KR20020044703A - An holder of test piece for testing tensile strength under glass-transition point - Google Patents

Abstract

PURPOSE: A sample holder is provided to achieve improved reliability for the test by protecting tap of sample from fracture caused when the tension test temperature is lower than the glass transition temperature. CONSTITUTION: A sample holder(21) is characterized in that a rubber pad(22) is attached to the surface of the sample holder contacting a sample(23) so as to measure the tensile force of the sample at a temperature lower than a glass transition temperature. The sample holder has a groove formed at the surface of the sample holder contacting the sample, wherein the groove is formed into a depth corresponding to the thickness of the rubber pad. Alternatively, the sample holder has a reinforcing plate inserted to the side surface of the sample holder so as to prevent the sample from being distorted or escaped during tension test.

Description

Specimen holder for tensile test below glass transition temperature {AN HOLDER OF TEST PIECE FOR TESTING TENSILE STRENGTH UNDER GLASS-TRANSITION POINT}

The present invention relates to a specimen holder for tensile testing below the glass transition temperature.

As the required performance of solid propellants or rigid elastomeric heat-resistant materials increases day by day, the materials that make up the type and composition of the propellants have also diversified. According to this need, the tensile properties of propellant materials should also be measured and evaluated in the operating temperature range, and in some cases, the propellant quality inspection test reference temperature may be at or below the glass transition temperature. do.

1 is a front view showing a part of a conventional tensile force test apparatus. The apparatus 10 is largely composed of upper and lower specimen holders 12a and 12b and a frame 11 for fixing each specimen holder up and down. In the figure, the specimen holder is a specimen to test the tensile force, and the specimen holder is a screw action grip to hold and fix both ends of the specimen. In this way, when the specimen is fixed by physical force, the force is concentrated in the part. The material exhibiting low temperature characteristics, that is, at low temperature, loses flexibility, such as rubber, and glassy or quasiglassy state, like hard plastic. If the material is very hardened, it may be damaged during the fixing of the specimen, or it may slip off during the test because of the hard material.

To solve this problem, a dumbbell-type specimen and a dumbbell-type specimen holder have been devised and tested for tensile strength. Most solid propellants, rigid elastomeric heat-resistant materials, or plastics have a low temperature tensile test reference temperature higher than the glass transition temperature, and the tab portion of the specimen in contact with the holder of the specimen used in the tensile test (both ends of the sub-test specimen). Since the cross-sectional area of) is much larger than the cross-sectional area of the target section (middle part of the sub-test specimen), no premature fracture occurs at the middle part of the tab.

However, if the test temperature is in the glass transition region or lower than the glass transition temperature, it will not break in the marking section where the cross-sectional area is smaller than that of the specimen tab portion, and will be prematurely premature in the middle part of the tab where the cross-sectional area is 1.6 times or more before reaching the fracture property limit of the material itself Because of the fracture, the specimen stress and elongation are lower than actual.

SUMMARY OF THE INVENTION An object of the present invention is to provide a specimen holder and a specimen holder that does not break during the tensile test when the low temperature test reference temperature is below the glass transition temperature in the tensile test of various rocket solid propellants, rigid elastic heat-resistant materials, plastics, and the like. It is intended to measure reliable tensile properties while reducing test errors by preventing premature fracture at the center of the contacting tab.

1 is a plan view showing a specimen holder of the present invention.

Figure 2 is a photograph showing a specimen holder of the present invention.

3A and 3B are photographs showing tensile test results when a rubber pad is not attached to a specimen holder and a rubber pad is attached to a specimen holder, respectively.

4A and 4B are photographs showing the tensile test results when the rubber pad is not attached to the specimen holder and the rubber pad is attached to the specimen holder, respectively.

5a and 5b are photographs showing the tensile test results when the rubber pad is not attached to the specimen holder and the rubber pad is attached to the specimen holder, respectively.

*** Explanation of symbols for main parts of drawing ***

21: specimen holder 22: rubber pad

Psalm 23

In order to achieve the above object, the present invention provides a device for a tensile test of a solid propellant or a hard elastic heat-resistant material, the rubber pad is attached to the contact surface of the specimen holder and the specimen to measure the tensile force of the specimen below the glass transition temperature Specimen holders for tensile testing below glass transition temperatures are provided.

Preferably, the specimen holder has a groove formed on the contact surface where the specimen holder and the specimen come into contact with each other by the thickness of the rubber pad, and the center of the specimen holder is fitted to the side surface of the specimen holder to prevent the specimen from twisting or falling off during the tensile test. It may be further provided with a cover plate protruding.

On the other hand, the rubber pad material may be used in various ways, HTPB (hydroxyl-terminated polybutadiene) rubber (glass transition temperature: -79 ℃) is suitable.

Figure 2a is a plan view showing a specimen holder of the present invention. One end of the dumbbell-shaped specimen 23 is fitted to the specimen holder 21, and a small curved groove is dug in the contact portion between the specimen and the specimen holder, and a rubber pad 22 is attached therein. Reference numeral 24 denotes a fastening groove for fitting the side face plate of the specimen holder.

The present invention prevents premature fracture at the tab mid-section of the specimen by attaching a rubber pad to the specimen holder surface and at the same time enables more reliable measurement of tensile properties. This prevents the measurement of values lower than the actual properties because the specimens are destroyed prematurely before the fracture properties limit of the material itself is reached. In addition, the present invention was to minimize the material and thickness of the rubber pad to be attached to the specimen holder, the surface is attached to maintain the continuity and the continuity of the specimen holder because it has a great effect on the reliability of the tensile test results.

Tensile tests of solid propellants or rigid elastomeric heat-resistant materials, such as Joint Army-Navy-NASA-Air Force (JANNAF) dumbbells, are used when the low temperature test temperature is lower than or similar to the glass transition zone or glass transition temperature. It breaks at the middle of the tab without breaking within the marking zone. This is due to the concentration of stress in the tab portion of the specimen in contact with the specimen holder, even though the area of this portion is larger than the area of the mark section. Therefore, the method of attaching an elastic rubber pad to the inner side of the specimen holder was conceived as a method to solve this stress concentration.

Since the rubber pad is in contact with the solid propellant or elastomeric heat resistant specimen while it is attached to the specimen holder, the glass transition temperature of the rubber pad must be lower than the glass transition temperature of the specimen material to eliminate stress concentrations acting on the tab portions of the specimen. do.

Therefore, as a result of examining rubbers of general-purpose materials from room temperature to low temperature, natural rubber (glass transition temperature: -59 ℃) and HTPB (hydroxyl-terminated polybutadiene) rubber (glass transition temperature: -79 ℃) were selected. As a result of the tensile test performed below the glass transition temperature of the specimen by attaching it to the inner surface of the specimen holder as shown in FIG. 1, the maximum stress and the maximum elongation increased by about 2 times as compared to the unattached case, as shown in Table 1 below. The fracture occurred in the tab section outside the marking section of the specimen. In other words, it was determined that the specimen was broken because the stress concentration was solved and the fracture limit was reached.

When the rubber pad is attached to the inner surface of the specimen holder, the fracture pattern of the specimen occurs in the range of the marking section or the boundary of the marking line.However, as the shape and dimension of the specimen surface are changed, discontinuity of the contact surface causes the test piece to be Prior to tensioning, the specimen was tensioned without complete contact between the contact surface between the holder and the specimen tab. In order to solve this problem, as shown in FIG. 2A, a groove was formed in the holder surface as much as the thickness of the rubber pad, and then the rubber pad was attached and tested.

In addition, if the thickness of the specimen is 12.5 mm, there is no problem, but type A and B regeneration propellants (type A has a glass transition temperature of -54 ° C., type B is -36 ° C., and regeneration has two main raw materials of the propellant. In the case of a rigid elastic heat-resistant material and the thickness of 2mm, the specimens were stabilized by applying a plate such as the left side of FIG. 2 to both sides of the grip holder in order to prevent twisting or falling out during tension.

As a result of examining the influence of the thickness of the rubber pad using the type A respiratory propellant, when the thickness is thick, the deformation of the rubber affects the elongation. In the case of 20 ℃, when HTPB rubber and natural rubber were used as pads, natural rubber was almost similar to the result of unattached, but HTPB rubber had a greater effect on the elongation rate at 20 ℃ than unattached. Able to know. The impact of this rubber pad was tested at -40 ° C, where the propellant specimens did not break outside of the marking section. The most similar results to the unattached results were found to be 0.5 mm thick.

As a result of examining the HTPB rubber and the natural rubber, in the case of natural rubber, even when the thickness was 0.90 mm at -60 ° C, the breakage of the tab portion was not resolved unlike the HTPB rubber. Since HTPB rubber is easier than natural rubber and its glass transition temperature is low, it is excellent in flexibility, and HTPB rubber is selected as a rubber pad material.

Therefore, the rubber material is HTPB rubber, and the thickness is set to 0.5 mm and tested by the specimen holder shown in FIG. 2. As a result, the maximum stress and the maximum elongation measured at a temperature lower than the glass transition temperature of the propellant are compared with those without the adhesive. An increased result was obtained. The results of the following examples are obtained under these test conditions.

In the following examples, the contents of the present invention will be described in more detail, but the scope of the present invention is not limited to these examples.

Example 1.

Type A and B regeneration propellants (type A: glass transition temperature -54 ° C, type B: -36 ° C) were tested at -60 ° C and -54 ° C below the glass transition temperature by a specimen holder with 0.5 mm HTPB rubber pad. As a result, when unattached, as shown in Table 1 and Table 2, the fracture of the specimen occurred at the middle of the gage boundary-tab or the gage boundary as shown in FIG. 3a. The fracture occurred within the zone, and the maximum stress and maximum elongation were more than doubled compared to the unattached cases.

Tensile test results of the Type A respiratory propellant below the glass transition temperature (glass transition temperature of the promoter: -54 ° C, hardness: 95) Rubber Name with Specimen Holder (Glass Transition Temperature) Rubber Pad Thickness (mm) Test temperature (℃) Tensile properties Specimen fracture pattern Stress (MPa) Maximum elongation rate (%) Elongation at Break (%) Modulus of elasticity (MPa) Display border-tab part Mark boundary Mark section Not attached 0 20 4.8 22 22 108 ◎ -60 19.9 1.1 1.1 1,784 ◎ HTPB Rubber (-78 ° C) 0.50 -60 41.6 2.4 2.4 1,746 ◎

Tensile test results of the Type B respiratory propellant below the glass transition temperature (glass transition temperature of the propellant: -36 ° C, hardness: 90) Rubber Name with Specimen Holder (Glass Transition Temperature) Rubber Pad Thickness (mm) Test temperature (℃) Tensile properties Specimen fracture pattern Stress (MPa) Maximum elongation rate (%) Elongation at Break (%) Modulus of elasticity (MPa) Display border-tab part Mark boundary Mark section Not attached 0 20 7.3 13 13 177 `◎ -54 18.5 0.9 0.9 2,186 ◎ HTPB Rubber (-78 ° C) 0.50 -54 50.3 2.8 2.8 1,815 ◎

This phenomenon occurs because the propellant is in the glass state below the glass transition temperature and exhibits brittleness. Therefore, the fracture of the specimen is prevented by the specimen holder before fracture occurs in the marking section where the cross section of the specimen is about 0.7 times smaller than the tab portion. The stress and elongation were increased by breaking the stress due to the stress concentration acting on the tab part. When the rubber pad is applied, the stress concentration is resolved as the stress concentration is solved.

Example 2.

NEPE (Nitrate Ester Polyether) mixed propellant (glass transition temperature -69 ° C) was tested at -70 ° C below the glass transition temperature by a specimen holder with a 0.5 mm HTPB rubber pad. This propellant was tested at −70 ° C. to demonstrate the effectiveness of the rubber pad because the glass transition temperature is below the low temperature test reference limit temperature (−60 ° C.). In the case of non-attachment as shown in FIG. 4A, the fracture of the specimen occurred in the middle of the gage boundary-tab or the gage boundary part as shown in FIG. 4A, whereas when the rubber pad was attached, the fracture occurred in the middle of the gage section as shown in FIG. 4B. As shown in Table 3, the tensile properties were increased by 1.6 times or more compared to the case where the maximum stress and the maximum elongation were not attached.

NEPE mixed solid propellant tensile test results below the glass transition temperature (glass transition temperature of the promoter: -69 ° C, hardness: 47) Specimen Holder Attached Rubber (Glass Transition Temperature) Rubber Pad Thickness (mm) Test temperature (℃) Tensile properties Specimen fracture pattern Stress (MPa) Percentage growth rate instead Elongation at Break (%) Modulus of elasticity (MPa) Mark boundary-tap part Mark boundary Mark section Not attached 0 20 0.6 37 49 2.4 ◎ -70 4.9 0.7 0.7 755 ◎ HTPB Rubber (-78 ° C) 0.50 -70 8.0 1.2 1.2 691 ◎

Example 3

The rigid elastic heat resistant material (glass transition temperature -23 ° C) was tested at -40, -60 ° C which was below the glass transition temperature by a specimen holder with a 0.5 mm HTPB rubber pad. In the case of -40 ℃, the unbonded fractured at the boundary line, but when the rubber pad is attached, it breaks within the boundary line and the interval, respectively, so that the rubber pad exhibits the function of relieving stress concentration even under the glass transition temperature. The maximum stress and maximum elongation were also increased as shown in Table 4. At -60 ° C, the tensile properties of both cases were slightly higher with rubber pads, but the difference was not large. However, when the fracture pattern of the specimen is not attached, the specimen is broken at the boundary of the mark as shown in FIG.

Effect of Rubber Pad and Thickness on Tensile Test of Rigid Elastic Heat Resistant (Glass Transition Temperature of Heat Resistant: -20 ° C, Hardness: 97) Specimen Holder Attached Rubber (Glass Transition Temperature) Rubber Pad Thickness (mm) Test temperature (℃) Tensile properties Specimen fracture pattern Stress (MPa) Percentage growth rate instead Elongation at Break (%) Modulus of elasticity (MPa) Mark boundary-tap part Mark boundary Mark section Not attached 0 20 14.6 45 46 80 ◎ -40 82.2 4.4 4.4 2,109 ◎ -60 82.6 3.8 3.8 2,286 ◎ ◎ HTPB Rubber (-78 ° C) 0.50 -40 85.0 5.0 5.0 2,043 ◎ -60 91.1 3.9 3.9 2,401 ◎

According to the present invention, when the tensile test temperature is lower than the glass transition temperature, the premature fracture phenomenon that occurred in the middle of the tab of the specimen was prevented because of brittleness, and the fracture occurred at the fracture limit, so that the stress and stress Elongation was further increased and measured. Thus, it is possible to measure more reliable tensile properties.

Claims (5)

A device for tensile testing of solid propellants or rigid elastic heat resistant materials, A specimen holder for tensile testing at or below the glass transition temperature, characterized in that a rubber pad is attached to a contact surface between the specimen holder and the specimen in order to measure the tensile force of the specimen at or below the glass transition temperature. The test piece holder according to claim 1, wherein the test piece holder has a groove formed on the contact surface between the test piece holder and the test piece by the thickness of the rubber pad. The method of claim 1, wherein the specimen holder is further provided with a cover plate having a central portion protruding so as to be fitted to the side surface of the specimen holder in order to prevent the specimen from twisting or falling off during the tensile test process Specimen holder for tension test The specimen holder of claim 1, wherein the rubber pad is made of HTPB (hydroxyl-terminated polybutadiene) rubber (glass transition temperature: -79 ° C.). The test piece holder according to claim 1, wherein the rubber pad has a thickness of 0.5 mm.