Apparatus for clamping a test sample without any bending moment

Test samples, especially samples made of ceramic materials, are tested under tension and/or compression loads while simultaneously avoiding the application of a bending moment to the test sample. For this purpose, each end of the test sample is held by a holding element having spherical outer surfaces received in spherical segment recesses of clamping jaws. Each test sample end is held by its holding element in a form-locking force-transmitting manner without a direct contact between the clamping jaws and the test sample. The application of bending moments to the test sample is avoided because the cooperating spherical surfaces permit an adjustment of the testing sample into a vertical position without any canting of the test sample.

FIELD OF THE INVENTION 
The invention relates to an apparatus for clamping a test sample without 
applying a bending moment to the test sample while permitting the 
application of a tensile and/or compression testing load to the sample. 
BACKGROUND INFORMATION 
Clamping devices of the above type are well known in the art. Such devices 
have sample holding elements with spherical outer surfaces which are 
received in spherical segment recesses of various types of clamping 
devices. Reference is made in this connection, for example, to German 
Patent Publication (DE-AS) 2,028,030, published on Nov. 30, 1972, and 
disclosing a clamping mechanism in which clamping jaws (2) are supported 
between sickle-shaped wedges (3) which in turn are slideable and 
adjustable between two clamping pressure applying elements (10) which are 
mounted on a base plate (9). The clamping elements (10) apply a sufficient 
pressure to hold the respective end of the test samples in such a way that 
the testing force or load can be applied to the test sample through a 
friction contact between the end of the test sample and the clamping jaws. 
Such a structure is relatively complicated and leaves room for 
improvement, especially with regard to avoiding the application of bending 
moments to the test sample. 
In connection with testing samples for tensile and/or compresssion 
strength, it is necessary that exactly defined loads can be applied to the 
sample being tested. It is especially important that the test sample is 
not exposed to bending loads which can occur unintentionally, for example, 
if the test sample is canting, or when the cooperating surfaces of the 
clamping mechanism are mismatched, for example, due to imprecisions in the 
manufacturing process, especially at the points where the sample contacts 
the clamping jaws. Avoiding the application of bending moments to the test 
sample is especially important in connection with ceramic test samples 
which are particularly sensitive to bending loads, since ceramics are 
brittle. 
In addition to the above mentioned German Patent Publication (DE-AS) 
2,028,030, reference is made to German Patent (DE-PS) 671,954, and German 
Publication (DE-AS) 2,306,393. These publications illustrate efforts to 
avoid the application of bending moments to a test sample in a clamping 
mechanism of a testing machine. Both known clamping mechanisms employ 
cooperating spherical surfaces. However, it has been found that the known 
clamping mechanisms are not suitable for testing sensitive components, for 
example, of ceramic materials which are be tested by tensile forces and/or 
compression forces exclusively. The known devices either permit only the 
application of tensile forces as is the case with the just mentioned two 
publications or a complicated structure is required as is the case in the 
first mentioned publication (DE-AS) 2,028,030. 
U.S. Pat. No. 1,341,431 (Morrow) discloses a grip for testing samples in 
which conical or spherical surface areas cooperate with each other. Morrow 
also wants to avoid the application of "lateral stress" to the test 
sample. Compression forces cannot be applied in the Morrow apparatus. The 
same considerations apply to the gripping device of U.S. Pat. No. 
2,896,448 (Haines) in which, for example, sheet metal to be tested is 
wound partially around a split pin which in turn is held in a cylindrical 
hole. Haines wants to distribute the applied testing stress evenly over 
the entire cross-section of the specimen. However, it is not certain that 
bending stress can be avoided because the clamping mechanism directly 
contacts the sides of the specimen. 
German Patent Publication (DE-OS) 3,316,218 discloses a mechanism for 
applying compressive loads to a test sample in a material testing 
apparatus whereby the cooperating surfaces are also spherical to assure a 
central introduction of the compressive testing force into the test 
sample. 
East German Patent Publication 71,217, published on Nov. 5, 1970 discloses 
a self-centering clamping mechanism for test samples in which a conical 
surface of the clamp body cooperates with a movable spherical surface. A 
ring contact is established in this manner to assure the self centering. 
East German Patent Publication 250,376, published on Oct. 8, 1987 discloses 
a clamping mechanism, especially suitable for testing sectional steel 
members, specifically the strength of welding seams between such sectional 
steel members. Suggestions toward the present combination are not found in 
the above discussed references. 
OBJECTS OF THE INVENTION 
In view of the foregoing, it is the aim of the invention to achieve the 
following objects singly or in combination: 
to provide a simple clamping mechanism which is easily operable and which 
will avoid the application of bending moments to a test sample; 
to provide a clamping mechanism capable of applying either a tension load 
or a compression load to a test sample while avoiding the application of 
bending loads in both instances; 
to provide a clamping mechanism which is particularly suitable for 
sensitive test samples such as ceramic test samples; and 
to avoid a direct contact between the test sample and the clamping jaws of 
the clamping mechanism. 
SUMMARY OF THE INVENTION 
The clamping apparatus according to the invention is characterized in that 
sample holding elements have spherical surfaces on the outside for 
cooperation with corresponding surfaces on the inside of clamping jaws and 
further surfaces for tightly holding an end of a test sample. The rigid 
connection between the holding element and an end of a test sample may be 
accomplished, for example, in that the holding element has a surface that 
fits into a hole of the test sample end. Similarly, the holding element 
may have a hole through which the end of the test sample passes. Both 
instances include a form-locking connection. 
The just described features of the invention using holding elements with 
spherical surfaces and attaching the sample end in a form-locking manner 
to these holding elements, assure a simple clamping structure that is 
easily manufactured and conveniently operable, especially for very brittle 
test samples. The cooperation of spherical surfaces on the clamping jaws 
and on the holding elements prevents the formation of bending moments, 
especially if the test sample is kept out of contact with the clamping 
jaws. The formation of bending moments is avoided during the clamping 
itself and also during the subsequent loading of the test sample because 
the ends of a test sample can assume a position which is free of any 
forces other than the testing force. The holding elements including their 
spherical surfaces, may be made of steel, ceramics, hard metals, or even 
of fiber composite materials, even conventional bearing balls are suitable 
for the present purposes. 
A practical embodiment of the invention employs balls or spheres for the 
holding elements and these spheres are received in segment spherical 
recesses or ball shells in the clamping jaws, which in turn are clamped 
together by a holding ring or the like which presses the clamping jaws 
against the balls. Preferably, the clamping jaws have an outer conical or 
wedge-shaped surface, while the clamping or holding ring has a 
correspondingly shaped conical or wedging countersurface forming an inner 
surface of the clamping ring for cooperating with the outer respective 
surface of the clamping jaws. Further, it is practical that the clamping 
jaws are received in or connected to force transmitting elements, such as 
a mounting fork secured to other components of the testing machine. The 
mounting fork holds the clamping jaws by a journal mounting, for example. 
The mutual adjustment of the sample ends relative to each other especially 
under testing load applying conditions, is improved if the holding 
elements, specifically the spherical surfaces of the holding elements or 
balls and/or the spherical surfaces of the segmented recesses in the 
clamping jaws, are coated with a material providing a low friction 
coefficient. Instead, or in addition to such a coating, a lubricant may be 
introduced between the just mentioned surfaces. 
The holding of round test samples is accomplished in a very practical 
manner by providing the holding elements or balls with a central 
through-going bore in which an end of the respective test sample is 
received and rigidly secured, for example, by welding or soldering or 
brazing, or by an adhesive bond, whereby a connection free of play is 
achieved between the holding element and the test sample. 
For securing flat test samples to the balls it is practical to provide the 
holding element with a ground, ring-shaped surface and to provide a 
respective bore in each end of the test samples. Each bore is a 
through-hole, so that the holding element passes through the hole, whereby 
the spherical surfaces of the respective holding element project on both 
sides of the test sample and the ground or even polished ring shaped 
surface of the holding element comes to rest against a respective surface 
of the through-hole in the test sample. The just mentioned ring surfaces 
may be cylindrical or conical to form a frustum portion between spherical 
end portions of each holding element. 
It is advantageous to select a slanting angle of the just mentioned ground 
ring surface and of the hole through the test sample in such a manner that 
these angles are small enough to provide a self-locking feature between 
the test sample and the holding element with regard to relative movements 
in a direction perpendicularly to the load application direction. 
Preferably, the surfaces cooperating in a self-locking manner slant in 
opposite directions. In other words, the self-locking surfaces at one end 
of the sample slant in one direction while the self-locking surfaces at 
the other end of the sample slant in the opposite direction. The 
self-locking feature makes sure that play between the holding elements and 
the test sample is avoided and that either tensile or compression forces 
can be applied to the test sample.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE 
OF THE INVENTION 
Referring to FIGS. 1, 1A, and 1B, a flat test sample 1 has an upper end 1a 
held in place by an upper clamping mechanism A, and a lower end 1b held in 
place by a lower clamping mechanism B. The upper clamping mechanism A and 
the lower clamping mechanism B are of similar construction, except for the 
wedging configuration of cooperating slanting surfaces as will be 
described in more detail below. 
Each end 1a, 1b of the test sample 1 has a through-bore or hole 9 for 
indirectly securing the test sample end to the respective clamping 
mechanism by means of a holding element such as shown at 2, 2' in FIG. 1. 
In the embodiment of FIG. 1, each holding element 2, 2' has a cylindrical 
ring surface 3, 3' formed in its periphery or equator between two 
spherical segments having spherical surfaces forming a complete sphere. 
The spherical surfaces form clamping surfaces while the cylindrical 
peripheral surface forms a holding surface for the test sample. For this 
purpose, the cylindrical surface 3, 3' fits into the respective hole 9 in 
the corresponding end 1a, 1b of the test sample 1 with a form-locking fit 
relative to forces F effective in the longitudinal axial direction of the 
test sample 1. The diameter of the holes 9 and the outer diameter of the 
the ring surfaces 3, 3' is selected to be as small as possible for the 
intended purpose and will depend on the size of the particular test sample 
1. The holding elements 2, 2' may be made, for example, of steel, ceramic 
materials, hard metals, or fiber composite materials. Even conventional 
spheres used in ball bearings are suitable for the present purpose. By 
making the through-holes 9 and the surfaces 3, 3' cylindrical, a desired 
distribution of the bearing pressure is achieved. However, it is quite 
possible to also have a line contact between the holding elements 2, 2' 
and the cylindrical through-hole 9. For this purpose, the holding elements 
2, 2' are entirely spherical without the above mentioned cylindrical 
surfaces 3, 3'. Where such surfaces 3, 3' are provided, they are smoothly 
ground for a form-locking fit with the respective smoothly ground inner 
surface of the through-holes 9. 
The upper clamping mechanism A comprises two clamping jaws 13a and 13b 
mounted in a fork 8 by a mounting pin 7. The fork 8 in turn is 
conventionally secured to the frame 10 of a conventional testing machine 
not shown in further detail. Similarly, the lower clamping mechanism B 
comprises two clamping jaws 13c and 13d mounted in a further mounting fork 
8' by a mounting pin 7'. The lower mounting fork 8' is secured to a lower 
frame portion 10' of the testing machine. Each clamping jaw has a 
spherical segment recess 4. These recesses are arranged in each pair of 
clamping jaws so that the recesses face each other in a pair. The recesses 
4 are deep enough to receive the corresponding holding element 2,2' in 
such a way that a gap G is formed between the two clamping jaws of a pair. 
The width of the gap G is larger than the thickness of the test sample 1 
so that the latter will not directly contact the clamping jaws. 
Each clamping jaw of the upper pair 13a, 13b has an outwardly flaring lower 
end portion to form outer wedging surfaces 5 which slant upwardly and 
inwardly relative to the longitudinal central vertical axis of symmetry of 
the apparatus. A clamping ring 6 similarly has upwardly and inwardly 
slanting surfaces for cooperation with the surfaces 5 to wedge the two 
clamping jaws of a pair to each other, thereby firmly holding without play 
the respective holding element 2. Similarly, each of the lower clamping 
jaws 13c and 13d has an upwardly tapering upper end with wedging surfaces 
5', which also cooperate with a respective clamping ring 6'. In both 
instances the slant of the surfaces 5, 5' is such, that the respective 
rings 6, 6' tend to increase the clamping force in response to gravity. In 
any event, the clamping pressure exerted by the rings 6, 6' is sufficient 
to hold the holding elements 2, 2' in place without play by firmly 
squeezing the spherical segment recesses 4, 4' against the spherical 
surface of the respective holding element 2, 2'. If necessary, mechanical 
means may be employed to force the rings 6, 6' downwardly for a tight 
clamping without play For example, the rings could engage the respective 
surface of the clamping jaws with a threading to increase the clamping 
force by rotating the respective clamping ring. 
In FIG. 1A the dashed line 6a shows a square cross-section which 
illustrates a modification of the clamping jaws. The clamping jaws 13a and 
13b have circular outer surfaces. However, it is quite possible to make 
these clamping jaws so that they have substantially a rectangular 
cross-section if viewed together as indicated by the dashed line 6a. In 
the embodiment having clamping jaws with a rectangular cross-section, the 
hole in the clamping ring will also be rectangular. 
In any event, the arrangement will be such that both the upper clamping 
ring 6 and the lower clamping ring 6' will tend to assume a lowermost 
clamping position under the influence of gravity. However, the invention 
is not limited to this arrangement. For example, it is possible that the 
upper slanting or conical surface 5 tapers downwardly and inwardly so that 
the upper ring 6 must be forced into a clamping position in an upward 
direction, for example, by the above-mentioned threading or by spring 
elements, pins, or the like. Even the two possible cross-sections shown in 
FIG. 1A are not critical. Other suitable configurations may be employed 
and the clamping jaws may even have the shape of clamping bails or the 
like. 
The spherical surfaces of the holding elements 2, 2' and/or the spherical 
surfaces of the segment recesses 4, 4' can be provided with a coating to 
reduce friction. For this purpose the coating will have a low friction 
coefficient. Synthetic materials, metal carbides, nitrides, or the like 
are suitable for this coating purpose. In addition, or instead, a 
lubricant may be provided between the just mentioned spherical surfaces. 
In this manner it is assured that the holding elements remain movable 
relative to the clamping jaws in a rotational manner to thereby permit the 
adjustment of the vertical position of the test sample 1 and to eliminate 
the application of any bending moments to the test sample 1. The 
movability of the holding elements relative to the clamping jaws in a 
rotatable manner, must be assured even when the testing loads are applied 
to the testing sample. 
Incidentally, the above mentioned frame members 10, 10' of the testing 
apparatus may be movable to apply the testing force, for example, as a 
tensile force F. The connection of the pairs of clamping jaws 13a, 13b and 
13c, 13d to the force applying components of the testing machine may be 
varied for particular purposes. The embodiment shown in FIGS. 1, 1A, and 
1B is suitable for testing the sample 1 by tensile forces F. 
In FIG. 2 the elements which are identical to those in FIG. 1 are provided 
with the same reference numbers. However, in FIG. 2 the holding elements 
12, 12' are spheres specifically constructed for holding a test sample 1' 
having a round cross-section at least at the clamping ends. The 
arrangement is constructed for applying tensile loads to the test sample 
1'. The holding elements 12, 12' are spheres having through-bores 15, 15' 
respectively. The ends of the test sample 1' are rigidly secured in the 
through-bores 15,15' of the respective spherical holding element, for 
example, by heat shrinking, welding, brazing, soldering, or by an adhesive 
bond as shown at 15a, 15b. The heat shrink fit may be used to secure the 
ends of the test sample in the respective bore of the holding elements. 
The just described connection between the holding elements and the ends of 
the test sample 1' may again be released by the application of heat, for 
example. The spherical segment recesses in the clamping jaws and the 
diameters of the holding spheres are again so dimensioned that a gap G is 
formed between the clamping jaws to avoid contacting the test sample 1' 
with the clamping jaws. 
FIG. 3 illustrates an embodiment in which the mounting of the clamping jaws 
13e and 13f to a machine frame member 10' is accomplished by mounting 
brackets 11 and 11' respectively and by machine screws 11a and 11b. Thus, 
the clamping jaws cannot journal around the pins as shown in FIGS. 1 and 
2. Accordingly, it is possible to apply a testing force that is either a 
tensile force or a compression force in the apparatus of FIG. 3. The 
connection of the test sample 1" in FIG. 3 to the holding element 14 may 
be accomplished either as described above with reference to FIG. 1 or with 
reference to FIG. 2. 
The connection of the clamping jaws 13e and 13f to the machine frame by the 
machine screws provides a form-locking and force-locking connection so 
that either tensile or compression forces may be applied to the test 
sample 1", while still avoiding the application of bending moments to the 
test sample by maintaining the gap G. 
FIGS. 4 and 5 illustrate together a test sample 15 with widened ends 16 and 
17. Each end is provided with a respective through-bore 18, 19 having 
slanted conical walls 18' and 19'. These conical walls 18' and 19' slope 
in the same direction as shown in full lines in FIG. 5. However, it is 
also possible that these conical walls slope in opposite directions as 
shown at 18" and 19' in FIG. 5. The sloping in opposite directions is 
preferred for a test sample which is to be tested both by tensile forces 
and compression forces. In that case, the slanting in opposite directions 
makes sure to hold the sample without play. However, the slanting angle 
.alpha. may be selected small enough to assure a self-locking feature 
between the sample and the respective holding element 20 shown also in 
FIG. 6. Preferably, the slanting angle .alpha. is within the range 
3.degree. to 10.degree.. 
The holding element 20 of FIG. 6 has two spherical surfaces 21 and 22 
interconnected by a cylindrical section 23 having an outer diameter 
fitting without play into the inner diameter of the through-bores 18, 19. 
The cylindrical section 23 adjoins a conical or frustum shaped section 24 
having a slanting angle corresponding to the slanting angle .alpha.. The 
construction of the holding element 20 is preferably such that the same 
holding element can be used regardless of the direction of the slant as 
shown at 18' or 18". FIG. 5 shows the same holding element 20 inserted 
from one side at the lower end of the sample and the dashed lines at the 
upper end of FIG. 5 show the insertion from the opposite side. The just 
described features prevent relative motions between the sample 15 and the 
holding elements 20 in directions perpendicularly to the load direction. 
Thus, a connection free of play between the holding and the sample is 
assured to permit the alternating application of compression and tension 
loads. Further, the connection between the holding element and the sample 
may be reinforced by an adhesive bond, either at certain points around the 
circumference of the holding element, or completely around the holding 
element. 
Although the invention has been described with reference to specific 
example embodiments, it will be appreciated, that it is intended to cover 
all modifications and equivalents within the scope of the appended claims.