Sandrea Brundidge-Young, Ph.D. (2013)
Dissertation Topic: Effect of Hypervelocity Impacts and UV Radiation on the Mechanical Performance and Fracture Behavior of Selective Aerospace Materials
Major Professor: Dr. Heshmat Aglan, Professor of Mechanical Engineering, Associate Dean, College of Engineering, Architecture and Physical Sciences
B.S.: Physics, Talladega College, Talladega, AL
M.S.: Mechanical Engineering, Tuskegee University, Tuskegee, AL
Employment: Birmingham, Alabama
Materials used in aerospace and aeronautical applications are some of the most advanced materials. Aerospace materials are exposed to extreme environments and come with some of the most rigorous safety requirements. Depending on the location of the spacecraft, the exterior surface materials of the craft are exposed to several harsh environmental hazards that can degrade these materials. Structures launched into low earth orbit are particularly susceptible to thermal cycling, atomic oxygen attack. ultraviolet radiation, and impacts from orbital debris and micrometeoroids. The focus of this research is to investigate the property effects of the space environment on aerospace materials.
The materials studied in this work were: (1) metals - an aluminum alloy and tungsten, (2) polymers - neat and nano-structured phenolic resin, woven carbon phenolic, woven aramid phenolic composite; neat and nano-structured epoxy composites, aramid epoxy composite, and (3) ceramic - silicon carbide. The nanofillers used were multi-walled carbon nanotubes (MWCNT) and alumina nanofibers (ANF). Materials were fabricated and/or machined and exposed to hypervelocity impacts (HVI) to simulate meteoroid/orbital debris in low earth orbit. In addition to meteoroid/orbital debris, some of the polymeric materials were subjected to ultraviolet radiation. The microstructure, mechanical performance, fracture resistance, and/or failure mechanisms of the systems were evaluated after hypervelocity impacts and ultraviolet radiation exposure; these properties were compared to those of the virgin materials. In some cases, attempts were made to correlate the test conditions to the associated damage in the form of crater size, penetration depth, cracking, etc. Although the majority of the work reported focuses on hypervelocity impacts, a low velocity impact experiment was conducted on structural ceramic tiles to better understand the impact phenomena and attempt to establish any correlation between low velocity and hypervelocity impacts.
The mechanical performance of the neat and nano-reinforced phenolic systems was similarly affected by UV radiation interacting with these systems. Gradual losses in flexural strength were observed in the neat and nano-filled phenolic systems after long term UV exposure. The type of reinforcement had more of an effect on the mechanical behavior of the epoxy based systems. All of the epoxy systems experienced losses in ductility, but the neat and ANF reinforced epoxy systems retained their ultimate strength after long term exposure; the strength of MWCNT filled epoxy decreased after the second month of exposure.
The effects of HVI on materials are primarily dependent upon the thickness of the target and the size and speed of the projectile. HVI typically manifest through cratering and/or perforation. Micron-sized, plasma driven particles produced the following effects: in the aluminum alloy, impact craters with ductile failure features and evidence of target-projectile interaction resulted; craters formed in phenolic resin had brittle fracture features (rough crater wall and spallation surrounding the crater); evidence of perforation by the projectile was seen in the woven carbon phenolic, along with fiber breakage and matrix cracking (indicative of brittle failure); and in the silicon carbide, large craters with brittle features (conical and radial lines, surface spallation) were formed. The performance of the materials were as follows upon impact of 1.5 - 3 mm diameter particles driven by a two-stage light gun: in the aluminum alloy, the impact-produced hole diameter increased with increasing target thickness; the tungsten displayed brittle fracture features (severe spallation around the circumference of the crater/hole) and crater/hole walls partially coated with deformed projectile debris; fiber breakage and delamination were observed in the area surrounding the penetration holes in the woven fabric thermoset composites; and brittle fracture features (large ridges emanating from the center and surface spallation) were observed in the silicon carbide.