Christopher M. Harvey

Mixed mode partition in one dimensional fractures

Taking a double cantilever beam (DCB) as a representative of one dimensional fracture, a unique pair of pure fracture modes I and II are successfully found in the absence of axial forces, which are orthogonal to each other with respect to the coefficient matrix of the energy release rate. Although the pair are pure modes there still exist interactions between them. The interactions result in energy flow between the two modes and are successfully determined. With the presence of axial forces, there are two independent pure modes I and two independent pure modes II, which are orthogonal to each other as well. They are found and used to partition the total energy release rate.

Numerical and analytical study of delamination in composite laminates

A robust numerical method is developed to study delamination in composite beam structures under lateral and axial loads. A tensor symmetrisation technique is used to formulate the beam element based on the Euler beam theory with full geometrical non-linearity to achieve high computational efficiency. Ply interfaces are modelled with high-stiffness springs. It is found that the beam element suffers from membrane locking for non-symmetric laminates. A method is found to overcome it. The model is used to simulate double cantilever composite beam structure tests and end notched flexure tests. Excellent agreement is observed with analytical and existing numerical and experimental data. The model is also used to study the buckling, post-buckling and delamination propagation in moderately thick composite beams. Satisfactory agreement is demonstrated between the present predictions and existing numerical and experimental data. It is noted that the through-thickness shear effect is significant for moderately thick composite laminates.

Adhesion toughness of multilayer graphene films

Interface adhesion toughness between multilayer graphene films and substrates is a major concern for their integration into functional devices. Results from the circular blister test, however, display seemingly anomalous behaviour as adhesion toughness depends on number of graphene layers. Here we show that interlayer shearing and sliding near the blister crack tip, caused by the transition from membrane stretching to combined bending, stretching and through-thickness shearing, decreases fracture mode mixity GII/GI, leading to lower adhesion toughness. For silicon oxide substrate and pressure loading, mode mixity decreases from 232% for monolayer films to 130% for multilayer films, causing the adhesion toughness Gc to decrease from 0.424 J m−2 to 0.365 J m−2. The mode I and II adhesion toughnesses are found to be GIc = 0.230 J m−2 and GIIc = 0.666 J m−2, respectively. With point loading, mode mixity decreases from 741% for monolayer films to 262% for multilayer films, while the adhesion toughness Gc decreases from 0.543 J m−2 to 0.438 J m−2.The reason why the surface adhesion of a graphene monolayer is much greater than that of graphene multilayers remains unclear. Here, the authors build a model to show interlayer sliding and fracture mode mixity cause the decrease in adhesion toughness of multilayer graphene.

Modeling of delamination propagation in composite laminated beam structures

A numerical method is developed to predict delamination propagation in composite laminated beam structures under lateral and axial loads. Full geometrical nonlinearity is included in the development of beam elements and the interfaces are modeled with imaginary interface springs. The one step crack closure technique, a contact algorithm and tensor symmetrization are employed in the formulation. It is found that asymmetric composite beam elements suffer from membrane locking and this is completely solved in the work. Also, the mode partitioning results are different to those from the existing mode partition theory. A new theory is developed which shows the flaw in the existing theory and demonstrates the validity of the imaginary interface spring model. In general, excellent agreement with existing numerical and experimental results is observed.

Determination of mode I and II adhesion toughness of Monolayer thin films by circular blister tests

This is a pre-copyedited version of a contribution published in Gdoutos E. (eds) Proceedings of the First International Conference on Theoretical, Applied and Experimental Mechanics. ICTAEM 2018 published by Springer The definitive authenticated version is available online via https://doi.org/10.1007/978-3-319-91989-8_38