Engineering Aspects of Crack-Tip Opening Displacement Fracture Toughness Testing
Authors: Wellman GW , Rolfe ST 
Published: December 2008

An investigation of the crack-tip opening displacement (CTOD) fracture behavior of four pressure vessel steels and one ship steel has been conducted. The purpose of the investigation was to determine the fracture behavior of the steels, to study the effects of CTOD testing variables, to examine various methods of evaluating test records, and to study the relationships among several fracture toughness parameters. The five steels studied, A508, A533, A516, A517, and A131, (Unified Numbering System [UNS] K12766, K12539, K02700, K11856, and K02102) had yield strengths from 240 to 750 MPa. In addition, three specimen sizes of the A533 and A516 steels were studied.

The CTOD fracture parameter provides a relatively simple method of extending fracture mechanics concepts from plane-strain linear elastic behavior to elastic-plastic behavior. Plane-strain linear elastic fracture toughness values (KIc as defined in ASTM Test Method for Plane-Strain Fracture Toughness of Metallic Materials E 399) can be obtained only at extremely low temperatures and large specimen sizes for the steels investigated. JIc (as defined in ASTM Test Method for JIc, a Measure of Fracture Toughness E 813) toughness values are currently applicable only to stable, fibrous tearing behavior. Thus, there is a large temperature region between the KIc and JIc toughness parameters where neither J nor K test results will meet their respective JIc or KIc test specifications. The CTOD fracture toughness parameter covers this temperature region between KIc and JIc as well as covering both the regions where KIc and JIc are valid. Thus the CTOD test method can be used throughout the entire service temperature range for pressure vessel and structural steel applications.

For the low to moderate strength steels tested, the macroscopic failure surface appearance changed from all brittle to a fibrous thumbnail initiation at a relatively consistent CTOD value. The change in appearance, which takes place between the lower and upper shelf, must occur at a much lower CTOD value for the high strength A517 steel. Correlations among Charpy V-notch (CVN), CTOD, J, and Kc values are presented. Generally it was not possible to obtain linear elastic plane-strain critical stress intensity factor KIc values for the specimen sizes studied, hence the use of Kc values. The results of the correlations increased with increasing temperature as expected.

Using the preferred specimen geometry of thickness t by depth 2t, there is a definite size effect in CTOD testing. On the upper shelf (fibrous tearing failure) a smaller specimen will give a smaller apparent toughness. In the lower transition region (brittle failure) the size effect is less clear. A smaller specimen (1/2t by t) will give a larger apparent toughness using average CTOD values, but there is no apparent size effect when the lowest CTOD test values are compared. Until this size effect is clarified, the British Standard requirement that the test specimen be the full thickness of the structure of interest is appropriate.

An extensive CTOD study of five structural steels has shown that this test method can be used to predict the elastic-plastic fracture toughness over the entire test temperature range, and that the results can be correlated with other fracture toughness test results such as CVN, J, and K.