ASTM D6815-09(2015)

Standard Specification for Evaluation of Duration of Load and Creep Effects of Wood and Wood-Based Products

ABSTRACT
This specification covers the procedure for testing and evaluating duration of load and creep effects of wood and wood-based materials relative to an accepted duration of load adjustment model. This specification is proposed for use for products that are covered by a consensus standard such as lumber, structural composite lumber and structural use panels. The procedure covered in this specification, however, is not intended to evaluate performance of products under impact loading.
SCOPE
1.1 This specification provides a procedure for testing and evaluating duration of load and creep effects of wood and wood-based materials relative to an accepted duration of load adjustment model. This specification was created for products that are currently covered by a consensus standard (for example, lumber, structural composite lumber, and structural-use panels). This procedure is intended to demonstrate the engineering equivalence to the duration of load and creep effects of visually graded lumber as specified in Practice D245 for a product under evaluation used in dry service conditions. This procedure is not intended to evaluate the performance of products under impact loading. Quantification of specific duration of load or creep factors is beyond the scope of this specification. For further guidance regarding the applicability of this specification refer to X1.1 in the Commentary.  
1.2 Use of the procedure in this specification to determine equivalence to the Practice D245 duration of load relationship is limited to solid wood and wood-based products whose long term load behavior is similar to that of solid wood. Equivalence demonstrated in this specification is dependent upon evaluation of a product's 90-day (minimum) creep-rupture performance. In this evaluation, three criteria must be satisfied: (1) adequate strength over a 90-day period, (2) decreasing creep rate, and (3) limited fractional deflection. A summary of the development of these criteria and the underlying assumptions behind them is provided in the Commentary in Appendix X1 and Appendix X2.  
1.3 Long term degradation phenomena not described by a creep-rupture model are not addressed in this specification (see Commentary X1.2.4).

General Information

• Status: Historical
• Publication Date: 31-Aug-2015
• ICS: 79.080 - Semi-manufactures of timber
• Technical Committee: D07 - Wood
• Drafting Committee: D07.01 - Fundamental Test Methods and Properties

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation:D6815 −09 (Reapproved 2015)
Standard Specification for
Evaluation of Duration of Load and Creep Effects of Wood
and Wood-Based Products
This standard is issued under the fixed designation D6815; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 This specification provides a procedure for testing and 2.1 ASTM Standards:
evaluating duration of load and creep effects of wood and D9 Terminology Relating to Wood and Wood-Based Prod-
wood-based materials relative to an accepted duration of load ucts
adjustment model. This specification was created for products D198 Test Methods of Static Tests of Lumber in Structural
that are currently covered by a consensus standard (for Sizes
example, lumber, structural composite lumber, and structural- D245 Practice for Establishing Structural Grades and Re-
use panels). This procedure is intended to demonstrate the lated Allowable Properties for Visually Graded Lumber
engineering equivalence to the duration of load and creep D1037 Test Methods for Evaluating Properties of Wood-
effects of visually graded lumber as specified in Practice D245 Base Fiber and Particle Panel Materials
for a product under evaluation used in dry service conditions. D2915 Practice for Sampling and Data-Analysis for Struc-
This procedure is not intended to evaluate the performance of tural Wood and Wood-Based Products
D3043 Test Methods for Structural Panels in Flexure
products under impact loading. Quantification of specific
duration of load or creep factors is beyond the scope of this D4442 Test Methods for Direct Moisture Content Measure-
ment of Wood and Wood-Based Materials
specification. For further guidance regarding the applicability
of this specification refer to X1.1 in the Commentary. D4761 Test Methods for Mechanical Properties of Lumber
and Wood-Base Structural Material
1.2 Use of the procedure in this specification to determine
D5457 Specification for Computing Reference Resistance of
equivalence to the Practice D245 duration of load relationship
Wood-Based Materials and Structural Connections for
is limited to solid wood and wood-based products whose long
Load and Resistance Factor Design
termloadbehaviorissimilartothatofsolidwood.Equivalence
E4 Practices for Force Verification of Testing Machines
demonstratedinthisspecificationisdependentuponevaluation
E6 Terminology Relating to Methods of Mechanical Testing
of a product’s 90-day (minimum) creep-rupture performance.
E177 Practice for Use of the Terms Precision and Bias in
In this evaluation, three criteria must be satisfied: (1) adequate
ASTM Test Methods
strengthovera90-dayperiod,(2)decreasingcreeprate,and(3)
2.2 Other References:
limited fractional deflection.Asummary of the development of
ANSI/AF&PA NDS-2005 National Design Specification
these criteria and the underlying assumptions behind them is
(NDS) for Wood Construction
provided in the Commentary in Appendix X1 and Appendix
X2.
3. Terminology
1.3 Long term degradation phenomena not described by a
3.1 Definitions—See Terminologies D9 and E6 and Prac-
creep-rupture model are not addressed in this specification (see
tices E4 and E177 for definitions of terms used in this
Commentary X1.2.4).
specification.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
This specification is under the jurisdiction ofASTM Committee D07 on Wood contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
and is the direct responsibility of Subcommittee D07.01 on Fundamental Test Standards volume information, refer to the standard’s Document Summary page on
Methods and Properties. the ASTM website.
Current edition approved Sept. 1, 2015. Published October 2015. Originally Available from American Forest & Paper Association (AF&PA), American
approved in 2002. Last previous edition approved in 2009 as D6815 - 09. DOI: Wood Council, 1111 19th Street NW, Suite 800, Washington, DC 20036, http://
10.1520/D6815-09R15. www.afandpa.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6815−09 (2015)
3.2 Definitions of Terms Specific to This Standard: mens shall be sampled from the initial test population. Long-
3.2.1 breadth—the dimension of the test specimen in the term and short-term test specimens shall have the same cross
direction perpendicular to the span and perpendicular to the section dimensions and length.
direction of an applied bending load.
NOTE 1—Matching is a technique that attempts to subdivide the initial
3.2.2 creep—the time-dependent increase of deformation of sample population into two or more separate groups that possess near
identical distributional form and scale for bending properties. Matching
the test material under a constant load.
specimens for the purposes of 5.1.1 should be done with care, considering
3.2.3 creep deflection—total measured deflection at a spe-
errors introduced by the process and the characteristics of the material
cific time minus the initial deflection. under test.
3.2.4 creep rate—the change in creep deflection over time.
5.1.2 Each test specimen shall be simply supported and
loaded by two equal concentrated forces spaced a distance of
3.2.5 creep-rupture—a failure phenomenon described by a
one-third the total span from the end supports (that is,
relationship between applied stress and time-to-failure.
third-point bending). Loads shall be applied in the product
3.2.6 depth—the dimension of the test specimen in the
orientation that represents the general intended use of the
direction perpendicular to the span and parallel to the direction
product.
of an applied bending load.
5.1.2.1 Forjoist-formmaterials,thespantodepthratioshall
3.2.7 dry service conditions—the conditions in most cov-
beasspecifiedinapplicableteststandards(seeNote2).Lateral
ered structures, where the moisture content of lumber will not
restraints shall be used when necessary to maintain lateral
exceed 19 % (1).
stability.The minimum test specimen cross section shall be 2.5
3.2.8 duration of load factor—a factor customarily used to in. (63.5 mm) in depth and 1.0 in. (25.4 mm) in width.
account for the effect of duration of load on the strength of
NOTE 2—For lumber sized products, span to depth ratios typically used
wood products.
for flexural tests range between 17 and 21.
3.2.9 failure—the point at which the test member can no
5.1.2.2 For sheathing-form materials, the test span shall be
longer support the applied constant load.
not less than 48 times specimen thickness or 24 in., whichever
3.2.10 fractional deflection—a ratio of total deflection to the is greater.The specimen width for all sheathing-form materials
initial deflection. shall not be less than 12 in. (305 mm).
5.1.3 Moisture content shall be measured on the short-term
3.2.11 initial deflection—the deflection at approximately
specimens immediately after destructive testing and on the
one minute after the application of load.
long-term specimens at the termination of the long–term test.
3.2.12 span—the distance between the centerlines of end
Measurement of moisture content shall be in accordance with
reactions on which a test specimen is supported to accommo-
Test Methods D4442. The average moisture content of all the
date a transverse bending load.
long-term test specimens shall not deviate more than 62%
from the average moisture content of all the short–term test
4. Significance and Use
specimens (see Note 3).
4.1 This specification provides a method for evaluating
5.1.4 The test environment temperature and relative humid-
duration of load and creep effects of wood and wood-based
ity shall be recorded daily (see Commentary X1.4.5).The daily
products subjected to bending stress. This method is intended
average temperature of the test environment shall not decrease
to demonstrate the engineering equivalence to the duration of
more than 5°C (9°F) below the temperature at which the
load and creep effects of visually graded lumber as specified in
short–term tests were conducted. At no time shall the test
Practice D245 for a product under evaluation. Equivalence is
environment reach a temperature less than 0°C (32°F).
based on evaluating a product’s creep-rupture performance
NOTE 3—Conditioning the short-term and long-term test material for at
over a minimum of 90 days and meeting the requirements of
least 30 days in the anticipated test environment conditions generally
this specification. This specification does not attempt to quan-
provides compliance with the 62 % moisture content change criterion.
tify the effect of damage accumulation or to establish product-
NOTE 4—In experiments where the temperature falls below the pre-
specific duration of load factors for the product under evalua- scribed limit, it may be possible to demonstrate the validity of the data by
continuing the experiment for an additional time period at least equal to,
tion.
and possibly greater than, the amount of time the temperature was below
the prescribed limit.
5. Test Methods and Acceptance Criteria
5.2 Short-Term Bending Tests:
5.1 Test Methods:
5.2.1 The loading rate for the short-term tests shall be such
5.1.1 A test population shall be sampled from production
that the sample target failure load would be achieved in
that is representative of the product under evaluation. Two
approximately 1 min. Failure load shall not be reached in less
matched test groups shall be selected, one for short-term
than 10 s nor more than 10 min. The procedures of Test
bending tests, and one for long-term creep-rupture bending
Methods D198 or D4761 shall be followed for joist-form
tests. A minimum sample size of 28 is required for each test
materials and Test Methods D1037 or D3043 for sheathing-
group. If further testing is contemplated, additional test speci-
form materials.
5.2.2 The sample standard deviation and the lower five
percent point estimate of the short-term test group (5 % PE)
The boldface numbers in parentheses refer to the list of references at the end of
this standard. shall be determined in accordance with Practice D2915.
D6815−09 (2015)
5.3 Creep-Rupture Bending Tests: critical order statistic (N ≥ N ) then the product under evalu-
90 c
ation fails to meet the adequate strength criterion with the
5.3.1 The creep-rupture test specimens shall be loaded such
that the average time to attain the pre-selected constant stress sample population, N.
level does not exceed the average time to failure of the
5.4.1.3 If the number of failures at 90 days is equal to the
short-term tests (see 5.2.1). Thereafter, the specimens shall be
critical order statistic (N = N)in 5.4.1.2, then additional
90 c
subjected to the constant stress for a minimum period of 90
testing may be conducted. In this case the sample population
days. During this period, mid-span deflection readings shall be
shall be increased by sampling an additional set of matched
taken for each test specimen, until the 90-day time period has
specimens in accordance with 5.1.1 sufficient to allow the use
elapsed or until the occurrence of a failure.At a minimum, the
of a higher non-parametric order statistic (see Note 6). The
deflection readings shall be taken at approximately one minute
additional specimens shall be tested for another 90-day test
after the application of the constant load (initial deflection),
duration. The adequate strength requirement of 5.4.1.1 is met
and at the end of one hour, day 1, day 7, day 14, day 30, day
when,attheendoftheadditionaltesting,thecombinednumber
60, and day 90. When better characterization of the creep rate
of specimen failures during these two test series (N com-
is desired, more frequent deflection measurements should be
bined) is less than the critical order statistic (N combined)
c
taken.Additionaldeflectionreadingsarerequiredwhenthetest
based on the combined number of specimens evaluated (N
extends beyond 90 days. When a specimen failure occurs,
combined).
time-to-failure shall be recorded.
NOTE 6—From Practice D2915 the order statistic for the lower 5 %
5.3.2 The specimens selected for these tests shall be tested
tolerance limit with 75 % confidence, N , for various sample populations,
c
at a constant stress level, f , as determined in accordance with
b
N, is as follows:
Eq 1 (see Commentary Appendix X1).
N 28 53 78 102
f 5 0.55 3 ~5% PE! (1)
N 1234
b
c
where:
5.4.2 Decreasing Creep Rate—All the test specimens that
f = minimum applied bending stress, and do not fail during the 90 day constant load time period shall
b
5 %PE = the lower five percent point estimate, as deter-
show a decreasing creep rate.
mined from the short-term bending tests in 5.2.
5.4.2.1 To determine a decreasing creep rate, the change in
creep deflection shall be calculated between a minimum of
The creep rate, fractional deflection (FD), and the total
three equally spaced time segments. The change in calculated
number of failures at 90 days (N ) (or greater) shall be used to
creep deflection shall progressively decrease for each speci-
evaluate the acceptance of the product.
men. For the three equal time periods of 0 to 30 days, 30 to 60
NOTE5—Examplesofacceptablecreepandcreep-rupturetestapparatus
days, and 60 to 90 days, the decreasing creep rate can be
are given in Refs 2,3.
expressed as (see Commentary Appendix X2):
5.4 Acceptance Criteria—The product is considered accept-
D 2 D .D 2 D .D 2 D (3)
30 i 60 30 90 60
able for using the duration of load and creep factors applicable
to lumber if the following three criteria are all satisfied: (1)
where:
adequate strength over the test duration, (2) decreasing creep
D = initial deflection (measured one-minute af-
i
rate, and (3) a limited fractional deflection.
ter application of the load in accordance
5.4.1 Adequate Strength—The total number of failures over
with 5.3.1), and
the test duration shall be used to determine acceptance.
D ,D ,D = deflections measured on 30th, 60th, and
30 60 90
5.4.1.1 The total number of failures at 90 or more days shall 90th day respectively.
be less than the critical order statistic, N , of the lower 5 %
NOTE 7—To better define the creep rate, additional segments with a
c
shorter frequency (for example, five 18-day segments) may be used.
non-parametric tolerance limit with 75 % confidence:
N ,N (2)
5.4.2.2 If the creep rate for a given specimen is not
90 c
decreasingattheendofthe90-dayperiod,thetimeperiodshall
where:
be extended for a minimum of 30 additional days. The change
N = number
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D6815 − 09 D6815 − 09 (Reapproved 2015)
Standard Specification for
Evaluation of Duration of Load and Creep Effects of Wood
and Wood-Based Products
This standard is issued under the fixed designation D6815; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This specification provides a procedure for testing and evaluating duration of load and creep effects of wood and
wood-based materials relative to an accepted duration of load adjustment model. This specification was created for products that
are currently covered by a consensus standard (for example, lumber, structural composite lumber, and structural-use panels). This
procedure is intended to demonstrate the engineering equivalence to the duration of load and creep effects of visually graded
lumber as specified in Practice D245 for a product under evaluation used in dry service conditions. This procedure is not intended
to evaluate the performance of products under impact loading. Quantification of specific duration of load or creep factors is beyond
the scope of this specification. For further guidance regarding the applicability of this specification refer to X1.1 in the
Commentary.
1.2 Use of the procedure in this specification to determine equivalence to the Practice D245 duration of load relationship is
limited to solid wood and wood-based products whose long term load behavior is similar to that of solid wood. Equivalence
demonstrated in this specification is dependent upon evaluation of a product’s 90-day (minimum) creep-rupture performance. In
this evaluation, three criteria must be satisfied: (1) adequate strength over a 90-day period, (2) decreasing creep rate, and (3) limited
fractional deflection. A summary of the development of these criteria and the underlying assumptions behind them is provided in
the Commentary in Appendix X1 and Appendix X2.
1.3 Long term degradation phenomena not described by a creep-rupture model are not addressed in this specification (see
Commentary X1.2.4).
2. Referenced Documents
2.1 ASTM Standards:
D9 Terminology Relating to Wood and Wood-Based Products
D198 Test Methods of Static Tests of Lumber in Structural Sizes
D245 Practice for Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber
D1037 Test Methods for Evaluating Properties of Wood-Base Fiber and Particle Panel Materials
D2915 Practice for Sampling and Data-Analysis for Structural Wood and Wood-Based Products
D3043 Test Methods for Structural Panels in Flexure
D4442 Test Methods for Direct Moisture Content Measurement of Wood and Wood-Based Materials
D4761 Test Methods for Mechanical Properties of Lumber and Wood-Base Structural Material
D5457 Specification for Computing Reference Resistance of Wood-Based Materials and Structural Connections for Load and
Resistance Factor Design
E4 Practices for Force Verification of Testing Machines
E6 Terminology Relating to Methods of Mechanical Testing
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
2.2 Other References:
ANSI/AF&PA NDS-2005 National Design Specification (NDS) for Wood Construction
This specification is under the jurisdiction of ASTM Committee D07 on Wood and is the direct responsibility of Subcommittee D07.01 on Fundamental Test Methods
and Properties.
Current edition approved Dec. 1, 2009Sept. 1, 2015. Published February 2010October 2015. Originally approved in 2002. Last previous edition approved in 20022009
as D6815D6815 - 09. — 02a. DOI: 10.1520/D6815-09.10.1520/D6815-09R15.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from American Forest & Paper Association (AF&PA), American Wood Council, 1111 19th Street NW, Suite 800, Washington, DC 20036, http://
www.afandpa.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6815 − 09 (2015)
3. Terminology
3.1 Definitions—See Terminologies D9 and E6 and Practices E4 and E177 for definitions of terms used in this specification.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 breadth—the dimension of the test specimen in the direction perpendicular to the span and perpendicular to the direction
of an applied bending load.
3.2.2 creep—the time-dependent increase of deformation of the test material under a constant load.
3.2.3 creep deflection—total measured deflection at a specific time minus the initial deflection.
3.2.4 creep rate—the change in creep deflection over time.
3.2.5 creep-rupture—a failure phenomenon described by a relationship between applied stress and time-to-failure.
3.2.6 depth—the dimension of the test specimen in the direction perpendicular to the span and parallel to the direction of an
applied bending load.
3.2.7 dry service conditions—the conditions in most covered structures, where the moisture content of lumber will not exceed
19 % (1).
3.2.8 duration of load factor—a factor customarily used to account for the effect of duration of load on the strength of wood
products.
3.2.9 failure—the point at which the test member can no longer support the applied constant load.
3.2.10 fractional deflection—a ratio of total deflection to the initial deflection.
3.2.11 initial deflection—the deflection at approximately one minute after the application of load.
3.2.12 span—the distance between the centerlines of end reactions on which a test specimen is supported to accommodate a
transverse bending load.
4. Significance and Use
4.1 This specification provides a method for evaluating duration of load and creep effects of wood and wood-based products
subjected to bending stress. This method is intended to demonstrate the engineering equivalence to the duration of load and creep
effects of visually graded lumber as specified in Practice D245 for a product under evaluation. Equivalence is based on evaluating
a product’s creep-rupture performance over a minimum of 90 days and meeting the requirements of this specification. This
specification does not attempt to quantify the effect of damage accumulation or to establish product-specific duration of load factors
for the product under evaluation.
5. Test Methods and Acceptance Criteria
5.1 Test Methods:
5.1.1 A test population shall be sampled from production that is representative of the product under evaluation. Two matched
test groups shall be selected, one for short-term bending tests, and one for long-term creep-rupture bending tests. A minimum
sample size of 28 is required for each test group. If further testing is contemplated, additional test specimens shall be sampled from
the initial test population. Long-term and short-term test specimens shall have the same cross section dimensions and length.
NOTE 1—Matching is a technique that attempts to subdivide the initial sample population into two or more separate groups that possess near identical
distributional form and scale for bending properties. Matching specimens for the purposes of 5.1.1 should be done with care, considering errors introduced
by the process and the characteristics of the material under test.
5.1.2 Each test specimen shall be simply supported and loaded by two equal concentrated forces spaced a distance of one-third
the total span from the end supports (that is, third-point bending). Loads shall be applied in the product orientation that represents
the general intended use of the product.
5.1.2.1 For joist-form materials, the span to depth ratio shall be as specified in applicable test standards (see Note 2). Lateral
restraints shall be used when necessary to maintain lateral stability. The minimum test specimen cross section shall be 2.5 in. (63.5
mm) in depth and 1.0 in. (25.4 mm) in width.
NOTE 2—For lumber sized products, span to depth ratios typically used for flexural tests range between 17 and 21.
5.1.2.2 For sheathing-form materials, the test span shall be not less than 48 times specimen thickness or 24 in., whichever is
greater. The specimen width for all sheathing-form materials shall not be less than 12 in. (305 mm).
5.1.3 Moisture content shall be measured on the short-term specimens immediately after destructive testing and on the
long-term specimens at the termination of the long–term test. Measurement of moisture content shall be in accordance with Test
Methods D4442. The average moisture content of all the long-term test specimens shall not deviate more than 62 % from the
average moisture content of all the short–term test specimens (see Note 3).
The boldface numbers in parentheses refer to the list of references at the end of this standard.
D6815 − 09 (2015)
5.1.4 The test environment temperature and relative humidity shall be recorded daily (see Commentary X1.4.5).The daily
average temperature of the test environment shall not decrease more than 5°C (9°F) below the temperature at which the short–term
tests were conducted. At no time shall the test environment reach a temperature less than 0°C (32°F).
NOTE 3—Conditioning the short-term and long-term test material for at least 30 days in the anticipated test environment conditions generally provides
compliance with the 62 % moisture content change criterion.
NOTE 4—In experiments where the temperature falls below the prescribed limit, it may be possible to demonstrate the validity of the data by continuing
the experiment for an additional time period at least equal to, and possibly greater than, the amount of time the temperature was below the prescribed
limit.
5.2 Short-Term Bending Tests:
5.2.1 The loading rate for the short-term tests shall be such that the sample target failure load would be achieved in
approximately 1 min. Failure load shall not be reached in less than 10 s nor more than 10 min. The procedures of Test Methods
D198 or D4761 shall be followed for joist-form materials and Test Methods D1037 or D3043 for sheathing-form materials.
5.2.2 The sample standard deviation and the lower five percent point estimate of the short-term test group (5 % PE) shall be
determined in accordance with Practice D2915.
5.3 Creep-Rupture Bending Tests:
5.3.1 The creep-rupture test specimens shall be loaded such that the average time to attain the pre-selected constant stress level
does not exceed the average time to failure of the short-term tests (see 5.2.1). Thereafter, the specimens shall be subjected to the
constant stress for a minimum period of 90 days. During this period, mid-span deflection readings shall be taken for each test
specimen, until the 90-day time period has elapsed or until the occurrence of a failure. At a minimum, the deflection readings shall
be taken at approximately one minute after the application of the constant load (initial deflection), and at the end of one hour, day
1, day 7, day 14, day 30, day 60, and day 90. When better characterization of the creep rate is desired, more frequent deflection
measurements should be taken. Additional deflection readings are required when the test extends beyond 90 days. When a
specimen failure occurs, time-to-failure shall be recorded.
5.3.2 The specimens selected for these tests shall be tested at a constant stress level, f , as determined in accordance with Eq
b
1 (see Commentary Appendix X1).
f 5 0.55 3 5% PE (1)
~ !
b
where:
f = minimum applied bending stress, and
b
5 % PE = the lower five percent point estimate, as determined from the short-term bending tests in 5.2.
The creep rate, fractional deflection (FD), and the total number of failures at 90 days (N ) (or greater) shall be used to evaluate
the acceptance of the product.
NOTE 5—Examples of acceptable creep and creep-rupture test apparatus are given in Refs 2,3.
5.4 Acceptance Criteria—The product is considered acceptable for using the duration of load and creep factors applicable to
lumber if the following three criteria are all satisfied: (1) adequate strength over the test duration, (2) decreasing creep rate, and
(3) a limited fractional deflection.
5.4.1 Adequate Strength—The total number of failures over the test duration shall be used to determine acceptance.
5.4.1.1 The total number of failures at 90 or more days shall be less than the critical order statistic, N , of the lower 5 %
c
non-parametric tolerance limit with 75 % confidence:
N ,N (2)
90 c
where:
N = number of specimen failures at the end of the 90-day test period, and
N = critical order statistic used to estimate the lower 5 % non-parametric tolerance limit based on the number of specimens
c
under long-term load (see Note 6).
For example, if 53 specimens are used in the creep-rupture tests, then N = 2 and no more than one specimen shall fail within
c
the 90-day period (N ≤ 1) for the product to be accepted as meeting the adequate strength criterion. Alternatively, if 28 specimens
are tested, then N = 1, and no failures shall occur (N = 0).
c 90
5.4.1.2 If the requirement of 5.4.1.1 is not met and the number of failures at 90 days is greater than or equal to the critical order
statistic (N ≥ N ) then the product under evaluation fails to meet the adequate strength criterion with the sample population, N.
90 c
5.4.1.3 If the number of failures at 90 days is equal to the critical order statistic (N = N ) in 5.4.1.2, then additional testing
90 c
may be conducted. In this case the sample population shall be increased by sampling an additional set of matched specimens in
accordance with 5.1.1 sufficient to allow the use of a higher non-parametric order statistic (see Note 6). The additional specimens
shall be tested for another 90-day test duration. The adequate strength requirement of 5.4.1.1 is met when, at the end of the
additional testing, the combined number of specimen failures during these two test series (N combined) is less than the critical
order statistic (N combined) based on the combined number of specimens evaluated (N combined).
c
D6815 − 09 (2015)
NOTE 6—From Practice D2915 the order statistic for the lower 5 % tolerance limit with 75 % confidence, N , for various samp
...