The crack tip opening displacement (CTOD) test is, like the tensile and notched bar impact test, a mechanical material test from the field of fracture mechanics. The CTOD test determines the resistance of a material to crack growth.
What CTOD standards are there?
- BS 7448 (Part 1-4): Fracture mechanical toughness tests
- ISO 12135: Metallic materials – Uniform test method for the determination of quasi-static fracture toughness
- ASTM E 1820 Standard test method for the measurement of fracture toughness
- WES 1108 Standard test method for the measurement of fracture toughness (CTOD)
- BS7910: Guide to methods for assessing the acceptability of defects in metallic structures
- ISO 15653 Metallic materials – Test methods for the determination of the quasi-static fracture toughness of welds
A CTOD specimen has similarities to a notched bar impact specimen. However, the specimen generally has the full thickness of the material, with the width being twice the thickness. A defined fatigue crack is introduced at the round notch base by means of prestressing. The specimen is then cooled to test temperature and then clamped and loaded in a three-point bending machine. The relationship between the crack tip expansion and the load is recorded. The CTOD specimen is then fractured and the crack surface and fracture pattern are examined.
Results are varied
The results of the CTOD are versatile. On the one hand, a load-crack opening displacement diagram is recorded and the type of curve is determined. On the other hand, the fracture pattern is examined macroscopically, whether the fatigue crack has a reasonably straight front and how long it is. Common target values are crack tip open displacement value min 0.25 mm and for welds (mostly in stress relieved condition) CTOD value min 0.2mm.
The crack tip open displacement test is not a standard in the mechanical material tests of structural steels and pressure vessel steels. For offshore steels such as S355G10+N/M (1.8813+N/M), S460G2+M/QT (1.8857+M/QT) or also S355NLO (1.8808+N), S355MLO (1.8808+M) but also S460MLO (1.8878) S460QLO (1. 8667), it is often used as an additional ordering option for EN 10225 (Weldable structural steels for fixed offshore structures) or NORSOK M-120 Material data sheets for structural steel. CTOD has also become more important for high-strength shipbuilding steels in recent years. In the meantime, there are steel grades which, as BCA (brittle crack arrestability) steels, require the crack tip opening displacement test on the base material and on the weld seams, and this mechanical material test is part of the manufacturers’ approval programme.
What are the areas of application for CTOD steels?
Possible applications include off-shore structures such as drilling rigs, production platforms, onshore and offshore wind farms, ships and assemblies for deep-sea piping systems.
EN 10225 Weldable structural steels for fixed offshore structures
The Crack Tip Opening Displacement (wmtr.com)
Crack Tip Opening Displacement -Tests – TWI (twi-global.com)
Crack Tip Opening Displacement Test – JFE Techno-Research Corporation (jfe-tec.co.jp)
Crack tip opening displacement (CTOD) testing – www.materialwelding.com
M-120 Material data sheets for structural steel