A SIMPLE PROCEDURE FOR SYNTHESIZING CHARPY IMPACT ENERGY TRANSITION CURVES FROM LIMITED TEST DATA

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1 International Pipeline Conference Volume 1 ASME 1996 IPC A SIMPLE PROCEDURE FOR SYNTHESIZING CHARPY IMPACT ENERGY TRANSITION CURVES FROM LIMITED TEST DATA Michael J. Rosenfeld, PE Kiefner & Associates, Inc. Worthington, Ohio ABSTRACT The im portance o f Charpy V-notch testing o f pipe has been w ell established in the pipeline industry. Until now, it has been necessary to perform a num ber o f tests in order to develop the toughness transition curve. A method is described which m akes possible forecasting the full-scale toughness transition from a single subsize test datum to an acceptable degree o f accuracy. This is potentially useful w here historical test results or material sam ples available for testing are lim ited in quantity. W orked exam ples illustrating the use o f the relationships are given. INTRODUCTION The Charpy V -notch (CV N ) im pact test is a method o f characterizing the notch toughness and resistance to fracture propagation o f a material. A lthough Charpy testing o f pipe m aterials has becom e a widely adopted practice in the pipeline industry, test results are subject to m isinterpretion due to effects o f specim en size and load rate on actual toughness transition behavior. The size dependency is readily accounted for by a sim ple mathematical procedure w hich m akes possible extrapolation o f the full-scale transition curve from a single subsize specim en test. This is potentially useful w here historical test data or m aterial sam ples available for testing (e.g., taken from pipe in service), are limited in quantity. TOUGHNESS TRANSITION BEHAVIOR Plain carbon and low-alloy steels exhibit a transition in toughness w ith tem perature, being brittle at "low" tem peratures and ductile at "high" tem peratures. Brittle fracture takes place at nominally elastic stress levels w ithout appreciable energy absorption occurring through deformation. Consequently, a brittle m aterial may fail catastrophically at low levels o f stress applied in the presence o f relatively small defects. A num ber o f infamous failures o f pipelines and other structures w ere the result o f brittle fracture propagation. By contrast, ductile fracture involves significant energy absorption through plastic m aterial deformation which inherently requires high levels o f applied stress to initiate. Consequently, ductile m aterials have high defect tolerance com pared to brittle materials. The low er part o f Fig. 1 illustrates the dramatic increase in absorbed im pact energy as the fracture m ode changes from brittle to ductile. The ductile impact energy level is sometim es referred to as the "upper-shelf toughness." The im pact energy is a m easure o f resistance to fracture propagation from a defect, but it is not an intrinsic m easure o f the ductility o f the material. D uctility involves plastic deformation, and is indicated in the CV N specim en by lateral expansion and by the proportion o f the fracture surface consisting o f shear. The u pper part o f Fig. 1 illustrates the dram atic increase in fracture surface shear area (as a percentage o f the total fracture surface area) as the fracture m ode changes from brittle to ductile. The transition in shear area and im pact energy over test tem perature tend to coincide. However, the actual tem perature range over w hich the transition occurs may vary greatly depending on metallurgical factors and loading rate. Ductile behavior is considered to occur at tem peratures above the "transition tem perature." A num ber o f definitions for the transition tem perature have been proposed or used. In the pipeline industry, the transition tem perature is generally regarded as the tem perature at w hich 85 percent o f the total fracture Copyright 1996 by ASME

2 surface area consists of shear. This is referred to as the shear-area transition temperature (SATT). IMPACT TOUGHNESS TESTS The Charpy V-Notch Impact Test The Charpy V-notch test is performed by impacting a notched bar specimen with a weighted pendulum having calibrated kinetic energy at the point of impact. The standard size CVN specimen is a rectangular bar with cross section measurements of 10 mm x 10 mm (0.394 inch * inch) and length between end supports of 40 mm.[11 A 2-mm-deep V- notch is machined across the specimen face opposite the impact point. The impact energy absorbed by the specimen as it fractures in three-point bending is measured by the height of the pendulum's swing on the follow-through (a higher swing indicating less energy absorbed in fracturing the specimen and hence lower notch toughness). When the pipe wall thickness is less than 10 mm, "subsize" specimens must be used. Only the specimen width is reduced, while the depth relative to the notch is maintained. One might expect absorbed impact energy at a given test temperature to be proportional to the specimen width. This may be the case at extreme positions (the upper or lower shelves) on the transition curve, however, in the transition area the situation is complicated by the fact that the width of the specimen reduces the constraint on crack-tip plasticity. Consequently, a subsize specimen may fracture with greater ductility than a full-size specimen at the same temperature. The problem this creates is that a smaller specimen will indicate that the brittle-ductile transition occurs at a lower temperature than a larger specimen. Likewise, test results obtained from a specimen that is significantly narrower than the full thickness of the pipe wall may lead one to underestimate the transition temperature unless the specimen size effect is accounted for. The Drop-Weight Tear Test The drop-weight tear test (DWTT) addresses the specimen size effect by using plate-like flattened specimens of full wall thickness. Hence, it reflects the full-scale temperature- and thickness-dependent resistance to crack propagation. The shear area transition temperature obtained from the drop weight tear test, TD, is taken as the true fracture propagation transition temperature of the full-scale pipe wall material. The standard DWTT specimen is 12 inches long, 3 inches deep, and contains a pressed-in notch 0.20 inch deep opposite the impact point)21 If the plates are thin, they may buckle upon impact. This can be prevented by using two specimens doubled up and welded together at their ends. The major disadvantage of the.dwtt is that 100 times the kinetic energy of the CVN test may be required to break the full-scale specimens. Consequently, a very large pendulum and correspondingly large and expensive test frame is needed, so few materials testing laboratories are equipped to perform the test. THE TRANSITION CURVE RELATIONSHIPS Adjustment of Transition Temperature for Specimen Thickness Fortunately, it is possible to avoid the difficulties of the drop-weight tear test by making suitable adjustments to CVN test results. Maxeypl developed an empirical graphical procedure to estimate the DWTT transition temperature, TD, from the SATT obtained from CVN tests, Tc, Fig. 2. The graphical procedure is emulated by the following relationships: T r + A T. D C size (i) and 0.55 Lw A T. = 6 6 s,ze (2) where tw is the pipe wall thickness and ^ is the width of the CVN specimen. Units are inches and degrees Fahrenheit. Agreement between the above equations and Maxey's graphical procedure is shown in Fig. 3. If we assume that TDis an inherent property of a given material in full-scale thickness tw, we can now make a prediction of the SATT of the material in any CVN specimen size "b" knowing the SATT in a specimen size "a". This is accomplished by equating Td in Equation 1 written in terms of specimen size "a" to Td written in terms of specimen size "b", giving the following expression: <JC\ = CTc) a ^ ) 55 (tc)a Units are inches and degrees F ( U (3) Adjustment of Charpy Impact Energy for Specimen Thickness Maxey demonstrated a relationship between the CVN impact energy and the shear area at temperatures below the upper shelf regime: Cv v- = 0.9(SA ) +0.1 (< < v where Cv' is the upper shelf impact energy, Cv is the impact energy at some temperature below the upper shelf regime, and SA is the shear area as a fraction of the net specimen fracture surface area. Alternatively, the shear area may be expressed as a function of the

3 impact energy: SA = 1.1 < v - o.i (5) Until now, it has still been necessary to perform at least half a dozen impact tests, and often more, in order to develop the transition curve. By observing that most simple brittle-ductile transitions have similar profiles, it seemed possible to develop general expressions that might be used to forecast the entire transition curve from a single point. It turns out that the transition curve of many plain carbon and low alloy steels can be adequately characterized using a Sigmoidal transition expressed as a function ofx=t-tc: SA where A and B are specimen-size dependent. We can readily invert Equation (6) to obtain the SATT from the shear area obtained at some other temperature, T: SA T_ = T - B In + A (7) 1 - SA Mathematically, -(A) is the transition center on the X=T-Tc axis (the value of X where SA=0.50), and B is the transition width (the range of X from SA=0.25 to SA=0.75) divided by ,(51 The ratio A/B is to establish SA=0.85 at X=0 (T=TC). The Sigmoidal curve fit is demonstrated in Fig. 4 using data from McNicol1*1. The typical SA transition curve is actually asymmetric, having a sharper "knee" at the upper shelf. Asymmetric transition expressions exist, but they are unwieldy to manipulate, while the Sigmoid is mathematically simple. In any case, the scatter of typical impact energy data simply does not justify a Swiss-watch approach to modelling the curves. The data in Fig. 4 were used to develop A and B values for 1/4-, 1/2-, and full-size CVN specimen sizes, while data from Ref. 3 were used to determine A and B for 2/3-size specimens. A and B are as given in Fig. 5. A and B for any subsize specimen can be determined recalling that a full-size specimen has a width of inch or 10 mm. The values given for A and B work in Equations (6) and (7) with temperature in degrees F. They should be divided by 1.8 when working with temperature in degrees C. The drop weight tear test transition curve is seen to be more abrupt than is the case with CVN transition curves. Effective values for the DWTT curve is A= 19 and B= 11 over a range of actual material thickness encompassing most pipeline applications. Generating the Complete Transition Curve One can now develop a complete impact energy and shear area transition curve in a given specimen size using Equations (4) through (7) starting with only two of the following four quantities: (a) CVN impact energy, Cv, at a test temperature T; (b) Shear area, SA, at a test temperature T; (c) The upper shelf impact energy, Cv' ; (d) The SATT, Tc. If only one of the four quantities is known, say impact energy at a single temperature with no information as to shear area of the specimen or the SATT or the upper shelf impact energy, the transition curve cannot be developed. Using Equations (1) and (2), it is possible to estimate the full-size or full-scale transition temperature from sub-size data. Equations (4) through (7) can then be used to forecast the full-scale transition curve. They may also be used to generate the full-size (10-mm-wide specimen) CVN transition curve for use with full-size specimen criteria or existing Kc-CVN correlations. Experimental Verification The methodology described above was verified against data from several sets of CVN impact tests of line pipe steels.15,151 The correlation between the actual and estimated CVN impact properties is shown in Fig. 6 and Fig. 7. The figures reflect that some scatter is to be expected even in the most complete set of CVN test data. Typical synthesized transition curves are compared to actual test data from Specimen C15T of Reference 6, Fig. 1. The curves were synthesized using only Tc and Cv'. The relationships given herein are applicable only to plain carbon or low alloy steels having a simple transition behavior. They are not validated for certain contemporary steels having a "rising upper shelf1. However, in the U.S., there are hundreds of thousands of miles of line pipe in the ground with more still being purchased for new construction which posess the typical simple transition behavior and which can be reliably modelled as given above. Although there is great utility in being able to perform only a single test, CVN tests are always susceptible to scatter. A confirmation test at the same or different temperature may be worthwhile where material availability permits. Load-Rate Temperature Shift Notch toughness as measured by the CVN test describes the ability of material to absorb energy dynamically and resist fracture propagation. Fracture toughness describes the ability of a material to resist

4 initiating a crack at a notch under static loading. It is expressed in terms of the critical crack-tip stress intensity, Kc, under conditions of slow loading and nominally elastic behavior. Unstable crack growth initiates when an applied crack-tip stress intensity exceeds Kc. Fracture toughness is useful for establishing tolerable crack sizes or establishing tolerable static stress levels given a minimum threshold of crack detection. Several statistical correlations exist between standard full-size C VN and Kc which can be used for such purposes17, but the use of those relationships is outside the scope of this discussion. Fracture toughness is not of interest in dealing with blunt or rounded defects such as metal-loss corrosion because in those cases failure is controlled by material strength. Although Kc-C VN correlations will not be reviewed here, what is significant is that Kc in carbon steel undergoes a temperature-dependent transition similar to that for CVN impact toughness. The transition temperature for Kc may be significantly lower than Tc, as shown in Fig. 8, indicating a loadrate effect. According to data presented in Reference 8, the load rate effect increases with decreasing yield strength. The static transition temperature, TK, can be estimated from the full-scale dynamic transition temperature, TD, as Tk = Td ATrate (8) AT rate, = 1.33S y with temperatures in degrees F and yield strength, ksi, in the range of 30 to 160 ksi (207 to 1,100 Mpa). The fact that lower strength grades may be greatly overstrength should be considered when estimating A Trate. Maxey 9 reports observed ATrate typically around -60 F (-33 C) for line pipe steels, which is a smaller shift than that given in Equation (8). A Trate may be used to verify ductile fracture initiation, which is a necessary precondition for the valid usage of the In-sec expression for longitudinal defects 9 and subsequent acceptance criteria such as ASME B31G. Ductile fracture initiation is verified by showing that the operating temperature exceeds the CVN transition temperature, Tc, corrected for size and rate effects, T ^ T + AT + AT roi op C size rale It is important to realize that a failure that initiates in a ductile manner may propagate in a brittle manner. This could be expected to occur where the operating temperature is above the static fracture initiation toughness transition temperature but below the fullscale fracture propagation transition temperature, or TK<Top<TD. EXAM PLES Exam ple Problem 1 Consider a company purchase specification that pipe must meet or exceed the CVN impact energy of 25 ft-lb and shear area of 60 percent in a full-size specimen at a temperature of 50 F. The pipe to be purchased has a wall thickness of inch, which is less than the width of a full-size CVN specimen (0.394 inch). What are acceptable test temperatures and test results for a subsize specimen that will be equivalent to meeting the full-size 25/60/50 specification? First, estimate the shear area transition temperature that would be given by a full-size specimen just meeting the specification. Using Equation (7) and A=55 and B=32 from the Fig. 5: 0.60 T = In = 92 F From Equation (4), the full-size upper shelf energy would likely be 25 < v = (0.9)(0.60) = 39.1 ft-lb The largest standard specimen that can be obtained from this material is a 2/3-size, with a width of inch. The shear area transition temperature of a 2/3-size specimen according to Equation (3) would be Tc = 92+(66)(0.281) 55[(0.394)- 7-(0.263)07] = 71 F The shear area at various test temperatures would be calculated from Equation (6), using Tc=71 F, and A=47 and B=27 from the Fig. 5. The 2/3-size specimen upper shelf energy would be Cv'=(2/3)(39.1) = 26.1 ft-lb. The impact energies at various test temperatures would be calculated from Equation (4), using the shear areas calculated above and Cv'=26.1ft-lb. The resulting curves are shown in Fig. 9 and Fig. 10. These represent minimum test or target values which would assure at least the same toughness in the actual pipe as would be assured by meeting the full-size 25/60/50 specification. It is not necessary to test at a specific temperature except that it should lie within the transition range shown for the 2/3-size specimen. The next smaller standard specimen is a 1/2- size, with a width of inch. The same procedure could be used to develop target test curves for that size, also shown in Fig. 9 and Fig. 10. Exam ple Problem 2 Consider a pipe in service with 30 inch OD

5 x inch N W T; the material is API 5L X52 but with an actual strength of 54 ksi; and operating at 55 F. It is desired to fully characterize toughness properties o f the material, but sufficient material can be obtained from a small hot tap coupon for a CVN test at a single temperature with one additional test for verification. Two 2/3-size specimens inch wide are tested at 32 F, producing an average impact energy of 9 ft-lb and shear area of 40 percent. First, extrapolate the 2/3-size CVN properties. From Equation (4), the 2/3-size specimen upper shelf impact energy is estimated to be 14 5 C ' = = 31.5 f t- lb v (0.9)(0.40)+ 0.1 From Fig. 5, A = 47 and B = 27 for 2/3-size specimens, and from Equation 7, In V = 90 F One can now develop the entire 2/3-size transition curve using Equation (6) and then Equation (4). Next, estimate the full-scale static and dynamic transition temperatures. From Equation (2), AT = (66)(0.312) 55 _ 100 = _ n F s,ze (0.263) 7 so TD= = 79 F. This indicates that the pipe operates below the fracture propagation transition temperature. From Equation (10), so A Trate=(1.33)(54)-187 = -115F, T k = = -36 F or using Maxey's A Tralc = -60 F, obtain TK = 15 F Either way, TK<Top<TD indicates that a fracture would initiate in a ductile manner (if loading is static or at a slow rate) but could propagate in a brittle manner. Estimate the full-size CVN properties. The full-size CVN SATT is estimated from Equation (3) as Tc = 90+(66)(0.312)055[(0.263)- 7-(0.394)-07 = 112 F The full-size shear area at 50 F is estimated to be 44.5 percent using Equation (6). Then Cv/Cv'=(0.9)(0.445)+0.1 = from Equation (4). The upper shelf impact energy is directly proportional to the net fracture surface area, so the full-size upper shelf energy is Cv' =(3/2)(31.5) = 47.3 ft-lb based on the estimate given above for the 2/3-size specimen given above. Then at 50 F, the full-size im pact energy will be 23.6 ft-lb. This pipe material does not meet the requirements of the 25/60/50 CVN specification referred to in Example 1. CONCLUSIONS It was shown that it is possible to describe the toughness transition behavior of typical carbon steel pipe material using a Sigmoidal transition function. Combined with established relationships between shear area and impact energy relative to the upper shelf, it was shown that it is possible to forecast the toughness transition curve to an acceptable degree of accuracy from a single test datum. Worked examples illustrating the use of the relationships were given. ACKNOWLEDGEMENT The author wishes to acknowledge the enormously helpful advice and opinion given by Mr. Bill Maxey o f Kiefiier & Associates, Inc. during the development o f the method described herein. REFERENCES 1. Standard Test Methods and Definitions for Mechanical Testing of Steel Products, ASTM Standard A Recommended Practice for Conducting Drop- Weight Teat Tests on Line Pipe, API RP 5L3. 3. Maxey, W. A., et al, "Brittle Fracture Arrest in Gas Pipelines," A.G.A. Report No. 135, Catalog No. L51436, April McNicol, R. C., "Correlation o f Charpy Test Results for Standard and Nonstandard Size Specimens," W elding Research Supplement, WRC 385-s, September TableCurve 2D U ser s Manual, Jandel Scientific, Eiber, R. I, et al, "Investigation o f the Initiation and Extent of Ductile Pipe Rupture," Battelle Report to USAEC, B M I-1908, June Roberts, R., and Newton, C., "Interpretive Report on Small-Scale Test Correlations with Klc Data," WRC Bulletin 265, February Rolfe, S. T., and Barsom, J. M., Fracture and Fatigue Control in Structures. Prentice-Hall, Maxey, W. A., et al, "Ductile Fracture Initiation, Propagation, and Arrest in Cylindrical Pressure Vessels," ASTM STP 514,1972.

6 Impact Energy, ft-lb Shear Area, % /100 T-TCiD deg F Figure 4. McNicol s Subsize and Standard Size Data Test Temperature, deg F Figure 1. Brittle-Ductile Transition Charpy Fractional Specimen Size Figure 5. Values of Sigmoidal Parameters A and B Figure 2. Temperature Correction for Thickness Effects Figure 3. Agreement Between Equation (2) and Graphical Method Figure 6. Correlation of Estimated Shear Area to Test Data

7 Estimateci Impact Energy, ft-lb ^ 0» 0 20 * too Test Impact Energy, ft-lb Tost Tempora ture, F Figure 9. Target Shear Area Curves for Example 1 Figure 7. Correlation of Estimated Impact Energy to Test Data Test Temperature, F Figure 10. Target Impact Energy Curves for Example 1 Figure 8. Load-Rate Effect on Brittle-Ductile Transition