Chapter6 Mixed Mode Evaluation of Different Grinding Depths in FRPC Repairs T.S. Chawla and M.N. Cavalli Abstract Variations in repair parameters and techniques have a significant influence on the mechanical properties of repaired fiberglass reinforced composite materials. A common method to repair damaged fiberglass wind turbine blades is to conduct hand layup repairs after grinding out the damaged portion. In one sided repair, the ply to which the damage extends depth wise is usually ground off completely and repairs are conducted on the top surface of the next ply. The scope of this work was to observe the effect on repair fracture toughness values when repairs are conducted with this top ply ground off partially. Mixed mode I–mode II testing was carried out on repaired fiberglass composite materials to determine the crack initiation fracture toughness values of two kinds of repairs; one with the top ply intact and other with the top ply partially ground off. The results from this testing were compared to those obtained from mode I testing conducted on similar repairs. Keywords Glass-fiber composites • Composite repairs • Mixed-mode testing • Fracture toughness • Out-of-autoclave 6.1 Introduction Continuous efforts are being made to improve the quality of repairs conducted on both engineering and advanced fiber reinforced composite parts and structures. The target is to restore the composite part to its initial structural integrity with minimal degradation in mechanical and cosmetic properties. Since fiber reinforced composites are subjected to different modes of loading while in service, it is pertinent that they be tested under various loading conditions. Delamination of repairs may be caused by a combination of loading modes. One such commonly tested mixed mode loading comprises examining effects of combined mode I and mode II loads. Initially, the Standard Test Method for Mixed Mode I–Mode II Interlaminar Fracture Toughness of Unidirectional Fiber Reinforced Polymer Matrix Composites (ASTM D 6671/D 6671M – 06) [1] was developed for evaluating the interlaminar fracture toughness of unidirectional fiber reinforced polymer matrix composites under mixed mode I–mode II loading. Though this method describes in details the mixed-mode bending (MMB) test as applicable to use with unidirectional carbon fiber tape laminates [2–5] with brittle and tough single-phase polymer matrices, it has also been extensively used to test the toughness of both glass fiber reinforced composites and materials bonded with adhesive joints [6–12]. For the presented work, mixed mode I–mode II testing was carried out to investigate fracture toughness of glass fiber reinforced polymer composite repairs conducted with variation in repair methods. This was in continuation of earlier work [13] involving mechanical characterization of vinyl ester resins with varying compositions. The previous work was carried out to improve the strength of repairs conducted on glass fiber reinforced polyester wind turbine blades. During characterization with mode I loading it was found that the fracture toughness of the repairs increased significantly when the top ply of the repair surface was ground off partially. This led to further investigation to see if similar effects due to variation in grinding depths could be seen under mixed mode loading. T.S. Chawla ( ) Plastics Engineering Technology, Engineering Technology Department, Western Washington University, Bellingham, WA 98225-9086, USA e-mail: Tanveer.Chawla@wwu.edu M.N. Cavalli Department of Mechanical Engineering, University of North Dakota, 243 Centennial Drive, Stop 8359, Grand Forks, ND 58202-8359, USA e-mail: matthewcavalli@mail.und.nodak.edu J. Carroll and S. Daly (eds.), Fracture, Fatigue, Failure, and Damage Evolution, Volume 5: Proceedings of the 2014 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-06977-7__6, © The Society for Experimental Mechanics, Inc. 2015 41
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