SAMPE Journal - May/June 2020 - 28
FEATURE / FIBER OPTIC SENSING
Figure 8. Algorithm used to develop software for plotting principal strains.
Figure 9. Measurement comparison between each leg of fiber Rosette 1 with the matching FSG rosette leg on Coupon A.
The dotted black line indicates a linear trendline.
strains and their orientation across a 3D model of
the part. This methodology removed the tedious
and time-consuming task of analyzing each segment individually. The main steps in the software's
algorithm are shown below in Figure 8.
RESULTS
Individual Fiber Optic Strain Rosettes
In order to verify the accuracy of distributed fiber
strain measurements when laid out in rosette configuration, comparisons were carried out between
fiber and FSG rosettes. As the legs of each rosette
were laid out in the same direction, a representative plot of fiber strain versus FSG measurement
output is shown in Figure 9. This is of fiber Rosette
1 (R1) on coupon A and demonstrates a linear relationship for all 3 leg pairs throughout the loading
range. The slope, the zero offset, and the R2 values
are given in Table 1.
Fiber Rosette 2 on Coupon A was bonded incorrectly (the fiber sensor was lifted off the coupon
surface) and measurements from this rosette are
not included in the analysis below. Additionally, the
28
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SAMPE JOURNAL
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Table 1. Trendline slope, intercept, and R2 value, for measurement comparison between each leg of fiber Rosette 1 with the matching FSG rosette leg
on Coupon A.
Slope (x 1e5)
Intercept (µε)
R2
Coupon A; Fiber Rosette ε1
6.0021
-3.25
0.9999
Coupon A; Fiber Rosette ε2
5.4797
-2.00
0.9903
Coupon B; Fiber Rosette ε1
5.7243
-5.68
0.9949
FSG on coupon B demonstrated erratic measurements during Test 1, possibly due to an intermittent electrical short in either the gage or the wiring
which resulted in faulty readings and are also not
included in the analysis below. For all other rosettes, the principal strains, e1 and e2, and the orientation angle, j, were calculated for each rosette
on the two coupons. The maximum strain, e1, for
all tests is positive which is expected since the rosettes were located on the tension face of the bent
test article. The minimum strain, e2, is negative due
to Poisson effects.
The calculated principal strains from the fiber
M AY/J U N E 2 0 2 0
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SAMPE Journal - May/June 2020
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