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Measured Surface Deformation and Strains in Thin Thermoplastic Prepreg Tapes Steered along Curved Paths without Adhesion Using StereoDIC

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Abstract

Stereo digital image correlation (StereoDIC) measurements of wrinkle shapes and surface deformations along with numerical simulation results of prepreg tapes steered along paths having different radii of curvature without adhesion to a Teflon-coated substrate are presented. Experiments are conducted at temperatures ranging from 25 to 350 °C on thermoplastic prepreg tape (APC-2/AS4) having various widths. For the baseline studies at room temperature (25 °C), finite element simulation results from the combined experimental-computational study show very good agreement for both the wrinkling mode shape and general trends in the surface strains. Measurements do indicate a sharp change in the longitudinal and transverse strains for the two smallest steering radii, indicating the development of mesoscale damage in the prepreg. Finally, results from the experiments confirm that there is no significant deviation in the deformation response of the prepreg tape when performing steering at temperatures from 25 to 110 °C. However, significant differences in the deformation pattern of the thermoplastic tape are observed above the glass transition temperature, Tg, of the matrix material.

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Notes

  1. Many previous investigators have used FEA along with full-field data to improve their understanding of specimen response, such as shown in [27] for the vibrational response of a structure.

  2. An estimate for the accuracy of the strain measurements using StereoDIC is obtained by performing image correlation of multiple speckle images taken in the reference configuration. Results confirm that the standard deviation in the strain measurements for the parameters noted above is ~ 150 με.

  3. The investigators obtained 120 image pairs during calibration so that any imperfect images (e.g. blurred) images could be discarded. Only 50 image pairs (5400 data points) were used in the analysis to obtain less than 30 parameters for the stereovision system [29]. This gives more than 200 equations per parameter, providing confidence in the resulting mean parameter values.

  4. Once the specimen reaches the required temperatures in the range 150 to 350 °C, comparison of the images acquired over the 30s time frame indicates that no significant additional creep was observed. Additional insight into the characteristic time for the thermoplastic prepreg tape specimens could be obtained through dynamic mechanical analysis experiments, which will be the subject of our future research.

  5. Since a step size of 1 pixel provides more data points close to the boundary, extrapolation of the data was performed using experimental data obtained with a step size of 1 pixel. However, the difference between the extrapolated boundary value of displacement using data obtained with a step size of 1 pixel and 7 pixels was small, on the order of 0.5%.

  6. FE analysis was performed using the as-measured displacements and rotations. The discrepancy noted above is highlighted to emphasize that StereoDIC provides a direct way to quantify displacement boundary conditions.

  7. Modeling of the tape for temperatures above Tg requires viscoelastic properties of the tape material. Measurement of viscoelastic properties and the associated finite element modeling of the tape above Tg will be the subject of a future publication.

  8. With regard to the highly localized changes in strain shown in Figs. 15, 16 and 17, the authors confirmed that these trends are not due to DIC processing parameters such as subset size, subset spacing or strain filter size. The high local gradients shown in Figs. 16 and 17 near the centerline are re-checked using a range of subset sizes, subset spacing and strain filters. In all cases, the steep gradients and rapid changes remained, though they slightly changed in magnitude and width. The local higher strain is also observed in approximately the same locations in a repeat test with a different speckle pattern size and viewing angle. Also, the local steep gradients of the strain are not visible for the higher radii of curvatures. Thus, the high gradients in strain are indicative of changes in material behavior rather than an artifact in the DIC measurement.

  9. These localized damage processes may be visible in an SEM or a high-resolution CT scan, but neither were performed during this study. Measurement and modeling of the evolution of such defects and their effect on the macroscale deformation patterns very well could be the subject of further research.

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Acknowledgements

Funding provided by Boeing Research Contract SSOWBRTW0915000 and associated matching funds provided by University of South Carolina Vice President for Finance Edward Walton via 15540 E250 is deeply appreciated. All materials and access to the Ingersoll AFP facility provided by the McNair Aerospace Center, University of South Carolina is gratefully acknowledged. The technical support and assistance of the McNair technical staff, particularly Mr. Burton Rhodes, Jr. and Prof. Ramy Harik during operation of the AFP, is also greatly appreciated. Finally, the support of Ms. Eileen Miller, Boeing Research and Technology in Charleston, SC, is gratefully acknowledged.

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  1. McNair Aerospace Center and Department of Mechanical Engineering, University of South Carolina, Columbia, SC, 29208, USA

    S. Rajan, M.A. Sutton, R. Wehbe, B. Tatting, Z. Gurdal & A. Kidane

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Appendix

Appendix

Fig. 19
figure 19

LEFT: StereoDIC measurement of out-of-plane displacement for (a) 9.5 mm and (b) 15.8 mm wide prepreg tape at 25 °C for a steering radius of 305 mm. RIGHT: Corresponding FE simulation results. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article)

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Fig. 20
figure 20

LEFT: StereoDIC measurement of longitudinal strain map for (a) 9.5 mm and (b) 15.8 mm wide prepreg tape at 25 °C for a steering radius of 305 mm. RIGHT: Corresponding FE simulation results. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article)

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Fig. 21
figure 21

LEFT: StereoDIC measurement of transverse strain map for (a) 9.5 mm and (b) 15.8 mm wide prepreg tape at 25 °C for a steering radius of 305 mm. RIGHT: Corresponding FE simulation results. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article)

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Fig. 22
figure 22

LEFT: StereoDIC measurement of in plane shear strain map for (a) 9.5 mm and (b) 15.8 mm wide prepreg tape at 25 °C for a steering radius of 305 mm. RIGHT: Corresponding FE simulation results. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article)

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Rajan, S., Sutton, M., Wehbe, R. et al. Measured Surface Deformation and Strains in Thin Thermoplastic Prepreg Tapes Steered along Curved Paths without Adhesion Using StereoDIC. Exp Mech 59, 531–547 (2019). https://doi.org/10.1007/s11340-019-00478-3

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