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Strain Sensor 1D: Case Study

06/17/2021

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Key Features

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 segmentsFigure 1. Aluminium specimen.
The red dots highlight the edges of
the bonded 100 mm fiber segments
In a case study, FSI 1D 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 specimenFigure 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%.