Local brittle zones are discrete microstructural regions in a weld heat affected zone (HAZ) that exhibit lower resistance to fracture initiation than surrounding material. See What are LBZs? for further discussion of this topic.
LBZs can result in fracture initiation under near linear elastic conditions during fracture toughness testing. For example, in terms of crack tip opening displacement (CTOD), fracture toughness can be of the order of 0.01mm in the LBZ when adjacent material can have a toughness of greater than 0.1mm at the same temperature. During testing, LBZs can result in specimen fracture or an arrested brittle fracture (pop-in). Consequently, LBZ behaviour is a warning of potentially low resistance to brittle fracture initiation in the structural component being assessed. However, LBZ behaviour does not necessarily mean that the structural component is at high risk of failure by brittle fracture. Where LBZ behaviour is observed, the results need to be carefully considered along with factors described below.
LBZs are not normally apparent during routine Charpy impact testing. (See What are LBZs?). However, low Charpy toughness can be due to presence of LBZs when they are favourably positioned within the specimen and the test temperature is appropriate.
LBZs can also result in fracture or pop-in with test specimen geometries other than the conventional, deeply notched three-point bend specimens used in CTOD testing (see BS 7448, ASTM E1820) such as: shallow notched bend specimens; single edge notch tension specimens; or structurally representative tests such as wide plate tension tests. However, because of reduced crack tip stress triaxiality or constraint in these geometries, LBZ behaviour may be less apparent and fracture toughness values may be higher than in conventional, deeply notched bend specimens. Consequently, when assessing the structural significance of LBZs, consideration needs to be given to differences in crack tip constraint between the test specimen and the crack in the structural component of interest. To assess these differences, it will be necessary to either:
- determine fracture toughness using a lower constraint test geometry (but ensuring at the same time that the microstructure giving rise to the LBZ is tested); or,
- conduct the fracture mechanics assessment allowing for the reduced constraint condition in the structure (e.g. using a constraint modified FAD [failure assessment diagram] according to BS 7910).
Both theoretical and experimental work shows that LBZ behaviour is influenced by the length of LBZ present along the crack tip front. Consequently, in addition to constraint effects, the structural significance of the LBZ will depend on the likelihood of a crack tip intersecting the LBZ and the length of LBZ present. Although LBZ behaviour produces low initiation (usually cleavage) fracture toughness, the arrest toughness of other regions of the HAZ could be significantly higher. This is because the HAZ is a composite of different microstructures, some of which have high toughness. Thus, brittle fracture initiation from a LBZ will be followed by crack advance along a front intersecting both LBZ and high toughness regions. The overall toughness of this 'composite' microstructure may be sufficiently high to arrest the brittle fracture. This has been observed in crack arrest tests conducted on HAZs that contain LBZs, and it is part of the explanation for the arrest of pop-ins observed in both conventional fracture mechanics tests (e.g. CTOD) and wide plate tests.
Finally, if the material adjacent to and ahead of the LBZ can be shown to have sufficient fracture toughness to arrest a brittle fracture initiating from the LBZ, the risk of brittle fracture in the structure will be eliminated. However, this will necessitate the determination of the arrest fracture toughness of the surrounding material. (A procedure for assessing the capability of the structure to arrest a fracture that has initiated from a LBZ is described in chapter III, Section III.12 of the British Energy R6 document). Furthermore, the significance of an arrested brittle fracture on other modes of failure, for example, fatigue crack growth, needs to be evaluated.
References
- ASTM E1820 Standard test method for measurement of fracture toughness.
- BS 7448-1: Fracture mechanics toughness tests. Part 1. Method for determination of KIc, critical CTOD and critical J values of metallic materials.
- BS 7910: Guide to methods for assessing the acceptability of flaws in metallic structures.
- R6 - 'Assessment of the Integrity of Structures containing Defects', British Energy.