found by visual

examination at position "0" is shown in Figure 13.

Metallographic examination of the crack-like indication at

position "0" was conducted with successive sections taken

starting from the lower part of the inside of the neck of the

cylinder. As shown in Figure 14, the crack-like indication was

found by metallographic examination to be a distinct crack.

Multiple cracks and branching cracks were found that originated

at the root of the metal folds in the cylinder neck. The maximum

depth of the crack measured on the metallographic specimen (d)

was approximately 0.023-inch and the metallographic examination

showed that the crack was approximately 1/ 2-inch long but did not

extend into the threaded area outside of the neck area of the

cylinder. This examination confirmed that crack-like

indications that were observed by visual examination in the neck

of the cylinder were definitely cracks and that multiple cracks

existed that could not be distinguished by visual observation

from the folds in the neck of the cylinder. The maximum

thickness of the cylinder neck is approximately 1-inch, so as

shown in Figure 15, the total depth of cracking is limited to

less than 5% of the wall thickness of the neck. This amount of

crack growth is not considered to be critical or to degrade the

structural integrity of the cylinder unless further crack growth

occurs. Due to the limited extent of cracking at position "0”,

the other cracks at positions "2" through ”9" were not examined

metallographically at this time. Only metallographic examination

was done on this cylinder (no three-point bend specimens were

tested)

.

6
Cylinder NO. T- 26 2190 : Visual examination showed the presence of crack- like

indications at positions "0", "2", "3”, "5", "6", "7", "9" and

"10". The crack-like indication at position "0" was about 1-inch

long extended into the first thread and the rest of the

crack-like indications were approximately 1/4- to 1/2-inch long

and did not extend into the threaded portion of the cylinder head

above the neck. An example of the most distinct crack found by

visual examination at position "0" is shown in Figure 16.

A three point bend specimen was made at position "0" and

broken open to delineate the crack. A cracked area was found at

this location but appeared to be less than 0.050-inch deep and

about 1-inch long. No metallographic examination was done of the

crack at position "0".

Metallographic examination of the crack-like indication at

position "6" was conducted with successive sections taken

starting from the lower part of the inside of the neck of the

cylinder. As shown in Figure 17, the crack-like indications were

found by metallographic examination to be distinct cracks.

Multiple cracks and branching cracks were found that originated

at the root of the metal folds in the cylinder neck. The maximum

depth of the crack at position "6" that was measured on the

surface of the metallographic specimen (d) was approximately

0.039-inch and the metallographic examination showed that the

crack was approximately 3/ 8-inch long but did not extend into

the threaded area outside of the neck area of the cylinder. This

examination did confirm that the crack-like indications were

definitely cracks and that multiple cracks existed that could not

be distinguished by visual observation from the folds in the neck

of the cylinder. The maximum thickness of the cylinder neck is

approximately 1-1/8-inch, so as shown in Figure 18, the total

depth of cracking is limited to less than 5% of the wall

thickness of the neck. This amount of crack growth is not

considered to be critical or to degrade the structural integrity

of the cylinder at this time. Due to the limited extent of

cracking at position "6", the other cracks at positions "2"

through "10" were not examined metallographically at this time.

Cylinder No. T-256255 : Visual examination showed the presence of crack-like

indications at positions "0", "3" and "8". The crack-like

indication at position "0" was about 1-inch long. The crack- like

indications at positions "3" and "8" were approximately 1/2-inch

long. None of the crack- like indications extended into the

threaded portion of the cylinder head above the neck . An example

of the most distinct crack- like indication found by visual

examination at position "0" is shown in Figure 19.

7
A three point bend specimen was made at position "0" and

broken open to delineate the crack. This crack was very distinct

as shown in Figure 20. The crack was about 0.30-inch deep and

5/ 8-inch long. The maximum thickness of the cylinder neck is

approximately 1-inch, so the crack is nearly 1/3 of the way

through the neck of the cylinder. This crack clearly shows the

extent of cracking that can occur in these DOT-3A1 type cylinders

and shows that even a crack of this depth does not extend into

the threaded area of the neck and would not be detected just by

looking only at the threaded area of the cylinder. This was the

largest crack that was detected in the eight cylinders that were

examined in this investigation. An analysis of the structural

significance of this crack was not made but it is reasonable to

expect that a crack of this depth would grow to the extent that

leakage of the cylinder would occur before the next required

hydrostatic test. No metallographic examination was done of the

crack at position "0". The scanning electron microscope

examination that was done of the fracture surface and did

indicate that the crack growth was by the slow crack growth mode

previously found in the high pressure aluminum cylinders (Ref. 2).

Metallographic examination of the crack at position "3" was

conducted with successive sections taken starting from the lower

part of the inside of the neck of the cylinder. As shown in

Figure 21, the crack-like indications were identified by

metallographic examination to be distinct cracks. Multiple

cracks and branching cracks were found that originated at the

root of the metal folds in the cylinder neck. The maximum depth

of the crack measured on the metallographic specimen (d) at

position "3" was approximately 0.020-inch deep and the

metallographic examination showed that the cracks were

approximately 3/ 4- inch long but did not extend into the threaded

area outside of the neck area of the cylinder. This examination

did confirm that the crack- like indications that were observed by

visual examination in the neck of the cylinder were definitely

cracks and that multiple cracks existed that could not be

distinguished by visual observation from the fold in the neck of

the cylinder.

Summary of Test Results for the DQT-3A1 2216 Cylinders : The results of these tests show conclusively that

substantial cracking can occur in DOT-3A1 cylinders at a service

pressure of 2216 psi after only five years of service. This is

particularly well shown by the substantial crack found in

cylinder No. T-256255. Other cracks in the same cylinder and in

the other two cylinders examined were less than 0.050-inch and

are not believed to be structurally significant at this time but

may be initiation sites for crack propagation that in the future

will degrade the structural integrity of the cylinder. From the

visual inspections done in this investigation, including the

8
examination on the inside of the cylinder heads at magnification

levels up to 30X, it is not possible to distinguish between the

large and significant cracks (such as the one that was

approximately 0.30-inch deep) and the many shallow cracks (less

than 0.050-inch) that are not thought to be structurally

significant. Therefore, visual examination alone is not

sufficient to distinguish between those cylinders that have

cracks that are likely to propagate to the point of leaking

before the next hydrostatic retest and those cylinders that have

cracks that are not structurally significant.

TYPE DOT-E-7235 2216 HOOP WRAPPED CYLINDERS

Three cylinders (serial numbers WK-31111, WK-32699, and

WK-31631 ) of this type were obtained. These cylinders were used

in air breathing service and were filled with compressed air at a

service pressure of 2216 psi. These cylinders were manufactured

(as indicated from the date of the first hydrostatic test stamped

on the cylinder) in the period of January 1982 through March

1982. They were removed from service at the time of the first

hydrostatic retest (required after 3 years service for

hoop-wrapped cylinders) because of the presence of crack-like

indications found on inspection of the threaded and neck area of

the cylinders.

Cylinder No. WK-31111 : Visual examination showed the presence of crack- like

indications at positions "0", "6" and "10". All the crack-like

indications were approximately 1/ 2-inch long and none extended

into the threaded portion of the cylinder head above the neck . An example of the most distinct crack found by visual examination

at position "0" is shown in Figure 22. No three point bend

specimens were taken from this cylinder.

Metallographic examination of the crack-like indications at

position "0" was conducted with successive sections taken

starting from the lower part of the inside of the neck of the

cylinder. As shown in Figure 23 distinct cracks were found in

the wall of the neck region of the cylinder. Multiple cracks and

branching cracks were found that originated at the root of the

metal folds in the cylinder neck. The maximum depth of the crack

at position "0" that was measured on the metallographical

specimen was approximately 0.031-inch and the metallographic

examination showed that the crack was approximately 1/4-inch long

but did not extend into the threaded area outside of the neck

area of the cylinder. This examination confirmed the crack- like

indications which were observed by visual examination in the neck

of the cylinder were definitely cracks and that multiple cracks

existed that could not be distinguished by visual observation

from the fold in the neck of the cylinder. The maximum thickness

9
of the cylinder neck is approximately 3 /4-inch, so as shown in

Figure 24, the total depth of cracking is limited to less than 5%

of the wall thickness of the neck. This amount of crack growth

is not considered to be critical or to degrade the structural

integrity of the cylinder at this time. Due to the limited

extent of cracking at position "0", the other crack- like

indications at positions "6" and "10" were not examined

metallographically at this time.

Cylinder No. WK-32699 : Visual examination conducted as part of this investigation

showed the presence of crack-like indications at positions "0"

and "2". Both crack-like indications were approximately 1/2-inch

long. Neither of the crack-like indications extended into the

threaded portion of the cylinder head above the neck . An example

of the most distinct crack found by visual examination at

position "0” is shown in Figure 25. A three point bend specimen

was cut from the cylinder at position ”0" and tested in an

attempt to delineate the crack at this position. This specimen

did not fracture at the correct position to determine the crack

size in this manner. However, the size of the crack at this

position could still be determined by metallographic examination.

Metallographic examination of the crack-like indication at

position "0" was conducted with successive sections taken

starting from the lower part of the inside of the neck of the

cylinder. As shown in Figure 26, the crack-like indication was

identified by metallographic examination to be a distinct crack.

Multiple cracks and branching cracks were found that seemed to

originate at the root of the metal folds in the cylinder neck.

The maximum depth of the crack measured on the metallographic

specimen (d) at position "0" was approximately 0.126-inch and the

metallographic examination showed that the cracks were

approximately 5/8-inch long but did not extend into the threaded

area outside of the neck area of the cylinder. This is a substantial crack and indicates that substantial crack

propagation had occurred during the three years that the cylinder

was in service. The maximum thickness of the cylinder neck is

approximately 3/4-inch, so as shown in Figure 27, the total depth

of cracking was approximately 8% of the thickness of the neck.

This crack clearly shows the extent of cracking that can occur in

this type of hoop wrapped cylinder and shows that even a crack of

this length and depth does not extend into the threaded area of

the neck and would not be detected just by only inspecting the

threads of the cylinder. An analysis of the structural

significance of this crack was not made at this time but it is

expected that a crack of this depth would grow to the extent that

leakage of the cylinder could occur before the next required

hydrostatic test. This examination did confirm that the cracklike indications observed by visual examination in the neck of

the cylinder were definitely cracks and that multiple cracks

10
existed that could not be distinguished by visual observation

from the fold in the neck of the cylinder. The other crack at

position "2" was not examined metallographically at this time.

Cylinder No. Wk-31631 : The detailed visual examination conducted as part of this

investigation showed the presence of crack- like indications at

positions "0, M "l" , "2" , ”3" , "5" , "6" "8", "9", "10", and "11. The

crack-like indications were approximately 1/2-inch long and that

none extended into the threaded area of the cylinder head above

the neck . An example of the most distinct crack-like indication

found by visual examination at position ”0" is shown in

Figure 28. A three point bend specimen was cut from the cylinder

at position "0" and tested in an attempt to delineate the crack

at this position. This specimen did not fracture at the correct

position to determine the crack size in this manner. However,

the size of the crack at this position could still be determined

by metallographic examination.

Metallographic examination of the crack-like indication at

position "0" was conducted with successive sections taken

starting from the lower part of the inside of the neck of the

cylinder. As shown in Figure 29, distinct cracks were found in

the wall of the neck region of the cylinder. Multiple cracks and

branching cracks were found that originated at the root of the

metal folds in the cylinder neck. The depth of the largest crack

measured on the metallographic specimen (d) at position "0" was

approximately 0.236-inch and the metallographic examination

showed that the crack was approximately 5/8-inch long but did not

extend into the threaded area outside of the neck area of the

cylinder. This is a substantial crack and indicates that

substantial crack propagation has occurred during the three years

that the cylinder was in service. The maximum thickness of the

cylinder neck is approximately 3/4-inch, so as shown in Figure

30, the total depth of cracking was approximately 15% of the

thickness of the neck. This crack clearly shows the extent of

cracking that can occur in these hoop wrapped cylinders and shows

that even a crack of this depth does not extend into the threaded

area of the neck and would not be detected by only looking at the

threads of the cylinder. An analysis of the structural

significance of this crack was not made at this time but it is

expected that a crack of this depth would be expected to grow to

the extent that leakage of the cylinder would occur before the

next required hydrostatic test. This examination did confirm

that the crack- like indications observed by visual examination in

the neck of the cylinder were definitely cracks and that multiple

cracks existed that could not be distinguished by visual

observation from the fold in the neck of the cylinder. The other

crack-like indications at positions "1" through "11", were not

examined metallographically at this time.

11
Summary of Results on DOT-E-7235 2216 Cylinders : Two of the three DOT-E-7235 2216 hoop-wrapped cylinders

showed significant cracks that were from 8% to 15% through the

thickness of the neck of the cylinder after only three years of

normal service. This indicates that this cylinder series is

definitely subject to crack propagation during normal service.

From the visual inspections done in this investigation,

including the examination on the inside of the cylinder heads at

magnification levels up to 30X, it is not possible to distinguish

between the large and significant cracks and the many shallow

cracks (less than 0.050-inch) that are not thought to be

structurally significant. Therefore, visual examination alone is

not sufficient to distinguish between cylinders that have cracks

that are likely to propagate to the point of leaking before the

next hydrostatic retest and those vessels that have cracks that

are not structurally significant.

DISCUSSION : The present investigation on a group of D0T-3A1 and

hoop-wrapped composite-aluminum (DOT-E-7235) cylinders shows that

all of the cylinders contained cracks in the neck area of the

cylinders, as shown in Table II. In all cases examined there

were multiple, branched cracks originating at the root of folds

in the neck of the cylinders. These cracks were generally less

than 0.050-inch deep and were up to approximately 1-inch long.

These shallow cracks can form the initiation sites for further

crack propagation during the life of the cylinder. Similar

cracking has been recently reported by investigations done by

Alcan Ltd. (Ref. 2) where it was concluded that the shallow

cracks are thought to be created at the time of or shortly after

the cylinder is manufactured.

In the DOT- 3 Al cylinders with a marked service pressure of

1800 psi used to transport carbide dioxide, only the shallow

cracks were found (less than 0.050-inch). However, in both the

DOT-3A1 and hoop-wrapped composite-aluminum (DOT-E-7235)

cylinders operated at 2216 psi service pressure with compressed

air, one or more of each type of cylinder had deeper cracks that

are believed to indicate the occurrence of substantial crack

propagation during service. This indicates that the higher

stresses produced in the cylinder neck at a service pressure of

2216 psi cause an increase in the crack growth rate. At the time

of the first required hydrostatic retest, cracks had propagated

up to approximately 1/3 of the way through the thickness of the

neck of the cylinders. This represents substantial crack

propagation during normal service. Such crack propagation can

reasonably be expected to extend to the point where these

cylinders could leak before the next required hydrostatic retest.

12
From the visual inspections done in this investigation,

including the examination on the inside of the cylinder heads at

magnification levels up to 30X, it is not possible to distinguish

between the large and significant cracks and the many shallow

cracks (less than 0.050-inch) that are not thought to be

structurally significant. Therefore, visual examination alone is

not sufficient to distinguish between those cylinders that have

cracks that are likely to propagate to the point of leaking

before the next hydrostatic retest and those cylinders that have

cracks that are not structurally significant at the time of

inspection.

CONCLUSIONS :

1. Shallow cracks (less than 0.050-inch deep) were found

in all of the D0T-3A1 and hoop-wrapped compositealuminum (DOT-E-7235) cylinders that were examined as

part of this investigation.

2. Deep cracks that may be structurally significant and

are expected to propagate in service were found in one

of the three D0T-3A1 and two of the three hoop-wrapped

composite-aluminum cylinders that had been operated at

a service pressure of 2216 psi. No deep cracks were

found in the D0T-3A1 cylinders designed for and marked

1800 psi service pressure, that were, in carbon dioxide

service

.

3. None of the cracks found in this group of cylinders

propagated beyond the neck of the cylinder and into the

threads of the cylinder even when the cracks were up to

approximately 1/3 of the way through the thickness of

the neck.

4. By any of the visual or optical inspection methods

used, it was not possible to distinguish between the

shallow cracks (less than 0.050-inch) that are not

thought to be structurally significant and the deep

cracks that have propagated through a large portion of

the cylinder neck and are thought to be structurally

significant.

13
RECOMMENDATIONS

:

1 . Additional inspections of cylinders that have been in

service for varying lengths of time should be conducted to

determine if all D0T-3A1 and hoop-wrapped compositealuminum (DOT-E-7235) cylinders are subject to cracking and

to determine if service pressure, time of use, and commodity

carried have a significant effect on the cracking.

2. As a minimum, careful visual inspection should be required

of the threaded and neck area of all aluminum and

hoop-wrapped composite-aluminum cylinders at the time of the

of the required periodic hydrostatic retest.

3. Continued efforts should be made to develop a method to

accurately determine the depth of the cracks so that only

those cylinders that have shown substantial crack

propagation will be rejected during inspection.

4. All newly manufactured cylinders should be carefully

inspected for the presence of cracks. Cylinders having

evidence of cracks should be rejected or treated to remove

the cracks.

14
REFERENCES

:

1. A.R. Woodward, "Examination of Type L45W Gas Cylinder

Manufactured by Luxfer USA Ltd.", Report by Luxfer Holdings

Ltd., May 1984.

2. N.J.H. Holroyd and A.R. Woodward, "Report on Study of

Cracking in Aluminum Fibre-Wound High Pressure Gas

Cylinders", Report by Luxfer Holdings Ltd., Aug. 1986.

15
TABLE I

DESCRIPTION OF CYLINDERS EXAMINED

Cylinder

Number

Type

of

Cylinder

Service

Pressure

(psi)

Gas Date of

Manufacture

U-53388 3A1 1800 C02 5/74

U-419288 3A1 1800 C02 5/85

T-262190 3A1 2216 Air 3/81

T-266763 3 Al 2216 Air 3/81

T-256255 3A1 2216 Air 12/80

WK-32699 Composite 2216 Air 3/82

WK-31631 Composite 2216 Air 2/82

WK-31111 Composite 2216 Air 1/82

16
TABLE II

EXTENT OF CRACKING IN EACH CYLINDER

Cylinder

Number

Cylinder

Type/

Pressure

Crack Depth

Measured on

Metallographic

Specimen

(d)

( in. )

Maximum

Crack

Length

(1)

( in. )

Crack

Depth

Through

Cylinder

Wall

(D)

( in. )

Crack

Depth/

Thick.

(D/t)

(%)

U-53388 3A1-1800 0.043 1/4 0.040 4

U-419288 3A1-1800 0.022 1/2 0.030 2

T-266763 3A1-2216 0.023 1/2 0.020 2

T-262190 3A1-2216 0.039 3/8 0.040 4

T-256255 3A1-2216 5/8 0.300 30

WK-31111 Hoop-Wrapped

2216

0.031 1/4 0.040 5

WK-32699 Hoop Wrapped

2216

0.126 5/8 0.060 8

WK-31631 Hoop-Wrapped

2216

0.236 5/8 0.110 15

d -- depth of crack measured on metallographic specimen

D -- depth of crack perpendicular to the inner surface of the

cylinder

1 -- length of crack along inner surface of cylinder

D/t -- depth of penetration of crack through thickness of cylinder

neck

17
.

.

o
Example of Type 3 A1 Cylinder

Fig 1
Example of Hoop-Wrapped Cylinder

Fig . 2
(Numbers Show Position of Cracks)

Macroscopic View of Inside of

Cylinder Head Mag. 3x

Fig 3
Example of Cracked Area in

Three-Point Bend Specimen

Fig. 4
Reference Surface

thickness (t)

Depth (d)

From Micrograph

Cracked

Area

Depth of Crack

1 to Surface (D)

Procedure for Sectioning

of Specimen

Fig. 5
Method of Measuring Crack Depth

on Metallographic Specimen

Fig

.

6
Crack-Like Indication

Cyl. No. U-53388

Fig. 7

Mag. lOx

i

,
Primary Crack in Cylinder Neck

Cyl. No. U-53388 Mag. 40x

Fig. 8
Extent of Primary Crack

Cyl. No. U-53388

Fig. 9
Crack-Like Indication

Cyl. No. U-419288 Mag. 6x

Fig. 10
Primary Crack in Cylinder Neck

Cyl. No. U-419288 Mag. 40x

Fig. 11
Extent of Primary Crack

Cyl. No. U-419288

Fig 12
Crack-Like Indication

Cyl. No. T-266763 Mag. lOx

Fig. 13
Primary Crack in Cylinder Neck

Cyl. No. T-266763 Mag. 40x

Fig. 14
Cracked

Area

Extent of Primary Crack

Cyl. No. T-266763

Fig. 15
Crack-Like Indication

Cyl. No. T-262190 Mag. lOx

Fig. 16
Primary Crack in Cylinder Neck

Cyl. No. T-262190 Mag. 40x

Fig. 17
Extent of Primary Crack

Cyl. No. T-262190

Fig. 18
Crack-Like Indication

Cyl. No. T-256255 Mag. 12x

Fig. 19
Extent of Primary Crack

Three-Point Bend Specimen

Cyl. No. T-256255

Fig. 20
Crack at Position ' 3

Cyl. No. T-256255 Mag. 40x

Fig. 21
Crack-Like Indication

Cyl. No. WK-31111 Mag. 12x

Fig. 22
Primary Crack in Cylinder Neck

Cyl. No. WK-31111 Mag. 40x

Fig. 23
Extent of Primary Crack

Cyl. No. WK-31111

Fig. 24
Cyl.

Crack-Like Indication

No. WK-32699 Mag. lOx

Fig. 25
Primary Crack in Cylinder Neck

Cyl. No. WK-32699 Mag. 40x

Fig. 26
Extent of Primary Crack

Cyl. No. WK-32699

Fig. 27
Crack-Like Indication

Cyl. No. WK- 31631 Mag. 12x

Fig. 28
Primary Crack in Cylinder Neck

Cyl. No. WK-31631 Mag. 18x

Fig. 29
Cracked Area

Extent of Primary Crack.

Cyl. No. WK-31631

Fig. 30
< BS-114A irev. 2 -eo

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

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

SHEET (See instructions) 86-3492 March 1987

4. TITLE AND SUBTITLE

Evaluation of Cracking in Aluminum cylinders

5. AUTHOR(S)

John H. Smith

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

DTRS 5683

8. Type of Report & Period Covered

Final

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

Research and Special Programs Administration

U.S. Department of Transportation

400 7th Street, S.W.

Wa dh-j not-nn t D 01 905Q0

10. SUPPLEMENTARY NOTES

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

111. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here) Cracking was first observed in the threaded area of high pressure (4500 psi)

fiberglass-wrapped composite-aluminum compressed gas cylinders and has

resulted in the leaking and rupture of a few of these cylinders in the last

three years (Ref. 1). More recently, the neck and threaded area of some fiberglass-wrapped composite-aluminum and some seamless aluminum (type 3A1

)

cylinders have been visually inspected during the normal periodic retest.

As a result of this visual inspection at the time of retesting, several

aluminum cylinders were reported to show evidence of cracking in the neck and

threaded area. A group of these cylinders was obtained for more detailed

laboratory examination to determine the extent of the cracking and to

attempt to assess the significance of the cracking. This report describes

the results of the laboratory tests on this group of cylinders that were

reported tO'have cracks.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) aluminum; aluminum alloy no. 6351 ; crack growths; cracking;

pressure vessels; sustained load cracking

13. AVAILABILITY 14. NO. OF

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