Table 3. Values of constants of equation (3) and (4) for API 5L X65 pipeline steel for static loading. Re μ(MPa) A(MPa) B (T-1) Rm μ(MPa) C(MPa) D (T-1) 434 1910 -0.01405 507 843 -0.0094 Dynamic yield stress (strain rate 102 s-1) is determined by the method of instrumented Charpy impact test [7]. The load versus time diagram is recorded and the load at general yielding PGY is evaluated. Fracture energy is defined on load displacement diagram until load at initiation, which is different from maximum load. Dynamic yield stress is then obtained using the Green and Hundy solution [8]. W R W a B P L e GY 4 ) ( 2 − = (5) where W is specimen’s width, B thickness, a is notch depth, and L is the constraint factor with a value of L=1.31. Results are fitted with Equations (3) and (4) and values of constants are reported in Table 4. One notes that higher loading rate leads to higher yield stress and transition temperature. Table 4. Values of constants of Equations (3) and (4) for API 5L X65 pipe steel for dynamic loading. Reμ,d (MPa) Ad(MPa) Bd (T-1) Rmμ,d(MPa) Cd (MPa) Dd (T-1) 541 1802 -0.0012226 658 691 0.010276 2.2. Brittle to ductile transition temperature for Charpy V test TK27 However, despite the introduction during the 1960’s of Fracture Mechanics tests to measure fracture resistance of materials, the practice of the Charpy impact test remains. It always gives a simple and inexpensive method to classify materials by their resistance to brittle fracture. The current trend is also to use these tests to measure fracture toughness and ductile tearing strength. The comparison of the two methods requires taking into account two major differences: • Charpy test uses a notched sample, and fracture mechanics tests use a pre-cracked specimen (but a pre-cracked Charpy specimens may also be used). • Charpy tests are dynamic tests, although the conventional fracture mechanics tests are static ones. Charpy energy versus temperature curve is fitted with the following relationship: ( ) ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ − = + CV CV CV CV CV C T D K A B tanh (6) where ACV, BCV, CCV, and DCV are constants. ACV represents Charpy energy at transition temperature Dcv, BCV is the energy jump between brittle and ductile plateaus and 2CCV is the temperature range of the Charpy energy transition. Transition temperature has been determined at conventional level of 27 joules and called TK27 and also at half the jump between brittle and ductile plateau (TK50 = DCV). Charpy impact tests have been performed on
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