Use of miniature specimens for creep-crack-growth testing

Measuring the condition of high-temperature components after many years of service is an important part of remaining life assessment. Tests of miniatu re specimens have been utilized for this purpose because they require that only small samples be removed from the components, which minimizes or elimi nates the need for costly repairs. Miniature-specimen techniques have been developed and validated for creep-rupture testing but not for creep-crack-g rowth testing. The objective of this research was to develop and validate a procedure for measuring creep-crack-growth behavior using miniature specim ens. Based on a past study of Type 316 stainless steel, the single-edge-not ch-tension (SENT) specimen configuration was chosen for the current work. S ENT specimens were designed and prepared from both base metal and weldments of 2-1/4Cr-1Mo and 1-1/4Cr-1/2Mo steels. The base metal was ASME SA 387, G rade P22 and Grade P11 plate, respectively. The 2-1/4Cr-1Mo steel weldment was a seam-weld hot reheat steam pipe that had been in service for 106,000 hours, while the 1-1/4Cr-1/2Mo steel weldment was from a pipe that had been weld repaired after 244,200 hours of service. SENT specimens with test sec tions of 10 x 5 x 1.5 mm, 15 x 7.5 x 1.5 mm, and 20 x 10 x 1.5 mm were eval uated. Tests were conducted under constant weight loading or constant load- line displacement loading. Specimens were heated to 538 degreesC using a st andard laboratory furnace. Crack length was measured using the DC electric potential drop method. The test results were analyzed to produce creep-crac k-growth rate (da/dt) as a function of the C* integral and compared with da ta from tests of half-size (IRT-CT) and standard full-size (IT-CT) compact tension specimens. For both base metals, the 1/2T-CT and three sizes of SEN T specimens gave results that agreed well with those of 1T-CT specimens. Fo r the 2-1/4Cr-1Mo steel weldments, SENT specimens produced good results. Ho wever, SENT specimens of the 1-1/4Cr-1/2Mo steel weldments exhibited cracki ng blunting and no creep crack growth. The reasons for the differences in b ehavior are discussed and guidance for application of miniature specimens t o creep-crack-growth testing is presented. [S0094-4289(00)01403-1].