Method heats specimens slowly and with sufficient control so that the critical temperature gradient in the specimens cannot occur before temperature equilibrium is reached

Andrae, R. W.

Carden, A. E.

Rowley, J. C.

English

NASA Technical Reports Server

Pyrolytic 
Graphite 
Insulation 
Water-cooled 
Probe 
Copper 
Washer 
0
Copper Nut 
Specimen 
Graphite 
Radiator 
Junc 1972	 B72-1O25 
NASA TECH BRIEF 
Space Nu clear Systems Office 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Repeatable Method of Thermal Stress Fracture Test

of Brittle Materials 
The problem: 
The thermal stress testing of graphite for the Nuclear 
Rocket required a simple reliable method of qualifying 
one sample against another with high accuracy. No test 
existed in which fracture is caused solely by a temper-
ature gradient under steady-state conditions. One test 
method involved a temperature gradient, but a mechan-
ical load was also needed. Another method utilized 
electric arc shock-induced stress; the specimen abruptly 
Fractured completely rather than cracking. Both of 0 	these techniques were not acceptable.
The solution: 
A method has been devised in which specimens are 
heated slowly and with sufficient control so that the 
critical temperature gradient in the specimens cannot 
occur before temperature equilibrium is reached, i.e., 
during the transient phase. 
How it's done: 
The test aparatus consists of a water-cooled copper 
probe for mounting the specimen, and a surrounding 
furnace containing a resistance heated graphite radiator. 
O
(continued overleaf) 
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The probe is insulated with pyrolytic graphite. The 
specimen is held in place by an integral shoulder of the 
probe, and kept in intimate contact with the probe by 
two copper washers. Each specimen is examined for 
manufacturing flaws before test, and again for thermally 
induced cracks after test, using an eddy current device 
which will readily detect even the smallest crack and a 
dye penetrant which will not only detect any crack, but 
also establish its size and shape. 
The object of a test is to find the radiator temperature 
at which the first crack is initiated. In a typical test a 
temperature is estimated at which fracture should occur, 
and the radiator is ramped-up to that value, usually in 
about five minutes. After ramp-down, the specimen is 
examined. If a crack has occurred, the radiator temper-
ature for the next specimen is lowered arbitrarily to 
establish a no-crack run. In a test of AUC graphite, only 
four to six specimens were needed to define the limits 
of crack/no-crack range to within 303 to 313K at 
around 2673K. This test series demonstrated the sensi-
tivity of the method to small changes in the material. 
Similar successful runs have been made with cermets 
and a wide range of metal carbide graphite particulate 
composites. 
This technique can best be used to compare similar 
materials differing in a single variable. The maximum 
radiator temperature is the only parameter measured; 
however, it is very valuable for qualitative ranking. 
Quantitative ranking is not possible because many other
factors which can contribute to cracking cannot be 
measured. The test fixture is uncomplicated and durable. 
The specimen geometry is simple and easy to fabricate, 
and the test results are readily reproducible and sensitive 
enough to detect component changes such as flour size. 
Notes: 
1. A wide variety of testing laboratories should find this 
method attractive in the evaluation of brittle coatings, 
linings of large furnaces, turbine and reentry body 
design, carbide machine tool fabrication and other 
related products 
2. Requests for further information may be directed to: 
Technology Utilization Officer 
AEC-NASA Space Nuclear Systems Office 
U.S. Atomic Energy Commission 
Washington, D.C. 20545 
Reference: B72-10258 
Patent status: 
No patent action is contemplated by the AEC or 
NASA.
Source: J. C. Rowley, R. W. Andrae and

A. E. Carden of
Los Alamos Scientific Laboratory

under contract to

Space Nuclear Systems Office

(NUC-1 1019)
S 
B72-10258	 Category 06


Repeatable method of thermal stress fracture test of brittle materials

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