Assessing the use of Circumferentially Notched Tension specimen for fracture toughness determination of ductile offshore steels

Assessing the use of Circumferentially Notched Tension specimen for fracture toughness determination of ductile offshore steels

Slachter, E.G.

Popovich, V.A. (mentor)
Riemslag, A.C. (mentor)
Hu, B. (mentor)

Mechanical, Maritime and Materials Engineering

Materials Science and Engineering (MSE)

2016-06-01

The Circumferentially Notched Tension (CNT) specimen is a potential candidate for determining the fracture toughness of highly constrained cracks, theoretically approaching plane strain conditions, even for small test specimen dimensions. This research aims to experimentally evaluate the use of the CNT test specimen geometry to fracture toughness test ductile steels. The process involves development of a fatigue precracking method designed to minimize eccentricity values, experimentally verifying a number of claims made in literature regarding the behaviour of the CNT specimen and investigating specimen dimensioning. All with the purpose to move toward implementing the CNT specimen as an established fracture toughness test geometry. Initiating a fatigue precrack by compressive fatiguing followed by tensile fatiguing to the required fatigue crack length proved to lead to ligament eccentricity values larger than those encountered in literature created using rotating bending. The prefatiguing method also proved sensitive to the material type. The resulting eccentricity values had no significant effect on the result of fracture toughness tests on S690QT steel. It is believed that the uncracked ligament realigns with the loading line without causing excessive damage on this ductile steel type. The bending distortion is thereby effectively cancelled. SEM photography on a specimen with a commonly encountered eccentricity magnitude of 0.2 mm showed that fracture is still initiated at the position of the longest fatigue crack length despite the relatively low eccentricity value. Comparing the results of fracture toughness tests on CNT specimen under two stress intensification rates of 3 MPa√m/s and 571 MPa√m/s showed an insensitivity of the CNT specimen to varying crosshead speeds. The conclusion is that the combined machine and specimen stiffness increase the displacement of the crack area during crack growth beyond the speed at which the crosshead moves. It is suggested that larger CNT specimens are required to approximate plane strain conditions for the S690QT steel tested. Experimental results of fracture toughness tests and critical stretch zone widths using SEM on 10 mm major diameter CNT specimens were comparable to SENB specimens with a width of 10 mm. These SENB specimen are known to be undersized and do not provide a plane strain fracture toughness. It is thought that areas of increased stress emerge at the crack tips and converge at the centreline of the CNT specimen. Literature source identified the linking up of these so called stress bands decreased crack tip constraint. The linking up of stress bands at experimental loads was confirmed to take place by 2-dimensional FEM simulations. An addition to the decrease in crack tip constraint may be the presence of a bending load. Further research is required to quantify the effect of bending on crack tip constraint levels. Critical stress intensity values obtained experimentally at the point of 2% crack extension proved to be consistent with little spread. This proves the potential of the CNT geometry to provide consistent fracture toughness values despite the presence of eccentric ligaments. More research is recommended in order to introduce the CNT specimen as a robust and efficient fracture toughness specimen.

circumferentially
notched
tension
CNT
fracture
toughness
steel
fatiguing

http://resolver.tudelft.nl/uuid:a610007f-e94e-401b-885c-4b7c29f50ca7

2017-06-01

Student theses

master thesis

(c) Slachter, E.G.