Method and apparatus for evaluating fold endurance and surface adhesion of sheet materials

An apparatus and method uses vacuum restraint, or other pressure differenl, to hold the ends of a sheet specimen to two opposing surfaces, thereby creating a fold in the specimen. As the opposing surfaces cycle along a parallel axis in opposite directions, the fold repeatedly rolls through a specific region of the specimen. The spacing between the opposing surfaces can be adjusted to increase or to decrease the radius of the fold. The velocity of the fold and the number cycles also can be controlled precisely. After repeated movement of the fold, the adhesion of surface treatments, coatings or printing on the specimen substrate begins to break down. Additionally, the integrity of the substrate itself may begin to deteriorate, if it is susceptible to deterioration and to varying degrees depending upon the susceptibility. The extent to which the specimen coating is degraded and substrate integrity reduced is a function of spacing, fold velocity and number of cycles.

Cross-references to related applications, if any: None. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to testing the endurance of 
flexible sheets and the adhesion of surface treatments, coatings, and 
print to the surface of such sheets, as well as the bonding of multilayer 
sheets. More specifically, the present invention relates to the creation 
of a small radius fold in a sheet specimen and methods of restraining the 
specimen with a pressure differential. The fold can be shifted 
progressively across the sheet in a controlled manner by manipulating the 
sheet specimen. 
2. Description of Related Art 
A number of fold endurance testers have been developed in the past. Some of 
these testers were capable of producing a sharp fold at one location with 
no sliding contact with the specimen. These devices merely folded and 
unfolded the sheet material repeatedly at a single line. However, a 
repeated fold at one location is relatively ineffective in the evaluation 
of printing durability or adhesion of other surface treatments. A moving 
line of fold or flexure is necessary for the efficient testing of sheet 
specimens. 
There are also devices which can cause a fold line to progress and recede 
over the surface of a specimen by means of rubbing or rolling contact with 
the specimen at the line of fold. If the line of fold is created and/or 
controlled by a roller, the minimum radius of the fold is severely 
limited. Since flexural stress in a folded specimen is proportional to the 
inverse of the fold radius (i.e., stress can be increased significantly by 
decreasing the radius of the fold), a roller severely limits a flexural 
stress which may be applied to the specimen. 
Smaller bend radii at the fold line are made possible by using a sliding 
contact at the inner radius on some devices. However, this approach limits 
the speed at which the test may be conducted, because of the heat 
generated by friction at the fold line. Moreover, the stress on the 
specimen in rubbing contact is an indefinable combination of cyclic 
flexure, shear at the surface, abrasion and wear. Thus, failure modes are 
inextricably mixed. 
The present invention is for a method and apparatus for testing the 
flexural durability and structural integrity of sheet materials and the 
adhesion and durability of surface treatments (such as printing and/or 
coatings) which permits testing without the use of rollers or rubbing 
surface contact, and permits testing with bend radii approaching zero, 
overcomes the shortcomings of earlier devices and represents a significant 
advancement in the art. In those applications where flexural endurance is 
used to determine the adhesion of printing, coatings, or other surface 
treatments, rubbing and/or sliding contact of the surface against itself 
is unacceptable. 
one immediate application of the present invention is in evaluating the 
print durability of currency notes. In this application, the present 
invention will replace a device that was intended only for testing of the 
structural integrity of the material being tested. It was never intended 
to test for the adhesion or durability of surface treatments. That device 
was labor intensive, time consuming and sensitive to operator 
manipulation. The present invention will reveal process defects not 
otherwise apparent by current methods. 
The inventors are unaware of any method or apparatus which produces a 
moving, 180.degree. fold line of extremely small inside radius in a sheet 
specimen. No mechanically clamped specimens are required in the process. 
The inventors believe that the present invention is likely to fulfill a 
similar need within specialized segments of the paper and plastic film 
industries. The invention represents a breakthrough in timely durability 
testing of currency, removing a barrier to the development of effective 
quality control that has existed for many years. As a result, it is likely 
that the present invention has potential for widespread use. The 
durability testing assists also in determining the structural integrity of 
the materials used in producing the sheets being tested as specimens. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is a first principal object of the present invention to provide a method 
and apparatus for testing the flexural durability of a sheet material 
specimen by moving a 180.degree. fold across the sheet material repeatedly 
without sliding or rolling contact against the face of the specimen at the 
inside radius of the fold. 
It is a second principal object of the present invention to provide a 
method and apparatus for testing the flexural durability of a sheet 
material specimen which also can test the durability of the bond between 
layers of a composite sheet material and the adhesion durability of 
printing, coatings and other surface treatments on the surface of the 
specimen. 
It is a different object of the present invention to provide a method and 
apparatus for testing the flexural durability of a sheet material specimen 
which can achieve very small bend radii in the specimen. 
It is one other object of the present invention to provide a method and 
apparatus for testing the flexural durability of a sheet material specimen 
which can be used to evaluate the print durability of currency notes. 
How these and other objects of the present invention are accomplished will 
be explained in the detailed description of the preferred and alternate 
embodiments of the invention in connection with the FIGURES. Generally, 
however, the objects of the invention are accomplished in an apparatus and 
method using vacuum suction to hold the ends of a sheet specimen to two 
opposing surfaces, thereby creating a fold in the specimen. As the 
opposing surfaces move in opposite directions, the fold in the specimen 
sheet continuously shifts. The opposing surfaces can be moved back and 
forth repeatedly, creating a moving fold in the specimen. The spacing 
between the opposing surfaces can be adjusted to increase or to decrease 
the radius of the fold. After repeated movement of the fold, the adhesion 
of surface treatments, coatings or printing on the surface of the specimen 
begins to break down. Additionally, the substrate itself may begin to 
deteriorate after repeated folding. The extent to which the surface 
coating is degraded and/or substrate integrity reduced can be measured in 
subsequent tests. 
Other variations, modifications, applications, advantages and ways in which 
the objects are accomplished will become apparent to those presently of 
ordinary skill in the art after reviewing the specification and are deemed 
to fall within the scope of the present invention if they fall within the 
scope of the claims which follow the description of the preferred and 
alternate embodiments.

In the FIGURES, like reference numerals refer to like components. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention relates generally to the testing of flexible sheets 
of material. Such sheets are tested to determine the structural integrity 
of the materials from which they are made. One such intended material 
includes cellulose, cellulosic materials, and combinations of cellulosic 
and other materials. The cellulosic materials can include wood pulp, 
cotton, flax, hemp, jute, ramie, and regenerated unsubstituted wood 
celluloses such as rayon. Combinations of said cellulosics and 
combinations of said cellulosics with other fibers such as polyesters, 
silk, nylons, plastics, acrylics, and the like also can be tested. 
In cases where printing, coating or other surface treatments are applied, 
the present invention also permits the testing of ink adhesion and the 
adhesion and durability of other surface coatings and treatments. The 
invention is generally shown in FIGS. 1-6. 
FIG. 1 illustrates an external view of one embodiment of the present 
invention. The external controls and features of the present invention are 
a matter of design choice. The view in FIG. 1 is provided for illustrative 
purposes. The testing device 100 includes an external frame 102 and a 
power source 104. A control mechanism 106 is mounted to external frame 
102. 
Control panel 106 includes an on/off switch 108 and a fuse 110. Controls 
for separation spacing, cycle count setting and resetting, vacuum control 
and fold velocity may also be provided. A cycle display 112 may also be 
provided to display the number of cycles completed. Device 100 includes a 
front entry aperture 114 having a shim 116 to assist in inserting a sheet 
for testing. Shim 116 slides in guides 118. 
It has been found that introduction of the specimen into the testing device 
100 can be simplified by folding the specimen over a thin sheet of metal 
shimstock and using the shimstock to insert the specimen into a space 
between perforated surfaces. Tapering of the entry aperture 114 further 
assists in this phase of the testing. Other guides can be used to permit 
easier entry and proper positioning of the shimstock insertion tool. 
FIGS. 2A-2C show the preferred embodiment of the present invention in a 
device 100. FIGS. 2A-2C illustrate the most basic embodiment of the 
present invention. Upper and lower stationary vacuum boxes 220 and 222, 
respectively, have vacuum chambers and seals 224 and 226, respectively. 
Seals 224 and 226 are effective about the entire perimeter of each vacuum 
box, preventing air from being drawn into the vacuum boxes at the upper 
surface of porous plate 228 and the lower surface of porous plate 230. 
An elevated pressure in the cavity in which the specimen 234 is held can 
also be used to create a pressure differential to hold specimens in place 
on the plates 228 and 230 instead of a vacuum. Alternatively, a 
combination of elevated pressure and vacuum also may be used. 
Electrostatic force also can hold a specimen in place on the translating 
plates. Appropriate electrostatic forces can be generated in conventional 
ways (e.g., in the same manner as that used with the plotting surface of 
x-ray recorder instruments and to motivate film-to-core attraction during 
automatic roll changes in a web-handling apparatus). 
Porous plates 228 and 230 are supported by a conventional slide means 232, 
which allows the plates 228 and 230 to translate in directions opposite 
one another. The distance between plates 228 and 230 is controlled 
precisely by slide means 232, which is of conventional design. Plates 228 
and 230 can be hinged to permit easy access to the space therebetween for 
insertion of a specimen and for cleaning and maintenance. 
The porosity of plates 228 and 230 is directional, so that air may pass 
freely through the thickness of the plate, while maintaining a pressure 
differential on the specimen 234. Plates 228 and 230 may be designed to be 
easily removable from the device, so that a folded specimen could be 
placed in the plates, and the plates inserted into the device. At the end 
of the test cycle, the plates would be removed from the device, and the 
test specimen retrieved. Such a configuration would facilitate periodic 
maintenance likely to be necessary to remove ink or other debris lost by 
the test specimen. 
FIG. 2A shows the edge view of a specimen of sheet material 234 folded in 
half and positioned so that the line of fold is directly below the sliding 
seals 224 and at right angles to the direction of motion of the porous 
plates 228 and 230. The specimen 234 need not be confined entirely to the 
space between the plates 228 and 230. The corners, or "tails" of the 
specimen 234 can overhang the ends of the plates 228 and 230 to facilitate 
removal of the specimen after testing. 
It may also be desirable to secure the tails of the specimen in some 
manner. Several methods of accomplishing this are acceptable. A constant 
vacuum can be applied to the ends of the specimen. The tail of the 
specimen might also be clamped mechanically by conventional means to the 
porous surfaces. One such configuration is shown in FIG. 2E. A clamp 242 
holds one end of the specimen 234 to plate 228, while a second clamp 244 
holds the other end of specimen 234 to plate 230. A clamp used in this 
configuration must be of a low profile so that it will not interfere with 
the very limited space between the plates 228 and 230 (or the nip between 
rollers or drums). Such mechanical means can be used to augment holding of 
the specimen by a pressure differential. 
The reduced pressure in the vacuum boxes 220 and 222 is communicated to the 
specimen 234 through the porous plates 228 and 230. The specimen 234 is 
thus drawn to the plates 228 and 230 and held firmly in contact with the 
surface of each plate. When the plates 228 and 230 are then moved, 
relative to one another, the line of fold in the specimen 234 rolls along 
the surface of the specimen 234, but remains stationary with respect to 
vacuum boxes 220 and 222. Positive and negative motions of equal length by 
the two perforated plates (or other directionally porous surfaces) cause 
the line of fold to be stationary within the apparatus and the rolling 
fold action to be repeated in a specified region of the specimen. If the 
motion of each of the two surfaces is designed to be independent of the 
other, the device can be programmed to perform tests in more than one area 
of the specimen. 
FIG. 2D illustrates one other variation on use of the invention. Linear 
translation of the perforated plates 228 and 230 may not necessarily be 
along axes orthogonal to the line of fold 236. In fact, the effect on the 
specimen 234 could be made more rigorous if the line of fold 236 is 
oriented at various angles to the axis of translation. FIG. 2D illustrates 
this feature. Angles .alpha. and .beta., which are measured from the line 
of fold 236 to the axis of linear translation (here indicated by lines 238 
and 240) need not be 90.degree., but must be equal in magnitude. 
For the line of fold 236 to remain stationary relative to the apparatus 
200, the travel of the linearly translating perforated plates 228 and 230 
must be equal in magnitude and opposite to one another relative to one 
another in direction. As is apparent, there is no limit to the magnitude 
of angles .alpha. and .beta.. If these angles are increased continuously, 
without accompanying linear translation, a radial line of fold pattern can 
be produced on the specimen 234. 
The pressures holding the specimen 234 in contact with plates 228 and 230 
are always located in the area immediately adjacent to the fold 236. This 
is the location where restraint is required to allow a fold to roll 
through a specific region of the specimen 234. This feature is further 
illustrated in FIGS. 2B and 2C, in which plates 228 and 230 have been 
moved in opposite directions relative to one another. At no time does the 
specimen 234 slide with respect to either plate 228, 230. The surface of 
plates 228 and 230 (or any other configuration used to hold the specimen) 
can be modified to enhance the restraint provided by differential 
pressure. Possible modifications include high friction or abrasive 
coating, or an array of needle points which actually pierce the specimen 
in an area remote from the testing area to prevent in-plane slipping. 
Small "curbs" on the engaging surfaces of the plates may also be used to 
prevent slippage of the specimen during testing. 
The spacing between surfaces which determines the radius of the fold can be 
changed to account for different material thicknesses or for changes in 
the degree of severity desired. So long as the space between the plates 
228, 230 is greater than twice the thickness of the specimen 234, there is 
no contact between inner faces of the specimen 234. The inside curvature 
can be very small, approaching zero, if desired. It is possible, and may 
be desirable, to initiate testing at a wide spacing, and then to change 
the spacing of the surfaces as testing progresses. This method likely will 
find particular application with testing of strong or stiff materials. The 
exercise process could begin gently at a large spacing, and become 
increasingly more severe as the specimen loses strength and stiffness. 
Several alternate embodiments also are contemplated with the present 
invention. FIG. 2F shows another parallel plate configuration in which 
rectangular pistons 270 have fixed positions and the perforated outer 
cylinders 272 oscillate in opposing directions to maintain a fold line 
276. 0-rings 274 maintain a vacuum in preselected sections of the 
cylinders 272. By including perforations on the noses of cylinders 272, 
the specimen may be clamped to the cylinder noses, as seen in FIG. 2G. 
This embodiment permits in-place visual or optical inspection of the 
specimen before and after testing. 
FIG. 3 shows a device 300 in which the porous plates 228 and 230 of FIGS. 
2A-2C have been replaced by porous drums 328 and 330. Drums 328 and 330 
are capable of restricted rotation about their axes 328a and 330a. 
Stationary vacuum boxes 320 and 322 are fixedly positioned within drums 
328 and 330, respectively. Seals 324 and 326 on the vacuum boxes prevent 
air from being drawn into the vacuum box past the inner diameter of each 
drum. Folded specimen 334 is held to the exterior surface of the drums 328 
and 330 by the vacuum communicated to those drums through the porous walls 
of each drum. 
Drums 328 and 330 are porous in a radial direction only. As is apparent 
from FIG. 3, if drums 328 and 330 are rotated in equal increments in the 
same direction (e.g., clockwise), the line of fold 336 will roll along the 
specimen 334 while the line of fold 336 itself remains fixed relative to 
the nip between the drums 328 and 330. This alternate embodiment shown in 
FIG. 3 is functionally equivalent to that of FIGS. 2A-2C. As is apparent 
from the geometry of the dual drum design of FIG. 3, as the drums become 
larger in size, this alternate embodiment becomes virtually identical to 
the preferred embodiment using a dual porous plate configuration. 
Another alternate embodiment is shown in FIGS. 4A-4E. Once again, this 
embodiment is functionally similar to that of FIGS. 2A-2C, except that the 
porous plates 228 and 230 have been replaced by two perforated continuous 
belts 428 and 430, which run over parallel rollers 438 and 440, 
respectively. With belts 428 and 430 moving in a direction generally 
indicated by arrows 442 in FIG. 4A, the folded specimen 434 is fed into 
the space between belts 428 and 430 until the line of fold 436 reaches a 
desired position at the seal line of the vacuum boxes 420 and 422. 
Seal boxes 424 and 426 define an area on belts 428 and 430, respectively, 
where a vacuum is applied through the directional porosity of the belts to 
hold a specimen 434 in a desired orientation and to move the specimen 434 
in a preselected manner, while maintaining the line of fold 436 in its 
preselected position. FIG. 4B illustrates the line of fold being in a 
preselected desired position relative to vacuum boxes 420 and 422 and seal 
boxes 424 and 426. 
With belts 428 and 430 moving in the direction generally indicated b? 
arrows 444, as seen in FIG. 4C, and arrows 446, as seen in FIG. 4D, it can 
be seen that the line of fold 436 will translate back and forth across the 
face of the specimen 434 in a manner similar to that illustrated in FIGS. 
2B and 2C with respect to the preferred embodiment. 
When a sufficient number of rolling fold cycles have been completed, the 
belts 428 and 430 may be driven in the direction generally indicated by 
arrows 448 in FIG. 4E to eject the specimen 434 from the apparatus 400. 
This embodiment has the advantage over the embodiment of FIGS. 2A-2C in 
that feeding and recovery of the specimen 434 is facilitated. The means by 
which the specimen 434 is held and flexed at the fold line 436, is 
functionally equivalent to that of the previous embodiments, however. 
The use of a translating plate 530 and a cylinder 528 may be combined, as 
seen in FIG. 5. Such a configuration would allow for visual inspection of 
the specimen 534 remote from fold 536 by a lens 550 or other optical 
viewing or analyzing device. In this embodiment, the plate velocity v must 
equal the speed r.omega. of the surface of cylinder 528 to maintain a 
constant line of fold 536. 
The apparatus and methods described thus far presume that the material of 
which the specimen is composed is sufficiently impermeable to air that a 
pressure differential can be applied across the specimen to hold it in 
place for testing. In cases where the specimen is composed of a 
semipermeable or permeable material, as seen in FIG. 6, a thin, flexible 
film 650 may be used. Film 650 renders specimen 634 impermeable and 
creates the holding effect without affecting the rolling fold test. It may 
be desirable, when inserting the specimen and film, to establish a small 
gap between the film 650 and the fold 636 of the specimen 634 to avoid 
undesirable rubbing or other contact between the two materials. 
Variations, modifications and other applications of the present invention 
will become apparent to those presently of ordinary skill in the art after 
reviewing the specification in connection with the FIGURES. Therefore, the 
above description of the preferred embodiment is to be interpreted as 
illustrative rather than limiting. The scope of the present invention is 
to be limited solely by the scope of the claims which follow.