Kevin Connelly

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.

Fracture Behavior of Polyurethane Foams

ABSTRACT Due to their high energy absorption capabilities, polyurethane (PU) foams have been widely used in many applications. The mechanical behavior of Polyurethane (PU) foams has been attracting the attention from engineers and researchers. But most of work was to study the compressive behavior of PU foams. Very little knowledge is available about the fracture behavior of the PU foams. In this paper, single edge notch bend (SENB) tests are conducted to study the mode-I fracture behavior of a rigid closed cell PU foam, PMDI 20, with a nominal density of 20 pcf (320 kg/m 3 ). The stress intensity factor K IC is calculated from the loading curves. The displacement and strain field around the crack tip is obtained using digital image correlation (DIC) technique. INTRODUCTION Polyurethane (PU) foam can absorb energy by undergoing a large amount of compressive deformation. It has been widely used in packaging and cushioning to protect sensitive objectives. However, they have a tensile failure strain of a few percent or even less than one percent. In our previous uniaxial compression tests, it has been shown that both tensile and compressive strains are developed inside the foam specimens due to the inhomogeneous deformation of the foam specimens during compression and that there is large tensile strain concentration at the interface of foam specimen and the protected object. The tensile deformation causes the failure of the foam specimen by the propagation of the crack [1]. Therefore, it is very important to study the fracture behavior of the foam specimens in order to effectively protect sensitive objects. Most rigid polymer foams are linear-elastic in tension. The fracture failure can thus be treated by the concept of linear-elastic fracture mechanisms [2]. The efforts of determining the fracture toughness of foam materials can be traced back to more than three decades ago. Fowlkes

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.

Coupled Thermal-Mechanical Experiments for Validation of Pressurized, High Temperature Systems

High fidelity finite element modeling of coupled thermal-mechanical failure processes in complex systems requires, as a precursor, high quality experimentation on several levels. The materials must be characterized such that the entire range of loading parameters is encompassed. Meaningful validation experiments must be developed that allow for the steady, incremental ascension of validation towards system level complexity and, eventually, predictability. This paper describes a combined experimental/modeling effort towards validating failure in pressurized, high temperature systems.

Effect of Applied Temperature and Strain Rate on Laser Welded Stainless Steel Structures

Sealed containers that hold organic substances can fail if organic material decomposition that occurs at elevated temperatures causes high enough pressures to cause a breach anywhere within the container or at welded or joined sections of the container. In this study, the response of stainless steel structures sealed by laser welding was of interest. Cylindrical can structures were constructed of two base materials, 304L stainless steel in tube and bar form, and joined by partial penetration laser welding. The base and weld materials contributed to the overall elastic–plastic response that led to failure in the weld region. The response of specimens constructed from sections of the cylindrical can structures was measured experimentally under thermomechanical loadings that investigated applied strain rate and temperatures (25–800 °C). Prior to testing, extensive measurements of the partial penetration weld geometry and cross section were completed on each specimen to enable correlation with measured response and failure. The experimental results of these sub-structure specimens tested at elevated temperatures are presented. Additionally, the material characterization results of the two 304L stainless steel materials used in constructing the cylindrical cans are presented.

Design and Implementation of Coupled Thermomechanical Failure Experiments

The importance of developing the capability to accurately and predictively model failure under combined thermal and mechanical loadings can not be overstated. Development of the necessary constitutive and failure models relies heavily on laboratory experiments that provide detailed information at several levels, from material characterization to laboratory scale validation experiments of increasing complexity, eventually leading up to full scale validation. This work is part of an interdisciplinary program that seeks to develop solutions to a large class of coupled thermomechanical failure problems. Coupled thermal-mechanical experiments with well-defined, controlled boundary conditions were designed and implemented through an iterative process involving a team of experimentalists, material modelers, computational developers and analysts.

Compression Testing of Aged Low Density Flexible Polyurethane Foam

Flexible open celled foams are commonly used for energy absorption in packaging. Over time polymers can suffer from aging by becoming stiffer and more brittle. This change in stiffness can affect the foam’s performance in a low velocity impact event. In this study, the compressive properties of new open-cell flexible polyurethane foam were compared to those obtained from aged open-cell polyurethane foam that had been in service for approximately 25 years. The foams tested had densities of 10 and 15 pcf. These low density foams provided a significant challenge to machine cylindrical compression specimens that were 1 “in height and 1” in diameter. Details of the machining process are discussed. The compressive properties obtained for both aged and new foams included testing at various strain rates (0.05. 0.10, 5 s−1) and temperatures (−54, RT, 74 °C). Results show that aging of flexible polyurethane foam does not have much of an effect on its compressive properties.

Mechanical Properties of 3-D LENS and PBF Printed Stainless Steel 316L Prototypes

Laser Engineered Net Shaping (LENS) and Powder Bed Fusion (PBF) are 3-D additive manufacturing (AM) processes. They are capable of printing metal parts with complex geometries and dimensions effectively. Studies have shown that AM processes create metals with distinctive microstructure features and material properties, which are highly dependent on the processing parameters. The mechanical properties of an AM material may appear to be similar to the corresponding wrought material in some way. This investigation focuses on the relationships among AM process, microstructure features, and material properties. The study involves several AM SS316L components made from 3D LENS and PBF printing. Specimens were taken from different locations and orientations of AM components to obtain the associated tensile properties, including yield, strength, and ductility, and to conduct microstructure analyses.

Failure of Laser Welded Structures Subjected to Multiaxial Loading: Experimental Development

A unique experimental capability was developed so combined mechanical and thermal loads could be imposed on specimens that are representative of laser welded structures. The apparatus, instrumentation and specimens were designed concurrently to yield the ability to apply a wide range of loading conditions that accurately replicate the multiaxial stress states produced in laser welded, sealed structures during pressurization at high temperatures up to 800 °C. Axial, radial and torsional loads can be applied individually or in combination, by direct or variable loading paths, to eventual failure of laser weld specimens. Several advantages exist for applying equivalent stress states by mechanical means rather than pressurization with gas, including: repeatability, controlled failure, safe experiments, assessment of loading path dependence, experimental efficiency and overall facility. The experimental design and development are described along with resulting measurements and findings from sample experiments.