CTOD test: Crack resistance of steel in the endurance test

The CTOD test is an important test standard for steel. It simulates the formation of cracks and helps to determine the material’s crack resistance. Read here how the test works and what advantages it offers.

CTOD – Crack Tip Open Displacement Test

The crack tip opening displacement test (CTOD test), also known as crack tip widening test, is a mechanical material test from the field of fracture mechanics. Similar to the tensile and notched bar impact test, it is used to characterize the properties of a material under load. The CTOD test aims to determine the resistance of a material to crack growth by measuring the crack tip opening deformation. This test provides important information about the toughness and breaking strength of the material.

What CTOD standards are there?

• BS 7448: (Part 1-4): Fracture mechanics toughness tests
• ISO 12135: Metallic materials – Uniform test method for the determination of quasi-static fracture toughness
• ASTM E 1820: Standard test method for measuring fracture toughness
• WES 1108: Standard test method for the measurement of fracture toughness (CTOD)
• BS7910: Guide to methods for assessing the acceptance of defects in metallic structures
• ISO 15653: Metallic materials – Test method for the determination of quasi-static fracture toughness of welds

A CTOD specimen is similar to an impact test 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 broken and the crack surface and fracture pattern are examined.

Different results in the CTOD test

The results of the crack tip opening displacement test are manifold. First, a diagram of the load-crack opening displacement is created and the shape of the curve is analyzed. In addition, the fracture pattern is examined macroscopically to determine whether the fatigue crack has a straight front and how long it is. The usual target values are a CTOD value of at least 0.25 mm and a crack tip opening displacement value of at least 0.2 mm for weld seams (usually in the stress-relieved condition).

The CTOD is not a standard for mechanical material testing 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 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 increasingly important for high-strength shipbuilding steels in recent years. There are now 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 manufacturer’s approval program.

What are the areas of application for CTOD steels?

Possible applications include off-shore constructions such as drilling rigs, production platforms, onshore and offshore wind farms, ships and assemblies for deep-sea piping systems.

Sources:
Wikipedia English ASTM.org EN 10225 Weldable structural steels for fixed offshore structures NORSOK M-120 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