A technique for characterizing process-induced fiber waviness in unidirectional composite laminates - Using optical microscopy

A technique has been developed, using optical microscopy, for the characterization of localized fiber waviness in composite laminates. Since most of the process-induced waviness which develops in unidirectional thermoplastic laminates is clearly discretized into little packets or fiber-wrinkled regions, the spatial distribution of fiber waviness can be estimated by surface inspection of the laminates. The waviness in these fiber-wrinkled regions is approximately sinusoidal so the waviness parameters chosen were amplitude and wavelength. The waviness in each of the fiber-wrinkled regions is approximately coherent; thus, one measurement of the local fiber geometry is enough to characterize the wave packet. Another issue this technique attempts to address is the thru-thickness waviness. To investigate the presence of waviness through the thickness a two pronged approach is presented: 1) a general survey in which an entire laminate was carefully sectioned into small pieces and inspected in cross-section for the existence of fiber waviness below the part surface and 2) a rigorous three-dimensional serial reconstruction of a ''typical'' fiber-wrinkled region to illustrate the nature of the fiber waviness in these zones. The laminates were surveyed using a statistical sampling routine and the fiber-wrinkled regions were carefully measured using microscopy and image analysis both on the surface and through the thickness of the plates. Results from a series of plates are included to demonstrate the application of this technique.