M. D. Thouless

Effects of pull-out on the mechanical properties of ceramic-matrix composites

Abstract The influence of fiber pull-out on the mechanical properties of fiber reinforced ceramics has been analyzed using an approach based on weakest-link statistics. The essential physics contributed by the statistics is the establishment of a relationship between the fiber failure site, which governs the pull-out length, and the properties of the fibers, the matrix, and the interface. This analysis involves the development of a stress/displacement law for fibers in the bridging zone of a matrix crack, thereby permitting a discussion of the crack growth resistance and its dependence on relevant microstructural variables.

Tunable elastomeric nanochannels for nanofluidic manipulation

Fluidic transport through nanochannels offers new opportunities to probe fundamental nanoscale transport phenomena and to develop tools for manipulating DNA, proteins, small molecules and nanoparticles. The small size of nanofabricated devices and the accompanying increase in the effect of surface forces, however, pose challenges in designing and fabricating flexible nanofluidic systems that can dynamically adjust their transport characteristics according to the handling needs of various molecules and nanoparticles. Here, we describe the use of nanoscale fracturing of oxidized poly(dimethylsiloxane) to conveniently fabricate nanofluidic systems with arrays of nanochannels that can actively manipulate nanofluidic transport through dynamic modulation of the channel cross-section. We present the design parameters for engineering material properties and channel geometry to achieve reversible nanochannel deformation using remarkably small forces. We demonstrate the versatility of the elastomeric nanochannels through tuneable sieving and trapping of nanoparticles, dynamic manipulation of the conformation of single DNA molecules and in situ photofabrication of movable polymeric nanostructures.

Mixed-mode fracture analyses of plastically-deforming adhesive joints

A mode-dependent embedded-process-zone (EPZ) model has been developed and used to simulate the mixed-mode fracture of plastically deforming adhesive joints. Mode-I and mode-II fracture parameters obtained from previous work have been combined with a mixed-mode failure criterion to provide quantitative predictions of the deformation and fracture of mixed-mode geometries. These numerical calculations have been shown to provide excellent quantitative predictions for two geometries that undergo large-scale plastic deformation: asymmetric T-peel specimens and single lap-shear joints. Details of the deformed shapes, loads, displacements and crack propagation have all been captured reasonably well by the calculations.

Growth and configurational stability of circular, buckling-driven film delaminations

Abstract A study is presented of delamination at the interface between a thin elastic film bonded to a substrate under conditions in which the film is subject to equi-biaxial compression. The focus is on initially circular delaminations. When the initial delamination is sufficiently large it buckles away from the substrate producing a blister which in turn induces a driving force on the interface crack tip. A two-part theoretical analysis of the coupled buckling/fracture problem is conducted: the axisymmetric growth of the circular blister, and the stability of the circular blister to nonaxisymmetric perturbations of the interface crack front. A simple criterion is identified which excludes the possibility of wide-spread delamination. Experiments are reported for a model film/substrate system (mica bonded to aluminum) chosen to allow visualization of the interface and to permit compressive stresses in the film to be generated over the full range of interest by exploiting the large thermal expansion mismatch of the system. The experiments bear out the theoretical prediction of a regime of axisymmetric growth which gives way to nonaxisymmetric blisters after a blister becomes sufficiently large. The study suggests that the wavy-circular and worm-like blister morphologies which are usually observed for delaminated films are a manifestation of the configurational instability of the interface crack front.

The edge cracking and spalling of brittle plates

Abstract The cracking and spalling processes that accompany the edge loading of brittle plates have been investigated. Experiments performed on glass and on PMMA have revealed systematic trends in crack location, crack propagation load, and in the onset of spalling. In particular, a steady state crack growth region has been identified wherein the cracks propagate parallel to the side surface. Calculations of mode I and mode II stress intensities have allowed comparison of the crack trajectories and crack propagation loads with experimental measurements. The general trends in cracking were found to be broadly consistent with predicted behavior governed by a zero mode II criterion and assuming that the cracks grow into a steady state trajectory. However, some quantitative discrepancies exist. These have been attributed to constraining tractions that develop upon distortion of the test specimens.

Numerical simulations of adhesively-bonded beams failing with extensive plastic deformation

Abstract An embedded-process-zone (EPZ) model was used to study the coupling between fracture of the interface and plastic deformation of the adherends in an adhesively-bonded joint. In this model, it was assumed that the primary role of the adhesive layer is to provide a traction-separation law for the interface. A series of experiments were performed in which thin, adhesively-bonded, symmetrical, double-cantilever beams made of an aluminum alloy were split by inserting different sizes of wedges along the interface. The parameters for the interfacial traction-separation law were determined by comparing the results of these experiments with numerical simulations using the EPZ model. It was found that once these parameters had been established for one thickness of specimen, the EPZ model could be used without further modification to predict the effect of the wedges on specimens made with different thicknesses of aluminum. These predictions showed excellent agreement with experimental observations. A subsequent series of tests involved monitoring the load, displacement and deformed shapes of a series of T-peel specimens made with the same combination of adhesives and adherends. Without changing any of the parameters determined from the wedge tests, the EPZ model gave excellent quantitative predictions for the results of these T-peel tests.

Stress development and relaxation in copper films during thermal cycling

The reliability of integrated-circuit wiring depends strongly on the development and relaxation of stresses that promote void and hillock formation. In this paper an analysis based on existing models of creep is presented that predicts the stresses developed in thin blanket films of copper on Si wafers subjected to thermal cycling. The results are portrayed on deformation-mechanism maps that identify the dominant mechanisms expected to operate during thermal cycling. These predictions are compared with temperature-ramped and isothermal stress measurements for a 1 μm-thick sputtered Cu film in the temperature range 25–450 °C. The models successfully predict both the rate of stress relaxation when the film is held at a constant temperature and the stress-temperature hysteresis generated during thermal cycling. For 1 μm-thick Cu films cycled in the temperature range 25–450 °C, the deformation maps indicate that grain-boundary diffusion controls the stress relief at higher temperatures (>300 °C) when only a low stress can be sustained in the films, power-law creep is important at intermediate temperatures and determines the maximum compressive stress, and that if yield by dislocation glide (low-temperature plasticity) occurs, it will do so only at the lowest temperatures (

The decohesion of thin films from brittle substrates

Abstract Experimental observations have been made concerning the decohesion of Cr films from glass substrates. The observations reveal that the films first split, the cracks then extend into the substrate and eventually acquire a steady-state trajectory parallel to the interface. A comparison of fracture mesurements with mechanics solutions for substrate cracks affirms a prior postulate that steady-state cracks grow along the plane for which KII = 0. Evaluations of KI, on that plane, deduced from measurements of crack velocities (in a controlled humidity environment) are also in good agreement with predicted values.

On the decohesion of residually stressed thin films

Abstract The propagation of cracks in a brittle substrate, as motivated by residual stress in the film, is analyzed. The results are used to predict trends in film decohesion with film thickness, residual stress, elastic properties and substrate toughness. The analysis is based on separate determinations of the strain energy release rate and mode I/modeII stress intensities for substrate cracks parallel to the interface. Experiments are performed on a system consisting of thin films bonded to SiO 2 substrates. Comparison between theory and experiment provides knowledge of the crack growth criterion and of trends in decohesion resistance.

The effects of geometry and material properties on the fracture of single lap-shear joints

A review of the mechanics of lap-shear joints is followed by a detailed analysis of the problem using a cohesive-zone approach. The cohesive-zone model allows not only the influence of geometry to be considered, but also allows the cohesive properties of the interface and plastic deformation of the adherends to be included in the analysis. The first part of the paper examines the strength of elastic joints, with an emphasis on the effects of geometry, the cohesive strength of the adhesive, and mode-mixedness. The cohesive-zone models show a transition to the predictions of linear-elastic fracture mechanics under conditions where these are expected to apply. The second part of the paper examines the effect of plasticity in the adherends, and looks at the transition between the elastic and plastic regimes. The final part of the paper makes comparisons between the predictions of the numerical calculations and experimental observations for a model system consisting of a commercial adhesive used to bond an aluminum alloy. Using cohesive-zone parameters previously determined for this particular combination of materials, the numerical predictions show excellent agreement with the experimental observations.