It might seem that theoretically the area of the crack would be under compression. OTOH, has anyone seen a crack that is open on the bottom side of a strut, closed at the top? Not that the pull is happening beneath the level of the strut, where the strings are.
On the one hand, the string tension should be pulling the tail of the plate upward, and rotating the front of the pinblock upward. But since the struts are above the plane of the strings, and the plate is firmly attached to the rim and pin block, the distribution of forces through the cast iron is probably more complex than that.
My guess would be that there was some defect in the casting, and that perhaps a microscopic hairline crack formed during cooling. Over time it grew a bit, unnoticeably, and then, when the balance between the forces and the material of the plate passed equilibrium, it grew suddenly. But it's the kind of thing we are unlikely ever to explain with any assurance.
The fact is that the plate actually moves under tension, as we can see by behavior of the wedge between the horn and the belly rail support (lower the tension and it falls out, add tension and it is held tightly), and by what happens when a string breaks (neighboring strings go sharp), or when you lower tension to a portion of the strings (remaining strings that are untouched go WAY sharp).
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Fred Sturm
University of New Mexico
fssturm@unm.eduhttp://fredsturm.nethttp://www.artoftuning.com"We either make ourselves happy or miserable. The amount of work is the same." - Carlos Casteneda
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Original Message:
Sent: 02-09-2019 11:22
From: Cy Shuster
Subject: Photo of piano Plate
Wouldn't that part of the plate be under extreme compression? Weird.
-Cy
Original Message------
Looks like you have a plate crack :-) These things do happen, and often they are unexplained.
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Fred Sturm
University of New Mexico
fssturm@unm.edu
http://fredsturm.net
http://www.artoftuning.com
"We either make ourselves happy or miserable. The amount of work is the same." - Carlos Casteneda
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