This is like an "unloader valve" (?) - which does exist - but with
additional characteristics(?)

The need...

I've got a hypothetical on-paper hydraulic device.

For fatigue-testing
- while "the hydraulic cylinder is always bigger than the sample
you are trying to test"
* has always meant a machine with a frame and parts distributed along
a central axis, dwarfing the size of the sample it's testing
* it also means the sample will always fit *inside* the hydraulic
cylinder which is testing it

I cycled up a high hill to get that inspiration, by the way, if you
were wondering...

For fatigue testing samples - it has to tension and release millions
of times.

If I had this valve I mention, you connect the cylinder directly to a
pump - the higher its capacity the faster - more strokes per second -
it will go - "strokes per second" - with "the valve" at the outlet,
dumping the oil in the cylinder and flow of the pump for the time
being back to the tank.

The set pressure of opening means you reach an aim maximum tension in
the sample.
That that valve stays fully open until the hydraulic pressure drops to
(very near) zero completely unloads the sample to no load.
The valve closes and the cycle repeats, etc.

Does such a valve exist?

There are computer-controlled systems with a pressure transducer and
the "dump" valve opening on command.
These are the "servo-hydraulic" systems which are familiar to many.
That might be the option it would be necessary to use, in reality.

However - still curious if there is a stand-alone valve device which
does what's wanted.

For accurate pressure control, the only thing I could think of was to
use a balanced open-close valve (sliding "bobbin" ?) - but with one
end pressurised by a "reference pressure system" with its own small
pump, large accumulator and pressure relief valve returning to the
tank.
With the cylinder pressure routed to the other side of the "balanced
valve".
So when the cylinder pressure exceeds the reference pressure by only a
small amount, the valve moves over to rapidly fully open a big dump
line to tank.
Then there has to be another mechanism / valve which only trips for
the valve to return to closed when the cylinder pressure is about the
same as atmospheric.
If proven to work well, the almost constant pressure in the reference
system could be taken as the peak pressure the cylinder reaches.
That reference pressure is freely adjustable by turning the adjuster
squeezing the spring on the relief valve of the "reference" system.

Thanks for considering.

Rich Smith