A Technique for Characterizing Process-Induced Fiber Waviness in Unidirectional Composite Laminates-Using Optical Microscopy

Abstract

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.

Nondestructive Evaluation (NDE) of Advanced Fiber Reinforced Polymer Composites

“…In addition, they concluded that they could qualitatively determine the degree of fiber waviness by utilizing changes in the ultrasonic wave travel time. Joyce et al (1997) extensively reviewed the literature for NDE techniques to determine fiber waviness in composites. They came to the conclusion that due to the complicated nature of acoustic waves propagation in wavy composites, the challenging problem of characterizing natural fiber waviness using ultrasonics remains largely unsolved.…”

Effects of defects on laminate quality and mechanical performance in thermoplastic Automated Fiber Placement-based process chains

“…They use microscopy to locate instability sites and determine the wrinkle heights as multiples of the cured ply thickness of their material and the angular inplane deviation of the 0 plies. The latter was done by measuring the aspect ratio of the elliptical shaped fibers, a principle that was originally introduced by Yurgartis [32] and applied and refined since then [15,[33][34][35]. Belnoue et al studied the formation of out-of-plane wrinkles during prepreg de-bulking and curing due to AFP gaps and overlap in [15].…”

An Infrared Thermoelastic Stress Analysis Investigation for Detecting Fiber Waviness in Composite Structures

“…[10] There is a large volume of literature confirming the large amounts of mechanical property reductions due to these defects [11][12][13][14] . Some have attributed the causes of fiber waviness in composites to the relationships between the mandrel and pressure considerations during cure [15,16] . Precure and postcure optical and ultrasonic techniques have been proposed for characterization with some success [15,[17][18][19][20] .…”

“…Some have attributed the causes of fiber waviness in composites to the relationships between the mandrel and pressure considerations during cure [15,16] . Precure and postcure optical and ultrasonic techniques have been proposed for characterization with some success [15,[17][18][19][20] .…”