Download Report

KOLSKY BAR WITH ELECTRICAL PULSE HEATING OF THE SAMPLE
Richard Rhorer, Debasis Basak, Gerald Blessing, Timothy Burns, Matthew Davies*, Brian Dutterer,
Richard Fields, Michael Kennedy, Lyle Levine, Eric Whitenton, Howard Yoon
National Institute of Standards and Technology (NIST)
100 Bureau Drive, Mail Stop 8223
Gaithersburg, MD 20899
*University of North Carolina at Charlotte (UNCC)
ABSTRACT
The accuracy of simulations for modeling of machining processes is often limited by insufficient knowledge of the material
4
properties during machining, which can involve strain rates on the order of 10 per second or higher, plus rapid material
heating. By adapting a traditional Kolsky bar (split-Hopkinson pressure bar) to an existing electrical-heating materials test
facility, we have developed at the National Institute of Standards and Technology (NIST) the capability to provide high strainrate, stress-strain data for rapidly preheated metal samples. By electrically heating the sample in ≈500 ms immediately before
the mechanical impact, we limit possible microstructural changes in the sample, and a stress-strain relationship can be
determined at different temperatures for various test materials. The paper will discuss the design and construction of this new
Kolsky bar facility and present preliminary data obtained from pulse heated dynamic tests.
Introduction
th
A manufacturing researcher of the late 19 century, F.W. Taylor, who spent his career studying how a cutting tool removes
metal, said [1]: “There are three questions which must be answered each day in every machine shop: a. What tool shall I
use? b. What cutting speed shall I use? c. What feed shall I use?” Modern machine shops are much different than the shops
of 1900, but they still need to answer the same questions in setting up a new process. Machine tools and the appropriate
tooling must be selected without the traditional trial-and-error because of the cost and lead-time required to set up a new
process. Manufacturing process models, illustrated generically in Figure 1, are often used to aid in-process design and tool
selection. The validity of the process model output depends in part on the material model used to describe the raw material
that is fed into the real manufacturing process.
For machining simulation, the validity of the
modeling process is often tested by making
measurements of temperatures and cutting
forces in real systems and then comparing
these experimental results to values
predicted by the models.
Experimental
mechanics related research is needed in
both the area of measuring the appropriate
material properties for simulation (such as
stress-strain relationships), and of obtaining
the experimental results needed to verify
machining models (such as thermal
gradients near the cutting tool tip and cutting
forces). Of course, ultimately we would like
to have a theoretical method of predicting
the needed material properties for input into
the machining simulation models, so that the
hundreds of different alloys and heat
treatments encountered in machining would
Raw
Material
Theoretical
Material
Material
Properties
Properties
Manufacturing Process
Output
Product
Real world
Comparison
experiments
Measure ments
Material
Data
Process Model
Product
Description
Measurements critical for developing
theoretical material property models
Fig. 1. Measurements required for modeling manufacturing processes.
not have to be individually measured. Although the theoretical prediction of properties remains a long-term goal, we currently
need to perform material tests on each of the materials of interest to support the machining simulations. However, the goal of
a fundamental understanding of dynamic material properties places an additional challenge on our measurement approaches.
Often the level of detail and precision of our dynamic material properties measurements is driven by the material scientists
working on the fundamental prediction of material properties. Therefore, simultaneously with providing data for machining
simulation, we are working with material scientists to develop appropriate measurements that aid in the fundamental
understanding of material properties.
th
Some of the earliest approaches of scientifically analyzing metal removal or machining processes in the mid 20 century, such
as the famous Merchant orthogonal machining model [2], assumed a perfectly plastic material description, where a single
value of flow stress was postulated. In recent times finite element modeling (FEM) methods have been developed to aid in the
simulation of machining processes [3]. These models can handle more complicated geometries, and can more completely
simulate the actual machining situations encountered in machine shops.
Also, FEM models can use a stress-strain
relationship (constitutive model) as a function of strain, strain-rate, and temperature. Most of the stress-strain relationship data
-1
currently available for commonly machined metals were obtained by testing at slow strain rates (less than 1.0 s ) whereas
4 -1
metal removal by machining involves strain rates of 10 s or higher [4]. In addition, machining involves rapid temperature
increases in the cutting zone up to the range of 1000 °C with high heating rates (50,000 °C/s or higher.) To obtain stress-strain
data to aid in the use of finite element models for machining processes, we have established at the National Institute of
Standards and Technology (NIST) a pulse-heated Kolsky bar (split-Hopkinson pressure bar).
Kolsky Bar Apparatus
The NIST Kolsky bar apparatus was fabricated in the NIST shops and assembled in a special projects building. The design
was based in part on discussions with researchers at The Johns Hopkins University and the Army Research Laboratory [5],
along with published information, such as the articles in the American Society of
Metals (ASM) Handbook [6]. The completed Kolsky bar in the lab at NIST is shown in
Figure 2. The apparatus consists of two long straight bars (each bar is 15 mm
diameter by 1.5 m long made from 350 Maraging steel), called the incident and
transmitted bars. In testing, the material sample is sandwiched between the two bars.
The bars are mounted in bearings to allow only pure axial motion. The apparatus
includes an air gun to accelerate a striker bar into the incident bar at velocities up to
40 m/s. The striker bar is a short piece (usually a 250 mm length) of the same
material as the incident and transmitted bars.
The bar system is mounted on a precision base structure 5 m long, made in five
sections. Each section consists of an H-beam (200 mm wide by 225 mm deep), with
steel plates welded on the ends. Each section is precision machined on both ends
and the top, then bolted together to form a long, straight, wide, flat surface for
mounting the bar supports. Two A-frames, which have casters and screw pads for
leveling, support each of the five base sections. The sections can be rolled into place
and then bolted together to form the Kolsky bar base structure.
Photo by Robert Rathe
Fig. 2. Loading a sample in the
NIST pulse-heated Kolsky bar.
The Kolsky bar technique has an advantage of being able to determine large plastic
strains in the sample by measuring the elastic strains, as a function of time, in the
bars. This strain data is then used to calculate the stress and strain in the sample as
a function of time. Measuring large dynamic plastic strains is difficult, but measuring
the elastic strains in the bars is straightforward using conventional variable resistance
strain gages. In operation, the output of the system is the oscilloscope-recorded
signals from the strain gages mounted in the centers of the incident and transmitted
bars.
For the NIST Kolsky bar, a single 1000 Ω metal foil gage with a 3 mm gage length (Micro-measurements∗ Model WK-06125AC-10C) is bonded in the center of each 1.5 m long bar. The output as a function of time for each gage (i.e., the change in
resistance proportional to the strain) is determined by using a standard single-arm Wheatstone-bridge powered by four 6 V dry
cells (to provide a noise-free power source of approximately 24 V DC). Before and after each series of tests the bridges are
calibrated by the standard parallel resistor technique using a precision resistor sized to provide a 5000 µstrain (or 0.5 % strain)
calibration pulse. The strain bridge outputs during the tests are recorded on an oscilloscope, using either a Nicolet Model 440*
or a Nicolet Model Odyssey XE*. The advantage of the modern oscilloscope is that traditional triggering concerns are
∗
Specific manufacturers have been mentioned to aid in the complete documentation of the equipment used for this project.
This is not an endorsement or recommendation in any way for a particular manufacturer’s equipment.
eliminated. The scope has the capability of writing directly to the hard disk drive for several minutes while recording on two
channels at one microsecond resolution. The scope is started before the test, and after the test has been completed, only the
few milliseconds of actual test data are located in the file and permanently saved.
Fig. 3. Typical strain gage output from the Kolsky bar apparatus.
A typical strain gage record is shown in Figure 3.
These digital voltage signals are transferred to a
spreadsheet (such as Excel) or a Matlab program
for data reduction.
The voltage signals are
converted to strain using a record of the voltage
output for a 5000 µstrain calibration pulse before
and after each test series (approximately 90
µstrain/mV). These strain signals from the gages on
the bars are then used to calculate the stress and
strain in the samples using the common approach
as presented in the ASM Handbook article by Gray
[6] and others. The reflected pulse is directly
proportional to the strain rate of deformation in the
sample. This signal can be integrated to calculate
the strain as a function of time. The transmitted
pulse is directly proportional to the stress in the
sample. The sample stress as a function of time is
then plotted versus the sample strain as a function
of time to produce the dynamic stress strain curves
as shown in Figure 4. By varying the projectile
impact velocity, different strain rates can be
obtained. In the current configuration of the NIST
Kolsky bar apparatus, strain rates from
-1
4 -1
approximately 500 s to 10 s can be produced.
For most metals the stress-strain curve shows a
higher yield stress for higher strain rates.
The NIST pulse-heated Kolsky bar is different from
many similar systems because metal bearings are
not used to support the rods.
A non-conducting
bearing made from acetal plastic (Delrin*) is used for
all the bearings except the center two which are
graphite lined metal sleeves. The support posts for
the center two bearings are electrically isolated from
the base structure and connected by welding cables
to the DC electric pulsing circuit. This support
bearing design allows the bars to be used to conduct
an electric pulse to the sample prior to the impact test
thereby providing the capability for dynamic stressstrain curves at controlled elevated temperatures.
Pulse Heating System and Thermal
Measurements
To generate material data useful for machining
simulation it is necessary to have the capability of
heating the samples. The NIST pulse-heated Kolsky
bar project approached the material heating from the
standpoint of heating the sample rapidly enough to
suppress microstructural changes in the material prior to the high strain rate test. Rapid pulse heating more closely simulates
the machining process than using a slower furnace to preheat the sample prior to an impact test.
Fig. 4. Comparison of preliminary results of a pulse heated test.
-1
The upper curve (room temperature) is at ≈ 2500 s and the lower
-1
curve (pulse heated to ≈ 580 °C) is at a strain rate of ≈ 4500 s .
The NIST Subsecond Thermophysics Lab has an existing capability that allows us to resistively heat the sample in a very
controlled fashion. The heating system power source is 2 V batteries and up to 24 batteries can be connected in series. The
total power capacity of this electrical system is sufficient to increase the temperature of a sample from room temperature to
above 1000 °C in less than 500 ms. The switching system consists of 20 computer controlled Field Effect Transistors (FETs) in
parallel. The system can be operated either in the time-control mode or the temperature-control mode. Changing the number
of batteries in the circuit and the resistance of a variable resistor controls the current through the sample. The voltage across
a calibrated standard resistor is used to measure the current through the circuit.
The measurement of the sample temperature during the tests is accomplished by two different measuring systems. A single
spot temperature of the sample is provided by a near-infrared micro-pyrometer (NIMPY). A video of thermal images of the
sample is recorded, simultaneously with the NIMPY, using a thermal camera. The NIMPY consists of a refractive 5x
microscope objective with a numerical aperture of 0.14 attached to a traditional microscope body. The thermal measurement is
performed with an InGaAs detector with ≈1 µs response time. The thermal camera (a 320 by 256 InSb array) is liquid nitrogen
cooled to reduce the dark current and is used without any filters with a 25 mm Si lens and sapphire window for protection.
Thermal images are digitally recorded by the camera and depending on the area of view and the integration time set in the
camera software, effective framing rates over 3000 frames per second can be obtained. Four still thermal camera frames
from a test series of over 2000 frames are shown in Figure 5. The first frame (a) is from shortly after the current is started in
the test, and the subsequent frames lead up to impact. These images can be used to evaluate the uniformity of the heating
without knowing the exact temperatures. The temperature calibration procedures are presented by Yoon [7].
(a)
(b)
(c)
(d)
Fig. 5. Uncalibrated thermal images of the Kolsky bar sample during the heating phase prior to impact showing
progression of heating: (a) after heating starts and 323 ms before impact, (b) 148 ms before impact, (c) 1.17 ms
Test
Samples
before impact, (d) one frame after impact—image partially blocked by shadow of bar.
The pulse heating capability of the NIST Kolsky bar produces unique requirements for the material samples. As presented by
Gray [6, p. 463] the traditional Kolsky bar samples are made so that the diameter is approximately 80 % of the diameter of the
bar and the ratio of diameter-to-length is two. For the NIST pulse-heated Kolsky bar a sample size of 4 mm diameter by 2 mm
length was selected. The diameter of the sample is approximately 27 % of the bar diameter; however, the diameter-to-length
ratio of two was maintained. The smaller sample diameter was required to produce a high current density in the sample
compared to the bars. The volume of the section of the bars carrying the current into the sample is on the order of 1000 times
the volume of the sample. This allows the sample to be heated while the bars remain close to room temperature. An analysis
of this heating approach is presented by Basak et al [8]. Preliminary tests of the heating of the 4 mm diameter by 2 mm
length stainless steel samples indicate that we can achieve samples temperatures of over 1000 °C in less than 500 ms with
the pulse heating system.
Test Samples
The traditional Kolsky bar sample is lubricated with a thin layer of grease to minimize friction between the ends of the bars and
the sample during testing. The grease also helps hold the sample in place prior to the test. Because we require good
electrical contact between the sample and the bars we have evaluated several alternatives to conventional grease, such as a
conducting grease (with nickel particles) and a thin graphite layer. The best heating to date has been with the graphite layer,
which provides a thin soft layer to increase
Misalignment between bars
S ample 2 mm thick
contact.
The preliminary testing has
produces
uneven
loading
of
sample
By 4 mm diameter
shown
that
uniform contact is difficult to
and uneven heating
achieve and therefore we have fabricated
samples with very flat and parallel
surfaces. The samples are fabricated in
the optical shop by lapping and polishing to
optical quality the 4 mm diameter surfaces
and achieving a parallelism of the surfaces
of less than 5 µm. The need for uniform
Incident bar 1. 5 m long by
contact on the sample surfaces also
Transmitted bar 1. 5 m long by
15 mm diameter
requires special care in the fabrication and
15 mm diameter
alignment of the Kolsky bars as illustrated
in Figure 6. The fabrication and alignment
S train gage 2
approach is presented by Rhorer et al [9].
S train gage 1
S ample
S triker bar
Fig. 6. Precision alignment needed for the pulse heated Kolsky bar.
The uniformity of the heating of the
samples is evaluated by thermal camera
imaging as shown in Figure 5. These images are single frames taken from a video recording of a complete test showing that
the sample is quite uniform at the time of impact, but appears to heat from the topside first. After obtaining these images we
are improving the contact between the sample and the ends of the bars. One approach being investigated is to preload the
outer ends of the Kolsky bars with elastic bands to provide increased contact pressure on the sample for better electrical
contact but not seriously affecting the strain wave propagation through the bars. Preliminary results of this preload indicate
some improvement in bar-sample contact.
Discussion and Future Work
The testing to date with the NIST pulse-heated Kolsky bar has demonstrated we can produce dynamic stress-strain data both
at room temperature and at elevated temperatures as shown in Figure 4. Comparison of the room temperature data with the
high temperature data indicates that flow stresses are considerably lower at high temperature resulting in greater plastic strain,
as expected. Work is underway to evaluate the uncertainty in the results and compare with the results of other researchers.
The long-range project goals include extensive tests with AISI 1045 steel, which is of interest because of its widespread usage
in automotive manufacturing.
The strain gage records as shown in Figure 3 are similar to many published records. These signals show that the strain rate
(proportional to the reflected pulse) is not exactly constant during the test. Therefore we have initiated an effort on this project
to pulse shape the incident pulse to produce a less dispersive strain wave. One of the authors (Burns) has been working to
adapt a method developed at Sandia National Laboratories by Frew, Forrestal and Chen [10] to design an appropriate pulse
shaper.
Additional areas of work include developing more uniform and consistent electrical contact between the samples and bar ends.
A method of lapping the bar ends in place is being developed which will allow the bars ends to be flat, smooth and parallel to
achieve the best possible contact with the sample. This lapping technique could also address any surface defects produced
during previous tests. Techniques such as plating the sample with a thin soft layer (possibly gold) are being considered so
that the sample will more completely contact over its entire surface and thereby allow more uniform heating. Work is also
underway to develop a method of rapidly heating the sample and briefly holding it at a preset temperature using a temperature
control algorithm and feedback from the micro pyrometer. This would allow a high degree of repeatability for preset
temperature tests.
Acknowledgements
The authors acknowledge the support of NIST ATP intramural funding, Jack Boudreaux, Program Manager, and the technical
and administrative support of the NIST Predictive Process Engineering Program, Kevin Jurrens Program Manager. Jack
Fuller of the NIST Optical Shop fabricates the special samples being used in the pulse-heated Kolsky bar apparatus.
References
1.
2.
Taylor, F.W., On the Art of Cutting Metals, Transactions of ASME 28, 31-350 (1906).
Merchant. E., Basic Mechanics of the Metal-Cutting Process, Journal of Applied Mechanics, ASME 66, A168-A175
(1944).
3. Marusich, T.D., and Ortiz, M., Modelling and Simulation of High-Speed Machining, International Journal for Numerical
Methods in Engineering 38, 3675-3694 (1995).
nd
4. Kalpakjian S., Manufacturing Processes for Engineering Materials, 2 Ed., Addison-Wesley, Reading, MA, 1991, p.
46.
5. Private communication. Matthew Davies, Brian Dutterer, Richard Fields, Lyle Levine, Timothy Burns, and others from
NIST met with K.T. Ramesh at The John Hopkins University, and with Wayne Chen of the University of Arizona and
Tusit Weerasooriya at the U.S. Army Research Laboratory in Aberdeen, MD, prior to starting the design of the NIST
Kolsky bar.
th
6. Gray, G.T. III, Classic Split-Hopkinson Pressure Bar Testing, ASM Handbook 8, 10 Ed, 462-469 (1998).
7. Yoon, H., Basak, D., Rhorer, R., Whitenton, E., Burns, T., Fields, R., and Levine, L., Thermal Imaging of Metals in a
Kolsky-Bar Apparatus, Thermosense 03, Orlando, April 2003 (to appear).
8. Basak, D., Yoon, H.W., Rhorer, R., and Burns, T., Microsecond Time-Resolved Pyrometry During Rapid Resistive
th
Heating of Samples in a Kolsky Bar Apparatus, 8 Temperature Symposium, Chicago, October 21-23, 2002.
9. Rhorer, R. L., Davies, M. A.*, Kennedy, M. D., Dutterer, B. S., and Burns, T. J., Construction and Alignment of a
Kolsky Bar Apparatus, American Society for Precision Engineering, Annual Conference, St. Louis, October 20 –23,
2002.
10. Frew, D. J., Forrestal, M. J., and Chen, W., Pulse Shaping Techniques for Testing Brittle Materials with a SplitHopkinson Pressure Bar, Experimental Mechanics 42, 93-106 (2002).