Determining the suitability and application of CNT test specimens for failure assesment of welded high strength steels

Determining the suitability and application of CNT test specimens for failure assesment of welded high strength steels: Are Circumferentially Notched Tensile test specimens suitable and applicable for failure assessment of (welded) S690QT steel?

Opraus, Thomas (TU Delft Mechanical, Maritime and Materials Engineering)

Riemslag, Ton (graduation committee)
Popovich, Vera (mentor)

Delft University of Technology

2017-06-27

Research has shown the Circumferentially Notched Tensile (CNT) Specimen to potentially be a size-independent fracture toughness specimen. This research focusses on expanding the practical applicability of the CNT specimen as well as using it to perform fracture toughness tests on S690QT steel and simulated CGHAZ material in S690.
Based on literature research the research goals of the research were to investigate different pre-cracking methods with the focus of creating cracks with little or no eccentricity, improve the accuracy of CTOD measurements, determine the equivalency between CNT and conventional fracture toughness, determine the size of the plastic zone at the crack tip specimen and find alternative methods of microstructure simulation.
In order to improve pre-cracking rotational bending (RB) and standalone compression – compression (C-C) fatigue were used, both lead to low eccentricity cracks, where the length of the crack could be influenced by changing the process parameters or specimen geometry. Room temperature CTOD measurements were performed on both, where RB specimen showed an average CTOD of 0.12 mm and C-C specimen of 0.18 mm. This difference is believed to be caused by the unique stress free situation at the crack tip of the C-C specimen.
By using digital image correlation (DIC) and optical microscopy, the most accurate approximation of the CTOD was determined by comparing extensometer data to optical measurements. Influence of the eccentricity of the ligament was on the fracture toughness was found to increase for brittle material. Equivalency between CNT and conventional specimen was determined by comparing both RB and C-C specimen’s CTOD at low temperature (-100 °C). C-C specimen were found to show comparable fracture toughness, while RB showed an increased value.
The size of the plastic zone was measured using EBSD, measurements on annealed S690 and low carbon steel failed to show distinctive plasticity, possibly caused by insufficient plastic strain.
Inductive heating was tested as a replacement for Gleeble heat treatment with promising results, however the Gleeble was used for producing all S690 CGHAZ samples. C-C was found to be the only viable pre-fatiguing method, creating average crack length extensions of 0.49mm and an eccentricity of 0.18 mm. CNT results showed a slightly lower room temperature CTOD compared to conventional specimen (0.12 mm vs 0.08 mm), low temperature results showed comparable CTOD values.
More research is recommended in further development of the new standalone C-C pre-fatiguing method, improving the inductive heating and cooling setup, expanding the DIC test setup, determining the plastic zone size by performing more EBSD measurements or by performing nanoindentation measurements and using CNT specimen on very brittle materials.

Fracture mechanics
Materials science
CNT specimen

http://resolver.tudelft.nl/uuid:03435526-a66f-478f-b177-eaef0938e108

Student theses

master thesis

© 2017 Thomas Opraus