Dimitra A. Ramantani

Buckling Behaviour of Carbon–Epoxy Adhesively-Bonded Scarf Repairs

The present work is dedicated to the experimental and numerical study of the buckling behaviour under pure compression of carbon–epoxy adhesively-bonded scarf repairs, with scarf angles varying from 2 to 45°. The experimental results were used to validate a numerical methodology using the Finite Element Method and a mixed-mode cohesive damage model implemented in the ABAQUS® software. The adhesive layer was simulated using cohesive elements with trapezoidal traction–separation laws in pure modes I and II to account for the ductility of the adhesive used. The cohesive laws in pure modes I and II were determined with Double Cantilever Beam and End-Notched Flexure tests, respectively, using an inverse method. Since in the experiments interlaminar and transverse intralaminar failures also occurred, cohesive laws to simulate these failure modes were also obtained experimentally following a similar procedure. Good correlations were found between the numerical predictions and experimental results for the elastic stiffness, maximum load and the corresponding displacement, plateau displacement and failure mode of the repairs.

Adhesively-bonded repair proposal for wood members damaged by horizontal shear using carbon-epoxy patches

In this work, a repair technique with adhesively bonded carbon-epoxy patches is proposed for wood members damaged by horizontal shear and under bending loads. This damage is characterized by horizontal crack growth near the neutral plane of the wood beam, normally originating from checks and shakes. The repair consists of adhesively bonded carbon-epoxy patches on the vertical side faces of the beam at the cracked region to block sliding between the beam arms. An experimental and numerical parametric analysis was performed on the patch length. The numerical analysis used the finite element method (FEM) and cohesive zone models (CZMs), with an inverse modelling technique for the characterization of the adhesive layer. Trapezoidal cohesive laws in each pure mode were used to account for the ductility of the adhesive used. To fully reproduce the tests, horizontal damage propagation within the wood beam was also simulated. A good correlation with the experiments was found. Regarding the effectiveness of the repair, for the conditions selected for this work, a full strength recovery was achieved for the bigger value of patch length tested.

Stress and Failure Analysis of Repaired Sandwich Composite Beams using a Cohesive Damage Model

The behavior of a repaired sandwich beam subjected to four-point bending is investigated numerically using the ABAQUS® software and specially developed interface elements including a cohesive mixed-mode damage model based on the indirect use of fracture mechanics. The two major repair configurations for sandwich structures, namely overlap and scarf repair, are studied. The interface elements, placed at the middle of the adhesive, parent laminate/adhesive and adhesive/patch interfaces, allow to obtain stress distributions at these locations as well as to simulate damage onset and growth. The influence of several geometrical parameters, such as overlap length and patch thickness for overlap repairs, and scarf angle for scarf repairs, is evaluated in terms of stress analysis and strength predictions. Conclusions were drawn about design guidelines of the sandwich composite repair.

Interlaminar fracture characterization of a carbon-epoxy composite in pure mode II

The interlaminar fracture toughness in pure mode II (GIIc) of a Carbon-Fibre Reinforced Plastic (CFRP) composite is characterized experimentally and numerically in this work, using the End-Notched Flexure (ENF) fracture characterization test. The value of GIIc was extracted by a new data reduction scheme avoiding the crack length measurement, named Compliance-Based Beam Method (CBBM). This method eliminates the crack measurement errors, which can be non-negligible, and reflect on the accuracy of the fracture energy calculations. Moreover, it accounts for the Fracture Process Zone (FPZ) effects. A numerical study using the Finite Element Method (FEM) and a triangular cohesive damage model, implemented within interface finite elements and based on the indirect use of Fracture Mechanics, was performed to evaluate the suitability of the CBBM to obtain GIIc. This was performed comparing the input values of GIIc in the numerical models with the ones resulting from the application of the CBBM to the numerical load-displacement (P-) curve. In this numerical study, the Compliance Calibration Method (CCM) was also used to extract GIIc, for comparison purposes.