UTILISING SHARP CRACKS TO DETERMINE FRACTURE TOUGHNESS
As more complex welding joints are used for heavy steel plates in large boilers, pressure vessels, and ship hulls and decking, the crack tip opening displacement (CTOD) test is used to determine a material’s fracture toughness in the presence of a crack. Fracture toughness is the critical stress intensity factor of a sharp crack where propagation of the crack suddenly becomes rapid and unlimited.
CTOD testing measures the resistance of a material to the presence of a flaw in terms of the load required to cause brittle or ductile crack extension (or to reach a maximum load condition) in a standard specimen containing a fatigue pre-crack. The result is expressed in toughness parameters such as KIc, J-integral, or CTOD.
Here at PTS, we have advanced testing equipment to measure the material’s fracture toughness for two primary types of configuration: Single-Edge Notched Bend (SENB) and Compact Tension (CT).
Our Fracture Toughness Testing Capabilities
CTOD Testing
The Crack Tip Opening Displacement Test, or CTOD Test, measures the resistance of a material to the propagation of a crack. CTOD is used on materials showing some plastic deformation before failure occurs. Accurate measurement of this displacement is essential to the test.
Single-Edge Notched Bend (SENB) or Single-Edge Notched Tension (SENT) are the most common specimen geometries used. It is often tested in accordance with BS ISO 12135, BS EN ISO 15653, and BS 7448.
J-Integral TESTING
The ASTM E1820 specification provides a standard method for determining all applicable toughness parameters from a single test. The J integral is a popular method of determining a material’s toughness by measuring the energy required to propagate a crack. Edge Notched Bend (SENB) and Compact Tension (CT) tests are commonly used in J-integral testing per ASTM E1820.
KIC TESTING
Linear-Elastic Plane-Strain Fracture Toughness KIC of Metallic Materials is often tested according to ASTM E 399 specifications. The KIC test is used to determine the brittle fracture toughness of metallic material.
It measures the resistance of a material to crack extension under low toughness conditions when there is little to no plastic deformation occurring at the crack tip.
Compact tension (CT) and Single-Edge Notched Bend (SENB) are the most common specimen geometries. Due to its economical use of material, the CT specimen is the more commonly employed testing method.
FATIGUE CRACK GROWTH RATE TESTING
Fatigue Crack Growth Rate (da/dN) Testing is used to determine the crack propagation rate once it is initiated under specified loading conditions.
PTS uses advanced crack-measuring techniques known as Direct Current Potential Drop (DCPD) to measure the change in resistance of a specimen, which correlates directly with crack growth. The graph of Cyclic Stress is plotted against the Crack Growth Rate, with stress intensity being the controlling variable.
Case study:
Client Introduction
The client specialises in the fabrication of offshore structures and had previously partnered with PTS for material testing.
pROBLEM
The client purchased an 80mm steel plate that did not comply with the API Specification 2W additional requirement SR 9 for pre-production qualification.
Solution
PTS provided the client with a one-stop solution for pre-qualification, including welding inspection and CTOD testing.
Review
PTS gives professional advice during the welding inspections and CTOD testing, willing to share technical information to improve the client’s understanding of CTOD testing.
Applications of a CTOD test
As determining fracture toughness is the key CTOD test purpose, this test benefits a wide range of industries and components:
Offshore structures
Pressure vessels & boilers
Shipbuilding
Structural steel
Pressure vessels & boilers
Testing typically uses the single specimen method with fatigue pre-cracking to simulate a sharp, natural crack.
Talk to Us Today
Our experts have decades of experience and a track record of successful applications. Our advanced equipment control, processes, and data acquisition methods enable us to guide you through even the most complex test results.
As a comprehensive CTOD testing laboratory Singapore clients trust, PTS can provide you with the best material testing programs tailored to your material, specification, and application requirements. For more information on our Fracture Toughness test, call us or send us an enquiry today.
Frequently Asked Questions
1. Why is CTOD testing required?
Why a CTOD test is required comes down to four main reasons:
- Welding procedure qualification – to verify that welded joints can tolerate crack-like defects without experiencing brittle fracture.
- Code and standard compliance – many international standards and codes (such as API 2W, BS 7448, ISO 12135, and EEMUA 158) mandate CTOD testing for critical applications.
- Fitness-for-service assessments – CTOD data supports engineering critical assessments (ECA) when defects are identified in service.
- Material selection and design assurance – to ensure sufficient fracture toughness for low-temperature, high-stress, or safety-critical service conditions.
2. What is the purpose of CTOD testing in welding?
The purpose of crack tip opening displacement test in welding is to confirm that weld metal and heat-affected zones (HAZ) possess fracture toughness. This is particularly important for thick-section welds used in offshore structures, pressure vessels, and pipelines, where residual stresses, metallurgical changes in the HAZ properties, and mismatches between weld metal and base metal can significantly influence crack initiation and propagation. CTOD testing provides assurance that welded joints can safely resist brittle fracture under service conditions.
3. What are the acceptance criteria for CTOD testing?
Crack tip opening displacement acceptance criteria are project-specific and there is no universal pass/fail value. The required CTOD value depends on the applicable governing code or standard (such as API RP 2Z, EEMUA 158, EN 10225), classification society rules (DNV, ABS, Lloyd’s Register), or client specifications. Typical minimum CTOD requirements commonly range from 0.15 mm to 0.25 mm at the specified test temperature, often -10°C or lower for offshore applications. Acceptance criteria should always be confirmed before testing.
