Owing to the high strength-to-weight and stiffness-to-weight ratios, composite materials are today essential building blocks for a wide range of industrial applications. However, their complex microstructures make it difficult to predict their failure mechanisms and residual lives under varying external loads. In situ health monitoring systems have received much attention in recent years as promising solutions for the above-mentioned limitations of composite materials. Here, we suggest a coupled health monitoring system in which infrared thermography and electrical resistance measurements are simultaneously applied for diagnosing the damage state of composite samples during tensile testing. In addition, we build a multiphysics simulation framework to model the interplay between the physical phenomena occurring in the three damage stages, involving crack propagation, variation in the temperature profile, and electrical resistance. The coupled electro-thermal monitoring system allows the estimation of the “damage stress (σD)”, which represents the onset of a micro-damage and may be correlated to the fatigue strength of the material and the assessment of the damage evolution process in glass fiber-reinforced polymer composites under quasi-static tensile loading. The proposed simulation framework enables the simultaneous understanding of the multiple physical phenomena occurring in the composite at the instance of crack nucleation and propagation.