4 half-bridge load cells with HX711 to measure strain or stress

[mortizmarquez]

Hello Everyone,

I have 4 50kg half bridge load cells connected to a load combinator and then this is connected to the load amplifier HX711, I'm following the tutorial for the HX711 in sparkfun but I'm interested in measure strain or stress to approximate fatigue.

I'm very new with arduino so I don't know how I can obtain strain or stress instead of weight

Thank you in advance

[MarkT]

strain is the ratio of deformation to original size, often expressed as ppm or %,
stress means simply force. Weight is force due to gravity.

A calibrated load cell is measuring force, any force sensor in the vertical direction can be used to measure weight.

To measure strain you need some sort of position sensing.

Internally a strain gauge measures its own strain as a proxy for the force upon it, but you want to
measure the strain in a test sample, which means you calibrate the strain guage to measure force,
and calibrate your strain calculations using the measured dimensions of the test piece.

[mortizmarquez]

Thank you for your reply,

I decide to continue by measuring the weight to get the stress, but I got two problems,

first by following the HX711 code in sparfun i got just 0 in the readings

and the other I'm wondering if this works for measuring the weights applied on a cantiléver beam

[DaveEvans]

MarkT:
stress means simply force

No, stress is force divided by area.

OP - No, you can't use those load cells in place of the strain gauges shown in your second image.

You could make a weighing platform with those load cells and attach the cantilever beam to the platform, to determine the weight applied to the beam.

And if you know the force applied to the simple rectangular cantilever beam shown in your second image, you can use basic mechanics of materials equations to calculate the stresses in the beam.

It would be better if you described the "big picture" - what exactly are you trying to accomplish?

[mortizmarquez]

Thanks for your reply dave Evans

In fact I'm trying to predict fatigue in a wing of a scale model A/C so I wanted to measure stress or strain to do it, I'm using cantiléver beam theory since the wing is attached to the fuselaje.

But to measure the stress or the deformation I want to apply known forces and also unknown forces

Here I attach a photo of the A/C I have

[DaveEvans]

Interesting. I had no idea that fatigue in model airplanes was worth worrying about or studying.

Your plane's wing is not a simple cantilever since the wire bracing carries much of the load (assuming the model behaves like the full-size plane).

In fact, depending on how the wing is attached to the fuselage, there may be very little or no bending stress in the wing at the fuselage attachment point. If that is the case, there is essentially no cantilever action at all (except outboard of the outermost wire bracing!) and the wing acts like a beam supported by springs (the wire bracing), pivoting around the attachment point.

And, the wire bracing introduces compression into some parts of the wing, so it is actually a "beam-column." Furthermore, depending on how the wire bracing is attached to the wing, the bracing itself may introduce bending moments into the wing.

Since you seem to not recognize these fundamental things, I suggest you take some courses on basic engineering mechanics of materials. That will be a good first (but very small) step toward fatigue analysis.

[eric_denny]

Should we connect the positive wire of the weight sensor to 5v and ground wire to ground

[DaveEvans]

This thread is nearly three years old. Please delete your post and start a new thread. And please include more information about what equipment you have and what you are trying to do.