GE
Inspection Technologies
Mobile Hardness Testing
Application Guide for
Hardness Testers
Dr. Stefan Frank
GE Inspection Technologies
2
Ultrasonics
Mobile Hardness Testing –
Application Guide
Dr. Stefan Frank
1. Introduction ............................................................................................................................................................ 4
1.1 What is hardness? .................................................................................................................................................... 4
1.2 Why hardness testing? .............................................................................................................................................. 5
1.3 On-site mobile hardness testing? ............................................................................................................................... 5
2. The UCI method (MIC 10, MIC 20) ........................................................................................................................... 5
2.1 The method ........................................................................................................................................................... 5
2.2 Selecting the suitable UCI probe ................................................................................................................................6
3. The Rebound method
(DynaPOCKET, DynaMIC, MIC 20) ............................................................................................................................. 8
3.1 The method ............................................................................................................................................................ 8
3.2 Selection of the suitable impact device ....................................................................................................................... 9
4. The optical Through-Indenter-Viewing method (TIV).................................................................................................... 10
4.1 The method .......................................................................................................................................................... 10
4.2 Selection of the suitable probe ................................................................................................................................12
5. The Hardness Testers – an Overlook ......................................................................................................................... 13
5.1 The DynaPOCKET .................................................................................................................................................. 13
5.2 The DynaMIC ........................................................................................................................................................ 13
5.3 The MIC 10 .......................................................................................................................................................... 13
5.4 The MIC 20 .......................................................................................................................................................... 14
5.5 The TIV ................................................................................................................................................................ 14
6. The different methods in the field ............................................................................................................................ 14
6.1 Selecting the test method ....................................................................................................................................... 14
6.2 Significance of indentation size ............................................................................................................................... 15
6.3 Relation between penetration depth and minimum thickness for coatings ...................................................................... 16
6.4 Hardness testing on welds (HAZ) ............................................................................................................................. 16
6.5 Test piece mass requirements .................................................................................................................................. 17
6.6 Wall thickness requirements .................................................................................................................................... 17
6.7 Surface quality ...................................................................................................................................................... 18
6.8 Handling, alignment, and fixing ............................................................................................................................... 18
6.9 Calibration ............................................................................................................................................................ 18
6.10 Verifying instrument performance............................................................................................................................. 20
7. Summary and help in choosing
the suitable test method ......................................................................................................................................... 20
7.1 The UCI method (MIC 20 / MIC 10) ......................................................................................................................... 21
7.2 Rebound method (MIC 20 / DynaMIC / DynaPOCKET) ................................................................................................ 21
7.3 Optical method – Through-Indenter-Viewing (TIV) ....................................................................................................... 21
7.4 Fundamental questions to the user .......................................................................................................................... 22
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GE Inspection Technologies
1. Introduction
Mobile hardness testing is on the ad-
vance: in these times of cost pressure
and higher quality requirements, it repre-
sents not only a quick but most of all an
economical supplement to stationary
hardness testing in the modern produc-
tion process. The application possibilities
are far ranging - this refers to both large
and smaller components, especially at
positions which are difficult to access.
There are three different physical methods
which are particularly recognized in the
field: the static UCI (Ultrasonic Contact
Impedance) method and the dynamic
rebound hardness testing method, as well
as the optical TIV (Through-Indenter-
Viewing) method. The decision as to which
method is to be used depends on the test
problem. Krautkramer offers five instru-
ment series for mobile hardness testing,
operating according to the UCI, the re-
bound or the TIV methods: DynaPOCKET,
DynaMIC, MIC 10, MIC 20, and TIV.
This Application Guide explains the basic
principles of these test methods and
compares them, using examples from the
field, e.g. hardness testing in the heat-
affected zone (HAZ) of welds.
In addition to this, the subjects critically
discussed are the factors liable to influ-
ence the measured values, such as sur-
face preparation at the test location or
the mass of parts to be tested as well as
their thickness.
1.1 What is hardness?
With regard to metallic materials, hard-
ness has always been (and still is) a sub-
ject of much discussion among metallur-
Fig. 1:
Hardness testing using the MIC 20 in combination with the test support
MIC 227 and a UCI probe in the heat-affected zone (HAZ) of a weld.
Fig. 3:
Hardness testing using the DynaPOCKET on the chain of an open-pit mining
excavator.
4
Fig. 2:
Hardness testing with a rebound hardness tester (DynaMIC) on the drive wheel
of a large hydraulic excavator.
Fig. 4:
Optical hardness testing using the TIV tester. Checking before final assembly.
Ultrasonics
gists, engineers, and material scientists.
It is no wonder therefore that there is a
wide range of definitions for the term
hardness. Attributes like wear resistance,
deformation behavior, tensile strength, as
well as modulus of elasticity or Young’s
modulus are connected with the term
hardness.
An exact description of the method must
be made if one wishes to compare the
obtained readings with each other in
order to achieve a usable hardness value.
However, if the reading depends on the
method, then the conclusion may quite
clearly be drawn that hardness is no
physical quantity but that it must be a
parameter.
Hardness testing is almost nondestructive
and in many cases used for determining
parameters to differentiate and describe
materials. For example, hardness values
can easily provide data on the strength
properties of a material.
The term hardness is generally understood
as being the resistance of a material
against the penetration of a body made of
a stronger material.
Hardness is therefore not a fundamental
quantity of a material but always a re-
sponse of the material to a certain load or
test method. A hardness value is calculat-
ed on the basis of the response of the
material to this load.
Depending on the test method, other
numerical values are then determined
which are due to and characterized by
• the shape and material of the indenter
• the type and size of the load,
e.g. test load.
The different test methods can be roughly
divided into two groups:
a) Static test method:
With this method of testing, the load is
applied statically or quasi-statically. After
removing the test load, the hardness
value is defined as a ratio of test load and
the surface or projected area of the per-
manent test indentation (Brinell, Vickers,
or Knoop). In tests according to Rockwell,
the hardness is determined by means of
the permanent penetration depth of a
body due to the test load.
b) Dynamic test method
As opposed to the static method, the load
is applied in the impact mode in this
case, and the hardness is determined on
the basis of the indenter’s ”loss of energy”.
It is normal practice – and often neces-
sary – to indicate the hardness values
using another scale than the one used for
measuring them. The following should
always be taken into account regarding
this:
• There are no generally applicable relati-
onships for the conversion of hardness
values from one to another
• Conversions are possible whenever the
conversion relationship has been deter-
mined by statistically backed compari-
son measurements
• Conversion relationships from national
and international standards apply to
certain material groups to a limited
extent.
The various conversion relationships, as
specified in standards DIN 50 150 and
ASTM E 140, are stored and can be
selected in the instruments of the
MICRODUR series (MIC 10 and MIC 20)
and in the rebound hardness testers
(DynaPOCKET, DynaMIC, MIC 20), as
well as in the optical TIV hardness
tester.
1.2 Why hardness testing?
Within the production and assembly
lines, the hardness of materials or com-
ponents is mainly tested for two reasons:
firstly, to determine the characteristics of
new materials, and secondly, for the
purpose of quality assurance by meeting
the required specifications.
1.3 On-site mobile hardness testing?
Conventional hardness testers according
to Rockwell, Brinell, or Vickers always
require the test piece be brought to the
tester. As this is not always possible for
practical reasons and, most of all, for
reasons of geometry, small and portable
hardness testers were developed that
enable quick on-site testing on the
component.
Different methods are applied here.
Most of all the portable hardness testers
according to the UCI, rebound, and TIV
method are successfully used in practical
operations in the field.
2. The UCI method (MIC 10, MIC 20)
Standardized according to ASTM A 1038
2.1 The method
As in Vickers or Brinell hardness testing,
the question as to the size of the test
indentation left in the material by a
Vickers diamond after applying a fixed
test load also arises in Vickers hardness
testing according to the Ultrasonic Contact
Impedance method (UCI for short).
However, the diagonals of the test inden-
tation are not determined optically for
the hardness value as usual, but the
indentation area is electronically detected
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GE Inspection Technologies
by measuring the shift of an ultrasonic
frequency. The UCI method can be illus-
trated by a small imaginary experiment. A
UCI probe essentially consists of a Vickers
diamond attached to the end of a metal
rod (Fig. 5). This rod is excited into longi-
tudinal oscillation by piezoelectric trans-
ducers. Imagine instead of the metal rod
(we refer to it as oscillation rod) a large
spiral spring held at one end and oscillat-
Piezo
Transducer
Piezo
Receiver
Oscillating
Rod
Vickers Diamond
Fig. 5:
Schematic description of the UCI method
(Test load / Oscillation rod / Vickers diamond /
Material to be tested).
ing at a resonant frequency of 70 kHz at
the free end. At the very top of this spring,
there is a contact plate, the Vickers dia-
mond. The test material, with which the
Vickers diamond comes into contact, can
also be imagined as being a system of
smaller spiral springs positioned vertically
to the surface - an atomic bond, two
atoms inter-linked via a “spring“. If only
one of these “atomic springs“ is touched
by the Vickers diamond - with very hard
material in which the diamond can only
slightly penetrate and consequently pro-
duce a small indentation – then an addi-
tional spring, i.e. mass, is coupled to the
large spiral spring. This produces a shift
in resonant frequency.
larger. Analogously, the largest frequency
shift is produced on soft test materials;
the diamond penetrates deeper into the
material leaving a large indentation.
The frequency shift is proportional to the
size of the test indentation produced by
the Vickers diamond. Therefore, the
diagonals of the test indentation are not
optically determined for the hardness
value, as is usually done, but the inden-
tation area is electronically detected by
measuring the frequency shift – taking
just a few seconds.
This is the whole secret of UCI hardness
testing: the frequency shift is proportional
to the size of the Vickers test indentation.
Equation 1 describes this basic relation in
comparison to the definition of the Vickers
hardness value.
Naturally, such a frequency shift likewise
depends on the spring constant of our
small ”atomic springs”.
When applied to the material to be tested,
this is known as the modulus of elasticity
or Young’s modulus. After completing the
calibration, the UCI method can be used
900
700
500
300
100
2 2.5 3 3.5 4 4.5 5
for all materials showing this modulus of
elasticity. The probes are factory-calibrated
on low-alloy or unalloyed steels; however,
modern test instruments can also be
quickly calibrated to other materials, such
as titanium or copper, at the test location.
2.2 Selecting the suitable UCI probe
To carry out a test according to the UCI
principle, a probe containing a rod with a
Vickers diamond attached to the contact
end is oscillated by piezoelectric ceramic
transducers at an ultrasonic frequency.
= frequency shift
A = area of indentation
E
= effective Young´s modulus
eff
HV = Vickers hardness value
F = Test Load
Equation 1:
The frequency shift is proportional to the indentation size of a Vickers indenter.
This frequency shift will become greater
when additional “springs“ are touched,
that means if the diamond penetrates
deeper into a material of medium hard-
ness, and the test indentation becomes
6
Frequency shift (kHz)
Fig. 6:
Frequency shift of the oscillation rod as a function of hardness (HV).
Ultrasonics
A spring applies the load, and the fre-
quency of the rod changes in proportion
to the contact area of the indentation
produced by the Vickers diamond. There-
The instrument constantly monitors the
frequency, calculates the value, and
instantaneously displays the hardness
value.
fore, the hardness value is not optically
determined by the diagonals of the in-
dentation, as would normally be the case
with a hardness tester measuring statically,
but by an electronic measurement of the
frequency shift within seconds.
The UCI method is best suited for testing
homogeneous materials. Six test loads are
employed by the various models of UCI
probes (table 1).
Fig. 7:
UCI-probes – different models.
Testload Available Probe Models Advantage or Benefit Typical Applications
98 N MIC 2010 Largest indentation; requires Small forgings, weld testing, HAZ
(10 kgf) Standard Length minimal surface preparation
Handheld Style
50 N MIC 205 Solves most general Induction or carburized
(5 kgf) Standard Length application problems mechanical parts, e.g. camshafts,
Handheld Style turbines, welds, HAZ.
MIC 205L 30 mm extended length Measurement in grooves, gear
Extended Length tooth flanks and roots
Handheld Style
10 N MIC 201 Load easy to apply; enables Ion-nitrided stamping dies and
(1 kgf) Standard Length controlled testing on a sharp molds, forms, presses, thin walled
Handheld Style radius parts
MIC 201L Measurement on complicated Bearings, tooth flanks
Extended Length geometries
Handheld Style
8 N MIC 211 Automatic load application Finished precision parts, gears,
(0.9 kgf) Motor Probe Style bearing raceways
3 N MIC 2103 Automatic load application Layers, e.g. copper or chromium
(0.3 kgf) Motor Probe Style layers on steel cylinders
(≥ 40 µm),
rotogravure cylinders, coatings,
hardened layers
(≥ 20µm)
1 N MIC 2101 The shallowest indentation These layers with a polished surface
(0.1 kgf) Motor Probe Style
Table 1:
UCI probe models, benefits, and typical applications.
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3. The Rebound method (DynaPOCKET, DynaMIC and MIC 20)
standardized according ASTM A 956
3.1 The method
Even with hardness testers operating
according to the Leeb’s or rebound
method, the size of the test indentation
generated is depended on the hardness
of material. However, it is in this case
indirectly measured via the loss of
Impact
m
▲
h
I
▲
m
▲
V
I
E
pot
= mgh
E
I
= mv
kin
2
/2
I
energy of a so-called impact body.
Figure 8 illustrates the physical principle
of rebound hardness testing.
A mass, in this case the impact body with
a tungsten carbide ball attached to its
tip, is fired against the test surface at a
Rebound
V
▲
m
R
m
▲
h
R
▲
d
= mv
2
/2 E
R
= mgh
pot
R
E
kin
defined speed by spring force. The im-
pact creates a plastic deformation of the
surface due to which the impact body
loses part of its original velocity, viz. i.e.
the softer the material, the greater the
loss in velocity. The velocity before and
after the impact is measured in a non-
contact mode. This is done by a small
permanent magnet within the impact
body (Fig. 9) generating an induction
voltage during its passage through a coil,
this voltage being proportional to the
velocity (refer to Fig. 10).
The Leeb’s hardness value HL, named
after the inventor of the rebound method
D. Leeb, is calculated from the ratio of
the impact and rebound velocities. Leeb
defined the hardness value as follows:
HL = VR / V1 · 1000
Fig 8:
The basic principle of rebound hardness testing:
d = diameter of indentation
E
= potential energy
pot
E
= kinetic ennergy.
kin
Impact body
Magnet
Tungsten
carbid ball
Coil
Fig 9:
Cross-section of an impact device.
Equation 2:
Hardness according to Leeb
m = mass
h
, hR = height before/after the impact
I
V
, VR = speed before/after the impact
I
Hardness Leeb
Time
Before Impact After impact
Fig 10:
Schematic course of the voltage signal generated by the impact body traveling through the coil. The signal
shows the voltage before and after the impact. (VDI Report No. 208, 1978).
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