The effects of manufacturing process parameters on structural strength, durability and damage tolerance are not well understood. In particular, the effects of inadequate design method and manufacturing process used to produce thick carbon/epoxy and glass/epoxy composite fatigue-critical, flight-critical components manifest themselves as defects such as wrinkles and porosity, and they impact the performance and the service life of these components. Such defects deteriorate the matrix-dominated properties resulting in degraded strength and fatigue structural behavior. Accurate measurements to quantify manufacturing defects are essential to establish the part condition and avoid assumptions of the worst-case scenario. As wrinkle geometry and porosity distributions in the fatigue-critical composite parts are three-dimensional, accurate 3-D measurement ability is required. The objective of this work is to show the ability of computed tomography (CT) for accurate three-dimensional measurement and characterization of defects in composite structures. Ability to measure wrinkles and porosity in thick structural details is demonstrated. Also, technology development requirements to convert the defect geometry data into finite element models for assessment of the effects of the defects are discussed.
Currently, composite designs adopt metal design philosophy and use the same factor of safety of 1.5 to determine the ultimate design load from the limit load even though composite parts are inherently more susceptible to variations in manufacturing processes than metal parts. In addition to material variation in the resin content, bulk factor, and fiber alignment, part fabrication process variations such as operator skill, tooling setup, humidity fluctuation, equipment control, etc. are common causes that contribute to variation in part quality. Consequently, the increased sensitivity of composite part quality to material and process variations lowers production yields. In order to increase production yields, heavy burden is placed on composite manufacturing communities to understand and control their processes. Production yields of greater than 90% remain a ‘hit-and-miss’ target.