Apparatus for performing high-temperature fiber push-out testing

The apparatus disclosed in the present invention measures the force at which a fiber resist the motion of an indenter driven at constant speed. This apparatus conducts these test in a vacuum of about 10.sup.-6 tort and at temperatures up to 1100.degree. C. Temperature and vacuum environment are maintained while controlling indenter motion, sample position, and providing magnified visual inspection during the test.

FIELD OF THE INVENTION 
The present invention is directed to a method of testing the mechanical 
strength of the interface between a reinforcing fiber and the surrounding 
matrix material. 
The interfacial behavior of a fiber reinforced composite is important 
because interfacial strength has a significant impact on the overall 
strength and toughness of the composite material. Various models have been 
proposed which relate the mechanical properties of the interface to those 
of the composite. As a result, fiber push-out testing has become an 
important tool for characterizing fiber debonding and sliding behavior in 
fiber-reinforced composite materials. 
Because the target use of many of the composite materials is at elevated 
temperatures, it is beneficial to extend the measuring range of 
interfacial testing to elevated temperatures, such as 1100.degree. C. 
It is, therefore, an object of the present invention to perform fiber 
push-out testing at elevated temperatures. 
It is a further object of this invention to generate data at composite 
service temperatures which could be used to optimize interfacial 
mechanical behavior while minimizing specimen oxidation at those elevated 
temperatures. 
It is another object of the present invention to evaluate the effects of 
residual stresses on fiber debonding and sliding. 
It is still a further object of the invention to maintain a magnified 
line-of-sight with the specimen during loading, so that the test can be 
monitored visually. 
It is still another object of the invention to determine the force at which 
a fiber resists the motion of an indenter driven at a constant 
displacement rate at elevated temperatures. 
DESCRIPTION OF THE RELATED ART 
Gass U.S. Pat. No. 2,892,342 is directed to a test apparatus which is used 
to determine the force necessary to induce failure in a plastic material. 
Eichenbrenner U.S. Pat. No. 3,795,134 is directed to a means of testing a 
metal specimen under conditions of heating. 
Underwood U.S. Pat. No. 4,537,060 discusses an apparatus including a cam 
driven flywheel which applies mass to a subject material. Suga U.S. Pat. 
No. 4,627,287, shows a xenon lamp, a focusing reflector, an air cooling 
means, and an arrangement which allows for the uniform heating of a 
specimen along the surface of a test structure. 
Tse U.S. Pat. No. 4,662,228 relates to a fiber push-out apparatus. O'Connor 
U.S. Pat. No. 4,926,118 describes a mechanism for testing materials in a 
heating environment. Wu U.S. Pat. No. 4,972,720 is directed to a method 
and apparatus for the thermal testing of composite materials in which heat 
applied to the subject causes it to deform. 
SUMMARY OF THE INVENTION 
The present invention is directed to an apparatus for performing push-out 
testing of a fiber reinforced matrix composite. An evacuated chamber is 
provided for heating a composite and an indenter by focusing the radiation 
from a halogen lamp through a quartz window onto the composite specimen 
and indenter. While the specimen temperature is remotely monitored through 
a thermocouple, a nearby remotely controlled translation stage used for 
sample positioning is maintained near room temperature by a water cooled 
copper plate underneath a heat isolating platform. 
During the testing a microscope connected to a television camera provides 
high-resolution remote monitoring on a television monitor. Alignment of 
the indenter and fiber is achieved by visually positioning the sample 
through a remotely controlled motorized translational stage inside the 
test chamber. Correct alignment is evaluated by visual inspection of the 
magnified image on the television monitor. Controlled indenter 
displacement inside the test chamber is performed using a linear motion 
feedthrough driven by a stepper motor outside the test chamber. A load 
cell connected to a displacement shaft inside the test chamber enables 
monitoring of the load applied to the composite specimen by sending 
electrical signals corresponding to the load through a data acquisition 
system, to a remote computer.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A schematic view of a fiber push-out apparatus is shown in FIG. 1. In the 
apparatus a cubical stainless steel chamber 10 with conflat-flanged ports 
on each face, houses a composite specimen 12. The specimen is supported by 
a sample support block 14 which has a set of three 300 .mu.m diameter 
grooves 16 underneath the sample 12 which allows the fibers to be pushed 
out without resistance from the support block. The sample and support 
block are spring-clamped to a heat isolating platform consisting of a 
machineable ceramic cylinder on top of a hollow stainless steel base. 
Underneath the heat isolating platform is a copper plate 18 cooled by 
water through pipes 22 which keeps the transitional stage 20 near room 
temperature. A turbopump exit area 24 facilitates the evacuation of the 
chamber, producing an inert environment. The inert environment facilitates 
the heating of the specimen without oxidation. The translational stage is 
controlled remotely through a joystick controller 26, enabling the 
specimen 12 to be placed under an indentor 28 for pushout testing. 
Both the specimen 12 and the indentor 28 are heated by a quartz halogen 
lamp 30. A radiant energy heating system composed of a two piece 
ellipsoidal reflector 32, focuses the radiant energy generated by the lamp 
30 onto the specimen 12 and indentor. The reflector is bisected by a 
quartz window halfway between the two focal points of the reflector. 
Bisecting the reflector places half of the reflector inside the chamber 
and the other half outside. The halogen lamp is in the portion of the 
reflector outside of the chamber enabling conventional cooling of the lamp 
by forced air convection. The quartz window 34 allows maximum transmission 
of the heating radiation produced by the halogen lamp. As a result, the 
quartz window 34 does not impede the quick heating of the specimen 16. A 
temperature sensing means 17 displays the temperature to an outside 
display 19. Sample temperatures as high as 1100.degree. are attained 
within 10 minutes. 
When preparing for a test a thin slice of a fiber reinforced composite is 
polished to smooth and expose the fiber ends in the matrix. A specific 
fiber is then indexed for fiber push-out testing. 
In the testing apparatus a flat-bottomed cylindrical tungsten carbide punch 
with a diameter of 100 .mu.m is used for pushing out a 142 .mu.m diameter 
SCS-6 SiC fiber. The indenter 28 was attached by a shaft 36 to a linear 
motion feed-through 38 which enables the control of the indenter from 
outside the chamber. The commercially available vacuum linear motion 
feedthrough was modified by the addition of a teflon bushing around the 
shaft entering the Vacuum as well as the addition of two thrust washers 
around the threaded shaft entering the feedthrough from the atmosphere 
side. These modifications eliminated both vertical and lateral play in the 
shaft that made the performance of the feedthrough unacceptable in its 
unmodified form. The linear motion feed-through 38 is driven by a stepper 
motor 40 which provides controlled loading by a computer 46, of the 
indenter 28 onto the specimen 12. The computer 46 is connected to the 
stepper motor 40 through a motor controller 42. The motor controller 42 
facilitates the uniform loading of the indenter onto the specimen and 
automatic retraction of the indenter at the completion of a test. 
During a typical test the stepper motor shaft rotated at 0.1 rpm (50,800 
microsteps/rev) which translates to 1.06 .mu.m/s linear motion of the 
indenter. A load cell 43 attached between the shaft 36 and the indentor 28 
produces and electrical signal proportional to the applied load which is 
collected by a data acquisition system 44. The information is then 
transferred to the computer 46 which collects stores and analyzes the raw 
data. 
A quartz window 48 tilted at 25.degree. from the vertical so that the 
window is perpendicular to the line of sight, minimizes the image 
distortion and reflections that can be caused by a viewing window. A long 
working distance optical microscope 50 is positioned facing the quartz 
window 48, to enable a continuous line of sight view while positioning and 
testing the specimen. An enlarged view of the specimen and indentor can be 
viewed remotely at a television monitor 54 which is connected to a 
television camera 52 attached to the microscope 50. 
FIG. 2 displays an enlarged view of the specimen 12 displaying the 
orientation of the fibers 13 a spring-loaded clamp (56) eliminates the 
need for any type of adhesive or glue. The sample clamp 56 secures the 
specimen in place against the support block 14. The fibers to be tested 13 
are aligned above the 300 .mu.m/width grooves 16 so that there is no 
resistance by the support block 14 when a fiber is pushed out by the 
indentor 28. 
FIG. 3 displays a graph of the average interfacial shear stress versus the 
linear motion feedthrough shaft displacement. A set of graphs are 
generated for the displacement of SCS-6 SiC fiber at different 
temperatures in degrees celsius. The debonding of the fiber from the 
surrounding composite material can be observed at the peaks in the curves 
58 and the friction associated with the fiber being pushed out is noted in 
60 as the curves flattens out. 
While the preferred embodiment of the invention is disclosed and described 
it will be apparent that various modifications may be made without 
departing from the spirit of the invention or the scope of the subjoined 
claims.