ISO/TS 20432:2022
(Main)Guidelines for the determination of the long-term strength of geosynthetics for soil reinforcement
Guidelines for the determination of the long-term strength of geosynthetics for soil reinforcement
This document provides guidelines for the determination of the long-term strength of geosynthetics for soil reinforcement. This document describes a method of deriving reduction factors for geosynthetic soil-reinforcement materials to account for creep and creep rupture, installation damage and weathering, and chemical and biological degradation. It is intended to provide a link between the test data and the codes for construction with reinforced soil. The geosynthetics covered in this document include those whose primary purpose is reinforcement, such as geogrids, woven geotextiles and strips, where the reinforcing component is made from polyester (polyethylene terephthalate), polypropylene, high density polyethylene, polyvinyl alcohol, aramids and polyamides 6 and 6,6. This document does not cover the strength of joints or welds between geosynthetics, nor whether these might be more or less durable than the basic material. Nor does it apply to geomembranes, for example, in landfills. It does not cover the effects of dynamic loading. It does not consider any change in mechanical properties due to soil temperatures below 0 °C, nor the effect of frozen soil. The document does not cover uncertainty in the design of the reinforced soil structure, nor the human or economic consequences of failure.
Lignes directrices pour la détermination de la résistance à long terme des géosynthétiques pour le renforcement du sol
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Standards Content (Sample)
TECHNICAL ISO/TS
SPECIFICATION 20432
First edition
2022-12
Guidelines for the determination
of the long-term strength of
geosynthetics for soil reinforcement
Lignes directrices pour la détermination de la résistance à long terme
des géosynthétiques pour le renforcement du sol
Reference number
© ISO 2022
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ii
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, abbreviated terms and symbols . 1
3.1 Terms and definitions . 1
3.2 Abbreviated terms . 2
3.3 Symbols . 3
4 Design procedure . 4
4.1 General . 4
4.2 Design lifetime . 4
4.3 Causes of degradation . 5
4.4 Design temperature . 5
5 Determination of long-term creep strain . 5
5.1 General . 5
5.2 Extrapolation . 6
5.3 Time-temperature superposition methods . 6
5.4 Isochronous curves . 7
5.5 Weathering, chemical and biological effects . 7
6 Determination of long-term strength . 8
6.1 Tensile strength . 8
6.2 Reduction factors . 8
6.3 Modes of degradation . 8
7 Creep rupture . 9
7.1 General . 9
7.2 Measurement of creep rupture: conventional method . 10
7.3 Curve fitting (conventional method) . 11
7.4 Curve fitting for time-temperature block shifting of rupture curves .13
7.5 Strain shifting and the stepped isothermal method . 14
7.6 Extrapolation and definition of reduction factor or lifetime . 15
7.7 Residual strength . 16
7.8 Reporting of results . 16
7.9 Procedure in the absence of sufficient data . 16
8 Installation damage . .17
8.1 General . 17
8.2 Data recommended . 17
8.3 Calculation of reduction factor . 18
8.4 Procedure in the absence of direct data . 18
8.4.1 General . 18
8.4.2 Interpolation from measurements with different soils. 18
8.4.3 Interpolation between products of the same product line . 19
8.4.4 Laboratory damage tests . 19
9 Weathering, chemical and biological degradation .19
9.1 General . 19
9.2 Data recommended for assessment . 20
9.3 Weathering . 20
9.4 Chemical degradation . 21
9.4.1 Causes of chemical degradation. 21
9.4.2 Evidence from service experience . 21
9.4.3 Accelerated chemical degradation tests .22
9.4.4 Oxidation of polyolefins . 26
9.4.5 Hydrolysis of polyesters . 27
iii
9.4.6 Procedure in the absence of sufficient data .29
9.5 Biological degradation .29
10 Determination of long-term strength .29
10.1 Factor of safety f .29
s
10.2 Design for residual strength .30
11 Reporting .30
Bibliography .31
iv
Foreword
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bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 221, Geosynthetics.
This first edition of ISO/TS 20432 cancels and replaces ISO/TR 20432:2007, which has been technically
revised. It also incorporates the Technical Corrigendum ISO/TR 20432:2007/Cor 1:2008.
The main changes are as follows:
— Subclause 7.4 has been modified to further detail and clarify the fitting of linear regression curves
to time-temperature block shifted creep-rupture test results.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
v
TECHNICAL SPECIFICATION ISO/TS 20432:2022(E)
Guidelines for the determination of the long-term strength
of geosynthetics for soil reinforcement
1 Scope
This document provides guidelines for the determination of the long-term strength of geosynthetics for
soil reinforcement.
This document describes a method of deriving reduction factors for geosynthetic soil-reinforcement
materials to account for creep and creep rupture, installation damage and weathering, and chemical
and biological degradation. It is intended to provide a link between the test data and the codes for
construction with reinforced soil.
The geosynthetics covered in this document include those whose primary purpose is reinforcement,
such as geogrids, woven geotextiles and strips, where the reinforcing component is made from polyester
(polyethylene terephthalate), polypropylene, high density polyethylene, polyvinyl alcohol, aramids
and polyamides 6 and 6,6. This document does not cover the strength of joints or welds between
geosynthetics, nor whether these might be more or less durable than the basic material. Nor does it
apply to geomembranes, for example, in landfills. It does not cover the effects of dynamic loading. It does
not consider any change in mechanical properties due to soil temperatures below 0 °C, nor the effect of
frozen soil. The document does not cover uncertainty in the design of the reinforced soil structure, nor
the human or economic consequences of failure.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 10318-1, Geosynthetics — Part 1: Terms and definitions
3 Terms, definitions, abbreviated terms and symbols
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 10318-1 and the following
apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1.1
long-term strength
load which, if applied continuously to the geosynthetic during the service lifetime, is predicted to lead
to rupture at the end of that lifetime
3.1.2
reduction factor
factor (≥ 1) by which the tensile strength is divided to take into account particular service conditions in
order to derive the long-term strength
Note 1 to entry: In Europe, the term 'partial factor' is used.
3.1.3
characteristic strength
95 % (two-sided) lower confidence limit for the tensile strength of the geosynthetic, approximately
equal to the mean strength less two standard deviations
Note 1 to entry: This should be assured by the manufacturer’s own quality assurance scheme or by independent
assessment.
3.1.4
block shifting
procedure by which a set of data relating applied load to the logarithm of time to rupture, all measured
at a single temperature, are shifted along the log time axis by a single factor to coincide with a second
set measured at a second temperature
3.1.5
product line
series of products manufactured using the same polymer, in which the polymer for all products in the
line comes from the same source, the manufacturing process is the same for all products in the line, and
the only difference is in the product mass per area or number of fibres contained in each reinforcement
element
3.2 Abbreviated terms
CEG carboxyl end group
DSC differential scanning calorimetry
HALS hindered amine light stabilizers
HDPE high density polyethylene
HPOIT high pressure oxidation induction time
LCL lower confidence limit
MARV minimum average roll value
OIT oxidation induction time
PA polyamide
PET polyethylene terephthalate
PP polypropylene
PTFE polytetrafluorethylene
PVA polyvinyl alcohol
SIM stepped isothermal method
TTS time-temperature shifting
3.3 Symbols
A time-temperature shift factor
i
b gradient of Arrhenius graph
a
d mean granular size of fill
d granular size of fill for 90 % pass (10 % retention)
f factor of safety
s
G, H parameters used in the validation of temperature shift linearity (see 7.4)
m gradient of line fitted to creep rupture points (log time against load); inverse of gradient of
conventional plot of load against log time.
M number averaged molecular weight
n
n number of creep rupture or Arrhenius points
P applied load
R ratio representing the uncertainty due to extrapolation
R ratio representing the uncertainty in strength derived from Arrhenius testing
f reduction factor to allow for chemical and biological effects
R,CH
f reduction factor to allow for the effect of sustained static load
R,CR
f reduction factor to allow for the effect of mechanical damage
R,ID
f reduction factor to allow for weathering
R,W
S sum of squares of difference of log (time to rupture) and straight line fit
sq
S , S , S sums of squares as defined in derivation of regression lines in 9.4.3
xx xy yy
σ standard deviation used in calculation of LCL
t time, expressed in hours
t time to 90 % retained strength
t design life
D
t degradation time during oxidation
deg
t induction time during oxidation
ind
t LCL of time to a defined retained strength at the service temperature
LCL
t longest observed time to creep rupture, expressed in hours
max
t Student’s t for n – 2 degrees of freedom and a stated probability
n–2
t time to rupture, expressed in hours
R
t time to a defined retained strength at the service temperature
s
T load per width
T batch tensile strength (per width)
B
T characteristic strength (per width) (see 6.1)
char
T unfactored long-term strength (see 9.4.3)
x
T long-term strength per width (including factor of safety)
D
T residual strength
DR
θ temperature of accelerated creep test
j
θ absolute temperature
k
T LCL of T due to chemical degradation
LCL char
θ service temperature
s
x abscissa: on a creep rupture graph the logarithm of time, in hours
x
mean value of x
x abscissa of an individual creep rupture point
i
x predicted time to rupture
p
y ordinate: on a creep rupture graph, applied load expressed as a percentage of tensile strength,
or a function of applied load
y value of y at 1 h (lg t = 0)
y
mean value of y
y ordinate of an individual creep rupture point
i
y value of y at time 0, derived from the line fitted to creep rupture points
4 Design procedure
4.1 General
The design of reinforced soil structures generally requires consideration of the following two issues:
a) the maximum strain in the reinforcement during the design lifetime;
b) the minimum strength of the reinforcement that could lead to rupture during the design lifetime.
In civil engineering design, these two issues are referred to as the serviceability and ultimate limit
state respectively. Both factors depend on time and can be degraded by the environment to which the
reinforcement is exposed.
4.2 Design lifetime
A design lifetime, t , is defined for the reinforced soil structure. For civil engineering structures this
D
is typically 50 years to 100 years. These durations are too long for direct measurements to be made in
advance of construction. Reduction factors have therefore to be determined by extrapolation of short-
term data aided, where necessary, by tests at elevated temperatures to accelerate the processes of
creep or degradation.
4.3 Causes of degradation
Strain and strength may be changed due to the effects of the following:
— mechanical damage caused during installation;
— sustained static (or dynamic) load;
— elevated temperature;
— weathering while the material is exposed to light;
— chemical effects of natural or contaminated soil.
4.4 Design temperature
The design temperature should have been defined for the application in hand. In the absence of a defined
temperature or of site specific in-soil temperature data, the design temperature should be taken as the
temperature which is halfway between the average yearly air temperature and the normal daily air
temperature for the hottest month at the site. If this information is not available, 20 °C should be used
as the default value.
Many geosynthetic tests are performed at a standard temperature of (20 ± 2) °C. If the design
temperature differs, appropriate adjustments should be made to the measured properties.
This document does not cover the effects of temperatures below 0 °C (see Clause 1).
5 Determination of long-term creep strain
5.1 General
The design specification may set a limit on the total strain over the service lifetime of the geosynthetic,
or on the strain generated between the end of construction and the service lifetime. In the second case,
the time at “end of construction” should be defined, as shown in Figure 1. When plotted against lg t,
even a one-year construction period should have negligible influence on the creep strain curve beyond
10 years.
Levels of creep strain encountered in the primary creep regime (creep rate decreasing with time) are
thought not to adversely affect strength properties of geosynthetic reinforcement materials.
Key
X time 3 load ramp period in creep test
Y strain 4 loading and creep of reinforcement in wall
1 laboratory creep test 5 new time = 0 for post construction creep
2 load ramp period on wall 6 wall construction time
Figure 1 — Conceptual illustration for comparing the creep measured in walls to laboratory
creep data
5.2 Extrapolation
Creep strain should be measured according to ISO 13431 and plotted as strain against the lg t. It
may then be extrapolated to the design lifetime. Extrapolation may be by graphical or curve-fitting
procedures, in which the formulae applied should be as simple as is necessary to provide a reasonable
fit to the data, for example, power laws. The use of polynomial functions is discouraged since they can
lead to unrealistic values when extrapolated.
5.3 Time-temperature superposition methods
Time-temperature superposition methods may be used to assist with extending the creep curves.
Creep curves are measured under the same load at different temperatures, with intervals generally
not exceeding 10 °C, and plotted on the same diagram as strain against lg t. The lowest temperature is
taken as the reference temperature. The creep curves at the higher temperatures are then shifted along
the time axis until they form one continuous “master” curve, i.e. the predicted long-term creep curve
for the reference temperature. The shift factors, i.e. the amounts (in units equivalent to lg t) by which
each curve is shifted, should be plotted against temperature where they should form a straight line or
smooth curve. The cautions given in 7.6 should be noted.
Experience has shown the strains on loading are variable. Since the increase in strain with time is
small, this variability can lead to wide variability in time-temperature shifting (TTS). The stepped
isothermal method (SIM) described in 7.5 avoids this problem by using a single specimen, increasing
the temperature in steps, and then shifting the sections of creep curve measured at the various
temperatures to form one continuous master curve.
If a more accurate measure of initial strain is required, five replicates are recommended at each load.
Some of these can be of short duration (e.g. 1 000 s). At a series of loads, fewer replicates at each load will
suffice if the data are pooled using regression techniques. One approach is to use regression analysis
to develop an isochronous load versus strain curve at 0,1 h. The creep curve should then be shifted
vertically to pass through the mean strain measured after 0,1 h.
If the lowest test temperature is below the design temperature, the shift factor corresponding to the
design temperature should be read off the plot of shift factor against temperature. The time-scale of the
master curve should then be adjusted by this factor.
5.4 Isochronous curves
From the creep curve corresponding to each load, read off the strains for specified durations, typically
1 h, 10 h, 100 h, etc., and including the design lifetime. Set up a diagram of load against strain. For each
duration, plot the points of load against strain for the corresponding durations (see Figure 2). These are
called isochronous curves. Where a maximum strain is permitted over the design lifetime, or between
the end of construction (e.g. 100 h) and the design lifetime, it is possible to read off the corresponding
loads from these curves. Where the strain is measured from zero, note that in geosynthetics strains are
measured from a set preload (defined in ISO 10319 and ISO 13431 as 1 % of the tensile strength) and
that some woven and particularly non-woven materials may exhibit considerable irreversible strains
below this initial loading. See Reference [2] for additional details on creep strain characterization.
Key
X strain
Y load
Figure 2 — Isochronous diagram
5.5 Weathering, chemical and biological effects
Creep strain is generally insensitive to limited weathering, chemical and biological effects. In addition,
creep strains are in general not affected by installation damage, unless the damage is severe, or unless
the load level applied is very near the creep limit of the undamaged material. In most cases, the load
level applied is well below the creep limit of the material. See Reference [3] for additional details on this
issue. Thus, no further adjustment is generally required beyond the effect of temperature.
Note, however, that artificially contaminated soils may contain chemicals, such as organic fuels and
solvents, which can affect the creep of geosynthetics. If necessary, perform a short-term creep test
according to ISO 13431 on a sample of geosynthetic that is immersed in the chemical or has just been
removed from it. If the creep strain is significantly different, do not use this geosynthetic in this soil.
6 Determination of long-term strength
6.1 Tensile strength
The characteristic strength, T , is taken as the basis for the long-term strength. T is typically a
char char
statistical value generated from the mean strength of production material less two standard deviations
sometimes referred to as the minimum average roll value (MARV), unless otherwise defined.
6.2 Reduction factors
T can then be divided by the following four reduction factors, each of which represents a loss of
char
strength determined in accordance with this Technical Specification, to arrive at the long-term strength
T :
D
— f is a reduction factor to allow for the effect of sustained static load at the service temperature;
R,CR
NOTE The effect of dynamic loads is not included.
— f is a reduction factor to allow for the effect of mechanical damage;
R,ID
— f is a reduction factor to allow for weathering during exposure prior to installation or of
R,W
permanently exposed material;
— f is a reduction factor to allow for reductions in strength due to chemical and biological effects
R,CH
at the design temperature (see 4.4).
In addition to the reduction factors, a factor of safety, f , takes into account the statistical variation
s
in the reduction factors calculated (see 6.1). It does not consider the uncertainties related to the soil
structure and the calculation of loads.
6.3 Modes of degradation
Degradation of strength can be divided into three modes according to the manner in which they take
place with time:
— Mode 1: Immediate reduction in strength, insignificant further reduction with time.
— Mode 2: Gradual, though not necessarily constant, reduction in strength.
— Mode 3: No reduction in strength for a long period; after a certain period, onset of rapid degradation.
For Mode 1, of which installation damage is an example, it is appropriate to reduce the tensile strength
by an appropriate time-independent reduction factor. For Mode 2, where there is a progressive
reduction in strength, the tensile strength will be reduced by a time-dependent reduction factor. For
Mode 3, it is not appropriate to apply a reduction factor to the tensile strength but rather to restrict the
service lifetime.
These modes are depicted schematically in Figure 3.
Key
X time
Y retained strength
1 mode 1
2 mode 2
3 mode 3
Figure 3 — Retained strength plotted against time for the three Modes of degradation
7 Creep rupture
7.1 General
Creep rupture, or lifetime under sustained load, is determined by measuring times to rupture of up to
at least 10 000 h. The results are extrapolated to predict longer lifetimes at lower loads and thereby the
reduction factor f .
R,CR
This procedure may be supported by measurements at higher temperatures. Conventional TTS of
results obtained on multiple specimens at elevated temperatures provides an improved prediction
of the long-term behaviour at ambient temperature. In the SIM, the temperature of a single specimen
is increased in steps. The sections of creep strain curve measured at each temperature step are then
combined to predict the long-term creep strain and rupture lifetime.
It should be noted that a creep rupture diagram depicts applied load plotted against time to rupture
and is not a statement of the loss of strength under continuous load. It has been predicted on the basis
of accelerated tests that many geosynthetics exposed to sustained load do not in fact significantly
diminish in strength until close to the end of their predicted life. When the strength equals the applied
load, the material ruptures (see Figure 4). Sustained load is therefore a Mode 3 form of degradation.
Key
X time
Y applied load, residual strength
1 creep rupture
2 residual strength
3 applied load
4 lifetime
Figure 4 — Creep rupture and residual strength as a function of time
The creep rupture curve shows the predicted lifetime corresponding to a particular applied load.
During that lifetime, the strength of the geosynthetic follows the residual strength curve, falling to
equal the applied load at the moment of rupture.
7.2 Measurement of creep rupture: conventional method
For limit state design, the creep rupture behaviour of the product should be measured according to
ISO 13431 with a minimum of 12 measurements. As a guide, at least four of the test results should have
rupture times between 100 h and 1 000 h, and at least four of the test results should have rupture times
of 1 000 h to 10 000 h, with at least one additional test result having a rupture time of approximately
10 000 h (1,14 years) or more.
Specimens should be tested in the direction in which the load will be applied in use. The tensile
strength of the same batch, T , of the material in the same direction should be determined according
B
to ISO 10319 using grips similar to those used for creep rupture testing. Loads applied during the creep
rupture tests should be expressed as a percentage of T . The nature of the failure should be observed
B
and recorded.
It is recommended that creep strain is measured as well as time to rupture, since this can assist with
conventional time-temperature strain shifting and in identifying any change in behaviour that could
invalidate extrapolation of the results. This practice will also permit laboratory creep data collected at
moderate differences (plus or minus 10 °C) in test temperature to be corrected to the desired reference
temperature. Similar moderate changes in reference temperature will be facilitated under this practice
as well.
The temperature should be as stated in ISO 13431 and ISO 10319; if a different temperature, for
example, the design temperature, is used then it should be the same for both tensile and creep rupture
measurements. Further tests at elevated temperature may be used for the purposes of TTS.
The creep rupture data for the product should be tabulated as:
— load per width T, as percentage of the batch tensile strength, T ;
B
— time to rupture, t , in h;
R
— lg t to rupture;
— observations on the failure, including the strain at failure or the strain at the point where the rate
of creep starts to increase (tertiary creep) and, where visible, the nature of the fracture surface, e.g.
ductile, semi-brittle or brittle and smooth;
— creep strain data, if available, particularly if conventional time-temperature strain shifting is
applied;
— whether the test was conventional (20 °C), time-temperature accelerated, SIM or was performed on
a similar material as supporting data.
Incomplete tests may be included, with the test duration replacing the time to rupture, but should be
listed as such. The procedure for handling incomplete tests is described in 7.3.
7.3 Curve fitting (conventional method)
The data, including any relevant supporting data, should be plotted as y = T (expressed as a percentage
of T ) against x = lg t , which should yield a linear plot (see Figure 5). This is referred to as a semi-
B R
logarithmic plot and has been shown to apply to polyester reinforcements. If the plot is not linear, it
may be necessary to plot the ordinate (y) as a function of applied load to achieve a linear plot. The use of
the function y = lg T, resulting in a double logarithmic plot, has been shown to apply to polyethylene and
polypropylene reinforcements. Where a function of T is used, it should preferably be based on a known
physical model.
Key
X logarithm of time to rupture (lg t )
R
Y load per width T, as % tensile strength
Figure 5 — Creep rupture diagram with straight line fit
Fit a straight line using statistical regression analysis. In the following, x equals lg t and y equals T or
R
a function of P. The creep rupture points, total number n, are denoted as (x , y ). Note that in contrast
i i
to most scientific plots, the independent variable is plotted on the y axis and the dependent variable is
plotted on the x axis. Formulae (1) to (3) therefore differ from those conventionally found by having x
and y interchanged.
The straight line fit (regression line) is given by the formula:
xx=+my()− y (1)
where
x y
∑ i ∑ i
x= and y=
n n
summed over all points (x , y ).
i i
m is given by the formula:
()xx− ()yy−
∑ ii
m= (2)
()yy−
∑ i
Because of the interchange of x and y, the gradient of the graph is equal to 1/m. For a semi-logarithmic
diagram, this should be expressed as percentage tensile strength per decade of time. The gradient
should be a negative value.
The intercept y on the line x = 0 (i.e. at lg t = 0; t = 1 h) is given by:
yy=−xm/ (3)
The accepted practice for incomplete tests is as follows. The regression should first be performed with
the incomplete tests excluded. The time to failure for an incomplete test should then be determined for
the corresponding value of T. If the predicted time to failure is less than the duration of the incomplete
test, the point may be added and the regression recalculated. If the predicted time to failure is greater
than the duration of the incomplete test, the point should continue to be excluded. In Figure 5 the
incomplete test shown by an open triangle is included since it lies to the right of the regression line.
Extend the regression line to the design lifetime, for example in Figure 5 where for a design lifetime of
1 000 000 h, T = 52 % of tensile strength. f = 1/52 % = 100/52 = 1,92
R,CR
Record the duration of the longest test that has ended in rupture, or the duration of the longest
incomplete test whose duration has been included in the regression calculation: this duration is denoted
as t .
max
7.4 Curve fitting for time-temperature block shifting of rupture curves
If data obtained at higher temperatures θ are to be included for the purposes of acceleration, tabulate
i
the values of y and t as in 7.3 together with the temperatures θ . For each temperature θ , assign a
i R i i
nominal shift factor A . Assign nominal values to the constants y and m. Include the test points derived
j 0
at 20 °C for which A = 0. Then proceed as follows.
i
For each measured value of t , calculate the shifted log time xt=+lg A .
R iR i
For each value of y , calculate the logarithm of the predicted time to rupture xy=− ym .
()
i
pi 0
For each pair of values, calculate the square of the difference xx− .
()
ip
Derive the sum of squares Si=∑ xx− .
()
sq ip
Using a spreadsheet optimization programme, minimize S as a function of all A , y and m.
sq j 0
Using the optimized values of A , recalculate the values of x .
j i
Plot y against the recalculated values of x . On the same diagram, using the optimised values of y and
i i 0
m, add the straight line fit as in 7.3. This line should be used for prediction of the time to rupture at
20 °C.
Plot the optimized values of A against θ . Check that the line passes through the point (20 °C, 0) and is
i i
then straight or lightly curved, such that if the curve is approximated by Formula (4):
AG=−θθ20 +−H 20 (4)
() ()
ji i
then –0,003 < G/H < 0,003. If not, the validity of the tests should be reviewed.
This curve may be interpolated or extended to derive the temperature shift factor A corresponding to
i
a different service temperature.
For example, in Figure 6, the regression creep rupture lines for 20 °C, 40 °C and 60 °C are assumed to be
parallel. The 40 °C and 60 °C lines and associated points have been shifted to the right until they coincide
with the 20 °C line to which they form an extension. Temperature steps ≤10 °C are recommended for PE
and PP.
This procedure assumes that the creep rupture curves at all temperatures are linear and parallel, which
has been found empirically to apply to polyester (semi-log plots) and polypropylene (log/log plots). It
[4]
should be pointed out that the theory of Zhurkov in the Bibliography, which assumes that the fracture
process is activated thermally with the additional effect of applied stress, predicts that the creep
rupture characteristics should be straight when plotted on a semi-logarithmic diagram, and that their
gradients should be stress-dependent. This theory has not provided a better fit to experimental creep
rupture data than the empirical method used here, but experience has shown that the shift factors can
be stress-dependent and block shifting ignores this.
7.5 Strain shifting and the stepped isothermal method
Long-term rupture data can be obtained through the use of the classical TTS of creep strain data.
Strain shifting as described in 5.2 can be applied to creep curves terminated in rupture. For example,
a creep strain versus lg t curve obtained under a given load at 60 °C and which terminates in rupture
can be shifted to longer times. Needed to accomplish this are creep strain curves at, say, 20 °C and
40 °C under the same load. The lower temperature curves can be terminated before rupture provided
that sufficient data are available to effect the TTS procedure properly. Because of the scatter in initial
strains mentioned previously, the strain tests should be replicated.
In the SIM, which is a special case of TTS, the temperature of the creep test is raised in a series of steps.
The sections of creep curve at the individual temperatures are then combined to form a continuous
determination as instructed in ASTM D 6992.
SIM can be considered for use in generating and extrapolating geosynthetic creep rupture data,
provided that the predictions are consistent with those based on conventional testing or time-
temperature block or strain shifting as described above. To this end, it is recommended that a minimum
of 12 data points, time-shifted to the reference temperature, be obtained from accelerated (TTS and
SIM) and conventional testing, with a minimum of
— three time-shifted durations between 1 000 and 100 000 h, and
— three time-shifted durations between 100 000 and 10 000 000 h.
In addition, a limited programme of conventional creep rupture tests obtained at the reference
temperature and therefore un-shifted (except as corrected per 7.2), should be performed in accordance
with 7.2. It is recommended that there should be four conventional creep rupture data points between
100 h and 10 000 h and one data point at 10 000 h or more. (The last data point may be an incomplete
test). This conventional creep rupture data envelope should then be compared to the envelope
determined from the accelerated data.
Linear regression analysis should be performed separately for the conventional and accelerated data
in accordance with 7.3 and 7.4. The value of f determined from the accelerated data at 2 000 h at
R,CR
t
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