Two dimensional stress relaxation testing device

The present invention relates to a two dimensional stress relaxation testing device in which a plus shaped sample is stretched in mutually perpendicular directions simultaneously to monitor the stress and also the relaxation process of any sheet material in both axes. In view of this advantage, the present device provides the closest approximation to the in situ failure of the material.

FIELD OF THE INVENTION
 The present invention relates to a two-dimensional stress relaxation
 testing device. The two-dimensional stress relaxation testing device has a
 potential industrial application as a quality control equipment in respect
 of such prominent industries like leather products, footwear, textile,
 polymeric industry to name but a few. Sheet materials like leather,
 polymers and textiles, under different application environments such as
 mechanical, thermal and hygrometric conditions exhibit various degrees of
 resiliance, shape retention, plastic set etc. This behavior of the above
 materials can be studied by analyzing their two dimensional stress
 relaxation which in turn would help in predicting its failure mechanism
 under actual application conditions.
 BACKGROUND OF THE INVENTION
 Several sheet materials like polymeric materials, leathers, textiles,
 engineering composite materials etc. are being widely used in several
 engineering environments. As reported by Ramanathan et al (Journal of the
 Indian Leather Technologists Association, 16, 293, 1968), these materials
 undergo various stresses in mutually perpendicular directions while in use
 for different applications. For example, the upper leather of a shoe is
 subjected to force in all directions. Ramanathan et al (Proceedings of the
 International Southern Biomedical Engineers Conference, Shrevepor, La.,
 USA, p. 1, 1982) reported that the force on the surface of the upper
 portion of leather is enormous while walking. Moreover, with the advent of
 fashions several combination of materials are being used in shoe
 manufacture. It is therefore necessary to understand the mechanical
 properties, stress relaxation, hysteresis and mechanism of failure of
 these materials by simulating the user conditions for efficient use of
 these materials.
 Presently the viscoelastic nature of materials is analysed by
 unidirectional testing using Universal Testing Machines (UTM), where a
 dumbbell shaped sample (1) gripped by two jaws (2) is pulled apart using
 motorized crosshead (3) till the sample fails, while the force developed
 during the process is sensed by a force transducer (5) and recorded by a
 recording device (4). As reported by Ridge and Wright (Biorheology,
 2,67,1964), Muthiah et al (Biorheology, 4,185,1967), the force generated
 per unit area during the deformation, which eventually leads to the
 fracture of the sample, is usually taken as a measure of the tensile
 strength of the material under consideration. In addition, the gradual
 decrease in the force, developed during deformation, to a given strain
 level, viz., stress relaxation, provides information on the rearrangements
 of the molecular structure within the sample, which depends on their
 viscous nature, as reported by Arumugam et al (Handbook of Advanced
 Materials Testing, Edited by N P Cheremisinoff and P N Cheremisinoff,
 Marcel Dekker Inc., New York, p 909, 1995). Similarly repeated
 deformations up to a certain strain level and back provide information
 about the plasticity of the sample, as reported by Vogel (Bioengineering
 and Skin, 4, 75,1988). Vogel (Journal of Medicine, 7, 177, 1976) has
 developed a theoretical model for the stress relaxation process for rat
 skin using uniaxial samples. As reported by Arumugam (Ph. D. Thesis,
 University of Madras, 1989) and Arumugam et al (Journal of Biosciences,
 19, 307,1994), all these characteristics depend on the rate at which the
 experiments are performed. Ambrazyavi et al (Pat. No. SU 1226123 dt.
 23/411986) have designed an apparatus to measure relaxation process after
 compression in polymers. Dzhunisbek et al (Pat. No. SU 998918 dt.
 23/4/1983) have also fabricated a device to measure the compression based
 stress relaxation coupled with friction in polymers. Methods involving
 monitoring of fall in pressure to detect relaxation have also been
 attempted for polymers as disclosed in Pat. No. SU 354317. Modifications
 induced during the relaxation process such as mechanical reduction in the
 thickness of the sample have also been tried by Dubovik et al (Pat. No. SU
 1186996 dt. 23/10/1985). The main limitation in all the above devices is
 that they are all designed to measure the relaxation process only along a
 single axis or dimension.
 In actual user conditions materials are generally exposed to forces acting
 in more than one dimension, which includes bending and stretching. Hence,
 the use of unidirectional sample for predicting the conditions for failure
 does not simulate the actual user conditions. In other words, deduction of
 realistic information and predicting or computing the mechanism of the
 failure of these viscoelastic materials from unidirectional tests has its
 limitations because of their high Poisson's ratio. For example, there is
 tremendous amount of lateral contraction due to the high Poisson's ratio
 of the leather while testing the sample in the conventional UTMs.
 Therefore, the extension and the breaking load observed for the sample
 under unidirectional test conditions are very different from the shoe
 undergoing a similar kind of stress during usage. Moreover, intermediate
 conditions of testing and the formation of surface cracks and their role
 in hastening or delaying failure during application cannot be studied by
 hitherto known techniques. In addition, there are inherent inhomogeneities
 in certain materials like leather, textiles etc., which necessitate
 performing uniaxial testing in more than one direction to get a complete
 understanding of the material properties.
 The main object of the present invention is to provide a two dimensional
 stress relaxation testing device, which obviates the drawbacks stated
 above.
 Another object of the present invention is to analyze stress related
 behavior like relaxation, hysteresis, fatigue and creep of materials
 simultaneously in two mutually perpendicular directions.
 Yet another object of the present invention is to perform dynamic testing
 of materials Still another object of the present invention is to reduce
 the number of test samples and time as well as labor while analyzing the
 stress related properties of materials.
 Yet another object of the present invention is to study the mechanical
 properties of sheet materials without any lateral contraction.
 Still another object of the present invention is to develop a device to
 provide accurate information on the failure of the materials as the
 lateral contraction is not present.
 SUMMARY OF THE INVENTION
 The main principle involved in this invention lies in using two pairs of
 juxtaposed grippers, mutually perpendicular to each other for holding the
 test specimen, thereby enabling the simultaneous measurement of stress
 related properties, especially stress relaxation of materials, avoiding
 the factor of Poisson's ratio, in mutually perpendicular directions while
 simulating the near exact application conditions of different materials of
 multifarious characteristics.

Various components, as shown with numerals in the FIG. 1A of the drawings
 accompanying this specification, are the following:
 1 refers to the dumbbell shaped sample
 2 refers to the jaws
 3 refers to the cross-head
 4 refers to the recording device and
 5 refers to the force transducer
 Various components, as shown with numerals in the FIG. 2 as well as FIG. 3
 of the drawings accompanying this specification, are the following.
 6 refers to the driving source.
 7 refers to the shaft connected to the driving source.
 8 refers to the grippers.
 9 refers to the slide ways, on which the grippers slide away from each
 other.
 10 refers to the arms connecting the shaft and the grips.
 11 refers to the specimen to be tested.
 12 refers to the force detector and
 13 refers to the attached output device.
 The device of the present invention comprises essentially a forearm
 mechanism which is driven by a driving source to move two pairs of
 grippers away from each other on slideways.
 Accordingly, the present invention provides a two dimensional stress
 relaxation testing device which comprises two pairs of juxtaposed grippers
 (8), mutually perpendicular to each other for holding the test specimen
 (11), each gripper being provided with a thimble to eliminate any
 slackness while testing, the grippers being driven over four slideways
 (9), away from each other, through forearm linkage mechanism, each arm
 (10) displacing one gripper slide in a horizontal plane, with the drive
 being fed by a shaft (7) from a driving source (6) and the force generated
 thereby being detected by four force detectors (12) mounted on a clamp,
 displaying the output in an output device (13), connected to an interface.
 DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In an embodiment of the present invention, the test specimen used may be
 selected from any sheet material from the group consisting of leather,
 polymer, textile, rubber and any composite thereof.
 In another embodiment of the present invention, the driving source may be
 selected from the group consisting of D.C. (Direct Current) motor,
 hydraulic drive and pneumatic drive.
 In yet another embodiment of the present invention, the speed of separation
 of the grips may be in the range of 0.01 mm to 1000 mm per second.
 In still another embodiment of the present invention, the gauge length used
 along two mutually perpendicular axes may be at least 5 mm.
 In yet another embodiment of the present invention, the grips used may be
 selected from the group consisting of mechanical, electrical, and
 pneumatic grips.
 In still another embodiment of the present invention, the control mechanism
 for the movement of the grips may be selected from gear assembly,
 hydraulic drive, and pneumatic drive.
 In yet another embodiment of the present invention, the force detectors may
 be selected from force transducers, and strain gauges
 In still another embodiment of the present invention, the output device
 used may be selected from the group consisting of computer interface,
 digital display, analog output, and chart recorder.
 The device has two pairs of juxtaposed grippers (8), mutually perpendicular
 to each other for holding the test specimen. The grippers are designed to
 move away from each other on mutually perpendicular linear motion
 slideways (9). Each gripper slide is provided with a thimble to eliminate
 any slackness in the specimen before initiating the experiment. The test
 sample (11) is held by the grippers (8) and is stretched at a rate ranging
 from 0.01 mm to 1000 mm per second in equal amounts along two mutually
 perpendicular directions. The drive to the gripper slides is provided by a
 driving source (6) through a vertical shaft (7) and a forearm linkage (10)
 mechanism, each arm displacing one gripper slide in the horizontal plane.
 The force generated is detected and measured by the force detectors (12),
 which send the output to output device. The simultaneous measurement of
 the force in the two mutually perpendicular directions gives the two
 dimensional stress relaxation. The loss of energy owing to the repeated
 stress or strain cycling of the sample can also be computed from the
 hysteresis loop obtained on the output device.
 The present invention will now be described with reference to the following
 illustrative but non-limitative examples:
 EXAMPLE 1
 A plus shaped test sample of length 20 mm in both the mutually
 perpendicular directions was cut with a die from a cow upper leather and
 the same was gripped to the device with the help of four mechanical
 grippers. The test sample was strained at 1 mm/min speed by using a DC
 motor as the driving source to 20% level and then the machine was switched
 off. The sample was allowed to relax for a period of 15 minutes, while
 continuously monitoring the force in the mutually perpendicular directions
 i.e., the X and Y axes, which was displayed. The values registered for
 both X and Y axes with time is given below in Table-1 .
 TABLE 1
 Load in Load in
 Time X Y Axis
 (sec.) Axis(Kg) (Kg)
 0 6.15 10.2
 15 2.5 7.8
 30 1.7 7.3
 45 1.6 7.1
 60 1.3 7
 120 1 6.8
 180 0.8 6.7
 300 0.5 6.5
 900 0.45 6.4
 EXAMPLE 2
 A plus shaped test sample of length 50 mm in both the mutually
 perpendicular directions was cut with a die from a polyethylene film and
 the same was gripped to the device with the help of four mechanical
 grippers. The test sample was strained at 50 mm/min speed by using a DC
 motor as the driving source to 80% level and then the machine was switched
 off. The sample was allowed to relax for a period of 15 minutes, while
 continuously monitoring the force on the sample The results obtained for
 the mutually perpendicular directions viz., the X and Y axes are given in
 Table-2 below.
 TABLE 2
 Load in
 Time X Load in Y
 (sec.) Axis(Kg) Axis(Kg)
 0 4.74 3.53
 15 3.4 2.5
 30 3.1 2.3
 45 3 2.2
 60 2.9 2.2
 120 2.8 2.1
 180 2.7 2.1
 300 2.7 2.1
 900 2.7 2.1
 EXAMPLE 3
 A plus shapes test sample of length 70 mm in both the mutually
 perpendicular directions was cut with a die from a textile material and
 the same was gripped to the device with the help of four pneumatic
 grippers. The test sample was strained at 100 mm/min speed by using a DC
 motor drive to 20% level and then the machine was switch off. The sample
 was allowed to relax for a period of 1 hour, while continuously monitoring
 the force on the sample, which was displayed. The results obtained for the
 mutually perpendicular directions i.e., the X and Y-axes are given in
 table 3 below.
 TABLE 3
 Load in
 Time X Axis Load in Y
 (sec.) (Kg) Axis (Kg)
 0 12.41 13.84
 15 7.6 8.7
 30 7.3 8.5
 45 7.2 8.2
 60 7.1 8.1
 120 7 8
 300 6.9 7.9
 1800 6.8 7.8
 3600 6.8 7.8
 The main advantages of the present invention are the following.
 1. It is possible to study the stress relaxation behavior as well as
 hysteresis of different materials in two mutually perpendicular directions
 simultaneously, thereby reducing the number of test samples and testing
 time required thereof.
 2. Stress relaxation experiments can be carried out without subjecting the
 test sample to lateral contractions seen in uniaxial tests. Thus the
 present method would give a more accurate and realistic data than the
 uniaxial tests by overcoming the limitation encountered in the materials
 with high Poisson's ratio. The sample data provided in Table 4 clearly
 brings out the dissimilarity between the two methods i.e., the faster rate
 of relaxation in the two dimensional test as against the uniaxial test for
 a goat upper leather.
 TABLE 4
 Load (Kg)
 Two-Dimensional
 Time (present method) Uniaxial
 (sec) X-axis Y-axis (conventional)
 0 6.15 10.2 1.22
 15 2.5 7.8 1.10
 30 1.7 7.3 1.08
 60 1.3 7 1.05
 120 1 6.8 1.04
 300 0.5 6.5 0.99
 900 0.45 6.4 0.96
 3. It is also possible to study the role of fiber orientation in the stress
 relaxation and dynamic testing conditions wherein the samples are
 subjected to repeated stress or strain cycling at high speed.
 4. lt is also possible to study the fatigue and flexing properties of sheet
 materials, polymeric materials and leather.
 5.The regional shape deformation in sheet materials, biological materials
 and leather can be studied in this instrument.