BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to apparatus, and an accompanying
method for use therein, for use in a conventional dynamic thermo-mechanical material
testing system in order to advantageously provide enhanced self-resistive specimen
heating that yields greater temperature uniformity throughout a specimen under test
than heretofore achieved.
Advantageously, our invention also finds use in a conventional
dynamic mechanical material testing system in order to self-resistively heat a test
specimen uniformly over its entire volume. Such heating can be controlled and synchronized
to a mechanical test program, e.g., a predefined series of mechanical deformations,
to impart a desired thermo-mechanical test program to the specimen for use in physical
simulation and/or other material testing applications.
2. Description of the Prior Art
Metallic materials play an indispensable role as an essential
component of an enormous number of
different products and hence occupy an extremely important
part of the world economy. As such, during manufacturing, various properties and
costs of these materials need to be carefully controlled to maximize their utility
and value in a given application.
Different metallic materials possess widely varying mechanical,
metallurgical and other properties. Different applications necessitate use of materials
with different properties. The specific properties required of a material for use
in a given application are first determined followed by selection of a specific
material that exhibits appropriate values of these properties.
During their initial production, metallic materials are
generally formed into slabs or ingots and then from there controllably deformed
into standard sized sheets, rods or coils using, e.g., conventional rolling, forging
and/or extruding operations. However, correctly configuring a rolling mill, forge
or extruder to properly deform production stock and impart desired physical and/or
metallurgical characteristics to the material can be a tedious, time-consuming and
expensive process -- particularly since a production machine needs to be taken out
of productive use for an extended time to properly adjust its operational parameters.
Consequently, to avoid such downtime, the art teaches the general concept of determining
properties of interest by testing relatively small specimens of each such material
under consideration. One such technique for doing so is so-called "physical simulation".
Ideally speaking, this technique, through use of a dynamic material testing system,
permits each such specimen to undergo appropriate mechanical deformation and, where
appropriate, simultaneous thermal processing that, collectively speaking and to
the extent possible, accurately mimic, in a small-scale environment, strains and
other phenomena that the same material (but of a far larger scale) would experience
through an actual production operation, such as rolling, extrusion or forging. Such
simulations, when accurately done, permit proper operational parameters of corresponding
production machinery to be readily ascertained and, concomitantly, minimize non-productive
downtime and its associated high costs.
One crucial property of metallic materials is their ability
to conduct electricity. Absent operation at superconductive temperatures, a metallic
object possesses a resistance to electrical current flow proportional to its length
and resistivity and inversely proportional to its cross-sectional area. Owing to
its resistance, the object will generate heat whenever an electric current is passed
through it. This form of heating, i.e., so-called "self-resistive heating", finds
use in a wide number of diverse applications. To the extent relevant here, dynamic
material thermo-mechanical material testing systems can employ self-resistive heating
to impart a desired thermal profile to each specimen prior to its being deformed
in order to more accurately simulate material temperatures that will be experienced
during a production operation.
Generally, in a conventional dynamic thermo-mechanical
material testing system, a compressive specimen is held between two anvils or, in
the case of a tensile specimen, gripped at each of its two ends in a jaw system.
Since the following discussion applies equally well to both compressive and tensile
testing, for simplicity, we will simply confine that discussion to compression testing.
For compression testing, the specimen is typically in the
form of, for example, a small cylinder of a given material and has a substantially
uniform circular cross-sectional area. Such specimens may be on the order of, e.g.,
10 mm in diameter and 15 mm long; though other sizes are readily used as well. An
electric current is serially passed from one anvil to another and hence generally
cross-wise end-to-end through the specimen to generate a rapid, but controlled,
heating rate throughout the specimen. Simultaneously therewith, various measurements
are made of the specimen. Depending upon the specific measurements being made, the
specimen either may or may not undergo controlled compressive deformation while
it is being heated. If the specimen is to be deformed, then this deformation can
be accomplished by moving one of the two anvils, at a controlled rate with respect
to the other, in order to squeeze the specimen by imparting a given compressive
force to the specimen. This process may be repeated several-times, at differing
amounts and rates of deformation, in order to impart a succession of different deformations
to the specimen, thus yielding differing and accumulating amounts of strain in the
specimen. Physical measurements, such as illustratively specimen dilation and temperature,
are typically made while heating or cooling and deformation are simultaneously occurring.
This testing not only reveals various static properties of the specimen material
itself, such as its continuous heating transformation curve, but also various dynamic
properties, such as illustratively hot stress vs. strain rates and hot ductility;
the dynamic properties being particularly useful in quantifying the behavior of
the material that will likely occur during rolling, forging, extrusion or other
material forming and/or joining operations. One system that provides excellent dynamic
thermo-mechanical testing is the GLEEBLE 3500 system manufactured by the Dynamic
Systems, Inc. of Poestenkill, New York (which also owns the registered trademark
"GLEEBLE" and is the present assignee). Advantageously, this system self-resistively
heats the specimen in order to generate transverse, essentially isothermal planes
along the entire specimen, i.e., the specimen material in each plane uniformly heats
as current passes longitudinally through that plane of the specimen. Consequently,
density of the electrical heating current will be relatively uniform throughout
that cross-section and, as such, will cause substantially uniform heating over that
entire dross-section. Examples of such systems are described in the following United
States patents, all of which are incorporated by reference herein:
6,422,090 (issued to H. S. Ferguson on July 23, 2002
);
5,195,378 (issued to H. S. Ferguson on March 23, 1993
); and
5,092,179 (issued to H. S. Ferguson on March 3, 1992
). See also
US 5315085 (issued to H.S Ferguson on May 24, 1994
).
In such systems, the anvils need to withstand the relatively
high forces imparted to the specimen without noticeably deforming themselves. Hence,
these anvils are physically much larger and considerably more massive than the specimens.
Consequently, for a given amount of self-resistive heating current serially passing
through both the anvils and the specimen, the anvils will attain a much lower temperature
than the specimen. As a result, longitudinal temperature gradients will appear end-to-end
along the specimen, with a central work zone of the specimen being hottest and specimen
temperature falling off towards each anvil -- even though the self-resistive heating
currents cause essentially isothermal planes to occur transversely across the specimen.
These gradients tend to limit the work zone of the specimen to its central region,
thus decreasing the effective length of the specimen for the simple reason that
the hotter work zone tends to increasingly soften and deform before the cooler regions
near the specimen ends do. This, in turn, tends to limit the maximum amount that
the specimen can be compressed during each deformation and hence the maximum strain
and strain rate that could be imparted to the specimen.
To eliminate these longitudinal gradients, conventional
anvil assemblies can be self-heated or separately heated. Unfortunately, owing to
the relatively large mass of the anvils compared to the specimen, the heating time
of the anvils would be considerably larger than that of the specimen which, in turn,
limits a maximum rate at which the specimen could be heated. This, in turn, slows
the thermal response of the entire system and is particularly problematic if more
than one test temperature is required for a given thermo-mechanical program, i.e.,
each successive deformation ("hit") is to be performed at a different specimen temperature.
Specifically, this requires that the anvils reach thermal equilibrium at each such
successive programmed temperature before each hit occurs. Depending on the temperature
excursions involved, the required anvil heating or cooling times could drastically
slow the overall response of the system. Modern material production equipment, such
as multi-stand rolling mills, often dynamically deforms materials at relatively
high-speeds. Hence, any appreciable diminution in the response of the test system,
such as those imposed by limits associated with the anvil heating and cooling rates,
could severely and adversely limit the production processes that could be accurately
simulated by such systems, thus potentially diminishing the attractiveness and cost-efficiencies
otherwise attainable through use of such systems.
As increasingly high-speed production processes are currently
being employed in industry, a concomitant need has recently arisen with conventional
thermo-mechanical material-testing-systems to impart increasingly higher amounts
of strain and strain rates to specimens. Therefore, a need exists in the art for
another approach which can be used in such systems for substantially, if not totally,
eliminating longitudinal thermal gradients from appearing in self-resistively heated
test specimens that are held in anvils (and jaw assemblies). Ideally, such an approach
should not appreciably slow the response of the system but still yield isothermal
planes in the specimens.
To address this need, the art teaches one approach to substantially
eliminate these thermal gradients; namely, by generating sufficient self-resistive
heat within each anvil to approximately equal that which would otherwise flow from
the specimen into the anvil. This, in turn, would eliminate much, if not substantially
all, of any temperature difference otherwise occurring between a top surface of
each anvil and the specimen bulk and hence preclude any appreciable longitudinal
thermal gradients from appearing in the specimen.
To effectuate this approach, the art teaches that each
anvil can be formed of an anvil stack having cylindrically shaped upper and lower
members separated by a foil interface having relatively high-resistance compared
to the anvil. The sides of the anvil stack are electrically and thermally insulated
from its supporting structure by a suitable insulating member, typically a woven
ceramic tubing or rigid ceramic sleeve. As a result-and during one-half cycle of
applied current, self-resistive heating current axially flows up from the support
through a base of the anvil stack, through the high-resistive foil interface, through
an anvil top and into the specimen end (and in an opposite direction during a next
half-cycle of applied current). Since the resistance of the foil interface is relatively
high compared to the anvil, passage of the heating current through the foil interface
causes it to self-resistively heat with the heat propagating throughout the entire
anvil, including the anvil top. The foil interface is typically formed of a stack
having a predefined number of graphite disks, with each disk being of an approximate
diameter of the anvil and of a given thickness to provide a desired resistance.
Unfortunately, the graphite disks, when exposed to the rather high impact forces
transmitted to the anvil stack during each hit, are rather compliant and tend to
deform and unevenly so, from one disk to the next, from each impact. The result
is that the abutting electrical contact amongst the graphite disks as well as the
resistance of each disk changes with each hit which, in turn, adversely affects
the passage of heating current and hence the heating of the specimen.
Therefore, a need still exists in the art for an approach,
for use in a dynamic thermo-mechanical material testing system, that can effectively
and substantially, if not totally, eliminate thermal gradients from appearing in
a specimen then under test while still permitting isothermal planes to occur in
the specimen, but without deforming, to any appreciable extent, if at all, under
the high impact force generated during each hit.
Furthermore, a considerable number of conventional, commercially
available dynamic material testing systems only provide mechanical deformation of
the specimen without any capability of thermally processing of the specimen. These
systems basically only compress a specimen held end-to-end between two anvils which
are themselves moved by a servo-hydraulic or screw driven actuators to controllably
squeeze the specimen. However, to accurately simulate production processes, the
specimen under test needs to undergo controlled uniform thermal processing synchronized
to the occurrence of the mechanical deformations. As such, these systems need to
be modified, by the addition of appropriate apparatus, to possess the capability
of providing accurate self-resistive specimen heating that establishes isothermal
planes across the specimen under test but without causing appreciable, if any, longitudinal
temperature gradients to appear along that specimen. Here too, this apparatus should
not deform, to any appreciable extent, if at all, under the high impact force generated
during each hit. Hence, a need also exists in the art to provide these capabilities
in such conventional mechanical testing systems.
Should these needs be met, then, the attractiveness-of
physically simulating high-speed production processes through dynamic material testing
equipment could very well increase, with advantageously substantial cost savings
flowing there from to their users.
SUMMARY OF THE INVENTION
The present invention advantageously overcomes the deficiencies
in the art by incorporating, into each anvil assembly between an anvil base and
an anvil top, a foil interface that has at least one multi-component composite layer,
with each constituent component therein having specific but differing thermal, electrical
and mechanical properties from the other.
This composite layer, generally in the shape of a disk
and in its preferred embodiment, is preferably concentrically oriented and illustratively
formed from two disks of different materials, but of the same thickness, with one
fitting within the other. One disk, being a thermal and electrical insulator but
with high compressive strength, forms a central portion of the composite layer;
while the other disk, being electrically resistive, forms an outer ring portion
concentrically situated around and peripherally abutting the outer surface of the
central portion. Electrical current only flows through the outer ring portion and
causes that portion to self-resistively heat, while the central portion provides
high impact strength and hence substantial resistance to deformation, thus preventing
the outer ring portion from deforming as a result of a hit. Illustratively, the
central portion is formed of a mica disk and the outer ring portion is formed of
a graphite ring. In fabricating the foil interface to have the proper resistance
and hence heating characteristics, the composite layer may be replicated as often
as necessary with suitable conductive disk, typically of tantalum (or-other suitable
conductive material), placed between successive composite layers (in the shape of
disks), for use within a given anvil stack. Furthermore, all the layers within the
stack need not have the same thickness, as one or more may have differing thicknesses,
in order to properly set the resistance of the entire foil interface. Also, the
materials used for the central and outer-ring portions may be reversed such that
the central portion is conductive, e.g., formed of graphite, while the outer ring-shaped
portion is formed of the high strength insulating material, e.g., mica. The sides
of the anvil stack are electrically and thermally insulated from its supporting
structure by a suitable insulating member, typically a woven ceramic tubing or rigid
ceramic sleeve.
The inventive anvil assembly can simply replace existing
anvil assemblies used in conventional dynamic thermo-mechanical material testing
systems in order to beneficially and advantageously impart substantially uniform
self-resistive heating, throughout the specimen bulk, that exhibits isothermal planes
but without-any appreciable-longitudinal-thermal gradients appearing and without
appreciably deforming, if- at all, as a result of each hit.
Furthermore, our inventive anvil assembly can also be readily
incorporated into conventional mechanical testing systems that lack a capability
to thermally process the specimen in order to provide the same benefits as would
arise through use of these anvil assemblies in a dynamic thermo-mechanical material
testing system.
To do so, a separate fixture, not part of the present invention,
is added to these mechanical testing systems. The fixture includes two opposing
inventive coaxially-oriented anvil assemblies, which collectively hold the specimen
and exerts sufficient force, through the anvils, onto the specimen to permit self-resistive
heating current to serially pass through the anvils and the specimen for heating
the specimen. This fixture utilizes separate supporting arms, each of which holds
a corresponding one of the anvil assemblies, situated on opposing sides of the specimen.
The fixture, through the action of springs, pneumatic cylinders or a combination
thereof, applies force to each arm sufficient to hold the specimen in position between
the opposing anvil assemblies and establish a good abutting electrical contact therebetween,
which permits the current to flow through the specimen but without causing substantially
any arcing, but with insufficient force to deform the specimen as it is being heated.
Separate opposing coaxially-oriented shafts (rams) existing within these systems
which heretofore were extended to simply squeeze the specimen itself are instead
controllably extended to strike the arms and squeeze them together, hence squeezing
the anvils together, to, in turn, generate each deformation "hit" in the specimen.
At the conclusion of each hit, these shafts are suitably retracted in preparation
for the next such hit on the fixture and hence on the specimen. The shafts are electrically
insulated from the fixture so that the self-resistive heating current can be controllably
and serially applied just through the fixture arms, the anvil assemblies and specimen.
Controlled current flow can occur before, during or after each hit to appropriately
and uniformly heat the specimen bulk to a desired temperature at the appropriate
time in its test procedure.
The fixture not part of the present invention can be easily
retrofitted to nearly any commercially available mechanical testing system by simply
being mounted, e.g., within a suitable vacuum/atmospheric tank, to existing supporting
columns in those systems, and preferably within a central working region of the
system, to permit an equal working extension of the existing shafts.
BRIEF DESCRIPTION OF THE DRAWINGS
The teachings of the present invention can be readily understood
by considering the following detailed description in conjunction with the accompanying
drawings, in which:
FIG. 1 depicts, in side view, conventional, commercially
available dual-ram servo-hydraulic mechanical testing system 100;
FIG. 2 depicts, in a sectional view and taken along lines
2-2 shown in FIG. 4, self-resistively heated anvil assembly 200 that incorporates
the teachings of the present invention;
FIG. 3 depicts, in accordance with our invention and in
exploded view, illustrative anvil stack 300 that forms part of anvil assembly 200
shown in FIG. 2; and
FIG. 4 depicts, also in side view, fixture 400, not being
part of the present invention that incorporates the anvil assembly 200 shown in
FIG. 2, which would be incorporated into the conventional system shown in FIG. 1.
To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements that are common
to the figures.
DETAILED DESCRIPTION
After considering the following description, those skilled
in the art will clearly -realize that the broad teachings of the invention can be
readily utilized in conjunction with a wide variety of material testing systems,
including both dynamic mechanical and thermo-mechanical material testing systems,
that controllably deform a test specimen to permit such systems to implement complex
thermal and mechanical programs for use in simulating a wide variety of production
processes and material applications. For simplicity, we will discuss our invention
in the context of its incorporation and use within a relatively simple and generic
dynamic mechanical material testing system. Based on that description, any one of
skill in the art can readily and easily ascertain how the teachings of our invention
can advantageously be incorporated into any one of a wide variety of existing thermo-mechanical
material testing systems.
FIG. 1 depicts, in side-view, commercially available servo-hydraulic
mechanical material testing system 100.
As depicted, system 100 is comprised of a frame illustratively
formed of base 101, columns 102 and 102a, upper crosshead 103 and lower crosshead
104. The system also contains lower hydraulic cylinder 105 and its shaft 106, and
upper hydraulic cylinder 107 and its shaft 108. Crossheads 103 and 104 are split
vertically at their horizontal ends extending inward to the columns. Bolts 119 and
119a, and 120 and 120a, extending through their corresponding crossheads, fixedly
clamp crosshead 103 and 104, respectively, to the columns.
As shown, this system has two hydraulic cylinders 105 and
107, as contrasted with many conventional systems that have only one cylinder usually
situated on lower crosshead 104. Lower cylinder 105 is mounted to a bottom surface
of lower crosshead 104 such that companion shaft 106 extends upward from the cylinder
through a hole (not specifically shown) in this crosshead. Upper cylinder 107 is
mounted to an upper surface of upper crosshead 103 such that its companion shaft
extends downward from this cylinder through a hole (also not shown) in the upper
crosshead. Hydraulic connections 109 and 110, 111 and 112 with suitable hydraulic
hoses (not specifically shown) connect cylinders 105 and 107, respectively, to separate
corresponding servo-hydraulic control systems (also not shown). These two servo-hydraulic
control systems are of a type commonly used in controlling servo-hydraulic testing
machines and are very well known in the art; hence, they will not be described in
any further detail. Feedback transducers 113 and 114, mounted to cylinders 105 and
107, respectively, provide shaft position information to the servo-hydraulic systems
for use in controlling the movement of the shafts. Although this figure contemplates
the use of servo-hydraulic controlled cylinders 105 and 107 for positioning shafts
106 and 108, these cylinders could be replaced with other suitable mechanical actuators,
such as ball screws, with appropriate position transducers and servo-control systems,
that collectively provide suitable actuator movement and can apply suitable forces
there through to corresponding anvils for deforming a specimen.
Area 306, generally centrally located and situated between
columns 102 and 102a, could contain a vacuum/atmospheric tank (not specifically
shown) with the inventive apparatus mounted inside the tank. Alternately, area 306a,
which is larger than area 306, includes part of columns 102 and 102a and affords
space, within the tank, to mount the inventive apparatus to the columns. In both
cases, the apparatus is centered vertically between shafts 106 and 108 (i.e., in
an approximate central working region of the system) and fixedly mounted to the
columns (either internally as in the case of area 306a or externally in the case
of area 306). A mechanical system that employs only one hydraulic cylinder, such
as cylinder 105, would require that the apparatus be oriented vertically off-center,
within area 306 or 306a, such that an anvil mounting base, situated opposite an
end of shaft 106, would be mounted to a rigid shaft (not specifically shown but
well known) rather than to a ram situated on an end of a shaft. In this instance,
shaft 106 would simply drive its ram toward the anvil mounted to the rigid shaft,
hence moving both anvils closer together. Shaft caps 115 and 116 are provided at
the ends of shafts 108 and 106, respectively, to abuttingly engage suitable and
corresponding anvil.mounting bases (not specifically shown in this figure). Caps
115 and 116 are situated on but are electrically isolated from shafts 108 and 106
by insulators 117 and 118, respectively. Each of shafts 106 and 108 is formed from
preferably austenitic stainless steel or 17-4 Ph heat treated stainless steel. Caps
115 and 116 are preferably formed of 17-4 Ph heat treated stainless steel. Insulators
117 and 118 are preferably fabricated of fiberglass, such as grade G-10, which has
adequate compressive strength to withstand a maximum force provided by servo-hydraulically
controlled cylinders 105 and 107 attached to distal ends of the shafts.
FIG. 2 depicts a cross-sectional view of self-resistive
anvil assembly 200 according to our inventive teachings, taken along lines 2-2 shown
in FIG. 4 and located along a centerline of the assembly, which prevents nearly,
if not totally, all longitudinal thermal gradients from appearing in a test specimen
held by the anvils as that specimen undergoes self-resistive heating. A dynamic
material testing system that utilizes our invention would employ two such anvil
assemblies. Since both assemblies are identical, we will merely discuss one of them.
As shown, anvil assembly 200 is mounted in support-arm
201. Anvil top 240 -is mounted on anvil base 241 with at least one relatively high-resistance
foil-interface 242 situated there between. Foil interface 242 (which is illustratively
shown in greater detail in FIG. 3 and will be described in detail below shortly)
can be formed from any of a number of different substances, such as graphite, tantalum,
mica or other well-known materials, that exhibit suitable electrical resistance
and other electrical properties, as well as appropriate mechanical properties. The
exact material used for the foil interface, as well as the number of layers that
fabricate foil interface 242, are not critical. The anvil top and base are each
typically formed of a high strength material, such as tungsten carbide with a cobalt
binder. The cobalt binder is appropriately varied, in its amount, to change the
properties of the anvil material and is usually in the range of 6 to 12 percent
of a resulting composite. A composite in this range produces a high temperature
material which retains its high strength at relatively high temperatures.
A resulting anvil stack consisting of anvil top 240, anvil
base 241 and foil interface 242 is tapered with an substantially upward sloping
conical taper to allow vertical mounting as shown, as well as inverted mounting
(opposite to that shown). The substantially conical shape imparts enhanced mechanical
stability to the stack. The stack is held together by and includes holder 243, preferably
either ceramic woven tubing or a rigid ceramic sleeve, that is tightly -placed -circumferentially
around the stack. The anvil stack is placed within cavity 244 in the support arm.
Tapered clamp 245, which has a taper complementary to that of the holder, secures
the stack including its holder to the arm. The holder, whether it is a woven ceramic
tubing or a rigid ceramic sleeve, substantially thermally and electrically insulates
the anvil stack from clamp 245. Clamp 245 is itself secured in place to arm 201
by fasteners (generally suitable bolts) 246.
The anvil stack is heated by serially passing electrical
heating current through the constituent components of the anvil stack and the specimen.
To prevent foil interface 242, specifically any of its layers, from deforming under
the forces it encounters during each hit, foil interface 242 is fabricated from
materials that are sufficiently hard to resist deformation as well as provide sufficient
resistance to self-resistively heat to a sufficient level to prevent appreciable,
if any, heat conduction from an end of the specimen under test into the top anvil.
The ceramic insulation, provided by holder 243, on the sides of the anvil stack
allows current flow only through arm 201, through the bottom of anvil base 241 and
upward through anvil stack to specimen 466 (specifically shown in FIG. 4). To enhance
abutting electrical contact between the anvil stack and support 201, a single soft
thin disk 247, typically copper, is situated between the anvil base and a lower
surface of cavity 244.
Foil interface 242, which serves as an interface between
the anvil top 240 and anvil base 241 of the stack, exhibits a sufficiently high
electrical resistance in order to cause substantial self-resistive heating at the
interface. When heating current serially passes through the anvil and specimen,
ideally, sufficient heat is produced by the foil interface and propagates through
anvil top 242 to cause the temperature of the anvil top to be at or very close to
the temperature of the specimen bulk, thereby precluding heat transfer from the
specimen to the anvil top. The temperature of the anvil top can be readily adjusted
by changing foil interface 242 by adding or subtracting individual foil interface
layers and/or changing the material of the foil interface layers to adjust the overall
resistance of the foil interface and thus adjust the thermal profile produced in
the anvil top. When the resistance and hence the configuration and the materials
of the foil interface are properly set, the thermal profile along the specimen will
be relatively flat or at a constant temperature, when the ends of the specimen are
in contact with anvils (both the anvil stack situated on one side of the specimen
and its complementary stack abutting against an opposite end of the specimen) at
the same temperature. Consequently, this substantially, if not totally, eliminates
any thermal gradients from appearing in the specimen bulk and particularly in the
region between the work zone of a specimen and its end that abuts against a top
surface of a corresponding anvil top.
FIG. 3 depicts, in accordance with our invention, an illustrative
embodiment of anvil stack 300 which forms part of anvil assembly 200 shown in FIG.
2, with foil interface 242 shown in an exploded view. As shown, anvil stack 300,
as with that shown in FIG. 2, is formed of anvil base 241, foil interface 242 and
anvil top 240, which are all contained within holder 243 here being a sleeve insulator.
This sleeve, is illustratively a Nextel sleeve which is a woven high-temperature
cloth fiber insulator which-reduces temperature loss to clamp 245 (See FIG. 2) and
provides electrical insulation on the sides of the anvil.
We have discovered, in accordance with our inventive teachings,
and empirically confirmed that excellent and uniform specimen heating and high impact
resistance can be had by use of at least one composite layer in foil interface 242
with a concentrically oriented multi-component arrangement. Specifically, that composite
layer is formed from two disks of different materials, but of the same thickness,
with one fitting within the other. One disk being a thermal and electrical insulator
but with high compressive strength, forms a central portion of the composite layer;
while the other disk, being electrically resistive, is appropriately shaped to form
an outer ring portion and is concentrically situated around and peripherally abutting
the central portion. The resulting composite layer is a single solid disk, formed
of two abutting concentrically aligned portions, that collectively exhibits significantly
differing mechanical, thermal and electrical properties between its outer ring and
central portions. Electrical current only flows through the outer ring portion and
causes that portion to self-resistively heat, while the central portion provides
high impact strength and hence substantial resistance to deformation thus preventing
the ring-shaped portion and the entire composite layer from, deforming as a result
of a hit and changing the resistance of the conducting path through the outer ring
portion. Alternatively, the materials used for the central and outer ring portions
may be reversed such that the central portion is resistive, e.g., made of graphite,
while the outer ring portion is formed of the insulating, high-strength material,
e.g., mica. In this case, the outer ring portion prevents deformation of the central
conductive portion during each hit. In either case, the outer ring size and diameter
of the central portion are appropriately chosen to obtain suitable thermal, mechanical
and electrical characteristics for these portions based on the specific materials
that form these portions.
As specifically shown in FIG. 3, foil interface 242 is
formed of three layered foil interface groupings 310A, 310B
and 310C, of which foil interface groupings 310A and 310B
are identical. For simplicity, we will specifically discuss foil interface grouping
310A. Grouping 310A contains ring portion 313, central portion
315, both of which form a single composite layer 320A, and disk 317 situated
there above. Central, portion 315, having a diameter on the order of .75" (approximately
1.9 cm) is formed of illustratively a solid mica disk which acts as both a thermal
and electrical insulator, and can withstand very high compressive forces without
deforming. This mica disk is approximately .004" thick (approximately .01 cm). Outer
ring portion 313, of the same thickness, is formed of graphite having an inner diameter
of .75" and an outer diameter of approximately 1" (approximately 2.5 cm). Disk 317
is preferably a solid tantalum foil disk with a 1" outer diameter and .004" thickness.
Foil interface grouping 310B is formed of outer ring portion 323 and
central portion 325 (which together form composite layer 320B) and disk
327 -- which are identical to ring portion 313 and central portion 315 (and composite
layer 320A), and disk 317. Foil interface grouping 310C is
identical to foil interface groupings 310B and 310A but without
a top tantalum disk, e.g., such as disk 327. The resulting composite layer 320C
formed of outer ring portion 333 and central portion 335 merely, as the top of foil
interface 242, abuts against an underside of anvil top 240. Using multiple foil
interface groupings to form foil interface 242 creates additional resistance and
hence heating in the anvil. The dimensions and materials and number of individual
foil interface groupings and the number of individual layers in each is not critical
as long as the entire resulting foil interface, here foil interface 242, exhibits
sufficient electrical resistance to generate an appropriate level of heat within
the anvil stack to offset self-resistive heat that would otherwise flow from an
end of the specimen under test into its abutting anvil top. While mica, graphite
and tantalum are specifically employed here, other materials with similar electrical,
mechanical and thermal characteristics could be substituted instead, though sized
accordingly.
Furthermore, while the composite layer is illustratively
shown as constituted of concentrically oriented central disk and outer ring portions
formed from just two disks, e.g., disks 315 and 313, respectively, clearly other
arrangements for the composite layer can be contemplated that exhibit other geometries.
One such geometry may envision a relatively large single disk of conductive material,
such as graphite, that has either, e.g., circular, triangular and/or rectilinear
cutouts there through in which complementary and correspondingly sized pieces of
high strength, insulating material, such as mica, are placed. Furthermore, different
materials may be placed in different cutouts to suitably alter the desired properties
of the entire composite layer. While we have empirically found that concentric arrangements
of the type described above for the composite layer provide highly satisfactory
results and are relatively simple to manufacture, we also recognize that equal performance
may be obtained from other composite layer geometries whether concentric or not.
Nevertheless, however configured, the resulting composite layer must possess the
requisite thermal, mechanical and electrical properties in terms of electrical resistivity/insulation
and/or high strength such that the entire anvil stack yields the-desired heating,
conductivity and high strength to impact forces.
FIG. 4 depicts an anvil assembly 200 shown in FIG. 2 along
with accompanying apparatus not part of the present invention that would be incorporated
into conventional mechanical testing system 100 shown in FIG. 1.
The specimen is held in a separate supporting fixture which
includes the anvil assembly and exerts sufficient force (in opposing directions),
through the anvils, onto the specimen to permit self-resistive heating current to
serially pass through the anvils and the specimen for heating the specimen. This
fixture utilizes separate supporting arms, each of which holds an anvil assembly,
situated on opposing sides of the specimen. The fixture applies force to each arm
to sufficiently hold the specimen in position between opposing anvil assemblies
and permit the heating current to flow through the specimen without causing substantially
any arcing but with insufficient force to deform the specimen as it is heated. Separate
opposing coaxially-oriented shafts are then controllably extended to controllably
strike the arms and move them together to compress the specimen and, as such, generate
each deformation "hit" in the specimen. At the conclusion of each hit, these shafts
are suitably retracted in preparation for the next such hit. Inasmuch as the shafts
are electrically insulated from the fixture, self-resistive heating current can
be controllably and serially applied through the fixture arms, the anvil assemblies
and specimen before, during or after each hit to appropriately and uniformly heat
the specimen bulk to a desired temperature. Since such a fixture can be easily retrofitted
to nearly any commercially available mechanical testing system, our invention uniquely
imparts self-resistive heating capability to such systems, and particularly through
use of our inventive anvil assembly, uniformly heats the specimen to produce isothermal
planes therein and without appreciable, if any, thermal gradients appearing along
the specimen length. This fixture is not needed in a dynamic material testing system
that already provides self-resistive heating capability. In that instance, use of
our inventive anvil assembly alone will suffice to provide uniform specimen heating
along with isothermal planes, and without longitudinal thermal gradients, throughout
the specimen bulk.
Specifically and as shown, fixture 400, including the anvil
assembly, is fixedly, vertically and centrally mounted between lower shaft 406,
with shaft cap 416, and upper shaft 408 with shaft cap 415. Shafts 406 and 408 move,
under servo-hydraulic control in the directions shown by arrows 407 and 409, respectively.
To prevent heating current from traveling through the shafts, shaft caps 415 and
416 are electrically insulated, by insulators 417 and 418, from shafts 408 and 406,
respectively.
This fixture is formed of support arms 201 and 451 that
are each constrained to move vertically up or down but independently of the other.
Heating current flows through each of the arms. Support arms 201 and 451 have anvil
assemblies 200 and 453 mounted in respective cavities (see FIG. 2 for cavity 244
in arm 201) and secured with fasteners 455 and 246, respectively. The anvil assemblies
are coaxially oriented. Each of the support arms is fabricated from a sufficiently
strong material which will endure repeated hitting at high force without deforming.
Typically, these support arms are made from a 303 or 304 series stainless steel
or 17-4 Ph heat-treated stainless steel. Both support arms 201 and 451 are water
cooled to prevent the arms and the components to which they are physically connected
from excessively heating due to the self-resistive heating of the anvil stacks and
the specimen, as well as any self-resistive heating that may occur within the arms
themselves due to the passage of heating current there through. Water ports 456
and 457 provide connections to the water cooling passages (not specifically shown)
in arms 201, while water ports 458 and 459 provide connections to water cooling
passages (also not specifically shown) in arm 451.
Support arms 201 and 451 are secured to slide plates 460
and 461 through insulation plates 462 and 463, respectively, using conventional
insulated fasteners (not shown). Slide plates 460 and 461 are free to move vertically
up and down along system mounting rails 464 and 465, being connected therebetween
through suitable pairs of linear bearings (conventional and not shown) which guide
this vertical motion. The rails and slide plates collectively form a mount for the
support arms. As such, arms 201 and 451 move in the vertical directions shown by
arrows 432 and 4.52, respectively. The slide plates are mounted to rails 464 and
465 through any of the many linear slide mechanisms readily available in the commercial
market place and which would restrict the motion of the slide plates to just vertical
motion. The linear bearings are used in side-by-side pairs to restrict this motion
to a single plane. This arrangement also permits support arms 201 and 451, anvil
assemblies 200 and 453 and specimen 466 situated therebetween to collectively and
freely move vertically up and down as a single unit. Additionally, arms 201 and
451, by moving independently of one another, permit specimen 466 to be appropriately
positioned therebetween and then to be compressed. Mounting rails 464 and 465 may
be mounted to the inside of a vacuum/atmospheric tank or to the columns 102 and
102a (see FIG. 1). As shown in FIG. 4, rails 464 and 465 are themselves rigidly
mounted to mounting brackets 492 and 493, and 494 and 495, respectively. The brackets
are suitably fashioned to attach the rails to the tank walls or to columns 102 and
102a (see FIG. 1) .
Electrical heating current for self-resistively heating
specimen 466 is supplied by transformer 477 and flows in a simple series path. Though
not critical, the transformer should possess a 440 volt, single phase 75 kVA primary
with a 5.7 to 10 volt paralleled secondary, preferably controlled by a tap switch,
and a 50 or 60-Hz operating frequency. The short circuit output current should be
on the order of 50 kA or more. The secondary winding of the transformer is typically
formed of one or two turns of a heavy copper casting. By varying the turns ratio
of the transformer in finite increments through the tap switch, specimens of different
sizes and shapes can be readily heated. Such a transformer is the model G4475NS61S
manufactured by Kirkhof Transformer of Grand Rapids, Michigan. Transformer 477 is
connected to a suitable and conventional servo controlled power source (not shown)
via leads 478 and 479.
The current path, for one-half cycle of alternating current
flow, starts at transformer output stab 475 and from there continues into bus 471.
This bus is connected, via fasteners 473, to the stab. From bus 471, the current
flows into one end of flexible conductor 467 which itself is physically and electrically
connected, through fasteners 470a, to the bus. Bus 467 is connected,
via fasteners 468, to arm 451 and hence the heating current is routed into this
arm. From the arm, the heating current is directed through anvil assembly 453 and
into specimen 466. Once the current passes through the specimen, it traverses through
anvil assembly 200 into support arm 201. This arm is itself electrically connected
through flexible conductor 469, to bus 472: Bus 472 is itself connected, via fasteners
470b, to conductor 469 and, via fasteners 474, to output stab 476 of the transformer.
Hence, the heating current will flow from arm 201, through flexible conductor 469,
into bus 472 and finally, via stab 476, back into the transformer. The current flow
will simply reverse its direction for the other half cycle of current flow. The
bus, flexible conductors, support arms, anvil bases and anvil tops all have extremely
low electrical resistance. Therefore, almost all of the heating occurs in specimen
466 and in the anvil stack, specifically within foil interface 242 situated within
each stack. The water cooling removes heat that occurs in the other components of
fixture 400 that are physically and thermally connected to the anvil assemblies.
The self-resistance heating is controlled through a conventional
servo-control system which utilizes feedback from a thermocouple having output leads
498 and which is affixed to a work zone of the specimen 466. A suitable pyrometer
may be used instead of the thermocouple. Output feedback signals provided by either
the thermocouple or the pyrometer (whichever is used) is provided as input, through
appropriate conventional signal conditioning circuitry, to an input of the servo-control
system. Generally speaking, the servo-control system has a predefined program of
temperature vs. time. The thermocouple output is compared continuously with the
program, and the power to the transformer is then adjusted to keep the specimen
temperature as close as possible to that desired and specified in the temperature
program. The thermal program is typically synchronized to a mechanical program to
provide complete thermal/mechanical control over the specimen. The servo-control
system is very similar to those used in the "GLEEBLE" material testing systems produced
by the present assignee and as described in illustratively
United States patents 6,442,090 (issued to H. S. Ferguson on July 23, 2002
);
5,195,378 (issued to H. S. Ferguson on March 23, 1993
); and
5,092,179 (issued to H. S. Ferguson on March 3, 1992
). Given the conventional nature of these control systems, they will not
be discussed in any further detail.
In order for specimen 466 be held in position and squeezed
with adequate force to provide a good electrical path for the heating current through
the anvil assemblies and the specimen, four springs are provided to apply forces
to the top and bottom of slide plates 460 and 461. Two springs are employed to provide
compressive force for plate 461, and the other two provide compressive force for
plate 460.
Compression spring 480 is mounted between plate 460 and
adjustment screw 482, which is threaded into internally threaded bracket 481. Adjusting
the screw in or up causes spring 480 to increasingly compress and hence exert increased
force onto slide plate 460 forcing the plate upward. Each of the three other springs
483, 486 and 489 are mounted and adjusted in a similar fashion through adjustment
screws 485, 488 and 491 threaded into internally threaded brackets 484, 487 and
490, respectively. Since springs 480 and 489 provide the additional force (preload)
to hold the specimen and maintain low enough electrical resistance for the heating
current, these springs may be stronger than springs 483 and 486. The preload obtained
from the forces of springs 480 and 489 minus any counterbalancing forces from springs
483 and 486 (net force) is the order of 200 to 600 N (newtons) depending on the
specimen size. The net force on the specimen, while not critical, is chosen to be
large enough to pass electrical current through the specimen without substantially
any arcing occurring yet small enough so as not to deform the specimen while it
remains at an elevated temperature. A centering force, collectively produced by
all the springs, should be enough to compensate for the weight of the slide plates,
support arms, the anvil assemblies and the specimen so that these components effectively
"float" in approximately mid-position along mounting rails 464 and 465. The range
of the desired centering force is the order of 50 to 100 N depending on the specimen
size.
Alternatively, suitable conventional pneumatic cylinders
(along with proper regulators and values), one for each slide plate, may be used
in lieu of springs 480 and 483 and adjustment screws 482, 485, 488 and 491. Use
of cylinders may be preferred inasmuch as spring force changes rapidly with deflection
(unless an extremely long spring is used) and springs generally require more maintenance
and must be mechanically adjusted to yield the proper forces. Furthermore, by simply
and appropriately setting the pressure within each cylinder, the forces required
to operate fixture 400 can be easily adjusted to an desired amount as can movement
of the support arms and anvils to readily permit loading and unloading of the specimen.
Since the force provided by shafts 406 and 408 onto support arms 201 and 451 during
each hit substantially exceeds the specimen holding force provided by the fixture
so as to controllably deform the specimen, the fixture must be resilient and permit
the support arms to move together even while they are holding the specimen. For
that reason, springs, pneumatic cylinders or other suitable force producing devices,
which have inherent resilience, are preferred over other devices, such as ball screws,
that could position the support arms and provide the requisite specimen holding
force but exhibit essentially no resilience.
Two air regulators would maintain correct pneumatic pressure
at all times regardless of the motion of cylinder rods pushing on slide plates 460
and 461. A typical installation would involve use of two air regulators, two pneumatic
cylinders and two 4-way, 3-position center-port blocked pneumatic control valves.
Any other suitable type of pneumatic valves can be used depending on the way the
valves are connected and operated. A typical installation involves attaching one
cylinder to bracket 484 and the other cylinder to bracket 490. An end of the cylinder
rod of the cylinders pushes on plates 460 and 461. One air regulator is set to provide
the desired compression force to maintain electrical contact between the anvils
and the specimen during self-resistive heating. The other air pressure regulator
is set to balance the weight of-the- assembly so-that,-in-operation, the support
arms remain relatively well-centered on the mounting rails.
To insert or remove the specimen from between the anvil
assemblies, these assemblies, i.e., arms 201 and 451, must be moved sufficiently
apart from each other to afford appropriate space between them for a user to suitably
manipulate the specimen. If just spring force is used to control the motion of slide
plates 460 and 461, then arms 201 and 451 must be separated manually using a suitable
bar to overcome the force generated by the springs and thus provide sufficient space
between the anvil assemblies to insert, position and remove the specimen.
Alternatively, if pneumatic cylinders are used instead of springs, then the pneumatic
control valves can simply be set to cause the support arms and hence the anvil assemblies
to readily separate allowing the specimen to be easily inserted, positioned and
subsequently removed. When the specimen is set in place, the pneumatic control valves
are simply returned to their original position. Clearly, use of pneumatic cylinders
greatly simplifies the process of loading and unloading the specimen over that required
with springs.
After the specimen is loaded between the anvil assemblies
and its thermocouple connected to the thermal servo-control system, appropriate
thermal and mechanical programs are placed into the control system. Testing is then
initiated and desired specimen measurements-begun. Through doing so,-the specimen
is brought to its first deformation temperature at a programmed rate and through
controlled self-resistance heating. The mechanical system then controllably moves
shafts 406 and 408 to strike the support arms and through movement of opposing anvil
assemblies 201 and 453 squeezes and compressively deforms the specimen at a programmed
rate and amount. Once this deformation is complete, shafts 406 and 408 are then
appropriately retracted. The specimen is then suitably heated or cooled at a programmed
rate, and, thereafter, to implement a next successive deformation, the shafts are
once again controllably moved to strike the support arms, and so forth.
Though we have described our inventive anvil assembly for
use with specimens that undergo compressive deformations, our invention multi-component
foil interface layer could be readily included within jaw assemblies for use with
self-resistively heated specimens that are to undergo tensile deformations in order
to provide, similarly with compressively deformed specimens, uniform heating throughout
the specimen bulk. However, with tensile testing, there are no impact forces imparted
to the specimen as the jaws are controlled to stretch the specimen apart by its
ends, through tension, rather than compressing it. As such, there is no need to
use a high strength component, for resisting impact forces, within any composite
layer situated within the foil interface.
Furthermore, while we have described retrofitting a dynamic
mechanical material testing system with a fixture, that utilizes our inventive anvil
assembly, to impart thermal processing capability to that system, through self-resistive
specimen heating coupled with the above-described benefits attainable through use
of that assembly, the fixture alternatively, if desired, could be outfitted with
conventional conductive anvils instead. In that case, self-resistive specimen heating
will still occur with isothermal planes developed in the specimen. Unfortunately,
longitudinal thermal gradients may well appear in the specimen and near its ends.
Lastly, though we have described our inventive anvil assembly
and fixture as being vertically oriented, they could alternatively be horizontally
positioned for use with horizontally oriented material testing systems, the direction
simply being governed by the direction of motion of the piston shafts (specifically
rams) used in these systems to deform the specimen. Hence, those skilled in the
art fully understand that when the term "vertical" is used herein, that term is
merely being used in a relative sense and would, where appropriate, encompass horizontal
orientations should the entire testing system be so oriented.
Although various embodiments which incorporate the teachings
of the present invention have been shown and described in detail herein, those skilled
in the art can readily devise many other varied embodiments that still incorporate
these teachings.