Stresses induced at the GaAs-Al2O3 interface by large ΔT excursions have been investigated by finite element simulation and have been correlated with experimental results. The effects of power and temperature cycling on crack propagation at the die attach are investigated. The FEA (finite element analysis) method is used to simulate the effect of die attach voids on the peak surface temperature and on the die stresses. These voids in the die attach are identified to be the major cause of die cracking. It was found that stresses developed on the die because of the environmental temperature changes and their dissipation as part of an effective thermal management is necessary to ensure reliable performance.

A mechanics study of the interface toughness of joints comprised of ceramic substrates joined by a thin ductile metal layer is carried out for arbitrary combinations of mode I and mode II loading. The crack lies on one of the metal/ceramic interfaces, and the mechanism of separation at the crack tip is assumed to be atomic decohesion. The SSV model proposed by Suo, Shih and Varias is invoked. This model employs a very narrow elastic strip imposed between the substrate and the ductile layer to model the expected higher hardness of material subject to high strain gradients and possible dislocation-free zone in the immediate vicinity of the crack tip. The criterion for crack advance is the requirement that energy release rate at the crack tip in this narrow elastic strip be the atomistic work of fracture. The contribution of plastic dissipation in the metal layer to the total work of fracture is computed as a function of the thickness and yield strength of the layer and of the relative amount of mode II to mode I. Ductile joints display exceptionally strong thickness and mixed mode dependencies.

We have calculated the work of adhesion (i.e. energy for rigid fracture) and peak interfacial stress for NiAl, Cr, and NiAl–Cr using self-consistent density functional calculations to obtain the complete energy vs. separation curve for each system. Our calculations indicate that the work of adhesion is largest for Cr and smallest for NiAI while those for interfaces of NiAI with Cr are intermediate. We have also estimated that segregation processes could alter the work of adhesion for the AlNi/Cr interface by up to 20% since Al tends to segregate to the free NiAl surface while Ni tends to segregate to the AlNi/Cr interface.

The effects of boron and sulfur impurities on the ideal cleavage fracture properties of Ni3Al under tensile stress are investigated for the first time using the full-potential linearmuffin- tin-orbital (FLMTO) total-energy method, with a repeated slab arrangement of atoms simulating an isolated cleavage plane. Results for the stress-strain relationship, ideal cleavage energies, ideal yield stress and strains with and without impurities are presented, and the electronic mechanism underlying the contrasting effects of boron and sulfur impurities on the ideal cleavage of Ni3Al is elucidated.

A precipitation-hardened Cu-0.26%Be alloy is used as a low-temperature model material for “dynamic embrittlement”, or quasi-static diffusion-controlled intergranular brittle fracture. This alloy is shown to undergo intergranular cracking in air at 150°C and to be almost free of this cracking in 2×10−6 Torr vacuum at 200°C. The time to failure is highly stress dependent. The temperature dependence of cracking was found to be 30 kcal/mole. This is about 50% greater than the activation energy for oxygen diffusion in copper, but the present experiments also include an unknown incubation time.

We have observed that a change in the bulk sulfur content of oxygen-free electronic copper markedly affects its high temperature (400–1000°C), high strain-rate (> 103 s−1) deformation and fracture behavior. These conditions are typical of those found in "jets" formed from the explosive deformation of copper shaped-charge liners. Specifically, an increase in the bulk sulfur concentration from 4 ppm to 8 ppm shortens the breakup time, tb, of the copper jets by nearly 20% as measured using flash x-ray radiographs recorded during breakup of the jets. At bulk concentrations of 4 ppm, the jet was observed to be uniform and axisymmetric with a breakup time of 186 µs. Jet particles exhibited length-to-diameter ratios of roughly 8:1. The addition of sulfur transformed the jet breakup behavior to non-uniform, non-axisymmetric rupture and reduced the breakup time to 147 µs. The length-to-diameter ratios decreased to roughly 5:1 in the sulfurdoped samples. Previously measured sulfur solubilities and diffusivities in copper at the temperatures where this material was processed indicates nearly all of the sulfur was localized to grain boundaries. Therefore, we infer that the increase in sulfur content at grain boundaries is directly responsible for the change in breakup performance of the shaped-charge jets.

The morphological features of 2024-T3 aluminum alloy were delineated using atomic force microscopy (AFM) during separate and combined actions of corrosion and fatigue.

In-situ AFM corrosion studies in hydrochloric acid environments without mechanical deformation showed accelerated dissolution in the vicinity of second phase precipitates leading to intergranular corrosion. During fatigue in air, AFM images revealed steps along grain boundaries, as well as parallel extrusions and intrusions during the early stages of fatigue life. At later stages of mechanical deformation persistent slip bands (PSBs) were observed on the sample's surface. Cracks were observed to nucleate and propagate along PSBs. For experiments where samples were subjected to the simultaneous action of a corrosive environment and mechanical deformation, intergranular cracking was observed during the early stages of fatigue life. The corrosive environment was observed to accelerate the crack nucleation process.

The present work analyzes the effects of a passive film formed during stress corrosion cracking on ductile/brittle fracture behavior, considering the interaction of a screw dislocation with a thin film-covered mode III crack under an applied remote load. Exact solutions are derived, and the results show that the crack stress field due to the applied load is enhanced by a harder film or abated by a softer film. The critical stress intensity factor for dislocation emission from the crack tip is greatly influenced by both the stiffness and thickness of the filn. A dislocation is more easily to be emitted from the crack tip if the covered film has a shear modulus larger than that of the substrate. The opposite is also true, i.e., a softer film makes dislocation emission more difficult. Both phenomena become more significant when the film thickness is smaller.

Criteria for progressive debonding at the fiber/matrix interface with friction along the debonded interface are considered for fiber-reinforced ceramic composites. The energy-based criterion is adopted to analyze the debond length, the crack-opening displacement, and the displacement of the composite due to interfacial debonding. The analytical solutions are identical to those obtained from the mismatch-strain criterion, in which interfacial debonding is assumed to occur when the mismatch in the axial strain between the fiber and the matrix reaches a critical value. Furthermore, the mismatch-strain criterion is found to bear the same physical meaning as the strength-based criterion.

Rising fracture resistance with crack extension (R-curve response) can lead to improvements in the mechanical reliability of ceramics. To understand how microstructures influence the R-curve behavior, direct observations of crack interactions with microstructural features were conducted on SiC whisker-reinforced alumina. The contribution of the dominant toughening mechanisms to the R-curve behavior of these composites is discussed using experimental and theoretical studies.

The tensile fracture of glass-bead/epoxy composites has been studied using a large number of glass volume fractions (q) from 0 to 0.25. Absorption of water from a saturated NaCl solution was used to destroy interface adhesion. The strength of the dry composites decreased up to q = 0.15, then became constant. The strength of the wet composites showed a plateau around q = 0.12, then an abrupt drop between 0.12 and 0.15, then became nearly constant. It is possible that the abrupt drop in strength occurs at the percolation threshold. However, electrical measurements on similar composites prepared with conducting (silver-coated) beads showed that the applicable percolation threshold (qc) was ≥ 0. 16, in agreement with the observation of anomalous water uptake only for q ≥ 0.18. It is concluded that the abrupt drop in strength occurs below qc, and is due to large finite interface clusters (or rather the more planar parts of such clusters) acting as nuclei for a progressive matrix fracture process. The plateau around q = 0.12 is due to favourable fibre-like interactions of smaller interface clusters, and gives the highest normalized (wet/dry) strength of about 0.9.

The fracture of a fibre reinforced graphite peek polymer is considered when dynamic load is applied. The energy-time diagram shows various critical points during cyclic tensile loading. A quantitative analysis based on the study of the fractal dimension of the chaotic attractor in the representative phase space will allow us to individuate the physical parame ters responsible of the stability range of the material. A new simple model has been derived and successfully tested from a non-linear chaotic scheme. The present approach appears very promising for a future interdisciplinary study for evaluating the physical response of real massive materials under stress.

Mechanical tests show that fibrous monolithic ceramics can exhibit graceful failure in bending due to marked crack deflection at silicon nitride/boron nitride interfaces. Model sandwich specimens were manufactured from polycrystalline silicon nitride with interphases consisting of mixtures of silicon nitride and boron nitride. The interfacial fracture resistances of the specimens were measured, and the results were correlated to the failure behavior of fibrous monolithic ceramics. It was found that the three modes of failure observed in fibrous monoliths can be correlated with the interfacial fracture resistance. In addition, anisotropy in the properties of the interphases were found to have a profound influence on crack deflection.

Silica doped alumina/aluminate materials present a combination of high strength and high toughness not achieved before in other alumina systems, except for transformation toughened alumina. We have associated the increase in toughness to crack bridging by anisotropically grown alumina grains with concurrent interfacial debonding of these grains. A HREM study of grain boundaries and hetero-interface structures in this material shows the absence of amorphous phases at grain boundaries. Local Auger electron analysis of fractured surfaces revealed the coexistence of Si and La at the grain facets exposed by the noticeable intergranular fracture mode of this material. It is concluded that a certain and important degree of boundaries weakness is related to both, presence of Si at the interfaces and existence of alumina/aluminate hetero-interfaces.

Effect of aggregate content on fracture behaviors of concrete is studied by testing on geometrically similar three-point bend beams. The results are analyzed by using a size effect method in which the fracture behavior of concrete is characterized by two parameters, fracture energy Gf and effective fracture process zone cf. Test results showed that with increasing volume fraction of aggregate in the range 45% - 75%: (1) the modulus of elasticity of concrete decreases slightly, (2) fracture energy Gf increases, but the rate is very small; (3) the size of the fracture process zone, cf, decreases, which may be explained by changes in coarseness of grain structures defined in terms of mosaic patterns.

Free standing alumina-13% titania samples were manufactured using high power water stabilized plasma spraying. Heat treatment was performed at 1450°C for 24 hours and then at 1100°C for another 24 hours. Four point bend tests were performed on the as-sprayed and heat-treated samples in both cross section and in-plane orientations with in situ acoustic emission monitoring to monitor the cracking during the tests. Catastrophic failure with less evidence of microcracking was observed for as-sprayed samples. Energy and amplitude distributions were examined to discriminated micro- and macro-cracks. It was found that the high energy (> 100) and high amplitude (say > 60 dB) responses can be characterized as macro-cracks. Physical models are proposed to interpret the AE responses under different test conditions so that the cracking mechanisms can be better understood.

With a recently developed numerical technique, we have investigated failure processes in real fiber composites. This technique utilizes 3D lattice Green's functions to calculate load transfer from broken to unbroken fibers, and also includes the important effects of fiber/matrix sliding. It is found that the failure processes are more complex than previously thought: The fibers that break or slide are not necessarily located around the largest defect cluster. Rather, the breaking/sliding fibers accumulate, form several large clusters, coalesce and finally cause the fiber composite to fail. Only in some cases does one large defect dominate the damage evolution. Based on our observations of the irregular evolution of the damage, a simple model is developed to describe these different failure modes, and the failure criterion of the fiber composite is also discussed.

The fracture toughness of interface reinforced with dps-b-dpmma copolymer between immiscible polymers of PS and PMMA is tested by asymmetric double cantilever beam. The local deformation field at the interfacial crack tip is determined by the technique of SIEM. Normal and tangential crack opening displacements are calculated. A weak singularity is shown to exist near crack tip. Direct observation on the fracture process inside an environmental scanning electron microscope shows the large effect of mode mixity.

The composition of the fracture surfaces of a composite made up of a polycrystalline organic nonpolymeric filler and a binder composed of a copolymer was studied by XPS. Because the binder and the filler of the composite each have at least one element not in common it is possible to easily distinguish between the binder and filler by XPS. A measure of the relative amounts of binder and filler on the fracture surfaces, therefore, could be made as a function of the sample temperature, T, and the strain rate during fracture. The ratio of filler to binder, F/B, increases with decreasing T at constant strain rate and is least sensitive to strain rate at T's below TG, the quasi static glass transition T. At higher T, F/B increases with strain rate at constant T. These results indicate that as the binder becomes stronger and stiffer due to a decrease in T or an increase in strain rate more of the fracture processes take place in the filler whose properties are expected to be less sensitive to T and strain rate. These results are related to the fracture properties as observed by uniaxial compression.