James Boileau

Measurement of the state of stress in silicon with micro-Raman spectroscopy

Micro-Raman spectroscopy has been widely used to measure local stresses in silicon and other cubic materials. However, a single (scalar) line position measurement cannot determine the complete stress state unless it has a very simple form such as uniaxial. Previously published micro-Raman strategies designed to determine additional elements of the stress tensor take advantage of the polarization and intensity of the Raman-scattered light, but these strategies have not been validated experimentally. In this work, we test one such stategy [S. Narayanan, S. Kalidindi, and L. Schadler, J. Appl. Phys. 82, 2595 (1997)] for rectangular (110)- and (111)-orientated silicon wafers. The wafers are subjected to a bending stress using a custom-designed apparatus, and the state of (plane) stress is modeled with ABAQUS. The Raman shifts are calculated using previously published values for silicon phonon deformation potentials. The experimentally measured values for σxx, σyy, and τxy at the silicon surface are in good ag...

A domain partitioning based pre-processor for multi-scale modelling of cast aluminium alloys

In this paper, a microstructural morphology based domain partitioning MDP methodology is developed for materials with non-uniform heterogeneous microstructure. The comprehensive set of methods is intended to provide a concurrent multi-scale analysis model with an initial computational domain that delineates regions of statistical homogeneity and inhomogeneity. The MDP methodology is intended to be a pre-processor to multi-scale analysis of mechanical behaviour and damage of heterogeneous materials, e.g. cast aluminium alloys. It introduces a systematic three-step process that is based on geometric features of morphology. The first step simulates high resolution microstructural information from low resolution micrographs of the material and a limited number of high resolution optical or scanning electron microscopy micrographs. The second step is quantitative characterization of the high resolution images to create effective metrics that can relate microstructural descriptors to material behaviour. The third step invokes a partitioning method to demarcate regions belonging to different length scales in a concurrent multiscale model. Partitioning criteria for domain partitioning are defined in terms of microstructural descriptors and their functions. The effectiveness of these metrics in differentiating microstructures of a 319-type cast aluminium alloy with different secondary dendrite arm spacings SDAS is demonstrated. The MDP method establishes intrinsic material length scales for the different SDAS, namely, 23, 70 and 100 µm, and consequently subdivides the computational domain for concurrently coupling macro- and micromechanical analyses in the multi-scale model.

A Multiresolution Transformation Rule of Material Defects

The ability to quantify the material damage at different length scales is critical in the multiscale analysis of material behavior from nanoscale to macroscale. In this article, on the basis of the equivalence of complementary elastic energy we propose a multiresolution rule that transforms different levels of material defects to the equivalent degradation of material properties. It facilitates a sequential memory-efficient processing of massive material defects in a multiresolution framework, and also supports a functionality of partial damage conversion to serve different needs in subsequent numerical analyses. Numerical simulation was conducted with different settings of material defects. The analysis results indicate the efficacy of the proposed method, offering a potential (i) to interface between multiscale material defects and (ii) as an effective method of homogenization for the determination of the damage variable in continuum damage mechanics.

Material damage evaluation with measured microdefects and multiresolution numerical analysis

This paper presents an alternative method of material damage evaluation based on the X-ray computer tomography-detected microdefects and multiscale computer simulation. This is achieved by developing a method of the digital diagnosis and full-field numerical calculation of material degradation in macroscopic material test specimens. The method comprised three basic components: (a) digital detection and processing of micro/mesoscale material defects of macroscopic material test specimens; (b) multilevel meshing and multilevel finite element analysis for evaluating local/global material degradation; and (c) synchronized experimental and numerical determination of material damage. The unique contributions of the proposed approach include (a) a multilevel finite element meshing and analysis scheme that makes the full-field estimation of material degradation in macroscopic test specimens computationally tractable on regular workstations, (b) full-field exploration of mesoscale material defects (i.e., those with a feature size from several micrometers to a few millimeters), which play a crucial role in failure analysis of engineering components, and (c) the proposed method offers a significantly better accuracy in estimating material degradation in terms of effective modulus than the conventional analytical models in continuum damage mechanics and micromechanics. Test results of aluminum alloys confirm the efficacy of our approach in the digital interrogation of material degradation.

A Microstructural and Mechanical Property Study of an AM50 HPDC Magnesium Alloy

As the need for weight saving and fuel economy has increased, so has the interest in using Aluminum-Manganese (AM) Magnesium alloys. A thorough examination of the existing literature found several competing conclusions on how elongation and microstructure correlate; therefore a study was performed on a high-pressure die cast (HPDC) AM50 alloy. This study focused on understanding the relationship between microstructure and mechanical properties in test bars manufactured from a complex-shaped test casting.The results from this study found a high degree of variability in the resulting tensile properties, especially in the elongation-to-failure data. To understand the role of microstructure on properties, an extensive analysis of the microstructures was performed. No difference in cell size through the sample cross-section was observed. Externally solidified cells (ESCs) were present in large numbers; however, no correlation could be determined between the location of the samples and the size and number of the ESCs. Porosity distribution was random across the sample cross-section. Examination of the fractures surfaces indicated that the fracture did not preferentially occur along the Mg17Al12 eutectic in the AM50 alloy.

The Dimensional Stability of Cast 319 Aluminum

The automotive use of cast aluminum has greatly increased during the past decade, especially for engine blocks and cylinder heads. One physical property that is important in elevated-temperature applications is longterm dimensional stability of the cast aluminum component. Certain cast aluminum alloys (like 319) can undergo dimensional changes when exposed to engine operating temperatures over long periods of time; when these changes occur, the shape of the casting is distorted and the performance of the component may be diminished. Thus, a study was conducted to measure dimensional growth changes in a cast 319-type aluminum alloy as a function of heat-treatment, exposure temperature, and exposure time at the given temperature. The results show that all three factors have a significant effect upon the dimensional stability.

Characterizing the Effect of 319 Aluminum Microstructure on Machinability: Part 1 — Model Development

This two-part paper is directed at the development of a machining force model that addresses key microstructural features of 319 Aluminum. In Part 1 of this paper, a machining force model is presented that incorporates microstructural effects. Secondary Dendrite Arm Spacing (SDAS) is identified as a significant microstructure feature of 319 aluminum in terms of machinability, and the SDAS is related to the solidification rate. A new material constitutive relationship that incorporates SDAS microstructure effects is proposed. In Part 2 of this paper, the results from material tests and machining experiments are presented. A new methodology for estimating parameters within the material constitutive model is described, and force model predictions are compared with the results from machining tests. The comparison reveals an excellent agreement between measured and predicted forces.Copyright

Characterizing the Effect of 319 Aluminum Microstructure on Machinability: Part 2 — Model Validation

In Part 1 of this paper, a machining force model was developed based on an enhanced version of Zheng’s Continuum Mechanics model that incorporates microstructural effects. Machining experiments identified Secondary Dendrite Arm Spacing (SDAS) as a significant microstructure feature of 319 aluminum in terms of machinability. A new material constitutive relationship that incorporates SDAS microstructure effects on the flow stress was proposed. In this part of the paper, disk turning tests are performed to simulate the orthogonal cutting process. The cutting forces obtained from some of these tests are used in concert with an inverse form of the continuum mechanics machining model to estimate the parameters in the material constitutive equation. The enhanced continuum mechanics orthogonal cutting model is then applied to predict cutting forces when machining Al319. Comparison of the model predicted and experimentally acquired cutting forces is demonstrated to show good agreement.© 2005 ASME