Stress intensity factor 3

[makeup]

Can anyone help to clarify an issue I have with the concept of stress intensity factor in fatigue analysis.

A crack will only grow when the value of 'K' reaches a critical value 'KIC'.

or

A material will fail completely if 'K' reaches 'KIC'

 

[metengr]

The second statement is correct, the material would fail once K reaches the critical stress intensity factor (KIc). The stress intensity (K) is a function of crack length, crack geometry and applied stress range. K is a dynamic value related to crack growth and applied stress, and once K reaches the critical stress intensity factor (KIc), the crack will propagate in an unstable manner under plane strain conditions.

 

[rb1957]

that's correct as far as it goes ...
Kic is the planein fracture toughness of a material, relevant to thick, heavy sections. thin sheet has a (much) higher toughness (called plane stress).

so the first statement is incorrect (cracks grow at stress intensities less than Kic; this is stable crack extension and occurs at stress intensities greater than "threshold")and the second is sometimes correct (some sections will fail at Kic, others will sustain higher stress intensities).

Also "fail" is a little tricky. in some structures (typically wings) the skin can initial dynamic crack growth (stress intensity exceeds the critical value) but then the crack is arrested by the stiffening elements (stringers) so that the structure doesn't necessarily "fail" when the crack goes unstable.

 

[rb1957]

correcting a couple of typos ...

that's correct as far as it goes ...
Kic is the plane strain fracture toughness of a material, relevant to thick, heavy sections. thin sheet has a (much) higher toughness (called plane stress).

so the first statement is incorrect ... cracks grow at stress intensities less than Kic; this is stable crack extension and occurs at stress intensities greater than "threshold"

and the second is sometimes correct ... some sections will fail at Kic, others will sustain higher stress intensities depending on geoemtry (thickness).

Also "fail" is a little tricky. in some structures (typically wings) the skin can initiate dynamic crack growth (stress intensity exceeds the critical value) but then the crack is arrested by the stiffening elements (stringers) so that the structure doesn't necessarily "fail" when the crack goes unstable.