Dynamic Facture of Hydrothermally Degraded Carbon-Epoxy Composites

Abstract

Structures used in naval applications are often subjected to marine or other aqueous environments during their work-life. It has been observed that many composites absorb moisture when exposed to such environments, which can lead to material damage and degradation of mechanical properties. The addition of salt to the water solution may have additional impacts on the material degradation over time. Furthermore, many of these applications require that the material be subject to stresswave loading environments like impact that may cause dynamic fracture. In this experimental study, the effect of water absorption on the Mode-I dynamic fracture behavior of carbon-epoxy composites is investigated. Specifically, the effects of moisture uptake and the role of saline on the critical dynamic SIF are investigated. Samples were hygrothermally soaked in an elevated temperature bath (70 °C) of either ASTM standard sea water or distilled water. The elevated temperature accelerates mass absorption, making experimental observation possible on a more reasonable timeline than natural aging. Soaking durations varied between 3.5 hours to excess of 4 months to explore the role of soak time on dynamic fracture behavior. Pre-cracked specimens were impacted using a unique long-bar striker device at 4 m/s, and the resulting Mode-I (opening mode) fracture behavior investigated. Digital image correlation (DIC) was used in conjunction with ultra-high-speed imaging to track the crack tip surface displacements, and an elastodynamic solution was leveraged to extract the stress intensity factors (SIF) at fracture initiation. The dynamic fracture toughness of the soaked samples was compared with results from a group of unsoaked, ambient condition samples. The results indicated that the ambient condition samples had a consistently higher Mode-I SIF than any of the soaked conditions. The difference between the ambient condition SIF and the soaked conditions was observed to be approximately 60% for the 3.5 hour soaks, 40% for the 2 day soaks, 50% for the 2 week soaks, and 80% for the samples soaked for longer than 4 months. The SIF reduction is thought to be due primarily to matrix and interface degradation. No statistically significant difference in the SIF was observed between samples soaked in DI water versus salt water.