Methods Manual for Salt Lake Studies/Measuring the hardness of playa crusts

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Authors: Fuland123, PSJ Coleman,

Overview[edit | edit source]

Playa crusts may comprise fine clays, gypsum, salt or calcite crusts, or fine powdery materials. The hardness of such surfaces may be relatively constant (eg gypsum crusts) or may vary markedly with wetting (e.g. clay pan surfaces). Hardness of crust materials may be measured in situ or by taking a piece of the crust away for testing. Vigar's needle (or similar) penetrometers are ideal for measuring a crust's resistance to penetration in situ.

Use of a needle penetrometer[edit | edit source]

This section needs a method description and photograph of the equipment

The section below needs some overview/description

Calculation of the Rockwell Hardness Number[edit | edit source]

During a Rockwell test, the force on the indenter is increased from a preliminary test force to a total test force, and then returned to the preliminary test force. The difference in the two indentation depth measurements, while under the preliminary test force, is measured as h (see Fig. 1).

The unit measurement for his mm. From the value of h, the Rockwell hardness number is derived. The Rockwell hardness number is calculated as:

For scales using a diamond conical indenter (see Tables 1 and 2): Rockwell Hardness = 100 - h/0.002 Rockwell Superficial Hardness = 100 - h/0.001 where h is in mm.

For scales using a ball indenter (see Tables 1 and 2): Rockwell Hardness = 130 – h/0.002 Rockwell Superficial Hardness = 100 – h/0.001 where h is in mm.

The Rockwell hardness number is an arbitrary number, which, by method of calculation, results in a higher number for harder material. Rockwell hardness values shall not be designated by a number alone because it is necessary to indicate which indenter and forces have been employed in making the test (see Tables 1 and 2). Rockwell hardness numbers shall be quoted with a scale symbol representing the indenter and forces used. The hardness number is followed by the symbol HR and the scale designation. When a ball indenter is used, the scale designation is followed by the letter “W” to indicate the use of a tungsten carbide ball or the letter “S” to indicate the use of a steel ball.

Examples: 52 HRC = Rockwell hardness number of 52 on Rockwell C scale 79 HR30N = Rockwell superficial hardness number of 79 on the Rockwell 30N scale 89 HRBW = Rockwell hardness number of 89 on the Rockwell B scale using a tungsten carbide ball indenter

A reported Rockwell hardness number or the average value of Rockwell hardness measurements shall be rounded in accordance with Practice E 29 with a resolution no greater than the resolution of the hardness value display of the testing machine. Typically, the resolution of a Rockwell hardness number should not be greater than 0.1 Rockwell units.

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