Henrik Myhre Jensen

Effects of residual stresses in the blister test

Abstract An analysis of the blister test, which is used as a method for measuring the fracture toughness of interfaces, is presented. The fracture toughness has been determined as a function of the critical loads needed to cause delamination. In most thin-film composites, residual stresses are present and the effect of these is incorporated in the analysis. For isotropic materials with equibiaxial residual stresses, and a pressure or a point load applied to the film, the fundamental shape of the delamination is circular due to symmetry. Closed-form solutions for the energy-release rate and mixedness of modes have been derived for circular crack fronts. One effect of compressive inplane stresses is a possible loss of stability of the circular crack front. Numerical results for the combinations of normal loads and residual stresses which result in crack front instabilities have been obtained and compared with experimental observations.

Mixed mode interface fracture criteria

Abstract The shielding of the full effect of mode II and III for interface cracks in brittle materials is demonstrated to be very important such that proper interface fracture criteria need to be modelled. A method for comparison of different phenomenological fracture criteria is discussed and applied to an experiment for a polyimide/glass interface. Finally, micromechanical models describing shielding for interface geometries with kinks are set up, allowing the phenomenological criteria to be linked up with measurable quantities such as the interface roughness.

Determination of size distributions in nanosized powders by TEM, XRD, and SAXS

Crystallite size distributions and particle size distributions were determined by transmissions electron microscopy (TEM), X-ray powder diffraction (XRD), and small-angle X-ray scattering (SAXS) for three commercially available TiO2 powders (P25, UV100, and TiO2_5u2009nm) and one SSEC produced powder (SSEC78). The theoretical Guinier model was fitted to the experimental obtained XRD data and compared to analytical expressions. Modeling of the XRD spectra showed a difference between the analytical size dependent expressions and the theoretical Guinier model. Primary particle size distributions were extracted from SAXS measurements by the hard sphere model including an interparticle interference factor. The sizes obtained from SAXS were smaller than the sizes obtained from the XRD experiments; however, a good agreement was obtained between the two techniques. Electron microscopy confirmed the primary particle sizes and the shapes obtained by XRD and SAXS. The SSEC78 powder and the commercially available powders showed different morphologies, but SSEC78, UV100, and TiO2_5u2009nm all consisted of both primary particles as well as a secondary structure comprised of nanosized primary particles agglomeration into larger clusters. P25 showed the largest primary particle size, but did not show a secondary structure.

Energy release rates and stability of straight-sided, thin-film delaminations

Abstract Straight-sided delamination of a thin, elastic film from an elastic substrate in analyzed. The loadings causing delamination are general in-plane stresses and a normal pressure on the film. Delamination is modelled as a crack propagation at the film/substrate interface and closed form expressions have been derived for the energy release rate and its separation into mode 1 and mode 2. The range of validity of these results is investigated by performing a stability analysis for the delamination shape considered. The results, which are related to both buckling delamination and blister tests, show strong dependence of the stability behaviour on the in-plane loading.

Crack initiation and growth in brittle bonds

A fracture mechanics based analysis of the strength of a bond between a thin plate and a substrate is presented. The bond edge is regarded as a crack front, which is loaded under combined mode I, II and III conditions. A numerical procedure is used to study crack initiation and growth. Results for bond strength are presented and effects of free plate edges and closely spaced bonds are studied. Crack growth at the interface is simulated by assuming a growth rate criterion, which is verified by ensuring that the fracture criterion is satisfied locally along the growing front. Experimental results are obtained and show good qualitative agreement with predictions.

Three-dimensional numerical investigation of brittle bond fracture

A fracture mechanics based analysis of interface bond failure is presented. The bond edge is regarded as an interface crack front loaded under combined mode 1, 2 and 3 loading, and results are obtained for the critical stress for initiation of bond failure and the location along the bond edge where failure is initiated. A numerical procedure is formulated to study the propagation of the interface crack following initiation. Assuming that the crack propagates at the interface, a criterion for propagation is formulated, and it is shown that the crack front shape predicted is consistent with the basic interface fracture mechanics assuming quasi-static crack propagation. Results for the bond strength are presented for different fracture criteria and different bond shapes.

Mechanics of Thin Film Structures

Cracking of thin films on substrates due to indentation is discussed. Previous results have been reviewed leading to a relationship between crack pattern and material parameters of the film. Interface fracture under steady-state conditions between thin films and substrates at corners is discussed. Crack front shapes are shown and it is shown that this mode of delamination is more critical than the plane strain edge crack. Possible explanations for the so-called telephone cord mode of buckling driven delamination are given.