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Stress Corrosion

Zhigang Suo's picture

A glass may withstand a static load for a long time (days, weeks, or years) and then, without warning, breaks suddenly. Here are salient empirical observations:

  • The delay time depends on the magnitude of the load: The smaller the load, the longer the delay time.
  • The phenomenon is environment-sensitive. Glass suffers delayed fracture in moisture, but not in vacuum. The lower the humidity, the longer the delay time.
  • The phenomenon is thermally-activated. The lower the temperature, the longer the delay time.

The phenomenon occurs to all materials to some degree in some environments. The phenomenon is known variously as

  • Static fatigue
  • Delayed fracture
  • Moisture-assisted cracking
  • Environment-sensitive fracture
  • Thermally-assisted cracking
  • Subcritical slow crack growth
  • Stress-corrosion cracking

This lecture describes a basic picture of stress corrosion due to Orowan (1944), a fracture-mechanics approach due to Wiederhorn (1966), and recent applications in thin-film structures.

These notes belong to a course on fracture mechanics

PDF icon Stress corrosion 2010 03 04.pdf339.34 KB


Bent F. Sørensen's picture

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.  

Zhigang Suo's picture

Dear Bent:  Thank you so much for describing your experience with fracture mechanics.  Posted here are notes I wrote for a lecture.  I am teaching a graduate course on fracture mechanics this semester, and have been trying to post notes for all lectures as we go.  The notes are not self-contained.  Most figures are missing.  I hope to come back to them to imrpove and update when I teach the course next time.  Your comments willl be valuable to me and to the students in the class.

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