Compression stress relaxation

Compression stress relaxation

Polymer Testing 2 (1981) 125-133 COMPRESSION STRESS RELAXATION R. P. BROWN Rubber & PlasticsResearch Association, Shawbury, Shrewsbury, Salop SY4 4...

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Polymer Testing 2 (1981) 125-133

COMPRESSION

STRESS RELAXATION

R. P. BROWN Rubber & PlasticsResearch Association, Shawbury, Shrewsbury, Salop SY4 4NR, UK

and F. N. B. BENNETT H. W. Wallace & Co. Ltd, 172 St Jame ' s Road, Croydon, Surrey CR9 2HR, UK

SUMMARY The requirements of apparatus to measure stress relaxation of rubbers in compression are outlined. Apparatus available commercially are described, in particular a universal semi-automatic force measuring head and jigs which accept button test pieces and have no sliding bearings to introduce [fiction.

1.

INTRODUCTION

An example of the importance of compression stress relaxation in a polymer is when the polymer is used as a sealing medium. Seals are used in a variety of circumstances, sometimes in contact with liquids and gases, which are persistant and aggressive. Seals may be called upon to continue working effectively in temperatures ranging from subzero to some hundreds of degrees Celsius. Seals may be buried in the ground for years, without attention, in contact with hydrocarbons on the one side, acid or alkaline soil, chalk, clay or water on the other. Usually the relative positions of the rigid surfaces containing the seal--for example the pipe--are fixed and the force between these surfaces and the seal is determined at least partly by the force exerted by the rubber. Since strain is fixed, the sealing force is determined by the modulus of the polymer, thus a knowledge of how modulus varies with time under service conditions will provide a basis for the design of sealing material which is most likely to give the required service life. Stress relaxation is a measure of decrease in modulus with time. This 125 Polymer Testing 0142-9418/81/0002-0125/$02.50 © Applied Science Publishers Ltd, England, 1981 Printed in Northern Ireland

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R.P. BROWN, F. N. B. BENNy11

decrease in modulus is caused by two principal mechanisms: 1. 2.

Physical, due to the visco-elastic nature of the material Chemical, due to chain scisson and further cross-linking.

The chemical mechanisms tend to predominate at high temperature over a long period of time. The two types of mechanism contribute to the decrease in modulus to a varying degree and it may not be a reliable method of test to use higher test temperatures over a shorter time to obtain information on elastomer properties at lower temperatures over a longer period of time. Although basically simple in concept, compression stress relaxation measurements can be difficult to carry out successfully in practice. Despite the value of such measurements having been long appreciated, the lack of accurate, reliable and relatively simple apparatus has to a large extent delayed adoption of the method. A compression stress relaxometer measures the reduction with time of the force exerted by a polymeric material against two rigid flat surfaces which are holding it in compression. The time factor may be hours, days or months and each test piece must be held compressed for the duration of the test. A large number of test pieces may be involved, and it would be uneconomic to have a force measuring device for each test piece, hence the apparatus to be described consists of numerous compression jigs and only one force measuring device. Ideally, the jigs should be simple, compatible with the need to avoid sliding friction and capable of withstanding the environment to which the test piece is subjected. The force measuring head must receive each jig in turn, measure force accurately within the specified time limit and present results in an acceptable manner with the minimum of operator involvement.

2.

STANDARD METHODS

An international standard, ISO 3384,1 covering compression stress relaxation measurements on button test pieces at normal or elevated temperatures was published in 1979. The standard test pieces are the accepted compression set buttons either 13 mm diameter and 6.3 mm thick or 2 9 m m diameter and 12.5 mm thick, with the smaller button preferred. The test piece is compressed between rigid plates with a surface finish not worse than 0.2 t~m, the preferred degree of compression being 25 +2%. The force exerted by the test piece (modulus) is measured as a function of time either by a dedicated force measuring element or by applying a slight extra compression. The extra compression must be no greater than 1N for balance type machines or 0.05 mm for stress-strain type machines and must be applied without overshoot in no more than 30 s.

COMPRESSION STRESS RELAXATION

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Two test procedures are specified: 1. The test piece is compressed at the test temperature and all measurements made at the test temperature 2. The test piece is compressed and all measurements made at standard laboratory temperature but the compressed test piece is stored between measurements at the test (elevated) temperature For the first method the jig must be capable of loading hot and arrangements made to measure at an elevated temperature. For the second method the jig and test piece must cool to standard laboratory temperature within 2 h. ASTM D13902 also uses button test pieces and has been in existence since 1956. The large compression set button is used and a particular design of stress relaxation jig specified. This apparatus is discussed in Section 3.2 but is basically a jig in which slight over-compression is applied to measure force using a standard compression (tensile) testing machine. Force measurements are made at room temperature as in the second ISO procedure but the times specified are not the same in the two standards. A draft ISO standard 3 concerning the stress relaxation of ring test pieces is awaiting publication but an equivalent British standard appeared in 1978. These methods are intended to be used particularly where the test piece is exposed to liquid environments. The British standard, BS 903: Part A34, 4 specifies either square or round cross-section ring test pieces. Essentially, the apparatus requirements are the same as in ISO 3384 but the compression plates are drilled to allow access of fluid. Three test procedures are specified, two being as in ISO 3384 and the third involving loading at standard laboratory temperature but making initial and subsequent force measurements at the test (elevated) temperature.

3.

COMPRESSION JIGS

A number of compression stress relaxation jigs have been designed and used in particular laboratories but few have found widespread use or are manufactured commercially. Comment here will be restricted to two well established designs and also a relatively new jig now available commercially. As mentioned before, for reasons of economy and convenience separate jigs with one force measuring head are usually used and jigs involving a dedicated force measuring system will not be considered. For most purposes it is desirable that the measurement of force is easy and rapid. Jigs of extremely simple construction have been designed, but they carry the penalty that the measurement of force is slow and rather complicated, and again, will not be considered here.

128

R.P. BROWN, F. N. B. BENNETT

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TOP PLATR

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Fig. 1. Diagram showing basic principle of compression jig.

The basic principle of a compression jig to be used with a separate force measuring head is shown in Fig. 1. The test piece is compressed between two plates, the top plate having a shaft which can be connected to a force measuring head. Except for the short times when force is measured the top plate is rigidly held against the body of the jig. To measure force the top plate is pressed very slightly downwards by the measuring head such that contact between the plate and jig body is broken and the force exerted by the test piece acts upon the measuring head. The slight extra compression of the test piece is ignored. 3.1. Lucas jigs The Lucas design of jig is shown in Fig. 2. The top compression plate is located only by a central contact point. This allows the plate to pivot about this point and hence, unless the test piece is homogeneous and accurately central in the jig, compression will be uneven. In practice the jig is restricted to ring test pieces which are thin in relation to their diameter, as specified in BS 903: Part

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Fig. 2. Lucas compression stress relaxation jig.

COMPRESSION STRESS RELAXATION

129

A34. 4 Despite this severe limitation of use, the Lucas jig is of relatively simple construction and, as the top plate is not laterally restricted, there is no friction. The central contact point also acts as an electrical contact which when broken by slight downward movement of the top plate (applying slight extra compression to the test piece) is arranged to give an indication that the force in the test piece is totally balanced by the force measuring device. This central ball electrical contact is a great improvement on the basic layout shown in Fig. 1 as there is no danger of a progressive and erratic break around the diameter of the plate. Lucas jigs were originally used with a simple electrical break circuit and a manually operated beam balance as the force measuring head, which required considerable care to obtain consistent results. However, the Lucas jigs can be used with the electronic measuring head (as described in Section 4), when it becomes a very convenient apparatus for use with ring test pieces. The jig is light in construction and hence can be heated or cooled fairly rapidly to comply with procedures B and C of BS 903: Part A34. However, it is very difficult, if not impossible, to load hot. 3.2. A S T M jigs The ASTM jig (Fig. 3) is very close to the basic design shown in Fig. 1. The electrical contact is made via a flange in contact with the top plate. The top plate is located in the body of the jig via vertical ball bushings which provide high lateral stability but may be a source of friction especially aSter the

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Fig. 3.

A S T M compression stress relaxation jig.

130

a . P . BROWN, F. N. B. BENNEqq"

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i Fig. 4.

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WalIace/RAPRA compression stress relaxation jig.

apparatus has been stored in liquids or at elevated temperatures for long periods. It is intended that the slight over-compression is applied by a standard compression machine, which also measures the force, and that the break in electrical contact is detected by a resistance meter. The jig uses 2 9 m m diameter, 12"5 mm thick test pieces but the design could be adapted to other geometries, although with small ring test pieces friction in the bushing would be significant. Because of the large electrical contact area and friction in the bushings it is doubtful if this jig would meet all the requirements of ISO 3384. It could be used in conjunction with the force measuring head described in Section 4. 3.3 Wallace R A P R A jigs This design (Fig. 4) was developed with the aims of overcoming the limitations and disadvantages of the Lucas and ASTM jigs. The test piece (A) is held between two fiat parallel plates (B and C). The bottom plate is designed to move in relation to the top plate to compress the test piece and the specified strain is applied to the test piece by tightening the bottom plate against a solid stop. Normally the jig is constructed to take 29 mm diameter, 12-5 mm thick test pieces but can also be made to take smaller buttons or rings. The position of the bottom plate is determined by distance pieces and the compression quickly applied by tightening the threaded plate (K).

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131

To measure the force exerted by the test piece the top plate is moved a very small amount in the direction of compression, breaking an electrical contact, in basically the same manner as previously described. The force measured comprises three components: 1. The force exerted by the test piece in its normally compressed state 2. The force exerted by the test piece due to over-compression during measurement 3. Any forces produced by the jig As the first component is the one of interest, the second and third must be very small compared with the first or must be accurately known. It is because of this requirement that both jig and force-measuring instrument must conform to certain specifications. The first requirement in the design of the jig is that the compression plates shall remain exactly parallel to, and a fixed distance from each other. In order to maintain parallelism with any test piece, a guiding system is necessary which must allow movement to compress the test piece by a very small amount to measure force. A simple plain sliding bush is unsuitable because of the uncertain effects of friction. Instead, a parallel spring diaphragm system is used which is entirely frictionless. Although this spring diaphragm will influence the force detected by the force-measuring instrument the value of this influence can be measured and allowed for. It has been said that the amount of over-compression of the test piece when measuring force must be very small. It is specified in ISO 3384 that overcompression be limited to 0.05 mm. The procedure for detecting this very small amount of movement is to sense the opening of an electrical contact. Provided the current passing through this contact and the contact area are sufficiently small, detection of contact opening will be very sensitive which means that, provided the jig body is rigid, the amount of over-compression will be confortably inside the limit set by the international standard. The force applied by the measuring head is applied to a horizontal rod (F). The electrical contact is shown at (G) and electrical insulation is provided by the ceramic plate at (H). The use of ceramic insulation ensures high resistance to fluids and high temperatures. It can be seen from the figure that the force applied to the test piece (and the force exerted by the test piece) is transmitted through the electrical contact (G). Because of the large force which may be involved the contacts are made of materials sufficiently hard to avoid deformation. The radiussed contact face is designed to provide a well defined point of contact. The jig is massively built to ensure stability. In particular the top piece against which the electrical contact pushes and the bolts which retain it are sufficiently rigid to prevent excessive over-compression.

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R. P. BROWN, F. N. B. BENNETI"

In summary, the main attributes of this jig are: 1. High lateral stability is achieved without friction 2. Any test piece geometry can be accommodated 3. The general construction and electrical contact design is such that over-compression is minimal and reproducible 4. Loading can be carried out quickly and at elevated temperatures

Fig. 5.

T h e W a l l a c e / R A P R A compression stress relaxometer.

COMPRESSION STRESS RELAXATION

4.

133

FORCE MEASURING HEAD

In principle, any compression machine, press or balance with appropriate capacity can be used in conjunction with a simple means of detecting break of an electrical contact. It should be simple in operation and must measure force accurately without undue over-compression. The Wallace/RAPRA forcemeasuring head is a semi-automatic device specifically designed for compression stress relaxation measurements. The measuring head (Fig. 5) has a ram powered by compressed air (consumption is small and a bottle may be used conveniently) to apply the slight over-compression of the test piece. An electronic load cell measures the force exerted by the test piece and a digital meter displays the force. Initial movement of the ram towards the jig is fast in order to save time but after contact with the jig and on reaching a selected force value, ram speed changes to slow. Immediately the jig contacts open, the change in resistance is sensed and automatically the ram reverses and returns to the start position at full speed whilst the force at that instant is displayed on the meter. Ram speeds, the force value for ram speed change, electrical resistance change level and force limits may be selected to suit test conditions. A safety device prevents accidental overload. By suitable choice of the contacting foot on the ram the head can be used with various designs of jig and with the addition of an oven to surround the jig, measurements can be made at elevated temperatures.

REFERENCES 1. 2. 3. 4.

ISO 3384: 1979. Rubber, vulcanised--Determination of stress relaxation in compression at normal and elevated temperatures. ASTMD1390-76. Rubber property--Stress relaxation in compression. ISO DIS.6056. Rubber, vulcanised--Determination of stress relaxation in compression (using ring test pieces). BS903: Part A34: 1978. Determination of stress relaxation of rings in compression.