1、Designation: F791 18Standard Test Method forStress Crazing of Transparent Plastics1This standard is issued under the fixed designation F791; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the year of last revision. A number in par
2、entheses indicates the year of last reapproval. A superscriptepsilon () indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the U.S. Department of Defense.1. Scope1.1 This test method covers the determination of the criticalcrazi
3、ng stress for a transparent plastic material when exposedto a specific solvent, chemical, or compound at a specifictemperature.1.2 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 a
4、ppro-priate safety, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.1.3 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for th
5、eDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2D618 Practice for Conditioning Plastics for TestingE691 Practice for Conducting an Interlaboratory Study t
6、oDetermine the Precision of a Test Method2.2 Other Method:ARTC (Aircraft Research and Testing Committee of theAircraft Industries Association of America, Inc.) Condi-tioning Method33. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 crazinga group of surface fissures that appear t
7、o besmall cracks in the material, although they are not.3.1.1.1 DiscussionCrazing is a form of yielding in poly-mers characterized by a spongy void filled fibrillar structure.The density in the craze changes resulting in a change in theindex of refraction, which causes light to be reflected off of t
8、hecrazes. This light reflection causes the crazes to sparkle whenviewed from certain angles. The crazes are sometimes randomand scattered with varied lengths and depths but usually areoriented perpendicular to a tensile stress. Crazing is difficult todetect. It becomes more pronounced when viewed wi
9、th a lightsource that is at an oblique angle.4. Significance and Use4.1 This test method provides a guide for evaluatingwhether a specific solvent, chemical, or compound is detrimen-tal to a transparent plastic as a result of a manufacturingprocess, a fabrication operation, or the operational enviro
10、n-ment.All transparent plastics are susceptible to crazing, thoughin widely varying degree and from a variety of causes. This testmethod is intended to allow establishment of the crazing stresswhen the simultaneous action of both load and a material thatwill cause crazing is applied, producing non-r
11、eversible damagethat limits the usage of that transparent plastic in a specificapplication.5. Apparatus5.1 Test Fixture, with fluorescent light source illustrated andconstructed as shown in Figs. 1 and 2.5.2 Drill Fixture constructed as shown in Fig. 3.5.3 Marking Fixture, constructed as shown in Fi
12、g. 3.5.4 Portable Specimen Rack, constructed in the manner asshown in Fig. 4 for handling and conditioning test specimens.5.5 WeightsA container and shot for the application ofweight on the rack as shown in Fig. 1.5.6 Filter Paper, quantity of 0.50 by 1.0-in. (12.7 by25.4-mm) pieces of filter, mediu
13、m-retention filter paper.6. Test Specimens6.1 The test specimen shall be machined from the transpar-ent plastic material to be evaluated. A minimum of six1This test method is under the jurisdiction of ASTM Committee F07 onAerospace and Aircraft and is the direct responsibility of Subcommittee F07.08
14、 onTransparent Enclosures and Materials.Current edition approved June 1, 2018. Published June 2018. Originallyapproved in 1982. Last previous edition approved in 2013 as F791 96(2013). DOI:10.1520/F0791-18.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer
15、Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available from Aircraft Industries Association, 1725 DeSales St. NW,Washington, DC 20034.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700,
16、 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 Worl
17、d Trade Organization Technical Barriers to Trade (TBT) Committee.1specimens for each solvent, chemical, or compound is re-quired. The preferred transparent plastic sheet material thick-ness is 6.35 6 0.64 mm (0.250 6 0.025 in.). Other thicknessmaterial is acceptable, but the actual thickness must be
18、recorded. Orientation of each test specimen within the testsheet or part must be recorded.6.2 The test specimens shall be 25.4 6 0.8 mm (1.00 60.03 in.) wide by 177.8 6 1.27 mm (7.00 6 0.05 in.) long bythickness.6.3 The edges shall be smooth machined surfaces withoutcracks, and the test specimen sur
19、face shall be free of defects orirregularities. If the test specimen has been machined tothickness, the non-machined surface shall be the test surface.7. Preparation of Apparatus7.1 Once the load for a particular stress is calculated, thatload will be the sum of the individual weights of the weightr
20、ack, rod, lead weights, container, shot, and the radiused nut.For convenience of assembly, the weight rack pan may bestamped with the total weight of the pan, rod, and nuts as aunit. A container, such as a12-pt (0.24-L) paint can with a6.4-mm (14-in.) hole drilled in the center of the bottom andinst
21、alled so it slides up and down on the rod, can serve as areceptacle for the lead shot (or alternative non-toxic media) toattain required weight.8. Conditioning8.1 Precondition the test specimens in accordance with oneof the following procedures:FIG. 1 Accelerated Crazing Test FixtureFIG. 2 Applicati
22、on of Test Liquid to Piece of Filter Paper on TopSurface of Test SpecimenFIG. 3 Fixtures for Specimen PreparationF791 1828.1.1 Practice D618 Procedure BForty-eight hours at50C (122F) followed by cooling to room temperature indesiccator over anhydrous calcium chloride for at least 5 h.Designate as Co
23、ndition 1 and test within 15 min.8.1.2 Two hours at 90C (194F), ambient cooled, andfollowed by 7 days at 23 6 1.1C (73.5 6 2F) and 50 6 5%relative humidity. Designate as Condition 2 and test within 1 h.8.1.3 ARTC MethodSixteen hours at 14C (25F) belowthe average heat deflection temperature, cool at
24、a rate notexceeding 28C (50F)/h and follow by 96 h at 23 6 1.1C(73.5 6 2F) and 50 6 5 % relative humidity. Designate asCondition 3 and test within 1 h.8.1.4 As received, no preconditioning. Designate as Condi-tion 4.NOTE 1The conditions listed above do not result in uniform moisturecontent for certa
25、in plastics. Moisture content reportedly has a strong effecton craze results for certain plastics. If other preconditionings are requiredto ensure uniform or desired moisture content, the use of these shall bereported in the test report.9. Calculation of Loads9.1 The width and thickness of each spec
26、imen shall bemeasured to the nearest 0.03 mm (0.001 in.). Enter this dataalong with the identification of the specimen in the requiredrecords.9.2 Calculate the load to be used with each specimen inaccordance with the following equation:P 5 S 3 B 3 D2!/6L! (1)where:P = load, N (lb.),S = maximum outer
27、 fiber stress, MPa (psi), determined bytest sequence in 10.5 10.8,or11.5 11.8,L = length of specimen from fulcrum to center of appliedload, mm (in.),B = width of specimen, mm (in.), andD = thickness of specimen, mm (in.).NOTE 2This equation is valid only for relatively small deflections. Forlarge de
28、flections, the dimension L shall be replaced by the actualhorizontal distance from the point of load application to the fulcrum in thedisplaced condition. A deflection of 25.4 mm (1 in.) at the point of loadapplication will result in an actual stress at the fulcrum which isapproximately 5 % less tha
29、n the expected stress, and a 38.1 mm (1.5 in.)deflection will result in an actual stress at the fulcrum which is approxi-mately 10 % less than the expected stress.10. Procedure ACraze Stress Iteration10.1 Place each specimen in the drill fixture and drill a 7.94mm (0.3125 in.) diameter hole at a dis
30、tance of 12.7 6 1.27 mm(0.50 6 0.050 in.) from one end and on the longitudinalcenterline of the specimen.10.2 Place each specimen in the marking fixture and draw apencil line on the edge of the specimen 101.6 mm (4.0 in.)from the center of the 7.94-mm (0.3125-in.) diameter hole andperpendicular to t
31、he length of the specimen (see mark in Fig.2).10.3 Measure the width and thickness of each specimen tothe nearest 0.03 mm (0.001 in.) at the pencil line. Handle eachspecimen only by its edges. Do not clean test specimens in anymanner during the time period between conditioning andtesting.10.4 Insert
32、 the conditioned specimen immediately in the testfixture with the pencil mark on the specimen in line with thecenter of the fulcrum. Raise the weights and insert the end ofthe rod through the 7.94-mm (0.3125-in.) hole in the testspecimen. Carefully center the load supporting nut (with a7.94-mm (0.31
33、25-in.) machined radius on the surface contact-ing the plastic) in the hole and tighten the nut. Align thespecimen edges so they are exactly perpendicular to thefulcrum and slowly lower the weights until the specimenaccepts the load.10.5 Test the first specimen at 27.58 MPa (4000 psi) outerfiber str
34、ess. Apply the load for 10 6 0.5 min and observe to besure no crazing has occurred. Place the 12.5 by 25-mm (12 by1 in.) filter paper directly over the fulcrum in the middle of thespecimen so there is a clear area along each edge to avoidinducing edge crazing. Apply the test chemical to the filterpa
35、per only. Use care so that the test material does not extendbeyond this area and defeat the purpose of the test. Keep thefilter paper moist with test chemical for the duration of the test,15 min, 30 min, or any duration desired. Remove the filterpaper after the test period and inspect for craze. Tur
36、n on thefluorescent lamps for inspection only to avoid undesiredheating of the test specimen. Terminate the testing of thatspecimen.NOTE 3It is recommended that a control test be run with each set ofcraze tests. This control test is conducted exactly the same as the othercraze tests, except that no
37、chemicals shall be applied to this controlspecimen during the craze test. This provides a baseline and allows adetermination of whether the crazing observed in the tests with thechemical applied is due to the chemical/stress combination, or is afunction of stress alone.10.6 If the first specimen is
38、crazed, test the second specimenat 13.79 MPa (2000 psi). If the first specimen did not craze, testthe second specimen at 20.68 MPa (6000 psi).10.7 If the second specimen does not craze at 2000 psi, testthe third specimen at 20.68 MPa (3000 psi). Test the fourth ata lower or higher stress depending o
39、n whether the thirdspecimen did or did not craze. Continue this procedure inFIG. 4 Specimen RackF791 183suitable increments until the critical crazing stress for specificsolvent, chemical, or compound is determined to the desiredaccuracy.10.8 Report the critical crazing stress as the stress midwaybe
40、tween the stress at which crazing was and was not observedon duplicate specimens. For example: Crazing was observed at27.58 MPa (4000 psi) and none at 24.13 MPa (3500 psi). Thecritical crazing stress is reported as 25.86 6 1.72 MPa (3750 6250 psi).10.9 In the examination of the crazing, note all cra
41、cks at theedge of the specimen as “edge crazing.” Disregard thiscondition when ascertaining the end point unless the edge ofcrazing grows and extends across the entire width of thespecimen.10.10 Testing shall be at the specified temperature 63C(65F).11. Procedure BCraze Stress Tracking11.1 Place eac
42、h specimen in the drill fixture and drill a7.94-mm (0.3125-in.) diameter hole at a distance of 12.7 61.27 mm (0.50 6 0.050 in.) from one end and on thelongitudinal centerline of the specimen.11.2 Place each specimen in the marking fixture and draw apencil line on the edge of the specimen 101.6 mm (4
43、.0 in.)from the center of the 7.94-mm (0.3125-in.) diameter hole andperpendicular to the length of the specimen (see the mark inFig. 2). On the side opposite the test surface, using an inksuitable for marking plastic, mark lines across the specimensurface perpendicular to the edge, from edge to edge
44、, at 6.4mm (0.25 in.) intervals, starting at the hole and progressing101.6 mm (4.0 in.) to the point at which the beam will rest onthe fulcrum.11.3 Measure the width and thickness of each specimen tothe nearest 0.03 mm (0.001 in.) at the pencil line. Handle eachspecimen only by its edges. Do not cle
45、an test specimens in anymanner during the time period between conditioning andtesting.11.4 Firmly press a 3.18 mm (0.125 in.) wide ribbon ofnon-compressible black butyl glazing tape sealant on the testsurface of the specimen along each edge to create a dam whichprevents the test chemical from coming
46、 in contact with thespecimen edges. Insert the conditioned specimen immediatelyin the test fixture with the pencil mark on the specimen in linewith the center of the fulcrum. Raise the weights and insert theend of the rod through the 7.94-mm (0.3125-in.) hole in the testspecimen. Carefully center th
47、e load supporting nut (with a 7.94mm (0.3125 in.) machined radius on the surface contacting theplastic) in the hole and tighten the nut. Align the specimenedges so they are exactly perpendicular to the fulcrum andslowly lower the weights until the specimen accepts the load.NOTE 4It is important that
48、 the butyl sealant does not itself causecrazing of the plastic material: KPTI-303 glazing tape butyl sealant hasbeen utilized for craze testing of acrylics and polycarbonates with noadverse effects. To evaluate crazing with butyl or other substituted dammaterial, conduct craze testing as described i
49、n this test method using thesealant material as the craze agent.11.5 Test the first specimen at 27.58 MPa (4000 psi) outerfiber stress. Use the equation of 9.2 to calculate the requiredload. Apply the load for 10 6 0.5 min and observe to be sureno crazing has occurred. At the completion of the 10 minutestabilization period, activate a stop watch or timer for referenceduring the test. Using an eye dropper, apply test chemical to thesurface of the specimen allowing it to run down the specimenbetween the butyl dams placed at each top surface edge of
