Original Article Effects of seawater exposure on mode II fatigue delamination growth of a woven E-glass/bismaleimide composite Zhao Yian, Seah Leong Keey and Chai Gin Boay Abstract The effects of seawater exposure on Mode II fatigue delamination growth of a woven E-glass Bismaleimide composite are investigated by end-notched flexure tests in this study. Dynamic mechanical analysis shows a strong plasticization effect of seawater immersion on the matrix material. Static tests of specimens after 2500 h of immersion in 50  C seawater reveal that the critical strain energy release rate (G IIC Þ is increased by 15.4%. This is mainly due to seawater effects of plasticization of the matrix, extension of the damage zone and enhancement of the ductility of the material. In fatigue tests, a practical formulation based on the Paris law is proposed to model the stable delamination growth rate for both dry and wet specimens. At a given normalized strain energy release rate, the delamination growth was found to be slower in wet specimens than in dry specimens. The examination on fracture surfaces by scanning electron micrographs also reveals evidence of the plasticization effect of seawater which improves the G IIC of wet specimens. Keywords Bismaleimide, environmental effect, fatigue crack growth, mode II delamination Introduction Fiber reinforced polymers are increasingly used in a variety of applications due to their superior mechanical properties such as high strength-to-weight ratio, good fatigue resistance and low susceptibility to corrosion. Woven fabrics are widely used for thick laminates in civil and marine industries due to the easy handling and lay-up during manufacturing. 1 However, polymeric composites generally have low interlaminar strength, which makes the occurrence of delaminations or cracks one of the major causes for material failure. Initial delaminations in composite laminates are com- monly induced by manufacturing defects and low vel- ocity impacts during service. 1,2 The existing delaminations may then propagate mainly in mode I (opening mode), mode II (shearing mode) or a combin- ation of both under dynamic loading even at much lower value than its critical static energy release rate (G C ). 3,4 The growth of delaminations would progres- sively reduce the stiffness and strength of the material, resulting in catastrophic failure of composite structures. Moreover, numerous studies have shown that environmental factors have significant effects on the mechanical properties of polymeric composites. 5–7 However, inconsistent data and conflicting results on the long-term water exposure effects on the Mode II critical energy release rate (G IIC ) were reported in pre- vious studies. 2,6–14 Due to the complex nature of the effect, G IIC varies considerably with materials, fluids and actual absorption conditions (e.g. temperature, immersion time, etc.). In addition, past efforts have been mostly devoted to static loading conditions while few attempts were made to characterize the water exposure effect on fatigue delamination behavior, espe- cially for mode II loading. Therefore, for marine appli- cations designed to take dynamic loading, it is essential School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore Corresponding author: Seah Leong Keey, School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. Email: MLKSEAH@ntu.edu.sg Journal of Reinforced Plastics and Composites 2016, Vol. 35(2) 138–150 ! The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0731684415609139 jrp.sagepub.com by guest on December 21, 2015 jrp.sagepub.com Downloaded from