creep creep creep

[Delwin D Fandrich]

Stephen Birkett wrote:
> 
> Del wrote:
> 
> I've clipped out the key points, only for ease of reading.
> > I do, of course, take exception to your assertion that "...the
> > deformation is asymptotic with time..."
> > ...but in a load carrying beam some level of deformation
> > continues as long as the applied load exists. It will never reach a
> > point of complete stability. If the applied load is great enough, or the
> > time long enough, any wood load carrying member will ultimately rupture.
> > Of course, if the beam is made strong enough in relation to the load,
> > the creep-rupture process can take quite a while.
> >
> This is incorrect. The visco-elastic ("creep") deformation of loaded
> wood *is* asymptotic with time (see e.g. fig.4-17 in the Wood
> Handbook)...this is to be expected from the microscopic theory re: what is
> happening internally.  Such time relationships are indeed bound by a
> maximum...in this case a maximum "creep" deformation. In the example in
> the fig. 4-17, even at 4,000 psi the maximum approx. doubling the initial
> elastic deformation is practically reached after about 350 days.

Fig. 4-17 is an illustration, not a plot of actual test results. In a
wood member under load the rate of “visco-elastic deformation certainly
decreases, but it never truly stops. The size and shape of load-carrying
wood beams have to be calculated for their long term load carrying
capacity. A beam adequate for a one year exposure to a given load may
not be adequate for a 10 year exposure to the same load. Or a 100 year
exposure. —ddf


> > And it is surely stretching reality a bit to claim that ...for
> > practical purposes we can imagine a worst case scenario of initial
> > deformation perhaps doubling over the first year of life of a piano,
> > then staying exactly the same for the remaining 149 years. I have a
> > problem with phrases like "final deformation has a maximum" and "staying
> > exactly the same for the remaining 149 years." Final deformation does
> > not have a maximum, unless you consider creep-rupture to be a maximum,
> > and no wood member under constant stress will remain exactly the same
> > for even one year, let alone 149 years.
> >
> Within a year or so after stringing, modern copies of big wood-framed
> pianos bend and groan into a condition that are exactly the same as the
> current state of their 150 year old ancestors, of which they are replicas.
> If "creep" deformation of the wood did indeed continue to increase with
> time we would expect all of these old pianos to have self-destructed by
> now...which is clearly not the case, in fact they are all highly stable.
> The aymptotic nature of "creep" deformation of a loaded wood member does
> reach a maximum, as described above - see the Wood Handbook or any
> textbook on visco-elastic phenomena for wood. Loaded wood beams that
> are subject to continuous sustained stress will *not* fail, no matter how
> long the duration of the stress, provided an additional appropriate safety
> margin is provided over and above the standard load strength to allow for
> the maximum expected creep deformation. (Doubling standard strength
> specifications is prbably sufficient in most practical cases).

Never is an awfully long time... —ddf


> > (OK, I promised to drop the subject of glues, but you're implying in the
> > above that I have been advocating using aliphatic adhesives in ways that
> > will ultimately result in the failure of a piano's structure.)...
> > Over the thirty-plus years that I've been in this business I have
> > witnessed - and repaired - far too many failed hide glue joints to trust
> > my structural work to its inconsistencies. When I glue something together,
> > I want to be sure that it stays glued. Which is why I prefer to use an
> > adhesive that is appropriate for the wood being used and the type of joint
> > in question.
> >
> I would definitely say that any piano relying on aliphatic glue for its
> structural integrity will ultimately fail...


Stephen,

I guess my only question at this point is this: Why do you insist on
implying that I want to put pianos together using only aliphatic glues?
What is it that I have said or written that gives you that impression? 

Even though the creep rate of modern aliphatics is low enough for most
applications in piano service work, it is not zero. I've tried to
indicate that I agree with you on this point. However, the creep rate of
both urea-formaldehyde and resorcinol resin adhesives is also zero. Both
of them form bonds that are both stronger and more reliable than those
formed by animal hide glue. Both have been around for better than 50
years (urea-formaldehyde since about 1930 and resorcinolics since the
early 1940's). No, that is not the same as 200 years, but surely it is
long enough to get a good feel for their long term reliability. Both of
these are used in modern piano construction with excellent success. 

—ddf