We follow the accumulation of microscopic damage ahead the crack tip in paper. The fiber debonding process varies even within each specimen because of large variation in fiber and bond properties. In general, stiff and weakly bonded fibers tend to debond as a rigid body while ductile or well bonded fibers pull out gradually in a process that propagates from the crack line to the fiber ends. Particularly in the latter case the network ruptures coherently rather than through debonding of single fibers. Experimental analysis and simulations show that fracture energy correlates closely with the size of the fracture process zone (FPZ) irrespective the nature of the debonding process. Only the cases of low bonding and stiff fibers seem to make an exception in that FPZ can grow in size without a corresponding increase in fracture energy.

An investigation of the brittle-to-ductile transition (BDT) in silicon has been conducted on essentially dislocation-free silicon test specimens made by photolithography. No pre-cracks or additional dislocation sources were introduced into the samples. Three-point bending tests of the samples reveals a well defined transition from brittle fracture of the specimens to complete yielding near 732°C at a crosshead displacement rate of 0.1 mm/min. Limited plasticity is observed below 732°C but is insufficient to prevent crack propagation suggesting that yielding is not dislocation mobility limited. Instead the transition may be controlled by the nucleation of a sufficient density of dislocations. Further support comes from the results of experiments conducted at temperatures below 732°C in which samples were rapidly pre-loaded within the linearly elastic regime, then immediately retested. This rapid pre-loading results in a lower transition temperature. This would not be expected if dislocation mobility controlled the BDT. Instead, it is believed that the transition only occurs when a sufficient density of dislocations has nucleated within the sample. In these experiments, the pre-loading event may increase the dislocation nucleation rate. The source of the dislocations in these defect free samples is still under investigation.

Mode I cracks introduced in Si at the ductile-brittle transition temperature (DBTT) have been examined extensively using transmission electron microscopy. Cross-sectional as well as plane-view specimens suitable for the observation were prepared using a focused ion beam technique. Many small dislocation loops nucleate at the fracture surface of a mode I crack during the propagation at DBTT.

The temperature dependence of unstable stacking fault free energies on glide and shuffle {111} planes in silicon is investigated using a finite temperature molecular dynamics approach which includes a full treatment of anharmonic vibrational effects. The results are compared to earlier zero temperature ab initio calculations in which finite temperature effects were estimated using a harmonic approximation to transition state theory (TST). The unstable stacking free energies are interpreted within the framework of Rice‘s dislocation nucleation criterium to characterize a possible change from shuffle to glide plane dominance in the context of dislocation nucleation processes at a sharp crack tip. Such a change may be related to the abrupt brittle-ductile transition observed in silicon.

Tensile tests on notched plates of single-crystalline silicon were carried out at high overloads. Cracks were forced to propagate on {110} planes in a <110> direction. The dynamics of the fracture process was measured using the potential drop technique and correlated with the fracture surface morphology. Crack propagation velocity did not exceed a terminal velocity of v = 3800 m/s, which corresponds to 83%7 of the Rayleigh wave velocity vR. Specimens fractured at low stresses exhibited crystallographic cleavage whereas a transition from mirror-like smooth regions to rougher hackle zones was observed in case of the specimens fractured at high stresses. Inspection of the mirror zone at high magnification revealed a deviation of the {110} plane onto {111} crystallographic facets.

Sol-gel derived thin nano-ceramic layers of TiO2 and Al2O3 are studied using scanning electron microscopy to reveal the microstructure and morphologies of the layers. The low-voltage scanning electron microscope with a field emission gun is equipped with an especially designed lens, where the specimen is placed at the location where the magnetic field is the largest. In such a way a maximum resolving power could be attained of 1.5 nm at 3kV accelerating voltage. The melt-spun layers were treated differently afterwards, i.e. by furnace and by laser curing. These heat treatments appeared to dictate the final morphologies of the layers to a large extent. Grain growth is observed for the furnace as well as the laser cured layers. The activation energy for grain growth of these layers is determined. Homogeneous dense layers may be obtained if the parameters in the curing process are selected adequately. If the parameters are chosen incorrectly, severely debonded layers may be obtained. Pre-heating the layers resulted in less blister formation. The mechanisms which may cause the layers to fail were examined in more detail.

In the present work, we analyze the behaviour of graphite-reinforced composite materials under periodic loading. We reconstruct the phase space of these physical systems by means of the tools of nonlinear system theory. Experimental measurements are used to calculate chaotic parameters as fractal and embedding dimensions. We use these quantities to characterize differences in the physical samples and modifications in their internal structure. This analysis can provide useful indications about the fracture process.

The long distance roughness of fatigue fracture surfaces of a nickel-based superalloy is reported for two samples of different grain size. Statistical analysis over a wide range of length scales, from a few nanometers to a few millimeters, using scanning electron microscopy and atomic force microscopy allows to obtain accurately the self-affine correlation length. Long distance fracture profiles of 14,000 points were obtained and digitized from overlapping electron micrographs at a resolution of 0.22 micrometers/point. We have also analyzed the long distance roughness of the mirror zone on a soda-lime glass using atomic force microscopy. In the case of the nickel superalloy, correlation lengths are found to correspond well to the grain size. This result gives information about the mechanism of crack propagation in heterogeneous materials and shows that the correlation length of fracture surfaces is of the order of the largest microstructural heterogeneity.

We use molecular dynamics simulation to investigate the evolution of a crack front in interfacial fracture in three dimensions. We find that when a crack passes through a localized region of heterogeneous toughness, crack front waves are initiated and propagate laterally. We also investigate the development of roughness of the crack front when the crack propagates in a region of heterogeneous toughness. We find that in steady state the mean square width W of the front scales with system size L as W∼L0.35, in agreement with recent theoretical predictions.

We have numerically studied various models for crack formation in the slow dynamical regime. Two different aspects are presented herein: (i) sputtering with reconstruction and (ii) internal destruction of a porous medium through the invasion of a secondary phase.

High-purity tantalum single crystal cylinders oriented with [011] parallel to the cylinder axis were deformed 10, 20, and 30 percent in compression. The engineering stress-strain curve exhibited an up-turn at strains greater than ∼20% while the samples took on an ellipsoidal shape during testing, elongated along the [100] direction with almost no dimensional change along [011]. Two orthogonal planes were selected for characterization using Orientation Imaging Microscopy (OIM): one plane containing [100] and [011] (longitudinal) and the other in the plane containing [011] and [011] (transverse). OIM revealed patterns of alternating crystal rotations that develop as a function of strain and exhibit evolving length scales. The spacing and magnitude of these alternating misorientations increases in number density and decreases in spacing with increasing strain. Classical crystal plasticity calculations were performed to simulate the effects of compression deformation with and without the presence of friction. The calculated stressstrain response, local lattice reorientations, and specimen shape are compared with experiment.

Discrete dislocation dynamics simulations have been performed to study dislocation activity in the vicinity of a crack tip and to get a better understanding of the decisive mechanisms of the brittle-to-ductile transition (BDT). The comparison of these simulations with fracture experiments on tungsten single crystals leads to a preliminary model for the BDT of this material. Many features predicted by this model are also found in other materials. Dislocation nucleation and the availability of active sources are shown to be limiting plasticity at low temperatures and partly in the semi-brittle regime. At elevated temperatures dislocation nucleation occurs easily, such that plasticity in this regime and the BDT itself, must be viewed as a thermally activated processes, which are controlled by dislocation mobility.

Mechanical properties of copper and aluminum have been studied using finite temperature molecular dynamics simulations. Atomic interactions have been described by a many-atom effective medium potential, which takes into account interactions up to third neighbors. The computed elastic constants showed good agreement with experimental data. Encouraged by these results the model was applied to study fracture in copper. Systems with a grain boundary and an initial cut serving as a crack seed have been studied. In the first case, crack nucleation and propagation took place exclusively at the grain boundary. In the second case, dislocation propagation was observed in one of the <110> directions, with a speed of about 60% of the longitudinal speed of sound. For thin systems crack propagation occurred through micro-void coalescence with a speed of about 30% of the Rayleigh wave speed in copper.

The incremental in-plane Green-Lagrange strain tensor was measured near a stationary crack tip in a cyclically work-hardened copper single crystal. Measurements were made on the surface of a four-point bend specimen, using a finite-deformation laser moiré interferometer. The measurement showed the existence of a narrow asymptotic field beyond a distance of 300 μm from the crack tip. The inner boundary of the asymptotic zone was almost fixed at a characteristic distance ahead of the crack tip. This length scale is thought to arise from a microstructural evolution near the crack tip. The inhomogeneous hardening due to glide-band clustering and patchy slip in a small volume near the crack tip triggered such an evolution. The outer boundary of the asymptotic zone radially grew with the increasing load. The deformation field was found to be very sensitive to additional mode II loading.

The dynamic fracture (spallation) of ductile metals is known to initiate through the nucleation and growth of microscopic voids. Here, we apply atomistic molecular dynamics modeling to the early growth of nanoscale (2nm radius) voids in face centered cubic metals using embedded atom potential models. The voids grow through anisotropic dislocation nucleation and emission into a cuboidal shape in agreement with experiment. The mechanism of this nucleation process is presented. The resulting viscous growth exponent at late times is about three times larger than expected from experiment for microscale voids, suggesting either a length scale dependence or a inadequacy of the molecular dynamics model such as the perfect crystal surrounding the void.

The mechanism of low macroscopic-plastic-strain, ductile fractures under various high triaxial stresses in constrained thin silver films was investigated. Particular emphasis was placed on investigating ductile fracture by unstable cavity growth. The various multi-axial loads to failure were experimentally measured. FEA analysis was used to determine the corresponding high triaxial stress-states within the interlayer, also considering cavity-cavity interaction, residual stresses and elastic incompatibility stresses across interfaces. These were compared to the stresses required for cavity instability. Ductile fracture under high triaxial stresses, associated with low macroscopic strains, appears to be explained by unstable cavity growth, where cavity growth may occur without large macroscopic (e.g., < 0.05) plastic strains.

In this work a study of the nature as well as an evalution of the thermal-mechanical stress in aluminum interconnects was carried out. A theoretical model discribes the atom flux which can be induced by the relaxation of the stress. Based on this theory an algorithm has been developed and integrated into the finite element simulation software. This algorithm allows the calculation of the mass flux divergence and prediction of the failure location before the damage occurs. For the verification of this algorithm an aluminum pad structure sputtered on thermal oxide layer was used. The failure location was correlated with in situ observation during the long term stress tests. Experimental results confirm that the observed structure degradations correspond with the simulations very well.

Numerous mechanisms have been identified as fundamental to the adhesion of thin metallic films. The primary mechanism is the thermodynamic work of adhesion of the interface, which in its most basic description is the difference between the surface energies of the two materials and that of the interface. This quantity is often described as leveraging the contributions of other mechanisms. One of the more important mechanisms is that of plasticity occurring in a process zone in the vicinity of the delamination boundary. A quantitative model to characterize the contributions of plastic energy dissipation has been developed and used to rationalize experimental adhesion assessments. This model incorporates the functional dependence of the film thickness and constitutive properties. Orders of magnitude increases in the practical work of adhesion were both observed and predicted. Experimentally, the films used for model comparison were sputter-deposited copper ranging from 40 to 3300 nm in thickness, with and without a thin 10 nm Ti underlayer. Nanoindentation induced delamination of the Cu from SiO2/Si wafers were evaluated in the context of composite laminate theory to determine adhesion energies ranging from 0.6 to 100 J/m2 for bare Cu and from 4 to 110 J/m2 for Cu with the Ti underlayer.

We present a study of the atomistic mechanisms of crack propagation along grain boundaries in metals and alloys. The failure behavior showing cleavage crack growth and/or crack-tip dislocation emission is demonstrated using atomistic simulations for an embedded-atom model. The simulations follow the quasi-equilibrium growth of a crack as the stress intensity applied increases. Dislocations emitted from crack tips normally blunt the crack and inhibit cleavage, inducing ductile behavior. When the emitted dislocations stay near the crack tip (sessile dislocations), they do blunt the crack but brittle cleavage can occur after the emission of a sufficient number of dislocations. The fracture process occurs as a combination of dislocation emission/micro-cleavage portions that are controlled by the local atomistic structure of the grain boundary. The grain boundary is shown to be a region where dislocation emission is easier, a mechanism that competes with the lower cohesive strength of the boundary region.

To test the hypothesis that the brittleness and ductility of crystalline materials are controlled by a competition between dislocation nucleation and cleavage failure at a crack tip, Rice et al. [1] designed a four-point bend copper/sapphire bicrystal which has two cracks that propagate along the interface in opposing directions. They predicted, on the basis of the Rice-Thomson model [2], that the specimen would exhibit a directionally dependent fracture behavior; that is, one crack would propagate more readily than the other. Beltz and Wang [3] carried out the experiments and reported that the specimen exhibits a directionally dependent fracture behavior in accordance with the predictions. In the present work, these experiments are repeated independently and it is shown that the orientation of the experimentally observed directionally dependent behavior is opposite that of the predicted orientation. Furthermore, we show that Beltz and Wang incorrectly measured the orientation of the directional dependence in their experimental results. A correct interpretation of their results is consistent with the present work.

Taking advantage of the transparent sapphire, the normal crack opening displacement (NCOD) profile of both cracks is measured with optical interferometry. The measurements show that, away from the very tip of the crack, the NCOD takes the form of a constant angle, irrespective of load level. The opening angle of the apparently brittle crack is smaller than that of the apparently ductile crack. Complementary measurements of the very tip of the crack with Atomic Force Microscopy show that the near-tip crack opening profiles of the brittle and ductile cracks differ significantly.

The specimen is analyzed with the Finite Element Method, taking into account the elastic and plastic properties of the single crystal constituents. Since the very essence of the observed phenomenon is one of crack growth, the two cracks are simulated as they grow quasistatically along the interface. The asymptotic deformation fields, characteristic of single crystals, of the growing interfacial cracks are identified. The stress and strain fields, as well as the NCOD, are calculated. We present a plausible explanation of the directional dependence of fracture on the basis of the continuum plastic fields surrounding the quasistatically growing cracks.