Designation: D6115 − 97 (Reapproved 2019)Standard Test Method forMode I Fatigue Delamination Growth Onset of UnidirectionalFiber-Reinforced Polymer Matrix Composites1This standard is issued under the fixed designation D6115; 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 test method determines the number of cycles (N)for the onset of delamination growth based on the openingmode I cyclic strain energy release rate (G), using the DoubleCantilever Beam (DCB) specimen shown in Fig. 1. This testmethod applies to constant amplitude, tension-tension fatigueloading of continuous fiber-reinforced composite materials.When this test method is applied to multiple specimens atvarious G-levels, the results may be shown as a G–N curve, asillustrated in Fig. 2.1.2 This test method is limited to use with compositesconsisting of unidirectional carbon fiber tape laminates withsingle-phase polymer matrices. This limited scope reflects theexperience gained in round robin testing. This test method mayprove useful for other types and classes of composite materials,however, certain interferences have been noted (see Section 6.5of Test Method D5528).1.3 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.3.1 Exception—The values provided in parentheses are forinformation only.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 establish appro-priate safety, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.1.5 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2D883 Terminology Relating to PlasticsD2584 Test Method for Ignition Loss of Cured ReinforcedResinsD2651 Guide for Preparation of Metal Surfaces forAdhesiveBondingD2734 Test Methods for Void Content of Reinforced PlasticsD3171 Test Methods for Constituent Content of CompositeMaterialsD3878 Terminology for Composite MaterialsD5229/D5229M Test Method for MoistureAbsorption Prop-erties and Equilibrium Conditioning of Polymer MatrixComposite MaterialsD5528 Test Method for Mode I Interlaminar Fracture Tough-ness of Unidirectional Fiber-Reinforced Polymer MatrixCompositesE4 Practices for Force Verification of Testing MachinesE6 Terminology Relating to Methods of Mechanical TestingE122 Practice for Calculating Sample Size to Estimate, WithSpecified Precision, the Average for a Characteristic of aLot or ProcessE177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE456 Terminology Relating to Quality and StatisticsE467 Practice for Verification of Constant Amplitude Dy-namic Forces in an Axial Fatigue Testing SystemE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test MethodE739 Practice for StatisticalAnalysis of Linear or LinearizedStress-Life (S-N) and Strain-Life (ε-N) Fatigue DataE1049 Practices for Cycle Counting in Fatigue AnalysisE1150 Definitions of Terms Relating to Fatigue (Withdrawn1996)31This test method is under the jurisdiction of ASTM Committee D30 onComposite Materials and is the direct responsibility of Subcommittee D30.06 onInterlaminar Properties.Current edition approved March 15, 2019. Published March 2019. Originallyapproved in 1997. Last previous edition approved in 2011 as D6115 – 97(2011).DOI: 10.1520/D6115-97R19.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at

[email protected] For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.3The last approved version of this historical standard is referenced onwww.astm.org.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.13. Terminology3.1 Terminology D3878 defines terms relating to high-modulus fibers and their composites. Terminology D883 de-fines terms relating to plastics. Terminology E6 defines termsrelating to mechanical testing. Terminology E456 and PracticeE177 define terms relating to statistics. Definitions E1150defines terms relating to fatigue. In the event of conflictbetween terms, Terminology D3878 shall have precedenceover the other terminology standards.3.2 Definitions of Terms Specific to This Standard:3.2.1 crack opening mode (Mode I)—fracture mode inwhich the delamination faces open away from each other andin which these faces do not undergo any relative sliding.3.2.2 cycles to onset of delamination growth, Na—the num-ber of fatigue cycles elapsed until the onset of delaminationgrowth from an implanted thin insert.3.2.3 fatigue delamination growth onset relationship,G–N—the relationship between the peak cyclic value of strainenergy release rate to the number of fatigue cycles until theonset of delamination growth, Na.3.2.4 mode I interlaminar fracture toughness, GIc—the criti-cal value of G for delamination growth because of an openingload or displacement.3.2.5 strain energy release rate, G—the loss of strainenergy, dU, in the test specimen per unit of specimen width foran infinitesimal increase in delamination length, da, for adelamination growing under a constant displacement. In math-ematical form:G 521bdUda(1)where:U = total elastic strain energy in the test specimen,b = specimen width, anda = delamination length.3.3 Symbols:3.3.1 a—delamination length.3.3.2 a0—initial delamination length.3.3.3 b—width of DCB specimen.3.3.4 C—compliance, δ/P, of DCB specimen.3.3.5 CV—coefficient of variation, %.3.3.6 da—infinitesimal increase in delamination length.3.3.7 dU—infinitesimal increase in strain energy.3.3.8 EII—modulus of elasticity in the fiber direction.3.3.9 G—strain energy release rate.3.3.10 GIc—opening mode I interlaminar fracture tough-ness.3.3.11 [GIc]av—average values of GIcfrom the quasi-statictests.3.3.12 GImax—maximum or peak cyclic mode I strain en-ergy release rate.3.3.13 G–N—relationship between the cyclic strain energyrelease rate and the number of cycles to onset of delaminationgrowth.3.3.14 h—thickness of DCB specimen.3.3.15 N—number of elapsed fatigue cycles.3.3.16 Na—application dependent value of N at whichdelamination growth onset will occur.3.3.17 N1a%—number of fatigue cycles for the value of Pmaxat N = 1 to decrease by 1 %.3.3.18 NaViS—number of fatigue cycles at which the onset ofdelamination growth is observed.3.3.19 N5a%—number of fatigue cycles for the value of Pmaxat N = 1 to decrease by 5 %.3.3.20 P—applied load.3.3.21 Pcr—value of load at the onset of delaminationgrowth from the insert in the quasi-static tests.3.3.22 Pmax—maximum cyclic load.3.3.23 R—ratio of minimum and peak loads Pmin/Pmax.3.3.24 SD—standard deviation.3.3.25 U—strain energy.3.3.26 Vf—fiber volume fraction, %.3.3.27 δ—load point deflection.3.3.28 δcr—value of displacement at the onset of delamina-tion growth from the insert in a quasi-static test.3.3.29 δmax—maximum value of cyclic displacement.3.3.30 δmean—mean value of cyclic displacement.3.3.31 δmm—minimum value of cyclic displacement.3.3.32 ∆—effective delamination extension to correct forrotation of DCB arms at delamination front.3.3.33 [∆]av—average value of ∆ from the quasi-static tests.4. Summary of Test Method4.1 The Double Cantilever Beam (DCB) shown in Fig. 2 isdescribed in Test Method D5528.4.2 The DCB specimen is cycled between a minimum andmaximum displacement, δmin, and δmax, at a specified fre-quency. For linear elasticity and small deflections (δ/a 0.4)the displacement ratio, δmin/ δmax, is identical to the R-ratio.The number of displacement cycles at which the onset ofdelamination growth occurs, Na, is recorded. The mode I cyclicstrain energy release rate, for example the maximum value,FIG. 1 DCB Specimen with Piano HingesD6115 − 97 (2019)2GImaxis calculated using a modified beam theory or othermethods described in Test Method D5528. By testing severalspecimens a relationship is developed between GImaxand Nafor the chosen frequency.5. Significance and Use5.1 Susceptibility to delamination is one of the majorweaknesses of many advanced laminated composite structures.Knowledge of a laminated composite material’s resistance tointerlaminar fracture under fatigue loads is useful for productdevelopment and material selection. Furthermore, a measure-ment of the relationship of the mode I cyclic strain energyrelease rate and the number of cycles to delamination growthonset, G–N, that is independent of specimen geometry ormethod of load introduction, is useful for establishing designallowables used in damage tolerance analyses of compositestructures made from these materials.5.2 This test method can serve the following purposes:5.2.1 To establish quantitatively the effects of fiber surfacetreatment, local variations in fiber volume fraction, and pro-cessing and environmental variables on G–N of a particularcomposite material.5.2.2 To compare quantitatively the relative values of G–Nfor composite materials with different constituents.5.2.3 To develop criteria for avoiding the onset of delami-nation growth under fatigue loading for composite damagetolerance and durability analyses.6. Interferences6.1 Linear elastic behavior is assumed in the calculation ofG used in this test method. This assumption is valid when thezone of damage or non-linear deformation at the delaminationfront, or both, is small relative to the smallest specimendimension, which is typically the specimen thickness for theDCB test.6.2 As the delamination grows under fatigue, fiber bridgingobserved in quasi-static testing (see Test Method D5528) mayalso occur. Fiber bridging inhibits the fatigue delaminationgrowth resulting in slower growth rates than if there was nobridging. This results in artificially high threshold values wherethe delamination ceases to grow or grows very slowly.4Inaddition, the rate of change of the delamination growth rateversus the peak cyclic strain energy release rate for the DCB isvery high. Therefore, small variations in the peak cyclic strainenergy release rate will result in large changes in the delami-nation growth rate. For these two reasons, this test method doesnot monitor the fatigue delamination growth rate. Instead, thistest method monitors the number of cycles until the onset ofdelamination growth from the end of a thin insert. A value ofG may be defined such that delamination growth will not occuruntil Nacycles have elapsed, where Nais defined by theapplication, Fig. 1.6.3 Three definitions to determine the number of cyclesuntil the onset of delamination growth were used during aninvestigative round robin. These include: (1) the number ofcycles until the delamination was visually observed to grow atthe edge, NaViS;(2) the number of cycles until the compliancehad increased by 1 %, N1%a(this is approximately equivalent toa 1 % decrease in the maximum cyclic load); and (3) thenumber of cycles until the compliance has increased by 5 %,N5%a(this is approximately equivalent to a 5 % decrease in themaximum cyclic load). The three techniques gave differentresults, but the N1%avalue is typically the lowest of the threevalues5and is recommended for generating a conservativecriterion for avoiding onset of fatigue delamination growth indurability and damage tolerance analyses of laminated com-posite structures. Because of the difficulties in visually moni-toring the end of a delamination during a fatigue test, the visualmethod is not included in this test method.6.4 The test frequency may affect results. If the test fre-quency is high, heating effects may occur in the composite. Toavoid these effects, frequency should be chosen to be between1 and 10 cycles per second (Hz) and should be chosen such thatthere is no temperature change of the specimen. Other testfrequencies may be used if they are more appropriate for theapplication. The test frequency shall be reported.6.5 The displacement ratio, δmin/ δmax, may have a largeeffect on the results. Because the DCB specimen cannot betested in compression the displacement ratio must remainwithin the following range: 0 ≤δmin/δmax 1. The displacementratio shall be reported. Large deflections may be considered byusing the corrections given in the Annex of Test MethodD5528.6.6 The application to other materials, lay-ups, and archi-tectures is described in Test Method D5528.7. Apparatus7.1 Testing Machine—A properly calibrated test machineshall be used that can be operated in a displacement controlmode. The testing machine shall conform to the requirementsof Practices E4 and E467. The testing machine shall be4Martin, R. H., and Murri, G. B., “Characterization of Mode I and Mode IIDelamination Growth and Thresholds in AS4/PEEK Composites,” CompositeMaterials: Testing and Design (9th Volume), ASTM STP 1059, S. P. Garbo, Ed.,1990, pp. 251 –270.5Preliminary data from D30.06 round robin.FIG. 2 G–N CurveD6115 − 97 (2019)3equipped with grips to hold the loading hinges, or pins to holdthe loading blocks, that are bonded to the specimen.7.2 Load Indicator—The testing machine load sensing de-vice shall be capable of indicating the total load carried by thetest specimen. This device shall be essentially free frominertia-lag at the specified rate of testing and shall indicate theload with an accuracy over the load range(s) of interest ofwithin 61 % of the indicated value. The peak cyclic load shallnot be less than 10 % of the full scale of the load cell. Section8.2 details how to estimate the expected peak cyclic load. If thecurrent load cell capacity of the test stand is too large, a lowload capacity load cell may be placed in series.7.3 Opening Displacement Indicator—The opening dis-placement may be estimated as the crosshead separation oractuator displacement provided the deformation of the testingmachine, with the specimen grips attached, is less than 2 % ofthe maximum cyclic opening displacement of the test speci-men. If not, then the opening displacement shall be obtainedfrom a properly calibrated external gage or transducer attachedto the specimen. The displacement indicat