Measurement of Crack Tip Displacement Fields in Mixed Mode I–Mode III Fracture

A new method has been devised to propagate stable subcritical mixed mode I–mode III cracks in soda–lime–silicate and borosilicate glasses. The mode I content was measured using an original optical tunneling technique and mode III content via multiple-beam interferometry. Measurement of the mode III displacement field allows values of mode III fracture toughness, K_III, to be derived. K_III vs crack velocity (v) diagrams have been constructed for both glasses, and, in the case of the soda–lime–silicate glass, v vs K_III relationships have been determined for a variety of humidities. Comparison has been made with published K_I vs v data for this glass.     

Direct comparison of the loss of load transfer to “E” and “S” glass fibres manufactured by drawing into different environments

Accelerated water uptake tests have been used to compare the onsets of destruction of the ability to transfer shear stress at fibre/matrix interfaces in epoxy matrix glass reinforced plastic (GRP) manufactured with each of four different fibres. The ability to transfer shear stress has been monitored directly by measurement of stress birefringence through and adjacent to individual fibres. Full theoretical and practical details of the experimental method are given. S glass fibres, drawn into an atmosphere of ammonia in an attempt to promote the deposition of primary amines and/or secondary amines, rapidly lose their ability to receive shear stress from the matrix. This is attributed to neutralization of CO₂ by NH₃ within interfacial pockets of dissolved water, and the associated generation of osmotic pressure. Commercial samples of S and E glass fibres and E glass fibres drawn into an atmosphere of ammonia, all survive much larger water uptakes although, in the case of both kinds of E glass fibre, immersion in boiling water eventually gives rise to interfacial pressure pockets. These pressure pockets are also attributed to osmosis, with the role of dissolved solutes tentatively ascribed to the modifying agents present in E glass formulations.     

Interferometric measurement of inhomogeneous swelling

Differential thermal contraction between matrix and fiber materials in composites and between adhesive and adherend materials in adhesive joints occurs during cooling from the resin cure temperature and gives rise to non-uniform internal stress fields. Resin swelling associated with water uptake during service gives rise to inhomogeneous stress, the magnitude and distribution of which is the subject of the present investigation. Thin layers of cured epoxy resin have been allowed to absorb water at their edges only, and the swelling normal to these layer planes has been measured by studying optical interference patterns. The stresses resulting from inhomogeneous swelling, a few tens of bars, are of similar magnitude to those resulting from inhomogeneous shrinkage during cooling from resin cure temperatures.     

High Resolution Optical Interference Measurements of Changes in Bond-Line Thickness—A Review

A novel moire optical interference method, originally developed¹ to measure the swelling displacement normal to the plane of a simple butt joint during water uptake at the joint edge, was subsequently used to investigate the unexpected occurrence of shrinkage and, at the same time, swelling during exposure to organic solvents. In later experiments, synergistic displacement fields generated during simultaneous exposure to water and organic solvents were studied. The same method has also been used to investigate the origin of adhesion failures at copper plated-through holes in thermally cycled multi-layer circuit boards.

The method employs a thin glass cover slip as one adherend. Changes in bondline thickness flex the cover slip thereby changing the gap between the latter and a nearby optical flat. By using monochromatic light to illuminate the specimen, interference between incident and reflected light is achieved. The small displacements of individual fringes associated with changes in gap size are used to generate moire patterns which bear a 1:1 relationship to the local changes in bondline thickness. Application of thin plate elasticity theory to the flexing of the cover slip yields the stress acting normal to the joint. The magnitude of this normal stress is strongly inhomogeneous, oscillating in sign and reaching compressive values as high as 50MNm^?2 for water uptake by a simple butt joint, and exceeding the yield strength of oxygen-free high conductivity copper (1MNm^?2) for the thermal expansion that accompanies simulated soldering of a multi-layer circuit board.

Attention is drawn to the high resolution achievable with the method. Changes in bond-line thickness of the order of Δ/10, where Δ is the wavelength of the light giving rise to the interference pattern, are readily resolved. It is pointed out that the method may well be sufficiently sensitive to detect the effects of different physical properties (thermal conductivity, specific heat capacity) of adherend surface on the nature of the cured adhesive and hence on the mechanical behaviour of the bond-line.     

Powder metallurgy by bidimensional compression

By subjecting encapsulated aluminum and aluminum alloy powders to hot bidimensional compression, the time needed to fabricate monolithic material can be reduced to seconds. The high shear stresses generated by bidimensional compression lead to high frictional forces between neighboring powder particles. These forces foster particle surface abrasion and cleaning and particle joining by time-independent mechanisms that rely on the generation locally of high temperature, frictional heat. Porosity and mechanical property measurements show that the compacts can be of near-theoretical density and of sound tensile strength and impact resistance. Formerly Ivan Racheff Chair of Excellence, Department of Materials Science and Engineering, University of Tennessee     

Internal Stresses Adjacent to a Hole Through a Multilayer Wiring Board

The normal displacement field close to copper plated-through holes (PTH) in multilayer laminated circuit boards that is generated during thermal expansion is strongly inhomogeneous. By adhering a thin, and therefore flexible, microscope cover slip to the top surface of the laminate, Fizeau optical interference fringes can be generated between the cover slip and an adjacent optical flat and used to map the displacement field; the precise distribution of these fringes changes when the distance between the two is changed by thermally expanding or contracting the laminate. Thin plate elasticity theory applied to the cover slip permits the distribution of normal stress in the laminate to be estimated. Moving radially outwards from the centre of a PTH the normal stress is found to oscillate in sign in a highly damped manner such that the amplitude is insignificant beyond the first cycle. The largest normal stresses in the laminate are found to act at a radial distance corresponding to the outer edge of the copper surface pad. Thus, with increase of temperature, there develops a circular locus of tensile normal stress in the laminate\pad combination at a radius less than, but immediately adjacent to, the edge of the pad and a locus of normal compression in the laminate alone at a radius slightly greater than that for the outside edge of the pad. Both the tensile and compressive peaks increase at a rate of 1.6 MPa/K. With decrease of temperature the opposite holds true except that the stressing rate is lower at 0.6 MPa/K. The fact that, independent of whether the temperature is increased or decreased from room temperature, there exist annular regions of tensile stress within and normal to the plane of the laminate explains the occurrence during changes of temperature of hidden delaminations, including, in multilayer circuit boards, debonding of interlayer copper conductors. The similarity of this configuration to bolt holes in composite structures is discussed.     

Effects of cooling rate on the shape memory effect thermodynamics of NiTi

The cooling rate of near-stoichiometric NiTi after annealing at 500° C is shown to have dramatic effects on the transformation thermodynamics. The slower the rate of cooling, the higher the transformation temperatures with less rhombohedral phase (R-phase) observed in room-temperature X-ray diffraction spectra. Given that this effect is due to the amount of time spent at intermediate temperatures, i.e an ageing effect, calorimetric analysis of specimens aged at 400 or 450° C revealed progressively higher transformation temperatures and latent heats with ageing. These observations are associated with the evolution of a secondary firstorder transformation to the R-phase. Analysis of stress-strain data of near-stoichiometric NiTi helices, water quenched from its secondary anneal at 500° C, indicated that a Carnot efficiency of 16% can be expected, compared with 13.5% for the same material when furnace cooled.     

The irreversibility of dimensional changes in epoxy adhesives undergoing uptake and expulsion of water

An optical interference method, developed to measure swelling inhomogeneities during water uptake by epoxy-based adhesive films¹, has now been used to study the extent of dimensional recovery during subsequent removal of the water responsible for swelling. A microscope cover slip is employed as marker to evaluate displacements normal to a resin film that is sandwiched between it and a rigid substrate. By placing an optical flat close to the free surface of the cover slip, a cavity is created within which optical interference can occur between light incident upon and light reflected from the specimen. Normal displacements in the resin cause similar displacements in the cover slip, i.e. the geometry of the cavity is altered, and this produces changes in the pattern of interference fringes. It is found that repeated exposure of the specimen to both wet and dry environments (distilled water at 62°C and dry air at 62°C) leads to reversible changes in the displacement field normal to the adhesive film when the exposure is relatively modest (～ 1 day at 62°C), but that prolonged exposure (> 2 days at 62°C) produces irreversible changes.     

Anisotropic glass-ceramics produced by extrusion through opposed dies

Oriented glass-ceramics have been produced in two morphologically different systems by extruding the green glasses through opposed dies at temperatures near their respective crystallization temperatures. The principal crystalline phase is a mica for one system and an asbestos for the other and, by using the crystals as markers, it has been possible to explore the pattern of flow and its effect on crystal orientation at various stages of the extrusion process. It is demonstrated that crystal alignment occurs during extensional flow between the dies and is modified by plug flow through the dies. Oriented glass-ceramics are expected to exhibit marked anisotropy in properties and, as far as mechanical properties are concerned, this has been confirmed by measurements of tensional modulus, fracture stress and indentation strength. The oriented crystals remain aligned during subsequent drawing down to fibres and by suitable heat-treatment can be complemented by a population of randomly oriented crystals.