ASTM D785-08 PDF

Due to a short overlap of Rockwell hardness scales by Procedure A, two different dial readings on different scales may be obtained on the same material, both of which may be technically correct. Generally, an uncured specimen has a hardness reading below normal. Readings on one material may be satisfactory for such a table, but there is no guarantee that other plastic materials will give corresponding readings because of differences in elasticity, creep, and shear characteristics. One make and model has a 25 to 1 leverage arm. If there are no material specifications, then the default conditions apply.

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A number in parentheses indicates the year of last reapproval. A superscript epsilon e indicates an editorial change since the last revision or reapproval. This standard has been approved for use by agencies of the Department of Defense. The values given in brackets are for information only.

It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Procedure A of this test method is equivalent to the test method in the main body of ISO Procedure B of this test method is equivalent to the test method in the integral annex part of ISO Terminology 3. Signicance and Use 4. A Rockwell alpha a hardness number represents the maximum possible remaining travel of a short-stroke machine from the net depth of impression, as the load on the indenter is increased from a xed minor load to a major load Procedure B.

Indenters are round steel balls of specic diameters. Rockwell hardness numbers are always quoted with a scale symbol representing the indenter size, load, and dial scale used.

This test method is based on Test Methods E Procedure A Section 11 yields the indentation of the specimen remaining 15 s after a given major load is released to a standard kg minor load.

Procedure B Section 12 yields the indentation of the indenter into the specimen after a s application of the major load while the load is still applied. Each Rockwell scale division represents 0. In practice, the Rockwell hardness number is derived from the following relationship: HR 5 2 e 1 2. Referenced Documents 2. Current edition approved March 1, Published March Originally approved in Last previous edition approved in as D - An a hardness number is equal to minus the instrument reading.

Due to a short overlap of Rockwell hardness scales by Procedure A, two different dial readings on different scales may be obtained on the same material, both of which may be technically correct.

Generally, an uncured specimen has a hardness reading below normal. Readings on one material may be satisfactory for such a table, but there is no guarantee that other plastic materials will give corresponding readings because of differences in elasticity, creep, and shear characteristics. If there are no material specications, then the default conditions apply.

Factors Affecting Reproducibility and Accuracy 5. Softer plastics and coarse-lled materials will have a wider range of variation. A large ball indenter will distribute the load more evenly and decrease the range of test results Note 2.

The sensitivity of the instrument decreases with an increase in the dial reading and becomes very poor for readings of and over due to the shallow indentation of the steel ball. Rockwell hardness readings over are not satisfactory and shall not be reported.

Readings between zero and are recommended, but readings to are permissible. For comparison purposes, it may be desirable to take readings higher than or lower than zero on any single scale. In such cases, Rockwell hardness readings may be reported, but the corresponding correct readings shall follow in parentheses, if possible. Such alternate readings are not always feasible when the specimen is subjected to constantly changing conditions or irreversible reactions.

NOTE 2—Molded specimens containing coarse ber llers, such as woven glass fabric, will inuence the penetration obtained. These variations in hardness may be reduced by testing with the largest ball indenter consistent with the overall hardness of the material.

One make and model has a 25 to 1 leverage arm. Otherwise the indenter will indent further into the material than when such vibrations are absent. Steel ball indenters that have nicks, burrs, or are out of round shall not be used. Dust, dirt or heavy oil act as a cushion to the load supporting members of the test equipment and cause erroneous readings of the instrument dial. The shoulders of the instrument housing, indenter chuck, ball seat in the instrument housing, capstan, capstan screw, and anvil shoulder seat should be kept clean and true.

The capstan and screw should be lightly oiled. Pitted anvil surfaces may be renished with grit paper. Generally, a molded nish will give a higher Rockwell reading than a machined face due to the high resin content or lled materials or better orientation and lower plasticizer content of unlled plastic materials.

Injection mold specimens in such a way that sink marks and warpage are minimized. Tubular or unsupported curved specimens are not recommended for plastic hardness testing. Such curved surfaces have a tendency to yield with the load and produce an unsymmetrical indentation pattern. In such cases, the hardest face is generally that one perpendicular to the molding pressure.

Specimens with ashing on the side supported by the anvil also may give erroneous results. A bulge, change in color, or other marking on the under surface of the test 2 D — 08 specimen closest to the anvil is an indication that the specimen is not sufficiently thick for precision testing.

Stacking of thin specimen is permitted provided they are at, parallel, and free from dust or burrs. The precision of the test is reduced for stacked specimens, and results should not be compared to a test specimen of standard thickness.

Apparatus 6. A at anvil at least 50 mm [2 in. To determine whether this condition is satised, the major load should be applied to the test specimen. If an additional load is then applied, by means of hand pressure on the weights, the needle should indicate an additional indentation. If this is not indicated, the major load is not being applied to the specimen, and a long-stroke PL machine or less severe scale should be used.

For the harder materials with a modulus around MPa [8 3 psi] or over, a stroke equivalent to scale divisions, under major load application, may be adequate; but for softer materials the long-stroke scale divisions under major load machine is required. Test Specimen 7. The specimen may be a piece cut from a molding or sheet. Care should be taken that the test specimen has parallel at surfaces to ensure good seating on the anvil and thus avoid the deection that may be caused by poor contact.

The specimen shall be at least 25 mm [1 in. The minimum width shall be 13 mm [12 in. NOTE 3—Specimen with a thickness other than 6 mm may be used if it has been veried that, for that thickness, the hardness values are not affected by the supporting surface and that no imprint shows under the surface of the specimen after testing.

The specimen may be composed of a pile-up of several pieces of the same thickness, provided that precaution is taken that the surfaces of the pieces are in total contact and not held apart by sink marks, burrs from saw cuts, or other protrusions and provided the hardness values are not affected by the stacking of thin specimens.

The operator should check the instrument manual for this adjustment. If more than one hardness scale is used in testing, choose a standardized test block for each scale used Note 4.

Make ve impressions on the test surface of the block. Compare the average of these ve tests against the hardness calibration of the blocks. If the error is more than 62 hardness numbers, bring the machine into adjustment as described in 8. If adjustment can not be achieved, more extensive servicing of the instrument may be needed.

To adjust the index lever, place a specimen plastic with low creep or soft metal on the anvil and turn the knurled elevating ring to bring the specimen in contact with the indenter.

Keep turning the ring to elevate the specimen until positive resistance to further turning is felt, which will be after the kg load is encountered. When excessive power would have to be used to raise, the specimen higher, set the dial so that the set position is at the top and take note of the position of the pointer on the dial.

If the pointer is between B 50 and B 70 on the red scale, no adjustment is necessary; if the pointer is between B 45 and B 50, adjustment is advisable; and if the pointer is anywhere else, adjustment is imperative. As the pointer revolves several times when the specimens is elevated, the readings mentioned above apply to the revolution of the pointer which occurs either as the reference mark on the gage stem disappears into the sleeve or as the auxiliary hand on the dial passes beyond the zero setting on the dial.

The object of this adjustment is to determine if the elevation of the specimen to the minor load does not cause even a partial application of the major load. Apply the major load only through the release mechanism.

Conditioning 9. NOTE 5—Operation of this test equipment above the below normal temperature is not recommended due to the change in viscosity of the dashpot oil and the close tolerance of the gage.

Calibration 8. Minor errors in leveling are not critical, but correct positioning is desirable. Adjust the dashpot on the Rockwell tester so that the operating handle completes its travel in 4 to 5 s with no specimen on the machine or load applied by the indenter to the anvil.

The major load shall be kg for this calibration. When so adjusted, the period taken for the mechanism to come to a stop with the specimen in place will vary from 5 to 15 s, 3 Number of Tests For anisotropic materials, at least ve hardness tests D — 08 shall be made along each principal axis of anisotropy, provided the sample size permits. Procedure A Rockwell hardness values are reported by a letter to indicate the scale used and a number to indicate the reading.

The choice of scales shall be governed by the considerations concerned with the total indentation readings and the nal scale reading for a particular material and scale see Table 1 and 5.

The full pressure of the major load is required to seat the indenter shoulder into the chuck. NOTE 6—The operation of the test instrument as described in this procedure may differ in detail from that of automated electronic instruments.

Thus, if the M scale indentation with the major load is divisions use the L Rockwell scale. In no case shall the clearance be less than 6 mm [14 in. Neither should tests be made so close to each other that the plastic surface is damaged from the previous indentation. In no case shall an indentation be made less than 6 mm [14 in. Never make duplicate tests on the opposite face of a specimen; if a specimen is turned over and retested on the opposite face, the ridges on the rst face will contribute to a softer reading on the second face.

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A number in parentheses indicates the year of last reapproval. A superscript epsilon e indicates an editorial change since the last revision or reapproval. This standard has been approved for use by agencies of the Department of Defense. The values given in brackets are for information only. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Procedure A of this test method is equivalent to the test method in the main body of ISO Procedure B of this test method is equivalent to the test method in the integral annex part of ISO

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