In recent years, the field of smart materials has witnessed considerable advancements, particularly with the development of self-sensing materials that demonstrate sensitive electrical responses to mechanical strains, facilitating real-time structural health monitoring. These self-sensing materials are typically heterogeneous composites. As such, they require advanced numerical simulation methods to thoroughly analyze how microstructural characteristics influence macroscopic response. This paper presents a coupled electro-mechanical response simulation model based on the finite element fracture phase field method, which is tailored to augment the design and application of piezoelectric smart materials. The proposed model uses a unified phase field damage formulation that simplifies both crack propagation and debonding at the matrix-fiber interface. This allows the model to accurately capture deformation, damage, and the electrical response of the composite under strain. Finally, we employed this model to examine the effect of varying interface thicknesses on sensing efficiency. Our findings indicate that an increase in interface thickness moderately enhances the sensing efficiency for strain and damage detection. Furthermore, this model offers strategic guidance for tailoring the microstructure of self-sensing materials to optimize their sensing performance.