Dynamic fracture behavior of hygrothermally degraded carbon epoxy composites

Abstract

Composite structures used in aerospace, automotive, and military applications are often subjected to aggressive marine or water environments during their service life that can cause material degradation over time. Many composites absorb moisture when exposed to wet environments which may affect their microstructure, consequently influencing their bulk material properties and performance. Additionally, certain applications may require them to be subjected to seawater environments where the addition of saline can further degrade the material over time. This experimental investigation examines the effects of moisture absorption and saline on the dynamic fracture behavior of unidirectional carbon epoxy composites. To accelerate mass absorption, samples were soaked in elevated temperature baths (70°C) of either ASTM standard seawater or distilled water. To explore the role of absorption duration on the dynamic fracture behavior, soaking time varied between 3.5 hours and longer than 4 months. Pre-cracked specimens were impacted using a long-bar striker device at 4 m/s, to instigate a dominantly Mode-I (opening) fracture response. Digital image correlation was used in conjunction with ultra-high-speed imaging to track the crack tip surface displacements and an elastodynamic solution for an orthotropic material leveraged to extract the stress intensity factor (SIF) up to and at initiation. The critical SIF of the soaked samples is compared with results from a group of ambient condition, unsoaked samples. Results suggest that moisture absorption causes the matrix to expand and introduce microcracks or delamination between plies as the composite swells to accommodate the additional water, inversely affecting the material resistance to fracture.