Influence of the Test Method on the Characterization of the Fatigue Delamination Behavior of a Composite Material under Mixed Mode I/II Fracture

Abstract

Composite materials manufactured by overlapping plies with certain specific geometries are likely to lose part of their strength due to the presence of internally delaminated regions. The aim of this paper is to experimentally evaluate the generation and propagation of these interlaminar cracks in a carbon-epoxy composite material subjected to fatigue loading under mixed mode I/II fracture. Two different test methods were used for this purpose: The standardized mixed-mode bending (MMB) test and the asymmetric double cantilever beam (ADCB) test, with the goal of exploring the viability of the ADCB test as a simpler alternative to perform than the MMB test, especially in fatigue testing. With this aim in mind and after prior static characterization of the material in which the critical values of the energy release rate were determined under both test methods, the levels of the energy release rate to be applied in fatigue tests were defined for two mode mixity ratios, GII/Gc = 0.2 and 0.4 (0.34 ADCB), and a fatigue loading ratio, R = Gmin/Gmax = 0.1. The G-N fatigue onset curves were subsequently obtained from these experimental data. The most relevant result of the study is that the fatigue limits obtained using the MMB method are generally more conservative than those obtained via the ADCB method.

Mixed-Mode Interlaminar Fracture Toughness of Glass and Carbon Fibre Powder Epoxy Composites—For Design of Wind and Tidal Turbine Blades

“…Hackle patterns, characteristic of mode I delamination are found on both the GFRP and CFRP fracture surfaces as shown in Figure 13 . These features were also observed in literature for mode I dominated delamination tests [ 42 , 43 ]. Hackle patterns, characteristic of mode II delamination [ 32 , 43 ] are also visible in the 25% mode ratio specimens but are far more present in the 75% mode ratio samples as shown in Figure 14 .…”

Improved Interlaminar Fracture Toughness and Electrical Conductivity of CFRPs with Non-Woven Carbon Tissue Interleaves Composed of Fibers with Different Lengths

“…Carbon fiber reinforced plastics (CFRPs) laminate have been widely used in weight-critical structures, such as aircraft, spacecraft, racing cars, etc., due to excellent mass-specific mechanical properties. Unfortunately, poor delamination toughness [1] in plies of composite laminate has become the important limiting factor in practical structure application. The extensive methods were obtained for improving the delamination toughness of CFRPs composites, including the techniques of toughening the matrix [2], Z-pins [3], stitching [4], and the surface modification of carbon fabrics [5,6] and interleaf [7].…”