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Industrial Engineering Letters www.iiste.org ISSN 2224-6096 (Paper) ISSN 2225-0581 (online) Vol.4, No.10, 2014 12 A Review Study of Mechanical Fatigue Testing Methods for Small-Scale Metal Materials Nabel Kadum Abd-Ali College of Engineering/Al-Qadisiyha University/ Al- Dewaniya /Iraq E-mail of the author corresponding: [email protected] Abstract A review of the mechanical fatigue testing techniques and evaluation methods for the small scale metal materials is presented. The concentration is on the different micro- and nanoscale testing techniques, classify, summarize, and compare all of these techniques that are currently available. Then, have a tendency to proven the most used and so popular. Furthermore, have a tendency to characterize the most studied materials and the attention way in them once changed with time. Finally, the perspectives of studies on fatigue of the small scale metal materials are offered. Keywords: Fatigue crack; multi-scale fatigue; MEMS/NEMS; fatigue testing method. 1. Introduction The attention on materials behavior at small length scales has profited great attention in the last two decades, as a couse of the increasing application, production, and commercialization of different types of micro- and nano- electromechanical systems (MEMS/NEMS). Due to their small size, short time to response, high performance and required low energy requirements, these devices are currently utilized in diversity of industrial, consumer and biomedical applications [1-4]. Such success has stimulated a further improvement of their design, in order to produce even more reliable and competitive components. The industries of the micro- electromechanical systems MEMS and nano-electromechanical systems NEMS used many materials such as metals, alloys and polymers (e.g., polyamide), not just the typical silicon- based materials of microelectronics. Moreover, different types of nanostructured materials are being an increasingly used. Acquaintance of the mechanical properties it’s very necessary to designing, fabricating and predicting the reliability of micro-devices. Additionally, with more spread of micro- electromechanical systems MEMS and nano-electromechanical systems NEMS, the development of new materials and innovative fabrication processes necessary to tighter design constraints enforced by the economic competition. As a conclusion, reliable and repeatable estimation of the mechanical properties of both currently used and emerging materials is needed. In this case, should define the characterization for standardized methodologies. Regrettably, the standard well-assessed techniques for mechanical characterization at macro-scale can’t be transferred to the micro/nano scale, as the machinery and equipment they include are not suitable for manipulating components with submillimeter size. Also, it can be shown that the mechanical properties of materials at smaller length scales can’t generally be resultant from bulk properties calculated by ordinary macro - scale methods [5, 6]. In reality the material properties change with specimen size has been well known for several years [7, 8]. In additional, the testing materials in small scales length consist from very small specimens and miniaturized testing structures. Such testing structures should be provided with high resolution measurement systems for accurately measuring loads and displacements. For matching such requirements, new and appropriate techniques and equipment have been established. Fatigue is the gradual structural damage resulting from the material exposure to periodic load and one of very significant failure modes in engineering structural materials, has been studied for more than one hundred years. In recent years, fatigue crack and their applications are gaining great interest among researchers who they are focused on the field of fatigue research, which is transition from the traditional engineering to using micro/nano technologies guides the fatigue's research interest to small scale materials [9]. On the small scale, the design and reliability of the materials do not follow the conventional theories since the fatigue behavior of small- scale materials has been proven to be different from that of bulk materials due to size effects. These small-scale materials are widely used in Micro electromechanical Systems (MEMS)/Microsystems, etc. high-tech fields [10]. Materials comprising of nanoscale microstructures are of technical and developed interest due to their unusual mechanical properties. In general, it can be categorized the small-scale materials depending on their dimensions to three kinds as shown below: (a) Thin films whose dimension in one direction is close to micrometers or less. (b) Thin fibers /wires whose dimensions in two directions are very small. (c) Small particles. Otherwise, these materials also can be classified depending on confinement for two types:
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Page 1: A review study of mechanical fatigue testing methods for small scale metal materials

Industrial Engineering Letters www.iiste.org

ISSN 2224-6096 (Paper) ISSN 2225-0581 (online)

Vol.4, No.10, 2014

12

A Review Study of Mechanical Fatigue Testing Methods for

Small-Scale Metal Materials

Nabel Kadum Abd-Ali

College of Engineering/Al-Qadisiyha University/ Al- Dewaniya /Iraq

E-mail of the author corresponding: [email protected]

Abstract

A review of the mechanical fatigue testing techniques and evaluation methods for the small scale metal materials

is presented. The concentration is on the different micro- and nanoscale testing techniques, classify, summarize,

and compare all of these techniques that are currently available. Then, have a tendency to proven the most used

and so popular. Furthermore, have a tendency to characterize the most studied materials and the attention

way in them once changed with time. Finally, the perspectives of studies on fatigue of the small scale metal

materials are offered.

Keywords: Fatigue crack; multi-scale fatigue; MEMS/NEMS; fatigue testing method.

1. Introduction The attention on materials behavior at small length scales has profited great attention in the last two decades, as a

couse of the increasing application, production, and commercialization of different types of micro- and nano-

electromechanical systems (MEMS/NEMS). Due to their small size, short time to response, high performance

and required low energy requirements, these devices are currently utilized in diversity of industrial, consumer

and biomedical applications [1-4]. Such success has stimulated a further improvement of their design, in order to

produce even more reliable and competitive components.

The industries of the micro- electromechanical systems MEMS and nano-electromechanical systems

NEMS used many materials such as metals, alloys and polymers (e.g., polyamide), not just the typical silicon-

based materials of microelectronics. Moreover, different types of nanostructured materials are being an

increasingly used.

Acquaintance of the mechanical properties it’s very necessary to designing, fabricating and predicting

the reliability of micro-devices. Additionally, with more spread of micro- electromechanical systems MEMS and

nano-electromechanical systems NEMS, the development of new materials and innovative fabrication processes

necessary to tighter design constraints enforced by the economic competition. As a conclusion, reliable and

repeatable estimation of the mechanical properties of both currently used and emerging materials is needed. In

this case, should define the characterization for standardized methodologies.

Regrettably, the standard well-assessed techniques for mechanical characterization at macro-scale can’t

be transferred to the micro/nano scale, as the machinery and equipment they include are not suitable for

manipulating components with submillimeter size. Also, it can be shown that the mechanical properties of

materials at smaller length scales can’t generally be resultant from bulk properties calculated by ordinary macro-

scale methods [5, 6]. In reality the material properties change with specimen size has been well known for

several years [7, 8]. In additional, the testing materials in small scales length consist from very small specimens

and miniaturized testing structures. Such testing structures should be provided with high resolution measurement

systems for accurately measuring loads and displacements. For matching such requirements, new and

appropriate techniques and equipment have been established.

Fatigue is the gradual structural damage resulting from the material exposure to periodic load and one

of very significant failure modes in engineering structural materials, has been studied for more than one hundred

years. In recent years, fatigue crack and their applications are gaining great interest among researchers who they

are focused on the field of fatigue research, which is transition from the traditional engineering to using

micro/nano technologies guides the fatigue's research interest to small scale materials [9]. On the small scale, the

design and reliability of the materials do not follow the conventional theories since the fatigue behavior of small-

scale materials has been proven to be different from that of bulk materials due to size effects. These small-scale

materials are widely used in Micro electromechanical Systems (MEMS)/Microsystems, etc. high-tech fields [10].

Materials comprising of nanoscale microstructures are of technical and developed interest due to their unusual

mechanical properties.

In general, it can be categorized the small-scale materials depending on their dimensions to three kinds

as shown below:

(a) Thin films whose dimension in one direction is close to micrometers or less.

(b) Thin fibers /wires whose dimensions in two directions are very small.

(c) Small particles.

Otherwise, these materials also can be classified depending on confinement for two types:

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Industrial Engineering Letters www.iiste.org

ISSN 2224-6096 (Paper) ISSN 2225-0581 (online)

Vol.4, No.10, 2014

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(1) Freestanding thin films, small dimensional foils (Fig. 1(a)) and thin wires (Fig. 1(b)).

(2) Thin films confined by a substrate (Fig. 1(c)).

Irrespective of any type of these materials at least one dimension of the material should be in the range from

micrometers to nanometers [11].

(a) Thin foil/film. (b) Fine wire.

(c) Substrate.

Fig.1. Schematic of the small-scale materials (a) freestanding thin foils/films, (b) fine

wires and (c) thin films confined by a substrate.

Nowadays, higher integration density in micro/nano systems is leading to material dimensions

(microstructural and geometrical dimensions) shrinking toward the submicrometer and nanometer scale.

Very few researches on mechanical characterization of micro-and nano-components were published

before the 1980s, but since, seems the micro-electromechanical systems MEMS, the number has become

significantly in increased. Numerous fatigue testing techniques for material specimens with characteristic size

ranging from few nanometers to several hundred micrometers, have been reported, investigated, developed,

described, and employed for determining mechanical fatigue testing techniques such as Uniaxial tension-tension,

Dynamic bending, Resonant vibration, Uniaxial tension-compression and Thermal-cyclic.

In this review we classify, describe, summarize, and compare all of these techniques that are currently

available. We prove what are the most used, why they are so popular, and what the actual vogue is. Furthermore,

we characterize the most studied materials and how the attention in them when changed with time.

2. Fatigue testing techniques and evaluation methods for small scale materials

Material mechanical properties are the basic input parameters for structural design of micro/ nano devices. The

accuracy of numerical modeling and simulation results depends on the accuracy of the material properties

provided as input. That is why, throughout the recent years several appropriate techniques are developed to

enhance the measuring accuracy or to increase the amount of the calculable mechanical quantities. We have a

tendency to present a classification of those techniques in Fig.2.

Fig.2. Classification of the fatigue testing techniques for materials at micro- and nano- scales.

To decrease the dimensions of materials into the micrometer nanometer scale. Some of new experimental

method investigated to measure fatigue properties, which is proposed that some of factors should be considered.

As shown below:

1- Ultra-low force load cell, closed-loop high resolution displacement actuator and PC-controlled data

acquisition system.

2- Reliable clamping method that can overcome the difficulty in mounting thin films without damaging.

3- Application of a simple and homogenous stress state which makes extraction of fatigue data and

comparison with bulk materials easier.

4- It is also anticipated to in-situ observe the fatigue damage process, such as fatigue crack initiation and

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growth during fatigue loading.

5- A simple sample fabrication process that allows the fabrication of small-scale materials suitable for

fatigue testing. Also the samples are easily prepared into transmission electron microscopy (TEM)

samples.

(a) Uniaxial cyclic loading (b) Dynamic bending

Fig.3. Flipchart of fatigue machine for small-scale materials through different testing methods:

(a) uniaxial cyclic loading, (b) Dynamic bending.

Many of the fatigue testing techniques presented in the literature are assembled by the researchers who

designed them. In the following sections are some emerging research efforts that focus on fatigue testing

methods, which are discussed in more detail.

2.1 Uniaxial tension-tension

Materials consisting of micro/nano scale microstructures are of scientific and industrial interest due to

their unusual mechanical properties. The mechanical properties of materials for Micro-Electro-Mechanical

System (MEMS) applications should be understood for the more development and commercialization of MEMS

devices. New test methods appropriate for small scale specimens, including testing techniques, specimen design

and equipment, are necessary to evaluate the mechanical properties of thin films as a result of these properties

dissent from those of bulk material [12-15].

Some researchers studied the load controlled fatigue results extending to over 1×105 cycles for electron

beam evaporated copper with specimen parameters the freestanding Cu films shape and used the thickness range

of 20 µm-100 µm. and using The piezoelectric actuators were controlling the closed-loop feedback allowed load-

controlled tests of fatigue with cycle duration of 15 Sec. New methods were expanded for uniaxial cyclic loading

of freestanding metal foils by substituting the magnetic vibrator to piezoelectric actuator. The basic principle of a

fatigue machine for small-scale materials was presented in Fig. 3(a). The plastic ratcheting happened in all of the

fatigue tests, to a much bigger degree in the specimens tested at the higher loads. Even though all these tests

have been run under load control, the fatigue failures at the highest loads appeared to be of the low cycle fatigue

type, as shown in Fig. 4. [16]. In additional, [17] designed methods for fatigue testing and tensile by using the

piezoelectric actuators and electron speckle interferometry to estimate the fatigue properties of thin Ni films.

Fig.4. Plot of maximum cyclic stress against the number of cycles to failure (S-N plot) for electron-beam-

evaporated copper thin-film specimens [16].

The experimental setup was a piezoelectric driven uniaxial stress-strain measurement system to

estimate fatigue properties of thin Ni films. The force calibration was linear over 0-1.50 N with a resolution of

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0.2 mN. The displacement range of the closed-loop piezoelectric actuator in the test apparatus was 0-90 µm, with

a displacement measurement resolution of 1.8 nm. The frequency response of the piezoelectric actuator was up

to 103 Hz, which is enough to do the fatigue test. To simplify grip specimen, three-axis micro positioning system

and a CCD with remote microscope were used to locate designed of glass grip appropriately. Thus, a micro

electromechanical system (MEMS) was the based devices that used to manage the fatigue testing of very small

specimens (20µm) for that attaching the specimen to the test equipment became very important.

While the tension–tension fatigue behavior of carbon nanotube wires have been studied by H.E. Misak

et al [18] and his coworker they conclude that the Three versions of CNT wires (30-, 60-, and 100-yarn) in the as

usual condition were tested to investigate their tension–tension fatigue behavior. Electrical conductivity of CNT

wires was also measured during fatigue tests. In order to establish fatigue-life diagrams (S–N curves), fatigue

tests were conducted at different stress levels (35%, 50%, 60%, 75% and 80%) of ultimate tensile strengths and

at 0.1 Hz and R of 0.1. The applied stress versus cycles to failure data showed a linear correlation on the semi-

log diagram then resulted equations are shown below in Fig. 5.

Fig.5. Applied stress vs. number of cycle relationships of 100-, 60-, and 30- yarns CNT wires [18].

Fatigue life was reduced significantly with an increasing number of yarns in the CNT wires. The major

damage under fatigue was caused by the formation of kink bands and tearing and breakage of strands in the wire.

These damage modes were caused by low resistance to shear force due to weak lateral bonds (van der Waals

forces, frictional forces, and mechanical interlocking) between CNTs. Electrical conductivity of CNT wires

increased with the increase in fatigue load and number of applied fatigue cycles. Micro CT density

measurements provided evidence that the increase in conductivity was due to the reduction of micro/nano voids

between and inside the yarns [19-22].

2.2 Dynamic bending

Instead of uniaxial tension-tension tests, mechanical properties of materials at micro/nanoscales can be

found from bending tests, which were sometimes preferred over tension tests, since they need smaller forces and

produce grander displacements. In this situation, micro-cantilever beams were affected by dynamic bending

under constant amplitude load control supplied by repeated indentation [23, 24].

Then, developed the fatigue machine for small-scale materials as shown in Fig.3 (b). Fatigue crack

growth tests were achieved by using a newly developed fatigue testing machine for micro sized specimens as

shown in Fig.6.Under constant amplitude load ( ,where ) of 2mN and stress ratios

( , where is the minimum load and is the maximum load applied over the

fatigue cycle) of 0.1, 0.3and 0.5. The fatigue machine has a displacement of 5nm and a load of 10µm. the

machine can used different waveforms for frequencies and cyclic loading up to 100 Hz. And used two types of

cantilever beam micro sized specimens with different breadth were all set from an electroless plated Ni-P

amorphous alloy thin film by focused ion beam machining as shown in Fig 7.

After seeing the striations on the fatigue fracture surfaces and fatigue crack propagation rates were

evaluated by a careful measurement of the striation spacing. Found the fatigue crack growth rates at stress ratios

of 0.3 and 0.5 were almost similar, but the fatigue crack growth rate at a stress ratio of 0.1 were lower compared

to the others at a given value of ∆ K as shown in Fig. 8, and Fig. 9. Shown the fatigue crack growth resistance

was depending on the breadth of specimen [23].

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Fig.6. Block diagram of fatigue testing machine for micro sized specimens[23].

Fig.7. Two types of micro sized cantilever beam specimens with different breadth (B) prepared by focused ion

beam machining. (a) B=10mm and (b) B=30mm [23].

Fig.8. Fatigue crack growth resistance curves for micro sized Ni-P amorphous alloy specimens with a B

of 10µm tested at different stress ratios [23].

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Fig.9. Fatigue crack growth resistance curves for micro size Ni-P amorphous alloy specimens with different B

tested at a stress ratio of 0.5 [23].

On the other hand, the cyclic loading of a free standing Ni-P amorphous alloy and SUS304 stainless

steel foils was carried out in[24] by consuming dynamic bending of micro cantilever beam. Before testing, the

diamond tip of the indenture was precisely located at the loading point of the micro beam by the repeated

calibrations using the CCD camera, and then the micro cantilever beam was dynamically bent under constant

load amplitude control. Based on the elastic beam theory, when the beam was deflected dynamically, the top

surface at the fixed end of the micro beam would be subjected to a tension-tension stress and the back surface of

the micro beam to a compression-compression stress. During dynamic bending tests, the micro beam stiffness

was recorded automatically. The fatigue life of the free standing small samples was determined based on the

variation of the micro beam stiffness during dynamic bending because crack initiation would cause a decrease in

the stiffness. While Fig. 10. Shown a process to manner cyclic tension-compression loading of thin Ag films

deposited onto a SiO2 substrate was projected by [25] out of a nanoindenter with a function of continuous

stiffness measurement, The dynamically elastic bending of the film/SiO 2 substrate composite causes cyclic

tension-compression loading to the thin Ag film at the top of the substrate. Fatigue failure of the Ag film was

controlled by the variation of the stiffness of the microbeam.

Fig.10. Schematic of the bilayer Ag-SiO2 beam deflection experiment. The indenter tip was used to deflect the

beam [25].

2.3 Uniaxial tension-compression

The most common tests involve uniaxial cyclic loading. The tension-compression cyclic loading tests

can also be studied on thin free-standing films. The new fatigue testing method over applying a uniaxial load to

metal films placed on a compliant substrate with a high elasticity and mechanical stability, as shown in Fig. 11

(a). The film/substrate-composite was strained by turn On/Off tensile load while the film was deformed

elastically and plastically by turning On/Off tension and in compression load, respectively. Additionally, to

estimate the applied plastic strain range, the cyclic stress-strain curves of the specimens were measured in

normal by X-ray diffraction goniometer, as shown in Fig. 11 (b). The advantages of this method are [26]:

1. A cyclic tension-compression load could be applied to the very small thickness metal films even less

than 10 nm.

2. The bulk-like fatigue sample of metal film/substrate composite simply invented and fixed to the fatigue

machine relatively large clamps.

3. Fatigue tests could be controlled via constant load range or total strain range.

4. By using conventional preparation methods can easily to get the samples for Transmission Electron

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Microscopy (TEM) from fatigued specimens.

(a) Cu film/polyimide specimen. (b) Film stress.

Fig.11. (a) Schematic of the Cu film/polyimide substrate composite for fatigue testing, (b) film stress Ϭx as a

function of substrate strain during the second and tenth cycle of the test on a 0.7 lm thick copper film. The

arrows indicate the development of the film stresses during loading and unloading [26].

The molecular dynamics (MD) simulations were utilized to realize the yield behavior of Nano

crystalline Ni and Cu with grain sizes ≤ 10 nm at high strain rates. The propos calculation flow stress values at a

strain rate of (109 s

-1) an asymmetry in the strength values in tension and compression for the Nano crystalline

metal more powerful in compression than in tension. Nevertheless, the tension–compression strength asymmetry

was detected to reduce as the grain size decreases to a size of 4 nm, after which, a further reduction in grain size

resulted to rise in the strength asymmetry values. The higher asymmetry at size of 2 nm was consistent with the

mechanical behavior observed in metallic glasses and can be referred to the amorphous nature of the Nano

crystalline metal. The effect of strain rate on the yield behavior of Nano crystalline metals as obtained from

molecular dynamics (MD) simulations was debated and compared with that informed in the literature obtained

by molecular statics simulations for quasi-static loading conditions [27].

Also, for enough small grain sizes with diameters ≤ 20 nm (reliant to the material) molecular dynamics

simulations (MD) and latest experimental researches had shown that inter-grain processes typically dominate

deformation. These processes can result in weakening of a metal with reducing the grain size. Nanocrystalline

metals with grain sizes in the inverse Hall–Petch regime had gained considerable attention due to their increased

strengths during deformation at high strain rates [28]. Whilst there had important progress in understanding

mechanisms of plastic deformation in Nano crystalline metals, understanding the effect of grain size and strain

rate on macroscopic deformation behavior at high strain rates was still in its infancy.

On the other hand, the influence of loading mode and strain amplitude on fatigue crack growth from

microscopic observations and analysis by comparing the micro-mechanisms of fatigue crack initiation and

propagation behaviors of an unnotched solid polycrystalline copper specimen under cyclic tension–compression

and torsion loadings were investigated [29]. The greatest significant finding displayed that the connection

between the matrix and persistent slip bands (PSBs) on the surface in crystalline materials is the favored location

for fatigue cracking [30-32]. This can be attributed to the plastic strain localization and high stress

engrossment in intrusions and extrusion initiated by cyclic slip irreversibility [32,33]. Moreover, the grain

boundaries (GBs) were furthermore significant sites for the fatigue crack initiation. Zhang et al. [34-37]

systematically examined the probability of the appearance of diverse grain boundaries (GBs) cracking, and

discovered that the intergranular fatigue cracking strongly depended on the interactions of persistent slip bands

(PSBs) with grain boundaries (GBs) in fatigued bicrystals instead of the grain boundaries (GBs) structure itself.

The SEM-ECC method was utilized to monitor styles of disintegration of the fatigued specimens. From

a great deal of observations, the most widespread dislocation patterns of coarse-grained copper after cyclic

tension–compression loadings with an matching effective usual strain amplitude were shown in Fig. 12.

Displayed the dislocation patterns in the interior grains and specimen surface under tension–compression fatigue,

respectively [29].

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Fig.12. Dislocation patterns of coarse-grained copper after cyclic deformation with equivalent effective normal

strain amplitude, (a) the internal area of tension–compression fatigue sample, (b) the external area of tension–

compression fatigue sample [29].

Fatigue cracks on the surface of polycrystalline copper under cyclic tension–compression and torsion

loadings with equivalent effective normal strain amplitude as shown in Figs13. And explained by the Slip band

(SB) – Grain boundaries (GB) damage mechanism [29].

Fig.13. Fatigue crack initiation and early propagation of coarse-grained copper during tension–compression

fatigue tests [29].

Hence, this method appears to be the most suitable one for fatigue realizations of thin films at room

temperature. Following such an approach, a number of fatigue behaviors investigations of thin metal films and

metallic multilayers are proceed.

Table.1: Methods of fatigue testing of small-scale materials.

Method Material Shape of

specimen Limitation

Uniaxial

tension-

tension [12-

22].

Cu, Ni,

Carbon

nanotube(CNT)

Free-standing

thin wires, dog-

bone shaped

foils,

-The transition from high cycle to low cycle fatigue

happen at around

5000 cycles.

- Stress decrease from 0.75 % to 0.5%.

Dynamic

bending

[23-25]

Ni-P

amorphous

alloys.

Stainless steel.

Silver.

Micro-sized

cantilever

beam

Machine parameters:

- Displacement =5nm.

- Load= 10µm.

- Frequency= 100 Hz

Uniaxial

tension-

compression

[26-37]

Cu/ Polyimide

Dog-bone shaped

films confined by

a substrate

(a) A cyclic tension-compression load could be

applied to the very small thickness metal films even

less than 10 nm.

(b) The bulk-like fatigue sample of metal

film/substrate composite simply invented and fixed

to the fatigue machine relatively large clamps.

(c) Fatigue tests could be controlled via constant

load range or total strain range.

(d) By using conventional preparation methods can

easily to get the samples for Transmission Electron

Microscopy (TEM) from fatigued specimens.

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Fig.14. Fatigue testing methods to characterize micro/nano samples.

It is worth pointing out that today the fatigue testing techniques are the most diffused and used

techniques for measure the fatigue at small scale metals. Thus, during the several types that is presented for

fatigue testing methods for small-scale materials. There is no one of them has enough merits as pointed

previously. So until now there is a long way to find novel experimental methods and special characterization

ways for the small-scale materials, especially for nano-scale wires and free-standing ultra-thin films. However,

there are many studies the mechanical fatigue testing techniques and evaluation methods have been carried out.

3. Conclusion

In the present paper, we reviewed the most relevant researches on mechanical fatigue testing techniques for

small-scales material. The most important testing techniques were identified, classified and briefly described. So

far, the main findings mentioned in previous sections are summarized below.

1. Current fatigue testing techniques to some extent can obtain fatigue properties of small-scale materials and

reveal size effects of the fatigue behavior of small-scale materials.

2. No matter whether small-scale materials are confined by substrates or not, the fatigue strength of the

material is significantly enhanced with decreasing length scale of the material. This is generally attributed to

the increase in yield strength with decreasing length scales including both microstructural and geometric

dimensions.

3. For thin metal films confined by a substrate, convincible experimental evidence shows that when either the

film thickness or the grain size is decreased below roughly 1 µm, the typical dislocation wall and cell

structures found in fatigued coarse-grained bulk materials no longer develop and are replaced by individual

dislocations. Similarly, the typical surface damage of fatigued bulk metals, such as extrusions and cracks

near extrusions, is gradually suppressed and replaced by damage that is localized at interfaces, such as

cracks, grooves, and voids along grain and twin boundaries. This gradual transition from damage

characteristic of bulk metals to damage localized at interfaces is attributed to constraints on dislocation

activity at submicron length scales.

Although the great work carried out for improving more accurate and simple methodologies nowadays,

there are still neither standard methods for mechanical fatigue testing of materials at micro and nano-scales.

Need new mechanical fatigue testing techniques to support the continuous advance of micro and nanoelectronics,

new testing methodologies are also needed to support the continuous advance of micro and nanoelectronics, as

well as biology, for a deeper comprehending of the behavior of materials like nanostructured, nanotubes and

biological samples. All the outcomes of the bibliographic enquiry conveyed out by the authors are summarized

in table or graphic format for very simple consultation. We wish this review help as an introduction to the new

researchers in field the mechanical fatigue testing techniques and evaluation methods for the small scale metal

materials.

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