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Experimental Mechanics

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08-03-2019

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

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

Published in: Experimental Mechanics | Issue 4/2019

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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, T_g, of the matrix material.

previous article Y-Shaped Cutting for the Systematic Characterization of Cutting and Tearing

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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.

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.

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.

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.

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%.

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.

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

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.

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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Title: Measured Surface Deformation and Strains in Thin Thermoplastic Prepreg Tapes Steered along Curved Paths without Adhesion Using StereoDIC
Authors: S. Rajan
M.A. Sutton
R. Wehbe
B. Tatting
Z. Gurdal
A. Kidane
Publication date: 08-03-2019
Publisher: Springer US
Published in: Experimental Mechanics / Issue 4/2019
Print ISSN: 0014-4851
Electronic ISSN: 1741-2765
DOI: https://doi.org/10.1007/s11340-019-00478-3

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