Mode I and II Interlaminar Fracture in Laminated Composites: A Size Effect Study

Abstract

This work investigates the mode I and II interlaminar fracturing behavior of laminated composites and the related size effects. Fracture tests on geometrically scaled Double Cantilever Beam (DCB) and End Notch Flexure (ENF) specimens were conducted to understand the nonlinear effects of the cohesive stresses in the Fracture Process Zone (FPZ). The results show a significant difference between the mode I and mode II fracturing behaviors. It is shown that, while the strength of the DCB specimens scales according to the Linear Elastic Fracture Mechanics (LEFM), this is not the case for the ENF specimens. Small specimens exhibit a pronounced pseudo-ductility with limited size effect and a significant deviation from LEFM, whereas larger specimens behave in a more brittle way, with the size effect on nominal strength closer to that predicted by LEFM. This behavior, due to the significant size of the Fracture Process Zone (FPZ) compared to the specimen size, needs to be taken into serious consideration. It is shown that, for the specimen sizes investigated in this work, neglecting the non-linear effects of the FPZ can lead to an underestimation of the fracture energy by as much as 55%, with an error decreasing for increasing specimen sizes. Both the mode I and II test data can be captured very accurately by Bažant's type II Size Effect Law (SEL).

Fabrication and Thermo-Electro and Mechanical Properties Evaluation of Helical Multiwall Carbon Nanotube-Carbon Fiber/Epoxy Composite Laminates

“…For higher a wt.% of HMWCNTs, discontinuous cracking, microcrack deflection, and pinning, or fiber/tow bridging of the crack, lead to highly nonlinear cohesive stresses, and hence the improved crack propagation resistance. Several researchers [ 73 , 74 , 75 ] have confirmed this underlying principle in the fracture of composite laminates. However, to capture and quantify the nonlinear stresses close to the crack tip, the introduction of a characteristic length scale associated with the size of the FPZ is necessary [ 73 , 74 , 75 , 76 ].…”

An isogeometric framework for the modeling of curvilinear anisotropic media

“…This is typically the case of structures featuring open and filled holes, sharp notches and other stress raisers. In such a case, a quasibrittle fracture mechanics framework should be preferred over a stress-based failure criterion [53][54][55][56][57]. All the required material properties such as longitudinal and transverse elastic moduli E 1 and E 2 , shear modulus G 12 , Poisson's ratio ν 12 , and the ply strengths: σ f 1t , σ f 1c , which are tensile and compressive strengths in the fiber direction, σ f 2t , σ f 2c , which are tensile and compressive strengths perpendicular to the fiber directions, and τ f 12 , that is shear strength are respectively assigned and tabulated in Table 1.…”

Delamination Fracture Behavior of Unidirectional Carbon Reinforced Composites Applied to Wind Turbine Blades

“…The size effect of the laboratory scale test is an important issue in these cases. Salviato et al [ 29 ] analyzed Bažant’s size effect law to take into account the nonlinear effects of the fracture process zone on DCB and ENF delamination testing. They concluded that applying this law the fracture behavior is properly determined.…”