Publications

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Subcritical crack growth can occur in a glass when the stress intensity factor is less than the fracture toughness if water molecules are present. A novel bi-material beam specimen is proposed to investigate environmentally assisted crack growth (EACG). Two materials with different coefficients of thermal expansion are diffusion bonded at high temperature and cooled to the room temperature which introduces residual stress in the beam. A Finite element (FE) model is developed and initially validated with an analytical model. Steady-state crack (SSC) depth at which mode II stress intensity factor (KII) is zero and the corresponding mode I stress intensity factor (KI) value are obtained for different material pairs and thickness ratios of the top and bottom materials using the FE model. Crack propagation path is also predicted. We finally modify the geometry of the specimen to generate non-constant KI values as the crack propagates.

Partial penetration laser welds join metal surfaces without additional filler material, providing hermetic seals for a variety of components. The crack-like geometry of a partial penetration weld is a local stress riser that may lead to failure of the component in the weld. Computational modeling of laser welds has shown that the model should include damage evolution to predict the large deformation and failure. We have performed interrupted tensile experiments both to characterize the damage evolution and failure in laser welds and to aid computational modeling of these welds. Several EDM-notched and laser-welded 304L stainless steel tensile coupons were pulled in tension, each one to a different load level, and then sectioned and imaged to show the evolution of damage in the laser weld and in the EDM-notched parent 304L material (having a similar geometry to the partial penetration laser-welded material). SEM imaging of these specimens revealed considerable cracking at the root of the laser welds and some visible micro-cracking in the root of the EDM notch even before peak load was achieved in these specimens. The images also showed deformation-induced damage in the root of the notch and laser weld prior to the appearance of the main crack, though the laser-welded specimens tended to have more extensive damage than the notched material. These experiments show that the local geometry alone is not the cause of the damage, but also microstructure of the laser weld, which requires additional investigation.

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Sealing glasses are ubiquitous in high pressure and temperature engineering applications, such as hermetic feed-through electrical connectors. A common connector technology are glass-to-metal seals where a metal shell compresses a sealing glass to create a hermetic seal. Though finite-element analysis has been used to understand and design glass-to-metal seals for many years, there has been little validation of these models. An indentation technique was employed to measure the residual stress on the surface of a simple glass-to-metal seal. Recently developed rate- dependent material models of both Schott 8061 and 304L VAR stainless steel have been applied to a finite-element model of the simple glass-to-metal seal. Model predictions of residual stress based on the evolution of material models are shown. These model predictions are compared to measured data. Validity of the finite- element predictions is discussed. It will be shown that the finite-element model of the glass-to-metal seal accurately predicts the mean residual stress in the glass near the glass-to-metal interface and is valid for this quantity of interest.

Microstructural variabilities are among the predominant sources of uncertainty in structural performance and reliability. We seek to develop efficient algorithms for multiscale calcu- lations for polycrystalline alloys such as aluminum alloy 6061-T6 in environments where ductile fracture is the dominant failure mode. Our approach employs concurrent multiscale methods, but does not focus on their development. They are a necessary but not sufficient ingredient to multiscale reliability predictions. We have focused on how to efficiently use concurrent models for forward propagation because practical applications cannot include fine-scale details throughout the problem domain due to exorbitant computational demand. Our approach begins with a low-fidelity prediction at the engineering scale that is sub- sequently refined with multiscale simulation. The results presented in this report focus on plasticity and damage at the meso-scale, efforts to expedite Monte Carlo simulation with mi- crostructural considerations, modeling aspects regarding geometric representation of grains and second-phase particles, and contrasting algorithms for scale coupling.

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Glass-to-metal seals are used extensively to protect and isolate electronic components. Small strains of just a few percent are typical in the metal during processing of seals, but generate substantial tensile stresses in the glass during the solidification portion of the process. These tensile stresses can lead to glass cracking either immediately or over time, which results in a loss of hermiticity of the seal. Measurement of the metal in the small strain region needs to be very accurate as small differences in the evolving state of the metal have significant influence on the stress state in the glass and glass-metal interfaces. Small strain tensile experiments were conducted over the temperatures range of 25-800 °C. Experiments were designed to quantify stress relaxation and reloading combined with mid-test thermal changes. The effect of strain rate was measured by directly varying the applied strain rate during initial loading and reloading and by monitoring the material response during stress relaxation experiments. Coupled thermal mechanical experiments were developed to capture key features of glass-to-metal seal processing details such as synchronized thermal and mechanical loading, thermal excursions at various strain levels, and thermal cycling during stress relaxation or creep loadings. Small changes in the processing cycle parameters were found to have non-insignificant effect on the metal behavior. The resulting data and findings will be presented.

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A Bayesian framework is developed for characterizing the unknown parameters of probabilistic models for material properties. In this framework, the unknown parameters are viewed as random and described by their posterior distributions obtained from prior information and measurements of quantities of interest that are observable and depend on the unknown parameters. The proposed Bayesian method is applied to characterize an unknown spatial correlation of the conductivity field in the definition of a stochastic transport equation and to solve this equation by Monte Carlo simulation and stochastic reduced order models (SROMs). The Bayesian method is also employed to characterize unknown parameters of material properties for laser welds from measurements of peak forces sustained by these welds.