Michael Thouless
These analytical studies were also accompanied by model experiments that helped establish the significance of mixed-mode failure criteria on interfacial delamination, including buckling and blistering phenomena [1992, 1993].
Although there were other analyses that followed in the literature, the first analyses to use linear-elastic fracture mechanics to link the crack spacing in brittle films to the fracture mechanics of the problem were provided in 1990 and 1992. A later extension of this work recognized the possibility of stable arrays of cracks penetrating into the substrate when there is a huge modulus mismatch between film and substrate [2011].
An appreciation that transverse shear has an effect on the delamination of beam-like geometries had been recognized for many years, but my students and I were the first to bring this effect into a rigorous interfacial mechanics framework [2004], and complete the linear-elastic fracture-mechanics framework for layered materials.
The toughness of composites and multi-layered materials is critically dependent on the role of interfaces in deflecting cracks. In 1989, I developed an early fracture-mechanics analysis that showed crack deflection would occur at an interface if the toughness of the interface is less than a quarter of the toughness of the matrix [22]. Subsequently, however, a cohesive-zone analysis raised the intriguing suggestion that the interfacial strength may be more significant than interfacial toughness in determining whether a crack will deflect into an interface [22]. This is important because it suggests that, for the design of flaw-tolerant composites, interfacial strength may be tailored to deflect cracks into tough, energy-absorbing interfaces, and it provides a very different insight into the design of interfaces than had been provided by earlier LEFM models (such as that of the 1989 paper, and that of others from the same period).
Cohesive-zone models have been used recently to explore interfacial fracture mechanics in the absence of stress singularities. These have been used to elucidate the relationship between interfacial properties, fracture-length scales and phase angles in mixed-mode fracture [2007, 2011].
For energy-efficient automotive structures, the ability to design joints between different light-weight materials will become increasingly important. Collaboration with the auto industry in this area has led to the development of cohesive models for fracture of adhesive joints. This is now a huge area of research in the field. Traditional fracture mechanics approaches for analyzing bonded structures are not appropriate for automotive applications in which large-scale plasticity can accompany fracture. Although, this is now a very common approach in the literature, a major intellectual advance for modelling adhesives was to adapt the concept of a cohesive zone to represent the entire adhesive layer [1999]. In particular, this allowed adhesive joints to be modelled for plastically deforming adherends, and for large-scale bridging. The key advances of this work included the development of experimental techniques to extract the mixed-mode cohesive strength and toughness for adhesive layers, and the demonstration of predictive capabilities under general loading conditions [2001, 2002, 2008, 2009]. This work was essentially the first time that good quantitative predictions had been realized when the deformation of adhesive joints is dominated by plasticity rather than elasticity.
The cohesive-zone approach has been expanded to include the fracture of bonded composites [2005, 2006] and ultrasonic welds [2005, 2006]. This latter work is significant because, historically, weld design has been based on a semi-empirical strength criterion, which has limited ability to accommodate different geometries.
Worked with a biomedical colleague has resulted in the use of fracture of thin films on soft polymers to develop devices for nano and biological applications. Cracking of an oxidized layer on PDMS to define lines of protein that cells could grow on. By stretching and relaxing the PDMS, the behaviour of cells to more or less protein could be studied [2005]. Nanocracks in plasma-treated surface layers have been used to create tunable nano-channels [2007, 2010]. A strain can be used to open and close nano-channels because of the compliant PDMS substrate. These flexible nano-channels can be used for sorting particles; and for studying DNA and chromatin by letting the DNA into the channels when open, and then closing the channels to stretch out the long-chain bio-molecules [2013].
Beyond these major themes of delamination, cracking of films, and cohesive-zone modelling, contributions have been made in a number of other areas. The first rigorous analysis of how statistical variations of fibre strength affect fibre pullout and the toughening of composites was developed in [1988], with the effects of interfacial properties on pullout also being examined [1989]. While at IBM, the first evidence that silicon is subject to stress-corrosion cracking during exposure to the HF etches that are used in silicon processing was presented [1990]. The importance of surface diffusion in the diffusive creep of thin films was recognized in [1993]. This was analyzed by incorporating coupling between surface diffusion and boundary diffusion to expand upon the classic Mullins' analysis for grain-boundary grooving. The fact that this coupling could be of practical significance because suppression of surface diffusion could suppress the creep relaxation of thin films, was subsequently demonstrated experimentally [1996]. These projects were associated with the first analysis of coupled creep mechanisms during the stress relaxation of metal films during thermal cycling [1993]. More work in coupled creep relaxation mechanisms has been provided in a recent study on the nuclear material zircaloy, used for cladding. Although creep mechanisms for this material have been studied for 40 years, no one had collected the data into a consistent creep-mechanism map. Nor had anyone demonstrated the incorporation of multiple creep mechanisms into finite-element codes so that the correct creep rates evolve naturally in analyses as the temperatures and stresses evolve [2013].