Most of the existing Structural Health Monitoring (SHM) systems are vulnerable to environmental and operational damages. Yet, majority of these systems cannot detect the size and location of the damage. Guided wave techniques are widely used to detect damage in structures due to its sensitivity to different changes in the structure. Finding a mathematical model for such system will help to implement a reliable and efficient low-cost SHM system. Moreover, it can lead to integrate the SHM with the Internet of Things (IoT) for rapid response, off-loading computational power, store data, and and remotely monitoring. In this paper, a mathematical model is proposed to detect the size and location of damages in physical structures using piezoelectric sensor. The proposed model combines both pitch-catch and pulse-echo techniques, and has been verified through out simulations using ABAQUS/Explicit finite element software.