Failure of metals I : brittle and ductile fracture

https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1048&context=usnavyresearch

Review of fracture toughness (G, K, J, CTOD, CTOA) testing and standardization

The scatter in KIC-results

The tensile deformation and fracture behaviour of the aluminium alloy 6061 reinforced with SiC has been investigated. In the T4 temper plastic deformation occurs throughout the gauge length and the extent of SiC particle cracking increases with increasing strain. In the T6 temper strain becomes localised and particle cracking is more concentrated close to the fracture. The elastic modulus decreases with increasing particle damage and this allows a damage parameter to be identified. The fraction of SiC particles which fracture is less than 5%, and over most of the strain range the damage controlling the tensile ductility can be recovered, indicating that other factors, in addition to particle cracking are important in influencing tensile ductility. It is suggested that macroscopic fracture is initiated by the SiC particle clusters that are present in these composites as a result of the processing. The matrix within the clusters is subjected to high levels of triaxial stress due to elastic misfit and the constraints exerted on the matrix by the surrounding particles. Final fracture is then produced by crack propagation through the matrix between the clusters.

Aspects of fracture in particulate reinforced metal matrix composites

Effect of microstructural refinement on the toughness of low carbon martensitic steel

Factors affecting the cleavage fracture stress and the fracture toughness K 1C in steel have been examined. A statistical method based on carbide induced cleavage fracture is proposed, by which it is possible to estimate the fracture toughness and study the effect of different variables. The predictions made are in excellent agreement with experimental results for a variety of microstructures.

Statistical model for carbide induced brittle fracture in steel

Prerequisites for precipitate cracking in a yielding ductile matrix have been examined. A statistical model based on the fibre loading model combined with weakest link fracture theory is presented. With the model it is possible to estimate the effect of different variables on the particle fracture probability quantitatively. The predictions made are in excellent agreement with experimental results for a variety of different precipitate types. The result can be applied to calculate the probability of cleavage fracture for steels.

Fracture of brittle particles in a ductile matrix

On the mechanisms of environment sensitive cyclic crack growth of nuclear reactor pressure vessel steels

The microstructural basis of the critical tensile stress criterion of cleavage fracture is discussed. It is shown that cleavage can be carbide induced in bainitic steels as well as in ferritic steels. The plastic work expended in brittle crack propagation is shown to be somewhat smaller for bainitic steels than for ferritic steels. This is proposed to be caused by the much higher dislocation density of the bainitic microstructure. The statistical nature of the cleavage fracture initiation process is shown to result in a test piece geometry dependent cleavage fracture stress.

Kari Törrönen

Papers

Statistical model for carbide induced brittle fracture in steel

Fracture of brittle particles in a ductile matrix

On the mechanisms of environment sensitive cyclic crack growth of nuclear reactor pressure vessel steels

On the Microstructural Basis of Cleavage Fracture Initiation in Ferritic and Bainitic Steels

Environment sensitive cracking in pressure boundary materials of light water reactors