Simplified single-specimen method for evaluating CTOA

Industrial application of Instrumented DWTT in evaluating material resistance to ductile fracture for modern pipeline steels

“…3, values of (A * σ f ) for three typical high strength and toughness pipeline steels are 0.261 GPa for X70, 0.292 GPa for X80 and 0.293 GPa for X90 respectively. Moreover, once given the material properties, σ f could be approximately evaluated as 1.3 times the average of yield and ultimate tensile strengths practically [6,9]. Therefore the unknown geometrical constant A* for these three steels were experimentally calculated about 0.30-0.35, verified very close to the commonly cited constant 1/3.…”

“…Where, S is the span between two support rollers (= 254 mm), r * is a rotation factor, for DWTT specimens with larger ligaments, it has been determined to be 0.57 for high-strength steels including API X70-X100 pipeline steel appropriately [6]. Thus, the regression value of impact energy vs. load within the stable cracking range that is defined by the crack length from a 1 to a 2 may be used to evaluate instantaneous CTOA C , only requiring values of r * and S.…”

“…In some works [3,6], the crack extension was estimated by limit loading hypothesis based on the change of the force over the propagation, compared with the maximum force at crack initiation, as Eq. (4).…”

Effect of fracture tunneling in DWT Tests

“…This is completely missing in the r à evaluation, whose value is essential for the critical conditions assessed during propagation in the DWTT (in the present case the critical CTOA). As a matter of fact, the r à enters directly in the energy models used to compute the CTOA [8,12]. In a pipe, subjected to crack propagation, the nominal stress ahead the crack tip keeps almost stationary so that the tunneling effect is not so critical and the exact crack tip positioning is poorly influent.…”

“…The output of these tests are the measurement of the energy lost during the steady state propagation of the crack. Through energy computation, performed with the application of a two ligament method [6,7] or a single test method [8,9], the ductile characteristics of the steel can be deduced. The tests are conducted in full thickness on a flattened specimen extracted from the pipe, so that the results are not suitable to characterize the material, but the pipeline, instead.…”