Steven P. Mates

Calibration of a two-color imaging pyrometer and its use for particle measurements in controlled air plasma spray experiments

Advances in digital imaging technology have enabled the development of sensors that can measure the temperature and velocity of individual thermal spray particles over a large volume of the spray plume simultaneously using imaging pyrometry (IP) and particle streak velocimetry (PSV). This paper describes calibration, uncertainty analysis, and particle measurements with a commercial IP-PSV particle sensor designed for measuring particles in an air plasma spray (APS) process. Yttria-stabilized zirconia (YSZ) and molybdenum powders were sprayed in the experiments. An energy balance model of the spray torch was used to manipulate the average particle velocity and temperature in desired ways to test the response of the sensor to changes in the spray characteristics. Time-resolved particle data were obtained by averaging particle streaks in each successive image acquired by the sensor. Frame average particle velocity and temperature were found to fluctuate by 10% during 6 s acquisition periods. These fluctuations, caused by some combination of arc instability, turbulence, and unsteady powder feeding, contribute substantially to the overall particle variability in the spray plume.

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.

Dynamic Tensile Behavior of a Quenched and Partitioned High Strength Steel using a Kolsky Bar

Dynamic tension tests were performed on a quenched and partitioned high strength steel grade, QP980, using a direct tension Kolsky Bar method. In this method, the steel incident bar consists of a tube section and a solid section of equal impedance mated through a threaded connection. The striker is pneumatically launched within the tube section into an impact cap to create the tensile loading pulse. The transmission bar, which is constructed of aluminum to improve the force measurement sensitivity, is not impedance matched to the incident bar, and as a result the wave analysis technique was modified accordingly. The sample geometry follows ISO 26203-1:2010. Strain-time histories of the specimens obtained by the wave analysis were compared to high speed DIC strain field measurements, and the latter were used to correct the compliance of the test setup. Material tests were performed parallel to, perpendicular to, and at 45° with respect to the rolling direction. Specimens were taken to failure and to several intermediate strain levels by using momentum traps on the incident and transmission bars. Specimen gauge length, gas pressure and striker bar length were changed to achieve different strain rates, covering the range needed for crash simulations. The dynamic behavior of the material is compared to its quasi-static behavior.

Evaluating the Relationships Between Surface Roughness and Friction Behavior During Metal Forming

The inhomogeneous distribution of surface asperities generated by deformation induces variability in the friction and initiates strain localizations during metal forming. The friction literature generally does not account for the strong influence localized variations in material properties have on the friction behavior. A prototype apparatus was developed that measures the friction behavior under simulated forming conditions and enables detailed characterization of the influences of the microstructure and the topographical conditions that occur under those conditions. The results demonstrate that the measurement system can resolve subtle real-time changes in the dynamic friction coefficient, and that a correlation could exist between the largest surface asperities and the largest variations in the measured friction coefficient.

Effect of fiber gripping method on the single fiber tensile test: II. Comparison of fiber gripping materials and loading rates

Single poly(p-phenylene terephthalamide) (PPTA) fiber tensile tests were carried out under quasi-static and high strain rate loading conditions using poly(methyl methacrylate) and rubber grips to investigate effects of grip materials and loading rates on fiber tensile properties. Differences in ultimate tensile strengths, failure strains, and moduli of PPTA fibers obtained by two different grip materials were insignificant. On the other hand, the fiber tensile properties showed significantly rate-dependent behaviors, which were graphically confirmed by kernel density plots as a non-parametric statistical analysis. Strength models considering three aspects (stochastic, fracture mechanics, and polymer chain domain behaviors) were also shown to link the loading rate effect in relation to fracture mechanisms.

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.

Effects of Microstructure on the Strain Rate Sensitivity of Advanced Steels

The dependence of the strain rate sensitivity of advanced ~1 GPa tensile strength steels on the phases present in their microstructures was studied by testing different steels at 0.005 and 500 s−1. The high strain rate tests were performed using a Kolsky bar setup, while the quasi-static tests were performed using a universal testing machine. The two main steels of interest were the Ferrite-Martensite DP980 and the Ferrite-Martensite-Austenite QP980; the latter being a transformation induced plasticity (TRIP) assisted steel. For comparison, ferritic CR5 mild steel and austenitic stainless steel 201 were also tested under the same conditions. Though the differences in the steel chemistries were not taken into account, the results obtained here suggest a strong relationship between the phase-content of the steel and its response to the changes in the loading rate. The relationships between the observed mechanical behavior and the phases present in the microstructure are discussed.

High-Strain-Rate Deformation of Ti-6Al-4V Through Compression Kolsky Bar at High Temperatures

In this paper, we present our first results from the study of the constitutive response of a popular Titanium alloy, Ti-6Al-4V, using a variation of the compression Kolsky Bar technique that employs electrical pulses to achieve high temperatures. Experiments are conducted at temperatures ranging from room temperature to 1000 °C at a strain rate of about 2200 s−1 and a heating rate of about 1500 °C/s. The dynamic stress-strain results demonstrate significant thermal softening in the alloy that could be described by Johnson-Cook equation with m = 0.8 up to 650 °C. Above 650 °C the rate of change in the flow stresses was faster, which is attributed to allotropic transformation that results in a change in the phase fractions of the hcp and bcc phases present in the alloy. Evidence of transformation is observed in the microstructure of post-compression specimens, which showed an acicular morphology formed from the high temperature bcc phase on quenching.

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