Richard L. Rhorer

DYNAMIC PROPERTIES FOR MODELING AND SIMULATION OF MACHINING: EFFECT OF PEARLITE TO AUSTENITE PHASE TRANSITION ON FLOW STRESS IN AISI 1075 STEEL

The Pulse-Heated Kolsky Bar Laboratory at the National Institute of Standards and Technology (NIST) has been developed for the measurement of dynamic properties of metals. With this system, a small sample can be pre-heated from room temperature to several hundred degrees C in less than a second, prior to rapid loading in compression at strain rates up to the order of 104 per second. A major focus of this research program has been on investigating the influence of the heating rate and time at temperature on the flow stress of carbon steels, for application to the modeling and simulation of high-speed machining operations. The unique pulse heating capability of the NIST Kolsky bar system enables flow stress measurements to be obtained under conditions that differ significantly from those in which the test specimens have been pre-heated to a high temperature more slowly, because there is less time for thermally activated microstructural processes such as dislocation annealing, grain growth, and solid state phase transformations to take place. New experimental results are presented on AISI 1075 pearlitic steel samples that were pulse-heated up to and beyond the austenite formation temperature of the material (723 °C). The data show that the flow stress decreased by about 50 % due to a phase transformation in the microstructure of the material from the stronger pearlitic phase to the weaker austenitic phase. As a result, the constitutive response behavior of the material cannot be modeled by a fixed-parameter constitutive model, like the Johnson-Cook flow stress model that is widely used in computer simulations of high-speed machining processes.

Smart machining systems: issues and research trends

Smart Machining Systems (SMS) are an important part of Life Cycle Engineering (LCE) since its capabilities include: producing the first and every product correct; improving the response of the production system to changes in demand (just in time); realizing rapid manufacturing; and, providing data on an as needed basis. Thereby, SMS improve the performance of production systems and reduce production costs. In addition, an SMS not only has to improve a particular machining process, but it also has to determine the best optimized solution to produce the part faster, better, at lower cost, and with a minimum impact on the environment. In addition, new software tools are required to facilitate the improvement of a machining system, characterized by a high level of expertise or heuristic methods. A global approach requires integrating knowledge/information about the product design, production equipment, and machining process. This paper first discusses the main characteristics and components that are envisioned to be part of SMS. Then, uncertainties associated with models and data and the optimization tasks in SMS are discussed. Robust Optimization is an approach for coping with such uncertainties in SMS. Current use of machining models by production engineers and associated problems are discussed. Finally, the paper discusses interoperability needs for integrating SMS into the product life cycle, as well as the need for knowledge-based systems. The paper ends with a description of future research trends and work plans.

Microsecond Time‐Resolved Pyrometry during Rapid Resistive Heating of Samples in a Kolsky Bar Apparatus

Analysis of machining processes is important in the understanding and improving of manufacturing methods. The modeling of machining processes relies on high‐strain rate, high‐temperature material properties. A split‐Hopkinson pressure bar (or Kolsky bar) is being installed in a NIST high‐current pulse‐heating facility. By heating the material sample rapidly with a controlled current pulse immediately before the mechanical impact in the bar, structural changes in the sample are inhibited, thus better simulating conditions during machining. A stress‐strain relationship can be determined at various temperatures for test materials. We describe the design and the development of a millisecond‐resolution split‐Hopkinson apparatus, where the sample is resistively heated by the passage of a sub‐second‐duration electric current pulse. The impact bar is constructed out of maraging steel and the sample is a cylinder of AISI 1045 steel. The current is transmitted through the oiled‐bronze sleeve bushing of the impact b...

Effects of fiber gripping methods on single fiber tensile test using Kolsky bar

Preliminary data for testing fibers at high strain rates using the Kolsky bar test by Ming Cheng et al. [1] indicate minimal effect of strain rate on the tensile stress-strain behavior of poly (p-phenylene terephathalamide) fibers. However, technical issues associated with specimen preparation appear to limit the number of samples that can be tested in a reasonable time. In addition, under the Kolsky bar testing condition fiber fracture may occur at the interface between the fiber and adhesive rather than in the gage section. In this study, the authors investigate the effects of different gripping methods in order to establish a reliable, reproducible, and accurate Kolsky bar test methodology for single fiber tensile testing. As many single fiber tests have been carried out associated with ballistic research, we compare the Kolsky bar test results with the quasi-static test results to determine the tensile behavior over a wide range of strain rates.

Thermal imaging of metals in a Kolsky-bar apparatus

Since the modeling of machining processes relies on high-strain-rate, high-temperature material properties, NIST has built a split-Hopkinson (or Kolsky) bar to determine the stress-strain behavior of rapidly heated materials at high temperatures. Our Kolsky bar has been constructed in the NIST high current pulse-heating facility, which enables electrically heating the samples within ~ 100 milliseconds time duration, immediately before the mechanical impact in the bar. Due to the rapid heating, we avoid possible structural changes in the sample, and a stress-strain relationship can be determined at different temperatures for various test materials. We describe the design and the development of the resistively-heated Kolsky-bar apparatus. The incident and the transmitted bars are constructed of 1.5 m long, 15 mm diameter maraging steel, and a typical sample is a 4 mm-diameter, 2 mm-long cylinder of 1045 steel. The sample is placed between the bars and held by friction. The current is transmitted through the graphite-sleeve bushings of the two bars. The non-contact temperatures are measured using an InGaAs near-infrared micro-pyrometer (NIMPY) and an InSb focal-plane (320 by 256) array (thermal camera). The NIMPY and the thermal camera are both calibrated using a variable-temperature blackbody, and the thermodynamic temperature of the metal is determined using the emissivity determined from the measured infrared spectral reflectance of the metal. Thermal videos of the electrically-heated and the room-temperature impacts will be shown with 1 kHz frame rates, and the changes in the stress-strain curves with the temperature of the samples will be discussed.

Identifying the Dynamic Compressive Stiffness of a Prospective Biomimetic Elastomer by an Inverse Method

Soft elastomeric materials that mimic real soft human tissues are sought to provide realistic experimental devices to simulate the human bodys response to blast loading to aid the development of more effective protective equipment. The dynamic mechanical behavior of these materials is often measured using a Kolsky bar because it can achieve both the high strain rates (>100s(-1)) and the large strains (>20%) that prevail in blast scenarios. Obtaining valid results is challenging, however, due to poor dynamic equilibrium, friction, and inertial effects. To avoid these difficulties, an inverse method was employed to determine the dynamic response of a soft, prospective biomimetic elastomer using Kolsky bar tests coupled with high-speed 3D digital image correlation. Individual tests were modeled using finite elements, and the dynamic stiffness of the elastomer was identified by matching the simulation results with test data using numerical optimization. Using this method, the average dynamic response was found to be nearly equivalent to the quasi-static response measured with stress-strain curves at compressive strains up to 60%, with an uncertainty of ±18%. Moreover, the behavior was consistent with the results in stress relaxation experiments and oscillatory tests although the latter were performed at lower strain levels.

Modeling and DIC Measurements of Dynamic Compression Tests of a Soft Tissue Simulant

Stereoscopic digital image correlation (DIC) is used to measure the shape evolution of a soft, transparent thermoplastic elastomer subject to a high strain rate compression test performed using a Kolsky bar. Rather than using the usual Kolsky bar wave analysis methods to determine the specimen response, however, the response is instead determined by an inverse method. The test is modeled using finite elements, and the elastomer stiffness giving the best match with the shape and force history data is identified by performing iterative simulations. The advantage of this approach is that force equilibrium in the specimen is not required, and friction effects, which are difficult to eliminate experimentally, can be accounted for. The thermoplastic is modeled as a hyperelastic material, and the identified dynamic compressive (non-linear) stiffness is compared to its quasi-static compressive (non-linear) stiffness to determine rate sensitivity.

Effect on Flow Stress of a Rapid Phase Transition in AISI 1045 Steel

New experimental data on AISI 1045 steel from the NIST pulse-heated Kolsky Bar Laboratory are presented. The material is shown to exhibit a nonequilibrium phase transformation at high strain rate. An interesting feature of these data is that the material has a stiffer response to compressive loading when it has been preheated to a testing temperature that is below the eutectoid temperature using pulse-heating than it does when it has been preheated using a slower heating method. On the other hand, when the material has been pulse-heated to a temperature that exceeds the eutectoid temperature prior to compressive loading on the Kolsky bar, it is shown to exhibit a significant loss of strength. A consequence of this behavior is that fixed-parameter constitutive models, such as the well-known Johnson-Cook model, cannot be used to describe this constitutive response behavior. An argument is made that the phase transition does not occur during high-speed machining operations, and suggestions are made as to how to modify the Johnson-Cook model of Jaspers and Dauzenberg for this material in order to obtain improved temperature predictions in finite-element simulations of high-speed machining processes.Copyright

NIST Mini-Kolsky Bar: Historical Review

The Society for Experimental Mechanics (SEM) has sponsored a series of technical paper sessions titled “Novel Testing Techniques” at their annual meetings. These sessions were organized by the Dynamic Behavior of Materials Technical Division of SEM and started in 2008. One of the novel techniques that we first learned about by attending SEM was the use of a small-size Kolsky bar system especially designed for the testing of polymer-single fibers. The Mini-Kolsky Bar was added to the National Institute of Standards and Technology (NIST) Kolsky Bar Laboratory based, in part, on the work presented at SEM conferences. A number of informal discussions at the annual SEM conferences added to the understanding and design details as we constructed our first small tension Kolsky bar. Subsequent developments of the NIST Mini-Kolsky bar, including improved gripping techniques were presented and discussed at SEM conferences. This paper reviews some of the work presented in the SEM’s Novel Testing Techniques sessions and discusses the history of additional follow-on work precipitated by the original papers.

Dynamic Flow Stress Measurements for Machining Applications

Metals undergo a combination of rapid loading and rapid heating during normal and high speed machining processes. Constitutive models for these materials, however, generally lack any information regarding kinetics of thermally-induced transformations, such as austenite formation in carbon steels, that can have profound effects on their mechanical viscoplastic behavior. The NIST electrically-pulse-heated Kolsky bar was developed specifically to probe material response under conditions approaching those present during machining operations. We have achieved heating rates in excess of 1,000 °C/s combined with strain rates above 1,000 s−1 with this system. This paper presents recent experimental results for AISI 1045 and AISI 1075 steel using the pulse-heated Kolsky bar, and examines some aspects of the uncertainty of the method.