Co-authors​ :

 Mohammad BURHAN (UNITED KINGDOM), Tommaso SCALICI (ITALY), Zahur ULLAH (UNITED KINGDOM), Zafer KAZANCI , Giuseppe CATALONOTTI  

The interlaminar stresses near the free edges of composite laminates are weakly singular due mismatch in material properties. Theses stresses can lead to delamination. The traditional Composite Plate Laminate Theory (CPLT) is not suitable for the evaluation of these free edge stresses due to its two-dimensional nature. Also, the exact solution of these stresses does not exist. Therefore, different closed-form [1] and numerical approaches [2] have been developed to compute interlaminar stresses. There exist non-local strength of material and fracture mechanics-based criterion to predict the free edge delamination. However, these depend on having the pre-requisite information about the characteristic length. In this study Finite Fracture Mechanics (FFM) [3] is used to predicts the delamination onset. In the FFM, both stress- and energy-based criterion are necessary and together they form a sufficient one. FFM yields both failure load and corresponding crack size. Dimensional analysis is performed to evaluate the two quantities (interlaminar stresses and incremental energy release rate) required for the FFM criterion for any arbitrary load and ply thickness. Energy release rate is calculated by considering a semi-elliptical crack emanating from the interface, using Virtual Crack Closure Technique (VCCT) through a three-dimensional numerical model. This semi-elliptical crack provides flexibility to calculate mixed-mode energy release rates. The interlaminar stresses are computed at the integration points within a thin resin-rich layer introduced at the interface, using a second finite element model without any crack. Using the equations from dimensional analysis, an average quadratic stress and mixed-mode energy-based criterion are established. The non-dimensionalised functions allow to solve the system of equations semi-analytically. Since the semi-elliptical crack has two dimensions, therefore the FFM system of equations lead to infinite many solutions. A hypothesis is made to introduce an additional inequality that the crack extension occurs homothetically, i.e., the crack retains its aspect ratio during extension. The unknown variables (failure load and two crack dimensions) are determined by solving an optimisation problem to find the minimum load corresponding to a specific crack and satisfying both the stress and energy criterion. Finally, the results obtained using FFM are compared to experimental results found in literature to validate the criterion using different laminate configurations. This approach can accurately predict free edge delamination onset provided the interlaminar strengths and the interfacial fracture toughness are available.