Y. Lawrence Yao

Micro Scale Laser Shock Processing of Metallic Components

Laser shock processing of copper using focused laser beam size about ten microns is investigated for its feasibility and capability to impart desirable residual stress distributions into the target material in order to improve the fatigue life of the material. Shock pressure and strain/stress are properly modeled to reflect the micro scale involved, and the high strain rate and ultrahigh pressure involved. Numerical solutions of the model are experimentally validated in terms of the geometry of the shock-generated plastic deformation on target material surfaces as well as the average in-depth strains under various conditions. The residual stress distributions can be further influenced by shocking at different locations with certain spacing. The potential of applying the technique to micro components, such as micro gears fabricated using MEMS is demonstrated. The investigation also lays groundwork for possible combination of the micro scale laser shock processing with laser micromachining processes to offset the undesirable residual stress often induced by such machining processes.

Analysis and Prediction of Edge Effects in Laser Bending

Laser forming of sheet metal offers the advantages of requiring no hard tooling and thus reduced cost and increased flexibility. It also enables forming of some materials and shapes that are not possible now. In single-axis laser bending of plates, the bending edge is found to be somewhat curved and the bending angle varies along the laser-scanning path. These phenomena are termed edge effects, which adversely affect the accuracy of the bending and result in undue residual stress. Numerical investigations are carried out to study the process transiency and the mechanism of the edge effects. Temperature dependency of material properties and strain-rate dependency of flow stress are considered in the numerical simulation to improve prediction accuracy. Numerical results are validated in experiments. Patterns of edge effects and resultant residual stress distributions are examined under a wide range of conditions. A more complete explanation for the mechanism of the edge effects is given.

THE RABIT: A RAPID AUTOMATED BIODOSIMETRY TOOL FOR RADIOLOGICAL TRIAGE

In response to the recognized need for high throughput biodosimetry methods for use after large-scale radiological events, a logical approach is complete automation of standard biodosimetric assays that are currently performed manually. The authors describe progress to date on the RABIT (Rapid Automated BIodosimetry Tool), designed to score micronuclei or &ggr;-H2AX fluorescence in lymphocytes derived from a single drop of blood from a fingerstick. The RABIT system is designed to be completely automated, from the input of the capillary blood sample into the machine to the output of a dose estimate. Improvements in throughput are achieved through use of a single drop of blood, optimization of the biological protocols for in situ analysis in multi-well plates, implementation of robotic-plate and liquid handling, and new developments in high-speed imaging. Automating well-established bioassays represents a promising approach to high-throughput radiation biodosimetry, both because high throughputs can be achieved, but also because the time to deployment is potentially much shorter than for a new biological assay. Here the authors describe the development of each of the individual modules of the RABIT system and show preliminary data from key modules. System integration is ongoing, followed by calibration and validation.

Numerical and experimental study of strain rate effects in laser forming

Experimental investigation and numerical simulation of the influence of the strain rate in laser forming are presented. To isolate and effectively study the strain rate effects, which are temperature dependent, a constant peak temperature method is developed with the aid of numerical modeling and solution. Under the condition of the constant peak temperature, the effects of strain rate on forming efficiency, residual stress and hardness of the formed parts are studied both experimentally and numerically. In the numerical model, the temperature dependence and strain-rate dependence of the flow stress and other material properties are considered. The simulation results are consistent with the experimental observations.

Adapting the γ-H2AX Assay for Automated Processing in Human Lymphocytes. 1. Technological Aspects

Abstract The immunofluorescence-based detection of γ-H2AX is a reliable and sensitive method for quantitatively measuring DNA double-strand breaks (DSBs) in irradiated samples. Since H2AX phosphorylation is highly linear with radiation dose, this well-established biomarker is in current use in radiation biodosimetry. At the Center for High-Throughput Minimally Invasive Radiation Biodosimetry, we have developed a fully automated high-throughput system, the RABIT (Rapid Automated Biodosimetry Tool), that can be used to measure γ-H2AX yields from fingerstick-derived samples of blood. The RABIT workstation has been designed to fully automate the γ-H2AX immunocytochemical protocol, from the isolation of human blood lymphocytes in heparin-coated PVC capillaries to the immunolabeling of γ-H2AX protein and image acquisition to determine fluorescence yield. High throughput is achieved through the use of purpose-built robotics, lymphocyte handling in 96-well filter-bottomed plates, and high-speed imaging. The goal of the present study was to optimize and validate the performance of the RABIT system for the reproducible and quantitative detection of γ-H2AX total fluorescence in lymphocytes in a multiwell format. Validation of our biodosimetry platform was achieved by the linear detection of a dose-dependent increase in γ-H2AX fluorescence in peripheral blood samples irradiated ex vivo with γ rays over the range 0 to 8 Gy. This study demonstrates for the first time the optimization and use of our robotically based biodosimetry workstation to successfully quantify γ-H2AX total fluorescence in irradiated peripheral lymphocytes.

Cooling Effects in Multiscan Laser Forming

Abstract For laser forming of sheet metal to become a practical production or rapid prototyping tool, multiple scans of the workpiece with the laser are necessary to achieve the required magnitude of deformation. Between consecutive scans, substantial waiting time is normally necessary for the workpiece to cool down so that a steep temperature gradient can be reestablished in the next scan. This paper first experimentally and numerically examines the effect of forced cooling on single-scan laser forming processes. Cooling effects under various conditions, including different laser power, scanning speed, nozzle offset, and cooling air pressure, are investigated. Cooling effects on microstructure change and other mechanical properties, including strength, ductility, and hardness, are also examined. A focus is to investigate the cooling effect on the deformation mechanism, including competing effects on temperature and flow stress. The investigation on multiscan laser forming shows forced cooling has the potential to significantly reduce the total forming time while having no undesirable effects on microstructure change and other mechanical behavior. Cooling in the buckling mechanism dominated laser forming process is also considered. The established numerical model for laser forming with forced cooling provides greater insights into the cooling effects on the deformation mechanism, helps predict such effects on final dimensional accuracy and mechanical properties, and can be extended to optimize the multiscan laser forming process.

Characterization of Plastic Deformation Induced by Microscale Laser Shock Peening

Electron backscatter diffraction (EBSD) is used to investigate crystal lattice rotation caused by plastic deformation during high-strain rate laser shock peening in single crystal aluminum and copper sample on (I 10) and (001) surfaces. New experimental methodologies are employed which enable measurement of the in-plane lattice rotation under approximate plane-strain conditions. Crystal lattice rotation on and below the microscale laser shock peened sample surface was measured and compared with the simulation result obtained from FEM analysis, which account for single crystal plasticity. The lattice rotation measurements directly complement measurements of residual strain/stress with X-ray micro-diffraction using synchrotron light source and it also gives an indication of the extent of the plastic deformation induced by the microscale laser shock peening.

Optimal Process Planning for Laser Forming of Doubly Curved Shapes

There has been a considerable amount of work carried out on two-dimensional laser forming. In order to advance the process further for industrial applications, however, it is necessary to consider more general cases and especially their process planning aspect. This paper presents an optimal approach to laser scanning paths and heating condition determination for laser forming of doubly curved shapes. Important features of the approach include the strain field calculation based on principal curvature formulation and minimal strain optimization, and scanning paths and heating condition (laser power and scanning velocity) determination by combining analytical and practical constraints. The overall methodology is presented first, followed by more detailed descriptions of each step of the approach. Two distinctive types of doubly curved shape, pillow and saddle shapes are focused on and the effectiveness of the proposed approach is validated by forming experiments.

Gas Jet–Workpiece Interactions in Laser Machining

Laser machining efficiency and quality are closely related to gas pressure, nozzle geometry, and standoff distance. Modeling studies of laser machining rarely incorporate gas effects in part because of the complex structure and turbulent nature of jet flow. In this paper, the interaction of a supersonic, turbulent axisymmetric jet with the workpiece is studied. Numerical simulations are carried out using an explicit, coupled solution algorithm with solution-based mesh adaptation. The model is able to make quantitative predictions of the pressure, mass flow rate as well as shear force at the machining front. Effect of gas pressure and nozzle standoff distance on structure of the supersonic shock pattern is studied. Experiments are carried out to study the effect of processing parameters such as gas pressure and standoff distance. The measured results are found to match and hence validate the simulations. The interaction of the oblique incident shock with the normal standoff shock is found to contribute to a large reduction in the total pressure at the machining front and when the nozzle pressure is increased beyond a certain point. The associated reduction in flow rate, fluctuations of pressure gradient and shear force at the machining front could lower the material removal capability of the gas jet and possibly result in a poorer surface finish. The laser cutting experiments show that the variation of cut quality are affected by shock structures and can be represented by the mass flow rate. @S1087-1357~00!01702-0#

Laser Bending of Tubes: Mechanism, Analysis, and Prediction

Laser bending of tubes is a process in which laser-induced thermal distortion is used to bend tubes without hard tooling or external forces. Mechanisms of the process are examined to better understand the deformation characteristics such as wall thickness variation, cross-section ovalization, bending radius, and asymmetry. Factors important to these characteristics are experimentally and numerically investigated. Temporal and spatial distributions of temperature and stress/strain obtained from experimentally validated simulation models are also used to better understand additional phenomena accompanying the process, and to help devise ways to improve the process such as reducing asymmetry. A closed-form expression for the bending angle is proposed.