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.
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.
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.
There has been increasing demand to understand the stress-strain response as well as damage and failure mechanisms of materials under impact loading condition. Dynamic tensile characterization has been an efficient approach to acquire satisfactory information of mechanical properties including damage and failure of the materials under investigation. However, in order to obtain valid experimental data, reliable tensile experimental techniques at high strain rates are required. This includes not only precise experimental apparatus but also reliable experimental procedures and comprehensive data interpretation. Kolsky bar, originally developed by Kolsky in 1949 [1] for high-rate compressive characterization of materials, has been extended for dynamic tensile testing since 1960 [2]. In comparison to Kolsky compression bar, the experimental design of Kolsky tension bar has been much more diversified, particularly in producing high speed tensile pulses in the bars. Moreover, instead of directly sandwiching the cylindrical specimen between the bars in Kolsky bar compression bar experiments, the specimen must be firmly attached to the bar ends in Kolsky tensile bar experiments. A common method is to thread a dumbbell specimen into the ends of the incident and transmission bars. The relatively complicated striking and specimen gripping systems in Kolsky tension bar techniques often lead to disturbance in stress wave propagation in the bars, requiring appropriate interpretation of experimental data. In this study, we employed a modified Kolsky tension bar, newly developed at Sandia National Laboratories, Livermore, CA, to explore the dynamic tensile response of a 4330-V steel. The design of the new Kolsky tension bar has been presented at 2010 SEM Annual Conference [3]. Figures 1 and 2 show the actual photograph and schematic of the Kolsky tension bar, respectively. As shown in Fig. 2, the gun barrel is directly connected to the incident bar with a coupler. The cylindrical striker set inside the gun barrel is launched to impact on the end cap that is threaded into the open end of the gun barrel, producing a tension on the gun barrel and the incident bar.
The increased demand for Liquefied Natural Gas (LNG) as a fuel source in the U.S. has prompted a study to improve our capability to predict cascading damage to LNG tankers from cryogenic spills and subsequent fire. To support this large modeling and simulation effort, a suite of experiments were conducted on two tanker steels, ABS Grade A steel and ABS Grade EH steel. A thorough and complete understanding of the mechanical behavior of the tanker steels was developed that was heretofore unavailable for the span of temperatures of interest encompassing cryogenic to fire temperatures. This was accomplished by conducting several types of experiments, including tension, notched tension and Charpy impact tests at fourteen temperatures over the range of -191 C to 800 C. Several custom fixtures and special techniques were developed for testing at the various temperatures. The experimental techniques developed and the resulting data will be presented, along with a complete description of the material behavior over the temperature span.
Instrumented, fully coupled thermal-mechanical experiments were conducted to provide validation data for finite element simulations of failure in pressurized, high temperature systems. The design and implementation of the experimental methodology is described in another paper of this conference. Experimental coupling was accomplished on tubular 304L stainless steel specimens by mechanical loading imparted by internal pressurization and thermal loading by side radiant heating. Experimental parameters, including temperature and pressurization ramp rates, maximum temperature and pressure, phasing of the thermal and mechanical loading and specimen geometry details were studied. Experiments were conducted to increasing degrees of deformation, up to and including failure. Mechanical characterization experiments of the 304L stainless steel tube material was also completed for development of a thermal elastic-plastic material constitutive model used in the finite element simulations of the validation experiments. The material was characterized in tension at a strain rate of 0.001/s from room temperature to 800 C. The tensile behavior of the tube material was found to differ substantially from 304L bar stock material, with the plasticity characteristics and strain to failure differing at every test temperature.
Coupled thermal-mechanical experiments with well-defined, controlled boundary conditions were designed through an iterative process involving a team of experimentalists, material modelers and computational analysts. First the basic experimental premise was selected: an axisymmetric tubular specimen mechanically loaded by internal pressurization and thermally loaded asymmetrically by side radiant heating. Then several integrated experimental-analytical steps were taken to determine the experimental details. The boundary conditions were mostly thermally driven and were chosen so they could be modeled accurately; the experimental fixtures were designed to ensure that the boundary conditions were met. Preliminary, uncoupled analyses were used to size the specimen diameter, height and thickness with experimental consideration of maximum pressure loads and fixture design. Iterations of analyses and experiments were used to efficiently determine heating parameters including lamp and heating shroud design, set off distance between the lamps and shroud and between the shroud and specimen, obtainable ramp rates, and the number and spatial placement of thermocouples. The design process and the experimental implementation of the final coupled thermomechanical failure experiment design will be presented.
Non-destructive detection methods can reliably certify that gas transfer system (GTS) reservoirs do not have cracks larger than 5%-10% of the wall thickness. To determine the acceptability of a reservoir design, analysis must show that short cracks will not adversely affect the reservoir behavior. This is commonly done via calculation of the J-Integral, which represents the energetic driving force acting to propagate an existing crack in a continuous medium. J is then compared against a material's fracture toughness (J{sub c}) to determine whether crack propagation will occur. While the quantification of the J-Integral is well established for long cracks, its validity for short cracks is uncertain. This report presents the results from a Sandia National Laboratories project to evaluate a methodology for performing J-Integral evaluations in conjunction with its finite element analysis capabilities. Simulations were performed to verify the operation of a post-processing code (J3D) and to assess the accuracy of this code and our analysis tools against companion fracture experiments for 2- and 3-dimensional geometry specimens. Evaluation is done for specimens composed of 21-6-9 stainless steel, some of which were exposed to a hydrogen environment, for both long and short cracks.
The mechanical properties of some materials (Cu, Ni, Ag, etc.) have been shown to develop strong dependence on the geometric dimensions, resulting in a size effect. Several theories have been proposed to model size effects, but have been based on very few experiments conducted at appropriate scales. Some experimental results implied that size effects are caused by increasing strain gradients and have been used to confirm many strain gradient theories. On the other hand, some recent experiments show that a size effect exists in the absence of strain gradients. This report describes a brief analytical and experimental study trying to clarify the material and experimental issues surrounding the most influential size-effect experiments by Fleck et al (1994). This effort is to understand size effects intended to further develop predictive models.