Publications

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Recent experimental investigations show that most models are not able to capture the ductile behavior of metal alloys in the entire triaxiality range, especially at low triaxiality. Modelers are moving beyond stress triaxiality as the dominant indicator of material failure and developing constitutive models that incorporate shear into the evolution of the failure model. Available data that cover low triaxiality range are rare and a series of critical experiments is needed. Here, experiments of smooth thin as well as notched tubular specimens of Al6061-T651 under combined tension-torsion loading were conducted. This provides a very basic set of data for phenomenological models. A full-field deformation technique, digital image correlation (DIC), was applied to these tests to allow measurement of the field deformation, including the notched area. The microstructural features of the tested specimens were characterized to better understand the different failure mechanisms which led to ductility variation in the aluminum alloy.

Recent experimental investigations show that most models are not able to capture the ductile behavior of metal alloys in the entire triaxiality range, especially at low triaxiality. Modelers are moving beyond stress triaxiality as the dominant indicator of material failure and developing constitutive models that incorporate shear into the evolution of the failure model. Available data that cover low triaxiality range are rare and a series of critical experiments is needed. Here, experiments of smooth thin as well as notched tubular specimens of Al6061-T651 under combined tension-torsion loading were conducted. This provides a very basic set of data for phenomenological models. A full-field deformation technique, digital image correlation (DIC), was applied to these tests to allow measurement of the field deformation, including the notched area. The microstructural features of the tested specimens were characterized to better understand the different failure mechanisms which led to ductility variation in the aluminum alloy.

This report contains the one-year feasibility study for our three-year LDRD proposal that is aimed to develop an experimental technique to measure the 3D deformation fields inside a material body. In this feasibility study, we first apply Digital Volume Correlation (DVC) algorithm to pre-existing in-situ Xray Computed Tomography (XCT) image sets with pure rigid body translation. The calculated displacement field has very large random errors and low precision that are unacceptable. Then we enhance these tomography images by setting threshold of the intensity of each slice. DVC algorithm is able to obtain accurate deformation fields from these enhanced image sets and the deformation fields are consistent with the global mechanical loading that is applied to the specimen. Through this study, we prove that the internal markers inside the pre-existing tomography images of aluminum alloy can be enhanced and are suitable for DVC to calculate the deformation field throughout the material body.

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This report describes a Laboratory Directed Research and Development (LDRD) project to use of synchrotron-radiation computed tomography (SRCT) data to determine the conditions and mechanisms that lead to void nucleation in rolled alloys. The Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL) has provided SRCT data of a few specimens of 7075-T7351 aluminum plate (widely used for aerospace applications) stretched to failure, loaded in directions perpendicular and parallel to the rolling direction. The resolution of SRCT data is 900nm, which allows elucidation of the mechanisms governing void growth and coalescence. This resolution is not fine enough, however, for nucleation. We propose the use statistics and image processing techniques to obtain sub-resolution scale information from these data, and thus determine where in the specimen and when during the loading program nucleation occurs and the mechanisms that lead to it. Quantitative analysis of the tomography data, however, leads to the conclusion that the reconstruction process compromises the information obtained from the scans. Alternate, more powerful reconstruction algorithms are needed to address this problem, but those fall beyond the scope of this project.

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Composite materials, particularly fiber reinforced plastic composites, have been extensively utilized in many military and industrial applications. As an important structural component in these applications, the composites are often subjected to external impact loading. It is desirable to understand the mechanical response of the composites under impact loading for performance evaluation in the applications. Even though many material models for the composites have been developed, experimental investigation is still needed to validate and verify the models. It is essential to investigate the intrinsic material response. However, it becomes more applicable to determine the structural response of composites, such as a composite beam. The composites are usually subjected to out-of-plane loading in applications. When a composite beam is subjected to a sudden transverse impact, two different kinds of stress waves, longitudinal and transverse waves, are generated and propagate in the beam. The longitudinal stress wave propagates through the thickness direction; whereas, the propagation of the transverse stress wave is in-plane directions. The longitudinal stress wave speed is usually considered as a material constant determined by the material density and Young's modulus, regardless of the loading rate. By contrast, the transverse wave speed is related to structural parameters. In ballistic mechanics, the transverse wave plays a key role to absorb external impact energy [1]. The faster the transverse wave speed, the more impact energy dissipated. Since the transverse wave speed is not a material constant, it is not possible to be calculated from stress-wave theory. One can place several transducers to track the transverse wave propagation. An alternative but more efficient method is to apply digital image correlation (DIC) to visualize the transverse wave propagation. In this study, we applied three-pointbending (TPB) technique to Kolsky compression bar to facilitate dynamic transverse loading on a glass fiber/epoxy composite beam. The high-speed DIC technique was employed to study the transverse wave propagation.

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The ductile failure in metals has long been associated with void nucleation, growth and coalescence. Many micromechanics-based damage models were developed to study the effects of the voids sizes, shape and orientation to the nucleation, growth and coalescence of voids. However, the experimental methods to quantitatively validate these models were lacking. This paper is aimed to experimentally investigate at the microscale and nanoscale the effects of the shapes, sizes, orientation and density to the nucleation, growth and coalescence of voids and their relation to the ductility of the metal. In this work, notched tensile specimens with various radii were designed along different orientations. These specimens were tensile loaded up to different percentage of ultimate failure strain. The deformed specimens were then sectioned both along and perpendicular to the loading direction to microscopically study the voids size, shape and density. On the other hand, microtensile specimens were made out of these already deformed specimens. Using the advanced imaging capabilities of AFM and SEM combined with in-situ loading, the growth and coalescence of voids were in-situ studied at the microscale and nanoscale.