Equotip Hardness Tester: The Correlation between Hardness and Texture

Hallo Pare Member.. ^_^
I am sorry for my late report, this report should be submitted before December. This report is actually the mini research I did in Geotechnical Measurement Engineering class under supervised by Prof. Satoru Kawasaki, Division of Sustainable Resource Engineering, Faculty of Engineering, Hokkaido University. The topic is about the physical properties measurement of rocks, especially hardness.  
Rocks are made of one or more minerals. Minerals are pure, solid, inorganic (nonliving) materials found in Earth's crust. Rocks have chemical compositions and physical properties related with their mineral properties. However, rocks with the same mineral ingredients may be has different physical properties due to variations in the amounts of minerals and the processes by which they are formed such as being burned, doughy, or just right. Rock material properties include rock type, color, particle size, texture, hardness, and strength.
Hardness is one of the physical rock parameter. According to Chandler (1999), hardness has a variety of meanings. To the metals industry, it may be thought of as resistance to permanent deformation. To the metallurgist, it means resistance to penetration. To the lubrication engineer, it means resistance to wear. To the design engineer, it is a measure of flow stress and to the mineralogist, it means resistance to scratching. Hardness may also be referred to as mean contact pressure. In the other definition, hardness is the subjective description of the resistance of an earth material to permanent deformation, particularly by indentation (impact) or abrasion (scratching) (ASTM D653 in USDA, 2012).
Hardness can be measured or tested by several classifications. The necessity of all these different hardness classification is due to the need for categorizing the great range of hardness from soft rubber to hard ceramics. Those classifications are as follows (Private communication with Kawasaki, 2014).
1.      Abrasive hardness : Deval, Los Angels, Taber
2.      Rebound hardness: Shore, Duloscope, Equotip
3.      Indentation hardness: Rockwell, Brinell, Vickers
4.      Scratch hardness: Mohs, Martens
The Equotip hardness tester is one of the useful testers developed so as to understand elastic properties and strengths of metallic materials. It is picked up as the characteristics of the Equotip hardness tester that a small sample is needed for the test, sampling time is short, and the tester is convenient for portable use (Kawasaki et al., 2001). The equotip devise is an electronic battery-operated spring-based devise (see Figure 1). The piston moves through a coil and causes a current through the coil. The voltage of the current which is proportional to the velocity of the piston, before impact (V1) and after impact (V2) are measured automatically and displayed as a ratio (V1/V2 × 1000) which is denoted by the unit: L (Hack et al., 1993).

Figure 1. Equotip Instrument


Figure 2. Rock Samples
According to the comprehensive understanding about hardness measurement, equotip tests were performed on five rock samples. The rock samples include granite, gravel sand, welded tuff, tuff, and fine sand (see Figure 2). The sample dimensions were approximately 30 mm diameter, and 80 mm long. There were 5 measurement of equotip rebound values for each sample. The results of hardness values tested by equotip measurement are shown in Table 1, while the average and standard deviation of the result are presented respectively in Figure 3 and Figure 4.
Table 1. Data of equotip hardness measurement on rock samples
No. Measurement
Granite
Gravel Sand
Welded Tuff
Tuff
Fine Sand
1
891
865
577
575
715
2
881
841
556
492
679
3
888
849
569
656
691
4
894
847
252
603
632
5
904
856
472
618
697
AVERAGE
891.6
851.6
485.2
588.8
682.8
Standard Deviation
8.444
9.209
136.948
61.504
31.228




Figure 3. Graphic of average value of equotip hardness tester

Based on Figure 3, the highest value of average hardness is granite sample followed with gravel sand, fine sand and tuff. While, the lowest value of hardness average data is welded tuff. In the contrary, according to Figure 4 the most highest value  of hardness standard deviation data is welded tuff, and the lowest value of hardness standard deviation data is granite.

Figure 4. Graphic of standard deviation value of equotip hardness tester

The correlation between average hardness and standardard hardness is invesely proportional. According to Verwall and Mulder (1993), it is generally accepted that surface rougness of the test specimen will affect result in rebound testing. It influences standard deviation value. The texture and rougness of each rock samples are presented in Figure 5.

Figure 5. Rock sample textures; 1. Granite, 2. Gravel sand, 3. Welded tuff,
4. Tuff, 5. Fine sand
The average value of hardness measurement is influenced by the particle size and texture of  each rock samples. Particel size refers to the size of the particles that make up a sedimentary or phyroclastic rock, while texture refers to the crystallinity and granualarity of igneous and crystalline metamorphic rocks. In the other hand, the standard deviation value  of hardness measurement is influenced by the rougness of surface samples. Granite is common type of felsic intrusive igneous rock which is granular and phaneritic in texture. This phaneritic texture is proved by the size of the crystallinity which is can be seen by naked eye. It forms by slow cooling of magma deep underground in the plutonic environment. Gravel sand texture is also phaneritic. While fine sand and tuff are aphanitic, which their component mineral crystals cannot detectable by the naked eye. Welded tuff texture is porphyritic. It has a range of particles size, from the largest agglomerates to very fine ashes.
Therefore it can be concluded that the hardness value of rock properties is affected by texture parameter. The biggest particle size and texture of the rock has the highest hardness value.

Reference:
Hack, H. R. G. K., Hingira, J., Verwaal., W., 1993, Determination of Discontinuity Wall Strength by Equotip and Ball Rebound Test, International Journal of Rock Mechanics Mineral Science & Geomechanics, Pergamon Press Ltd.
Harry Chandler, 1999, Hardness Testing, 2nd Edition, ASM International
Kawasaki, S., Yoshida, M., Tanimoto, C., Masuya, T., 2001, The Development of Property Evaluation Method for Rock Materials based on the Simple Rebound Hardness Test: Investigation on the Effects of Test Conditions and Fundamental Properties, Rock Mechanics – a Challenge for Society, Sarkka & Eloranta (eds), Sweets & Zeitlinger Lisse.
United States Department of Agriculture, 2012, National Engineering Handbook Part 631 Geology Chapter 4 Engineering Classification of Rock Materials, (Amend. 55, January 2012), Washington, DC
Verwaal, W., and Mulder, A., 1993, Technical Note Estimating Rock Strength with the Equotip Hardness Tester, International Journal of Rock Mechanics Mineral Science & Geomechanics, Pergamon Press Ltd


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