How to measure stress/tension on a rope?

I think you need to say if you are allowed to cut the rope in order to insert a transducer (load cell etc).
Also the question appears a bit homework-like! So if it isn't, perhaps you should tell us a bit more about the application so that people are not put off replying.

Without access to the string, it can't be done in any way that could justifiably be called "electronically". You're talking about doing some serious physics here. Like: shoot a lot of x-ray intensity at it and have an expert interpret the diffraction pattern for you to determine the lattice length of the string, from that (assuming you at least know the material) the deformation and from that, in turn, the tension. Or shoot acoustic energy at it over a wide spectrum and try to find its resonance frequency.

(a) Imagine an 5-100kg (we do not know exact weight) object is hanged with a
piece of string/rope/wire and swings randomly.
(b) We do not have access to either end of this string.
(c) How can we measure, electronically,em the stress/tension on the string?

Make a loop in the middle of the string and put an electronic scale or load cell in there.
Mitch

You have described a "pendulum" : oscillation frequency is proportional to pendulum length and weight on the end.
If you know the time for one oscillation, the gravitational acceleration, length of the rope, you should be able to solve for the mass of the pendulum (weight + rope).
Once you have the mass at the end and the velocity of it you should be able to calculate the force exerted on the rope through centrifugal force.
Sounds like a lot of mucking around.

Yes.
Umm, no. But thanks for playing.
Hint: where is the "weight" in 2*pi*sqrt(L/g)?
Regards,
-=Dave

Pendulum, yes.
But length only, not weight. Therefore, probably no help here.
Perhaps if you knew the length and weight of the string, you could calculate the center of mass by the period of the swing, then from that the mass of the object.
But my last Physics class was about 30 years ago...
Rufus

No, this is not homework, and I'm not a student.
Imagine a parachute. How can you measure stress/tension on a parachute line? You can not access the either end of the string/line. (One end connectected to parachute, the other is connected to carabiner.)

Maybe you're trying to design the ultimate way to hang people? :D

Can we contact the string/line at all? Do we know what it is made of?
If so we could potentially measure stretch over a small sample if we can access it before the load is applied and can previously know the stretch characteristics.

Unless the rope is infinitely rigid (wonderful first year physics assumption), the period and the swing itself will vary with the mass, because the rope will stretch with the angular acceleration. I don't want to do the math, even if I could.
Not that this applies to this case, but just to be complete.

I'm sure Gary Peek would know. He's an occasional poster on this group and has used our controllers to measure tension on parachute shrouds.

My company makes these
Eric

On Fri, 9 Sep 2005 01:56:11 +1000, "Tony Limson" put finger to keyboard and composed:
Would it be acceptable to fit a sensor to the harness and assume the load is equally distributed amongst the risers?
Otherwise, is this what you are looking for?
Collecting Parachute Test Drop Data: -- Franc Zabkar
Please remove one 'i' from my address when replying by email.

Lots of people make load cells designed to measure tension on lines. One compact version---albeit for higher loads, is shown at:
Mark Borgerson

You probably could make a simple gizmo to measure the strain using inexpensive force sensors like below (search for force sensors). You would just slip the parachute line in the gizmo sideways and when the string is pulled tight the gizmo flexes pinching the sensor in preportion to the pull on the line.

Try 2*pi*sqrt(L/g)= 2*pi*sqrt(m/k)
where m = mass
thanks for playing.
Regards,
-=Mark

[...re:2*pi*sqrt(L/g)...] Assume a spherical horse moving through a vacuum... ;-)
Regards,
-=Dave

You might try calling the U.S. Navy NAWS, China Lake, Ca. at 760-939-9011 and ask for the parachute department. I vaguely recall seeing something about the group being disbanded but it may still be in business and I would imagine they've instrumented a parachute at some time or another.