ME 215 – Engineering Materials I

Dr. Oğuzhan YILMAZAssistant Professor

Mechanical EngineeringUniversity of Gaziantep

Chapter 7

Brittle Fracture and Impact Properties

2

Introduction

A great deal of attention was directed to the brittle failure of welded ships and tankers.

Failures occured during winter months and when the are in heavy seas and anchored at dock.

This fact focussed on that normally ductile mild steel can become brittle under certain conditions.

Therefore, researches aimed to understand the mechanism of brittle fracture and fracture in general.

While the brittle failure of ships concentrated great attention to brittle failure in mild steel.

Brittle failures in tanks, pressure vessels, pipelines, and bridges have been noticed.

3

Introduction

There are five kinds of fracture in metals based on the nature of process:

I. Ductile,

II. Brittle,

III. Adiabatic shear,

IV. Creep,

V. Fatigue fracture.

Three basics factors contribute to a brittle-cleavage type of fracture:

(1) triaxial state of stress(2) Low temperature (3) High strain rate or rapid rate of loading

4

Introduction

There are circumstances under which certain ductile materials behave

as brittle.

Two important cases of this type of failure (i.e. brittle failure of ductile

materials) are:

1.Fatigue failure (which was studied previously)

2.Brittle fracture (which is going to be treated here).

5

Introduction

Common examples of catastrophic failures of structures caused by

brittle fracture are:

- Welded ships & tankers made of mild steel (during World War II)

- Rails of railways during cold winter periods.

6

Introduction

Brittle fractures in steel structures usually occur without visible or audible

warnings at stresses less than nominal Sy value.

Such fractures usually initiate at sharp notches and crack-like defects,

and may subsequently propagate through a complete structure at faster

than speed of sound.

Load-time history for an instrumented Charpy test (Dieter, 1988)

7

Brittle Fracture

Radiating pattern of markings is important as they point back towards the

origin of fracture, allowing the point of crack initiation to be traced (Fig. 1).

origin of fracture

radiating markings

Figure 1

8

Brittle Fracture

Fig. 2 shows the crack initiation and propagation with herringbone type

surface markings. The direction of crack propagation is the opposite to

the direction of crack initiation.

direction of crack propagation

direction of crack initiation

Figure 2

9

Impact Properties

Many engineering components are subjected to suddenly applied loads

and they are expected to transmit or absorb this impact load.

The energy of impact load can be absorbed by part as elastic or plastic

deformation.

In design stage, it is aimed that this energy of impact load is absorbed as

elastic deformation.

After load is passed, this elastic strain is released or transmitted, and the

structure does not suffer permanent deformation.

10

Impact Properties

However, the elastic range may be exceeded due to unexpected service

conditions or faulty design. In such cases, most ductile metals exhibit

some plastic deformation in two ways:

(1)it can redistribute the stress (thus, reducing harmful effects

(2)the visible appearance of plastic deformation itself can be a warning

for taking further precautions.

In a brittle metal structure, no noticeable deformation is observed and

fracture happens without warning.

Due to this fact, necessary cautions must be taken when using brittle

metals (e.g. using large safety factors).

11

Impact Properties

However, serious problems can arise when a ductile metal fractures in a

brittle manner without any prior plastic deformation.

Many metals which show a ductile behaviour in static tensile tests

exhibit a brittle behaviour under impact loading at low temperatures.

Thereby, the information from tensile tests is not enough to predict the

behaviour in such cases.

12

Impact Properties

The property of a material relating to work required to cause rupture is

toughness, which depends on the ductility and ultimate strength.

It is known that a high-rate of loading results in an increase in strength,

but a reduction in ductility. When forces are applied suddenly for very

short time intervals, another effect of such forces is to produce stress

waves.

13

Impact Properties

Not all materials respond in the same way to variations in strain rate.

For instance, a slowly applied point load shatters the glass while a high-

speed bullet punctures a fairly clean hole.

Similarly, sealing wax behaves in a ductile manner at low strain rate, but

snaps into two under a sharp blow.

14

Impact Velocity

The toughness of a material does not vary greatly over a considerable

range in striking velocity. However, above some critical speed (varying

from material-to-material), the energy required for rupture of a material

appears to decrease rapidly. This critical velocity is associated with rate

of propagation of plastic strain and is effected by the specimen length.

ρEVp E : Young’s modulus, MPa

ρ : mass density, kg/mm3

Velocity of stress wave (Vp) should

be distinguished from the velocity

of particles in stressed zone (Vx):

EVS x

In elastic region, velocity of

plastic wave propagation in

a cylindrical bar (Vp) is:

Following equation shows that stress (S) depends

on particle velocity (Vx) in addition to E and ρ:

e

epx dVV0

e : plastic strain

corresponding to Vx

15

Specimen Shape

The specimen shape also has a marked effect upon its capacity to resist

impact loads. A plain ductile bar will not fracture under an impact load at

normal temperatures. If the specimen is notched, fracture can happen

under a single blow.

Figure 3

Many different notch

configurations used

in impact tests are

suggested in ASTM

E23 & DIN 50115.

However, Charpy

and Izod are the two

standard classes of

specimens used for

notched-bar impact

testing (Fig. 3).

16

Impact Testing (Pendulum Type)

In pendulum type impact testing, the

impact load is produced by swinging

of an impact weight (W = m * g) from

initial height (h0) through the arc of a

circle, thus striking and fracturing the

notched specimen (Fig. 4). After that,

the weight reaches maximum height

(h1). Neclecting frictional losses, the

energy used to fracture the

specimen (U) is then approximately

defined as:

Figure 4

Absorbed Energy

(energy to rupture)

= Initial Potential Energy

(energy before rupture)

– Final Potential Energy

(energy after rupture)U = m * g * (h0 – h1)

The absorbed energy (U), indicated on the scale of tester, is expressed

in joule (i.e. N*m) or kg*m in metric system and in inch-pounds in British

system. This energy value is sometimes called “impact toughness”.

17

Charpy and Izod Type Impact Tests

The Charpy specimen is supported at the ends

and struck in the middle (Fig. 5).

However, the Izod specimen is a cantilever

beam with a notch on the tension side to ensure

fracture when the impact load is applied (Fig. 6).

Figure 5

Figure 6

Charpy and Izod type impact tests bring out the

notch behavior (Brittleness vs Ductility) by

applying a single overload of stress.

The notch behavior in an individual test applies

to specimen size, notch geometry and testing

conditions. Thus, such a behavior cannot be

generalized to the other specimen sizes or

conditions.

Charpy

Izod

18

Impact Fracture

brittle fractureductile fracture

19

Factors Affecting Impact Properties

Impact toughness values are greatly influenced by the testing conditions.

-The most pronounced is the effect of temperature on notch behaviour of

material.

- Tangential striking velocity should not be less than 3 m/s nor more than

6 m/s.

- Rigidity of testing machine and its parts are important since some

energy is absorbed by the machine itself.

20

Factors Affecting Impact Properties1

.

Temperature:

The notched-bar impact test has the greatest

importance in determining “ductile-to-brittle

transition” of a metal.

This transition occurs at a temperature below

which the material is brittle and fractures with a low

energy absorption & low ductility, and above which

it is ductile.

The transition actually covers a range of temperatures in which degree of

brittleness increases gradually as temperature falls.

It is very difficult to make a universal definition of transition temperature as

two different materials having the same transition temperature may have

different failures.

Therefore, there are many definitions of this temperature.

21

Factors Affecting Impact Properties

Fig. 7

Ta: The average temp. corresponding to minimum impact strength (15 ft/lb).

Tb: The lowest temp. (Fracture Transition Plastic - FTP) at which the specimen exhibits 100% shear fracture.

Tc: The temp. (Fracture Appearance Transition Temperature - FATT) at which 50% of fracture is ductile.

Td: The average temp. between ductile and brittle fracture, i.e. (Tb+Tf)/2.

Te: Like Ta, it is a special temp. (Ductility Transition Temperature - DTT) based on an arbitrary low-impact energy toughness.

Tf: The temp. (Nil Ductility Temperature - NDT) for 100% brittle fracture.

The definitions of transition temperature (Fig. 7) are as follows:

Figure 8

22

Factors Affecting Impact Properties

2. Composition: In metals, the impact testing is mostly applied to steel,

testing of nonferrous materials is seldom.

- The main factors influencing brittleness of steels are:

composition, heat treatment and section size.

- The greater is the hardness,

the higher is the transition temperature.

Considering the effect of composition in

steels, carbon content plays important

role (Fig. 8).

The optimum combination of properties

in quenched and tempered low alloy

steel occurs for 0.3 - 0.4 % C.

The effect of other elements on

impact properties can be found

in the textbook.

23

Factors Affecting Impact Properties

4

.

Microstructure: The shape of carbide

precipitates in steel has a great effect on

impact toughness.

A tempered martensitic structure has the

best combination of strength and fracture

toughness. Tensile properties of such

structures of the same carbon content

and the same hardness are alike, but

great variations in their impact toughness

with temperature.

3. Grain Size: As the grain size increases, transition temperature

increases and fracture stress decreases. Thereby, it is possible to

improve ductility and toughness of steel by obtaining ultrafine grain size.

24

Factors Affecting Impact Properties

5. Orientation: The orientation of test bar in a formed product affects both

the impact energy and the value of Fracture Appearance Transition

Temperature,FATT, as well as the tensile ductility. For rolled products,

orientation does not have a great influence on FATT.

Effect of specimen orientation of Charpy transition-temperature

curves (Dieter, 1988)

25

Ductile-to-Brittle Transition (Embrittlement)

1. Hydrogen Embrittlement: Hydrogen produces severe embrittlement in

many metals. Even very small amount of hydrogen can cause cracking

in steel and titanium. It may be introduced during melting and entrapped

during solidification, or it may be picked up during heat treatment, acid

pickling, electroplating or welding.

2. Temper Embrittlement: Tempering some steels within 450 - 590 °C

results in temper brittlement, which is manifested by increase in impact

transition temperature. It is due to segregation of certain elements to

grain boundaries, giving local hardening to fracture.

3. Blue Brittleness: Low-carbon steels exhibit two types of aging which

causes an increase in transition temperature: quench aging & strain

aging. Strain aging is the slow increase in hardness in steels finished

by cold work (mainly cold rolling). Blue brittleness is attributed to strain

aging caused by heating cold worked steel to around 205 °C.

26

Alternative Impact Tests

It is not possible to obtain realistic results by conventional tests if specimens

are of thicknesses greater than Charpy and Izod specimens. So, there are

two alternative tests of practical importance:

1. Drop Weight Test: This test (ASTM E208) is

employed to determine NDT of ferritic steels

of 15.9 mm or thicker. A simple rectangular

specimen is subjected to a single impact load

(free-falling weight with energy of 340-1630

J) at selected temperatures to determine

max. temp. at which specimen breaks (Fig.

9).

Figure 9

A crack-starter weld (63.5 mm long & 12.7 mm

wide) is deposited on tension side of

specimen. An artificial notch is cut at the centre

of weld bead length to start crack, and the

specimens are tested to determine the NDT.

27

Alternative Impact Tests

2. Dynamic Tear Test: This test (ASTM E604)

is used to determine resistance of a material

to rapid progressive fracturing. Single-edge

notched beam is impact loaded in 3-point

bending (Fig. 10). The notch is machined to

start a crack. The impact energy is imparted

by a swinging pendulum or a drop-weight of

highly sufficient capacity at a test velocity of

4.9 - 8.5 m/s. Hence, the total energy loss

during separation is recorded.

Figure 10

Figure 11

At temperatures below NDT, the fracture is

flat and completely brittle without any shear

lips (Fig. 11). Above NDT, absorbed energy

increases, surface begins to develop shear

lips becoming progressively more dominant.

At FTE and above, fracture is fully ductile.

28

Impact Testing of Plastics

The impact tests for plastics can be divided into two groups:

a) using instruments where energy is imparted by a swinging pendulum.

b) using free-falling weights or other impactors to impart energy.

The pendulum type machines are similar to those

used for testing metals, but smaller in size and

capacity to comply with low-energy requirements of

plastics.

Charpy and Izod type plastic specimens were

given in Fig.4.

As a standardized type, ISO Charpy specimen is:

a rectangular bar (120 mm long, 15 mm wide, 10

mm thick), which can be tested without or with

rectangular notch (2 mm wide and 3.3 mm high).

29

Impact Testing of Plastics

It is more difficult to interpret the results of impact testing for plastics:

1. The test may be too severe (may cause brittle behavior unrealistically).

2. The test result may be dependent more on crack propagation resistance

than ability to resist crack initiation.

3. Test conditions may give misleading results even on a comparative basis.

4. Test conditions may probably be unrelated to service conditions.