Damage Evolution in Composites with Varied Fiber-Matrix Interfaces Helen Inglis 1 , Thomas Mackin 1 and Steven Seghi 2 1 Department of Mechanical and Industrial Engineering, 2 Department of Materials Science and Engineering University of Illinois at Urbana-Champaign 1206 West Green Street Urbana, IL 61801 Abstract A glass-epoxy composite system with an activated carbon fiber coating has been developed as an alternative to conventional fiber-matrix coupling. In order to investigate whether varied interfaces affected damage mechanisms, systems with three different activated carbon surface chemistries and three different coating thicknesses were tested. Damage evolution was observed using thermoelastic imaging to quantify the role of the interface on the properties of the composite. Introduction In an attempt to provide increased moisture resistance and allow for varying degrees of chemical bonding to polymer matrices in the glass fiber-polymer matrix composite system, a new interface coating was developed. Activated carbon was deposited on glass fibers. These activated carbon coatings were then chemically modified to produce three different surface chemistries, highly basic, slightly acidic and highly acidic. Varying the surface chemistries allowed control over the strength of the carbon-matrix interface. Initial testing indicated that the thickness of the carbon interface might also affect the physical response of the composite [1], so the thickness of the carbon coating was also varied. Control over the strength of the carbon-matrix interface was expected to provide some control over the damage mechanism of the composites. If the carbon-matrix interface is strong, a crack can only propagate by breaking fibers, with some degree of fiber bridging in its wake (a Class I damage mechanism). [2] However, if the bond is weak enough, a crack will preferentially run along the interface between fiber and matrix. This second damage mechanism, known as Class III damage, [2] is preferable to the first, as it results in redistribution of load across the remaining section rather than failure of the section. [3,4] Thermoelastic stress analysis (TSA) is a method of stress imaging which produces full-field stress maps of structures under cyclic load. [5] Under adiabatic conditions, the change in temperature in a loaded specimen is directly proportional to the change in stress. This is called the thermoelastic effect. This very small temperature change can be measured using infrared imaging. The signal in phase with the applied loading gives a measure of stress changes in the specimen while the signal out of phase with the applied loading is related to hysteretic heating. Hysteretic heating indicates the presence of nonlinear mechanisms, such as frictional heating, and hence is an indication of damage in the structure. [3] The mechanisms of damage and failure in these composite systems with varied fiber-matrix interfaces were investigated experimentally. Specimen Preparation 95% unidirectional E-glass fibers were used for this study. The carbon coating on the unsized fibers was created by solution coating the fibers with phenolic resin in ethanol. Decreasing the percentage of phenolic in the ethanol decreased the thickness of the carbon coating deposited. 70nm, 30nm, and 25nm thicknesses were used in this study. The phenolic coating was dried to remove the ethanol and begin the cross-linking procedure. The cross-linking was completed by heating the coated fibers. To produce a slightly acidic chemistry, the cured coatings were carbonized by heating in dry nitrogen. [6] This activation produced an oxidized surface with oxygen content of 3.8% and nitrogen content of 0.14%. [7] To increase the amount of oxidation and hence produce an acidic coating, some of the slightly acidic coated fibers were submerged in nitric acid then rinsed in de-ionized water and dried. [8] This treatment produced oxygen contents of 12.8%, with corresponding nitrogen content of 2.2%. [7] The third chemical modification of the activated carbon involved carbonizing the coating in dry ammonia gas instead of dry nitrogen. [9] This procedure, creating a basic coating, contained 6.5% nitrogen and 1.7% oxygen. [7] The nine different composite systems were fabricated in 6” square plates using hand wet lay-up and vacuum bag techniques. 5” by 1” double edge-notched tensile specimens were cut from these plates. One sample of each type of composite system was tested. Tabs were bonded to the specimens to distribute load from the test machine grips. The front surface of the specimens was sprayed with one layer of ultra-flat black spray paint to ensure uniform emissivity.