TWI Knowledge Summary

Fracture toughness testing

by Henryk Pisarski

Background
Standards
Test specimens
Instrumentation and loading
Fracture toughness parameters
References

Background

The resistance to fracture of a material is known as its fracture toughness. Fracture toughness generally depends on temperature, environment, loading rate, the composition of the material and its microstructure, together with geometric effects (constraint). ^[1] These factors are of particular importance for welded joints, where the metallurgical and geometric effects are complex ^[2,3]

Fracture toughness is a critical input parameter for fracture-mechanics based fitness-for-service assessments. Although fracture toughness can sometimes be obtained from the literature, or materials properties databases, it is preferable to determine this by experiment for the particular material and joint being assessed.

Various measures of 'toughness' exist, including the widely used but qualitative Charpy impact test. Although it is possible to correlate Charpy energy with fracture toughness, a large degree of uncertainty is associated with correlations because they are empirical. It is preferable to determine fracture toughness in a rigorous fashion, in terms of K (stress intensity factor), CTOD (crack tip opening displacement), or J (the J integral); see also What is a fracture toughness test? Standards exist for performing fracture mechanics tests, with the most common specimen configuration shown in Fig.1 (the single-edge notch bend SENB specimen). A sharp fatigue notch is inserted in the specimen, which is loaded to failure. The crack driving force is calculated for the failure condition, giving the fracture toughness.

Fig.1. Fracture mechanics testing

Standards

Various national Standards have been developed for fracture toughness testing:

The British Standard BS 7448 ^[4] includes four parts, for testing of metallic materials, including parent materials, weldments, high strain rates (dynamic fracture toughness testing, to be published in 2005) and resistance curves (R-curves for ductile tearing). BS 7448: Part 2 is the first Standard worldwide to apply specifically to weldments.
A series of American ASTM Standards cover K, CTOD, J testing (including R-curves), ASTM E1290 (CTOD testing) and ASTM E1820 (K, J & CTOD). None specifically address testing of welds. together with a summary of applicable terminology. ^[5-8]
A series of international (ISO) standards are being developed. ISO 12135 covers all aspects of fracture testing (K, J & CTOD) of plain material. Standards are being prepared on testing of welds (ISO/CD 15653) and stable crack growth in low constraint specimens (ISO/CD 15653). The latter is mainly concerned with testing thin, sheet material.
The European Structural Integrity Society (ESIS) has published procedures for R-curve and standard fracture toughness testing of metallic materials. ^[9-10] Currently, a draft unified testing procedure (ESIS P3-04), which includes weld testing, is being developed. (These are not standards in the usual sense, but rather testing protocols that have been agreed by experts).

Although different standards have historically been published for determining K, CTOD and J, the tests are very similar, and generally all three values can be established from one test. See Are there any differences between fracture toughness tests carried out to BS7448 and E1820?

Test specimens

The most widely used fracture toughness test configurations are the single edge notch bend (SENB or three-point bend), and the compact (CT) specimens, as shown in Fig.2. The compact specimen has the advantage that it requires less material, but is more expensive to machine and more complex to test compared with the SENB specimen. Also, special requirements are needed for temperature control (e.g. use of an environmental chamber). SENB specimens are typically immersed in a bath for low temperature tests. Although the compact specimen is loaded in tension, the crack tip conditions are predominantly bending (high constraint). If limited material is available, it is possible to fabricate SENB specimens by welding extension pieces (for the loading arms) to the material sample. (Electron beam welding is typically used, because the weld is narrow and causes little distortion).

Fig.2. Examples of common fracture toughness test specimen types

Other specimen configurations include centre-cracked tension (CCT) panels, single edge notch tension (SENT) specimens, and shallow-crack tests. These specialised tests are associated with lower levels of constraint, and can be more structurally representative than standard SENB or CT specimens.

The position and orientation of the specimen is important. In particular, the location and orientation of the notch is critical, especially for welded joints. Typically, the notch (fatigue pre-crack) is positioned such that a chosen microstructure is sampled. The orientation of the notch is defined with respect to either the weld axis for welded joints, or the rolling direction or forging axis for other components.

In standard SENB & C T specimens (see Fig.1), the notch depth is within the range 45-70% of the specimen width, W, giving a lower-bound estimate of fracture toughness, because of the high level of crack tip constraint generated by the specimen design.

A notch is machined into the fracture toughness specimen, following which a fatigue crack is grown by applying cyclic loading to the specimen. Specialised high frequency resonance or servo-hydraulic machines are often used for this process.

The fracture mechanics test standards include many checks to ensure that results are credible. These include restrictions on the fatigue crack size, position and shape, together with limitations on the maximum allowable fatigue force (this is to ensure that the crack-tip plastic zone produced during fatigue pre-cracking is small in comparison with the plastic zone produced during testing). Many of these checks can only be performed after testing.

Instrumentation and loading

During fracture toughness testing, the force applied to the specimen and specimen displacements and loading rate (using load cells and displacement transducers), together with the temperature are recorded.

One of the displacements is the crack-mouth opening. This is measured using a clip gauge either attached to knife edges mounted at the crack mouth (see Fig.1) or integral knife edges machined into the notch. These gauges comprise two cantilevered beams on which are positioned four strain gauges. By measuring the elastic strains and calibration it is possible to infer the crack-mouth opening.

Fracture toughness tests are performed in universal hydraulic test machines, generally using displacement control.

Fracture toughness parameters

The following are the fracture toughness parameters commonly obtained from testing

K (stress intensity factor) can be considered as a stress-based estimate of fracture toughness. It is derived from a function which depends on the applied force at failure. K depends on geometry (the flaw depth, together with a geometric function, which is given in test standards for each test specimen geometry).
CTOD or (crack-tip opening displacement) can be considered as a strain-based estimate of fracture toughness. However, it can be separated into elastic and plastic components. The elastic part of CTOD is derived from the stress intensity factor, K. In some standards, the plastic component of CTOD is obtained by assuming that the specimen rotates about a plastic hinge. The plastic component is derived from the crack mouth opening displacement (measured using a clip gauge). The position of the plastic hinge (defined by r _p ) is given in test standards for each specimen type. Alternative methods exist for estimating CTOD, which make no assumption regarding the position of the plastic hinge. These require the determination of J from which CTOD is derived. ^[6,7] CTOD values determined from formulations assuming a plastic hinge ^[4] may differ from those determined from J. ^[6,7]
J (the J-integral) is an energy-based estimate of fracture toughness. It can be separated into elastic and plastic components. As with CTOD, the elastic component is based on K, while the plastic component is derived from the plastic area under the force-displacement curve.

References

Items 1-3 are not in the public domain but are available to TWI Industrial Member companies.

M G Dawes: 'An Introduction to K, CTOD and J Fracture Mechanics Analyses and Toughness, and the Application of these to Metal Structures'.
H G Pisarski: 'Update on Fracture Toughness Test Methods for Welded Joints'.
H G Pisarski: 'A Review of HAZ Toughness Evaluation'.
BS7448: 'Fracture Mechanics Toughness Tests'
- Part 1:1991: 'Method for determination of Kic, Critical CTOD and Critical J values of metallic material'.
- Part 2: 1997: 'Method for Determination of Kic, Critical CTOD and Critical J Values of Welds in Metallic Materials'.
- Part 3: 'Method for determining dynamic fracture toughness' (to be published in 2005)
- Part 4 1997: 'Method for Determination of Fracture Resistance Curve and Initiation values for Stable Crack Extension in Metallic Materials'. British Standards Institute, London.
ASTM E399-90: 'Standard Test Method for Plane Strain Fracture Toughness of Metallic Materials'. American Society of Testing and Materials, Philadelphia, 1990.
ASTM E1290-02: 'Standard Test Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness Measurement'. American Society of Testing and Materials, Philadelphia, 2003
ASTM E1820-05: 'Standard Test Method for Measurement for Fracture Toughness'. American Society of Testing and Materials, Philadelphia, 2005.
ASTM E1823-96: 'Technology Relating to Fatigue and Fracture Testing'. American Society for Testing and Materials, Philadelphia, 1996.
ESIS P1-92: 'ESIS Recommendation for Determining the Fracture Resistance of Ductile Materials' European Structural Integrity Society, 1992.
ESIS P2-92: 'ESIS Procedure for Determining the Fracture Behaviour of Materials'. European Structural Integrity Society, 1992.