1、Designation: D 6815 02aStandard Specification forEvaluation of Duration of Load and Creep Effects of Woodand Wood-Based Products1This standard is issued under the fixed designation D 6815; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revi
2、sion, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This specification provides a procedure for testing andevaluating duration of load and creep effect
3、s of wood andwood-based materials relative to an accepted duration of loadadjustment model. This specification was created for productsthat are currently covered by a consensus standard (forexample, lumber, structural composite lumber and structural-use panels). This procedure is intended to demonst
4、rate theengineering equivalence to the duration of load and creepeffects of visually graded lumber as specified in Practice D 245for a product under evaluation used in dry service conditions.This procedure is not intended to evaluate the performance ofproducts under impact loading. Quantification of
5、 specificduration of load or creep factors is beyond the scope of thisspecification. For further guidance regarding the applicabilityof this specification refer to X1.1 in the Commentary.1.2 Use of the procedure in this specification to determineequivalence to the Practice D 245 duration of load rel
6、ationshipis limited to solid wood and wood-based products whose longterm load behavior is similar to that of solid wood. Equivalencedemonstrated in this specification is dependent upon evaluationof a products 90-day (minimum) creep-rupture performance.In this evaluation, three criteria must be satis
7、fied: (1) adequatestrength over a 90-day period, (2) decreasing creep rate, and (3)limited fractional deflection. A summary of the development ofthese criteria and the underlying assumptions behind them isprovided in the Commentary in Appendix X1 and AppendixX2.1.3 Long term degradation phenomena no
8、t described by acreep-rupture model are not addressed in this standard (seeCommentary X1.2.4).2. Referenced Documents2.1 ASTM Standards:D 9 Terminology Relating to Wood2D 198 Test Methods of Static Tests of Lumber in StructuralSizes2D 245 Practice for Establishing Structural Grades and Re-lated Allo
9、wable Properties for Visually Graded Lumber2D 1037 Test Methods for Evaluating Properties of Wood-Base Fiber and Particleboard Panel Materials2D 2915 Practice for Evaluating Allowable Properties forGrades of Structural Lumber2D 3043 Test Methods for Structural Panels in Flexure2D 4442 Test Methods f
10、or Direct Moisture Content Measure-ment of Wood and Wood-Base Materials2D 4761 Test Methods for Mechanical Properties of Lumberand Wood-Base Structural Material2D 5457 Specification for Computing the Reference Resis-tance of Wood-Based Materials and Structural Connec-tions for Load and Resistance Fa
11、ctor Design2E 4 Practices for Force Verification of Testing Machines2E 6 Terminology Relating to Methods of Mechanical Test-ing2E 177 Practice for Use of the Terms Precision and Bias inASTM Test Methods23. Terminology3.1 DefinitionsSee Terminologies D 9 and E 6, and Prac-tices E 4 and E 177 for defi
12、nitions of terms used in thisspecification.3.2 Definitions of Terms Specific to This Standard:3.2.1 breadththe dimension of the test specimen in thedirection perpendicular to the span and perpendicular to thedirection of an applied bending load.3.2.2 creepthe time-dependent increase of deformation o
13、fthe test material under a constant load.3.2.3 creep deflectiontotal measured deflection at a spe-cific time minus the initial deflection.3.2.4 creep ratethe change in creep deflection over time.3.2.5 creep-rupturea failure phenomenon described by arelationship between applied stress and time-to-fai
14、lure.3.2.6 depththe dimension of the test specimen in thedirection perpendicular to the span and parallel to the directionof an applied bending load.1This specification is under the jurisdiction of ASTM Committee D07 on Woodand is the direct responsibility of Subcommittee D07.01 on Fundamental TestM
15、ethods and Properties.Current edition approved Sept. 10, 2002. Published November 2002. Originallypublished as D 6815-02. Last previous edition D 681502.2Annual Book of ASTM Standards,Vol1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United State
16、s.3.2.7 dry service conditionsthe conditions in most cov-ered structures, where the moisture content of lumber will notexceed 19 % (1).33.2.8 duration of load factora factor customarily used toaccount for the effect of duration of load on the strength ofwood products.3.2.9 failurethe point at which
17、the test member can nolonger support the applied constant load.3.2.10 fractional deflectiona ratio of total deflection to theinitial deflection.3.2.11 initial deflectionthe deflection at approximatelyone minute after the application of load.3.2.12 spanthe distance between the centerlines of endreact
18、ions on which a test specimen is supported to accommo-date a transverse bending load.4. Significance and Use4.1 This specification provides a method for evaluatingduration of load and creep effects of wood and wood-basedproducts subjected to bending stress. This method is intendedto demonstrate the
19、engineering equivalence to the duration ofload and creep effects of visually graded lumber as specified inPractice D 245 for a product under evaluation. Equivalence isbased on evaluating a products creep-rupture performanceover a minimum of 90 days and meeting the requirements ofthis specification.
20、This specification does not attempt to quan-tify the effect of damage accumulation or to establish product-specific duration of load factors for the product under evalua-tion.5. Test Methods and Acceptance Criteria5.1 Test Methods:5.1.1 A test population shall be sampled from productionthat is repre
21、sentative of the product under evaluation. Twomatched test groups shall be selected, one for short-termbending tests, and one for long-term creep-rupture bendingtests. A minimum sample size of 28 is required for each testgroup. If further testing is contemplated, additional test speci-mens shall be
22、sampled from the initial test population. Long-term and short-term test specimens shall have the same crosssection dimensions and length.NOTE 1Matching is a technique that attempts to subdivide the initialsample population into two or more separate groups that possess nearidentical distributional fo
23、rm and scale for bending properties. Matchingspecimens for the purposes of 5.1.1 should be done with care, consideringerrors introduced by the process and the characteristics of the materialunder test.5.1.2 Each test specimen shall be simply supported andloaded by two equal concentrated forces space
24、d a distance ofone-third the total span from the end supports (that is,third-point bending). Loads shall be applied in the productorientation that represents the general intended use of theproduct.5.1.2.1 For joist-form materials, the span to depth ratio shallbe as specified in applicable test stand
25、ards (see Note 2). Lateralrestraints shall be used when necessary to maintain lateralstability. The minimum test specimen cross section shall be 2.5in. (63.5 mm) in depth and 1.0 in. (25.4 mm) in width.NOTE 2For lumber sized products, span to depth ratios typically usedfor flexural tests range betwe
26、en 17 and 21.5.1.2.2 For sheathing-form materials, the test span shall benot less than 48 times specimen thickness or 24 in., whicheveris greater. The specimen width for all sheathing-form materialsshall not be less than 12 in. (305 mm).5.1.3 Moisture content shall be measured on the short-termspeci
27、mens immediately after destructive testing and on thelongterm specimens at the termination of the longterm test.Measurement of moisture content shall be in accordance withTest Methods D 4442. The average moisture content of all thelong-term test specimens shall not deviate more than 62%from the aver
28、age moisture content of all the shortterm testspecimens (see Note 3).5.1.4 The test environment temperature and relative humid-ity shall be recorded daily (see Commentary X1.4.5).The dailyaverage temperature of the test environment shall not decreasemore than 5C (9F) below the temperature at which t
29、heshortterm tests were conducted. At no time shall the testenvironment reach a temperature less than 0C (32F).NOTE 3Conditioning the short-term and long-term test material for atleast 30 days in the anticipated test environment conditions generallyprovides compliance with the 62 % moisture content c
30、hange criterion.NOTE 4In experiments where the temperature falls below the pre-scribed limit, it may be possible to demonstrate the validity of the data bycontinuing the experiment for an additional time period at least equal to,and possibly greater than, the amount of time the temperature was below
31、the prescribed limit.5.2 Short-Term Bending Tests:5.2.1 The loading rate for the short-term tests shall be suchthat the sample target failure load would be achieved inapproximately 1 min. Failure load shall not be reached in lessthan 10 s nor more than 10 min. The procedures of TestMethods D 198 or
32、D 4761 shall be followed for joist-formmaterials and Test Methods D 1037 or D 3043 for sheathing-form materials.5.2.2 The sample standard deviation and the lower fivepercent point estimate of the short-term test group (5 % PE)shall be determined in accordance with Practice D 2915.5.3 Creep-Rupture B
33、ending Tests:5.3.1 The creep-rupture test specimens shall be loaded suchthat the average time to attain the pre-selected constant stresslevel does not exceed the average time to failure of theshort-term tests (see 5.2.1). Thereafter, the specimens shall besubjected to the constant stress for a minim
34、um period of 90days. During this period, mid-span deflection readings shall betaken for each test specimen, until the 90-day time period haselapsed or until the occurrence of a failure. At a minimum, thedeflection readings shall be taken at approximately one minuteafter the application of the consta
35、nt load (initial deflection),and at the end of one hour, day 1, day 7, day 14, day 30, day60, and day 90. When better characterization of the creep rateis desired, more frequent deflection measurements should betaken. Additional deflection readings are required when the test3The boldface numbers in
36、parentheses refer to the list of references at the end ofthis standard.D 6815 02a2extends 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 testedat a constant stress level, fb, as determined in accordance withEq 1
37、(see Commentary Appendix X1).fb5 0.55 3 5%PE! (1)where:fb= minimum applied bending stress, and5 %PE = the lower five percent point estimate, as deter-mined from the short-term bending tests in 5.2.The creep rate, fractional deflection (FD), and the totalnumber of failures at 90 days (N90) (or greate
38、r) shall be used toevaluate the acceptance of the product.NOTE 5Examples of acceptable creep and creep-rupture test appara-tus are given in Refs 2,3.5.4 Acceptance CriteriaThe product is considered accept-able for using the duration of load and creep factors applicableto lumber if the following thre
39、e criteria are all satisfied: (1)adequate strength over the test duration, (2) decreasing creeprate, and (3) a limited fractional deflection.5.4.1 Adequate StrengthThe total number of failures overthe test duration shall be used to determine acceptance.5.4.1.1 The total number of failures at 90 or m
40、ore days shallbe less than the critical order statistic, Nc, of the lower 5 %non-parametric tolerance limit with 75 % confidence:N90, Nc(2)where:N90= number of specimen failures at the end of the 90-daytest period, andNc= critical order statistic used to estimate the lower 5 %non-parametric toleranc
41、e limit based on the numberof specimens under long-term load (see Note 6).For example, if 53 specimens are used in the creep-rupturetests, then Nc= 2 and no more than one specimen shall failwithin the 90-day period (N90# 1) for the product to beaccepted as meeting the adequate strength criterion. Al
42、terna-tively, if 28 specimens are tested, then Nc= 1, and no failuresshall occur (N90= 0).5.4.1.2 If the requirement of 5.4.1.1 is not met and thenumber of failures at 90 days is greater than or equal to thecritical order statistic (N90$ Nc) then the product underevaluation fails to meet the adequat
43、e strength criterion with thesample population, N.5.4.1.3 If the number of failures at 90 days is equal to thecritical order statistic (N90= Nc) in 5.4.1.2, then additionaltesting may be conducted. In this case the sample populationshall be increased by sampling an additional set of matchedspecimens
44、 per 5.1.1 sufficient to allow the use of a highernon-parametric order statistic (see Note 6). The additionalspecimens shall be tested for another 90-day test duration. Theadequate strength requirement of 5.4.1.1 is met when, at theend of the additional testing, the combined number of speci-men fail
45、ures during these two test series (N90combined) is lessthan the critical order statistic (Nccombined) based on thecombined number of specimens evaluated (N combined).NOTE 6From Practice D 2915 the order statistic for the lower 5 %tolerance limit with 75 % confidence, Nc, for various sample populatio
46、ns,N, is as follows:N 28 53 78 102Nc123 45.4.2 Decreasing Creep RateAll the test specimens thatdo not fail during the 90 day constant load time period shallshow a decreasing creep rate.5.4.2.1 To determine a decreasing creep rate, the change increep deflection shall be calculated between a minimum o
47、fthree equally spaced time segments. The change in calculatedcreep deflection shall progressively decrease for each speci-men. For the three equal time periods of 0 to 30 days, 30 to 60days and 60 to 90 days, the decreasing creep rate can beexpressed as (see Commentary Appendix X2):D302 Di. D602 D30
48、. D902 D60(3)where:Di= initial deflection (measured one-minuteafter application of the load per 5.3.1),andD30,D60,D90= deflections measured on 30th, 60th and90th day respectively.NOTE 7To better define the creep rate, additional segments with ashorter frequency (for example, five 18-day segments) ma
49、y be used.5.4.2.2 If the creep rate for a given specimen is notdecreasing at the end of the 90-day period, the time period shallbe extended for a minimum of 30 additional days. The changein calculated creep rate for the additional time segment(s) after90 days shall progressively decrease relative to the precedingsegment.NOTE 8The creep rate may fluctuate due to environmental changes inrelative humidity or temperature, or both. Extending the test beyond the90-day period in a controlled environment may demonstrate that thebeams were not exhibiting tertia
