Micromechanics of crack nucleation during indentations

An analysis for the nucleation of microcracks from the inhomogeneous flow lines in soda-lime glass under Vickers indentations is considered. The minimum loads for crack nucleation are shown to depend on the hardness,H, and the critical stress intensity factor,K _IC. Unlike the Lawn and Evans analysis, the present model does not require the presence of any fortuitous flaws of critical dimensions in the material, since the flaws are nucleated by the deformation in the deformed zone.     

Transient effects during rapid crack propagation

An analysis is described for rapid crack propagation in an axially loaded strip and for a driven wedge. Each has a dynamic steady state, but this is usually not achieved because the initial boundary conditions of crack length and speed are violated. It is shown that the induced transient behaviour may be described using a perturbation analysis. In the strip case there is a nonlinear oscillating solution, while for the wedge a damped harmonic motion results. This revised version was published online in July 2006 with corrections to the Cover Date.     

Acoustic energy during pop-in crack propagation

The acoustic energy emitted during a brittle multiple pop-in fracture process in DCB specimens has been measured and its values have been correlated with the mechanical parameters. An approximate formulation has also been developed in order to explain the behaviour of the emitted acoustic energy to the temperature.     

Dynamic crack nucleation and propagation in polycrystalline aluminum aggregates subjected to large inelastic deformations

The major objective of this work has been to apply a new compatibility-based fracture theory to the investigation of dynamic failure of polycrystalline metals and alloys. To model the nucleation and propagation of failure surfaces at the microstructural scale, under large deformations and dynamic loading conditions, a general fracture criterion based on the integral law of compatibility is used. This new fracture criterion, was coupled with rate-dependent dislocation-density based crystalline plasticity formulations to elucidate the microstructural mechanisms related to the evolution of intergranular and transgranular failure and to understand how grain sizes and strain-rate sensitivity affect aggregate strength, ductility, and dynamic damage tolerance. It is shown that cracks commonly nucleate at triple junctions and at grain boundaries as intergranular cracks, and that slip bands through grains result in transgranular cracks.     

Dislocation nucleation and propagation during thin film deposition under compression

In this paper, we study the nucleation of dislocations and their subsequent propagation, during thin film deposition, using the three-dimensional (3D) molecular dynamics (MD) method. Aiming to reveal the generic mechanisms, the case of tungsten on a substrate of the same material is investigated. The substrate is under uniaxial compression along the [1 1 1] direction, with the thermodynamically favored surface being horizontal. The simulation results indicate that the nucleation starts with a surface step where an atom is squeezed to the layer above, generating a half-dislocation loop at the surface. It may then either propagate into the film or become the bottom of a sessile dislocation loop. In the first case, the dislocation loop, having a Burgers vector on a (1 0 1) glide plane, propagates along the direction on the surface, and extends to about two atomic layers along the [1 1 1] direction. In the second case, the missing layer propagates along the [1 0 0] direction on the surface, extending to about four atomic layers along the [1 1 1] direction. In this case, the sessile dislocation has a Burgers vector on the plane (0 1 1).     

Secondary cracking process during fatigue crack propagation

Using the scanning electron microscope, fatigue striations in secondary microcracks normal to the main fatigue cracks have been observed on fracture surfaces of fatigue specimens made from 2024 and 2618 aluminium alloys, a Ti-6A1-V titanium alloy, and an Incolloy 901 nickel-base refractory alloy. Tentative explanations of these observations are presented in the paper together with a short discussion.     

Analysis of crack development during processing of laminated ceramic tubes

Models for the strain energy release rate in crack extension and tunnelling are taken from the literature with minor modification to take account of the compressive stresses in multi-layer ceramic tubes or composites. These models are applied to predict the potential for crack development in co-extruded tubes fabricated from combinations of alumina, zirconia (fully and partially stabilized) and zirconia toughened alumina. Experimental results show that the models predict the failure in the composites closely and thus form useful tools in the design of such structures. The main aspect of the predictive capacity is in the selection of the layer numbers and the layer thicknesses to prevent failure. The significance of crack-free layers is improved mechanical properties, in particular strength, and potential improvement in fracture toughness and device performance.     

A three-dimensional graphical aid to analyze fatigue crack nucleation and propagation phases under fatigue limit conditions

A three-dimensional diagram is presented in which the fatigue limit of notched components is plotted as a function of the notch stress concentration factor K _t and the ² a/a ₀ ratio, , a and a ₀ being a shape factor, the notch depth and the El Haddad–Smith–Topper length parameter, respectively. Intersections with the planes normal to the axes allow the display of the different influence of crack nucleation and propagation on the fatigue limit of notched components.     

Analysis of crack propagation during tooth interior fatigue fracture

Tooth interior fatigue fracture is a failure mode that is initiated as a fatigue crack in the interior of the tooth of a gear. TIFF cracks have been observed in case hardened idler gears. A fracture mechanical analysis of a TIFF crack is performed utilising FEA. A 3D TIFF crack is modelled at a position in the tooth that corresponds with an observed crack surface. The different material properties in the case and the core, determined by mechanical testing, are considered, as well as the residual state of stress due to case hardening. Various crack lengths are analysed to estimate crack propagation both into the core and into the case. The following results have been found:

• A TIFF crack initiated slightly under the case layer will propagate into the case layer where it stops.

• The main crack propagation will take place in the core.

• The crack propagation is only a small portion of the total life (order of 10⁵ cycles).

• After reaching the case layer the TIFF crack eventually deflects toward the tooth root and the upper part of the tooth falls off. The crack deflection is due to redistribution of contact loading. Several gear teeth pairs are simultaneously in contact and the cracked tooth is loaded less than the uncracked during this stage of life.

Energy loss in Homalite 100 during crack propagation and arrest

Dynamic photoelastic experiments were conducted with a modified compact tension specimen to study energy loss in Homalite 100 during the fracture process. The method of analysis utilizes an energy balance in the system. The results showed that approximately 40% of the initial strain energy was lost due to damping during propagation and after arrest.

Total energy loss was then partitioned into energy loss during propagation and energy loss after arrest by studying oscillations in the stress intensity factor in the post arrest period. Results indicated almost equal energy loss during propagation and after arrest.     

Effects of imperfections on localized bulging in inflated membrane tubes

The problem of localized bulging in inflated membrane tubes shares the same features with a variety of other localization problems such as formation of kink bands in fibre-reinforced composites and layered structures. This type of localization is known to be very sensitive to imperfections, but the precise nature of such sensitivity has not so far been quantified. In this paper, we study effects of localized wall thinning/thickening on the onset of localized bulging in inflated membrane tubes as a prototypical example. It is shown that localized wall thinning may reduce the critical pressure or circumferential stretch by an amount of the order of the square root of maximum wall thickness reduction. As a typical example, a 10 per cent maximum wall thinning may reduce the critical circumferential stretch by 19 per cent. This square root law complements the well-known Koiter's two-thirds power law for subcritical periodic bifurcations. The relevance of our results to mathematical modelling of aneurysm formation in human arteries is also discussed.     

The character of crack propagation during fatigue failure of polymethylmethacrylate

Photographs of the surface of specimens fatigued to fracture in bending at various stress amplitudes were taken. A similarity in the character of the formation and propagation of cracks in material deformed in bending and in tension was observed. It was shown that the crack size increases with rising temperature and with increasing loading time.     

Process region influence on energy release rate and crack tip velocity during rapid crack propagation

A finite element model of a plate with an edge crack is investigated. A cell model of the material, with the cell size representing some characteristic intrinsic material length, is adopted. The size of the process region depends on the number of cells that have reached a state which is unstable at load control. The results show that the growth of the process region is a main factor responsible for the lack of a unique relation between the small scale yielding energy release rate and the crack tip velocity and also for the observed constant crack velocities that are significantly below the Rayleigh wave velocity. A rapidly propagating crack appears to meet an increase of the energy flow to the crack edge per unit of time by increasing the size of the process region rather than increasing its edge velocity.     

Mechanisms of crack nucleation in ice

This paper reviews the literature on the mechanisms by which cracks are nucleated in polycrystalline ice. In the absence of preexisting cracks, crack nucleation is the first step in mechanical failure. A variety of mechanisms have been discussed, mostly involving the propagation of microcracks or precursors from initial sizes below the level of convenient detection to easily observable cracks. True crack nucleation without preexisting precursors requires that stresses be locally concentrated to levels matching the theoretical cleavage or cohesive strength. Candidate mechanisms for this concentration of stress include: dislocation glide (leading to pile-ups of dislocations on particular slip planes); grain-boundary-sliding (leading to stress concentrations at the triple junctions at the edge of grain-boundary facets); thermal expansion of extraneous inclusions (such as produced by progressive freezing of brine pockets upon cooling); and elastic anisotropy of the ice crystals. Both dislocation glide and boundary sliding are kinetic processes in which the stress redistribution occurs at a finite, temperature-dependent rate. These processes also contribute temperature-dependent internal friction and anelasticity and their operation can therefore be independently measured. Elastic anisotropy leads to stress concentrations in an athermal manner and therefore becomes more important at very low temperatures and high loading rates. It has been shown, however, that in the absence of stresses due to brine pockets, the inherent elastic anisotropy of ice is not sufficient to nucleate cracks in purely two-dimensional models, such as perfectly columnar grains in plane strain. Calculated nucleation energies are far beyond available thermal activation energies and some additional stress concentrating effects are required. Irregularities in the third dimension (e.g. jogs in column boundaries) have not been completely investigated. Although inclusions (e.g. brine pockets or particulate inclusions) may sometimes be important, we should not neglect the kinetic processes mentioned above. Various experiments on fresh-water ice have provided evidence that crack nucleation is usually associated with grain boundaries in a manner consistent with grain-boundary sliding and the associated stress concentration fields.