Impact Response of Curved Composite Laminates: Effect of Radius and Thickness

This paper presents the results of drop-weight impact testing (5 J to 30 J) on curved +/- 55 degrees E-glass-epoxy laminates of varying radii and wall thickness. Three radii (75 mm, 100 mm, and 125 mm) on laminates with an effective wall thickness of 2.5 mm, and three wall thicknesses (2.5 mm, 4.1 mm, and 6.6 mm) with a radius of 100 mm were investigated. The damage pattern remained consistent, with the exception of the damage area, across the tested energies and was dominated by internal matrix cracking and multiple delaminations. However, no damage was recorded following a 5 J impact on the 2.5 mm thick laminates with 100 mm and 125 mm radii, all energy was absorbed elastically, while the laminate with a 75 mm radius of curvature developed a damage area of over 80 mm(2). The thicker laminates showed a reduced overall damage area but a greater number of delaminations. The relationship between laminate thickness and delamination threshold load was found to be in line with impact testing of flat plates, varying with the laminate thickness to the 3/2 power. However, the simplified beam theory and a fracture mechanics model developed for the prediction of delamination threshold of flat plates was found to underestimate the delamination threshold load (DTL) of the curved laminates studied by about 40%. An increase in the laminate's flexural modulus of a factor of two is required to bring the model's predictions in line with the DTL values measured experimentally, highlighting how curvature can enhance bending stiffness and alter damage evolution. Finally, a significant finding is that the DTL of the curved plates is around 15% lower than the value measured for the whole cylindrical pipe of same specifications. Testing curved sections rather than a whole pipe could reduce effort, but further work is required to confirm this statement.