Mode II Delamination Fracture Toughness of Unidirectional Graphite/Epoxy Composites

Abstract

The mode II delamination fracture toughness of a ductile and a brittle unidirectional graphite/epoxy composite has been studied using the end-notched flexure (ENF) test and the end-loaded split laminate (ELS) test. The stress field in the vicinity of the crack tip of a split laminate beam under mode I and mode II (ELS test) conditions has been determined by means of a finite-element analysis. Also, the micromechanics of mode II fracture have been studied during in situ and postmortem observations of the fracture process. Both the ENF and ELS tests give similar values for GIIc. However, because elastic material behavior is assumed in the analysis, the GIIc results for a ductile composite are somewhat uncertain because permanent deformation is observed. The ELS test provides a pure shear stress state in the vicinity of the crack tip. The formation of hackles in composites made with brittle resins provides a more tortuous path for the crack leading to an increased resistance to delamination under mode II conditions compared to mode I. However, extensive resin deformation and yielding play a more significant role in the fracture resistance for mode II loading of composites made using a rubber-toughened epoxy.

Towards a physics-based relationship for crack growth under different loading modes

“…To this aim, uncovering the relationship between the macroscopic resistance to crack propagation and the micromechanisms of delamination is of utmost importance [16].…”

“…This is based on the concept presented in the work of Neuber [25,26] and also developed later by Lazzarin and colleagues [27,28], in which stresses distributed around the crack tip are said to provide support to the highly stressed area at the tip of the crack. Besides, the stress distribution ahead of the crack tip, which changes with the loading mode, was shown to determine the damage mechanisms acting on fracture and observed on the fracture surfaces for ply delaminations [7,14,16]. Therefore, in order to account for the damage mechanisms when characterizing the energy dissipated in fracture, the stress distribution around the crack tip must be considered.…”

“…In order to address this issue, studies available in literature have tried to connect the microscopic damage features with the macroscopic behaviour of damage growth. To this aim, these studies used the SERR and analyses of fracture surfaces to study the effects of resin toughness, resin layer thickness and loading mode in the resistance to delamination [13][14][15][16][17][18]. A commonly reported result in these studies is that the fracture toughness was observed to be higher for mode II delaminations than for mode I delaminations.…”

Strain-Energy Release Rate Analysis of Adhesive-Bonded Composite Joints with Prescribed Interlaminar Crack

“…A number of researchers have performed delamination experiments on fiber reinforced composites over the past years [18][19][20]. Several standards and test methods for measuring interlaminar fracture toughness have been published recently.…”

Analysis of Mode II Crack in Bilayered Composite Beam

“…In this relation, analytical solutions are needed for the determination of parameters describing the crack behaviour [6,7,8]. Mode II crack behaviour has been explored in many works where the cantilever beam specimen is made of one composite material and, besides, the crack arms have the same heights [9,10,11].…”