Standard Test Method for Fracture Strength in Cleavage of Adhesives in Bonded Metal Joints 1

Transcription

1 Designation: D Standard Test Metod for Fracture Strengt in Cleavage of Adesives in Bonded Metal Joints 1 Tis standard is issued under te fixed designation D 3433; te number immediately following te designation indicates te year of original adoption or, in te case of revision, te year of last revision. A number in parenteses indicates te year of last reapproval. A superscript epsilon (e) indicates an editorial cange since te last revision or reapproval. 1. Scope 1.1 Tis test metod (1, 2, 5, 6, 9) 2 covers te determination of fracture strengt in cleavage of adesives wen tested on standard specimens and under specified conditions of preparation and testing (Note 1). 1.2 Tis test metod is useful in tat it can be used to develop design parameters for bonded assemblies. NOTE 1 Wile tis test metod is intended for use in metal-to-metal applications it may be used for measuring fracture properties of adesives using plastic aderends, provided consideration is given to te tickness and rigidity of te plastic aderends. 1.3 Te values stated in SI units are considered to be te standard. Values in parenteses are for information purposes. 1.4 Tis standard does not purport to address all of te safety concerns, if any, associated wit its use. It is te responsibility of te user of tis standard to establis appropriate safety and ealt practices and determine te applicability of regulatory limitations prior to use. 2. Referenced Documents 2.1 ASTM Standards: A 167 Specification for Stainless and Heat-Resisting Cromium-Nickel Steel Plate, Seet, and Strip 3 A 366/A366M Specification for Steel, Seet, Carbon, Cold- Rolled, Commercial Quality 3 B 36 Specification for Brass Plate, Seet, Strip, and Rolled Bar 4 B 152 Specification for Copper Seet, Strip, Plate, and Rolled Bar 4 B 209 Specification for Aluminum and Aluminum-Alloy Seet and Plate 5 B 265 Specification for Titanium and Titanium Alloy Strip, Seet, and Plate 6 1 Tis test metod is under te jurisdiction of ASTM Committee D-14 on Adesives and is te direct responsibility of Subcommittee D14.80 on Metal Bonding Adesives. Current edition approved May 10, Publised August Originally publised as D Last previous edition D Te boldface numbers in parenteses refer to te references at te end of tis test metod. 3 Annual Book of ASTM Standards, Vol Annual Book of ASTM Standards, Vol Annual Book of ASTM Standards, Vol Annual Book of ASTM Standards, Vol D 907 Terminology of Adesives 7 E 4 Practices for Force Verification of Testing Macines 8 E 399 Test Metod for Plane-Strain Fracture Tougness of Metallic Materials 8 3. Terminology 3.1 Definitions: Many of te terms used in tis test metod are defined in Terminology D Definitions of Terms Specific to Tis Standard: crack-extension force, G, te system isolated (fixed load-displacement) loss of stress field energy for an infinitesimal increase, d A, of separational area. In equation form, GdA 52dU T (1) were U T 5 total elastic energy in te system (component or test specimen). In te test specimens of tis metod, te crack front is nearly straigt troug te specimen tickness, B, so tat da 5 B da, were da is an infinitesimal forward motion of te leading edge of te crack. Completely linear-elastic beavior is assumed in te calculations (See Annex A1) of G used in tis metod, an allowable assumption wen te zone of nonlinear deformation in te adesive is small relative to specimen dimensions and crack size Wen te sear stress on te plane of crack and forward to its leading edge is zero, te stress state is termed opening mode. Te symbol for an opening mode G is G I for plane-strain and G 1 wen te connotation of plane-strain is not wanted opening mode fracture tougness, G 1c te value of G just prior to onset of rapid fracturing wen G is increasing wit time opening mode crack arrest tougness, G 1a te value of G just after arrest of a run-arrest segment of crack extension It is assumed tat te dimensions of te part containing te crack are large compared to te run-arrest segment wic precedes crack arrest and tat te quasi-static stress field enclosing te crack tip just after crack arrest can be assumed in calculating G 1a. 4. Summary of Test Metod 4.1 Tis test metod involves cleavage testing bonded specimens suc tat a crack is made to extend by a tensile force 7 Annual Book of ASTM Standards, Vol Annual Book of ASTM Standards, Vol Copyrigt ASTM, 100 Barr Harbor Drive, West Consoocken, PA , United States. 1

2 acting in a direction normal to te crack surface. 4.2 Load versus load-displacement across te bondline is recorded autograpically. Te G 1c and G 1a values are calculated from tis load by equations tat ave been establised on te basis of elastic stress analysis of specimens of te type described below. Te validity of te determination of G 1c and G 1a values by tis test metod depends upon te establisment of a sarp-crack condition in te bondline in a specimen of adequate size. Tis test metod will measure te fracture strengt of a bonded joint wic is influenced by aderend surface condition, adesive, adesive-aderend interactions, primers, adesive-supporting scrims, etc., and in wic of te above possible areas te crack grows. 5. Significance and Use NOTE 2 Crack growt in adesive bond specimens can proceed in two ways: (1) by a slow-stable extension were te crack velocity is dictated by te crossead rate or (2) by a run-arrest extension were te stationary crack abruptly jumps aead outrunning te crossead-predicted rate. Te first type of crack extension is denoted flat; te second type peaked because of te appearance of te autograpic record. Te flat beavior is caracteristic of adesives or test temperatures, or bot, for tese adesives were tere is no difference between initiation, G 1c, and arrest, G 1a. For example, te rubber modified film adesives tested above 17.8 C (0 F) all exibit flat autograpic records. Peaked curves are exibited for all modified materials tested below 73 C ( 100 F) and in general for unmodified epoxies. It sould be noted tat bot peaked and flat beaviors are determined from a crack-lengt-independent specimen. For oter specimens or structures were G increases wit a at constant load te onset of crack growt would result in rapid complete fracturing watever te adesive caracteristics. 5.1 Te property G 1c (and G 1a if relevant) determined by tis test metod caracterizes te resistance of a material to slow-stable or run-arrest fracturing in a neutral environment in te presence of a sarp crack under severe tensile constraint, suc tat te state of stress near te crack front approaces tritensile plane strain, and te crack-tip plastic region is small compared wit te crack size and specimen dimensions in te constraint direction. It as not been proven tat toug adesive systems fully meet tis criteria. Terefore, data developed using equations based on tis assumption may not represent plane-strain fracture values. Comparison of fracture tougness between adesive systems widely different in brittleness or tougness sould take tis into consideration. In general, systems of similar type tougness (3, 4, 7, 8, 10) can be compared as can te effect of environment on tougness of a single system. A G 1c value is believed to represent a lower limiting value of fracture tougness for a given temperature, strain rate, and adesive condition as defined by manufacturing variables. Tis value may be used to estimate te relation between failure stress and defect size for a material in service werein te conditions of ig constraint described above would be expected. Background information concerning te basis for development of tis test metod in terms of linear elastic fracture mecanics may be found in Refs (6) and (7) Cyclic loads can cause crack extension at G 1 values less tan G 1c value. Furtermore, progressive stable crack extension under cyclic or sustained load may be promoted by te presence of certain environments. Terefore, application of G 1c in te design of service components sould be made wit awareness of te G increase for a prior crack wic may occur in service due to slow-stable crack-extension. 5.2 Tis test metod can serve te following purposes: In researc and development to establis, in quantitative terms, significant to service performance, te effects of adesive composition, primers, aderend surface treatments, supporting adesive carriers (scrim), processing variables, and environmental effects In service evaluation to establis te suitability of an adesive system for a specific application for wic te stress conditions are prescribed and for wic maximum flaw sizes can be establised wit confidence For specifications of acceptance and manufacturing quality control, but only wen tere is a sound basis for specification of minimum G 1c values. Te specification of G 1c values in relation to a particular application sould signify tat a fracture control study as been conducted on te component in relation to te expected istory of loading and environment, and in relation to te sensitivity and reliability of te crack detection procedures tat are to be applied prior to service and subsequently during te anticipated life. 6. Apparatus 6.1 Testing Macine, conforming to te requirements of Practices E 4. Select te testing macine suc tat te cracking load of te specimens falls between 15 and 85 % of te full-scale capacity and tat is provided wit a suitable pair of self-aligning pinned fixtures to old te specimen. 6.2 Ensure tat te pinned fixtures and attacments are constructed suc tat tey will move into alignment wit te test specimen as soon as te load is applied. 6.3 For a discussion of te calculation of separation rates see Annex A1. 7. Test Specimens 7.1 Flat Aderend, conforming to te form and dimensions sown in Fig. 1, cut from test joints as in Fig. 2, prepared as prescribed in Section Contoured Double-Cantilever Beam (CDCB), conforming to te form and dimensions sown in Fig Te following grades of metals are suggested for te test specimens (Note 3): Metal ASTM Designation Brass B 36, Alloy 260 (6), quarter ard temper Copper B 152, cold rolled, Type 110, ard temper Aluminum B 209, Alclad 2024, T3 temper, mill finis Steel A 366, regular matte finis Corrosion-resisting steel A 167, Type 304, No. 2B finis Titanium B 265, Grade Test at least twelve specimens, representing at least four different joints. NOTE 3 Since it is unacceptable to exceed te yield point of te metal in flexure during test, te permissible tickness of te specimen will vary wit type of metal, and te general level of strengt of te adesive being investigated. Te minimum permissible tickness in a uniform symmetrical aderend may be computed from te following relationsip: 5Π6 Ta BF (2) ty 2

3 FIG. 1 Flat Aderend Specimen 5 tickness of metal normal to plane of bonding, mm (or in.), F ty 5 tensile yield point of metal (or te stress at proportional limit) MPa (or psi), T % of te maximum load to start te crack in te adesive bond, N (or lbf), a 5 crack lengt at maximum load, mm (or in.), and B 5 bond widt, mm (or in.). 8. Preparation of Test Joints 8.1 Cut seets of te metals or contoured aderends prescribed in and to recommended size (Figs. 2 and 3). All edges of te metal panels and specimens must be flat, free of burrs, and smoot (4.1-µm (160-µin.) maximum) before te panels are surface-treated and bonded. Clean, treat, and dry te seets or contoured aderends carefully, in accordance wit te procedure prescribed by te manufacturer of te adesive. Prepare and apply te adesive in accordance wit te recommendations of te manufacturer of te adesive. Apply te adesive to te faying surface of one or bot metal seets. Ten assemble te seets, faying surface to faying surface in pairs, and allow te adesive to cure under conditions prescribed by te manufacturer of te adesive. 8.2 It is recommended tat eac flat aderend test joint be made wit sufficient area to provide at least five test specimens. FIG. 2 Test Joint 9. Preparation of Test Specimens 9.1 For flat aderend test specimens, trim joint area in accordance wit Fig. 2. Ten cut test specimens, as sown in Fig. 1, from te joints, Fig. 2 (Note 4). Ten cut oles for load pins as sown in Fig Contoured double-cantilever specimens are ready for test as bonded. NOTE 4 Do not use lubricants or oils during te cutting process. For aluminum it is suggested tat te specimens be roug cut 3.2 mm ( 1 8 in.) over-size using a four-pitc band saw traveling at approximately 4.2 m/s (800 ft/min) followed by finis dimensioning to a 1-in. wide 3.2-µm (125-µin.) surface using a five-blade 15-deg carbide fly cutter at 1115 rpm and to m/s (3 to 7-in./min) feed rate. 10. Procedure 10.1 Test specimens, prepared as prescribed in Section 8, in an atmospere maintained at % relative umidity and C ( F). Tests at oter tan ambient temperature may be run if desired. It is suggested tat specimens be conditioned for a minimum of 10 min and a maximum of 30 min at te temperature of test to assure equilibrium. Te manufacturer of te adesive may, owever, prescribe a definite period of conditioning under specific conditions before testing. 3

4 FIG. 3 Contoured Double-Cantilever Beam Specimen 10.2 Determine te following test specimen dimensions Distance from center of 6.4-mm (0.25-in.) insidediameter pin oles to close end of specimen Widt of test specimen, b Tickness of test specimen 127 mm (5 in.) from pin end and 227 mm (9 in.) from pin end Bond line tickness 125 mm (5 in.) from pin end and 227 mm (9 in.) from pin end Load te specimen in te test macine and pin in position using te 6.4-mm (0.25-in.) inside-diameter pin oles. Balance te recorder or cart, or bot. Set te test macine at a crossead separation rate _ cosen to keep time-to-fracture in te order of 1 min, see 6.1 and Annex A1. For example, 2 mm/min (0.08 in./min) gives fracture in 1 min for a CDCB 1 2-in. wide m 5 90-in. 1 aluminum aderend specimen aving a 3-in. long starter crack Te cart recording sould be suc tat maximum load occurs on te record and tat at least 13 mm ( 1 2 in.) of motion is represented on te abscissa (_) for eac 100 mm (4 in.) of ordinate motion (P). For load-time records a cart speed rate sould be used suc tat te slope of te load versus time record is similar to tat specified for load versus loaddisplacement (for example, 5 mm/min (0.2 in./mm)) Apply load to specimen until Point A is reaced. (See Point A, Fig. 4 for flat aderend and Fig. 5, Point A for contoured double-cantilever specimen.) Point A is te load at wic te crack begins to grow rapidly. Ten stop loading and follow crack growt curve on te cart. Wen te load as leveled off at an approximate constant value (te crack as stopped growing), determine and record te following values: FIG. 4 Typical Flat Aderend Test Load to start crack, L (max), N (or lbf), Load wen crack stops, L (min), N (or lbf), and Distance from loading end of specimen to te stationary crack tip in millimetres (or inces) Repeat 10.4 to yield five determinations on eac specimen. 4

5 11. Calculation 11.1 Flat Aderend Specimen: Calculate te fracture tougness, G 1c (from load to start crack), in joules per square metre or pounds-force per inc as follows: G 1c 2 ~max!#@3 a # Calculate fracture tougness, G 1a (from arrest load), as follows: L(max) L(min) E B a FIG. 5 Typical Contoured Double-Cantilever Beam Test G 1a L 2 ~min!#@3 a # 5 load to start crack, N (or lb), 5 load at wic crack stops growing, N (or lb), 5 tensile modulus of aderend, MPa (or psi), 5 specimen widt, mm (or in.), 5 crack lengt, mm (or in.) ( 5 distance from crack tip to pin ole centers), and 5 tickness of aderend, normal to plane of bonding mm (or in.) ( mm (0.50 in.) unless oterwise specified) Contoured Double-Cantilever Specimen: Calculate te fracture tougness, G 1c (from load to start crack), in joules per square metre or pounds-force per inc, as follows: G 1c L 2 2 # Calculate te fracture tougness, G 1a (from arrest load), as follows: G 1a L 2 2 # (3) (4) (5) (6) a 5 crack lengt, mm (or in.) ( 5 distance from crack tip to pin ole centers), 5 tickness of aderend, normal to plane of bonding, mm (or in.), m 5 3 a 2 / 3 +1/, (Note 3) (Note 5), L(max) 5 load to start crack, N (or lbf), L(min) 5 load at wic crack stops growing, N (or lbf), E 5 tensile modulus of aderend, MPa (or psi), B 5 specimen widt, mm (or in.), NOTE 5 Te purpose of te contoured double-cantilever specimen is to make te measurement of fracture tougness G 1 independent of crack lengt a. To develop a linear compliance specimen, its eigt is varied so tat te quantity 3a is constant. Hence, 3a m (7) Tere are, of course, any number of m values tat can be used in designing a specimen. A convenient contour for testing adesives is m 5 90 in. 1, as sown in Fig. 3. Te very ig m number or low-taper angle would cause a large bending stress on te plane of te crack if te specimen were monolitic. Because of te low modulus of te adesives compared wit tat of te aderends, tese bending stresses are not significant. If bulk specimens of te adesive materials are to be tested, te bending stresses tend to cause one or te oter arm to break off. Tis problem is minimized by using lower m numbers, tat is, by making te beams stiffer, and adding side grooves to te specimens to direct te crack in te desired plane of extension. Wen te specimens are made stiffer, te description of m as 5 3 a 2 / 3 +1/ is satisfactory for designing linear compliance specimens but cannot be used to calculate G 1c because te assumptions used in beam teory become increasingly invalid as te beam eigt to lengt ratio increases. In place of m an experimental value determined from compliance calibrations and designated as m8 is required. Hence, te tougness for monolitic specimens aving low m values is defined as G 1c 5 L 2 ~max!@8#@m8# 2B (8) n Eb B n 5 specimen widt at crack plane, and b 5 gross specimen widt. 12. Report 12.1 Report te following information: Complete identification of te adesive tested, including type, source, date manufactured, manufacturers code number, form, etc., Complete identification of te metal used, its tickness, and te metod of cleaning and preparing its surfaces prior to bonding, Application and bonding conditions used in preparing te specimens, Conditioning procedure used for specimens prior to testing, Test temperature, Loading rate used, Time-to-fracture, Cart speed used, Number of specimens tested, Number of joints represented, Bondline tickness (Note 4), Individual G 1c and G 1a (fracture tougness to start 5

6 crack and fracture tougness from arrest load) values for eac specimen, Maximum, minimum, and average values for G 1c and G 1a, and Te nature of te failure, including te average estimated percentages of failure in te coesion of te adesive, contact failure, voids, and apparent adesion to te metal. NOTE 6 Report te average tickness of adesive layer after formation of te joint witin 0.01 mm ( in.). Describe te metod of obtaining te tickness of te adesive layer including procedure, location of measurements, and range of measurements. 13. Precision and Bias 13.1 Te following data sould be used for judging te acceptability of results (95 % confidence limits) (Note 7): Repeatability Duplicate test results by an individual sould be considered suspect if tey differ by more tan 10 % Reproducibility Te average result reported by one laboratory sould be considered suspect if it differs from tat of anoter laboratory by more tan 10 %. NOTE 7 Tese precision data are approximations based on limited data, but tey provide a reasonable basis for judging te significance of results. Care must be taken to control variation in bondline tickness and to measure te crack lengt accurately. Te ability to measure te crack tip and its geometry as well as actual variation in te material properties of some adesive may result in performance wic will ave greater scatter. 14. Keywords 14.1 adesive; bonded joint; cleavage; double-cantilever beam; fracture strengt ANNEX (Mandatory Information) A1. CALCULATION OF SEPARATION RATES A1.1 Fracture tests are generally designed so tat te onset of crack extension occurs in about 1 min from te time monotonically increasing loading begins. Due to compliance and compliance cange differences for different specimen geometries specific ranges of separation rate are required to conform tis time to fracture specification. Tus, te calculation of separation rates for a particular test specimen sall be done using te following expressions. For contoured doublecantilever beams (CDCB): 3200 CB/2 =m8,ḋ, CB/2 =m 8 (A1.1) D 5 displacement of te load (load-displacement), mm (or in.), Ḋ 5 load-displacement rate, mm (or in.)/min, B 5 specimen widt, m8 5 defined in Section 11, C 5 specimen compliance, MPa (or psi); a function of crack lengt, namely: C 5 8/EB [(3 ( a o ) 2 / 3 +1/)+m8 (a a o )] E 5 tensile modulus (defined in Section 11), a 5 crack lengt, mm (or in.) (defined in Section 11), a o 5 lengt of constant-eigt section of te front part of te specimen from te center-line of te loading oles to te point at wic te contoured section begins, and 5 aderend tickness, mm (or in.) (defined in Section 11). NOTE A1.1 Te constants 3200 and are in units of psi =in. and require all units in te equation to be in similar units. If MKS, metric conversion is desirable 3200 and psi =in. are 3.51 and MPa m 3/2. A1.1.1 For example, for 1 2-in. tick, 1 2-in. wide aluminum m in. 1 aderends, te expression for Ḋ and C becomes 84C < Ḋ < 416 C C 5 100/ /10 6 ~a ! (A1.2) A1.1.2 For a crack lengt of 3 in. a rate of 0.08 in./min will cause crack growt to occur in 1 min if G Ic is 10 lb/in. For a 3-in. long crack, 0.025,Ḋ, (A1.3) and te value of 0.08 is witin te range specified. Tis expression for Ḋ in terms of C will give fracture times in te order of 1 min for G Ic values between 1 and 25. (Ḋ sould be selected for a given adesive tougness to give time-to-fracture values close to 1 min.) A1.1.3 Te value of Ḋ sould be increased periodically as te crack extends suc tat it conforms to te expression. If te crack were to be at 6 in.: 0.053,Ḋ, 0.26 (A1.4) Te value of 0.08 in./min would still be witin te above range; owever, fracture times would be increased to 2 min (G Ic 5 10 lb/in.). Tis in itself is not considered a violation of specifications, but if fracture times were to be sortened to 1 min, Ḋ would ave to be increased to 0.17 in./min. A1.1.4 In practice, te crack would be run for some distance, for example 2 in., and te loading rate increased to reduce te fracture time to an acceptable value. A1.1.5 Te calculation of Ḋ for uniform double-cantilever beam specimens can be done in muc te same manner; for example: 3200 CB CB 2 3 ~a 1 0.6! 1 1,Ḋ, 2 3 ~a 1 0.6! 1 1 (A1.5) 2Œ 2 2Œ 2 6

7 3 ~a 1 0.6! C 5 8/EBS 1 D a A1.1.6 For a 3-in. long crack in a 1 2-in. tick 1 2-in. wide aluminum aderend specimen: ,Ḋ, (A1.6) In order to keep Ḋ witin te tolerance limits crack lengt would ave to be monitored wic, of course, would ave to be done to determine initial values of G. D 3433 A1.2 It sould also be noted tat Ḋ, te load-displacement, is not identical wit jaw separation, altoug for low loads using a relatively stiff testing macine tey will be close. For tose tests were it is determined tat tere is a substantial difference between Ḋ and jaw separation rate te jaw separation rate sould be increased to conform wit time-to-fracture requirements. Subsequent tests sould be made using watever correction factor is determined for te particular test macine. REFERENCES (1) Ripling, E. J., Mostovoy, S. and Patrick, R. L., Application of Fracture Mecanics to Adesive Joints, ASTM STP 360, ASTM, (2) Ripling, E. J., Mostovoy, S., and Patrick, R. L., Measuring Fracture Tougness of Adesive Joints, Materials, Researc, and Standards, ASTM, Vol 64, No. 3, (3) Mostovoy, S., and Ripling, E. J., Fracture Tougness of an Epoxy System, Journal of Applied Polymer Science, Vol 10, 1966, pp (4) Mostovoy, S., and Ripling, E. J., Influence of Water on Stress Corrosion Cracking of Epoxy Bonds, Journal of Applied Polymer Science, Vol 13, 1969, pp (5) Mostovoy, S., Bersc, C. F., and Ripling, E. J., Fracture Tougness of Adesive Joints, Journal of Adesion, Vol 3, 1971, pp (6) Ripling, E. J., Corten, H. T., and Mostovoy, S., Fracture Mecanics: A Tool for Evaluating Structural Adesives, Journal of Adesion, Vol 3, 1971, pp (Also publised in SAMPE Journal, 1970). (7) Ripling, E. J., Bersc, C., and Mostovoy, S., Stress Corrosion Cracking of Adesive Joints, Journal of Adesion, Vol 3, 1971, pp (Also publised in SAMPE Journal, 1970). (8) Mostovoy, S., and Ripling, E. J., Te Fracture Tougness and Stress Corrosion Cracking Caracteristics of an Adesive, Journal of Applied Polymer Science, Vol 15, 1971, pp (Also publised in SAMPE Journal, 1970.) (9) Mostovoy, S., and Ripling, E. J. Effect of Joint Geometry on te Tougness of Epoxy Adesives, Journal of Applied Polymer Science, Vol 15, 1971, pp (Also publised SAMPE Journal, 1970.) (10) Mostovoy, S., and Ripling, E. J., Effect of Temperature on te Fracture Tougness and Stress Corrosion Cracking of Adesives, Applied Polymer Symposium No. 19, 1972, pp Te American Society for Testing and Materials takes no position respecting te validity of any patent rigts asserted in connection wit any item mentioned in tis standard. Users of tis standard are expressly advised tat determination of te validity of any suc patent rigts, and te risk of infringement of suc rigts, are entirely teir own responsibility. Tis standard is subject to revision at any time by te responsible tecnical committee and must be reviewed every five years and if not revised, eiter reapproved or witdrawn. Your comments are invited eiter for revision of tis standard or for additional standards and sould be addressed to ASTM Headquarters. Your comments will receive careful consideration at a meeting of te responsible tecnical committee, wic you may attend. If you feel tat your comments ave not received a fair earing you sould make your views known to te ASTM Committee on Standards, 100 Barr Harbor Drive, West Consoocken, PA Tis standard is copyrigted by ASTM, 100 Barr Harbor Drive, West Consoocken, PA , United States. Individual reprints (single or multiple copies) of tis standard may be obtained by contacting ASTM at te above address or at (pone), (fax), or service@astm.org ( ); or troug te ASTM website (ttp:// 7