Fiber-reinforced polymer-matrix composites are relatively simple two-phase materials, for which a straightforward correlation between microstructure and mechanical properties can may times be established. However, certain manufacturing processes render a much more complex structure, requiring more sophisticated characterization procedures. This is the case, in particular, for composites manufactured by filament winding, in which process parameters such as the number of fiber layers, the fiber orientation angle in each layer, and layer relative stacking come into play. This paper presents a methodology for complete characterization of such a material, based on scanning electron microscopy (SEM) and image analysis. Figure 1 shows some typical images obtained in backscattered electrons mode (BSE). The diversity is quite evident and no representative average behavior can be derived. Thus, a mosaic of 4 x 7 = 28 images was assembled and digitally aligned. Figure 2a shows the resulting image after careful background correction to eliminate borders between the tiles. This image reveals the complete microstructure while preserving enough resolution to reveal individual fibers.Quantitative analysis of this microstructure was then developed. Besides typical parameters such as fiber/matrix area fraction and fiber size distribution, the developed method allowed measurement of fiber orientation angle and detection of individual fiber layers, distinguished by fiber orientation. The excellent contrast between fiber and matrix in BSE SEM images obviated fiber discrimination by thresholding. Touching fibers were separated by the well-known watershed method. In this way, the vast majority of the individual fibers was discriminated. Shape detection was then employed to distinguish layers containing fibers with different orientations, varying from perfect circles to elongated ellipses. A sequence of morphological operations allowed a layer by layer separation, as shown in Figure 2b.Once the layers were correctly discriminated, several size, shape and orientation parameters were measured for individual fibers in each layer, and average values per layer were calculated, as shown in Table 1. Layers 2, 6 and 10 present large standard deviations in the measurements, due to distorted objects created by incomplete fiber separation. As in these layers the fibers were strongly inclined in relation to the surface, they appeared as very elongated touching ellipses, for which the watershed method did not perform well.Even though the image analysis procedure required ad-hoc parameters adjusted to the specific microstructure analyzed, the same sequence can be applied in similar systems, with relatively simple adjustments of processing parameters [1]. References[1] The support of CNPq, Brazilian Research Council is gratefully acknowledged.
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