Re: stress vs. strain determines crack limit

Dear seferiad:

"seferiad" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> Hello,
> With respect to classic power-law crack growth in brittle materials,  the
> critical limit is given by
>
> K = Y*S*a^1/2,  where Y is a constant, S is the applied stress, and a is
the
> crack length (which is taken as a square root).
>
> My question is : What is the true independent variable (i.e., stress or
> strain) that determines when the material fractures?  This equation
suggests
> that it is the stress, since the strain is not given, but that might be
more
> for convenience since we tend to measure stress, not strain.
>
> Assuming the stress-strain relationship were perfectly linear it wouldn't
> matter, but since materials fracture in a regime in which the
stress-strain
> is not linear, it changes the interpretation of fracture data.

Strain is a function of the material, so once you have selected a material
that "degree of freedom" is lost.  I would vote for stress to be the
independent variable.

David A. Smith