Bahattin Kilic

Toward the development of miniaturized imaging systems for detection of pre-cancer

In this paper, we describe the progress toward the development of miniaturized imaging systems with applications in medical imaging, and specifically, detection of pre-cancer. The focus of the article is a miniature, optical-sectioning, fluorescence microscope. The miniature microscope is constructed from lithographically printed optics and assembled using a bulk micro-machined silicon microoptical table. Optical elements have been printed in a negative tone hybrid glass to a maximum depth of 59 /spl mu/m and an rms surface roughness between 10-45 nm, fulfilling the requirements of the miniature microscope. Test optical elements have been assembled using silicon-spring equipped mounting slots. The design of silicon springs is presented in this paper. Optical elements can be assembled within the tolerances of an NA=0.4 miniature microscope objective, confirming the concept of simple, zero-alignment assembly.

The sandia fracture challenge: Blind round robin predictions of ductile tearing

Existing and emerging methods in computational mechanics are rarely validated against problems with an unknown outcome. For this reason, Sandia National Laboratories, in partnership with US National Science Foundation and Naval Surface Warfare Center Carderock Division, launched a computational challenge in mid-summer, 2012. Researchers and engineers were invited to predict crack initiation and propagation in a simple but novel geometry fabricated from a common off-the-shelf commercial engineering alloy. The goal of this international Sandia Fracture Challenge was to benchmark the capabilities for the prediction of deformation and damage evolution associated with ductile tearing in structural metals, including physics models, computational methods, and numerical implementations currently available in the computational fracture community. Thirteen teams participated, reporting blind predictions for the outcome of the Challenge. The simulations and experiments were performed independently and kept confidential. The methods for fracture prediction taken by the thirteen teams ranged from very simple engineering calculations to complicated multiscale simulations. The wide variation in modeling results showed a striking lack of consistency across research groups in addressing problems of ductile fracture. While some methods were more successful than others, it is clear that the problem of ductile fracture prediction continues to be challenging. Specific areas of deficiency have been identified through this effort. Also, the effort has underscored the need for additional blind prediction-based assessments.

Peridynamic Theory for Thermomechanical Analysis

Thermomechanical modeling for interconnects and electronic packages is a difficult challenge, especially for material interfaces and films under 1 ¿m dimension. Understanding and prediction of their mechanical failure require the simulation of material behavior in the presence of multiple length scales. However, the classical continuum theory is not capable of predicting failure without a posterior analysis with an external crack growth criteria and treats the interfaces having zero thickness. A new nonlocal continuum theory referred to as peridynamic theory offers the ability to predict failure at these length scales. This study presents a new response function as part of the peridynamic theory to include thermal loading. After validating this response function by comparing against the displacement predictions in benchmark problems against those of finite element method, the peridynamic theory is used to predict damage initiation and propagation in regions having dissimilar materials due to thermomechanical loading.

Effects of Nanoparticles on Stiffness and Impact Strength of Composites

The effects of nanoparticles on impact stiffness and strength are investigated. Nanoparticles are prepared from nanoclay particles embedded into carbon fiber/epoxy wet lay-up laminates. Specimens are characterized via low-velocity impact tests and following Cscanner damage analyses. The results show that the area of damage is reduced 46% when 5% nanoparticle is used in the composite. Modeling of the nanocomposites is performed by peridynamic simulations and compared with experimental measurements. The mode of damage and the improvement of the impact resistance of nanocomposites are captured successfully via numerical simulations.

Global-Local Finite Element Analysis of Bonded Single-Lap Joints

Adhesively bonded lap joints involve dissimilar material junctions and sharp changes in geometry, possibly leading to premature failure. Although the finite element method is well suited to model the bonded lap joints, traditional finite elements are incapable of correctly resolving the stress state at junctions of dissimilar materials because of the unbounded nature of the stresses. In order to facilitate the use of bonded lap joints in future structures, this study presents a finite element technique utilizing a global (special) element coupled with traditional elements. The global element includes the singular behavior at the junction of dissimilar materials with or without traction-free surfaces.

Analysis of Brazed Single-Lap Joints Using the Peridynamics Theory

Design requirements for high-temperature applications can be usually satisfied by C/C and C/C-SiC materials. The strength of the brazed joint is controlled by increasing the thickness of the braze layer. The thickness of the braze layer is on the order of the micro scale, and the mismatch in material properties among the adherends and braze is a major source of high stress concentrations. Such stresses lead to interlayer delamination and cracking, which can lead to premature failure. The focus of this study is to analyze the effect of material properties, the braze layer, and joint geometry on the crack initiation and growth in the joint. Because the joint has macro- and micro-length scale components, its analysis requires the use of a theory at the meso-scale that accounts for the interaction of these length scales. The peridynamics theory permits time-dependent analysis at multiple length scales, damage is part of the constitutive model, and the material response is determined at the bond level. This feature allows initiation and propagation of failures inside the material without resorting to crack initiation or growth criteria.

Coupling of peridynamic theory and finite element method

The finite element method is widely utilized for the numerical solution of structural problems. However, damage prediction using the finite element method can be very cumbersome because the derivatives of displacements are undefined at the discontinuities. In contrast, the peridynamic theory uses displacements rather than displacement derivatives in its formulation. Hence, peridynamic equations are valid everywhere, including discontinuities. Furthermore, the peridynamic theory does not require external criteria for crack initiation and propagation since material failure is invoked through the material response. However, the finite element method is numerically more efficient than the peridynamic theory. Hence, this study presents a method to couple the peridynamic theory and finite element analysis to take advantage of both methods. Peridynamics is used in the regions where failure is expected and the remaining regions are modeled utilizing the finite element method. Then, the present approach is demonstrated through a simple problem and predictions of the present approach are compared against both the peridynamic theory and finite element method. The damage simulation results for the present method are demonstrated by considering a plate with a circular cutout.

ANALYSIS OF BLISTER TESTS BY USING HYPERSINGULAR INTEGRAL EQUATIONS

It is essential to determine the fracture characteristics of adhesive bonds. The blister test is commonly used to measure the adhesion strength and to evaluate the performance of adhesive bonding. The test procedure involves a thin film bonded to a substrate except for a circular disbond where either a uniform pressure or a point load is applied. The applied loading is increased gradually until its critical value at which adhesive failure occurs. The concept of energy release rate can be applied to predict adhesive failure. However, the calculation of energy release rate requires accurate prediction of singular stresses or stress intensity factors at the crack (disbond) front based on the theory of elasticity. The previous solutions based on the theory of elasticity permit only the determination of energy release rate which is not directly suitable for comparison with experimental measurements. These solutions employ the derivative of the crack surface opening displacements as the primary unknowns, thus leading to singular integral equations with Cauchy-type singularity. The present study employs the crack opening and sliding as primary unknowns rather than their derivatives, thus leading to hypersingular integral equations. Solution to these singular integral equations permits the determination of not only the complex stress intensity factors but also the crack opening displacements. The analysis predictions are compared with both the finite element simulations and previous benchmark solutions.

Thermomechanical Fatigue Life Prediction Analysis

The life prediction of solder joints under thermal cyclic loading requires a two-stage analysis: (1) a three-dimensional finite element analysis to compute the stress, strain, and strain energy field, and (2) a life prediction analysis. As shown in Fig. 2.1, a typical thermal cycle is described by its starting, maximum, and minimum temperatures, as well as the durations of time for ramp-up, ramp-down, and dwell at maximum and minimum temperatures. The starting temperature is the reference (stress-free) temperature of the thermal cycle. Four thermal cycles in the simulation are usually sufficient to ensure the stability of the hysteresis loop.

Mechanical Bending Fatigue Life Prediction Analysis

The fatigue life prediction of solder joints under cyclic bending requires a three-stage analysis: (1) a three-dimensional finite element analysis with linear material properties for computing the assembly stiffness and imposed strain values, (2) a one-dimensional combined creep and time-independent plastic deformation analysis under cyclic loading for computing the inelastic strain energy, and (3) a life prediction analysis.