Stress Corrosion

Dear Zhigang,

Thanks for posting an important and interesting topic. One potential problem is that stress-corrosion cracking can actually be overlooked in fracture mechanics testing. This can happen for specimen configurations (e.g., the single edge-notched beam) for which the energy release rate increases with crack length. Then, once the crack growth initiates, the energy release rate will increase leading to fast fracture. The experimentalist will have little chance of noticing that the material possesses stress corrosion cracking and he may end up simply characterise the materials fracture properties in terms of a (fast fracture) fracture energy (the value of the energy release rate  G  corresponding to fast fracture). With such test methods is thus difficult to notice and possibly impossible to characterise subcritical crack growth. Furthermore, since crack growth from a machined notch often requires a higher load than for propagation of a truly sharp crack tip, the fracture energy is likely to be overestimated.

An illustrative example to show the importance of recognising subcritical crack growth: In an earlier paper on characterisation of sub-critical crack growth in a ceramic material for a SOFC (solid oxide fuel cells) component (Kumar, A. N., and Sørensen, B. F., 2002, "Fracture energy and stable crack growth in surface treated Yttria stabilised Zirconia for SOFC applications", Materials Science and Engineering A333, 380-9), we exemplified this by designing a SOFC multilayer component against tunneling cracking due to residual stresses induced by thermal expansion mismatch. Using a value of  G  corresponding to crack arrest, the multilayer could be designed against tunnelling cracking. If the multilayer had been designed using  Gc  obtained from fast fracture, the multilayer specimen would have cracked across its entire width by subcritical crack growth in less than one hour.

This point illustrates the importance of recognising stress corrosion cracking both in the characterisation and modelling of brittle materials. 

The key to study subcritical crack growth is thus to use test specimen configurations that enables crack arrest. That is specimens for which the energy release rate decreases with crack length. Examples are DCB specimens loaded by the use of fixed wedges (Wiederhorns approach) or pure moments, or double torsion specimens.