Greetings again, Mr. Fandrich
Thanks so much for taking the time to explain so that I can understand your thinking about the matter.
I’m not quite sure I get either the point or the question but I'll take a shot at this and see what happens.
My read is you have the jist of of, but, specifically, I was asking for comment on this issue I wrote of:
If it is claimed it has better tuning stability, then the board core is being constrained..., a more constrained system will experience more stress, and higher stress generally speaking, leads to more damage, somewhere.
So the literal questions are: Is this true or not? If not, why is this system special or what am I missing?
In more dramatic terms, if we are moving from freezing ice in a rubber container to a ceramic container (the more constrained system), the stresses will be actually higher and something may give, the ice will fissure or the ceramic container will fracture so that stress is relieved.
The mechanism is clear that on axis grains of the laminate are constraining the core, and to the extent one says that tuning stability is improved, I am saying that stresses in that system will be actually higher than in the conventional board, and if a conventional board yields or fatigues, then the laminate system will too, and if the stresses are higher, then moreso.
You of course have many observations you wrote of, and probably more that you have not discussed, but I can address my concerns with respect to what your wrote of here.
All soundboard panels are subject to stress. If you think about it, what is the conventional solid spruce panel/rib configuration but a constrained two-ply laminate. The solid spruce panel is still subject to perpendicular-to-grain compression and tension even though there is no cross ply surface veneer.
In a round about way, this example can be an illustration supportive of my point. In humid conditions, conventional boards fail at the rib/board interface in no small part, I think, because of the various mismatches at that interface. Of couse, what the adhesives are doing matters too in this mix. Specifically for humid conditions, Steinway screws the ribs on to deal with this, to an extent. Of course we need to be quantitative about this, and we do, below.
Wood is moderately compliant—spruce can handle up to about 1% perpendicular-to-grain compression and about 0.5% stretch without exhibiting physical damage. If you start with wood that is very dry—say, 4%—and constrain that piece of wood in the perpendicular-to-grain direction and move it to an atmosphere that raises the moisture content to 18% you’re going to exceed that limit. If you hold it there for any length of time you’re going to have a problem. If you start at 6.5% or 7% and raise it to the same 18% moisture content you will have considerably less perpendicular-to-grain compressive stress and you might get by with it. At least you’ll get by with it longer.
I have seen quite clear data on this too, for example, HERE
See p 471, the figure on bottom right.
The solid diagonal line indicates the stress free interface state for various starting relative humidity (RH), such as I suppose would be found at the center region of a rib crown board. If humidity goes up the board expands, constrained by the rib, it goes into compression. Depending on what your staring humidity was, you can see by the upper data points and line that yield happens with a 10% to 30% RH swing to the upside. That graph is showing yield for perfectly constrained tangential wood (and I assume the axial dimension of the of the rib is nearly a perfect constrainer in its longitudinal/length direction), but let's face it, even the highest quality boards are rarely perfectly quarter-sawn, and sorry, I have think it may be at times less perfectly so if it is laminated with a face veneer.
Now, if we are saying the laminated board moves less with humidity, it is more perfectly constrained, the stresses on the core are higher, so I'm saying this rational is telling me that the core in the laminated board is more likely to yield than the conventional board.
(Keep in mind too that materials, including wood, experience fatigue with cyclic stress at levels below their yield point)
But I’m not sure if this really matters in laminated soundboard construction. Essentially what we have is a thin piece of lumber-core plywood. This is the stuff upright case parts and grand lids and other flat parts were made of for a hundred-plus years before MDF came along. Longer than that in other woodworking endeavors. Even while the solid spruce soundboards inside these pianos were self-destructing their lids were doing just fine. Were there macro- or micro-cracks developing in the (usually poplar) core material? I have no idea; I’ve never seen any signs of them but I’ve never examined any of this wood through a microscope either.
I need to go on a little tangent here first before addressing this. This is a bit of a complaint about the ink that is spent on the fact that laminated boards don't crack.
A key point: cracking is not their particular failure mechanism.
So if we are looking for macro cracking as a metric, we are looking in the wrong place.
Back to your paragraph above. It might be a bit of a tangent, because mahogany is the veneer of choice which has very special properties as a rainforest wood, a wood of choice for high humidity applications, and the substrate and mounting protocol is different too.
That said, I observe checking, not lacquer checking, but veneer checking on old pianos. Checking is the failure mechanism of the thin wood laminates due to humidity cycling. The laminate soundboard has thicker top and bottom surfaces with more all-around strength. So yet again, the system is different.
The core panel in a laminated soundboard panel is also constrained. Do macro- or micro-cracks develop in this wood? Again, I have no idea. They might but I’ve never seen any evidence of such. As with the above-mentioned lumber-core plywood, though, I’ve not examined any of them through a microscope. Some damage might well be there—if I get the chance someday I’ll look—but even if I do detect some such, my question then becomes: so what?
I think we will all agree that it is the sound that matters over the time in the use environment. I have to think that collapsed wood cell structure is not a good thing, damps sound, etc, but I have not studied this matter.
There are hundreds of thousands of pianos in service with laminated soundboard panels in them.
Wow, that's a lot. where does this data come from?
So, while your question is intellectually interesting I can see it having any particular relevance in the real piano world. That said, if I’ve missed your point we can try again.
The thing is, if your read a book like "The Toyota Way" and read about double loop learning, you come away with the understanding that to have good quality results, one has to understand fundamentally what is going on. Yes, observables on old sound boards are good. experience as you have with changing the lamination angle is good. But to understand such things as, what are the tolerances? to understand the implications, before they happen, of controlled and uncontrolled changes in the process, the theoretical understanding is necessary too. When I see the conundrum, where the laminated board is potentially in a higher stress state, you know, I think it is a valid concern. I am not bringing up the effects of the phases of the moon here!
When we watched Japan come up, we thought they were doing nothing but copying without understanding.
Changes in attitude were driven in view of this, buy the US gov't no less.
In the time of pre mid-70’s: Only non-commercial true research was supported
then, mid-70’s: about-face to focus only on ‘applied’ science (misreading, and then 'thinking' they were emulating Japan)
USA across the board, started to make junk, with many quality problems.
Then, there was a drifting to realization in the late ‘80s that “you have to understand what it is you are doing”
The organizations that survive are the organizations that keep learning, fundamentally understanding. Actually, I think your are generally on to this, so all is good.
Spiel-off, thank you for listening, best regards,