Showing papers by "Vadim V. Silberschmidt published in 2006"

Analysis of material response to ultrasonic vibration loading in turning Inconel 718

Abstract: The paper is focused on the analysis of the surface layer formed on a workpiece treated with ultrasonically assisted turning (UAT) in comparison to conventional turning (CT). Various experimental methods are used to study the difference between the two machining techniques: nanoindentation, light microscopy and scanning electron microscopy (SEM). The experimental part of the paper studies the material response to CT and UAT in terms of material's hardness, residual stresses, and changes in the microstructure. The difference in the distribution of residual stresses in the machined surface layer is further studied by means of numerical (finite element) simulations. A three-dimensional thermomechanically coupled finite element (FE) model of both UAT and CT is used to study temperature distributions in the process zone and thermally induced stresses. Numerical results are compared with the obtained experimental data.

Modelling of Ag3Sn coarsening and its effect on creep of Sn-Ag eutectics

Abstract: A new constitutive model, which can account for the solder's microstructure and its evolution, is proposed to describe the creep behaviour of the Sn–Ag eutectic phase. In this model, the threshold stress, being a function of the particle size, volume fraction and distribution of Ag 3 Sn intermetallic compound (IMC), is introduced to build the relationship between the creep behaviour of the Sn–Ag solder and its microstructure. Evolution of the eutectic phase's microstructure is accounted for in terms of the coarsening model. Both the creep strain rate and hydrostatic stress's influence are taken into account in the IMC coarsening model. The proposed model is implemented into the commercial finite element code ABAQUS. The creep deformation due to the applied stress and IMC coarsening are discussed in the case of a flip chip solder joint. The obtained results show that the shape of the solder joint influences the particle distribution caused by heterogeneous coarsening. The solder joint is softened due to microstructure evolution over a long range of time.

Effect of micro-randomness on macroscopic properties and fracture of laminates

Abstract: Composite materials demonstrate a considerable extent of heterogeneity. A non-uniform spatial distribution of reinforcement results in variations of local properties of fibrous laminates. This non-uniformity not only affects effective properties of composite materials but is also a crucial factor in initiation and development of damage and fracture processes that are also spatially non-uniform. Such randomness in microstructure and in failure evolution is responsible for non-uniform distributions of stresses in composite specimens even under externally uniform loading, resulting, for instance, in a random distribution of matrix cracks in cross-ply laminates. The paper deals with statistical features of a distribution of carbon fibres in a transversal cross-sectional area in a unidirectional composite with epoxy matrix, based on various approaches used to quantify its microscopic randomness. A random character of the fibres’ distribution results in fluctuations of local elastic moduli in composites, the bounds of which depend on the characteristic length scale. A lattice model to study damage and fracture evolution in laminates, linking randomness of microstructure with macroscopic properties, is discussed. An example of simulations of matrix cracking in a carbon fibre/epoxy cross-ply laminate is given.

Stresses in Ultrasonically Assisted Turning

Abstract: Ultrasonically assisted turning (UAT) is a novel material-processing technology, where high frequency vibration (frequency f ≈ 20kHz, amplitude a ≈15μm) is superimposed on the movement of the cutting tool. Advantages of UAT have been demonstrated for a broad spectrum of applications. Compared to conventional turning (CT), this technique allows significant improvements in processing intractable materials, such as high-strength aerospace alloys, composites and ceramics. Superimposed ultrasonic vibration yields a noticeable decrease in cutting forces, as well as a superior surface finish. A vibro-impact interaction between the tool and workpiece in UAT in the process of continuous chip formation leads to a dynamically changing stress distribution in the process zone as compared to the quasistatic one in CT. The paper presents a three-dimensional, fully thermomechanically coupled computational model of UAT incorporating a non-linear elasto-plastic material model with strain-rate sensitivity and contact interaction with friction at the chip–tool interface. 3D stress distributions in the cutting region are analysed for a representative cycle of ultrasonic vibration. The dependence of various process parameters, such as shear stresses and cutting forces on vibration frequency and amplitude is also studied.

Grain features of SnAgCu solder and their effect on mechanical behaviour of micro-joints

Abstract: SnAgCu alloy, which promises compatible properties with Sn-Pb solder, has been identified as one of the most potential Lead-free solders for electronic interconnections. However, due to the miniaturization of solder joints, a micro-joint of this material contains only few grains. In this case, the mechanical behaviour of solder alloys shifts from the polycrystal-based to single-crystal based. Since P-Sn, the matrix of SnAgCu solder, has a contracted body-centred tetragonal structure, its grains are expected to have anisotropic properties, which are important, the reliability of a micro-joint. The present paper studies the inelastic anisotropic behaviour of this material. In order to analyse the effect of grain features, solder joints at different size are formed under the different cooling rate. An in-situ shear test is then performed to correlate the mechanical behavior of a joint to its microstructural features. The results show that the decrease in the joint's dimension results in the diminishment of the number of grains, and that the inelastic behaviour of SnAgCu grains is orientation-dependent.

Effect of material’s randomness on scaling of crack propagation in ceramics

Abstract: Crack propagation in real (quasi)brittle materials demonstrates various signs of stochasticity; a tortuous character of fracture surfaces, multiple cracking and crack branching observed in experiments are a vivid confirmation of it. Traditional approaches of fracture mechanics represent cracks as geometrically smooth objects with straight (or curved) crack fronts, thus usually neglecting morphology of real cracks. An introduction of a direct account for stochastic features of brittle materials into modelling schemes leads to a more adequate description of real fracture processes. The effect of the material’s randomness on crack propagation in ceramics is studied based on the approach, combining these random features with continuum damage mechanics (CDM) and fracture mechanics. CDM describes a macroscopic manifestation of various failure processes developing at lower length scales. The numerical mode-I fracture analysis, based on discretization of a specimen’s cross-section, containing a sharp notch, into rectangular elements, provides detailed information on slow crack propagation. A necessity to describe a crack with its length changing along the front presupposes a transition from a unique stress-intensity factor to a set of its local values. A computational procedure for simulation of crack-damage interaction and crack propagation in alumina specimens at tension is suggested on the basis of a modification of a lattice scheme unified with ideas of CDM and local stress-intensity factors. Inhomogeneity of material properties is modelled in terms of various random spatial distributions of the initial damage in the specimen’s cross-section. Characterization of complicated morphology of cracks is implemented by means of scaling analysis of the crack-front shape.

Micromechanical analysis of effective properties of plasma sprayed alumina coatings

Abstract: With the use of an image analysis technique, microstructure of plasma sprayed alumina coatings is quantitatively characterised using such parameters as the size, shape and number (density) of microvoids and the porosity level in the coatings. A Monte-Carlo simulation approach is proposed to model the characterised microstructure of alumina coatings with various levels of porosity. Based on this approach, effective elastic properties of the coatings with low porosity (less than 10%) are calculated using an analytical method. The effect of microstructure on material properties is investigated quantitatively and qualitatively. The numerical results are compared with the data obtained from nanoindentation and bending tests.

Analysis of Stress Distribution in SnAgCu Solder Joint

Abstract: SnAgCu solder is a promising lead-free material for interconnections in electronic packages. However, its melting temperature (490°K) is considerably higher than that of the traditional SnPb solder (456°K). At the same time, SnAgCu has much better creep resistance at high temperature. These properties may cause large residual stresses during manufacturing processes due to the mismatch of thermal properties of electronic components that can influence the reliability of solder joints in electronic packages. This paper studies the residual stresses in solder joints in a flip chip package under different cooling conditions and their influence on the subsequent cyclic test by means of a finite element approach. The results show that the initial temperature of 453°K is high enough to induce residual stresses due to manufacturing procedures. Simulations, based on traditional creep-fatigue models, demonstrate that the residual stresses affect the mechanical behaviour of solder joints in several initial thermal cycles but have little effect on their reliability.

Analysis of damage evolution in thick ceramic coatings

Abstract: This paper analyses the damage evolution process in thick ceramic coatings on cylindrical bodies exposed to uniform heating for two cases of the substrate material under conditions of plane strain. Alumina coatings with three different types of the initial through-thickness distribution of manufacture-induced porosity (uniform as well as with increasing and decreasing porosity levels with the distance from the interface) are studied. The analysis is based on a general computational scheme to determine damage evolution parameters, which incorporates a solution of the appropriate problem of thermoelasticity. It is shown that the type of the substrate interplays with the coating's thickness in its influence upon the character and rate of damage accumulation in coatings. A number of other important features of the damage evolution process in thick ceramic coatings linked to the coating's thickness are also revealed.

Computational modeling of ultrasonically assisted turning

Abstract: Ultrasonically assisted turning (UAT) is an advanced machining technique, where high frequency vibration (frequency f ≈ 20 kHz, amplitude a ≈ 15 µm) is superimposed on the movement of the cutting tool. Compared to conventional turning (CT), this technique allows significant improvements in processing intractable materials, such as high-strength aerospace alloys, composites and ceramics. Superimposed ultrasonic vibration yields a noticeable decrease in cutting forces, as well as a superior surface finish [1].