‘’FAILURE’’

Part 1

IE-114 Materials Science and General Chemistry

Lecture-8

OUTLINE

- Mechanisms of crack propogation for both ductile and

brittle modes of fracture

- Impact fracture testing techniques

- Strengths of Brittle Materials

- Fracture toughness and Plain Strain Fracture Toughness

Failure

The inability of a material to;

(1) Perform the intended function

(2) Meet performance criteria although it may still be operational

(3) Perform safely and reliably even after deterioration

Examples of Failure: Yielding, wear, buckling, corrosion and fracture

Fracture: The seperation of solid under stress into two or more parts.

Ductile Brittle

Most of the metal alloys are ductile, but ceramics are brittle. Polymers

exhibits the two types of fracture.

Mixture of ductile and brittle fracture may also be seen

(1) Ductile Fracture

Ductile fracture of a metal occurs after

extensive plastic deformation.

Characterized by slow crack propogation

(a) For extremely soft metals: (b) Most common tensile fracture profile:

There is about 100% reduction in area.

Pure gold or lead at RT or some metals

Polymers and inorganic glasses at

high T.

Necking is followed by frature.

A typical fracture process has two steps:

1) Crack formation

2) Propagation (this is the step determining the mode of the

fracture)

Fracture point

Stages of cup-and-cone DUCTILE fracture

Initial necking Small cavity

formation

Coalescence of cavities

to form a crack

Crack

propagation Final shear fracture at a

45o angle relative to

tensile direction

Cup-and-cone fracture of an aluminum alloy

Ductile fracture proceeds slowly as the crack

length is extended. This type of crack is called

stable.

Equiaxed dimples formed during

microvoid coalescence

Characteristic Features of Ductile

Fracture Surfaces

Shear dimples resulting from shear

loading

Dimples are characteristics features of ductile failure

Each dimple is one half of a microvoid formed and then seperated during the

fracture process

(2) Brittle Fracture

Ceramics fracture in a brittle manner with little or no plastic deformation at room

temperature

The motion of the crack is perpendicular to applied stress.

Many metals with the HCP crystal structure (i.e. Zn) fractures in a brittle

manner (limited number of slip systems)

BCC metals such as -iron, Mo, W fracture in brittle manner at low

temperatures and high strain rates

Cracks spread rapidly with very little accompanying plastic

deformation. This type of cracks are called unstable.

Fracture Point

Brittle crystalline materials have successive and repeated breaking of atomic bonds along specific crystallographic planes (cleavage

planes) and fracture cracks pass through grains. The process is

called cleavage and type of fracture is called transgranular.

Intergranular:

When grain boundary contains brittle

film or segregated detrimental

particles

Cracks propogates along the grain

boundaries

Transgranular:

Most brittle fractures in polycrystalline metals

are transgranular

Cracks propagate across the matrix of

grains

Characteristic Features of Brittle

Fracture Surfaces

In many cases, brittle fracture in metals occurs due to existance of defects,

low operating temperatures, or high strain rates

Defects may be formed either in manufacturing stage or develop during service

Manufacturing ( forging, rolling, extrusion and casting) defects;

- Large inclusions

- Poor microstructure

- Porosity

- Tears

- Cracks

- Voids

- Sharp corners

Brittle fracture initiates at the defect location (stress risers) - The fracture of a metal starts at a place where the stress-concentration is highest (which

may be at the top of a crack)

Brittle Fracture in Metals

Brittle Fracture Surfaces seen in Steels

Fracture surfaces of materials that failed in a brittle manner will

have their own characteristics

V-shaped ‘chevron’ marking

Radial fan shaped ridges

Cracks initiated at sharp corner

Cracks initiated at surface crack

Cup-cone fracture in Al Brittle fracture: mild Steel

Soft metals at RT (Au, Pb)

Metals, polymers,

inorganic glasses at high T.

• B is most common mode.

• Ductile fracture is desired.

Why?

A B C

Very Moderately Brittle

Brittle fracture:

no warning.

Note:

Remnant of

microvoid

formation

and

coalescence.

Brittle: crack failure

Plastic

region

crack + plastic

Toughness and Impact Testing

The test is used to measure the impact energy (amount of energy a material can absorb before fracturing) , which is also called notch toughness.

Standardized tests:

Charpy

Izod

- Used to measure the impact energy (or notch toughness)

- Standart V-notch specimens are used

Standart test specimen

The load is applied as an impact blow

from a weighted pendulum hammer. It is

released from a fixed height (h).

Pendulum strikes and fractures the

specimen at the notch. The pendulum

continues its swing rising to maximum

height h’. The energy absorption,

computed from the difference between h

and h’ is a measure of the impact energy.

Impact Testing

IMPACT TESTING APPARATUS

IZOD

CHARPY

final height

initial height

IZOD

Ductile to Brittle Transition Temperature(DBT)

Ductile-to-brittle trasnsition temperature is determined by

conducting Charpy or Izod test at various temperatures

Low temperatures, high stress values, and fast loading rates may

all cause a ductile material to behave in a brittle manner

Ductile to brittle transition temperature of Steel used in Titanic was 32oC, the seawater

temperature at the time of accident was -2oC

Important in material selection for components that operate in cold

environment

BCC metals (e.g., iron at T < 914°C)

Imp

ac

t E

ne

rgy

Temperature

High strength materials ( y > E/150)

polymers

More Ductile Brittle

Ductile-to-brittle transition temperature

FCC metals (e.g., Cu, Ni)

This temperature is often defined as:

-The temperature at which the absorbed energy assumes some value (e.g. 20 J)

or

- The temperature corresponding to some given fracture appearance (e.g. %50

fibrous)

brittle

(shiny)

ductile

(fibrous)

Fracture surfaces of V-notched charpy impact specimens

Metal alloys with FCC structures (Al and Cu) remain ductile even at extremely low T.

BCC and HCP alloys experience this transition.

Decreasing the grain size lowers the transition T and increasing C

content raises the CVN transition of the steels.

Factors that influence the ductile to brittle transition:

Composition

Heat treatment

Processing

SURFACE CRACK INTERNAL CRACK

This research area is about the relationships between material properties, stress level, the presence of crack producing flaws and crack propagation mechanisms.

The measured fracture strengths for brittle materials are lower than those predicted by theoretical calculations.

Microscopic flaws or cracks already existing within the material.

Fracture Mechanics

The flaws are sometimes called

stress raisers due to their ability to

amplify the applied stress in their locale.

The fracture of a material starts at a place where the stress (or stress

concentration) is maximum

If the crack has an elliptical shape and is oriented perpendicular to the

applied stress;

σm : maximum stress at the crack tip

σ0 : magnitude of the nominal applied tensile stress

ρt : radius of curvature of the crack tip

a : length of a surface crack or half of the length of an internal crack

The ratio of maximum stress at the crack tip to nominal applied stress:σm/ σnominal

This factor shows the degree to which an external stress is amplified at the tip of a

crack.

m = 2o(a/t)1/2

Kt = m/ o = 2(a/t)1/2

Maximum Stress at the crack tip, m

* The effect of stress raiser in brittle fracture is more significant than in ductile fracture of the

materials. In ductile material, the plastic deformation indicates the point when maximum

stress exceeds the yield strength. This causes a more uniform distribution of stress in the

vicinity of the stress raiser. But this does not occur in brittle materials.

Stress concentration factor, Kt

The critical stress (σc) required for crack

propagation in a brittle material:

If the magnitude of a tensile stress at the tip of a flaw exceeds the value of this

critical stress, then a crack forms and then propagates, which results in fracture.

(m> c)

c=(2Es/a)1/2

E: Modulus of elasticity

s : specific surface energy

a : one half of the length of an internal crack

Griffith Theory of Brittle Fracture

Energy balance between release of elastic

strain energy during propogation of crack and

surface energy

Above equation applies only to completely brittle materials. But, some metals

which fail in a brittle manner will experience some plastic deformation. So, s in

above equation is replaced by p + s

p : plastic deformation energy associated with crack extension

Condition for Crack Propogation in brittle material

Fracture Toughness, Kc

Fracture Toughness (Kc) is a measure of materials resistance to brittle

fracture when a crack is present.

σc : critical stress for crack propagation

a : crack length

Y : dimensionless parameter and its value depends

on both crack and specimen sizes and geometries,

and load application.

Fracture of a metarial starts at a place where the stress concentration is highest (e.g. at

the top of a sharp crack)

The critical value of stress concentration is Kc and depends on applied load and width of

the crack

Kc = Yca

SURFACE CRACK INTERNAL CRACK

Y=1 for aplate of infinite width having a through-

thickness crack

Y= 1.1 for aplate of semi-infinite width having an

edge crack lentgh of a.

If the thickness is much greater than the crack dimensions, Kc becomes independent of thickness and plain strain conditions exists. This means that when a load operates on a crack in the manner shown in there is no strain component perpendicular to the front and back faces.

B 2.5 (KIC/y)2

B: specimen thickness

KIC: plain strain fracture toughness

KIc = Ya

Plain Strain Condition:

Plain Strain Fracture Toughness, KIc

For relatively thin specimens, the value of Kc depends on the thickness.

The magnitude of KIC decreases with increasing strain rate and

decreasing temperature

KIC decreases as yield strength is improved by solid solution or

dispersion additions or by strain hardening.

KIC increases with reduction in grain size.

KIc

is a fundamental material property

and depends on:

Temperature

Strain rate

Microstructure

Room Temperature Yield Strength and Plane Strain

Fracture Toughness Data for Some Materials

Brittle materials have relatively low fracture tougnhness (Kıc)

Ductile materials have high Kıc values