John S. Korellis

A microstructurally based method for stress estimates

Abstract A method is described in which quantitative microstructural analysis is used to estimate the local stress and strain states occurring within near-surface layers due to frictional contact. Quantitative estimates of local stress and strain have applications in friction and wear models, in finite element analysis of sliding interfaces, and as a basis for formulating and evaluating models on a local scale. This method is illustrated for three cases of dry sliding on nominally flat surfaces. Sliding tests were performed on a flat plate friction tester, developed at Sandia, which used copper friction samples and a steel test platen. The evolving friction coefficients were measured as a function of normal load and sliding speed. Microstructural analyses included both scanning and transmission electron microscopy (SEM and TEM) of the cross-sectioned friction samples. The sliding-induced dislocation substructures were quantitatively characterized and measured as a function of subsurface depth and normal load. Two simple relationships between the size scale of the dislocation substructure and the flow stress were used to estimate the material properties and the stress state as a function of depth and normal load.

Improved Kolsky tension bar for high-rate tensile characterization of materials

A new Kolsky tension bar has been re-designed and developed at Sandia National Laboratories, CA. The new design uses the concept that a solid striker is fired to impact an end cap attached to the open end of the gun barrel to generate dynamic tensile loading. The gun barrel here serves as part of the loading device. The incident bar that is connected to the gun barrel and the transmission bar follow the design similar to the Kolsky compression bar. The bar supporting and aligning systems are the same as those in the Kolsky compression bar design described by Song et al (2009 Meas. Sci. Technol. 20 115701). Due to the connection complication among the gun barrel, bars and specimen, stress-wave propagation in the new Kolsky tension bar system is comprehensively analyzed. Based on the stress-wave analysis, the strain gage location on the incident bar needs to be carefully determined. A highly precise laser-beam measurement system is recommended to directly measure the displacement of the incident bar end. Dynamic tensile characterization of a 4330-V steel using this new Kolsky tension bar is presented as an example.

Improved Kolsky-bar design for mechanical characterization of materials at high strain rates

A Kolsky apparatus with numerous modifications has been designed for mechanical characterization of materials at high strain rates. These modifications include employing a highly precise optical table, pillow blocks with Frelon®-coated linear bearings as bar supports and a laser system for better precision bar alignment, etc. In addition, the striker bars were coated with Teflon® to minimize the friction with the gun barrel after removal of the conventional plastic sabots. This new design significantly simplifies the alignment process, improving the final alignment and calibration in the bar system; both are critical for validity and accuracy of the resulting data. An example of a dynamic experiment on a 6061 aluminum specimen by using this newly designed Kolsky bar is also presented.

A newly developed Kolsky tension bar.

Mechanical characterization of materials requires highly precise and reliable experimental facilities. At 2009 SEM conference, we presented a newly developed Kolsky compression bar at Sandia National Laboratories, Livermore, CA. Comparing the compression bar, development of Kolsky tension bar is much more challenging. In this study, besides remedies for the Kolsky compression bar design were used for the new tension bar, the loading device facilitating tension wave was newly designed. The newly developed Kolsky tension bar was demonstrated reliable and precise for investigation of stress-strain behavior as well as damage and failure response of materials under impact loading conditions.

Experimental results of single screw mechanical tests: a follow-up to SAND2005-6036.

The work reported here was conducted to address issues raised regarding mechanical testing of attachment screws described in SAND2005-6036, as well as to increase the understanding of screw behavior through additional testing. Efforts were made to evaluate fixture modifications and address issues of interest, including: fabrication of 45{sup o} test fixtures, measurement of the frictional load from the angled fixture guide, employment of electromechanical displacement transducers, development of a single-shear test, and study the affect of thread start orientation on single-shear behavior. A286 and 302HQ, No.10-32 socket-head cap screws were tested having orientations with respect to the primary loading axis of 0{sup 0}, 45{sup o}, 60{sup o}, 75{sup o} and 90{sup o} at stroke speeds 0,001 and 10 in/sec. The frictional load resulting from the angled screw fixture guide was insignificant. Load-displacement curves of A286 screws did not show a minimum value in displacement to failure (DTF) for 60{sup o} shear tests. Tests of 302HQ screws did not produce a consistent trend in DTF with load angle. The effect of displacement rate on DTF became larger as shear angle increased for both A286 and 302HQ screws.

Shear Deformation of High Density Aluminum Honeycombs

The orthotropic crush model has commonly been used to describe the constitutive behavior of honeycomb [1]. To completely define the model parameters of a honeycomb, experimental data of axial crushes in T, L, and W principal directions as well as shear stress-strain curves in TL, TW, and LW planes are required. The axial crushes of high-density aluminum honeycombs, e.g., 38 pcf (pound per cubic foot), under various loading speeds and temperatures have been investigated and reported [2]. This paper describes experiments and model simulations of the shear deformation of the same high-density aluminum honeycomb. Results of plate shear test, beam flexure test, and off-axis compression are presented and discussed.Copyright

Experimental Study of Voids in High Strength Aluminum Alloys

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 [1-2]. However, the experimental methods to quantitatively validate these models were lacking. In this work, the ductility and the microstructural characteristics of the voids in high strength aluminum is investigated. The particular material interested in this work is high strength rolled Aluminum 7075-T7351.

Study of Damage Evolution in High Strength Al Alloy using X-Ray Tomography

Many micromechanics-based damage models were developed to mimic the macroscopic response of materials through matching measures such as toughness and failure strain [1]. However, there is a lack of microstructural experimental data to identify the roles of the initiation, growth and coalescence of voids to damage and failure. This paper is aimed to experimentally investigate the microstructure of the material and understand the damage processes leading to failure. Experiments using X-Ray Computed Tomography (XRCT) 3D imaging technique with in-situ loading were conducted [2 - 5].

Experiments for Calibration and Validation of Plasticity and Failure Material Modeling: 6061-T651 Aluminum

Experimental data for material plasticity and failure model calibration and validation were obtained from 6061-T651 aluminum, in the form of a 4-in. diameter extruded rod. Model calibration data were taken from smooth tension, notched tension, and shear tests. Model validation data were provided from experiments using thin-walled tube specimens subjected to path-dependent combinations of internal pressure, extension, and torsion.

Experiments for calibration and validation of plasticity and failure material modeling: 304L stainless steel.

Experimental data for material plasticity and failure model calibration and validation were obtained from 304L stainless steel. Model calibration data were taken from smooth tension, notched tension, and compression tests. Model validation data were provided from experiments using thin-walled tube specimens subjected to path dependent combinations of internal pressure, extension, and torsion.