Effect of displacement loading rates on mode-I fracture toughness of fiber glass-epoxy composite laminates

Abstract

Size-effect method is used in determining mode-I fracture characteristics of woven fiber glass/epoxy composite laminates for varying loading rates. Tensile testing of geometrically similar single-edge notch (SEN) specimens with three widths 30 mm, 40 mm and 50 mm is carried out for four different displacement loading rates namely 1, 10, 100, 500 mm/min. For each width D and loading rate, the crack-length a is varied as 0.125D, 0.25D, 0.375D and 0.5D. A total of 186 specimens are tested. The peak stresses σNu, the initial Youngs Modulus Exx and the shear modulus Gxy are calculated. The fracture toughness KIC, critical strain energy release rate Gf and material characteristic length Cf are calculated using linear regression analysis of non-linear fracture mechanics equations. The values of Gf and Cf first decrease and then increase monotonically as loading rate changes from 1 to 500 mm/min. This behavior was explained using Bažant size-effect equation relating nominal strength and specimen size. The study revealed change in modes of failure characterized by a jump in brittleness number as loading rate increases from 1 mm/min to 10 mm/min followed by decrease in brittleness number as loading rate increases from 10 mm/min to 500 mm/min. This conclusion was supported by highly magnified images of failed specimens using scanning electron microscope. A numerical algorithm to determine crack growth resistance curves (R-curves) from peak load is also implemented. The R-curves are geometry as well as loading rate dependent and give Gf,Cf values which agree qualitatively and to some extent quantitatively with the linear regression approach.