Strain Sensor: Case Study

06/17/2021

Andrea Mannarino - Director of Data Science
Welcome to Nova Scientia,
I’m Andrea and today I’ll demonstrate you our strain sensing technology.
The test set up is represented by a tensile test machine; an aluminium specimen instrumented with a fibre optics; our acquisition unit and a visualisation software.
Instrumented Specimen
This is our aluminium specimen and it is designed to have a constant strain region.
A fibre runs all along the specimen in and out and it’s fully embedded in it.
Visualisation Software
This is our visualisation software:
On the top you can visualise the different fibre segments and you can select what segments would you like to visualise. This bar will change colour according to the elongation of each segment.
At the bottom, you can actually see the time series of the elongation and as you can see, the system responds in real time to the perturbations that I’m applying to the specimen.
Test
We will now demonstrate our strain sensing technology on a tensile test machine.
The test is composed by four phases: a pre-load, a ramp to a target load of 3000 Newtons, a steady state condition at 3000 Newtons and then an unload phase.
The pre-load and unload phases will not be recorded by the test machine, but we will be able to record them via our technology.
I will now start the test.
As you can see nothing is displayed on the test machine, while we start to visualise something on our visualisation. Now the test machine started logging and as you can see, we also visualise the same trend. Now we wait to reach steady state conditions.
As you can see, the segment that we have selected is in the constant strain region.
We have now reached the steady state condition. I will now unload the specimen to return to the original conditions.
As you can see from our visualisation software, the condition returned to the original stage.
Our resolution is 20 nanostrain and we are able to identify the stiffness of the specimen with an accuracy of 0.05%.
Contact us
If you are interested to know more, please contact us.

Key Features

20nε Resolution
Independently from Sampling Frequency
0.05% Accuracy
compared to nominal Youngs modulus
High Repeatability

Strain Sensor 1D: Case Study

Figure 1. Aluminium specimen.<br>The red dots highlight the edges of<br>the bonded 100 mm fiber segments

Figure 1. Aluminium specimen.
The red dots highlight the edges of
the bonded 100 mm fiber segments

In this case study, FSI strain sensors are used to measure uniaxial deformation of an Aluminium specimen.
We achieve 20nε resolution, independently from the sampling frequency and observe high repeatability.
The specimen’s Youngs modulus obtained experimentally is accurate when compared to the used theoretical value, with a deviation of 0.05%.

The FSI sensors (1 SM fibre with 4 segments, 100mm each) are bonded onto an Aluminium specimen with cross-sectional area of 52.5mm2 and length of 250mm, which is then mounted on an Instron test machine and loaded in tension up to the desired level, in the test setup of Figure 1. The goal of the test is to characterise the fibre elongation when subjected to load and validate the measurements against the in-house preliminary calculations used to design the specimen.

The test is organised in 4 phases and repeated 3 times, to verify the sensor repeatability:

Phase 1: preloading to 100με at a slow load rate (between 0 and 10 seconds)
Phase 2: faster load ramp (between 10 and 30 seconds)
Phase 3: constant load (between 30 and 90 seconds)
Phase 4: unloading (from 90 seconds)

Test Results

Figure 2. Left: measured deformation vs time.<br>Right: zoom during Phase 3 (constant load)

Figure 2. Left: measured deformation vs time.
Right: zoom during Phase 3 (constant load)

In Figure 2 is reported the average of the three experiments carried out. When looking at Phase 3, the specimen deformation, measured through the elongation of the fibre, is comparable to the expected value of 816με, calculated using basic material science theory.

Figure 3. Estimation of specimen's stiffness

During Phase 3, it can be appreciated from Figure 2 (right side) that the deformation noise is significantly small. Under constant load, the experimental standard deviation of the deformation signal is 20nε,
independently from the sampling frequency.

In Phase 4 instead, one can remark that the measured signal goes back to its unloaded value.

The Young modulus of the specimen is calculated through its strain-force characteristic, displayed in Figure 3. We obtain 69.34GPa against a theoretical value of 69.30GPa, with an accuracy of 0.05%.