This paper identifies the material and processing parameters which most significantly influence the development of in-plane waviness in laminates. Thin laminates of unidirectional, T300 carbon-fiber/polysulfone matrix prepreg were processed in an autoclave and a custom-made water-cooled chamber, which allowed fast cooling rates. Multivariate regression analysis of process-induced waviness was performed for combinations of the select process variables and their interactions to identify those factors responsible for waviness development. Of the eight parameters investigated – hold temperature, hold time, pressure, length, width, thickness, cooling rate, and tool plate material – only three affected the development of fiber waviness: length, cooling rate, and tool plate material. Length affects not only the number of wrinkles and wrinkle distribution, but also the average amplitude of the waviness. Cooling rate affects the wavelength and amplitude of the waviness, as well as the number of wrinkles. Tool plate material primarily affects the number of wrinkles, without showing a significant effect on the average wave geometry. There is also an interaction between tool plate material and cooling rate in producing fiber waviness. For the three relevant parameters, the possible waviness-inducing mechanisms are tool plate/part coefficient of thermal expansion (CTE) mismatch, temporal temperature gradients (or cooling rates), and spatial temperature gradients. The tool plate/part CTE mismatch proved to be the most important mechanism driving fiber waviness in plates, although changes in cooling rates also dramatically affected the quantity of waviness which developed. Spatial temperature gradients were negligible for this study. The tool plate/part CTE mismatch-driven axial buckling loads on the fibers were substantial in the outermost laminate plies, or skin, but negligible in the laminate core. Waviness was limited to the surface or skin plies, even in identically-processed thick laminates. This study confirmed that if the fibers experience axial loads – albeit a small fraction of their Young’s modulus – while the matrix is unable to provide some level of transverse fiber support, the fibers will microbuckle resulting in waviness (in-plane or out-of-plane depending upon the laminate constraint).

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.

A detailed survey was conducted of the localized fiber waviness which develops in unidirectional thermoplastic laminates (T300/P1700) in order to determine how part length affects the distribution of fiber waviness. Eleven laminates of varying length were manufactured using identical processing histories. Each plate exhibited discrete wrinkle regions concentrated in the lengthwise center of the part. The amplitude, wavelength, and distribution of these regions were characterized for each plate processed, while penetration depths were studied for sample plates of each length. The results indicate that the waviness severity increases slightly with increasing part length. Furthermore, the major component is in-plane and the waviness frequently penetrates several plys below the surface.

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