The origins of fiber print-through in lightweight composite optics

NASA and the U. S. Air Force are looking to improve space borne telescopes by reducing mirror weight. One commonly attempted solution is to fabricate Carbon Fiber Reinforced Polymer (CFRP) mirrors using a mirror replication technique. These attempts have been hindered by the well-known fiber print-through phenomenon. The resulting sinusoidal surface distortion is fiber print-through, where chemical and thermal shrinkage during cure have been hypothesized to be the dominant causes. Although successful mitigation of fiber print-through via a polished resin layer method has been proven, an additional resin layer reduces the heat transfer through the mirror thickness that would be necessary for high-energy laser applications and also carries structural disadvantages. The purpose of this research was to quantify the dominating causes of fiber print-through and its contribution to the total surface roughness of a composite (where total roughness includes the elements of print-through and other surface anomalies that contribute to diffuse reflection). In order to quantify the causes of fiber print-through, a number of CFRP samples with varying fiber type, diameter and cure schemes were fabricated. The dominating causes of fiber print-through were then found by measuring fiber print-through, via microscopic interferometry, and determining which variables had the greatest influence on print-through.