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. Scope 1.

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A number in parentheses indicates the year of last reapproval. This standard has been approved for use by agencies of the U. Department of Defense. The values given in parentheses are for informationonly. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.

ProcedureAof 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 methodin the integral annex part of ISO Referenced Documents2. Significance and Use4. Indenters are roundsteel balls of specific diameters. Rockwell hardness numbersare always quoted with a scale symbol representing theindenter size, load, and dial scale used. This test method isbased on Test Methods E Procedure A Section 11 yieldsthe indentation of the specimen remaining 15 s after a givenmajor load is released to a standard kg minor load.

Procedure B Section 12 yields the indentation of the indenterinto the specimen after a s application of the major loadwhile the load is still applied. Each Rockwell scale divisionrepresents 0. Current edition approved Oct. Published October Originallyapproved in Last previous edition approved in as D - Due to a short overlap ofRockwell hardness scales by Procedure A, two different dialreadings on different scales may be obtained on the samematerial, both of which may be technically correct.

Generally,an uncured specimen has a hardness reading below normal. Readings on one material may be satisfactory forsuch a table, but there is no guarantee that other plasticmaterials will give corresponding readings because of differ-ences in elasticity, creep, and shear characteristics.

If there are no material specifications, then the default condi-tions apply. Factors Affecting Reproducibility and Accuracy5. Softer plastics and coarse-filled materials will have a widerrange of variation. A large ball indenter will distribute the loadmore evenly and decrease the range of test results Note 2. Thesensitivity of the instrument decreases with an increase in thedial reading and becomes very poor for readings of andover due to the shallow indentation of the steel ball.

It isdesirable to use the smallest ball and highest load that ispractical because of this loss of sensitivity. Rockwell hardnessreadings over are not satisfactory and shall not be reported.

Readings between zero and are recommended, but read-ings to are permissible. For comparison purposes, it maybe desirable to take readings higher than or lower than zeroon any single scale. In such cases, Rockwell hardness readingsmay be reported, but the corresponding correct readings shallfollow in parentheses, if possible.

Such alternate readings arenot always feasible when the specimen is subjected to con-stantly changing conditions or irreversible reactions. NOTE 2—Molded specimens containing coarse fiber fillers, such aswoven glass fabric, will influence the penetration obtained. These varia-tions in hardness may be reduced by testing with the largest ball indenterconsistent with the overall hardness of the material.

Otherwise the indenterwill indent further into the material than when such vibrationsare absent. Steel ball indenters that have nicks, burrs, or are out ofround shall not be used. Dust, dirt or heavy oilact as a cushion to the load supporting members of the testequipment and cause erroneous readings of the instrument dial.

The shoulders of the instrument housing, indenter chuck, ballseat in the instrument housing, capstan, capstan screw, andanvil shoulder seat should be kept clean and true. The capstanand screw should be lightly oiled. Pitted anvil surfaces may berefinished with grit paper. Generally, a molded finishwill give a higher Rockwell reading than a machined face dueto the high resin content or filled materials or better orientationand lower plasticizer content of unfilled plastic materials.

Injection mold specimens in such a way that sink marks andwarpage are minimized. Tubular or unsupported curved speci-mens are not recommended for plastic hardness testing. Suchcurved surfaces have a tendency to yield with the load andproduce an unsymmetrical indentation pattern. One make andmodel has a 25 to 1 leverage arm.

Specimens with flash-ing on the side supported by the anvil also may give erroneousresults. Abulge, changein color, or other marking on the under surface of the testspecimen closest to the anvil is an indication that the specimenis not sufficiently thick for precision testing.

Stacking of thinspecimen is permitted provided they are flat, parallel, and freefrom dust or burrs. The precision of the test is reduced forstacked specimens, and results should not be compared to a testspecimen of standard thickness.

A flat anvil at least 50 mm 2 in. To determine whether this condition is satisfied, themajor load should be applied to the test specimen. If anadditional load is then applied, by means of hand pressure onthe weights, the needle should indicate an additional indenta-tion. If this is not indicated, the major load is not being appliedto the specimen, and a long-stroke PL machine or less severescale should be used. Test Specimen7. The specimen may be a piece cutfrom a molding or sheet. Care should be taken that the testspecimen has parallel flat surfaces to ensure good seating onthe anvil and thus avoid the deflection that may be caused bypoor contact.

The specimen shall be at least 25 mm 1 in. NOTE 3—Specimen with a thickness other than 6 mm may be used if ithas been verified that, for that thickness, the hardness values are notaffected by the supporting surface and that no imprint shows under thesurface of the specimen after testing.

The specimen may be composed ofa pile-up of several pieces of the same thickness, provided that precautionis taken that the surfaces of the pieces are in total contact and not heldapart by sink marks, burrs from saw cuts, or other protrusions andprovided the hardness values are not affected by the stacking of thinspecimens. Minor errors in leveling are notcritical, but correct positioning is desirable.

Adjust the dashpot on the Rockwell tester so thatthe operating handle completes its travel in 4 to 5 s with nospecimen on the machine or load applied by the indenter to theanvil. The major load shall be kg for this calibration. Whenso adjusted, the period taken for the mechanism to come to astop with the specimen in place will vary from 5 to 15 s,depending upon the particular specimen, the indenter, and theload used.

The operator should check the instrument manualfor this adjustment. If more than onehardness scale is used in testing, choose a standardized testblock for each scale used Note 4. Make five impressions onthe test surface of the block. Compare the average of these fivetests against the hardness calibration of the blocks. If the erroris more than 62 hardness numbers, bring the machine intoadjustment as described in 8. If adjustmentcan not be achieved, more extensive servicing of the instru-ment may be needed.

To adjust the index lever, place aspecimen plastic with low creep or soft metal on the anvil andturn the knurled elevating ring to bring the specimen in contactwith the indenter.

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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.

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