1. Introduction and overview 2. Film stress and substrate curvature 3. Stress in anisotropic and patterned films 4. Delamination and fracture 5. Film buckling, bulging and peeling 6. Dislocation formation in epitaxial systems 7. Dislocation interactions and strain relaxation 8. Equilibrium and stability of surfaces 9. The role of stress in mass transport.

Thin Film Materials: Stress, Defect Formation and Surface Evolution

In a numerical micromechanical study of void nucleation, a framework is used where constitutive relations are specified independently for the matrix, the void-nucleating particles and the interface. Plane strain analyses are carried out for a doubly periodic array of circular cylindrical particles. The particles are taken to be rigid and the elastic-plastic deformations of the matrix are described in terms of continuum crystalline plasticity, using a planar crystal model that allows for three slip systems. Comparison is made with void-nucleation predictions based on a corresponding flow theory of plasticity with isotropic hardening. The crystal model can give rise to shear localization at the particle-matrix interface and shear localization, which leads to large localized strains in the matrix, is found to inhibit decohesion. The role of the triaxiality of the stress state in determining whether decohesion or localization occurs first is investigated. A parameteric study is also carried out for a crystal matrix using two descriptions of the interface shear behaviour; one is periodic in the shear displacement across the interface, while the other allows for shear decohesion.

https://iopscience.iop.org/article/10.1088/0965-0393/1/2/001/pdf

Void nucleation by inclusion debonding in a crystal matrix

Calculations are reported for the mixed mode toughness of an interface joining an elastic-plastic solid to a solid which does not yield plastically. A potential function of the components of the crack face displacements is used to generate the tractions along the interface where the fracture processes causing separation occur. The two main parameters characterizing this potential are the work of separation per unit area and a peak normal stress. This description of the interface separation process is embedded within the continuum description as a boundary condition on the interface linking the adjoining solids. Small-scale yielding in plane strain is considered with the remote field specified by the magnitude and phase of the mixed mode stress intensity factors. Crack growth resistance curves are computed for a range of the most important nondimensional material parameters and for various combinations of remote mixed mode loading. Particular emphasis is placed on the ratio of the steady-state interface toughness to the “intrinsic” work of separation as it depends on plastic yielding and on the combination of modes 1 and 2. Plasticity enhances the interface toughness for all modes of loading, but substantially more so in the presence of a significant mode 2 component of loading than in near-mode 1 conditions.

The influence of plasticity on mixed mode interface toughness

Adhesion and debonding of multi-layer thin film structures

http://www.eng.usf.edu/~volinsky/VolinskyActaMatReview2002.pdf

Interfacial toughness measurements for thin films on substrates

Substantial progress has been made on the mechanics of interface fracture. An engineering program has emerged which allows the fracture resistance of interfaces to be measured and utilized. This recent development, assessed in an Acta-Scripta Metallurgica Proceedings, is discussed.

Several results have been obtained on the plasticity aspects of interface cracks in which one (or both) of the constituent materials can deform plastically. The crack tip fields are members of a family parametrized by plastic mode mixity parameter ξ. The J integral scales each member field. Analyses of finite width crack geometries loaded by remote tension show that the effects of load, ligament plasticity and geometry on the near-tip fields are adequately accounted for by the J integral. The progress is summarized.

Cracks on bimaterial interfaces: elasticity and plasticity aspects

An experimental technique whereby pure mode I, mode II, and combined mode I-mode II fracture toughness values of ceramic materials can be determined using four-point bend specimens containing sharp, through-thickness precracks is discussed. In this method, notched and fatigue-precracked specimens of brittle solids are subjected to combined mode I-mode II and pure mode II fracture under asymmetric four-point bend loading and to pure mode I under symmetric bend loading. A detailed finite element analysis of the test specimen is performed to obtain stress intensity factor calibrations for a wide range of loading states. The effectiveness of this method to provide reproducible combined mode I-mode II fracture toughness values is demonstrated with experimental results obtained for a polycrystalline Al2O3. Multiaxial fracture mechanics of the Al2O3 ceramic in combined modes I, II, and III are also described in conjunction with the recent experimental study of Suresh and Tschegg (1987). While the mode II fracture toughness of the alumina ceramic is comparable to the mode I fracture toughness KIc, the mode III fracture initiation toughness is 2.3 times higher than KIc. The predictions of fracture toughness and crack path based on various mixed-mode fracture theories are critically examined in the context of experimental observations, and possible effects of fracture abrasion on the apparent mixed-mode fracture resistance are highlighted. The significance and implications of the experimental methods used in this study are evaluated in the light of available techniques for multiaxial fracture testing of brittle solids.

Mixed-mode fracture toughness of ceramic materials

Various stages of kink band formation, propagation and band width broadening were recorded by a high resolution video camera. Based on these observations, the easiest modes of deformation have been identified and these form the basis of a new kinematic model for kinking. Theoretical predictions for kink band orientation and compression strength under steady-state band broadening are made. The conditions at incipient kinking and the incipient kinking stress are investigated. The relevance of the incipient kinking stress and the band broadening stress are discussed.

Kink band formation and band broadening in fiber composites under compressive loading

The problem of a crack approaching a bimaterial interface is considered in this paper. Attention is focused on an interface between two elastoplastic solids whose elastic properties are identical and whose plastic properties are different. For the case of a crack approaching a bimaterial interface perpendicularly, it is shown by recourse to detailed finite element analyses that the near-tip “driving force” for fracture is strongly influenced by whether the crack approaches the interfaces from the lower strength or the higher strength materials. Specifically, it is demonstrated that the crack-tip is “shielded” from the remote loads when it approaches the interface from the weaker material, and that the effective J-integral at the crack tip is greater than the remote J when it approaches the interface from the stronger material. This plasticity effect determines whether a crack approaching the bimaterial interface continues to advance through the interface or is arrested before penetrating the interface. These theoretical findings are substantiated using controlled experiments of fatigue crack growth perpendicular to a ferrite—austenite bimaterial interface. The effect of the non-singular T-stress, acting parallel to the crack plane, on shielding and amplification of the stress fields is also discussed.

Fracture normal to a bimaterial interface: Effects of plasticity on crack-tip shielding and amplification

A theory is proposed for cleavage cracking surrounded by pre-existing dislocations. Dislocations are assumed not to emit from the crack front. It is argued that the pre-existing dislocations, except for occasional interceptions with the crack front, are unlikely to blunt the major portion of the crack front, so that the crack front remains nanoscopically sharp, advancing by atomic decohesion. The fracture process therefore consists of two elements: atomic decohesion and background dislocation motion. An elastic cell, of size comparable to dislocation spacing or dislocation cell size, is postulated to surround the crack tip. This near-tip elasticity accomodates a large stress gradient, matching the nanoscopic, high cohesive strength to the macroscopic, low yield strength. Consequences of this theory are explored in the context of slow cleavage cracking, stress-assisted corrosion, fast running crack, fatigue crack growth, constraint effects, and mixed mode fracture along metal/ceramic interfaces. Computational models and experiments to ascertain the range of validity of this theory are proposed.

C.F. Shih

Papers

Cracks on bimaterial interfaces: elasticity and plasticity aspects

Mixed-mode fracture toughness of ceramic materials

Kink band formation and band broadening in fiber composites under compressive loading

Fracture normal to a bimaterial interface: Effects of plasticity on crack-tip shielding and amplification

A theory for cleavage cracking in the presence of plastic flow