Dynamic mode II delamination in through thickness reinforced composites

Through thickness reinforcement (TTR) technologies have been shown to provide effective delamination resistance for laminated composite materials. The addition of this reinforcement allows for the design of highly damage tolerant composite structures, specifically when subjected to impact events. The aim of this investigation was to understand the delamination resistance of Z-pinned composites when subjected to increasing strain rates. Z-pinned laminated composites were manufactured and tested using three point end notched flexure (3ENF) specimens subjected to increasing loading rates from quasi-static (~0m/s) to high velocity impact (5m/s), using a range of test equipment including drop weight impact tower and a split Hopkinson bar (SHPB). Using a high speed impact camera and frame by frame pixel tracking of the strain rates, delamination velocities as well as the apparent fracture toughness of the Z-pinned laminates were measured and analysed. Experimental results indicate that there is a transition in the failure morphology of the Z-pinned laminates from quasi-static to high strain rates. The fundamental physical mechanisms that generate this transition are discussed

Deformation of the Fermi surface in the extended Hubbard model

The deformation of the Fermi surface induced by Coulomb interactions is investigated in the t-t'-Hubbard model. The interplay of the local U and extended V interactions is analyzed. It is found that exchange interactions V enhance small anisotropies producing deformations of the Fermi surface which break the point group symmetry of the square lattice at the Van Hove filling. This Pomeranchuck instability competes with ferromagnetism and is suppressed at a critical value of U(V). The interaction V renormalizes the t' parameter to smaller values what favours nesting. It also induces changes on the topology of the Fermi surface which can go from hole to electron-like what may explain recent ARPES experiments.Comment: 5 pages, 4 ps figure

Mode II fracture energy in the adhesive bonding of dissimilar substrates: carbon fibre composite to aluminium joints

The end-notched flexure (ENF) test calculates the value of mode II fracture energy in adhesive bonding between the substrates of same nature. Traditional methods of calculating fracture energy in the ENF test are not suitable in cases where the thickness of the adhesive is non-negligible compared with adherent thicknesses. To address this issue, a specific methodology for calculating mode II fracture energy has been proposed in this paper. To illustrate the applicability of the proposed method, the fracture energy was calculated by the ENF test for adhesive bonds between aluminium and a composite material, which considered two different types of adhesive (epoxy and polyurethane) and various surface treatments. The proposed calculation model provides higher values of fracture energy than those obtained from the simplified models that consider the adhesive thickness to be zero, supporting the conclusion that the calculation of mode II fracture energy for adhesives with non-negligible thickness relative to their adherents should be based on mathematical models, such as the method proposed in this paper, that incorporate the influence of this thickness

Numerical and Experimental Analysis of Double-Sided Stepped Lap-Repaired CFRP Laminates Under Tensile Loading

Adhesive layer plays a critical role in the strength restoration of the scarf repaired carbon fibre-reinforced plastic (CFRP) laminates. In this work, Araldite 2015 is used. Hence, it is crucial to model the behaviour of adhesive layer accurately in case of numerical model. Modelling of adhesive layer by cohesive zone law characterises the fracture behaviour of the bonded joint accurately. In this paper, cohesive zone law parameters for mode I and mode II are determined by comparing numerical predictions to experimental observations of a double cantilever beam (DCB) for mode I and end notched flexure (ENF) for mode II fracture test. In this work, Araldite 2015 (supplied by Huntsman) is used for repair work. Strain energy release rate for both mode I and mode II is determined by performing DCB and ENF test, respectively. Traction–separation law for mode I is generated by direct method which involves differentiation of the relation between the strain energy release rate and crack tip opening displacement which is measured using digital image correlation (DIC) technique. Traction–separation law for mode II is generated by inverse method which involves fitting the numerical and experimental load–displacement curves. The obtained cohesive law is used to model the adhesive layer in numerical analysis of double-sided stepped lap joint repair of CFRP laminate subjected to tensile loading. The numerical predictions are validated by comparing the load–displacement curve obtained from the experimental study. A good agreement exists between numerical and experimental results confirming that the proposed cohesive law for mode I and mode II can be applied to model adhesive layer with CFRP as adherend