Astm stp 1114 1991

STP 1114 Elastic-Plastic Fracture Test Methods: The User's Experience (Second Volume) James A Joyce, editor ASTM Publication Code Number (PCN) 04-011140-30 1916 Race Street Philadelphia, PA 19103 Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized A S T M P u b l i c a t i o n C o d e N u m b e r ( P C N ) : 04-011140-30 ISBN: 0-8031-1418-4 ISSN: 055-8497 Copyright 1991 A M E R I C A N SOCIETY F O R TESTING A N D M A T E R I A L S , Philadelphia, PA All rights reserved This material may not be reproduced or copied, in whole or in part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent of the publisher, Photocopy Rights Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by the A M E R I C A N SOCIETY F O R TESTING A N D M A T E R I A L S for users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service, provided that the base fee of $2.50 per copy, plus $0.50 per page is paid directly to CCC, 27 Congress St., Salem, M A 01970; (508) 744-3350 For those organizations that have been granted a photocopy license by CCC, a separate system of payment has been arranged The fee code for users of the Transactional Reporting Service is 0-8031-1418-4/91 $2.50 + 50 Peer Review Policy Each paper published in this volume was evaluated by three peer reviewers The authors addressed all of the reviewers' comments to the satisfaction of both the technical editor(s) and the ASTM Committee on Publications The quality of the papers in this publication reflects not only the obvious efforts of the authors and the technical editor(s), but also the work of these peer reviewers The ASTM Committee on Publications acknowledges with appreciation their dedication and contribution to time and effort on behalf of ASTM Printed in Baltimore, MD August 1991 Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized Foreword The papers in this publication, Elastic-PlasticFracture Test Methods; The User's Experience (Second Volume), were presented at a symposium held in Lake Buena Vista, Florida, 8-9 November 1989 The symposium was sponsored by ASTM Committee E24 on Fracture Testing James A Joyce, U.S Navy Academy, presided as chairman and is editor of this publication Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions auth Contents Overview Experience with the Use of the New ASTM E - - - w ALAN VAN DER SLUYS A N D C H A R L E S S W A D E A Comparison of the J-Integral and CTOD Parameters for Short Crack Specimen T e s t i n g - - W I L L I A M A SOREM, ROBERT H DODDS, JR., AND STANLEY T ROLFE 19 Normalization: An Experimental Method for Developing J-R Curves ZHEN ZHOU, K A N G L E E , R U B E N HERRERA~ A N D J O H N D L A N D E S 42 Quantification of Engineering Limits to J Control of Ductile Crack G r o w t h - - J A M E S A J O Y C E 57 Specimen Size Requirements for Elastic-Plastic Crack Growth Resistance C u r v e s - J R O B I N G O R D O N A N D R I C H A R D L J O N E S 81 A Fracture Instability Data Qualification Limit BRUCE D MACDONALD, R H O B E R D I C K , A N D A L H I S E R , JR 102 Development of Eta Factors in Elastic-Plastic Fracture Testing Using a Load Separation Techuique MONIR H S H A R O B E A M , J O H N D L A N D E S , A N D RUBEN HERRERA 114 Obtaining J-Resistance Curves Using the Key-Curve and Elastic Unloading Compliance Methods: An Integrity Assessment Study SABU J JOHN 133 Nonincremental Evaluation of Modified J-R Curve NAOTAKE OHTSUKA 150 Experience in Using Direct Current Electric Potential to Monitor Crack Growth in Ductile M e t a l S - - M A R K P LANDOW AND CHARLES W MARSCHALL 163 Analysis of Deformation Behavior During Plastic Fracture JUN MING HU AND PEDRO ALBRECHT 178 Fracture Toughness and Fatigue Crack Initiation Tests of Welded PrecipitationHardening Stainless Steel JOHN H UNDERWOOD, RICHARD A FARRARA, G P E T E R O ' H A R A , J O H N J Z A L I N K A , A N D J O H N R S E N I C K 197 Experience with J Testing of Type 304/308 Stainless Steel W e l d m e n t - - S T E P H E N M G R A H A M , W R A N D O L P H L L O Y D , A N D W A L T E R G R E U T E R 213 Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproduction Key-Curve Analysis of Linde 80 Welds KENNETH K YOON, W ALAN VAN DER S L U Y S , A N D A R T H U R L L O W E , JR Observations in Conducting J-R Curve Tests on Nuclear Piping Materials-CHARLES W MARSCHALLAND MARK P LANDOW 225 238 Effect of Residual Stress on the J-R Curve of HY-100 Steel ANDREA D GALLANT, ISA B A R - O N , A N D F L O Y D R T U L E R 260 Dynamic Fracture Toughness of Modified SA508C12 in the Ductile-to-Brittle Transition R e g i o n - - M A R I E T M I G L I N , C S C O T T W A D E , J A M E S A J O Y C E , AND W ALAN VAN DER SLUYS Discussion The Application of the Multispecimen J-Integral Technique to Toughened Polymers DONALD D HUANG 273 289 290 Fracture Toughness of Polycarbonate as Characterized by the J-Integral HENRY L BERNSTEIN 306 Determination of Jt~ for Polymer Using the Single Specimen M e t h o d - - w A t N C H U N G ,AND J A M E S G W I L L I A M S 320 Author Index 341 Subject Index 343 Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized STP1114-EB/Aug 1991 Overview User experience with elastic-plastic test methods dates to 1981 when the first test standard in this field, ASTM E 813-81, Jic, A Measure of Fracture Toughness, became a part of the ASTM Standards This original standard provided a starting point for standards development in elastic-plastic fracture mechanics throughout the world In 1983 the first symposium on User's Experience with Elastic-Plastic Fracture Test Methods was sponsored by ASTM Committee E24 and held in Knoxville, Tennessee Papers and discussion presented at this symposium was published in A S T M STP 856 in 1985 The work presented included not only criticism of E 813 but also new and improved test techniques and many suggestions for improvement of elastic-plastic test technology This forum of new work and criticism had direct application to the development of a dramatically improved version of E 813 as well as the completion of a second test standard, ASTM E 1152, Determining J-R Curves, both of which were first included in the ASTM Book of Standards in 1987 Much work has continued in the field of elastic-plastic fracture mechanics, and the new work is again having a direct impact on the ASTM test standards The Second Symposium on User Experience with Elastic-Plastic Fracture Test Methods was held in Orlando, Florida, in November of 1989 to again bring together the experts with experience to share in testing of elastic-plastic and fully plastic materials Papers presented cover experiences with the test standards, suggestions for improvements and modifications, possible redefinition of the limits of applicability, and applications to a range of materials including polymers Generally the presentations and discussions at this symposium demonstrate a higher level of satisfaction with the E 813-87 standard than there was with the E 813-81 standard Many suggestions for improvements were made and will become a basis for a continued evaluation of elasticplastic test standards The editor would like to acknowledge the assistance of Dorothy Savini of ASTM, E M Hackett and J P Gudas of DTRC, Annapolis, Maryland, in planning and organizing the symposium I thank the authors for making their presentations and submitting their formal papers which make up this publication, and I thank the attendees whose open discussions, questions, and comments resulted in a stimulating symposium I especially thank the reviewers who read and critiqued the papers and who have helped me ensure a high degree of professionalism and technical quality in this publication I wish to thank Portia Wells and Inez Johnson of the U S Naval Academy Mechanical Engineering Department for their aid with document preparation and correspondence associated with both the symposium and this publication, and I wish to thank ASTM publications staff for their many contributions, including supplying deadlines, suggestions, and advice during the preparation of this special technical publication James A Joyce Mechanical Engineering Department, U S Naval Academy, Annapolis, MD 21402; symposium chairman and editor Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Copyright9 by ASTM International www.astm.org Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authori W A l a n Van Der Sluys I and Charles S Wade Experience with the Use of the New ASTM E 813-87 REFERENCE: Van Der Sluys, W A and Wade, C S., "Experience with the Use of the New ASTM E 813-87," Elastic-Plastic Fracture Test Methods: The User's Experience (Second Volume), A S T M STP 1114, J A Joyce, Ed., American Society for Testing and Materials, Philadelphia, 1991, pp 2-18 ABSTRACT: In this paper the impact of recent changes in ASTM Test Method for Jlc, a Measure of Fracture Toughness (E 813) are evaluated J~c was determined from a large number of J-R curves using both the 1981 and the 1987 versions of ASTM E 813 The value of Jic is usually from 10 to 15% higher when measured according to the new version of the standard The scatter in the measured Jtc values was not affected by the revisions Although the revisions to the standard removed a number of difficulties with its use, one problem still remains to be resolved ASTM E 813 should be revised to include some guidance for correcting ao so that the blunting line fits the data in the early portion of the J-R curve when a J-R curve from ASTM Test Method for Determining J-R Curves (E 1152-87) is used KEY WORDS: elastic-plastic fracture, test methods, J-R curve, Jic test standards, fracture toughness The Jic value of a material was first defined in R e f in 1972 This p a r a m e t e r is now used as a measure of a material's resistance to the initiation of ductile testing In 1981, the A S T M issued the Test M e t h o d for Jic, a Measure of Fracture Toughness (E 813-81) This m e t h o d was extensively revised and reissued in 1987 The objective of this paper is, in part, to evaluate the impact on m e a s u r e d values of Jic m a d e by the changes to A S T M E 813 in the 1987 revision Two m a j o r modifications were m a d e to the A S T M E 813-81 version in creating the A S T M E 813-87 version The most significant involved changing the m e t h o d of determining the value of Jic from the J - R curve The 1981 version of the m e t h o d uses the intersection of the blunting line and a linear line fit to a portion of the J - R curve as the measuring point This procedure was changed in the 1987 version of the m e t h o d to use the intersection of a p o w e r law fit to the same portion of the data and a construction line parallel to the blunting line that is offset by an amount representing 0.2 m m (0.008 in.) of crack extension The second m a j o r revision to the 1981 version modified the equation used to evaluate J from load, displacement, and crack length information The expression used in the 1981 version evaluated J from the total area under the load displacement curve The expression was changed so that the elastic and plastic parts of J are evaluated separately in the 1987 version The elastic term is evaluated from the elastic stress intensity, K, defined in A S T M Test M e t h o d for Plane-Strain Fracture Toughness of Metallic Materials (E 399-83) The plastic t e r m is determined from the plastic portion of the area under the load displacement 1Scientist and group supervisor, respectively, Babcock & Wilcox, Research and Development Division, Alliance, OH 44601 Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Copyright9 by ASTM International www.astm.org Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized VAN DER SLUYS AND WADE ON CHANGES IN ASTM E 813-87 curve The combination of the modified relationship for calculating J and the new procedure for determining Jic were intended to improve the accuracy in calculating J and decrease the variability in Jic Differences observed in data sets analyzed by both versions of the method will be discussed in this paper In addition to the two revisions just described, ASTM issued a new standard in 1987, A S T M Test Method for Determining J-R Curves (E 1152-87) ASTM E 813-87 allows the use of the J-R curve determined by ASTM E 1152-87 for the determination of J~ A second objective of this study is to evaluate problem areas that still exist in the method and to recommend solutions to these problems The method of correcting a0 so that the blunting line fits data in the initial portion of the J-R curve is still a problem in the standard A discussion of this problem and difficulties meeting validity criteria will be included in this paper Finally, various procedures for fitting mathematical models to a J-R curve will be reviewed The procedures will be evaluated in terms of the goodness of the fit to the J-R curve and the ability to extrapolate the J-R curve from small-sized specimens Comparison of Data The important issue to be addressed is the effect of the changes in the method on the measured value of J~c Difficulties were encountered with the 1981 version that were identified at the 1983 user's experience symposium [2] One major problem with the 1981 version was a significant variation in JIc with repeated evaluation of the same data set By omitting alternate points between the exclusion lines, variations in valid measures of J~c were as high as 10% for a given test This problem is related to the use of a linear fit to the data between the 0.15-mm (0.006-in.) and 1.5-mm (0.060-in.) exclusion lines for the determination of JIc' The shape of a J-R curve between the exclusion lines is often best represented by a power law relationship rather than a linear relationship In this situation, the linear relationship is strongly influenced by the number and spacing of points between the exclusion lines In the 1981 version, J~ was determined from the intersection of a linear fit to the data between the exclusion lines and the theoretical blunting line Therefore, J~c was also sensitive to the number and spacing of points on the J-R curve that fell between the exclusion lines As a solution to this problem, the 1987 version uses a power law fit to the data between the exclusion lines This relationship is much less sensitive to the number and spacing of points between the exclusion lines The intersection of the power law fit and a construction line define J~c The construction line has a slope equivalent to the theoretical blunting line but is offset by an amount representing 0.2 mm (0.008 in.) of crack extension A second concern identified in the 1983 symposium was scatter in JIc values obtained from the analysis of data sets generated from testing several specimens from the same material The modifications made in the 1987 version of the method were intended to address these concerns To reveal the changes in measured J~ values that are induced by the modifications to the method, results from a large number of J tests were reviewed Data generated in several testing programs were used to make the comparisons It was desired to evaluate test results over a range in measured J~c values Therefore, the data reviewed includes that obtained from tests conducted for O R N L that were reported in Refs and and represent relatively low Jic results for ferritic materials Data obtained in a ferritic steel piping program conducted for both Babcock & Wilcox (B&W) and the Electric Power Research Institute (EPRI) and reported in Ref was also used in the JIc comparison This data set contained a range in J~c results For those tests that were conducted prior to 1987, the results were reanalyzed using ASTIvI E 813-87 procedures For tests completed according to the 1987 version of Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions ELASTIC-PLASTIC FRACTURE TEST METHODS ASTM E 813, the results were reanalyzed to the 1981 version of the method As will be discussed later, a procedure was used that resulted in a consistent correction of the initial crack length, a0 This correction method provides for good agreement between the data in the initial portion of the J-R curve and the blunting line The method described in ASTM E 1152-87 for determination of a0 can result in inappropriate placement of the blunting line and erroneous J~ values All J tests used in this comparison were conducted using the computer-controlled singlespecimen technique described in Ref Load and displacement data were stored directly Crack length information was inferred from unloading compliance data The data presented in Figs ! and are used to evaluate the changes in the measured values of J~ produced by the modifications of the method Figure presents the Jlc values determined on seven different materials over a range in test temperatures all on the Charpy upper shelf The materials included in this figure are four submerged-arc-weld metals (Refs 3-5), two ferritic steels [5], and a manual metal weld [5] In all cases, the values analyzed to the 1987 method are higher than those calculated in accordance with the 1981 version of the method The difference in the submerged-arc-weld metal data ranges from a to 30% increase in the measured value of Ji~ from the 1981 to the 1987 versions The average increase is 11% for the 12 results reported In the case of the ferritic materials and the manual weld, the increase ranges from to 32% The average increase is 18% for the six values reported Figure shows the results from two series of tests conducted at 149~ (300~ on submerged-arc-weld metal [3,4] These two weldments were fabricated using the same welding procedures and with the same heat of weld wire and lot of flux They were each subjected to identical post-weld heat treatment cycles There is significant scatter in these test results from each weldment However, the difference between the results of the two test series is not significant Bars are shown in the figure showing the plus and minus one standard deviation about the mean value of J~ The 1987 version of the analysis resulted in an increase of the measured J~ value of approximately 10% as compared to the 1981 analysis However, use of the 1987 analysis procedure did not reduce the scatter in the measured Jlr data as evidenced by the standard deviations -2500 40~ o93c 121C }V8A SUB ARC WELD ~ 149C [3 HIGH MN MO WELD SA - * r 2000 sA 1o6c "& E 7015-AI Z WELD 300 O P E N POINT E813-81 C L O S E D POINTS E - ~ 1500 I 200 1000 oI " [] o ,W ~oo: 81 O ,I ~* " o!, o -7 500 A& 0i MATERIAL TESTS FIG Jlc values determined u,sing A S T M E 813-81 compared with values obtained using A S T M E 813-87 for several materials Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized 331 CHUNG AND WILLIAMS ON POLYMERS 3.51 i 2.5 / / ~.-'"- t' i / -< "1 -j~" 11 ,w / ]/~ pO 1" ~ 0.5pI 0.0 f / ASTM Blunting Line / - / E813-81 E813-87 / I 0.2 J= 3.54 A a 0.3o , 0.4 , 9 , 0.6 0.8 , 1.0 , , 1.2 1.4 , , 1.6 2.0 1.8 Aa, m m FIG 12 J-R curve for H D P E from single specimen J tests with loading rate of I mm/min pronounced in the blunting region The J-R curves obtained from the two methods are compared and shown in Figs 16 to 19 Despite the scatter of the data, the R-curves acquired from both methods show good agreement All the numerical results are summarized in Table 20 18 / 16 / i : • ." ," "" o ~ 14 i't I= 12 V 10 "~ /' / Z 86 / ~- I ~r;2 i't'1 - ASTM Blunting Un~ ~,3-~1 / / - - " E813-87 / i 0.05 0.10 0.15 0.20 -^ J= 20"4 A a 0'34 u 0.25 - u 0.30 u 0.35 I 0.40 " I 0.45 I 0.50 " ! 0.55 " 0.60 Aa, mm FIG 13 J-R curve for PVDF from single specimen J tests Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized 332 ELASTIC-PLASTIC FRACTURE TEST METHODS 50 45" 40" 35" 9"*"~ii~ 30v o.o.O.o.o ~ "" 25" - ; 2015" ASTM Blunting Line E813-81 E813-87 J= 39.4 A a 0.6 / " os -2" ~.o.-" 10 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 i ! 1.0 1.1 1.2 Aa, mm FIG 14 J-R curve for M D P E from single speciment J tests i i 00 9~ "~2 sl il ~ = }ll Loading Rate: * mal, 0.5mm/min 9 mm/min 5mm/min J | u ! i | ! w ! 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Aa, mm FIG 15 J-R curves for H D P E from single specimen J tests at various loading rates Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized 333 CHUNG AND WILLIAMS ON POLYMERS 9 e~ E eO o ee oql 42 O9 Io b9 Single Specimen (SS) Multiple Specimen (MS) i i i i i i i i i 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 ~a, mm FIG 16 Comparison of J-R curves from multiple and single specimen J tests for HDPE 30 0 20 10 80 o 0.2 I Single Specimen (SS) Multiple Specimen (MS) I I I ! | I 0.4 0.6 0.8 1.0 1.2 1.4 1.6 ~a, mm FIG 17 Comparison of J-R curves from multiple and single specimen J tests for PVDF Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further repro 334 ELASTIC-PLASTICFRACTURE TEST METHODS 50 0 E 13 4O 0 30 0 8o 20 O~ ,0~i~.~ **~ n i 0.2 014 0~6 018 i0 Aa, mm Single Specimen (SS) Multiple Specimen (MS) 112 1~4 116 1~8 2.0 FIG 18 Comparison of J-R curves from multiple and single specimen J tests for MDPE 9 ~, ; o9 ,o m , ,a'-nu 13 &O " [] -, & at nn m~ [] A i ,o ,;",, I ~I o , ~ , ~ "1& ~ MS SS Loading Rate: , ~ o o O.~,n & 0.2 0.4 0.6 0.8 1.0 1.2 1.4 I n I 5mrn/min 1.6 1.8 2.0 Aa, mm FIG 19 Comparison of J-R curves from multiple and single specimen J tests for H D P E at various loading rates Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized CHUNG AND WILLIAMS ON POLYMERS 335 TABLE Main results Jlc, kJ/m~ Material PVDF MDPE HDPE Multiple Specimen Single Specimen Loading Rate, mm/min E 813-81 E 813-87 E 813-81 E 813-87 0.5 0.5 1.0 5.0 10.1 26.5 2.6 2.6 2.1 15.0 28.5 3.0 2.9 2.7 10.2 26 2.3 2.8 2.7 14.5 31 2.8 2.8 3.0 Discussion Application of the ASTM E 813 Test Method For results to be considered valid in this method, the J-controlled growth condition must be fulfilled, and this is achieved by limiting the maximum crack extension to less than 6% of the uncracked ligament [14] In the ASTM E 813 standard, the 1.5 mm offset blunting line is used also for data exclusion Combining these two restrictions, it can be implied that the specimen should have a minimum width of 50 mm and a thickness of 25 mm This may impose problems in producing specimens of such a size for polymers It is suspected that the 6% criteria is too stringent, and, on examination of the work performed by other authors [1-4] and the results from this study, it is apparent that a geometry independent R curve can exist at crack growths of up to about 10% of the uncracked ligament It has been also proposed that the limit can be relaxed to 15% for metals, and a size independent R-curve still be obtained [15] Therefore, it is suggested here that a maximum crack extension of 10% of the ligament is considered to be acceptable for polymers The nine point averaging technique used for crack extension measurements is considered to be more appropriate than the maximum point taken as the ~a by most other investigators [1-5] in the past Since J is a two dimensional analysis a straight crack front is assumed so that the fracture area can be represented by the crack length However, J is the energy per unit area and this area change is better described by the nine point averaging technique if the amount of curvature changes with Aa The data extrapolation procedure in the E 813-81 method can lead to an artificial size effect on the Jic value as observed in Ref The same effect could also result if the amount of crack extension is small [9], and this is illustrated in Fig 20a It can be seen that the Jic value depends on the amount of crack extension and the distribution of the data points In order to reduce the bias on the J~c determination, ~ic, it is suggested that the data exclusion and distribution schemes recommended in the standards should be followed if the E 81381 protocol is adopted However, the curve fitting practice in the E 813-87 method will provide a mor~ consistent Jic value [9] as shown in Fig 20b Blunting Line In E 813, the blunting line plays a crucial role in defining the JTc value However, it has been reported that the conventional blunting line does not adequately reflect the blunting mechanisms for polymers [1,2,8,9] and the same phenomenon can be seen in Figs to The reasons for this are still uncertain, but it has been explained as the effect of plastic constraint at the crack tip [2] From the results, this is considered to be inappropriate for Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions au 336 ELASTIC-PLASTIC FRACTURE TEST METHODS I j)/ d~ ~ c A B ! gJzc qualified data nonqualified data R curve for data A points to B R curve for data points to R curve for data C points to ~j Uncertainty in Jxc lC determination ,~ Aa(mm) a) E - J A B ~Jlc ~ D C ~11 II ii |11 u')o 9~ ~ I o '~ ~ Aa(mm) b) E - FIG - - E r r o r in determination o f the Jic value the materials tested here since the constraint factor will be less than in the cases of H D P E and PVDF It is believed that the problem lies on the crack length measurement and the properties of the materials Crack extension measurements from the fracture surfaces are always difficult since the changes of fracture surface morphology are taken as crack advance For materials such as H D P E which craze before fracture, the craze zone is usually included in the crack length measurements, and this is bound to give a less steep blunting line as shown in Fig 21a in which ,~ab > B/2 In case of the tougher polymers such as MDPE, the material at the crack tip flows due to viscoelasticity, a layer of plastically deformed and strain hardened material will be formed, and a larger crack opening displacement ~, COD, and a steeper blunting line will result, as shown in Fig 21b in which Aa~ < B/2 Figure 21c shows the usual assumption of smooth blunting in which Aab = B/2 together with the other two types of blunting line Crack blunting measured from the side of the midsection by Copyright by ASTM Int'l (all rights reserved); Wed Dec 23 18:49:20 EST 2015 Downloaded/printed by University of Washington (University of Washington) pursuant to License Agreement No further reproductions au CHUNG AND WILLIAMS ON POLYMERS 337 (a) Crazing Effect A a b > ~i12 - Aa b - (b) Crack tip Stretching Aab< 8/2 (c) Smooth blunting Aa b - / Aa b = a/~, > % / / = J/q/=2t~ b /~>2~ i j~