Investigations on the Machining Characteristics of Silica-Phenolic Ablative Tiles Bonded to a Metal Substrate

Abstract

Abstract Thermal protection systems are of great importance to the aerospace industry as they prevent the structural member from being heated up to a great degree, which otherwise may lead to undesired failures. “Silica phenolics” is one such ablative composite that is widely used for rocket nozzle and re-entry applications. Though the composite is fabricated to a near net shape, secondary processing, such as machining, becomes inevitable for maintaining dimensional tolerances. Usually, these composites are bonded to a structural member that needs protection from high temperatures. Subsequently, these composites, after bonding to the substrate, are machined to the required geometries using single-point cutting tools to suit the functional requirements. The present work addresses the single-point machining of ablative tiles bonded to a metallic mandrel to simulate the real-time conditions of machining ablative composites bonded to a rocket nozzle. The present work also adopts a special cutting strategy that varies the uncut thickness on different segments of a circular workpiece during its rotation, which simulates reality. Variation of cutting forces was captured at each of these ablative composite segments for different cutting speeds and depth of cut to understand the influence of uncut thickness and width of cut. Possible hypotheses on the mechanism of material removal were analyzed to understand the defects that would arise out of cutting edge interaction with the ablatives. The study revealed that the cutting forces during machining were much lower, and the debonding of the ablatives from the substrate was not observed. The results of the shear lap test also indicated that the debonding force was comparably higher than the observed cutting forces.