Shore Scleroscope Test
h Invented by A. F. Shore, it is done by
dropping a diamond tipped hammer
by its own weight from a fixed height
and reading rebound height (Fig. 8).
Figure 8
h Higher the rebound, harder the metal.
h There are two models (scales) having
certain height & weight of hammer:
1. Model C (251.2 mm & 23 g)
2. Model D (17.9 mm & 36 g)
h Hardness number is indicated based on ASTM A448 as follows:
HSc XX (Model C), HSd XX (Model D).
Material
t (mm)
Hard steel
0.15
h Due to its portability, the testers can be used for various size of
Cold-rolled steel
0.25
parts, including small parts (e.g. thin sheets) and large parts
Semi-hard brass strip 0.25
(e.g. steel rolls). Table shows the minimum material thickness.
h Surface finish of the part is important for reliable measurement.
Annealed brass strip 0.38
h Tests should not be made more than once on the same spot due to cold working occurring
around that spot.Thus, the indentations must be at least 0.51 mm apart from each other.
Durometer
h This is an instrument for measuring
indentation hardness of elastomers,
rubbers, polymers (ASTM D2240-68).
Figure 9
h Shore durometer has Models A & D
for testing soft and hard materials,
respectively. They differ in sharpness
of indenters and magnitude of load
applied by calibrated spring (Fig. 9).
h Spring force (in grams) is 56 + 7.66 HA (for Model A) and 45.36 HD (for Model D)
where HA and HD are the hardness readings taken within 1 second of pressing.
h Similar to Rockwell testing, the hardness is a measure of the depth of indentation.
The indenter is pressed against the surface, and the indentation depth is measured
by an indicating device.
h The hardness reading is 100 when depth is zero. Reading is zero when the indenter
meets no resistance (i.e. when the depth is h = 2.5 ± 0.04 mm).
2
Scratch Test
h In scratch test, a hard indenter pressed onto part
surface and drawn steadily in horizontal direction.
h Thus, the resistance of a material against wear
due to grinding and scratching is measured.
h Moh’s scale defines the hardness of
minerals numbered consecutively from
1 to 10 based on their relative hardness
(e.g. calcite can scratch talc).
h The extended version of this scaling is
available by Ridgway.
h Both scales are not quite objective.
Thus, other scratch methods have been
proposed as alternative.
No.
1
2
3
4
5
6
7
8
9
10
Moh’s Scale
Reference Material
Talc
Gypsum
Calcite
Fluorite
Apatite
Feldspar
Extended Scale by Ridgway
No. Reference Material
1 Talc
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Orthoclase (feldspar)
7 Vitreous pure silica
Quartz
8 Quartz
Topaz
9 Topaz
10 Garnet
11 Fuser zirconia
Sapphire or corundum 12 Fuser alumina
13 Silicon carbide
14 Boron carbide
Diamond
15 Diamond
Scratch Test
h Bierbaum’s test has been the most widely-known test, but
discontinued due to lack of reproducibility. A diamond indenter
is drawn under a vertical load steadily along surface (Fig. 10),
and Bierhaum Scratch Hardness (BSH) is defined as:
BSH
F w2
F : applied load (kg)
w : measured width of scratch (mm)
F
Figure 10
w
specimen
h The shape of indenter can be adopted to particular requirements. It can be conical
with angles of 90°, 120°, 150° (similar to Brale indenter in Rockwell test) or pyramidal
with an interfacial angle of 120° (similar to diamond indenter in Vickers test).
h The scratch produced by moving the indenter is compared with a reference scratch.
Marten’s scratch hardness is the required load to produce scratch of 0.01 mm.
There are also reference scratches for soft or thin materials.
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Special Purpose Hardness Tests
1. Ultrasonic Hardness Test: A magnetostrictive diamondtipped rod vibrating at ultrasonic frequencies (> 20 kHz) is
in contact with the surface at a load of 3.3 kg.
The change in vibration frequency is read on a scale in
terms of Rockwell or Vickers number.
The tester is portable, and suitable for superficial testing
(only for steel) due to shallow indentation (< 0.013 mm).
2. Hot-hardness Test: It is used for measuring the hardness of metals at elevated
temperatures. Some degree of success has been obtained in correlating hothardness with strength. Dedicated testers as well as some Rockwell testers (models
6JR, 6JS, 6JT) can be used.
There are difficulties with this test:
ƒ Both indenter and specimen must be at the same temperature. A cold indenter will
cause a local cooling, causing similar effect of quenching and resulting in errors.
ƒ Test must be carried out in vacuum or inert atmosphere due to oxidation of surface.
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Highlights of Hardness Tests
h Hardness tests are relatively “nondestructive” although this term is very subjective.
Even though extent of indentation is rather small, its destructive effect is dependent
upon size of the part (i.e. testing of parts with thin sections is particularly difficult).
h Hardness measurements are strongly affected by surface quality. The general rule
is: lighter the test load, higher the degree of surface finish required. Rockwell test is
relatively insensitive to surface finish. With accurate definition of indentation diameter,
Brinell test is also not highly influenced by scratches and roughness. Good finish is
required in Vickers and microhardness testing (metallographic finish).
h The advantages of hardness tests can be summarized as follows:
ƒ Hardness tests are relatively simpler and easier than other mechanical tests.
ƒ Special specimens are not needed (machine parts can be tested directly).
ƒ Certain tests can be conducted on-site as well as in the production line.
ƒ Tests can be made quickly (taking about seconds to few minutes).
ƒ Hardness can be measured on parts of small thickness or in very thin layers.
ƒ Quality level of similar materials may be graded according to hardness.
ƒ Hardness can be related to strength (Sut and Suc for ductile and brittle materials).
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Choice of the Testing Method
h The followings should be considered in selection of the most appropriate method:
1. Degree of accuracy required: This is depending on scope of the test (to determine
the properties that are directly or indirectly related with hardness). Hardness of a part
following a heat-treatment is a directly related property. Assessment of the tensile
strength based on a hardness value is an indirect way.
2. Speed of testing: This involves the time for conducting the test as well as the time
required to prepare the surface. This issue is significant when the test is employed for
quality control. Special automated high-speed testers are available for this purpose.
Order of tests (fastest to slowest): Shore Scleroscope, Rockwell, Brinell, Vickers
3. Size of the part: This is the issue whether the part could be accomodated by tester.
Too large/ heavy parts and the miniature parts cannot be tested on standard testers.
h Shape and size of the indenter and magnitude of the load are chosen according
to structure and properties of the material:
ƒ For materials with heterogeneous structure of large inclusions (such as gray iron or
nonferrous bearing alloys), large-size indenter should be used.
ƒ For materials with fine homogeneous structure, tests should be made at small spots
using smaller indenters in order to represent the bulk of material sufficiently.
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Choice of the Testing Method
h Ferrous materials: Practically all ferrous materials of suitable thickness can be
tested by standard test methods. Among them, Brinell test gives better results with
course grained materials because of the averaging provided by Ø10 mm ball. BHN is
used to determine approximate tensile strength of steels:
Sut
HB 0.35 kg mm 2 (for steels with BHN above 175)
Sut
HB 0.33 kg mm 2 (for steels with BHN below 175)
h Hardened and tempered steels: Rockwell and Vickers tests produce more accurate
results. For thin parts, Rockwell A scale or superficial testing can be employed.
h Case hardened steels: For such steels, outside layer (case) is harder than inner
portion (core). The hardness is concerned with the depth of hardened layer, thereby
the indentation should not disturb this layer. Thus, the variation of hardness across
the case depth can only be done by microhardness testing. Vickers and Rockwell
(with suitable scales depending upon case thickness) can be used.
h Steel forgings: Brinell test (with Ø10 mm ball and 3000 kg load) gives a good
relationship between BHN and tensile strength of forged steels (as explained above).
Choice of the Testing Method
h Cast iron: Brinell test is preferred due to nonuniformity of cast iron. MacKenzie
suggested a correlation between BHN and compressive strength of gray cast iron:
Suc
1.274 HB 1.85
103 kg mm 2
h Plastics: Following ASTM procedures are used for determining hardness of plastics
(The relationship between hardness and strength of plastics is not so determinate):
ƒ D2240-68 (Durometer test)
ƒ D2583-67 (Barcol Impressor)
ƒ D785-65 (Rockwell test with R, L, M, E, K scales)
h Elastomers: Rubber and rubber-like materials are tested on Durometer. Model A is
used with soft rubbers whereas hard rubber products are tested using Model D.
h Very hard materials: The difficulty is that very high contact pressures are required to
produce an indentation of measurable size. Diamond indenters are used, thus
selection of inappropriate scales reduce the life of diamond. Knoop scale is usually
employed for ceramics whereas Rockwell A scale is accepted for cemented carbides.
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