1、Designation: D 6068 96 (Reapproved 2002)e1Standard Test Method forDetermining J-R Curves of Plastic Materials1This standard is issued under the fixed designation D 6068; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of l
2、ast revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.e1NOTEEditorially corrected figure reference in paragraph 7.3 in November 2002.1. Scope1.1 This test method covers the determinati
3、on of theJ-integral versus crack growth resistance (J-R) curves forpolymeric materials.1.2 This test method is intended to characterize the slow,stable crack growth resistance of bend-type specimens in sucha manner that it is geometry insensitive within limits set forthin this test method.1.3 The re
4、commended specimens are the three-point bend(SE (B) and pin-loaded compact tension (C (T) specimens.Both specimens have in-plane dimensions of constant propor-tionality for all sizes. Specimen configurations other than thoserecommended in this test method may require different proce-dures and validi
5、ty requirements.1.4 This test method describes a multiple specimen methodthat requires optical measurement of crack extension fromfracture surfaces. It is not recommended for use with materialsin which the crack front cannot be distinguished from addi-tional deformation processes in advance of the c
6、rack tip.1.5 The values stated in SI units are to be regarded as thestandard.1.6 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 and health practices and determ
7、ine the applica-bility of regulatory limitations prior to use.NOTE 1There is no equivalent ISO standard.2. Referenced Documents2.1 ASTM Standards:D 618 Practice for Conditioning Plastics for Testing2D 4066 Classification System for Nylon Injection and Ex-trusion Materials PA3D 5045 Test Methods for
8、Plane-Strain Fracture Toughnessand Strain Energy Release Rate of Plastic Materials4E 399 Test Method for Plane-Strain Fracture Toughness ofMetallic Materials5E 616 Terminology Relating to Fracture Testing5E 1152 Test Method for Determining J-R Curves5E 1737 Test Method for J-Integral Characterizatio
9、n of Frac-ture Toughness5F 1473 Test Method for Notch Tensile Test to Measure theResistance to Slow Crack Growth of Polyethylene Pipesand Resins63. Terminology3.1 DefinitionsTerminology related to fracture testingcontained in Terminology E 616 is applicable to this testmethod.3.2 Definitions of Term
10、s Specific to This Standard:3.2.1 J-integral, J (FL1)a mathematical expression, aline or surface integral over a path that encloses the crack frontfrom one crack surface to the other, used to characterize thelocal stress-strain field around the crack front. See TerminologyE 616 for additional discus
11、sion.3.2.2 J-R curvea plot of resistance to stable physical crackextension, Dap.3.2.3 net thickness, BN(L)the distance between the rootsof the side grooves in side grooved specimens.3.2.4 original crack size, a0(L)the physical crack size atthe start of testing.3.2.5 original uncracked ligament, b0(L
12、)the distancefrom the original crack front to the back edge of the specimen(b0= Wa80).3.2.6 physical crack extension, Dap(L)an increase inphysical crack size (Dap=apa0).3.2.7 physical crack size, ap(L)the distance from areference line to the observed crack front. The distance may bea calculated aver
13、age of several measurements along the crackfront. The reference line depends on the specimen geometryand is normally defined as in 3.2.10. The reference line isdefined prior to specimen deformation.3.2.8 specimen span, S(L)the distance between specimensupports for the SE(B) specimen.1This test metho
14、d is under the jurisdiction of ASTM Committee D20 on Plasticsand is the direct responsibility of Subcommittee D20.10 on Mechanical Properties.Current edition approved November 10, 2002. Published January 2003. Origi-nally approved in 1996. Last previous edition approved in 1996 as D 6068 - 96.2Annua
15、l Book of ASTM Standards, Vol 08.01.3Annual Book of ASTM Standards, Vol 08.02.4Annual Book of ASTM Standards, Vol 08.03.5Annual Book of ASTM Standards, Vol 03.01.6Annual Book of ASTM Standards, Vol 08.04.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2
16、959, United States.3.2.9 specimen thickness, B(L)the side-to-side dimensionof the test specimen (shown in Fig. 2).3.2.10 specimen width, W(L)a physical dimension on atest specimen measured from the rear surface of the specimento a reference line (for example, the front edge of a bendspecimen or the
17、load line of a compact specimen).4. Summary of Test Method4.1 This test method describes a multiple specimen tech-nique for determining the J-R curve for polymeric materials.The J-R curve consists of a plot of J versus crack extension inthe region of J-controlled growth as determined by the dataqual
18、ification requirement of 9.2.4.2 This test method uses optical measurements of cracklength and crack extension on the fracture surfaces after eachtest.5. Significance and Use5.1 A J-R curve produced in accordance with this testmethod characterizes the crack growth resistances of a widerange of tough
19、 polymers and polymer blends (1-5)7that cannotbe obtained in sufficient size and thickness for valid character-ization by linear elastic fracture mechanics in Test MethodsD 5045.5.2 The J-R curve characterizes, within the limits set forth inthis test method, the resistance of a polymeric material to
20、 slowstable crack growth after initiation from a preexisting sharpflaw.5.3 A J-R curve can be used as an index of materialtoughness for blend or alloy design, material selection, mate-rials processing, and quality assurance (6).5.4 The J-R curves from bend specimens represent lowerbound estimates of
21、 J capacity as a function of crack extension,and have been observed to be conservative relative to thoseobtained from specimen configurations under tensile loading.5.5 The J-R curves for a given material of constant micro-structure tend to exhibit lower slope (flatter) with increasingthickness. Thus
22、, it is recommended that the largest possiblespecimen with representative microstructure be used.5.6 The J-R curve can be used to assess the stability ofcracks in structures in the presence of ductile tearing, withawareness of the differences that may exist between laboratorytest and field condition
23、s.5.7 A J-R curve may depend on the orientation and propa-gation of the crack in relation to the anisotropy of the materialwhich may be induced by specimen fabrication methods.5.8 Because of the possibility of rate dependence of crackgrowth resistance, J-R curves can be determined at displace-ment r
24、ates other than that specified in this test method (7).6. Apparatus6.1 Measurements of applied load and load-line displace-ment are needed to obtain the total energy absorbed by thespecimen. Load versus load-line displacement may be recordeddigitally or autographically.6.2 Testing MachineThe J-integ
25、ral tests are to be con-ducted under displacement control to maximize the attainableamount of stable crack extension in the test specimens.NOTE 2The extent to which the crack grows in a stable manner isdependent on the machine stiffness (8) and the mode of control of loading(9).6.3 Bend Test Fixture
26、A suggested fixture for SE (B)specimens is shown in Fig. 2. The fixture may have eitherstationary or moving rollers of sufficiently large diameter tominimize excessive plastic indentation.7The boldface numbers given in parentheses refer to a list of references at theend of the text.FIG. 1 Specimen C
27、onfigurationsFIG. 2 Bending RigD 6068 96 (2002)e126.4 Grips for C (T) SpecimensAsuggested clevis and pinarrangement for gripping compact specimens is given in TestMethod E 399. This arrangement accommodates specimenswith B = 0.5 W.6.5 Displacement Measurement:6.5.1 Load-line displacement measurement
28、s are needed tocalculate J from the area under the load-displacement record.6.5.2 The remote displacement measurement can be per-formed using the stroke or position transducer on the testingmachine. Data obtained in this manner must be corrected forextraneous displacements (such as indentation effec
29、ts, pinpenetration, or machine compliance) by conducting a separateindentation measurement described in 8.7.6.5.3 A direct displacement measurement can be performedusing a separate displacement transducer. This arrangement isshown in Fig. 2 for SE (B) specimens. For C (T) specimens, thedisplacement
30、gage should be placed in the notch on the loadline.7. Specimen Configuration, Size, and Preparation7.1 Specimen Size:7.1.1 The initial selection of specimen size and dimensionscan only be based on J results estimated from previousexperience. Generally, the largest available specimens arerecommended
31、for testing in order to obtain a larger portion ofthe J-R curve and to obtain the most conservative estimate ofcrack growth resistance.7.1.2 Any thickness may be used with the understandingthat the J-R curve will be limited by the maximum crackextension considerations of 9.2 and that the J-R curve i
32、s onlyappropriate for the thickness that is being evaluated.7.2 Specimen Configurations:7.2.1 The recommended SE (B) and C (T) specimens aresimilar to the configurations in Test Methods D 5045 and areshown in Fig. 1. The specimens can be modified to permitload-line displacement measurement. Suggeste
33、d modificationsare given in Test Method E 1152.7.2.2 All in-plane dimensions are proportional to the speci-men width, W. The thickness is nominally B = 0.5 W.7.2.3 The original crack length, a0, shall be greater than 0.5W, but less than 0.65 W.7.2.4 The span, S, to width, W, ratio in SE (B) specimen
34、sshall be 4.7.2.5 Side GroovesSpecimens may need side grooves topromote straighter crack fronts during testing. The sidegrooves should be equal in depth and have an included angle of45 6 5 with a root radius of 0.25 m 6 0.05 mm. The totalthickness reduction may not exceed 0.20 B. Side grooves mustbe
35、 used when the crack front requirements of 9.2.3 cannot bemet with plane sided specimens.7.2.6 Alternative specimens may have 2 # W/B # 4.7.3 Indentation Correction SpecimensSeparately pre-pared unnotched test specimens are used for indentationdisplacement and energy corrections. The specimens are s
36、hownin Fig. 3.7.4 Conditioning:7.4.1 Condition the test specimens at 23 6 2C and 50 65 % relative humidity for not less than 40 h prior to test inaccordance with Procedure A of Practice D 618, for those testswhere conditioning is required. In cases of disagreement, thetolerances shall be 61C and 62
37、% relative humidity.7.4.2 Note that for some hygroscopic materials, such asnylons, the material specifications (for example, SpecificationD 4066) call for testing “dry as molded specimens.” Suchrequirements take precedence over the above routine precon-ditioning to 50 % relative humidity and require
38、 sealing thespecimens in water vapor-impermeable containers as soon asmolded and not removing them until ready for testing.7.5 Notching:7.5.1 The objective of the notching procedure is to obtainthe sharpest possible crack with minimal damage to thematerial in advance of the crack tip.7.5.2 Machine a
39、 pre-notch into the specimen to a depth of0.5 W using either a saw or a single-point flycutter.7.5.3 Create a natural crack by inserting a razor blade intothe pre-notch and tapping it into the specimen and forcing thecrack to grow in advance of the razor blade tip.7.5.4 If a natural crack cannot be
40、successfully generated bytapping the razor blade, slide a fresh, unused razor blade acrossthe root of the machined pre-notch.7.5.5 The length of the razor crack shall not be less than 5 %of the total original crack length, a0.7.5.6 Alternative Notching Techniques:7.5.6.1 Fatigue pre-notching is perm
41、issible. Suggestednotching conditions are given in Test Method E 1152. Becauseof the possibility of hysteretic heating leading to subsequentdamage, frequencies less than 4 Hz are recommended.FIG. 3 Arrangements for Finding Indentation DisplacementD 6068 96 (2002)e137.5.6.2 Pressing a fresh razor bla
42、de into the notch is alsopermissible provided that damage to the material is minimized.Suggested notching conditions and equipment are given in TestMethod F 1473.8. Procedure8.1 Testing ProcedureThe objective of this procedure is todevelop a J-R curve consisting of J-integral values at spacedcrack e
43、xtensions, Dap, as described in 9.3.2. In the multi-specimen method, each test specimen is to develop a singlepoint on the J-R curve. A series of specimens are loaded todifferent displacements using crosshead or displacement con-trol. The resulting crack fronts are marked (as described inAppendix X1
44、) and the crack extensions are measured from thefracture surface. An independent indentation measurement isalso conducted to correct for non-fracture related energydissipation. The J value is then calculated from the indentationcorrected energy for fracture. Each specimen has thus provideda set of J
45、, Dapvalues to describe the J-R curve.8.2 Measure specimen dimensions B, BN, and W to thenearest 0.050 mm or 0.5 % accuracy, whichever is larger.8.3 Because of the viscoelastic nature of polymers, the J-Rcurve may be dependent on test temperature and displacementrate. Therefore, record these conditi
46、ons with the results.8.3.1 Provided that stable, well-defined crack growth can beachieved, any test temperature may be used.8.3.2 Similarly, any test speed that leads to stable, well-defined crack growth may be used. However, test speeds thatlead to loading times (time to maximum load) that are less
47、 than1 ms are not recommended for this procedure due to dynamiceffects on the loading signal that can lead to erroneous results.8.3.3 For general characterization, the suggested test condi-tions in the standard laboratory atmosphere are 23 6 2C and50 6 5 % relative humidity. The suggested test speed
48、 is 1mm/min.8.4 Number of SpecimensAminimum of seven specimensare used to generate the power law fit to the data. All shall bemachined to the same dimensions. The initial precrack lengthsshould be as consistent as possible. The objective is to replicatethe initial portion of the load versus load-lin
49、e displacementtraces as much as possible.8.5 Take each specimen through the following steps:8.5.1 Load to a selected displacement level that is judged toproduce a crack extension in a desired position on the J-Rcurve in accordance with 9.3.2. Use displacement or clip gagecontrol in order to control the amount of crack growth andminimize crack growth instability. Record the load versusload-line displacement curve.8.5.2 Unload the specimen, mark the crack front (seeAppendix X1), and break the specimen to expose the fracturesurface.8.6 Crack Length and Crack
