Semiautomatic rupture modulus tester for vacuum or inert atmospher

Arias, A.

English

NASA Technical Reports Server

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Reprinted frOIll TilE thvlf:\\, OF SCIf:1\TIFI ~ '~\i~IENTS, Vol. 34, ="io. , 911 - 914. Aug~st, 1963 h ~//- tJ/f 
- ' ._- -, - , Printed in U.~. A. V -..? II,"" • .r, V ( ~ . c.~~/ . . 
Semiautomatic Modulu f Ru ture Tester for OperatIOn III Vacuum 
\ or Inert Atmospheres 
2e«.. ~; ;' /4- ALA ~ 2-/ 3~ 64 10 075 
Lewis Research Cenle". National Auonaltlics and Space AdministratiE2.1;, Cleveland, Oh-io CCot;; #O/ltG' (R~:d29 April 1963 ; and In final forlll, 27 May 196312 c:..t- I {; () 7 r-
The construction and operation of semiautomatic modulus of rupture tester for determining the modulus of 
rupture up to 2400°C in either vacuun or inert atmospheres are described. One of the unique features of the ap-
paratus is its ability to test up to 18 Sp'l'cimens without being reloaded. This paper includes a description of the 
auxiliary equipment used in conjunction itb the tester and a typical curve of modulus of rupture against tempera-
ture obtained with this tester. flztrffO (!./ 
INTR ODUCTION 
\ T HE modulus of rupture or flexural strength for'-'\. beam of rectangular cross ection wi th freely Supp(l)rted 
ends is defined by the equation \ 
where S is the modulus of rupture in units of force per unit \ 
area, P is the force aL rupture applied to the beam at " 
midspan, L is the span between supports, b is the width of 
tlle beam, and It is the height of the beam. 
The modulus of rupture is a useful criterion for com-
parison of the relative trengths of brittle mat rials up to 
temperatures at which the materials deform plastically. 
Oxides, carbides, borides, and many other ceramic-type 
materials are commonly tested in this manner. The ease 
with which the test specimens can be prepared, the speed 
with which the tests can be carried out, and the relativc 
simplicity of the te t equipment required for room-tcm-
perature testing, among other factors, accowlt for the 
popularity of this testing method. l:-ligh-temperature te t-
ing for the modulus of rupture ha , in common WIth other 
testing procedures, the di advantage that time is required 
fOl heating and cooling the furnace as well as for loading 
and unloading of test specimens. It is the purpose of thi 
paper to describe a modulus of rupture tester for high-
temperature use in which a large number of specimen CRn 
be tested in the same run without opening the furnace. In 
addition, the furnace used in onjunctiol1 with the te ler i 
described. 
FURNACE 
The high-temperature vacuum furnace u ed i howl1 
schematically in FIg. 1. This vacuum furnace IS indtlction 
heated with a graphite susceptor and is mowlted between 
the crosshead and the table of a 20 OOO-Ib capacity, Tmius 
Olsen Electomatic ten ile machine. The furnace i up-
ported by four leg of adjustable height, whIch are bolted 
to the floor. Except for the top and bottom plun O'er , no 
part of the furnace touches the tensile machine. 
The 6i-in.-i.d., 16-in.-Iong graphite susceptor and tlJe 
carbon susceptor extensions re t on the water-cooled 
bottom plate and are kept in position by means of pins 
(not shown) . The susceptor is located inside a lO-in.-i.d., 
32-in.-long fused silica tube, which fits into annular grooves 
in the water-cooled top and bot Lorn plates. Silicone rubber 
gaskets in these grooves seal the heating chamber vacuum 
tight. Carbon black insulation fills the annular space be-
tween the susceptor assembly and the silica tube. Glass or 
graphite cloth backed by a segmented graphite ring pre-
vents the carbon black insulation from being blown out 
during evacuation of the furnace. The furnace connects to 
th vacuum system through a port in the water-cooled 
J manifold that i welded to the top plate. Four thick tie rods 
\ between the top and bollom plates align the furnace and 
j ensure a rigid a sembly. Power is provided through a water-
cooled induction coi l wrapped around the silica tube. 
I The modulu of rupture tester rests on a carbon block 
I in ulator, which, in turn, rests on the water-cooled boltom 
I plunger. Both the tester and the carbon block clear the susceptor assembly by about /6 in. A Teflon bellows 
a embly between the bottom plu nger and the bottom plate 
provides vacuum tightness without appreciable effect on 
Ih vertical motions of the bottom plunger and prevents it 
TO VACUUM SYSTEM 
THERMOCOUPLE TUBE 
CARBON ROD- -
CARBON 
RADIA TIDN SHIELDS 
~IODULUS OF RUPTURE 
TESTER (SEE FIG 2) -
_f COOLING WATER 
-,,~ - ACTUATOR 
• - - WATER-COOLED 
TOP PLUNGER 
- - MANIFOLD 
'---ACTUATOR 
EXTENSION ROD 
- CARBON BLACK 
INSULA TlDN 
• GRAPHITE SUSCEPTOR 
- - CARBON BLOCK 
FUSED SILICA TUBE SUSCEPTOR EXTENSION 
WA TER-COOLED 
FURNACE LEG- BOTTOM PLUNGER 
LEAD COUNTERWEIGHT TEFLON BELLOWS 
THREADED COLLARr.Li.-i+--1:...L-+!-...LJ~COOLING WATER 
FIG. 1. High-lemperature vacuum furnace. 
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912 
from rotating. A threaded collar allows vertical adjustments 
of the boltom plunger. A lead counterweight balance the 
atmospheric pressure on the plunger when the furnace is 
under vacuum. 
The force required La break the LesL pecimen i tran 
milted from the crosshead to the tesLer through the water-
cooled top plunger and a carbon rod. Couplings bet \V en 
the top plunger and the carbon rod and between the carbon 
rod and the tester complete the force-transmiLling link. 
The fiat lower portion of the actuator fils 100 ely into a slot 
in the adapter, which i aLLached to the actuator exten ion 
rod. The actuator assembly allows transmis ion of ro-
tational motion to the lester without interfering with it 
vertical motions. 
A thermocoupl tube with a tung Len-rhenium Lher- '/ 
mocouple and beryllium oxide insulator can reach inside 
the tester to determine pecimen temperatures. The ther-
mocouple tube enters the furnace through a Conax filling, 
and the tube can be rai ed above the carbon radiation 
shields for operation at temperature above the range of 
the thermocouple assembly. Temperatures can also be 
determined with an optical pyrometer by sighting the f 
l pecimen through a glass pri m in the furnace cover. A 
system of vacuum valves (not hown) in the tube where \ 
the prism is located allow removal and cleaning of the f 
pri m while the furnace is in operation. 
The water-cooled furnace over is provided with O-rings J 
for sealing all its openings vacuum tight. This cover is l , 
provided with an internal flange and with external guide 
pins for accurate centering and location. 
Crosshead motions are sensed by a defiectometer con-
nected to the top plunger through an adjustable-length 
link. The eros head motion and the load are recorded on 
the tensile machine recorder. 
Up to about 1200°C vacuums of less than 0.01 p. Hg are 
readily obtained. A the teml erature increases, the pres-
sure grad ually build , up to about 10 p. Hg at 200°C be-
cause of evaporation of volatile matter from the carbon 
parts. This pressure is expected to lecrease with use a the 
volatile matter i driven out of the carbon parts. 
The maximum power required to hold the furnace at 
2800°C is about 20 kW. 
Calibration of the tensile machine plus the furnace 
system by means of dead weights show d that, in add ition 
to the tensile machine percent error, frictional force in 
the furnace and variations in atmospheric pres ure in-
troduce an error of less than ±2 lb in determining the 
actual loads applied to the test specimens. 
This furnace design lends itself to a high degree of 
versatility. By the use of various internal parts, the appara-
tus may serve as a hot press, tensile tester, or compression 
tester in addition to its primary design purpose as a 
modulu of rupture tester. So far, our particular furnace 
ha been used for modulus of rupture testing and as a hot 
CASE 
I 
/ 
I 
./ 
/ 
/ 
I 
/ ( 
) ( 
SIGH/ TiNG HOLE~, 
, A...::irJc _ "" .. 
PIN ~, 
,I""SHEAR PIN 
",.0' ,-ACTUATOR 
/' EXTENSION ROD 
THERMocfouPLE" , 
HOLE - -r-- ____ ~ 
F KEY 
,~STUD 
, 
I 
SET ,SCREW- c- .~~OOM'~~ I 
PU~H ROD-- __ • ,~ARM 
,-CARRIAGE 
.",_'--'·""Ir-REAR WALL fRAME- -~_ f 
,'DE COVER~\ 
I ~ _ 
I RODS-"-- -
PIN- - -- - ""' 
--c 
--8 
- A 
~SPECIMEN STACK 
FRONT WALL 
"-SPECIMEN 
- HOUSING 
FIG. 2. SemiauLomatic modulus of rupLure Lester. 
pre s. During hot pressing, a load maintainer in the tensile 
machine keeps the load at any desired level, and the cro s-
head motions can be followed WiUl the defiectometer to 
determine the specimen densification with time, pres ure, 
and temperature. 
Metered argon or helium can be used instead of a 
vacuum as an atmosphere or for rapid cooling blow 1500°C. 
Overheating of the silica t.ube above thi temperature pre-
clude the 11 e of argon or helium except at low pre ure. 
MODULUS OF RUPTURE TESTER 
The modulus of rupture tester to be described here is of 
the centerpoint-loading type and differs from the familiar 
type of modulus of rupture tester in that it is provided 
with a magazine for storage of specimens, a means of 
moving the spe imens into po ition, and a means for re-
moving the broken specimens. This modulus of rupture 
ester is shown in Fig. 2. 
\ The te ter is mad entirely of graphite. The housing o.d. 
is '0 in. and the height is 8! in. The lid is secured to the 
housing, and at the same time centered, by means of 
graphite bolts. The load is applied to the specimen through 
a pu h rod , which slide up and down the center hole in the 
lid. This push rod i prevented from rotating by a et 
screw whose end fits into a groove in the pu h rod. 
Th , specimens rest on t-in.-diam rods located in V 
grooves in the base. These rods are fastened to the ba e by 
means of pins (not shown) . The V grooves in the ba e are 
accurately machined to provide a 2.000-in. span between 
the cen terlines of the rod . 
A carriage can slide back and forth on a tra k cut in the 
base. The carriage is actuated by rotating the actuator 
extension rod, the motion being tran mitted to the carriage 
r----------~---- ----------- --- -- --- -- -----
;VI 0 D U L 1 
through an arm and a stud. The ann is secured to the 
actuator exten ion rod with a key. The stud is fastened to 
the carriage and extend below the carriage to fit into a slot 
(not shown) in the ba e. The size of this slot limits the 
backward motion of the carriage, so as to prevent the 
carriage from touching the su ceptor. 
The magazine for storage of specimens is formed by the 
rear wall, the front wall, and the two ide irames. These 
side frames are provided with vertical grooves on which the 
front and rear walls of the magazine fit. The side frames are 
held in place by long bolts that pass through the lid and 
the ba e and screw into the bollom of the housing. The 
rear wall is provided with an opening at the bottom that 
just allows passage of the horizontal step B of the carriage 
(Fig. 2). The bottom piece in the stack of specimens in 
the magazine re ts on the rods. In thi position, the bottom 
piece is too high to pass through the opening in the rear 
waU, but it can pass through a bottom opening in the 
front wall when pushed by vertical surface A of the 
carriage. 
The side cover is held in place by mean of pins. Thi 
cover prevents the broken specimen pieces from entering 
the annular space between the te ter and the su ceptor 
and, at the same time, provides a "blackbody" enclosure 
for more accurate optical temperature measurements 
through the sighting hole in the lid. There is a hole in the 
side cover through which the thermocouple tube can be 
inserted. 
The pecimens used in this particular setup are prismatic 
bars of rectangular cross section 2.35 in. long, 0.500 in. 
wide, and 0.250 in. high. As already mentioned, tbe span 
between the !-in. rods on which the pecimens are broken 
is 2.000 in. The distance between the ide frame , which 
limits the hteral motion of the test sp cimens, is 2.40 in. 
Specimen higher or lower than 0.250 in. can be used pro-
vided the bottom opening of the front wall is enlarged or 
shortened accordingly. 
The magazine i :filled with test bar either through the 
rear by removing the lid and the rear wall or through the 
bottom opening in the front wall with the aid of a spatula. 
The device operates as follows. With the tester installed 
in the furnace as shown in Fig. 1 and with the specimen in 
the position shown in Fig. 2, the specimen is loaded at 
midspan by the push rod, which is actuated by the cross-
head, until the specimen breaks. The motion of the cross-
head i reversed until it returns to its original position, as 
determined by reading the recorder chart. As the actuator 
is ro tated, the vertical urface A of the carriage pushes the 
bottom test bar in the magazine through the bottom 
opening of the front wall. This new specimen pushes the 
broken specimen to the edge of the rods, from where it 
falls to the bottom of the hou ing. At the end of its forward 
travel, vertical surface C of the carriage will be touching 
the rear wall, while the new test bar will be in its test posi-
OF RUITURE 
60.000 
w S 40.000 
t-
~ 
a: 
... 
o 
IS 20.000 § 
::; 
o 
o 
400 
o 
o o 
o 
800 1200 1600 2000 
TEMP. ·c 
FIG. 3. Plot of modulus of rupLure against temperature 
for LiLanium-modified zirconia. 
913 
tion right under the push rod and ready for breaking. 
While the carriage was pushing the specimen into test 
10 ition, the bottom specimen in the stack dropped on 
horizontal surface B of the carriage but was prevented from 
moving with the carriage by the front wall. When the 
motion of the carriage is reversed, this test bar drops on the 
rods and is ready for a repetition of the sequence described 
above. 
The capacity of the magazine is 18 pieces of the size 
mentioned above. 
The apparatus described above has been operated at a 
total force of 1000 Ib without ill efJects and at temperatures 
up to 2400°C without noticeable dimensional changes. 
I Full-load runs with graphite test specimens were carried 
out. 0 trouble was encountered with the graphite speci-
mens up to the maximum te t temperature of 2400°C. The 
graphite test specimens (G grade, National Carbon Com-
pany) were prepared without particular care about dimen-
sions or surface conditions since the only objective of this 
te t \Va to determine the suitability of the feeding mech-
anism. Plastic yielding in tllis graphite was readily ob-
erved in the curve of crosshead motion against load in the 
recorder. The fir t indication of plastic deformation was 
obtained in the 2200°C test. This plastic deformation was 
later confirmed by determining the radius of curvature of 
the broken specimens. 
Run were also made with titanium-modified zirconium 
oxide pecimens. These specimens could be fed easily by 
the actuating mechanism at all temperatures up to 1400°C, 
at which temperature the specimens started sticking to 
each other. Subsequent runs in which the top specimens in 
the magazine were separated from each other with small 
piece of 5-mil tung ten wire made it possible to operate 
the feeding mechanism up to 1800°C, at which temperature 
the specimens started sticking to the graphite rods and to 
the carriage. These troubles were caused by evaporation of 
titanium metal from the test specimens followed by re-
action of the titanium with the graphite parts. Considering 
the reactivity of titanium, the device was operating under 
really disadvantageous conditions. This kind of trouble is 
not expected to occur with the great majority of ceramic 
materials. 
\ 
914 ALA 
A typical curve of modulus of rupture against tempera-
ture obtained with the aid of this tester is shown in Fig. 3. 
This curve was obtained in an 8-h run in which about 20 
min were allowed for each specimen to reach an equilibrium 
temperature. This curve is included for illustration pur-
poses only, and data regarding specimen preparation, 
curve shape, and so forth will be included in a separate 
study. Regarding the unusual shape of the curve, it will 
suffice to say that the zirconia in this material undergoes 
a phase transition from monoclinic to tetragonal which 
starts at about 950°C on heating. 
ARIA 
In cases where the thermal history 01 the specimen may 
influence the modulus of rupture values, the tests can be 
carried out "on the run" since the temperature gradient 
in specimens of the dimensions used is small and is not 
expected to alter the modulus of rupture significantly. In 
the great majority of cases, however, the materials used 
are either sintered or hot pressed well above the maximUID 
testing temperature, and soaking at temperature before 
testing should not significantly affect the results since 
grain growth, sintering, and so forth are strongly tempera-
ture dependent and should be negligibly small compared 
with the same effects at the sintering temperature. 


Semiautomatic Modulus of Rupture Tester for Operation in Vacuum or Inert Atmospheres

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