United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-93/092 July 1993
&EPA Project Summary
Stress Cracking Behavior of
HOPE Geomembranes
And its Prevention
Robert M. Koerner, Yick G. Hsuan, and Arthur E. Lord, Jr.
The project summarized here focused
on the stress cracking behavior and
testing of geomembranes made from
high-density polyethylene (HOPE). The
project included a literature study of
stress cracking failures in the field and
exhumation of geomembranes at vari-
ous sites of opportunity. In total,
twenty-five cases of stress cracking
have been documented since the intro-
duction of this particular material in
1980.
Because of the limitations of the cur-
rently used ASTM D 1693 "bent strip"
test method for evaluating stress crack-
ing behavior of HOPE geomembranes,
a new test was developed—the notched,
constant-tensile-load (NCTL) test. Eigh-
teen commercially available sheet ma-
terials and seven field-exhumed
geomembranes were evaluated using
the test. The results led to the recom-
mendation that for an HOPE
geomembrane to be acceptably crack
resistant, the transition time to the on-
set of brittle behavior must be greater
than 100 hr. The results also indicated
that the NCTL test is much more chal-
lenging than is the bent strip test.
In addition, two related tests, a single-
point, notched, constant-tensile-load
(SP-NCTL) test and a seam, constant-
tensile-load (SCTL) test was estab-
lished. The recommendations for both
of these tests were that no failure
should occur within 200 hr at an ap-
plied stress of 30% of yield stress of
the sheet for five replicate tests.
The implementation of this set of
three tests is oriented toward resin sup-
pliers, manufacturers, designers, and
owners involved with HOPE geomem-
branes. By employing the recommen-
dations made in the full report, the in-
cidence of stress cracking of HOPE
geomembranes in the field should be
significantly reduced.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, Ohio, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
HOPE is a widely used polymer for
manufacturing the geomembranes used
in waste containment facilities, partly be-
cause the relatively high crystallinity of the
material results in excellent chemical re-
sistance. This high crystallinity, however,
increases the tendency of the material to
stress crack, i.e., the failure of the
geomembrane under stress in a brittle
manner so that it exhibits little or no elon-
gation adjacent to the failure surfaces.
The initial task of the project was a
literature search and subsequent field in-
vestigations of stress crack occurrences
involving HOPE geomembranes. Subse-
quent tasks focused on laboratory testing.
The limitation of the currently used ASTM
D 1693 "bent strip" test method led to the
investigation of an alternative test to evalu-
ate stress cracking—the NCTL test. The
NCTL test procedure, its reproducibility,
and its evaluation of commercial sheets
and field-exhumed samples were deter-
Printed on Recycled Paper
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mined with the use of commercially avail-
able geomembranes and field-exhumed
geomembranes that showed varying de-
grees of stress cracking. The test results
led to a quantitative recommendation for
acceptance of HOPE geomembranes. In
addition, we also compared the NCTL test
with the conventionally used bent strip
test.
Because of the long testing time re-
quired to conduct the NCTL test, a shorter
test, the SP-NCTL test, was developed.
The SCTL test was also developed to
evaluate stress cracking resistance of dif-
ferent types of HOPE geomembrane
seams.
Survey on Occurrences of
HOPE Stress Cracking in the
Field
From both the literature and field inves-
tigations it was found:
• stress crack incidents had occurred,
• cracking generally occurred or was
initiated at the seams,
• the geomembranes were exposed,
i.e., non soil covered, in all cases,
• all parts of the United States (location
- hot/cold, humid/arid, etc.) were
involved,
• the actual service life varied 1 to 10
yr, and
• a variety of different resins and
manufacturers were involved.
NCTL Test Procedure
As developed in this project, NCTL test
specimens are dumbbell shaped with di-
mensions according to ASTM D 1822. A
controlled notch is introduced into one side
of the surface to produce a "ligament" of
80% of the nominal thickness of the sheet.
After fixing the specimens in position in
the test device and loading them to vari-
ous percentages of their yield stress they
are immersed in a constant temperature
bath maintained at 50°C. The bath is filled
with a surface active wetting liquid. Fail-
ure occurs as either ductile elongation (at
the higher stress levels) or as brittle fail-
ure (at the lower stress levels). The failure
times of the test specimens are recorded
to an accuracy of 0.1 hr.
To obtain the test response resulting
from a series of differently stressed test
specimens in the NCTL test, the logarithm
of percent yield stress is plotted against
the logarithm of average failure time for
the entire set of test specimens evaluated
(see Figure 1). The resulting curve con-
sists of two uniquely linear regions. The
linear portion of the curve at high stresses
and short failure times is called the "duc-
tile" region." The linear portion at low
stresses and long failure times is called
the "brittle region." The portion of the
curves that separates these two regions
is called the 'transition region." Four unique
characteristics, or parameters, can be ob-
tained from the resulting curve:
• time for onset of the brittle region,
defined as the transition time,
• stress at onset of the brittle region,
defined as the transition stress,
• slope of the ductile portion of the
curve, and
• slope of the brittle portion of the curve.
Of these, the "transition time" was se-
lected as the parameter of primary focus.
Reproducibility of NCTL Test
Results
To evaluate the tests reproducibility,
three complete sets of NCTL tests were
performed on a sample of the same type
and lot of a 2-mm (80-mil) HOPE geo-
membrane. The results indicated that the
coefficient of variation of the transition
times was within 12%. This was consid-
ered acceptable reproducibility at least
within a single test laboratory.
The test method has also been evalu-
ated by independent laboratories in con-
nection with ASTM interlaboratory tests to
establish precision and bias. Although the
tests are not concluded, the results to
date are very promising.
NCTL Test Results and
Recommendation
The results from the 18 commercial
geomembranes indicated a wide variation in
their transition time values, ranging from 10 to
5000 hr. In contrast, there was much less
variation in the transition stress, the variation
ranging from 28% to 40% of yield stress.
Thus it was concluded that transition time
should be the parameter of choice for com-
paring and contrasting stress cracking resis-
tance of different HOPE geomembranes. The
transition times from the NCTL test of the
seven field-exhumed geomembranes ranged
from 4 hr to 97 hr. Since the field
geomembranes had actually shown various
degrees of stress cracking during their ser-
vice periods, the recommended acceptance
criteria must be greater than 97 hr, which was
the best of the field samples. The nearest
round number that is higher than 97 hr is 100
hr, which was selected as being the minimum
recommended acceptable transition time. It
should be noted that of the 18 commercial
geomembranes, 10 are higher than 100 hr
transition time and thus are acceptable under
this recommendation.
Comparison between NCTL
Test and Bent Strip Test
Results
The currently used test method that
qualifies the stress cracking resistance of
HOPE Membranes is ASTM D 1693, the
bent strip test. The results, given as either
pass or fail, are based on a criterion de-
fined by the National Sanitation Founda-
tion in NSF #54. Fourteen of the 18 com-
mercial geomembranes were so tested
and only one geomembrane failed the cri-
terion. In contrast, 7 out of the same 14
geomembrane samples failed the NCTL
test when using the criterion of 100 hr
transition time. Of the 7 field-exhumed
geomembranes, 4 failed the bent strip test.
In contrast, the NCTL test criterion dis-
qualified all 7 of the geomembranes since
the acceptance criterion used this set of
data as its control group. This indicates
that the bent strip test cannot adequately
evaluate the stress cracking resistance of
HOPE geomembranes.
Single-Point, Notched,
Constant-Tensile-Load (SP-
NCTL) Test
An SP-NCTL test was developed to de-
crease the testing time of the complete
NCTL test so that the stress cracking re-
sistance of the geomembrane could be
confirmed on a pass/fail basis within a
relatively short period of time. The con-
cept of the test was to select a single,
applied stress level slightly below the ini-
tial onset of the brittle region of the
geomembrane and then observe whether
the failure time of the test specimens ex-
ceeded a specified value. The single ap-
plied stress level has been set at 30% of
the yield stress of the material, and the
minimum time recommended for accep-
tance is 200 hr. Five replicate test speci-
mens should be evaluated, and all should
pass the minimum recommended value.
Seam Constant-Tensile-Load
(SCTL) Test
An SCTL test was also developed to
evaluate the quality of an HOPE
geomembrane seam to see whether it has
a stress crack resistance comparable to
the sheet material. The sheet material and
the seam can be directly compared only if
their stressed geometries are the same.
Hence, seam test specimens notched as
described above were used to develop
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the SCTL test. Paralleling the single-point
test, the selected applied stress was 30%
of the yield stress of the sheet material
and the minimum failure time recom-
mended for acceptance is 200 hr. Five
test specimens should be evaluated, and
all should pass the minimum recom-
mended value.
Conclusion
Because of the concern about HOPE
geomembranes stress cracking, a suitable
laboratory test is essential to evaluate com-
mercially available resins, their formula-
tion, and their manufacture. In this project,
the NCTL test was developed and used to
evaluate 18 commercially available
geomembranes and 7 field-exhumed
geomembranes. The results revealed a
large variation in the transition time of the
onset of brittle failure. An evaluation of
the test results led to a recommendation
that the transition time for an acceptable
HOPE geomembrane should be 100 hr or
longer.
The comparison between the NCTL test
and the currently used bent strip test indi-
cated that the bent strip test is inadequate
to assess the stress cracking resistance
of HOPE geomembranes. The NCTL test
is much more challenging and authorita-
tive.
Since completing the NCTL test curve
in its entirety is time consuming, a single-
point test (SP-NCTL) has been developed
as a QC/QA test. In this test, five speci-
mens are to be tested under an applied
tensile load equal to 30% of the yield
stress. None of the test specimens should
fail within 200 hr.
To evaluate HOPE geomembrane
seams, the SCTL test was developed. The
targeted use of the test is evaluating re-
search and development methods for new
seams or new seaming devices. Five
notched-seam specimens are tested un-
der a tensile load equal to 30% of yield
stress of the sheet. As with the single-
point test, none of the specimens should
fail within 200 hr.
The full report was submitted in fulfill-
ment of Cooperative Agreement No. CR-
815692 by Drexel University under the
sponsorship of the U.S. Environmental Pro-
tection Agency.
Percent
Yield
Stress
100 -
50-
25
10
"Knee"
0.1 1 10
(a) A Bi-Linear (or "Knee") Response Curve
100 -
100 1000
Failure Time (hr)
i
10,000
'Nose*
50 -
Percent
Yield
Stress
10
0.1 1
(a) An Overshoot (or "Nose") Response Curve
1000
Failure Time (hr)
10,000
"Step"
Yield
Stress 25 —
10
\
"\
, r<
1 I I I 1
0.1 1 10 100 1000
\.
I
10,000
(c) A Tri-Linear (or "Step") Response Curve
Failure Time (hr)
Figure 1. Typical response curves resulting from a complete notched constant tensile load (NCL T) test.
•&U.S. GOVERNMENT PRINTING OFFICE: W93 - 75047I/M020
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P.M. Koerner, Y.G. Hsuan, andA.E. Lord, Jr., are with Drexel University,
Philadelphia, PAW 104.
David A. Carson is the EPA Project Officer (see below).
The complete report, entitled "Stress Cracking of High Density Polyethylene
Geomembranes and Its Prevention," (Order No. PB93-196616AS; Cost:
$27.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
EPA/600/SR-93/092
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