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This article describes an automated method for the measurement of crack initiation and propagation in composite materials during modeI double cantilever beam (DCB) testing under different environmental conditions. The method uses the time-domain reflectometry (TDR)-DCB system, which transmits a high-frequency pulse through a transmission line integrated within the composite test coupon and measures impedance discontinuities generated due to the presence of a crack. Using this system, real-time crack propagation in the specimen can be monitored, and the critical fracture toughness parameters (GIC) can be calculated in a variety of environmental conditions. TDR-DCB test method was used for the measurement of GIC for dry and wet (water-saturated) DCB samples made from E-glass fiber/vinyl ester composites under dry conditions (room temperature (RT) at 94°C) and wet conditions (RT at 60°C). For all test conditions, TDR signals showed that crack initiation and propagation was the dominant mechanism in identifying impedance changes in the material. Changes in dielectric properties of the specimen due to the test conditions, whether from water uptake, temperature, or a combination of the two, did not significantly affect signal quality