Solid freeform fabrication as a method for creation of structures with multiple materials

Abstract

Solid freeform fabrication (SFF) methods enable the creation of new structures with multiple materials. While the ability to put any material in any location during the building process allows the freedom to create most any combination, it does not readily suggest the best combination for a given task. The performance demands of a structural member differ from those of a sensor and hence would have different criteria for optimal structures. The methods used herein begin to show how the interaction of the different materials impacts their performance experimentally and can be modeled to determine preferred structures. In this work structures were formed by SFF comprising multiple materials with two areas examined: metal-ceramic monoliths and embedded polymer sensors within composite structures. The Metal-ceramic monoliths consisted of a SFF ceramic preform which was subsequently infiltrated with metal resulting in a graded structure, pure ceramic on one side and nearly pure metal on the other. These structures showed improved toughness over pure ceramic structures when tested in bending. Different metal-ceramic interface gradings were modeled based on the experimental samples, including variations of ceramic content. The model showed that the optimal structure was dependent on the orientation during mechanical testing, or application, as well as the ceramic content of the monolith. Embedded poly vinylidene fluoride (PVF2) sensors were used to monitor internal stresses in composite systems. The PVF2 sensors were shown to be capable of detecting damage over the range light tapping to severe impact. More importantly the sensors were able to detect barely visible impact damage (BVID), which can lead to deterioration of mechanical performance without visible evidence. Additionally the PVF2 sensors were used to monitor cure of epoxy systems by sensing the modulus of the matrix. It was shown that for a fixed impact level the sensor response varied as the relative modulus of matrix to sensor changed. Modeling confirmed that when the sensor modulus is much higher than the matrix the stress level in the sensor is higher. The model also showed that the stress level in the sensor is dependent on the geometry and loading, with smaller sensors performing better.