ASTM_D_5323_-_92_2011.pdf

Designation:D532392(Reapproved 2011)Standard Practice forDetermination of 2%Secant Modulus for PolyethyleneGeomembranes1This standard is issued under the fixed designation D5323;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This practice presents a technique for calculating the2%secant modulus for polyethylene geomembranes between0.5 and 5 mm(20 and 200 mil)using Test Method D638.1.2 This practice will facilitate modulus comparisons ofsimilar materials by standardizing the method for deriving thepoints on the stress-strain curve from which the calculationsare performed.1.3 The values stated in SI units are to be regarded as thestandard.The values given in parentheses are for informationonly.1.4 This standard does not purport to address all of thesafety concerns,if any,associated with its use.It is theresponsibility of the user of this standard to consult andestablish appropriate safety and health practices and deter-mine the applicability of regulatory limitations prior to use.2.Referenced Documents2.1 ASTM Standards:2D638 Test Method for Tensile Properties of Plastics3.Terminology3.1 Definitions:3.1.1 modulus of elasticity,MPa(FL2),nthe ratio ofstress(nominal)to corresponding strain below the proportionallimit of a material,expressed in force per unit area,such asmegapascals(pounds-force per square inch).3.1.1.1 DiscussionThe stress-strain relations of manyplastics do not conform to Hookes law throughout the elasticrange,but rather deviate therefrom even at strains well belowthe elastic limit.For such materials,the slope of the tangent tothe stress-strain curve at a low strain is usually taken as themodulus of elasticity(or elastic modulus).Since the existenceof a true proportional limit in polyethylene is questionable,andwith the impracticality of measuring it reliably,the use ofsecant modulus for comparative evaluations is preferred.3.1.2 secant modulus,nthe ratio of stress(nominal)tocorresponding strain at any specified point on the stress-straincurve.3.1.2.1 DiscussionThe measurement units for secantmodulus may change,depending on the standard used.For thepurposes of this practice,the measurement units shall be forceper unit area(FL2),such as megapascals(pounds-force persquare inch).4.Significance and Use4.1 Where to draw the tangent to determine the modulus ofelasticity is often unclear when performing tensile tests withpolyethylene geomembranes.This problem results in a widevariation in test results and therefore makes this propertyunreliable for comparisons.4.2 A secant modulus based on 2%strain can be usefulwhen making comparisons between materials,in qualitycontrol,and in comparing the same sample after being sub-jected to a nonstandard environment.4.3 Secant modulus is an approximation of modulus ofelasticity and generally results in a lower value than that for themodulus of elasticity.4.4 Although the technique for measuring 2%secant modu-lus is described here,other percent secant moduli can bemeasured by this practice.5.Procedure5.1 Follow the test procedure described in Test MethodD638.5.1.1 A cross-head speed of 50 mm/min(2 ipm)is recom-mended for determining secant modulus,regardless of the typeof geomembrane being evaluated.Faster cross-head speedsreduce resolution of the points on the curve.5.1.2 High resolution of load and cross-head movement isimportant for obtaining accurate and reproducible values.Where possible,use settings on the testing equipment that willmagnify this region.1This practice is under the jurisdiction of ASTM Committee D35 on Geosyn-thetics and is the direct responsibility of Subcommittee D35.10 on Geomembranes.Current edition approved June 1,2011.Published July 2011.Originally approvedin 1992.Last previous edition approved in 2006 as D532392(2006).DOI:10.1520/D5323-92R11.2For referenced ASTM standards,visit the ASTM website,www.astm.org,orcontact ASTM Customer Service at serviceastm.org.For Annual Book of ASTMStandards volume information,refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 5.2 Determine the load at 2%strain.5.2.1 Industry standard practice uses Test Method D638Type IV specimens and permits the test to be conductedwithout an extensometer.Hence,strain up to the yield pointwill be based on a gage length of 33 mm(1.3 in.).Thisrepresents the reduced area of the specimen.A gage length of33 mm(1.3 in.)requires a cross-head movement of 0.66 mm(0.026 in.)for 2%strain.5.2.2 Do not compare test results obtained with the use ofextensometers to those obtained without the use of extensom-eters.5.3 Calculate the 2%secant modulus as follows:2%secant modulus5stressstrain(1)where:stress=force/area