| Summary: | Micro-level composite properties are a key determinant of the long-term performance and life span of marine-based composite structures. In this paper, the effect of the aging process in salt water at room temperature and elevated temperature on the elastic modulus and hardness of composite specimens have been investigated through indentation testing. To do this, carbon/epoxy (CE) and glass/epoxy (GE) samples were immersed in artificial sea water with 3.5% salinity at room temperature and 60 °C for 90 days. Matrix only and fibre/matrix (fibre constraint) cells have been examined by two different approaches, namely standard indentation testing and continuous stiffness measurement (CSM) methods to measure elastic modulus and hardness in micro level. It was indicated that degradation at 60 °C had a higher effect on elastic modulus than the room temperature aging process. According to the experimental results from the standard method indentation in matrix cells, the average modulus in the aged GE in 60 °C was 3.47 ± 0.1 GPa (22.3% loss). The average modulus in the reference CE samples was 4.7 ± 0.1 GPa, while those values for aged samples at room temperature and 60 °C showed 38.8% and 51.3% losses, respectively. The hardness for the reference GE specimen was 0.189 ± 0.004 GPa, and after the aging process, it dropped by 2.1% (room temperature) and 27.51% (60 °C). Based on the continuous stiffness measurement for matrix cells, the mean value of modulus for the dry GE and CE matrix material cells was calculated as 4 GPa and 3.7 GPa, respectively. During the aging process, the hardness of CE samples decreased from 0.048 GPa to 0.038 GPa at room temperature and 60 °C, respectively, while aging of GE samples at both temperatures after 90 days caused a 31% loss in the hardness compared to the reference specimen. It was evident from the CSM experiments in the fibre constraint cells that the indentation modulus of the dry epoxy matrix constituent increased by 7.5% and 14% by the neighbouring stiff glass and carbon fibre regions respectively. The indentation characterization in GE samples thorough the standard method in the fibre constraint cells showed a 12.58% reduction in modulus at room temperature, and 25.8% at 60 °C compared to the reference specimen.
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