Re: stress vs. strain determines crack limit

> Returning to the original question, it can be argued that
> strain is what is imposed on the material and stress is 
> its response to it. On balance I would prefer to regard 
> strain as the "independent variable", but engineers
> are used to working with stress so this tends to be 
> more commonly used.
>
> Anyway, just my 'two penny worth' to the thread.
>
> Regards
> Martin
> --
> http://www.analysis.demon.co.uk
> http://www.fracturetraining.co.uk


If you are to use an Instron displacement driven test machine, then you 
are indeed imposing strain.

If you are jumping in the air and grab and swing on a vine, then you are 
doing a load controlled loading of the structure.

Then there are situations which are fully dynamic, such as falling onto 
a matress from 20 feet and the loading in this case is neither simple 
strain controlled nor simple load controlled.  Inertia and stiffness 
both are key parameters.

If you are a tester by orientation, you will generally tend to think in 
the way that your test machine operates.

So, an Instron man may believe that the basic material response is 
strain controlled.

An MTS or similar pnuematic computerized machine can think of 
deformation as either strain controlled (via the fancy computerized 
feedback in the MTS machine) or load controlled as old fashioned 
pnuematic machines used to be designed.

At one time, the Instron philosophy was more modern when the 
electromechanical machines first began replacing the old pnuematic 
machines. Now, the Instron philosophy is no longer the more modern one 
as the MTS type machines are quite brilliant in the total spectum of 
conditions they bring to testing.

As an aside, how do you evaluate impact fracture? [And we will skip the 
many different kinds of impact fracture as just extra complications.] It 
seems hard to imagine impact fracture as primarily a strain controlled 
loading environment.

I still advocate that the oversimplifications we impose on fracture are 
one of the reasons it is still technically difficult and seemingly 
fragmented.

Jim

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