Integrated Damage Sensing in Fiber-Reinforced Polymer Matrix Composites Using Nano- and Micro-Scale Materials

Abstract

Composite materials have significantly contributed to advances in state-of-the-art aircraft and spacecraft design, resulting in lighter and more flexible structures which have pushed the boundaries of performance and efficiency while maintaining a high standard of safety. However, in-situ sensing of the state of the composite structure (i.e. damage and strain) while in service typically relies on externally bonded sensors and actuators which are limited by size, space, and aerodynamic requirements. To circumvent these issues, more recent attention has turned to embedding sensing components within composite structures during fabrication. However, this increases the probability of failure due to the addition of separate materials within the already damage-susceptible interlaminar region. Consequently, multifunctional composite materials which are capable of efficiently performing the tasks of multiple subsystem components without adding mass or bulk or affecting the structural integrity of the material are needed for high performing engineering structures. Although some progress has been made in the field of multifunctional composite materials, current techniques are either limited to conductive fiber reinforcement or dependent on complex processes that require extreme temperature or chemical environments. Therefore, the use of simple and scalable methods for the development of alternative multifunctional materials with integrated structural health monitoring capabilities for a wider class of fiber reinforcement would overcome multiple barriers facing the field of advanced composite materials.

This work investigates the use of scalable and automatable processes to fabricate self-sensing fiber-reinforced composite materials. The first set of composites investigated are designed such that the structural components possess similar functionality to piezoelectric sensors, thus forming a sensing structural composite. To accomplish this, piezoelectric zinc oxide (ZnO) nanowires are grown directly onto electrically insulating fabric which is then sandwiched between layers of conductive carbon fiber that act as electrodes; thus allowing for electrical measurements to be taken directly from the outer surfaces of the composite. By passively measuring the voltage across the composite, multiple damage modes are detected through voltage emissions which occur as a result of the direct piezoelectric effect. This methodology is further validated using a piezoelectric prepreg formed using dehydrofluorinated polyvinylidene fluoride (DHF PVDF) infused in woven fiberglass which is again sandwiched between conductive carbon fiber layers, thus demonstrating that the investigated principles can be extended to alternative composites containing integrated piezoelectric components.

In addition to piezoelectric materials, this work also considers the integration of piezoresistive functionality within electrically insulating structural composites through the use of laser induced graphene (LIG). The LIG is first directly printed onto woven aramid fabric plies which are then combined to form a piezoresistive composite capable of self-sensing both strain and damage in-situ. Furthermore, a transfer-printing process is developed to integrate LIG within commercial fiberglass prepregs. The resultant fiberglass composites are shown to be capable of detecting both tensile and flexural strain and damage in addition to being able to localize damage both through the thickness as well as in the two-dimensional plane of the composite. Further investigation of the fiberglass composites containing LIG also establishes the ability of electrical resistance measurements to track fatigue damage in the composites. Trends in the resistance measurements are also used to estimate the fatigue life of the samples for the first time in piezoresistive fiberglass-reinforced composites. The result of this work is thus two groups of self-sensing multifunctional composites that are fabricated using simple, scalable processes.