Abstract

We measure the spatial distribution of the mechanical stress induced inside translucent glass/epoxy composites by means of polarization-sensitive optical coherence tomography. The Stokes parameters determined from two orthogonal polarization components of the backscattered light allow the internal stress to be identified in terms of its magnitude and principal direction based on a birefringence light scattering model of glass/epoxy composites. Measurement examples show the particular case of stress concentration near a through hole and the internal structural damages caused by excessive tensile loading.

© 2003 Optical Society of America

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References

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Appl. Opt. (1)

Appl. Phys. A (1)

W. Zhang, Y. Wang, �??Polarization optical behavior of optically heterogeneous fiber-reinforced composites,�?? Appl. Phys. A 59, 589-595 (1994).
[CrossRef]

Experimental Mechanics (1)

J. Cernosek, �??On Photoelastic Response of Composites,�?? Experimental Mechanics 15, 354-358 (1975).
[CrossRef]

Opt. Lett. (4)

Optics and Lasers in Engineering (1)

J. P. Dunkers, F. R. Phelan, D. P. Sanders, M. J. Everett, W. H. Green, D. L. Hunston, and R. S. Parnas, �??The application of optical coherence tomography to problem in polymer matrix composites,�?? Optics and Lasers in Engineering 35, 135-147 (2001).
[CrossRef]

Other (1)

S. C. Tan, Stress concentrations in laminated composites, (Lancaster, Technomic, 1994).

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Figures (8)

Fig. 1.
Fig. 1.

Schematic of the PS-OCT system configured for testing composites. SLD: super-luminescent diode, LP: linear polarizer, F-R: Fresnel-Rhomb prism, BS: non-polarizing beam splitter, PBS: polarizing beam splitter, SMF: single-mode fiber, Det: detector. Top right circle: Orientation of the composite structure to be examined.

Fig. 2.
Fig. 2.

Two-dimensional images of Stokes parameters for a glass/epoxy composite under different loading conditions; A) σ=0 MPa and B) σ=130 MPa. The physical size of each image is 1.0mm (depth) × 0.3mm(width). The averages plots on the right display the means of the Stokes parameters averaged over the entire width. All images take the same color map drawn at the bottom.

Fig. 3.
Fig. 3.

Calibration of the material-fringe value f L using six different stress values.

Fig. 4.
Fig. 4.

Stress concentration measurement for a composite sample with a hole under tensile loading, σnorminal=118 MPa. Left, the sample composite with a hole, 2a=5.5 mm and 2a/W=2.85. Right, two-dimensional images of Stokes parameters along the line A-B are presented. The physical size of each image is 1.0 mm (depth) × 4.5 mm(width), and the pixel size is 8×50 µm.

Fig. 5.
Fig. 5.

Experimental data for stress concentration of a unidirectional composite with a hole of 2a=5.5 mm and 2a/W=2.85.

Fig. 6.
Fig. 6.

Matrix cracking caused by drilling at the hole boundary. Two-dimensional images of Stokes parameters along the line A-B are presented. Arrows in the s0 image indicate damages accompanying strong backscattering, and the s3 image shows that the gradient of residual stresses. The physical size of each image is 1.0 mm (depth) × 0.3 mm(width), and the pixel size 8×10 µm.

Fig. 7.
Fig. 7.

Comparison of two-dimensional images of Stokes parameters taken before and after failure by excessive loading. Upper four images were taken before failure and below four images after failure. Arrows in S0 image indicate delaminations. The physical size of each image is 1.0 mm (depth) × 0.5 mm(width), and the pixel resolution is 8×10 µm.

Fig. 8.
Fig. 8.

Two-dimensional stress distribution for a composite sample with a hole under tensile loading. The conditions of external loading and the sample dimensions are the same as those of Fig. 4. The two-dimensional image of stress along the line A-B is presented in terms of the stress magnitude (a) and the principal direction (b). The physical size of each image is 0.72 mm (depth) × 3.2 mm (width), and the pixel size is 8×50 µm.

Tables (1)

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Table 1. Photoelastic properties of the constituents of UGN150 glass/epoxy composite.

Equations (11)

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δ ( z ) = 2 π σ · z f L
A H = R ( z ) sin ( 2 π σ z f L ) exp [ ( Δ z c ) 2 ] cos ( 2 k 0 Δ z + 2 φ )
A v = R ( z ) cos ( 2 π σ z f L ) exp [ ( Δ z c ) 2 ] cos ( 2 k 0 Δ z )
S 0 = A H 2 + A V 2
S 1 = A H A V sin ( A H A V )
S 2 = A H A V cos ( A H A V )
S 3 = A H 2 A V 2
S 0 R ( z )
S 1 = S o sin ( 4 π σ z f L ) sin 2 φ
S 2 = S o sin ( 4 π σ z f L ) cos 2 φ
S 3 = S o cos ( 4 π σ z f L )

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