Measurement of CTOA of Pipe Steels Using MDCB and DWTT Specimens

Abstract

Two types of specimen for crack tip opening angle (CTOA) measurement have been investigated for pipeline applications, i.e., the modified double cantilever beam (MDCB) (at NIST) and the drop-weight tear test (DWTT) specimen (at CANMET). Results of effects of specimen types, thicknesses and loading rates on CTOA are summarized and discussed. The main observations include: (i) For both MDCB and DWTT specimens tested at quasi-static loading rate, crack front tunnelling (i.e., with a deep triangular crack-tip shape) was present in high-strength steels; (ii) For DWTT specimens, CTOA values measured optically at the surface were significantly higher than those from the simplified single-specimen method (S-SSM) and those measured at mid-thickness [on sections cut using electric discharge machining (EDM)]; and (iii) CTOA values from surface measurement of MDCB specimens were comparable to those derived from S-SSM of DWTT specimens, but the surface values of DWTT were higher than those of MDCB specimens.

Crack Tunnelling and Crack Tip Opening Angle in Drop-Weight Tear Test Specimens

“…flat or slant). Note that DWTT specimens may show a transition from flat to slant fracture in quasi-static tests with decrease in thickness in the gauge section (Xu et al, 2010); furthermore, interior CTOA values measured at the midthickness were lower than those measured at the surface. The interior CTOAs at the midthickness were comparable to those estimated from the S-SSM.…”

“…For a propagating crack, the CTOA has been proposed as a fracture propagation parameter for aerospace sheet and pipeline steel, and has gained special attention for use in high-strength steel pipelines (Horsley, 2003). To characterize CTOA of pipe steels using laboratory-scale specimens, the drop-weight tear test (DWTT) has been suggested as a most suitable and practical mill test (Xu and Tyson, 2008;Xu et al, 2010). In tandem with DWTT, a simplified single-specimen method (S-SSM) has been developed at the authors' laboratory to derive CTOA from load (P) vs. load-line displacement ( ) obtained from an instrumented test (Xu et al, 2007).…”

“…The assistance of Mr. D. Ashe in performing digital image photography and analysis for quasi-static tests is gratefully acknowledged. Discussions and collaboration with NIST (Boulder, CO), as reported by Xu et al (2010), have been helpful in focusing this work.…”

A new data analysis technique to examine crack tip opening angle values tested in API X65 pipe steel

“…The first author has been using the latter test technique to experimentally measure the CTOA fracture parameter over the past decade. The test method has been successfully used to measure the stable CTOA levels of different API (X65, X70, X80 and X100) steels by the first author and many other researchers . However, a limiting issue in this test approach is the high number of test data (photographs) that should be accurately monitored and subsequently analysed for precise CTOA estimation.…”

Effect of fracture tunneling in DWT Tests

“…With a particular emphasis on ductile fracture propagation, crack tunneling can be evidenced [14], interesting an extension that is comparable or even greater than the specimen thickness itself.…”