CTOA Results for X65 and X100 Pipeline Steels: Influence of Displacement Rate

Abstract

The toughness and plasticity of steel generally decreases with increasing testing rate. The crack tip opening angle (CTOA) was measured on two types of commercial pipeline steels API-X65 and API-X100, at a range of displacement rates to characterize rate effects. The testing was conducted at quasi-static and dynamic rates. The crosshead displacements in our test matrix ranged from 0.002 mm/s in the quasi-static mode to approximately 8000 mm/s in the dynamic mode. The quasi-static tests were conducted in a servo-hydraulic uniaxial test machine using Modified Double Cantilever Beam (MDCB) specimens. The dynamic experiments were made on a similar servo-hydraulic uniaxial test machine using the same type of specimen and with the addition of a disc spring setup for the fastest rate. The results of these tests indicate that the rate effect has negligible influence on the CTOA values measured for these materials for the reported rates. The CTOA values measured for the two materials show a small but convincing difference. The resistance to fracture was found to be higher for the X65 steel, as indicated by a higher CTOA and lower crack growth velocity. This paper presents results on the influence of displacement rates from quasi-static to dynamic for the X65 and X100 grade pipeline steels, and discusses a method for optimizing the reduction of the CTOA data.

CTOA Measurements of Welds in X100 Pipeline Steel

“…If, for example, we compare values for the 8 mm thick specimens, we see that the mean value for CTOA from the girth-weld HAZ is 10.73°, whereas it is 6.96° from the seam-weld HAZ. This value is much lower, and in fact less than the ~9° value for the same X100 base metal [12], as one would expect for the "weak link." The welding process is certainly different in the seam weld and girth weld, but other factors may be affecting these values.…”

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

“…[23] and 13.6 AE 1.0 in Ref. [8] for similar X65 steels (Table 3). However, the measured (quasi-static) CTOA values from this research are less than dynamic CTOA data (14.2 ) from Ref.…”

“…The issues of fracture initiation, propagation and arrest in high pressure gas transportation pipelines have been the subject of study for more than six decades . Different elastic and elastic–plastic fracture criteria, including stress intensity factor ( K ), energy release rate ( G ), crack tip opening displacement, crack tip opening angle (CTOA) and J‐integral, have been proposed to characterise the ductile fracture in these structures . Similarly, different fracture test specimens have been suggested and used in laboratory to measure such fracture criteria.…”