The Double Cantilever Beam (DCB) Test at Forty David L. Sponseller P.E. and Thomas E. Sponseller

Omni Metals Laboratory, Ann Arbor, Michigan, U.S.A.

Summary

Developed by R. B. Heady in the early 1970s, the double-cantilever-beam (DCB) test for measuring sulfide fracture toughness was adopted by some laboratories in 1975. Now, four decades later, it seems appropriate to review the nature, evolution, and experience of this powerful test for measuring the sulfide stress cracking (SSC) resistance of steels used in sour oil and gas wells throughout the world. The quantitative nature and high sensitivity of the test make it uniquely capable of discerning the effects of variations in steel chemistry, and of the many processing parameters involved in the production of oil country tubular goods. Because of its accuracy in rating the SSC resistance of a product and its virtual insensitivity to the method by which the specimen surface is finished, the DCB test has no peer as a quality control tool. Examples are given and the current utilization of the test by producers and users is discussed.

Introduction

For over 60 years the oil and gas industry has been contending with geologic formations bearing harmful amounts of hydrogen sulfide. The extent of this problem has dramatically increased during this time, requiring ever more attention from exploration and production groups. Managing the problems posed by H2S required measuring the H2S resistance of candidate steels and performing extensive quality control testing of mill products. The three main tests for measuring the SSC resistance of low alloy steels were developed in the United States during the first 20 years of what might be called the 'sour era'. These are as follows:

3-Point Bend Test, 1955

Fraser et al. (1)

("Shell bent beam" or "Sc” test); Became NACE Method B in 1990

Tension Test, 1960;

Baldy et al. (2)

Became NACE Method A in 1977.

Double-cantilever-beam (DCB) Test, 1974

Heady (3)

Became NACE Method D in 1990.

This document focuses on the last of these tests, which has now been in use for 40 years. At this major milestone the document is presented as a review of its history and its place in helping assure safe exploration and production operations involving sour oil/gas formations.

Nature of the Test

In 1969 R.B. Heady was given the challenge to develop a test for measuring the resistance of oil well tubulars to hydrogen sulfide based on the rather new fracture mechanics methodology. His extensive analysis and experimentation resulted in a long double-armed and side-grooved plate specimen that gave decreasing KIssc values as hardness and yield strength increased. The specimen dimensions were 1/4 x 1 x 5 inches, with a 1/16-inch-thick slot and side-grooves 0.040 inch deep (16%). Figure 1 shows the key stages of the testing process: 1) removal of the blank from tubular, 2) the machined specimen with hardness impressions, 3) the specimen with wedge inserted, 4) exposure in the aqueous test solution, 5 & 6) measurement of the liftoff load, 7) splitting the DCB, and 8) microscopic examination of the SSC fracture for possible anomalies.

Most commonly, four replicate specimens from a pipe sample are machined and tested. A typical set of SSC fractures is seen in Figure 2. The low angle shadowing reveals fracture topology, which typically changes from coarse to flat-fibrous as the SSC crack grows to its final length. After measuring the liftoff load ‘P’ and crack length ‘a’ and entering them in the equation, the sulfide fracture toughness KIssc is obtained.

When the pipe wall has been extensively worked and has a uniform macrostructure with low twist angle the crack lengths are nearly equal, as seen in Figure 2, and the variations in KIssc are quite small. Standard deviations as low as 1.5 percent of the group average have been observed in groups of four specimens. More typically the macrostructure is relatively coarse and has significant variation from point to point. This sometimes leads to either or both of two main anomalies in the fracture surface:

1) Non-Planar: Figure 3 shows a side view of two specimens. The upper specimen is a normal flat crack, while the lower specimen’s crack deviates severely from the midplane. This is shown at higher magnification in Figure 4. The cause is a macrostructure in the crack initiation zone, Figure 5, reflecting the twist angle from rotary piercing (revealed by annealing to develop a ferrite/pearlite microstructure) that pulls the SSC crack sharply out of the mid-plane. This is an extreme example of a non-planar crack. Non-planar fractures raise the KIssc value, all the more as the deviation increases. Specimens are deemed invalid when the deviation extends beyond the corner of the sidegroove, as in this case. The crack-guiding effect of segregation bands also may be seen in the specimen, annealed after testing, of Figure 6. The upper photomicrograph shows a transverse section through the specimen, with vertical steps at the fracture surface as the fracture tends to follow segregation bands. In the lower photo we had just barely polished into the broad, flat fracture surface, clearly revealing the tendency of the fracture to follow the segregation bands.

2) Edge cracks: Figure 7 shows a heavily edge-cracked specimen. As with the non-planar cracks, edge cracks are caused by the segregation bands arranged in a twist a pattern that result from rotary piercing. As this twist pattern guides the main crack away from the midplane the stress-concentrating effect of the sidegroove tries to bring the cracking process back to midplane by forming a new crack at the bottom of the sidegroove that radiates inward and forward from the initiation point. The ligament between the main crack and the edge crack (“leaf spring”, or “bridge”) absorbs part of the wedge force, impeding crack growth. This tends to give shorter cracks and false, high values of KIssc. Though most errors are much smaller, errors as high as 57 percent have been observed. For this reason the DCB standard requires the invalidation of specimens that are excessively edge-cracked. The basis for invalidating specimens is described in Par. 11.6.1 of NACE TM0177-2005. (4).

It has been shown that the incidence of edge cracking increases with increasing hardness and with the weight/ft of the tubular, which is a proxy for the degree of chemical segregation. (5)

3) HIC Pinning: A third anomaly, seldom observed, is the pinning of the crack front by an HIC

crack. A normal fracture and a much shorter pinned fracture are shown in Figure 8. This blocking of normal crack growth gives an artificially high KIssc value and requires rejection of the test.

When the testing laboratory is careful to disregard those specimens with the types of invalid fractures described above they are rewarded with highly accurate ratings of the pipe’s SSC resistance that afford fine discrimination between products of nearly equal quality.

Evolution of Test

In 1982 a new NACE work group, now named 085c, was formed to write a standard procedure for the DCB test that R.B. Heady had developed in the 1970’s. The committee approved the following changes proposed by the founding chairman (DLS):

• Shortening the specimen from 5 to 4 inches. • Thickening the specimen from 1 / 4 to 3/8 inch. • Deepening the sidegroove from 16 to 20 percent of thickness. • Increasing the slot thickness from 1/16 to 3/32 inch. • Changing the wedge design from single- to double-tapered. • Performing hardness measurement before, not after, the bath exposure. • Allowing the use of an alternative EDM crack starter instead of the chevron starter. • Allowing the use of carbon steel sideplates to promote hydrogen charging for CRA’s. • Increasing the hydrogen charging from 5 minutes MWF to continuous.

The group then organized a round robin test program to evaluate seven low-alloy steels and CRA’s by the DCB test.

The participation in these tests by 14 laboratories did much to expand the interest in the DCB test. The results of this round robin are summarized in Table 1. Average values of KIssc ranged from 17.23 ksi-√in. for 4137H to 66.10 ksi-√in. for C-276. Scatter was high, indicating the need for tightening up test conditions in the standard, as well as reflecting the inexperience of most participants with the relatively new test.

Following this main round robin a ‘mini’ round robin was performed to amass more data on the popular C-90 casing grade.

All participants tested the samples at three arm displacements, 0.020, 0.030, and 0.040 inch. The group average hardness and sulfide fracture toughness were 22.97 ± 0.71 HRC and 34.02 ± 2.24 ksi-√in, respectively. (6). The ‘mini’ round robin gave the unexpected result that every laboratory found a positive slope of KIssc vs. arm displacement. The increasing displacements boosted KIssc, on average, by approximately 0.2 ksi-√in. per 0.001 inch increase in displacement. (It has been theorized that the displacement effect results from the rise in pH during the longer period of crack growth for larger displacements.) To help promote uniformity of results among different laboratories, the work group provided central values, with ± 0.005 inch ranges, for arm displacements when writing the NACE DCB standard. These displacement ranges were based on crack length data observed by chairman DLS in extensive DCB testing during the conduct of many alloy development projects at the Climax Molybdenum laboratory in Ann Arbor, Michigan. These displacement guidelines have served the industry well without change in the NACE values in the ensuing years since the first DCB standard was included in TM0177, in1990. For the API standard sour grades, C-90, T-95, and C-110, the displacement limits have since been tightened up significantly, the current limits in API document 5CT for tests in the 100% H2S NACE Solution A being:

C-90: 0.030 +0.001/-0.002 inch

T-95: 0.028 +0.001/-0.002 inch

C-110: 0.020 +0.001/-0.002 inch.

The main improvements approved by the work group since its founding are as follows:

Tightening of the temperature range to 75 ± 3 °F. Lining up the location of the EDM crack starter tip with that of the chevron base,

1.500 inches from the slotted end of the specimen. Specifying limits on the conditions for fatigue precracking (if used) to avoid

causing false, high KIssc values. Requiring hardness testing. Limiting the maximum deviation of the SSC fracture surface, which should not

rise beyond the corner of the sidegroove. Limiting the extent of edge cracking, as detailed in TM0177-2005, Method D,

Par. 11.6.1. Invalidating specimens whose crack front is pinned by a HIC crack.

Including metallic-looking dry cracks that form after removal from the bath (mainly mild solutions) with the black crack in measuring crack lengths.

Extending the bath exposure for mild solutions by three days, to 17 days, to eliminate most or all dry cracks.

Providing mechanical assurance by checking accuracy of arm displacement and liftoff curve measurements using the compliance calculation. (Figure 9).

Enabling the determination of Klimit , from tests at several values of Kapplied, so as to cancel out the enhancement of KIssc measurements caused by the arm-displacement effect. (Figure 10).

Providing for the testing of thin-walled tubulars by: o Authorizing ¼-inch-thick and 3/16-inch-thick subsized specimens o Permitting the testing of full-wall-thickness specimens and providing the

equation for computing KIssc for such specimens.

The above-listed changes in the test procedure have markedly reduced the scatter in KIssc values, thus greatly increasing the value of the test.

Usefulness of DCB Test

The DCB is unique among SSC tests in that it directly gives a numerical rating of a product’s resistance to a hydrogen sulfide environment, rather than merely a fail / no-fail result. Moreover the values of KIssc are generally quite accurate and reproducible, permitting the effects of small changes in alloy chemistry, processing, or exposure conditions to be discerned. This gives the test a unique ability to solve problems or answer questions for producers and users, as explained here:

Producers

Pipe mills find the DCB test quite helpful in alloy development programs to improve existing grades or develop new ones. Only the DCB test can quantitatively evaluate the effects of changes in the concentrations of C, Mn, Cr, Mo, and Nb plus other minor and residual elements, on SSC resistance. In Table 2 one sees that our tests at OMNI revealed consistently higher KIssc values in pipes having 0.002/0.003% S than pipes having 0.004/0.005% S, in both standard and mild environments. We have seen other examples of this high sensitivity to the effect of sulfur concentration on SSC resistance.

Any of the nine major processing steps is fertile ground for improving the SSC resistance of the final tubular product, as follows:

Steelmaking process Billet diameter Induction stirring at the caster Temperature and time in rotary furnace Rotary piercing parameters Pipemaking scheme and parameters Austenitizing temperature, time, and quench method Tempering temperature, time, and cooling method

Straightening and stress-relieving parameters

Additionally, the DCB test is a powerful and accurate quality control test for assuring that tubulars meet the minimum KIssc values specified for Grades C-90, T-95, and C-110 in API document 5CT, or specified in the purchase agreement. Although more expensive than the Method A test, it has the advantages of requiring less than half the bath time, and being less sensitive to the method of surface preparation.

Users

Most major users are taking advantage of the sensitive discriminating power of the DCB test to solve specific problems and advance the state of the art in their exploration and production activities. Some uses are the following:

Evaluating new alloys or new grades, and new pipe sizes/weights. Ranking pipe lots by their KIssc value so that the best can be used at coolest

spots in the well bore and vice versa. Testing the suitability of a product in specific FFP conditions of various H2S

concentrations, pH values, chloride concentrations, and temperatures. For example, the variation of KIssc with H2S concentration, at various pH values, is shown in Figure 11. It can be seen that the percentage change in KIssc is much lower than that of the H2S concentration, the relationship being more logarithmic than linear. Also, the effect of temperature on KIssc is seen in Figure 12.

Taking advantage of the fracture mechanics basis of the DCB test to assure a safe design, once the minimum detectable flaw size is known, and KIssc data are acquired in the actual FFP conditions.

Monitoring the SSC resistance of expandable tubulars.

Extent of Adoption by Industry

Respondents to a survey conducted for this paper show that industry has embraced the DCB test extensively. From survey results it is estimated that approximately 100,000 tests have been performed to date. This includes the 13,000-plus tests performed at OMNI.

Producers

Most respondents use the DCB test for alloy development and for process development. They like the fact that it gives numerical ratings, not just fail / no-fail results,

The DCB test is not used as much as the Method A test for quality control, but mills are seeing increasing demands for quality control either with both Method A and Method D, or with the Method D alone. Its shorter bath time allows the pipe to be shipped sooner.

Users

Respondents like the numerical ratings from DCB tests, contrasted with the simple fail / no-fail results provided by Method A tests.

Some users employ the DCB test for acceptance testing.

Users find the DCB test very useful in qualifying the following:

New producers New alloys from current producers Higher-strength grades.

They like the fact that DCB testing can quantitatively predict a product’s performance in FFP conditions, and its ability to withstand souring of a field as time goes on and when water flooding is used.

Summary

The history, nature, and current usage of the DCB test have been presented. Its accuracy has been markedly improved through the sustained efforts of the members of NACE Work Group 085c since the committee’s founding in 1982. This has given the oil and gas industry and its suppliers a test that accurately rates the product tested; its discriminating power makes it especially useful in alloy and processing development, and in assessing the ability of a product to perform safely in specific field (FFP) conditions. The DCB test has been accepted by API, enabling producers to qualify the API grades of OCTG by this test. Virtually all major producers and users have embraced the DCB test; after the impressive progress the test has made in the four decades since it was developed by R.B. Heady, its use by all ‘players’ in the oil and gas industry will undoubtedly continue to grow in the years ahead.

Acknowledgements:

The authors are indebted to A.T. Schmidt, W.T. Sponseller, and W. Gram, for their valued assistance in OMNI operations, and to those producers and users that have generously supplied information regarding the utilization of the DCB test by Industry. The authors gratefully acknowledge the sponsorship of DCB research on which some of the information is based.

References

[1] Fraser, J.P, Eldredge, G.G, and Treseder, R.S. , “ Laboratory and Field Methods for Quantitative Study of Sulfide Corrosion Cracking”, H2S Corrosion in Oil and Gas Production - A Compilation of Classic Papers, Tuttle and Kane, 1981

[2] Baldy, M.F. , “Sulfide Stress Cracking of Steels for API Grade N-80 Tubular Products” – H2S

Corrosion in Oil and Gas Production - A Compilation of Classic Papers, Tuttle and Kane, 1981

[3] Heady, R.B. , “ Evaluation of Sulfide Corrosion Cracking Resistance in Low Alloy Steels” ,

H2S Corrosion in Oil and Gas Production - A Compilation of Classic Papers, Tuttle and Kane, 1981

[4] NACE Standard TM-0177-05, Standard Test Method, “Laboratory Testing of Materials for

Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments”, 2005

[5] Sponseller, D.L, Padfield, C.J, Urband, B.E. and Prideco, L.P, “Factors Affecting Crack

Path, Edge Cracking, and K1ssc Rating during Testing of Low-Alloy Steels by the NACE Double-Cantilever Beam Method of TM0177-96(D)”, Corrosion 2003, No.03103

[6] Sponseller, D.L. , “Interlaboratory Testing of Seven Alloys for SSC Resistance by the Double

Cantilever Beam Method” , Corrosion Engineering, Vol. 48, No. 2, pp. 159-171

Table 1. Results of 1980’s Round Robin Program

Alloy Number of Data Sets

Average Hardness, HRC

Round Robin Mean

Standard Deviation of Laboratory Means

Standard Deviation as a Percentage of Round Robin Mean

Low Alloy Steels Cast 4130 2 24.2 36.24 6.67 18.4% C-90 15 23.0 33.51 4.74 14.2% AISI 4137H 4 30.7 17.23 4.29 24.9%

Stainless Steels, Nickel- and Titanium-Base Alloys CA-6NM (Cast) 12 21.0 43.15 8.62 20.0%

Duplex St. Steel 10 31.4 39.69 3.58 9.0%

C-276 2 39.7 66.10 1.98 3.0% Ti-8-1-1 2 38.1 42.90 2.97 6.9%

Table 2. DCB Test Sensitivity to Differences in Sulfur Concentration of C-90 Pipes

Grade KIssc, ksi-√in.

Standard Conditions API Mild Conditions Lower Sulfur

(0.002-0.003%) 36.83A 45.68A

Higher Sulfur (0.004-0.005%) 33.31B 40.04B

Penalty from Higher Sulfur Level -3.52 (-9.6%) -5.64 (-12.3%)

A: Average of 3 pipes; B: Average of 2 pipes

Figure 1. The steps in DCB testing.

Figure 2. Typical set of DCB specimens after splitting. Low-angle illumination reveals texture of the SSC fracture.

Figure 3. Planar and severely non-planar DCB cracks.

Figure 4. Higher magnification view of non-planar crack

Figure 5. View of segregation causing non-planar crack.

Figure 6. Crack-guiding effect of segregation. The top photo shows a transverse section along the length of the crack. The bottom photo shows a top view of the crack surface, after polishing

inwards.

Figure 7. DCB specimen exhibiting extensive edge cracking (an invalid specimen).

Figure 8. HIC crack pinning of crack front. A non-pinned specimen from the same lot is shown below.

Figure 9. Typical plot of compliance criteria.

Figure 10. Determining Klimit by varying the arm displacement of C-110 specimens.

2,00

2,50

3,00

3,50

4,00

4,50

5,00

5,50

6,00

500 700 900 1100 1300 1500

Cra

ck L

engt

h/H

alf H

eigh

t

Lift-off Load/Thickness

δmax

δmin

Min. KIssc

Figure 11. Effect of H2S concentration on KIssc in varying test solutions.

Figure 12. Effect of temperature on KIssc Reduction Factor, R, in various test solutions.

0

10

20

30

40

50

60

70

1 10 100

KIs

sc (k

si-in

0.5 )

% H2S

75 F

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

20 22 24 26 28 30HRC

R =

KIs

sc @

40°

F /

KIs

sc @

75°

F

1% H2S pH = 2.7

1% H2S pH = 3.5

1% H2S pH = 4.5

10% H2S pH = 2.7

10% H2S pH = 4.5

100% H2S pH = 2.7 (Standard)