1. API - WI 2348 Methodology to provide Mechanical Assurance in DCB test
Update to the RGSSP
API - WI 2348 Methodology to provide Mechanical Assurance in DCB test
Supporting for Work Group 085c
Double –Cantilever-Beam (DCB) Test
API Summer Meeting
Washington DC, Jun 26th 2013
8:00 am – 12:00 pm

2. Summary of activities to date – FEA models
Update to the RGSSP
Numerical analysis through FEA model of DCB specimens (2D and 3D) with Chevron and EDM. Contribution from USS, V&M and Tenaris
Main Conclusions
EDM line up (shorter) configuration has the smallest difference considering variation of P/B vs. crack length with respect to Chevron notch
EDM lineup specimen has more similar initial cracking driving force to Chevron starter specimen than current standard EDM specimen. EDM line up configuration has been adopted by NACE TM0177 ballot
For cracks exceeding 2 inches long, the percentage variation of lift load is lower than 3%.Therefore the same variation in compliance is expected
Difference of 5% in Compliance between Current equation and 3D Model could be attributed to a slight geometric difference between them (currently under review) and will be verified after Experimental program.

3. Summary of activities to date – Experimental program
Update to the RGSSP
Experimental program on DCB specimens with EDM notch is being conducted at CRC (Shell Lab in Calgary – Canada). This work has been donated by Shell. Activity under progress
Scope of work and experimental procedure
Material:
Grade C110 ODxWT177.8x11.51 mm.
Mechanical Properties :
YS = 113 ±2ksi ; UTS = 126,5±2ksi; mean Hardness cross section 25.5±1 RC.
Compliance determination:
Load and line displacements will be measured by a clip-on gage monitored during the test. An average value of compliance will be obtained for each specimen/condition.
Compliance determination will be performed at Shell Canada labs (CRC).
Evaluated geometries:
Two EDM configuration with four discrete crack lengths per each EDM configuration will be evaluated. Triplicates will be performed for each geometry. Total number of specimens 24.
EDM 1
EDM 2

4. Scope of work and experimental procedure
Summary of activities to date – Experimental program
Update to the RGSSP
Scope of work and experimental procedure
COD linearity
Compliance determination as machined specimens (24), prior to fatigue pre-cracking.
Compliance measurement after pre-cracking to a specified crack length (24).
The maximum load at maximum COD must be less than the maximum load of the fatigue pre-cracking cycles.
Frozen of fracture and crack length measurement. Measurement of DCB crack length will be side visual, by staining and by opening the fracture surface.
Option to pre-crack 3 specimens to NACE pre-crack length and measure compliance prior to extending crack to final specified crack length
Compile the data and determine the best curve fitting the data to obtain a compliance formula for the EDM DCB . Document the work.
Note: Procedure has been discussed/agreed with T. Perez and K. Szklarz

5. Summary of activities to date – Experimental program
Update to the RGSSP
DCB geometry details
Configuration to allow attachment of a load-line crack opening displacement (COD) gauge will be used.

6. Summary of results to date
Summary of activities to date – Experimental program
Update to the RGSSP
Summary of results to date
Compliance determination as machined specimens (24), prior to fatigue pre-cracking.
Checking of the as received specimens has reached following results compared with old data (1993) coming from Chevron notch specimens:
For the short crack started EDM specimens (EDM 2 – Y=38,1 mm)
Differences reported are 10,2 % to 13,2% (Ave 11%)
For the longer crack started EDM specimens (EDM 1 – Y= 41,3 mm)
Differences are 6% to 7,8% (Ave 7%)
Comments and Action Items defined:
As the results were unexpected, Karol Szklarz suggested to check the previous work on Chevron notch (performed in 1993), by doing some more tests (4 o 6 specimens) on the same material.
Tenaris will be machining in the upcoming weeks 6 specimens and then sent it to CRC to be tested.
Shell is checking funding approval to make these additional tests at CRC.

7. Summary of results to date
Summary of activities to date – Experimental program
Update to the RGSSP
Summary of results to date
Compliance measurement after pre-cracking to a specified crack length (24 specimens).
For the longer EDM (EDM 1 - Y=41,3 mm) fatigue cracking and compliance for approximately 40, 50 and 60 mm have been done.  70 mm is yet to come.  However the Shell lab has not yet reported the final crack lengths. According Karol Szklarz estimations the compliance could range from 7% lower for as-received to about 2 % lower for the 60 or 70 mm crack (as compared to existing equation).
For the shorter EDM (EDM 2 – Y=38 mm) no fatigue cracking has been done so far.
Pending work:
Shell to report final crack length on EDM 1 (longer) in order to complete Compliance measurement on the 24 specimens after pre-cracking.
Shell to perform pending fatigue cracking of EDM 2 (shorter) and to report them.
Option to pre-crack 3 specimens to NACE pre-crack length and measure compliance prior to extending crack to final specified crack length will be evaluated.
Compile the data and determine the best curve fitting the data to obtain a compliance formula for the EDM DCB . Document the work.
Reasonable estimation indicate that testing program should be completed before next API winter meeting to be presented.

8. Summary of activities to date – Young modulus
Update to the RGSSP
Proactive discussion has been performed to get consensus on proposal for changing Young’s modulus that is being currently used in DCB compliance analysis.
Based on contributions from Vallourec, NSSMC and Tenaris primary consensus achieved was to recommend : 210 GPa (30,5 x106 psi) for T95 and C110 material.
However, taking into account that API 5C3 standard has stated a Young modulus value of 30,0 x 106 psi, a final consensus was achieved to recommend such value as the right one to be used for Compliance check.
Therefore the Young modulus value of 200 GPa (29,0 x 106 psi) currently used on DCB compliance analysis, is recommended to be changed to a 207 GPa (30,0 x 106 psi )
Proposal was submitted to the API RGPP (Resource Group on Performance Properties) to be considered. RGPP accepted the proposal.

9. Summary of activities to date – Young modulus
Update to the RGSSP
Following recommended changes on NACE TM ballot has been adopted.
Quote
TM 0177 Ballot - Page Determination of Ki applied
Where it say :
E = elastic modulus of DCB material. If a measured elastic modulus is not available, 2.0 x 105 MPa shall be used for carbon and low alloy steels;
Should be changed as follow:
E = elastic modulus of DCB material. If a measured elastic modulus is not available, 2.07 x 106 MPa (30,000 ksi) shall be used for carbon and low alloy steels;
TM 0177 Ballot - Page 59 – Appendix D
Where it say
E = measured elastic modulus or default of 2.0 x 105 MPa (29,000 ksi) for carbon and low alloy steels.
Should be changed as follow:
E = measured elastic modulus or default of 2.07 x 106 MPa (30,000 ksi) for carbon and low alloy steel
Unquote

10. Summary of activities to date – K limit vs. K applied
Update to the RGSSP
K limit and K applied issues regarding effect of specimen thickness, initial crack length and linear-elastic and elastic-plastic deviation were discussed .
Following bullets as a proposal to work has been considered:
Definition of the range of arm displacements to be used for K-limit determination.
Effect of specimen thickness, initial crack length and other test variables.
After discussions on WI meeting at Houston June 3th, Karol Szklarz suggested to use the current experimental program to perform further test by extension of the lift of load and to determine the linear –elastic or elastic-plastic deviation and how much is it, by open the arms after testing. He will prepare the protocol for the CRC experts.