YIELD & TENSILE STRENGTH OF
STEEL & ALUMINIUM USING MICROINDENTATION
Prepared by
Duanjie Li, PhD & Pierre Leroux
6 Morgan, Ste156, Irvine CA 92618 · P: 949.461.9292 · F: 949.461.9232 · nanovea.com
Today's standard for tomorrow's materials. © 2014 NANOVEA
INTRODUCTION
The yield strength (YS) is the stress at which a material begins to deform plastically and will not returns to its
original shape when the applied stress is removed. The ultimate tensile strength (UTS) is the maximum stress
that a material can withstand while being pulled before breaking. These two properties indicate the upper
limit to the load that can be applied to mechanical parts and structure, and play important roles in materials
production including annealing, forging, rolling and pressing.
IMPORTANCE OF YS and UTS MEASUREMENT USING INDENTATION TECHNIQUE
Traditionally the YS and UTS have been tested using a tensile testing machine, a large instrument requiring
enormous strength to pull apart the test specimen. It is both costly and time-consuming to properly machine
many test coupons for a material – each sample can only be tested once. Small defects in the sample could
possibly create noticeable variance in the test result. The different configuration and alignment of the tensile
testers in the market often result in substantial variations in the mechanics of testing and their outcomes.
Nanovea’s innovative indentation method directly provides the YS and UTS values comparable to that
measured by conventional tensile tests. This measurement opens up a whole new realm of possibilities for all
industries. The simple experimental setup significantly cuts the time and cost for sample preparation,
compared to the coupons with a complex shape for tensile tests. The small indentation size makes it possible
to perform multiple measurements on one single sample. It can prevent the influence of defects in the tensile
test coupons created during sample machining. Moreover, it allows YS/UTS measurements on small samples
and localized areas, vital for YS/UTS mapping and local defect detection of pipelines or auto structure.
MEASUREMENT OBJECTIVE
In this application, the Nanovea Mechanical Tester, in indentation mode is used to measure the YS and UTS of
metal alloy samples, including stainless steel SS304 and aluminum Al6061. The test samples were chosen for
their commonly recognized YS and UTS values to show the reliability of the indentation method.
Fig. 1: Indenter and depth sensor on the test sample.
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MEASUREMENT PRINCIPLE
Nanoindentation is based on the standards for instrumented indentation, ASTM E2546 and ISO 14577. It uses
an established method where an indenter tip with a known geometry is driven into a specific site of the
material to be tested, by applying an increasing normal load. When reaching a pre-set maximum value, the
normal load is reduced until complete relaxation occurs. The load is applied by a piezo actuator and the load
is measured in a controlled loop with a high sensitivity load cell. During the experiment the position of the
indenter relative to the sample surface is precisely monitored with high precision capacitive sensor.
The resulting load/displacement curves provide data specific to the mechanical nature of the material under
examination. Established models are used to calculate quantitative hardness and modulus values for such
data. Nanoindentation is especially suited to load and penetration depth measurements at nanometer scales
and has the following specifications:
Maximum displacement (Dual Range) : 50 m or 250 m
Depth Resolution (Theoretical)
: 0.003 nm
Depth Resolution (Noise Level)
: 0.15 nm
Maximum force
: 400 mN
Load Resolution (Theoretical)
: 0.03 N
Load Resolution (Noise Floor)
: 0.3 N
Analysis of Indentation Curve
Following the ASTM E2546 (ISO 14577), hardness and elastic modulus are determined through
load/displacement curve as for the example below.
Pmax
S = dP/dh
ht
hc
hmax
Fig. 2: Load-displacement curve of nanoindentation.
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Hardness
The hardness is determined from the maximum load, Pmax, divided by the projected contact area, Ac:
H
Pmax
Ac
Young’s Modulus
The reduced modulus, Er, is given by:
Er 

S
2
Ac
Which can be calculated having derived S and AC from the indentation curve using the area function, AC being
the projected contact area. The Young’s modulus, E, can then be obtained from:
1 1   2 1   i2


Er
E
Ei
Where Ei and  i are the Young’s modulus and Poisson’s ratio of the indenter and  the Poisson’s ratio of the
tested sample.
How are these calculated?
A power-law fit through the upper 1/3 to1/2 of the unloading data intersects the depth axis at ht. The
stiffness, S, is given by the slope of this line. The contact depth, hc, is then calculated as:
hc  hmax 
3Pmax
4S
The contact Area Ac is calculated by evaluating the indenter area function. This function will depend on the
diamond geometry and at low loads by an area correction.
For a perfect Berkovich and Vickers indenters, the area function is Ac=24.5hc2. For Cube Corner indenter, the
area function is Ac=2.60hc2. For Spherical indenter, the area function is Ac=2πRhc , where R is the radius of the
indenter. The elastic components, as previously mentioned, can be modeled as springs of elastic constant E,
where σ is the stress, E is the elastic modulus of the material, and ε is the strain
given the formula:
that occurs under the given stress, similar to Hooke's Law. The viscous components can be modeled as
dashpots such that the stress-strain rate relationship can be given as
the viscosity of the material, and dε/dt is the time derivative of strain.
, where σ is the stress, η is
Since the analysis is very dependent on the model that is chosen, Nanovea provides the tool to gather the
data of displacement versus depth during the creep time. The maximum creep displacement versus the
maximum depth of indent and the average speed of creep in nm/s is given by the software. Creep may be
best studied when loading is quicker. Spherical tip is a better choice.
Other possible measurements by Nanovea Mechanical Tester:
Stress-Strain & Yield Stress, Fracture Toughness, Compression strength, Fatigue testing and many others.
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TEST CONDITIONS & PROCEDURES
The YS/UTS tests were performed on the Nanovea Mechanical Tester in the Microindentation mode. The tip
used was a cylindrical flat diamond tip of 200 μm diameter. SS304 and Al6061 alloys were selected for their
extensive industrial application and commonly recognized YS and UTS values, in order to show the great
potential and reliability of the indentation method. They had been mechanically polished to a mirror-like
surface finish prior to the tests so as to avoid the influence of surface roughness or defects to the test results.
Depth versus load was recorded during the test. The test conditions are listed in Table 1. More than ten tests
were performed on each sample to ensure repeatability of the test values.
Al6061
SS304
Material
30
40
Applied Force (N)
60
80
Loading rate (N/min)
60
80
Unloading rate (N/min)
Table 1: Test conditions of YS & UTS for SS304 and Al6061.
RESULTS AND DISCUSSION
The load-displacement curves of the SS304 and Al6061 alloy samples are shown in Fig. 3 with the flat
indenter imprints on the test samples inset. Through analysis of the “S” shape of the loading curve using a set
of special algorithm developed by Nanovea, the YS and UTS are automatically calculated by the software as
summarized in Table 1. The YS and UTS values obtained by conventional tensile tests are listed for
comparison.
Fig. 3: Load-displacement curves of SS304 and Al6061 samples. The flat indenter imprints on the test
samples are inset.
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Material
SS304
Al6061
Yield strength (MPa)
Indentation Tensile test
277±21
275
231±18
Ultimate tensile strength (MPa)
Indentation
Tensile test
609±16
620
241
301±9
300
Table 2: YS and UTS of SS304 and Al6061 measured using indentation and tensile tests.
The results show that the YS and UTS values of the SS304 and Al6061 alloys measured using the indentation
and tensile techniques are in a good agreement. The YS and UTS tested by indentation mode of Nanovea
Mechanical Tester exhibit superior precision and repeatability. Compared to conventional tensile test
procedure, the indentation technique for YS and UTS evaluation in this study is substantially more time- and
cost-effective. It does not require expensive complicated sample machining, enables fast quantitative
mapping of YS and UTS values on small samples, and provides a solution of detecting localized mechanical
defects of mechanical parts in service.
CONCLUSION
In this study, we showcased the capacity of Nanovea Mechanical Tester in evaluating YS & UTS of stainless
steel and aluminum alloy sheet samples. The simple experimental setup significantly cuts the time and cost
for sample preparation required for tensile tests. The small indentation size makes it possible to perform
multiple measurements on one single sample. This method allows YS/UTS measurements on small samples
and localized areas, providing a solution for YS/UTS mapping and local defect detection of pipelines or auto
structure.
The Nano, Micro or Macro modules of the Nanovea Mechanical Tester all include ISO and ASTM compliant
indentation, scratch and wear tester modes, providing the widest and most user friendly range of testing
available in a single system. Nanovea's unmatched range is an ideal solution for determining the full range of
mechanical properties of thin or thick, soft or hard coatings, films and substrates, including hardness, Young’s
modulus, fracture toughness, adhesion, wear resistance and many others.
In addition, optional 3D non-contact profiler and AFM Module are available for high resolution 3D imaging of
indentation, scratch and wear track in addition to other surface measurements such as roughness.
To learn more about Nanovea Mechanical Tester or Lab Services.
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