Effects of Environmental Degradation on Flexural Failure Strength of Fiber Reinforced Composites T. Nakamura & R.P. Singh & P. Vaddadi Received: 11 August 2005 / Accepted: 7 November 2005 / Published online: 22 February 2006 # Society for Experimental Mechanics 2006 Abstract Fiber-reinforced composite laminates are often used in harsh environments that may affect their long-term durability as well as residual strength. In general, environmental degradation is observed as matrix cracking and erosion that leads to deterioration of matrix-dominated properties. In this work, cross-ply laminates of carbon fiber reinforced epoxy were sub- jected to environmental degradation using controlled ultraviolet radiation (UV) and moisture condensation and the post-exposure mechanical properties were evaluated through elastic modulus and failure strength measurements. Additionally, both degraded and unde- graded were subjected to cyclic fatigue loading to investigate possible synergistic effects between envi- ronmental degradation and mechanical fatigue. Exper- imental results show that the degradation results in reduced failure strength. Greater effects of degradation are observed when the materials are tested under flexural as opposed to uniaxial loading. Based on strength measurements and scanning electron micros- copy, we identified various damage modes resulting from exposure to UV radiation and moisture conden- sation, and cyclic loading. The principal mechanisms that lead to reduction in mechanical properties are the loss of fiber confinement due to matrix erosion, due to UV radiation and moisture condensation, and weakened/ cracked ply interfaces due to mechanical fatigue. An em- pirical relationship was established to quantify the specific influence of different damage mechanisms and to clarify the effects of various degradation conditions. Keywords Composites . Environmental degradation . UV radiation . Moisture . Fatigue . Residual strength Introduction Fiber reinforced composites offer significant advantage over traditional materials. However, composites are susceptible to degradation by moisture, temperature, ultraviolet (UV) radiation, thermal cycling and me- chanical fatigue. For example, when exposed to humid environments, carbon-epoxy composites absorb mois- ture, which leads to changes in the thermophysical, mechanical and chemical characteristics of the epoxy matrix by plasticization and hydrolysis [1–3]. These changes in polymer structure can lower both the elastic modulus and the glass transition temperature [4–6]. Furthermore, moisture absorption induced dilatational expansion can also cause irreversible damage at the fiber-matrix interface and along interlaminar bound- aries due to material property mismatch. For the increased reliability and durability of these materials, their capacity for sustained performance under harsh and changing environmental conditions must be quan- tified. A recent study [7] examined the physical, chemical and mechanical degradation of an IM7/997 carbon fiber reinforced epoxy composite following exposure to UV radiation and/or water vapor conden- sation. Based on observations of physical and chemical degradation, it was concluded that these two environ- ments operate in a synergistic manner, which causes severe microcracking and erosion of the epoxy matrix. Experimental Mechanics (2006) 46: 257–268 DOI 10.1007/s11340-006-6067-7 T. Nakamura ()) . R.P. Singh (SEM member) . P. Vaddadi Department of Mechanical Engineering, State University of New York at Stony Brook, NY 11794, USA e-mail: toshio.nakamura@sunysb.edu SEM