Adhesively bonded methods (single lap, scarf, stepped) have emerged as versatile methods to repair the damaged composite components due to low weight penalty and super joint efficiencies^[1,2]. However, a prominent problem with the use of bonded-repaired structures is their susceptibility to environments such as hygrothermal conditions^[3]. Hence, studying the effect of hygrothermal conditions on the mechanical behavior of bonded-repaired composite structures is of significance.

Considering the high cost of long-term environmental aging tests, numerical approach for failure prediction of adhesively repaired composite is necessary. Since hygrothermal aging is a complex phenomenon that operates at different time and spatial scales, and the incurred degradation strongly depends on the material components involved and on the exposure environment, it is critical to assess the material degradation accurately in terms of safety evaluation.

Therefore, a progressive damage model considering the gradient of moisture content was proposed to predict the failure strength and analyze the damage evolution of adhesively repaired composite. Once the moisture contents were obtained from diffusion analysis, the material parameters of composite laminates and adhesive layer degraded differently according to the moisture content based on Tsai equations and a degraded cohesive zone model, respectively. This model was validated by comparing with experimental results and showed a good consistency. Through the numerical approach, the effects of hygrothermal aging on the damage mechanism and parametric analysis were subsequently conducted.