Creep test coupon for metal matrix composites

A test coupon for metal matrix composites is provided with means for structurally isolating the reinforcing fibers in the gage section from the reinforcing fibers in the clamped end regions of the coupon, when the latter is subjected to creep testing, so that the results of creep testing can be more accurately indicative of the creep properties associated with the fiber-reinforced metal matrix composite material in the coupon gage section. More particularly, the test coupon of the invention is provided with a plurality of slots or cut-out regions disposed in a section located between the clamped end regions and the centrally located gage section. The plurality of slots, which can be provided in longitudinally and/or laterally staggered arrays, effectively causes structural isolation of the reinforcing fibers in the gage section from the clamped reinforcing fibers located in the end regions of the coupon.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to test coupons for determining creep 
characteristics for metal matrix composite structures, and more 
particularly to methods for forming and using such coupons and to 
configurations of the coupon itself. 
2. Background of the Invention 
The use of creep test coupons to determine structural properties and 
behavioral characteristics is not new. For many years, such coupons have 
been provided as samples of monolithic materials requiring investigation. 
Testing of the samples has been accomplished by subjecting them to 
compressive and/or tensile forces in appropriate testing apparatus, 
thereby making it possible to obtain data indicative of the character of 
materials in question. 
Recently, new composite materials known as "metal matrix composites" 
(MMC's) have been developed for use in the aerospace industry, and there 
has arisen a great and urgent need for investigation of the properties of 
these materials as well. 
In order to accomplish this task, the industry typically has performed 
creep and other types of material characteristic testing using a coupon of 
substantially rectangular shape having simple straight, parallel sides 
(see FIG. 1) or a coupon formed in a dog-bone shape (see FIG. 2). 
To date, the latter configuration has been widely accepted as the industry 
standard. FIG. 3 illustrates this configuration of coupon when used with 
fiber-reinforced metal matrix materials. As is well known, the reinforcing 
fibers are embedded in the metal matrix material. The plurality of fibers 
include one or more layers of fiber sets, where in each layer the fibers 
are disposed at a predetermined angle (between 0.degree. and 90.degree.) 
relative to the longitudinal axis Z--Z of the coupon. Each set of 
angularly oriented fibers typically extends along a portion (if not all) 
of the length of the coupon. Those sets of fibers having no angular 
orientation relative to the coupon longitudinal axis (i.e., zero (0) 
degrees orientation) typically span the length of the coupon from one end 
region to the opposite end region. 
Exemplary of such an angular arrangement of reinforcing fibers in a test 
coupon are the fiber sets 310, 320 and 330 shown in FIG. 3. Fiber set 310 
is oriented at +45.degree. relative to the longitudinal axis Z--Z of the 
coupon, fiber set 330 is oriented at -45.degree. relative to the 
longitudinal axis, and fiber set 320 is positioned at an angular 
orientation of approximately zero degrees relative to the longitudinal 
axis Z--Z. The FIG. 3 representation of fiber sets does not purport to 
identify upper, middle and lower sets of fibers, but rather only is 
illustrative of three possible angular orientations of the fiber sets. 
Holes 302, 304 located in opposite end regions of the coupon serve as 
alignment means for facilitating attachment of the coupon in the testing 
fixture. As stated above, the three fiber sets depicted in FIG. 3 are 
merely intended to serve as illustrative examples of the infinite number 
of angular orientations possible. The number of fiber sets in any given 
test coupon, the different angular orientations of these fiber sets, and 
the relative placement of these fiber sets one above the other are all 
considered to be design considerations which depend on which, and to what 
degree of magnitude, strength-of-material characteristics are being 
developed. 
Testing of coupons is typically accomplished by first securing opposite 
ends of the coupons in fixture clamps, and then subjecting the coupon to 
tensile or compressive forces, as the case warrants, to obtain the creep 
(or stress or strain) characteristics of the material involved. 
One of the most disturbing problems attendant the use of the known 
fiber-reinforced MMC coupon configurations is that, after the coupon is 
clamped and the testing takes place, the test results produce creep curves 
which suggest that the section of fiber-reinforced metal matrix compound 
material located between the clamped end regions (i.e., in the gage 
section) is just as strong as the fibers disposed within the metal matrix 
compound material. In other words, an accurate reading of the test results 
with a high level of confidence cannot be obtained in carrying out 
standard testing procedures with the presently known test coupon 
configurations. 
A situation in which the fiber-reinforced metal matrix compound material of 
the gage section would be expected to be as strong as the fibers in the 
MMC material would be where the coupon was perfectly fastened at both end 
regions and subjected only to a pure tension load. However, it would be 
presumptuous to believe that perfect loading or perfect clamping of the 
coupon could be attainable in practice. Indeed, it is believed that, in 
using the conventional coupon configurations and currently known 
procedures of clamping and testing coupons fabricated from 
fiber-reinforced metal matrix compound materials, creep strain (known to 
be recoverable in a monolithic material) would be trapped. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a novel 
coupon configuration which will permit a more accurate determination of 
structural properties of matrix metal composite (MMC) materials, while at 
the same time overcoming all the deficiencies and disadvantages of the 
known coupon configurations for such MMC materials. 
Another object of the present invention is to provide a new design for 
fiber reinforced MMC material test coupons in which the fiber-reinforced 
material located in the section of the coupon between the clamped end 
regions (i.e., the gage or measurement section) is physically isolated 
from the fibers at the clamped end regions. 
Still another object of the invention is to provide a test coupon 
configuration which is envisioned as the next industry standard for tests 
on fiber-reinforced materials. 
Yet another object of the invention is to provide a novel method for 
fabricating test coupons manufactured from fiber-reinforced materials 
whereby creep behavior of the materials can be accurately determined. 
These and other objects are accomplished through the application of the 
surprising results discovered by the present inventor that the structural 
effect of reinforcing fibers in the gage section of a test coupon 
subjected to creep testing can be isolated from the reinforcing fibers 
located in the clamped end regions of the coupon, and therefore the 
results of creep testing will be more accurately indicative of the creep 
properties associated with the fiber-reinforced metal matrix composite 
material in the centrally located gage section. 
More particularly, in accordance with the principles of the present 
invention, these objects are accomplished by the fabrication of a test 
coupon having slots or cut-out regions disposed in a section located 
between the clamped end regions and the centrally located measurement 
section. The test coupon has any shape determined to be most appropriate 
for achieving the desired results, e.g., rectangular, square, circular, 
elliptical, etc. That which is most important is to provide a perimeter of 
isolating slots or perforations about the centrally located measurement 
section of the test coupon.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIGS. 4-6, wherein identical or similar references 
numerals represent identical or similar structural components, there are 
shown three novel configurations of a test coupon embraced by the present 
invention. 
In the configuration of FIG. 4, coupon 400 includes three primary portions; 
a first set of clamp portions 420, 420, a second gage portion 460, and a 
third set of medial portions 440 interposed between each clamp portion and 
the gage portion. The gage portion is bounded on opposite sides by a pair 
of circular protruberant regions 462 each of which is provided with an 
aperture 464 by means of which an extensometer or similar device can be 
attached to measure relative elongation of the gage portion. Stress relief 
areas may optionally be provided (note areas 466) to separate the 
protruberant regions 462 from the medial portions 440. It is to be 
understood that the regions 462 and apertures 464, as well as the stress 
relief areas might also be provided on any of the test coupons disclosed 
herein and contemplated by the present invention. 
Each of the medial portions 440 includes a slot 410, 412 with an elongated 
opening which extends laterally across the coupon in the direction of the 
external face of the width. Each slot is configured such that the regions 
of metal matrix compound material in the medial portion located between 
the edges of the coupon and the edges of the slot together have a linear 
lateral dimension greater than the lineal lateral dimension of the slot. 
The effect of providing the slots 410 and 412 is to isolate the fibers of 
the clamp and medial regions from the fibers of the gage portion. Looking 
at FIG. 4, it can be seen that the zero degree fibers 442 and off-axis 
fiber sets 444 and 446 in the clamp and medial portions are interrupted by 
the slots 410 and 412, and consequently neither extend into the gage 
portion nor impact on the fiber set 442 contained in the gage portion. 
Indeed, the fiber set 442 in the gage portion has been effectively 
structurally isolated from the fiber sets 442, 444, 446 and 448 in the 
clamp portion of the coupon. p The "slotted" design of test coupon 
embraced by the present invention permits load transference to the gage 
portion through the metal matrix compound and the off axis fiber sets 
(i.e., note fiber sets 444 and 446 in FIG. 4) which are not affected by 
the slots. Further, as a result of this construction, the gage portion 
will carry a higher stress level due to the lateral dimension being 
smaller across the gage section than the sum of the lateral dimensions of 
material at the location of the slot(s). 
Referring now to FIG. 5, wherein the same or similar reference numerals 
represent the same or similar structural features, there is shown a second 
configuration for the test coupon with which this application is 
concerned. This second configuration is substantially identical to the 
configuration of FIG. 4, with the exception of the array AA of slots 
extending across the medial portions. The slots shown extend in a 
direction normal to the longitudinal axis Z--Z of the coupon. Each array 
of slots includes two "regions" 580, 590 of slots. It is contemplated that 
region 580 would include at least one slot (only one slot is shown in this 
Figure), and region 590 would include at least two slots 592, 592'. The 
slots in each of the two regions extend in the direction normal to the 
longitudinal axis Z--Z such that two "virtual rows" aa, bb are defined 
(see bottom of FIG. 5). These "virtual rows" are spaced apart and are 
effectively parallel to one another. The slots in the two "virtual rows" 
are staggered in the direction perpendicular to the Z--Z axis, with a slot 
in one "virtual row" being disposed longitudinally above or below a region 
594 of metal matrix compound material surrounding or located adjacent to 
the adjacent end portions of the slots 592, 592' in the other "virtual 
row". It is understood that either of the one or the other "virtual rows" 
may contain one or more slots, but the concept which applicant wishes to 
convey is that the slots in one "virtual row" are offset in a direction 
perpendicular to the longitudinal axis from the slot(s) in the other 
"virtual row". Of course, the invention also contemplates the provision of 
more than two "virtual rows" of slots located at predetermined positions 
along the longitudinal axis of the coupon. 
A third configuration of test coupon 600 is disclosed in FIG. 6 where, once 
again, the same or similar reference numerals represent the same or 
similar structural features. Here the coupon has a substantially square 
gage portion 660 and is secured to a test fixture via four clamp portions 
620, 620', 620", 620'", each clamp portion being disposed adjacent a 
respective one of the edges of the gage portion. Due to the substantial 
area of the clamp portions which must be secured in the test fixture, and 
the fact that, in this coupon configuration, each of the clamp portions 
are structurally independent from any one of the other clamp portions, a 
plurality of alignment holes 602 are provided to facilitate securing the 
coupon in the fixture. In contrast to the coupon configuration of FIG. 5, 
the test coupon of this embodiment is designed to be subjected to loads 
along two axes (i.e., "biaxially") either alternately or concurrently, and 
includes several "virtual rows" of slots, the emphasis being on making 
sure that the gage portion be as completely isolated from the reinforcing 
fibers provided in the clamp portions of the coupon as possible, while 
insuring that the sum of the lateral dimensions of material between slots 
be greater than the lateral dimension across the gage portion. 
It is contemplated that the number of "virtual rows" of slots could be 
chosen according to the needs of the situation at hand and the 
requirements of the tests being performed. It is also contemplated that 
the interruptions in continuity of the MMC material provided in the test 
coupon could take the form of perforations or sets of perforations, with 
one set being staggered longitudinally and/or laterally relative to the 
other set(s). 
In all the embodiments of the invention disclosed and described above, it 
is important that the totality of solid portions of the coupon (in each 
medial section) which separate the slots or perforations be of greater 
magnitude than the lateral dimension across the gage section. In other 
words, the sum of all the lateral dimensions of the solid sections 
(between the lateral edges of the slots or perforations) across the width 
of each medial portion must be greater than the lateral dimension across 
the gage section. 
FIG. 7 comparatively illustrates creep test results using actual test data 
for similarly configured unnotched test coupons subjected to stress-loaded 
conditions specified at a temperature of about 1200.degree. F. The 
vertical axis represents increasing axial creep strain, while the 
horizontal axis represents the parameter of increasing time. 
Line 710 depicts test results obtained using an unnotched fiber reinforced 
MMC test coupon provided with four layers of fiber arrays and subjected to 
a stress of approximately 38.5 ksi. In this coupon, each of the fiber 
arrays is aligned with the longitudinal axis of the test coupon, and the 
fibers are gripped at opposite ends of the coupon by the test fixture. 
Under application of a tensile load, the coupon behaves as if the only 
support for the applied load is the fibers, and as shown, curve 710 shows 
effectively no change in creep strain (and no material degradation) with 
increasing time until failure of the coupon. 
Line 720 depicts test results obtained using an unslotted fiber-reinforced 
MMC test coupon having layers of fiber arrays arranged at 45.degree. 
angles to the coupon longitudinal axis, and in a symmetrical manner (e.g., 
in a coupon with four layers of fiber arrays, layer 1 would be disposed at 
+45.degree., layer 2 would be disposed at -45.degree., layer 3 would be 
disposed at -45.degree. and layer 4 would be disposed at +45.degree.). 
Curve 720 shows minimal creep resistance with increasing time, with an 
ultimate failure. Stress level is approximately 8.82 ksi, only about 23% 
of the stress level applied in the situation relating to line 710. 
Line 730 depicts test results obtained using an unslotted fiber-reinforced 
MMC test coupon having layers of fiber arrays arranged at 90.degree. 
angles to the coupon longitudinal axis. Stress level applied to this 
coupon is approximately 4.5 ksi. In this case, the fibers in the gage 
section of the coupon effectively have no load-carrying function, since 
even though the coupon ends are gripped in the fixture (see the 
description above relating to the curve 710), there are no fibers in the 
gage section which structurally communicate with the fibers in the clamp 
portion. Nevertheless, in this situation, creep strain increases with 
increases in time since the load-bearing cross section of the test coupon 
gage portion is substantially diminished by the presence of the fiber 
arrays. 
Line 750 shows the curve which is expected to be obtained using a slotted 
fiber-reinforced MMC test coupon of the type shown in FIGS. 4 or 5 of the 
present application.