FRACTURE BEHAVIOUR

OF NANOCOMPOSITES

-FATIGUE

SHEETHAL P

2ND M.Sc BPS

CBPST, KOCHI

Compared to microparticles , nanoparticles

have some unique features. Firstly, higher

specific surface area can promote stress transfer

from matrix to nanoparticles, Secondly; the

required loadings of nanoparticles in polymer

matrices are usually much lower than those of

micro-fillers (typically 10–40 vol. % for the latter).

Therefore, man y intrinsic merits of neat

polymers, such as low weight, ductility, good

processability, and transparency (e.g . for epoxy)

will be retained after the addition of nanoparticles.

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Fracture Modes Simple fracture is the

separation of a body into 2 or more pieces in response to an applied stress that is static (constant) and at temperatures that are low relative to the Tm of the material.

Classification is based on the ability of a material to experience plastic deformation.

SEM micrographs of fracture surface of

composite material

Types of Fracture Brittle Fracture

Ductile Fracture

Fatigue Fracture

Creep Fracture

Ductile fracture

Accompanied by significant

plastic deformation

Brittle fracture

Little or no sudden

plastic deformation

Fracture Mechanism

Imposed stress Crack Formation Propagation

Ductile failure has extensive plastic deformation in the vicinity of the advancing crack. The process proceeds relatively slow (stable). The crack resists any further extension unless there is an increase in the applied stress.

In brittle failure, cracks may spread very rapidly, with little deformation. These cracks are more unstable and crack propagation will continue without an increase in the applied stress.

5

=

e

E2

Equation governing fracture mechanisms

e is half of the crack length,

is the true surface energy

E is the Young's modulus.

the stress is inversely proportional to the square root of

the crack length.

Hence the tensile strength of a completely brittle material

is determined by the length of the largest crack existing

before loading.

For ductile materials (additional energy term p involved,

because of plastic deformations

8

Ductile vs Brittle FailureVery

Ductile

Moderately

DuctileBrittle

Fracture

behavior:

Large Moderate%AR or %EL Small

• Ductile fracture is

usually more desirable

than brittle fracture.

Ductile:

Warning before

fracture

Brittle:

No

warning

99

• Ductile failure:

-- one piece

-- large deformation

Figures from V.J. Colangelo and F.A. Heiser, Analysis of

Metallurgical Failures (2nd ed.), Fig. 4.1(a) and (b), p. 66 John

Wiley and Sons, Inc., 1987. Used with permission.

Example Of Failures

• Brittle failure:

-- many pieces

-- small deformations

Fracture behaviour

•Fracture behaviour of polymer nano composites,including the various

toughening and fracture mechanisms and the effects of nanopartticle

aspect ratio and dispersion.

The interfacial interactions and the difference of relaxation time between

clay and polymer chains have significant influences on the fracture

strength of polymers.

The failure strength of carbon nanotube systems under biaxial tensile

torsional loads is significantly different from what occurs under uniaxial

tensile loading

for eg In Al-Si nanocomposites,the failure occurs by void damage

accumilation, culminating in crack formation.

Fracture toughness testing

The fracture toughness of the specimens was determined by conforming to the procedure outlined in the standard ASTM D 5045-99.

The critical stress intensity factor, KICand strain energy release rate, GIC were determined according to linear elastic fracture mechanics principles

Specimens were loaded under plane-strain condition in three-point bending until failure occurred from an initially prepared sharp precrack.

Testing was conducted using a MTS 810 universal tester (MTS Systems Corporation, Eden Prairie, MN, USA) at a crosshead speed of 0.2 mm min−1:

where f is a geometric factor, p is the failure

load, B is the specimen thickness, w is the

specimen width, and a is the overall crack

length.

K1=(p/BW-1/2) f With X= a/w

F(x)=6x1/2 { 1.99-x(1-x)(2.15-3.9kx-12.7x2/(1+2x) (1-x)1/3}

fracture behavior of surface treated

montmorillonite/epoxy nanocomposites.

mechanical properties of clay-reinforced

nanocomposites are significantly affected by the

dispersion of clay particles in the matrix.

The effect of surface-treatment of Influence of clay

concentration on fracture toughness of nanocomposites

Montmorillonite (MMT) on the fracture behavior of

MMT/epoxy nanocomposite was investigated. For this

purpose, fracture tests were performed using samples

with three different clay concentration level.

After fracture tests, SEM analysis was made on the

fracture surfaces to examine the fracture mechanism. It

was found that the MMT treatment using 3-

aminopropyltriethoxysilane enhanced the fracture

toughness increased of the MMT/epoxy nanocomposite.

This is due to the improved intercalation effect and

interfacial strength between MMT and epoxy matrix.

.

SEM micrograph

of a neat

epoxyfracture

surface

Fracture surface

micrographs (SEM) of

nanocomposites made by

ultrasonic dispersion of (a)

1 wt%, (b) 2 wt%, and (c)

3 wt% clay (high

magnification pictures) and

(d) 1 wt%, (e) 2 wt%, and (f)

3 wt% clay (low

magnification pictures) in

epoxy.

Influence of clay

concentration on fracture

toughness of

nanocomposites.

Clear evidence of the distorted and perturbed crack path can be seen in Figures , These tortuous paths were caused by a crack deflection mechanism when the path of a propagating crack was impeded by the uniformly distributed nanoparticles (i.e., both the intercalated parallel platelets and partially exfoliated platelets)

The above observations lead to the inference that in this particular epoxy system, the occurrence of crack deflection mechanism only provided insignificant energy dissipation . Void formation and cavitation were not observed,

so there was an excellent interfacial interaction between the epoxy matrix and clay in an exfoliated and intercalated structure as had been predicted.

• size of clay aggregates increases with

increasing clay concentration, while at the same

time interparticle distance decreases and

roughness increases

At higher concentration, the clay agglomerates

may act as stress concentrators during the

fracture process and instigate localized matrix

shear yielding around the clay inclusions or cause

interfacial failure at the epoxy-clay interface.

•nanoclay assembled into uniformly distributed

closely spaced microstructures in the epoxy

matrix. It is conjectured that these intercalated

clay assemblies were in fact very efficient in

inhibiting the crack propagation

Fatigue Failure

It has been recognized that a material subjected to a repetitive or fluctuating stress will fail at a stress much lower than that required to cause failure on a single application of load. Failures occurring under conditions of dynamic loading are called fatigue failures

. Fatigue failure is characterized by three stages

Crack initiation

Crack propagation

Final fracture

Fatigue Fatigue is one of the primary reasons for

failure in structural materialsNano particles are believed to improve the fatigue behaviour without sacrificing stiffness of polymer composites

The mechanisms for the enhanced fatigue resistance may include crack planning,crack tip deflection,and partice debonding

Addition of nano particles results in an order of magnitude reduction in fatigue crack propagation rate for systems

Fatigue crack propagation rate can be reduced by reducing the diameter and length and improving their dispersion

. Fatigue failure is a multi-stage process. It begins with

the initiation of cracks, and with continued cyclic

loading the crack propagates, finally leading to the

rupture of the component or specimen.

For homogenious materials the fatigue behavior is

often characterized by an early crack that dominates

the damage development and lead to final fracture

For inhomogeneous materials, such as fiber-or

particulate-reinforced polymers, the fatigue damage at

an early stage is often diffuse in nature, as the crack

can be initiated

The S-N Curve A very useful way to visual the failure for a specific

material is with the S-N curve.

The “S-N” means stress verse cycles to failure,

which when plotted using the stress amplitude on

the vertical axis and the number of cycle to failure

on the horizontal axis.

An important characteristic to this plot as seen is

the “fatigue limit”.

6

10

14

16

22

18

26

30

34

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The point at which the curve flatters out is termed as fatigue limit and is well below the normal yield stress. The significance of the fatigue limit is that if the material is loaded below this stress, then it will not fail, regardless of the number of times it is loaded. Materials such as aluminium, copper and magnesium do not show a fatigue limit; therefore they will fail at any stress and number of cycles. Other important terms are fatigue strength and fatigue life. The fatigue strength can be defined as the stress that produces failure in a given number of cycles.The fatigue life can be defined as the

number of cycles required for a material to fail at a certain stress.

FATIGUE BEHAVIOUR OF POLYMER

NANOCOMPOSITES

TEM images of the

Polyamide

nanocomposites

containing 1 wt.%

nanoclay (a),

5 wt.% nanoclay (b)

and 15 wt.% nanoclay

(c).

.

The correlation between structure and fatigue behaviour of

polymer systems in terms of their resistance

against crack propagation were presented

An improvement in the resistance against crack propagation

was achieved by adding up to 5wt.% nanoclay for exfoliated

clay dispersion in the Polyamide.

A further increase of the clay content lead to an embrittlement

of the material due to the formation of agglomerates and

intercalated particles,

which act as stress concentrators in the polymer

matrix.

This study also demonstrated the potential of using nanoclay

to reinforce Polyamide and reduse the fatigue crack

propagation behaviour. It is worth

mentioning that not only the amount of the nanoclay content

has a significant influence on the mechanicalbehaviour, but

especially the dispersion of the nanoclay platelets within the

polymer matrix is most important.

SEM OBSERVATIONS OF THE FRACTURE

SURFACES

The fracture surfaces under cyclic stresses consist of the crack initiation region followed by a smooth region leading to steps or river-like pattern.

The fatigue fracture surfaces also contain a series of concentric crack growth bands surrounding the surface source. These bands are caused by intermittent growth of the crack due tobreakdown of a craze.

The discontinuous crack growth bands are followed by a region that show radial tear lines, secondary fracture features and increasingsurface roughness

Fracture surfaces of the nanocomposite

specimens were also similar for those

broken at higher stress levels and those

broken at lower stress levels.

Crack initiated from subsurface on the

fracture surface of nanocomposite (see

Fig. 4(c)).

For this material, the uniform dispersion

of nanofibers inside the polymer is

important and in some areas

agglomeration of nanofibers exists.

Estimation of Fatigue Life of Epoxy-

alumina Polymer Nanocomposites

Epoxy alumina polymer nanocomposites were

synthesized by in-situ polymerization technique.

Dispersion of rod shaped alumina nanoparticles,

having a length less than 50 nm and diameter in

the range of 10 nm, in the epoxy matrix was

achieved using ultrasonication.

Nanocomposites having 0.5, 1 and 1.5 wt % of

alumina nanoparticles were prepared. Good

dispersion of alumina nanoparticles in epoxy matrix

was observed through transmission electron

micrographs of the nanocomposites

It was observed that the addition of alumina

nanoparticles provides good improvement in

fatigue life of epoxy.

An increment of three to four times in the

fatigue life of nanocomposites having 1.5%

alumina particles was observed over that of

neat polymer at low stress levels. Whereas

increment in fatigue life of nanocomposites

decreased at higher stress level .

Fracture surfaces of specimens in

fatigue were examined with the help of

scanning electron microscope in order

to investigate mechanisms

responsible for the increase in fatigue

life.

Roughness of fractured surfaces of

nanocomposites were more in

comparison to that of neat epoxy

showing consumption of higher energy

for fatigue failure causing an

increment in the fatigue life of

nanocomposites

Fatigue behavior of Epoxy/ SiO2

NanocompositesReinforced with E- glass Fiber

Blackman et al studied the fracture and fatigue behavior of nano-

modified epoxy polymers

. Further, design engineers would clearly prefer both the initial

toughness and the long-term cyclic-fatigue properties to be

significantly enhanced by the presence of the toughening phase

in the epoxy polymer.

introduction of nano-silica particles into the epoxy polymer has

increased both the initial toughness, as measured by the fracture

toughness, and also significantly improved the cyclic-fatigue

behaviour of the epoxy polymer

Thermal fatigue and creep fracture

behaviours of a nanocomposite solder

Creep and thermal fatigue behaviours of joints soldered by a tin base nano composite solder were characterised at different temperatures

Comparison with Sn60Pb40 solder

The result shows that the nano composite solder has much better creep resistance and thermal creep fatigue property than the Sn60Pb40 solder

This is due to the uniformly dispersed nano sized Ag particles that have provided effective impediment to dislocation movement and grain boundary sliding,in addition to the alloying effect

Sn60Pb40 solder joints deform dominantly by transgranular sliding,while the nano composite solder joints creep by intergranular mechanism through grain boundary sliding and voids growth