Types of Test-Pieces

The number and type of test pieces exposed for each measurement point will depend on the property being measured. If measurement is non-destructive, e.g., loss of mass or colour, then the same specimens can be used throughout the test, being replaced in the oven after each measurement. If the measurement is destructive then one set of specimens must be prepared for each combination of duration and temperature. It is recommended to expose at least two reserve sets in case the threshold level has not been reached by the end of the last planned duration. Usually the number specified in the relevant test method standard is chosen but, again, the more the better. An example and some of the problems are described in Section 12.2. 

In the interests of standardisation it is desirable to limit as far as possible the variety of test piece sizes allowed. Success in this direction has not always been possible, as illustrated by the tensile test pieces detailed in ASTM D412. However, there would be no need to limit dimensions at all if it were not a fact that the size of test pieces can affect the magnitude of the result obtained, or at least the variability. In the case of tensile tests, the difference in level between results from rings and dumb-bells has already been mentioned. The variability of the two types of test pieces has been found to be similar. The measured tensile strength has a tendency to decrease with increasing cross-sectional area of the test piece and it is desirable to make comparisons only between groups of test pieces of nominally the same type and thickness. The difference between the results from type 1 and 2 dumb-bells is not normally significant but Bartenev and... 

ISO 694310 for fatigue in tension specifies two different types of test pieces, rings and dumb-bells, which correspond to the geometries used on commercially available apparatus. There is, in principle, little difference between the two forms of test piece but dumb-bells are necessary for studying directional effects. They are also easier than rings to cut from sheet, but normally a specially moulded sheet is required such that the dumb-bells... 

The dumb-bells specified are the same as those for tensile stress/strain tests except the preferred thickness is 1.5 mm. The ring is also the same as the tensile ring, which means that the bulk of the two types of test piece are different. 

As a result of these preliminary tests, the following experimental conditions have been adopted for both types of test piece ... 

Standard test methods for the determination of Izod impact strength are listed in Table 12.10. For example, ISO R180 normalizes the notch length. The velocity of the striker on impact has been standardized at 3.5 10 per cent m/s with impact energies of 1.0, 2.75, 5.5, 11.0 and 22.0 J. Four types of test pieces are permitted. 

ISO R179 Two striking energy levels Four types of test pieces are allowed. First three are as in British Standards. Fourth type is 125 mm long by 13 mm square Same notch types as above. Molded notches are permitted.Machined notches are preferred. 

Tensile impact can also be carried out with a pendulum apparatus equipped with a device to hold the test piece so that on impact by the pendulum the test piece is strained in tension. There are two types of test piece holder, one where the test piece is held on the frame of the apparatus and the other where it is held in the head of the pendulum. In both approaches corrections have to be made for the energy used in moving part of the test piece support. 

The desired energy to rupture the test piece is then simply the dilTcrencc between the uncorrected energy read from the maximum swing of the pendulum after impact and the above correction energy. As for the Charpy and Izod tests, the result is normalized with respect to the area of the test piece cross section, although unlike the other pendulum impact tests, there arc several types of test piece that are used. Fig. 16 illustrating these. 

Figvire 5.2 shows different types of test-pieces which can be used to measure the tear strength. The tearing energy, T, can be calculated from easily measurable applied forces or strains, which make them convenient for experimental purposes. 

The usual way in which the life of differently made bonds can be compared is to prepare types of test pieces and then place them in a jig, which will allow a strain to be placed upon the rubber and the bond edge (see Durability creep rupture). The test pieces are then subjected to contact with a hostile environment such as hot water or salt fog (see ASTM B I 17). The resultant failures generally depend upon the type of surface preparation used and may be used to monitor these procedures. 

Different types of test pieces for tribological and mechanical testing were prepared by means of electrical discharge machining. The powders for sintering HP-NbC and the NbC-8Co were pure and essentially free of tantalum. 

All ceramic materials are elastic, and hence show very little bending under load. They do not exhibit any creep under load. The modulus of rupture type of test is the routine test most commonly used in the ceramic industry, and gives the figure generally quoted for the strength of the material. It must be remembered that the value obtained for any particular body depends on the cross-sectional area of the test piece thus figures quoted from test results may be higher than those obtained on actual articles, which usually have a thicker section than the test piece. 

One type of test measures the inhibition of respiration of bacterial cells on pieces of pig skin by the substance under test. Here, the factor of correlation between cell death and cessation of respiration should be borne in mind. 

A type of test for determining abrasion resistance. Fixed knives, under constant load, scrape across a rotating rubber test piece. The resulting loss in weight is a measure of the abrasion resistance. 

The great majority of durability tests (indeed all types of test) are made on test pieces rather than on the complete product. There are several reasons for this, not least the cost of... 

Tests for scorch and rate of cure should be distinguished from tests for degree of cure or optimum cure measured on the vulcanised material. The latter type of test estimates degree of cure by measuring the physical properties of test pieces vulcanised for various times, tensile properties, swelling and set measurements being the parameters most commonly used. 

The standard briefly covers the significance of hardness in terms of its relation with modulus, and the practical use of hardness tests. The hardness tests for rubber that are standardized by ISO are introduced and the distinction between dead load and durometer type instruments is explained to help with selection of a test method for particular circumstances. The effect of test piece, use of standard hardness blocks and comparison of hardness scales is also outlined. 

A loose abrasive powder can be used to impinge on the rubber rather in the manner of a shot blasting machine, or tumbled with the rubber test pieces in a rotating drum. These are logical ways to simulate the action of sand or similar abradants impinging on the rubber in service, as may be the case with conveyor belts or tank linings, but this type of test is not very common. 

This standard specifies a procedure for determining the dynamic cushioning performance of cellular rubber materials and rigid and flexible plastics by measuring the peak deceleration of a mass when it is dropped on a test piece. The test is intended primarily for quality assurance. However, since this type of test is also used to obtain design data notes are given in Annex A to assist in this objective. The method is applicable solely to packaging materials. 

These two types of tests, the one on sand cast, and the other on chill cast test pieces, seemed indispensable in order to show the different results obtained by the two methods. 

The standard method for compressive tests are listed in Table 12.3. For example, the ISO Standard 604 allows four types of test specimens 1) the right square prism, 2) the right rectangular prism, 3) the right cylinder and 4) the right circular crown tube. The test specifies for each of these test specimens, the load-bearing surfaces be parallel to each other within 0.1% of the height of the test piece. 

In BS903, Part A9, the ISO method is one of four standard tests, the others being the DuPont, the Akron, and the Taber or rotary platform double head type machine. Like the ISO method, the DuPont test is one of continuous abrasion against a replaceable abrasive paper, this time of silicon carbide rather than of aluminum oxide, but there is no built-in procedure to avoid abrasive smearing and progressive changes in abrasive power other than a rotation of test pieces to even out differences. The Akron method involves the wear... 

Equation (10.5) is more generally applicable than Eq. (10.6) because it is not restricted to linearly elastic materials. It constitutes a criterion for tensile rupture of a highly elastic material having a cut in one edge of length, /, in terms are of the fracture energy, Gc- Two important examples of test pieces of this type are (1) the ASTM tear test piece for vulcanized rubber (ASTM D624-54) and (2) a typical tensile test piece that has accidental small nicks caused, for example, by imperfections in the surface of the mold or die used to prepare it. 

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